Authors Jim O’Gorman Joe O’Gorman Mati Aharoni Raphaël Hertzog
License CC-BY-SA-3.0
Kali Linux
Revealed
Mastering the Penetration Testing
Distribution (2021)
Kali Linux
Revealed
Mastering the Penetration Testing
Distribution (2021)
by Raphaël Hertzog, Jim
O’Gorman, Mati Aharoni, and Joe
O’Gorman
Kali Linux Revealed
Copyright © 2021 Raphaël Hertzog, Jim O’Gorman, Mati Aharoni, and Joe O’Gorman
This book is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
è https://creativecommons.org/licenses/by-sa/3.0/
Some sections of this book borrow content from the ”Debian Administrator’s Handbook, Debian Jessie from
Discovery to Mastery” written by Raphaël Hertzog and Roland Mas, which is available here:
è https://debian-handbook.info/browse/stable/
For the purpose of the CC-BY-SA license, Kali Linux Revealed is an Adaptation of the Debian Administrator’s
Handbook.
”Kali Linux” is a trademark of Offensive Security. Any use or distribution of this book, modified or not, must
comply with the trademark policy defined here:
è https://www.kali.org/trademark-policy/
All Rights Not Explicitly Granted Above Are Reserved.
ISBN: 978-0-9976156-0-9 (paperback)
OffSec Press
230 Park Ave, 3rd Fl West
New York NY 10169
USA
www.offensive-security.com
Library of Congress Control Number: 2017905895
The information in this book is distributed on an ”As Is” basis, without warranty. While every precaution
has been taken in the preparation of this work, neither the authors nor OffSec Press shall have any liabil-
ity to any person or entity with respect to any loss or damage caused or alleged to be caused directly or
indirectly by the information contained in it.
Because of the dynamic nature of the Internet, any Web addresses or links contained in this book may have
changed since publication and may no longer be valid.
Printed in the United States of America.
Table of Contents
1. About Kali Linux 1
1.1 A Bit of History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Relationship with Debian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 The Flow of Packages ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Managing the Difference with Debian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Purpose and Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Main Kali Linux Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4.1 A Live System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4.2 Forensics Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4.3 A Custom Linux Kernel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4.4 Completely Customizable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4.5 A Trustable Operating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4.6 Usable on a Wide Range of ARM Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.5 Kali Linux Policies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
.
1.5.1 Network Services Disabled by Default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
..
1.5.2 A Curated Collection of Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2. Getting Started with Kali Linux 13
2.1 Downloading a Kali ISO Image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.1 Where to Download ............ .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.2 What to Download . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.3 Verifying Integrity and Authenticity . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Relying on the TLS-Protected Website . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Relying on PGP’s Web of Trust . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.4 Copying the Image on a DVD-ROM or USB Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Creating a Bootable Kali USB Drive on Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Creating a Bootable Kali USB Drive on Linux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Creating a Bootable Kali USB Drive on OS X/macOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2 Booting a Kali ISO Image in Live Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.1 On a Real Computer ................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 In a Virtual Machine ................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Preliminary Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
VirtualBox
VMware Workstation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3. Linux Fundamentals 47
3.1 What Is Linux and What Is It Doing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.1.1 Driving Hardware ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.1.2 Unifying File Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.1.3 Managing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.1.4 Rights Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 The Command Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2.1 How To Get a Command Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2.2 Command Line Basics: Browsing the Directory Tree and Managing Files . . . . . . . . . . . . . . . . . . 52
3.3 The File System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.1 The Filesystem Hierarchy Standard .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.2 The User’s Home Directory ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.4 Useful Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.4.1 Displaying and Modifying Text Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.4.2 Searching for Files and within Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.4.3 Managing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.4.4 Managing Rights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.4.5 Getting System Information and Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.4.6 Discovering the Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4. Installing Kali Linux 65
4.1 Minimal Installation Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2 Step by Step Installation on a Hard Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2.1 Plain Installation ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Booting and Starting the Installer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Selecting the Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Selecting the Country . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Selecting the Keyboard Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Detecting Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Loading Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Detecting Network Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Configuring the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
User Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Configuring the Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Detecting Disks and Other Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Partitioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Copying the Live Image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
IV Kali Linux Revealed
Configuring the Package Manager (apt) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Installing Metapackages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Installing the GRUB Boot Loader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Finishing the Installation and Rebooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.2.2 Installation on a Fully Encrypted File System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Introduction to LVM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Introduction to LUKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Setting Up Encrypted Partitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
End of the Guided Partitioning with Encrypted LVM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.3 Unattended Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.3.1 Preseeding Answers ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
With Boot Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
With a Preseed File in the Initrd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
With a Preseed File in the Boot Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
With a Preseed File Loaded from the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.3.2 Creating a Preseed File ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.4 ARM Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.5 Troubleshooting Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
5. Configuring Kali Linux 107
5.1 Configuring the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.1.1 On the Desktop with NetworkManager ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.1.2 On the Command Line with Ifupdown ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.1.3 On the Command Line with systemd-networkd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
5.2 Managing Unix Users and Unix Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.2.1 Creating User Accounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.2.2 Modifying an Existing Account or Password . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.2.3 Disabling an Account . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.2.4 Managing Unix Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.3 Configuring Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.3.1 Configuring a Specific Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.3.2 Configuring SSH for Remote Logins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.3.3 Configuring PostgreSQL Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Connection Type and Client Authentication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Creating Users and Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Managing PostgreSQL Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.3.4 Configuring Apache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
Configuring Virtual Hosts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
Common Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
5.4 Managing Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Table of Contents V
6. Helping Yourself and Getting Help 127
6.1 Documentation Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
6.1.1 Manual Pages .......... . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
6.1.2 Info Documents ......... . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.1.3 Package-Specific Documentation . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.1.4 Websites . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.1.5 Kali Documentation at kali.org/docs/ . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.2 Kali Linux Communities . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.2.1 Forums on forums.kali.org . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.2.2 #kali-linux IRC Channel on Freenode . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.3 Filing a Good Bug Report . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.3.1 Generic Recommendations . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
How to Communicate . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
What to Put in the Bug Report . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Miscellaneous Tips . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.3.2 Where to File a Bug Report . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
6.3.3 How to File a Bug Report . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Filing a Bug Report in Kali . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Filing a Bug Report in Debian . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
Filing a Bug Report in another Free Software Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7. Securing and Monitoring Kali Linux 155
7.1 Defining a Security Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
7.2 Possible Security Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.2.1 On a Server ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.2.2 On a Laptop ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.3 Securing Network Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
7.4 Firewall or Packet Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
7.4.1 Netfilter Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
7.4.2 Syntax of iptables and ip6tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
7.4.3 Creating Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.4.4 Installing the Rules at Each Boot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
7.5 Monitoring and Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
7.5.1 Monitoring Logs with logcheck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
7.5.2 Monitoring Activity in Real Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
7.5.3 Detecting Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Auditing Packages with dpkg --verify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Monitoring Files: AIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
VI Kali Linux Revealed
8. Debian Package Management 175
8.1 Introduction to APT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
8.1.1 Relationship between APT and dpkg .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
8.1.2 Understanding the sources.list File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
8.1.3 Kali Repositories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
The Kali-Rolling Repository . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
The Kali-Dev Repository . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
The Kali Linux Mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
8.2 Basic Package Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
8.2.1 Initializing APT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
8.2.2 Installing Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
Installing Packages with dpkg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
Installing Packages with APT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
8.2.3 Upgrading Kali Linux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
8.2.4 Removing and Purging Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
8.2.5 Inspecting Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Querying dpkg’s Database and Inspecting .deb Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Querying the Database of Available Packages with apt-cache and apt . . . . . . . . . . . . . . . . . . . . 191
8.2.6 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
Handling Problems after an Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
The dpkg Log File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
Reinstalling Packages with apt --reinstall and aptitude reinstall . . . . . . . . . . . . . . . . . . . 194
Leveraging --force-* to Repair Broken Dependencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
8.2.7 Frontends: aptitude and synaptic .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
Aptitude................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
Synaptic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
8.3 Advanced APT Configuration and Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
8.3.1 Configuring APT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
8.3.2 Managing Package Priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
8.3.3 Working with Several Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
8.3.4 Tracking Automatically Installed Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
8.3.5 Leveraging Multi-Arch Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
Enabling Multi-Arch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
Multi-Arch Related Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
8.3.6 Validating Package Authenticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
8.4 APT Package Reference: Digging Deeper into the Debian Package System . . . . . . . . . . . . . 210
8.4.1 The control File ................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
Dependencies: the Depends Field ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
Pre-Depends, a More Demanding Depends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
Recommends, Suggests, and Enhances Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
Conflicts: the Conflicts Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
Table of Contents VII
Incompatibilities: the Breaks Field .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
Provided Items: the Provides Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
Replacing Files: The Replaces Field .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
8.4.2 Configuration Scripts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
Installation and Upgrade Script Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
Package Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
8.4.3 Checksums, Conffiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
8.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
9. Advanced Usage 227
9.1 Modifying Kali Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
9.1.1 Getting the Sources ...... .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
9.1.2 Installing Build Dependencies . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
9.1.3 Making Changes . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
Applying a Patch . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
Tweaking Build Options . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
Packaging a New Upstream Version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
9.1.4 Starting the Build . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
9.2 Recompiling the Linux Kernel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
9.2.1 Introduction and Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
9.2.2 Getting the Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
9.2.3 Configuring the Kernel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
9.2.4 Compiling and Building the Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
9.3 Building Custom Kali Live ISO Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
9.3.1 Installing Pre-Requisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
9.3.2 Building Live Images with Different Desktop Environments . . . . . . . . . . . . . . . . . . . . . . . . . 242
9.3.3 Changing the Set of Installed Packages.......... . . . . . . . . . . . . . . . . . . . . . . . . . 243
9.3.4 Using Hooks to Tweak the Contents of the Image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
9.3.5 Adding Files in the ISO Image or in the Live Filesystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
9.4 Adding Persistence to the Live ISO with a USB Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
9.4.1 The Persistence Feature: Explanations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
9.4.2 Setting Up Unencrypted Persistence on a USB Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
9.4.3 Setting Up Encrypted Persistence on a USB Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
9.4.4 Using Multiple Persistence Stores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
9.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
9.5.1 Summary Tips for Modifying Kali Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
9.5.2 Summary Tips for Recompiling the Linux Kernel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
9.5.3 Summary Tips for Building Custom Kali Live ISO Images . . . . . . . . . . . . . . . . . . . . . . . . . . 253
10. Kali Linux in the Enterprise 257
10.1 Installing Kali Linux Over the Network (PXE Boot) . . . . . . . . . . . . . . . . . . . . . . . . . . 258
10.2 Leveraging Configuration Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
VIII Kali Linux Revealed
10.2.1 Setting Up SaltStack .......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
10.2.2 Executing Commands on Minions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
10.2.3 Salt States and Other Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
10.3 Extending and Customizing Kali Linux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
10.3.1 Forking Kali Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
10.3.2 Creating Configuration Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
10.3.3 Creating a Package Repository for APT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
10.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
11. Introduction to Security Assessments 283
11.1 Kali Linux in an Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
11.2 Types of Assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
11.2.1 Vulnerability Assessment .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
Likelihood of Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
Overall Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
In Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292
11.2.2 Compliance Penetration Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292
11.2.3 Traditional Penetration Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
11.2.4 Application Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
11.3 Formalization of the Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
11.4 Types of Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
11.4.1 Denial of Service .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
11.4.2 Memory Corruption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
11.4.3 Web Vulnerabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
11.4.4 Password Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
11.4.5 Client-Side Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
11.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
12. Conclusion: The Road Ahead 305
12.1 Keeping Up with Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
12.2 Showing Off Your Newly Gained Knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
12.3 Going Further . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
12.3.1 Towards System Administration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
12.3.2 Towards Penetration Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
Index 308
Table of Contents IX
Preface
You have no idea how good you have it.
In 1998, I was an up-and-coming hacker, co-founding one of the earliest professional white hat
hacking teams. We were kids, really, with dream jobs, paid to break into some of the most secure
computer systems, networks, and buildings on the planet.
It sounds pretty sexy, but in reality, we spent most of our time hovering over a keyboard, armed
with the digital tools of our trade. We wielded a sordid collection of programs, designed to map
networks and locate targets; then scan, exploit, and pivot through them. In some cases, one of
us (often Jim Chapple) would write custom tools to do wicked things like scan a Class A network
(something no other tool could do, at the time), but most often we would use or modify tools
written by the hacker community. In those pre-Google days, we frequented BugTraq, AstaLaVista,
Packet Storm, w00w00, SecurityFocus, X-Force, and other resources to conduct research and build
our arsenal.
Since we had limited time on each gig, we had to move quickly. That meant we couldn’t spend a
lot of time fiddling with tools. It meant we had to learn the core tools inside and out, and keep the
ancillary ones on tap, just in case. It meant we had to have our tools well-organized, documented,
and tested so there would be few surprises in the field. After all, if we didn’t get in, we lost face
with our clients and they would take our recommendations far less seriously.
Because of this, I spent a lot of time cataloging tools. When a tool was released or updated, I’d go
through a routine. I had to figure out if it would run on the attack platform (some didn’t), and
whether it was worthwhile (some weren’t); I had to update any scripts that relied on it, document
it, and test it, including carrying over any changes made to the previous version.
Then, I would shake out all the tools and put them in directories based on their purpose during an
assessment. I’d write wrapper scripts for certain tools, chain some tools together, and correlate
all that into a separate CD that we could take into sensitive areas, when customers wouldn’t let us
take in attack machines or remove media from their labs.
This process was painful, but it was necessary. We knew that we had the ability to break into any
network—if we applied our skills and expertise properly, stayed organized, and worked efficiently.
Although remaining undefeated was a motivator, it was about providing a service to clients who
needed us to break into networks, so they could plug gaps and move money toward critical-but-
neglected information security programs.
We spent years sharpening our skills and expertise but we wouldn’t have been successful without
organization and efficiency. We would have failed if we couldn’t put our hands on the proper tool
when needed.
That’s why I spent so much time researching, documenting, testing, and cataloging tools, and at
the turn of the 21st Century, it was quickly becoming an overwhelming, full-time job. Thanks to
the Internet, the worldwide attack surface exploded and the variety and number of attack tools
increased exponentially, as did the workload required to maintain them.
Starting in 2004, the Internet exploded not only as a foundation for business but also as a social
platform. Computers were affordable, more consumer-friendly and ubiquitous. Storage technol-
ogy expanded from megabytes to gigabytes. Ethernet jumped from hundreds of kilobits to tens
of megabits per second, and Internet connections were faster and cheaper than ever before. E-
commerce was on the rise, social media sites like Facebook (2004) and Twitter (2006) came online
and Google (1998) had matured to the point that anyone (including criminals) could find just about
anything online.
Research became critical for teams like ours because we had to keep up with new attacks and
toolsets. We responded to more computer crimes, and forensic work demanded that we tread
lightly as we mucked through potential evidence. The concept of a live CD meant that we could
perform live forensics on a compromised machine without compromising evidence.
Now our little team had to manage attack tools, forensic tools, and a sensitive area tool distribu-
tion; we had to keep up with all the latest attack and exploit methodologies; and we had to, you
know, actually do what we were paid for—penetration tests, which were in high demand. Things
were spinning out of control, and before long, we were spending less time in battle and much more
time researching, sharpening our tools, and planning.
We were not alone in this struggle. In 2004, Mati “Muts” Aharoni, a hacker and security profes-
sional released “WHoppiX” (White Hat Knoppix), a live Linux CD that he billed as “the ultimate
pen testing live CD,” It included “all the exploits from SecurityFocus, Packet Storm and k-otik,
Metasploit Framework 2.2, and much, much more.”
I remember downloading WHoppiX and thinking it was a great thing to have around. I downloaded
other live CDs, thinking that if I were ever in a real pinch, live CDs could save my bacon in the field.
But I wasn’t about to rely on WHoppiX or any other CD for real work. I didn’t trust any of them
to fulfill the majority of my needs; none of them felt right for my workflow; they were not full,
installable distributions; and the moment I downloaded them they were out of date. An aged
toolset is the kiss of death in our industry.
I simply added these CD images, despite their relatively massive size, to our arsenal and kept up
the painful process of maintaining our “real” toolkit.
But despite my personal opinions at the time, and perhaps despite Muts’ expectations, WHoppiX
and its descendants had a seismic impact on his life, our industry, and our community.
XII Kali Linux Revealed
In 2005, WHoppiX evolved into WHAX, with an expanded and updated toolset, based on “the more
modular SLAX (Slackware) live CD.” Muts and a growing team of volunteers from the hacker com-
munity seemed to realize that no matter how insightful they were, they could never anticipate all
the growth and fluctuation of our industry and that users of their CD would have varied needs in
the field. It was obvious that Muts and his team were actually using WHAX in the field, and they
seemed dedicated to making it work. This was encouraging to me.
In 2006, Muts, Max Moser, and their teams consolidated Auditor Security Linux and WHAX into
a single distribution called BackTrack. Still based on SLAX, BackTrack continued to grow, adding
more tools, more frameworks, extended language support, extensive wireless support, a menu
structure catering to both novice and pro users, and a heavily modified kernel. BackTrack became
the leading security distribution, but many like me still used it as a backup for their ”real tools.”
By early 2009, Muts and his team had extended BackTrack significantly to BackTrack 4. Now a full-
time job for Muts, BackTrack was no longer a live CD but a full-blown Ubuntu-based distribution
leveraging the Ubuntu software repositories. The shift marked a serious evolution: BackTrack 4
had an update mechanism. In Muts’ own words: “When syncing with our BackTrack repositories,
you will regularly get security tool updates soon after they are released.”
This was a turning point. The BackTrack team had tuned into the struggles facing pen testers,
forensic analysts and others working in our industry. Their efforts would save us countless hours
and provide a firm foundation, allowing us to get back into the fight and spend more time doing
the important (and fun) stuff. As a result, the community responded by flocking to the forums
and wiki; and by pitching in on the dev team. BackTrack was truly a community effort, with Muts
still leading the charge.
BackTrack 4 had finally become an industrial-strength platform and I, and others like me, breathed
a sigh of relief. We knew firsthand the “pain and sufferance” Muts and his team were bearing,
because we had been there. As a result, many of us began using BackTrack as a primary foundation
for our work. Yes, we still fiddled with tools, wrote our own code, and developed our own exploits
and techniques; and we researched and experimented; but we did not spend all our time collecting,
updating, validating, and organizing tools.
BackTrack 4 R1 and R2 were further revisions in 2010, leading to the ground-up rebuild of Back-
Track 5 in 2011. Still based on Ubuntu, and picking up steam with every release, BackTrack was
now a massive project that required a heroic volunteer and community effort but also funding.
Muts launched Offensive Security (in 2006) not only to provide world-class training and penetra-
tion testing services but also to provide a vehicle to keep BackTrack development rolling, and
ensure that BackTrack remained open-source and free to use.
BackTrack continued to grow and improve through 2012 (with R1, R2, and R3), maintaining an
Ubuntu core and adding hundreds of new tools, including physical and hardware exploitation
tools, VMware support, countless wireless and hardware drivers, and a multitude of stability im-
provements and bug fixes. However, after the release of R3, BackTrack development went rela-
tively, and somewhat mysteriously, quiet.
Preface XIII
There was some speculation in the industry. Some thought that BackTrack was getting “bought
out”, selling its soul to a faceless evil corporate overlord for a massive payout. Offensive Secu-
rity was growing into one of the most respected training companies and a thought leader in our
industry, and some speculated that its success had gobbled up and sidelined the key BackTrack
developers. However, nothing could be farther from the truth.
In 2013, Kali Linux 1.0 was released. From the release notes: “After a year of silent development,
Offensive Security is proud to announce the release and public availability of Kali Linux, the most
advanced, robust, and stable penetration-testing distribution to date. Kali is a more mature, se-
cure, and enterprise-ready version of BackTrack.”
Kali Linux was not a mere rebranding of BackTrack. Sporting more than 600 completely repack-
aged tools, it was clearly an amazing toolset, but there was still more to it than that. Kali had been
built, from the ground up, on a Debian core. To the uninformed, this might not seem like a big
deal. But the ripple effects were staggering. Thanks to a massive repackaging effort, Kali users
could download the source for every single tool; they could modify and rebuild a tool as needed,
with only a few keystrokes. Unlike other mainstream operating systems of the day, Kali Linux
synchronized with the Debian repositories four times a day, which meant Kali users could get
wickedly current package updates and security fixes. Kali developers threw themselves into the
fray, packaging and maintaining upstream versions of many tools so that users were constantly
kept on the bleeding edge. Thanks to its Debian roots, Kali’s users could bootstrap an installation
or ISO directly from the repositories, which opened the door for completely customized Kali in-
stallations or massive enterprise deployments, which could be further automated and customized
with preseed files. To complete the customization trifecta, Kali Users could modify the desktop
environment, alter menus, change icons, and even replace windowing environments. A massive
ARM development push opened the door for installation of Kali Linux on a wide range of hardware
platforms including access points, single-board computers (Raspberry Pi, ODROID, BeagleBone,
and CubieBoard, for example), and ARM-based Chromebook computers. And last but certainly
not least, Kali Linux sported seamless minor and major upgrades, which meant devotees would
never have to re-install customized Kali Linux setups.
The community took notice. In the first five days, 90,000 of us downloaded Kali 1.0.
This was just the beginning. In 2015, Kali 2.0 was released, followed by the 2016 rolling releases.
In summary, “If Kali 1.0 was focused on building a solid infrastructure, then Kali 2.0 is focused on
overhauling the user experience and maintaining updated packages and tool repositories.”
The current version of Kali Linux is a rolling distribution, which marks the end of discrete ver-
sions. Now, users are up to date continuously and receive updates and patches as they are created.
Core tools are updated more frequently thanks to an upstream version tagging system, ground-
breaking accessibility improvements for the visually impaired have been implemented, and the
Linux kernels are updated and patched to continue wireless 802.11 injection support. Software De-
fined Radio (SDR) and Near-Field Communication (NFC) tools add support for new fields of security
testing. Full Linux encrypted disk installation and emergency self-destruct options are available,
XIV Kali Linux Revealed
thanks to LVM and LUKS respectively, USB persistence options have been added, allowing USB-
based Kali installs to maintain changes between reboots, whether the USB drive is encrypted or
not. Finally, the latest revisions of Kali opened the door for NetHunter, an open-source world-class
operating system running on mobile devices based on Kali Linux and Android.
Kali Linux has evolved not only into the information security professional’s platform of choice,
but truly into an industrial-grade, world-class, mature, secure, and enterprise-ready operating
system distribution.
Through the decade-long development process, Muts and his team, along with the tireless dedi-
cation of countless volunteers from the hacker community, have taken on the burden of stream-
lining and organizing our work environment, freeing us from much of the drudgery of our work
and providing a secure and reliable foundation, allowing us to concentrate on driving the industry
forward to the end goal of securing our digital world.
And interestingly, but not surprisingly, an amazing community has built up around Kali Linux.
Each and every month, three to four hundred thousand of us download a version of Kali. We come
together on the Kali forums, some forty-thousand strong, and three to four hundred of us at a time
can be found on the Kali IRC channel. We gather at conferences and attend Kali Dojos to learn how
to best leverage Kali from the developers themselves.
Kali Linux has changed the world of information security for the better, and Muts and his team
have saved each of us countless hours of toil and frustration, allowing us to spend more time and
energy driving the industry forward, together.
But despite its amazing acceptance, support, and popularity, Kali has never released an official
manual. Well, now that has changed. I’m thrilled to have come alongside the Kali development
team and specifically Mati Aharoni, Raphaël Hertzog, Devon Kearns, and Jim O’Gorman to offer
this, the first in perhaps a series of official publications focused on Kali Linux. In this book, we
will focus on the Kali Linux platform itself, and help you understand and maximize the usage of
Kali from the ground up. We won’t yet delve into the arsenal of tools contained in Kali Linux, but
whether you’re a veteran or an absolute n00b, this is the best place to start, if you’re ready to dig
in and get serious with Kali Linux. Regardless of how long you’ve been at the game, your decision
to read this book connects you to the growing Kali Linux community, one of the oldest, largest,
most active, and most vibrant in our industry.
On behalf of Muts and the rest of the amazing Kali team, congratulations on taking the first step
to mastering Kali Linux!
Johnny Long
February 2017
Preface XV
Foreword
The sixteen high-end laptops ordered for your pentesting team just arrived, and you have been
tasked to set them up—for tomorrow’s offsite engagement. You install Kali and boot up one of the
laptops only to find that it is barely usable. Despite Kali’s cutting-edge kernel, the network cards
and mouse aren’t working, and the hefty NVIDIA graphics card and GPU are staring at you blankly,
because they lack properly installed drivers. You sigh.
In Kali Live mode, you quickly type lspci into a console, then squint. You scroll through the
hardware listing: “PCI bridge, USB controller, SATA controller. Aha! Ethernet and Network con-
trollers.” A quick Google search for their respective model numbers, cross referenced with the
Kali kernel version, reveals that these cutting-edge drivers haven’t reached the mainline kernel
yet.
But all is not lost. A plan is slowly formulating in your head, and you thank the heavens for the
Kali Linux Revealed book that you picked up a couple of weeks ago. You could use the Kali Live-
Build system to create a custom Kali ISO, which would have the needed drivers baked into the
installation media. In addition, you could include the NVIDIA graphics drivers as well as the CUDA
libraries needed to get that beast of a GPU to talk nicely to hashcat, and have it purr while cracking
password hashes at blistering speeds. Heck, you could even throw in a custom wallpaper with a
Microsoft Logo on it, to taunt your team at work.
Since the hardware profiles for your installations are identical, you add a preseeded boot option to
the ISO, so that your team can boot off a USB stick and have Kali installed with no user interaction—
the installation takes care of itself, full disk encryption and all.
Perfect! You can now generate an updated version of Kali on demand, specifically designed and
optimized for your hardware. You saved the day. Mission complete!
With the deluge of hardware hitting the market, this scenario is becoming more common for
those of us who venture away from mainstream operating systems, in search of something leaner,
meaner, or more suitable to our work and style.
This is especially applicable to those attracted to the security field, whether it be an alluring hobby,
fascination, or line of work. As newcomers, they often find themselves stumped by the environ-
ment or the operating system. For many newcomers Kali is their first introduction to Linux.
We recognized this shift in our user base a couple of years back, and figured that we could help
our community by creating a structured, introductory book that would guide users into the world
of security, while giving them all the Linux sophistication they would need to get started. And so,
the Kali book was born—now available free over the Internet for the benefit of anyone interested
in entering the field of security through Kali Linux.
As the book started taking shape, however, we quickly realized that there was untapped potential.
This would be a great opportunity to go further than an introductory Kali Linux book and explore
some of the more interesting and little-known features. Hence, the name of the book: Kali Linux
Revealed.
By the end, we were chuffed with the result. The book answered all our requirements and I’m
proud to say it exceeded our expectations. We came to the realization that we had inadvertently
enlarged the book’s potential user base. It was no longer intended only for newcomers to the
security field, but also included great information for experienced penetration testers who needed
to improve and polish their control of Kali Linux—allowing them to unlock the full potential of
our distribution. Whether they were fielding a single machine or thousands across an enterprise,
making minor configuration changes or completely customizing down to the kernel level, building
their own repositories, touching the surface or delving deep into the amazing Debian package
management system, Kali Linux Revealed provides the roadmap.
With your map in hand, on behalf of myself and the entire Kali Linux team, I wish you an exciting,
fun, fruitful, and “revealing” journey!
Muts
February 2017
XVIII Kali Linux Revealed
Introduction
Kali Linux is the world’s most powerful and popular penetration testing platform, used by security
professionals in a wide range of specializations, including penetration testing, forensics, reverse
engineering, and vulnerability assessment. It is the culmination of years of refinement and the
result of a continuous evolution of the platform, from WHoppiX to WHAX, to BackTrack, and now
to a complete penetration testing framework leveraging many features of Debian GNU/Linux and
the vibrant open source community worldwide.
Kali Linux has not been built to be a simple collection of tools, but rather a flexible framework
that professional penetration testers, security enthusiasts, students, and amateurs can customize
to fit their specific needs.
Why This Book?
Kali Linux is not merely a collection of various information security tools that are installed on a
standard Debian base and preconfigured to get you up and running right away. To get the most
out of Kali, it is important to have a thorough understanding of its powerful Debian GNU/Linux
underpinnings (which support all those great tools) and learning how you can put them to use in
your environment.
Although Kali is decidedly multi-purpose, it is primarily designed to aid in penetration testing.
The objective of this book is not only to help you feel at home when you use Kali Linux, but also to
help improve your understanding and streamline your experience so that when you are engaged
in a penetration test and time is of the essence, you won’t need to worry about losing precious
minutes to install new software or enable a new network service. In this book, we will introduce
you first to Linux, then we will dive deeper as we introduce you to the nuances specific to Kali
Linux so you know exactly what is going on under the hood.
This is invaluable knowledge to have, particularly when you are trying to work under tight time
constraints. It is not uncommon to require this depth of knowledge when you are getting set up,
troubleshooting a problem, struggling to bend a tool to your will, parsing output from a tool, or
leveraging Kali in a larger-scale environment.
Is This Book for You?
If you are eager to dive into the intellectually rich and incredibly fascinating field of information
security, and have rightfully selected Kali Linux as a primary platform, then this book will help
you in that journey. This book is written to help first-time Linux users, as well as current Kali
users seeking to deepen their knowledge about the underpinnings of Kali, as well as those who
have used Kali for years but who are looking to formalize their learning, expand their use of Kali,
and fill in gaps in their knowledge.
In addition, this book can serve as a roadmap, technical reference, and study guide for those pur-
suing the Kali Linux Certified Professional certification.
General Approach and Book Structure
This book has been designed so that you can put your hands on Kali Linux right from the start.
You don’t have to read half of the book to get started. Every topic is covered in a very pragmatic
manner, and the book is packed with samples and screenshots to help make the explanations more
concrete.
In chapter 1, “About Kali Linux” [page 2], we define some basic terminology and explain the pur-
pose of Kali Linux. In chapter 2, “Getting Started with Kali Linux” [page 14], we guide you step-by-
step from the download of the ISO image to getting Kali Linux running on your computer. Next
comes chapter 3, “Linux Fundamentals” [page 48] which supplies the basic knowledge that you
need to know about any Linux system, such as its architecture, installation process, file system
hierarchy, permissions, and more.
At this point, you have been using Kali Linux as live system for a while. With chapter 4, “Installing
Kali Linux” [page 66] you will learn how to make a permanent Kali Linux installation (on your hard
disk) and with chapter 5, “Configuring Kali Linux” [page 108] how to tweak it to your liking. As
a regular Kali user, it is time to get familiar with the important resources available to Kali users:
chapter 6, “Helping Yourself and Getting Help” [page 128] gives you the keys to deal with the
unexpected problems that you will likely face.
With the basics well covered, the rest of the book dives into more advanced topics: chapter 7,
“Securing and Monitoring Kali Linux” [page 156] gives you tips to ensure that your Kali Linux
installation meets your security requirements. Next, chapter 8, “Debian Package Management”
[page 176] explains how to leverage the full potential of the Debian packaging ecosystem. And
in chapter 9, “Advanced Usage” [page 228], you learn how to create a fully customized Kali Linux
ISO image. All those topics are even more relevant when you deploy Kali Linux at scale in an
enterprise as documented in chapter 10, “Kali Linux in the Enterprise” [page 258].
XX Kali Linux Revealed
The last chapter, chapter 11, “Introduction to Security Assessments” [page 284], makes the link
between everything that you have learned in this book and the day-to-day work of security pro-
fessionals.
Acknowledgments of Raphaël Hertzog
I would like to thank Mati Aharoni: in 2012, he got in touch with me because I was one out of
dozens of Debian consultants and he wanted to build a successor to BackTrack that would be based
on Debian. That is how I started to work on Kali Linux, and ever since I have enjoyed my journey
in the Kali world.
Over the years, Kali Linux got closer to Debian GNU/Linux, notably with the switch to Kali Rolling,
based on Debian Testing. Now most of my work, be it on Kali or on Debian, provides benefits to the
entire Debian ecosystem. And this is exactly what keeps me so motivated to continue, day after
day, month after month, year after year.
Working on this book is also a great opportunity that Mati offered me. It is not the same kind
of work but it is equally rewarding to be able to help people and share with them my expertise
of the Debian/Kali operating system. Building on my experience with the Debian Administrator’s
Handbook, I hope that my explanations will help you to get started in the fast-moving world of
computer security.
I would also like to thank all the Offensive Security persons who were involved in the book: Jim
O’Gorman (co-author of some chapters), Devon Kearns (reviewer), Ron Henry (technical editor),
Joe Steinbach and Tony Cruse (project managers). And thank you to Johnny Long who joined to
write the preface but ended up reviewing the whole book.
Acknowledgments of Jim O’Gorman
I would like to thank everyone involved in this project for their contributions, of which mine were
only a small part. This book, much like Kali Linux itself was a collaborative project of many hands
making light work. Special thanks to Raphaël, Devon, Mati, Johnny, and Ron for taking on the
lion’s share of the effort. Without them, this book would not have come together.
Acknowledgments of Mati Aharoni
It has been a few years since Kali Linux was first released, and since day one, I have always dreamt
of publishing an official book which covers the Kali operating system as a whole. It is therefore
a great privilege for me to finally see such a book making it out to the public. I would like to
sincerely thank everyone involved in the creation of this project—including Jim, Devon, Johnny,
Introduction XXI
and Ron. A very special thanks goes to Raphaël for doing most of the heavy lifting in this book,
and bringing in his extensive expertise to our group.
Acknowledgments of Joe O’Gorman - 2021 Revision
Kali Linux has changed a lot since Kali Linux Revealed first released back in 2017. From additional
developers to moving away from using root as the default user, lots of progress has been made
over the years. Kali Linux Revealed was due for an updated revision, and I am honored for the
opportunity to have contributed to this epic project during the revision process.
XXII Kali Linux Revealed
Keywords
Linux distribution
Debian derivative
Purpose
Features
Policies
Chapter
About Kali Linux 1
Contents
A Bit of History 2 Relationship with Debian 4 Purpose and Use Cases 5 Main Kali Linux Features 8
Kali Linux Policies 10 Summary 11
Kali Linux1 is an enterprise-ready security auditing Linux distribution based on Debian
GNU/Linux. Kali is aimed at security professionals and IT administrators, enabling them to con-
duct advanced penetration testing, forensic analysis, and security auditing.
What is a Linux Although it is commonly used as a name for the entire operating system, Linux is
Distribution? just the name of the kernel, a piece of software that handles interactions between the
hardware and end-user applications.
The expression Linux distribution, on the other hand, refers to a complete operating
system (OS) built on top of the Linux kernel, usually including an installation pro-
gram and many applications, which are either pre-installed or packaged in an easily
installable way.
Debian GNU/Linux2 is a leading generic Linux distribution, known for its quality and
stability. Kali Linux builds on the work of the Debian project and adds over 400 special-
purpose packages of its own, all related to information security, particularly the field
of penetration testing.
Debian is a free software project providing multiple versions of its operating system
and we often use the term distribution to refer to a specific version of it, for exam-
ple the Debian Stable or Debian Testing distributions. The same also applies to Kali
Linux—with the Kali Rolling distribution, for example.
1.1. A Bit of History
The Kali Linux project began quietly in 2012, when Offensive Security decided that they wanted to
replace their venerable BackTrack Linux project, which was manually maintained, with something
that could become a genuine Debian derivative3 , complete with all of the required infrastructure
and improved packaging techniques. The decision was made to build Kali on top of the Debian dis-
tribution because it is well known for its quality, stability, and wide selection of available software.
That is why I (Raphaël) got involved in this project, as a Debian consultant.
The first release (version 1.0) happened one year later, in March 2013, and was based on Debian
7 “Wheezy”, Debian’s stable distribution at the time. In that first year of development, we pack-
aged hundreds of pen-testing-related applications and built the infrastructure. Even though the
number of applications is significant, the application list has been meticulously curated, drop-
ping applications that no longer worked or that duplicated features already available in better
programs.
During the two years following version 1.0, Kali released many incremental updates, expanding
the range of available applications and improving hardware support, thanks to newer kernel re-
leases. With some investment in continuous integration, we ensured that all important packages
1
https://www.kali.org/
2
https://www.debian.org/
3
https://wiki.debian.org/Derivatives/Census
2 Kali Linux Revealed
were kept in an installable state and that customized live images (a hallmark of the distribution)
could always be created.
In 2015, when Debian 8 “Jessie” came out, we worked to rebase Kali Linux on top of it. While
Kali Linux 1.x avoided the GNOME Shell (relying on GNOME Fallback instead), in this version we
decided to embrace and enhance it: we added some GNOME Shell extensions to acquire missing
features, most notably the Applications menu. The result of that work became Kali Linux 2.0,
published in August 2015.
Xfce is Kali Linux’s A desktop environment is a collection of graphical applications that share a common
Default Desktop graphical toolkit and that are meant to be used together on user workstations. Desk-
Environment top environments are generally not used in servers. They usually provide an applica-
tion launcher, a file manager, a web browser, an email client, an office suite, etc.
Xfce4 is one of the most popular lightweight desktop environments, perfect for our
live ISO images and is included on the main installer ISO images provided by Kali
Linux (together with GNOME5 , KDE6 ). You can select a desktop environment of your
choosing during installation. Otherwise, post-installation you have the additional
options of; i37 , i3-gaps8 , MATE9 , Enlightenment10 , LXDE11 .
In parallel, we increased our efforts to ensure that Kali Linux always has the latest version of all
pen-testing applications. Unfortunately, that goal was a bit at odds with the use of Debian Stable12
as a base for the distribution, because it required us to backport many packages. This is due to the
fact that Debian Stable puts a priority on the stability of the software, often causing a long delay
from the release of an upstream update to when it is integrated into the distribution. Given our
investment in continuous integration, it was quite a natural move to rebase Kali Linux on top of
Debian Testing13 so that we could benefit from the latest version of all Debian packages as soon
as they were available. Debian Testing has a much more aggressive update cycle, which is more
compatible with the philosophy of Kali Linux.
This is, in essence, the concept of Kali Rolling. While the rolling distribution has been available
for quite a while, Kali 2016.1 was the first release to officially embrace the rolling nature of that
distribution: when you install the latest Kali release, your system actually tracks the Kali Rolling
distribution and every single day you get new updates. In the past, Kali releases were snapshots of
the underlying Debian distribution with Kali-specific packages injected into it.
4
https://www.xfce.org/
5
https://www.gnome.org/
6
https://kde.org/
7
https://i3wm.org/
8
https://github.com/Airblader/i3
9
https://mate-desktop.org/
10
https://www.enlightenment.org/
11
https://www.lxde.org/
12
https://www.debian.org/releases/stable/
13
https://www.debian.org/releases/testing/
Chapter 1 — About Kali Linux 3
A rolling distribution has many benefits but it also comes with multiple challenges, both for those
of us who are building the distribution and for the users who have to cope with a never-ending
flow of updates and sometimes backwards-incompatible changes. This book aims to give you the
knowledge required to deal with everything you may encounter while managing your Kali Linux
installation.
1.2. Relationship with Debian
The Kali Linux distribution is based on Debian Testing14 . Therefore, most of the packages available
in Kali Linux come straight from this Debian repository.
While Kali Linux relies heavily on Debian, it is also entirely independent15 in the sense that we
have our own infrastructure and retain the freedom to make any changes we want.
1.2.1. The Flow of Packages
On the Debian side, the contributors are working every day on updating packages and uploading
them to the Debian Unstable distribution (Unstable is also known as sid16 ). From there, packages
migrate to the Debian Testing distribution once the most troublesome bugs have been taken out.
The migration process also ensures that no dependencies are broken in Debian Testing. The goal
is that Testing is always in a usable (or even releasable!) state.
Debian Testing’s goals align quite well with those of Kali Linux so we picked it as the base. To add
the Kali-specific packages in the distribution, we follow a two-step process.
First, we take Debian Testing and force-inject our own Kali packages (located in our kali-dev-only
repository) to build the kali-dev repository. This repository will break from time to time: for in-
stance, our Kali-specific packages might not be installable until they have been recompiled against
newer libraries. In other situations, packages that we have forked might also have to be updated,
either to become installable again, or to fix the installability of another package that depends on
a newer version of the forked package. In any case, kali-dev is not for end-users.
kali-rolling is the distribution that Kali Linux users are expected to track and is built out of kali-dev
in the same way that Debian Testing is built out of Debian Unstable. Packages migrate only when
all dependencies can be satisfied in the target distribution.
14
https://www.debian.org/releases/testing/
15
https://www.debian.org/derivatives/
16
https://www.debian.org/releases/sid/
4 Kali Linux Revealed
1.2.2. Managing the Difference with Debian
As a design decision, we try to minimize the number of forked packages as much as possible. How-
ever, in order to implement some of Kali’s unique features, some changes must be made. To limit
the impact of these changes, we strive to send them upstream, either by integrating the feature di-
rectly, or by adding the required hooks so that it is straightforward to enable the desired features
without further modifying the upstream packages themselves.
The Kali Package Tracker17 helps us to keep track of our divergence with Debian. At any time, we
can look up which package has been forked and whether it is in sync with Debian, or if an update
is required. All our packages are maintained in Git repositories18 hosting a Debian branch and a
Kali branch side-by-side. Thanks to this, updating a forked package is a simple two-step process:
update the Debian branch and then merge it into the Kali branch.
While the number of forked packages in Kali is relatively low, the number of additional packages
is rather high: in January 2021 there were almost 50019 . Most of these packages are free software
complying with the Debian Free Software Guidelines20 and our ultimate goal would be to maintain
those packages within Debian whenever possible. That is why we strive to comply with the Debian
Policy21 and to follow the good packaging practices used in Debian. Unfortunately, there are also
quite a few exceptions where proper packaging was nearly impossible to create. As a result of
time being scarce, few packages have been pushed to Debian.
1.3. Purpose and Use Cases
While Kali’s focus can be quickly summarized as “penetration testing and security auditing”, there
are many different tasks involved behind those activities. Kali Linux is built as a platform, because
it includes many tools covering very different use cases (though they may certainly be used in
combination during a penetration test).
For example, Kali Linux can be used on various types of computers: obviously on the laptops of
penetration testers, but also on servers of system administrators wishing to monitor their net-
work, on the workstations of forensic analysts, and more unexpectedly, on stealthy embedded de-
vices, typically with ARM CPUs, that can be dropped in the range of a wireless network or plugged
in the computer of target users. Many ARM devices are also perfect attack machines due to their
small form factors and low power requirements. Kali Linux can also be deployed in the cloud to
quickly build a farm of password-cracking machines and on mobile phones and tablets to allow
for truly portable penetration testing.
17
https://pkg.kali.org/derivative/kali-dev/
18
https://gitlab.com/kalilinux/packages
19
https://pkg.kali.org/derivative/kali-roll/
20
https://www.debian.org/social_contract
21
https://www.debian.org/doc/debian-policy/
Chapter 1 — About Kali Linux 5
But that is not all; penetration testers also need servers: to use collaboration software within a
team of pen-testers, to set up a web server for use in phishing campaigns, to run vulnerability
scanning tools, and other related activities.
Once you have booted Kali, you will quickly discover that Kali Linux’s main menu is organized by
theme across the various kinds of tasks and activities that are relevant for pen-testers and other
information security professionals as shown in Figure 1.1, “Kali Linux’s Applications Menu” [page
6].
Figure 1.1 Kali Linux’s Applications Menu
These tasks and activities include:
• Information Gathering: Collecting data about the target network and its structure, identify-
ing computers, their operating systems, and the services that they run. Identifying poten-
tially sensitive parts of the information system. Extracting all sorts of listings from running
directory services.
• Vulnerability Analysis: Quickly testing whether a local or remote system is affected by a
number of known vulnerabilities or insecure configurations. Vulnerability scanners use
databases containing thousands of signatures to identify potential vulnerabilities.
• Web Application Analysis: Identifying misconfigurations and security weaknesses in web
applications. It is crucial to identify and mitigate these issues given that the public avail-
ability of these applications makes them ideal targets for attackers.
6 Kali Linux Revealed
• Database Assessment: From SQL injection to attacking credentials, database attacks are a
common vector for attackers. Tools that test for attack vectors ranging from SQL injection
to data extraction and analysis can be found here.
• Password Attacks: Authentication systems are always a go-to attack vector. Many useful
tools can be found here, from online password attack tools to offline attacks against the
encryption or hashing systems.
• Wireless Attacks: The pervasive nature of wireless networks means that they will always
be a commonly attacked vector. With its wide range of support for multiple wireless cards,
Kali is an obvious choice for attacks against multiple types of wireless networks.
• Reverse Engineering: Reverse engineering is an activity with many purposes. In support
of offensive activities, it is one of the primary methods for vulnerability identification and
exploit development. On the defensive side, it is used to analyze malware employed in tar-
geted attacks. In this capacity, the goal is to identify the capabilities of a given piece of
tradecraft.
• Exploitation Tools: Exploiting, or taking advantage of a (formerly identified) vulnerability,
allows you to gain control of a remote machine (or device). This access can then be used
for further privilege escalation attacks, either locally on the compromised machine, or on
other machines accessible on its local network. This category contains a number of tools
and utilities that simplify the process of writing your own exploits.
• Sniffing & Spoofing: Gaining access to the data as they travel across the network is often ad-
vantageous for an attacker. Here you can find spoofing tools that allow you to impersonate
a legitimate user as well as sniffing tools that allow you to capture and analyze data right
off the wire. When used together, these tools can be very powerful.
• Post Exploitation: Once you have gained access to a system, you will often want to maintain
that level of access or extend control by laterally moving across the network. Tools that
assist in these goals are found here.
• Forensics: Forensic Linux live boot environments have been very popular for years now.
Kali contains a large number of popular Linux-based forensic tools allowing you to do ev-
erything from initial triage, to data imaging, to full analysis and case management.
• Reporting Tools: A penetration test is only complete once the findings have been reported.
This category contains tools to help collate the data collected from information-gathering
tools, discover non-obvious relationships, and bring everything together in various reports.
• Social Engineering Tools: When the technical side is well-secured, there is often the possi-
bility of exploiting human behavior as an attack vector. Given the right influence, people
can frequently be induced to take actions that compromise the security of the environment.
Did the USB key that the secretary just plugged in contain a harmless PDF? Or was it also a
Trojan horse that installed a backdoor? Was the banking website the accountant just logged
into the expected website or a perfect copy used for phishing purposes? This category con-
tains tools that aid in these types of attacks.
Chapter 1 — About Kali Linux 7
1.4. Main Kali Linux Features
Kali Linux is a Linux distribution that contains its own collection of hundreds of software tools
specifically tailored for their target users—penetration testers and other security professionals.
It also comes with an installation program to completely setup Kali Linux as the main operating
system on any computer.
This is pretty much like all other existing Linux distributions but there are other features that
differentiate Kali Linux, many of which are tailored to the specific needs of penetration testers.
Let’s have a look at some of those features.
1.4.1. A Live System
Alongside the main installer ISO images, Kali Linux offers a separate live ISO image to download.
This allows you to use Kali Linux as a bootable live system. In other words, you can use Kali Linux
without installing it, just by booting the ISO image (usually after having copied the image onto a
USB key).
The live system contains the tools most commonly used by penetration testers, so even if your day-
to-day system is not Kali Linux, you can simply insert the disk or USB key and reboot to run Kali.
However, keep in mind that the default configuration will not preserve changes between reboots.
If you configure persistence with a USB key (see section 9.4, “Adding Persistence to the Live ISO
with a USB Key” [page 246]), then you can tweak the system to your liking (modify config files,
save reports, upgrade software, and install additional packages, for example), and the changes
will be retained across reboots.
1.4.2. Forensics Mode
In general, when doing forensic work on a system, you want to avoid any activity that would alter
the data on the analyzed system in any way. Unfortunately, modern desktop environments tend
to interfere with this objective by trying to auto-mount any disk(s) they detect. To avoid this
behavior, Kali Linux has a forensics mode that can be enabled from the boot menu: it will disable
all such features.
The live system is particularly useful for forensics purposes, because it is possible to reboot any
computer into a Kali Linux system without accessing or modifying its hard disks.
8 Kali Linux Revealed
1.4.3. A Custom Linux Kernel
Kali Linux always provides a customized recent Linux kernel22 , based on the version in Debian
Unstable. This ensures solid hardware support, especially for a wide range of wireless devices.
The kernel is patched23 for wireless injection support since many wireless security assessment
tools rely on this feature.
Since many hardware devices require up-to-date firmware files (found in /lib/firmware/), Kali
installs them all by default—including the firmware available in Debian’s non-free section. Those
are not installed by default in Debian, because they are closed-source and thus not part of Debian
proper.
1.4.4. Completely Customizable
Kali Linux is built by penetration testers for penetration testers, but we understand that not ev-
eryone will agree with our design decisions or choice of tools to include by default. With this in
mind, we always ensure that Kali Linux is easy to customize based on your own needs and prefer-
ences. To this end, we publish the live-build24 configuration used to build the official Kali images
so you can customize it to your liking. It is very easy to start from this published configuration
and implement various changes based on your needs thanks to the versatility of live-build.
Live-build includes many features to modify the installed system, install supplementary files, in-
stall additional packages, run arbitrary commands, and change the values pre-seeded to debconf.
1.4.5. A Trustable Operating System
Users of a security distribution rightfully want to know that it can be trusted and that it has been
developed in plain sight, allowing anyone to inspect the source code. Kali Linux is developed by a
small team25 of knowledgeable developers working transparently and following the best security
practices: they upload signed source packages, which are then built on dedicated build daemons.
The packages are then checksummed and distributed as part of a signed repository.
The work done on the packages can be fully reviewed through the packaging Git repositories26
(which contain signed tags) that are used to build the Kali source packages. The evolution of each
package can also be followed through the Kali package tracker27 .
22
https://pkg.kali.org/pkg/linux
23
https://gitlab.com/kalilinux/packages/linux/-/blob/kali/master/debian/patches/series
24
https://gitlab.com/kalilinux/build-scripts/live-build-config
25
https://www.kali.org/about-us/
26
https://gitlab.com/kalilinux/packages
27
https://pkg.kali.org/
Chapter 1 — About Kali Linux 9
1.4.6. Usable on a Wide Range of ARM Devices
Kali Linux provides binary packages for the armel, armhf, and arm64 ARM architectures. Thanks
to the easily installable images provided by Offensive Security, Kali Linux can be deployed on
many interesting devices, from smartphones and tablets to Wi-Fi routers and computers of various
shapes and sizes.
1.5. Kali Linux Policies
While Kali Linux strives to follow the Debian policy whenever possible, there are some areas where
we made significantly different design choices due to the particular needs of security profession-
als.
1.5.1. Network Services Disabled by Default
In contrast to Debian, Kali Linux disables any installed service that would listen on a public net-
work interface by default, such as HTTP and SSH.
The rationale behind this decision is to minimize exposure during a penetration test when it is
detrimental to announce your presence and risk detection because of unexpected network inter-
actions.
You can still manually enable any services of your choosing by running sudo systemctl enable
service. We will get back to this in chapter 5, “Configuring Kali Linux” [page 108] later in this
book.
1.5.2. A Curated Collection of Applications
Debian aims to be the universal operating system and puts very few limits on what gets packaged,
provided that each package has a maintainer.
By way of contrast, Kali Linux does not package every penetration testing tool available. Instead,
we aim to provide only the best freely-licensed tools covering most tasks that a penetration tester
might want to perform.
Kali developers working as penetration testers drive the selection process and we leverage their
experience and expertise to make enlightened choices. In some cases this is a matter of fact, but
there are other, more difficult choices that simply come down to personal preference.
Here are some of the points considered when a new application gets evaluated:
• The usefulness of the application in a penetration testing context
• The unique functionality of the application’s features
10 Kali Linux Revealed
• The application’s license
• The application’s resource requirements
Maintaining an updated and useful penetration testing tool repository is a challenging task. We
welcome tool suggestions within a dedicated category (New Tool Requests) in the Kali Bug Tracker28 .
New tool requests are best received when the submission is well-presented, including an explana-
tion of why the tool is useful, how it compares to other similar applications, and so on.
1.6. Summary
In this chapter we have introduced you to Kali Linux, provided a bit of history, run through some
of the primary features, and presented several use cases. We have also discussed some of the
policies we have adopted when developing Kali Linux.
Summary Tips:
• Kali Linux29 is an enterprise-ready security auditing Linux distribution based on Debian
GNU/Linux. Kali is aimed at security professionals and IT administrators, enabling them
to conduct advanced penetration testing, forensic analysis, and security auditing.
• Unlike most mainstream operating systems, Kali Linux is a rolling distribution, which means
that you will receive updates every single day.
• The Kali Linux distribution is based on Debian Testing30 . Therefore, most of the packages
available in Kali Linux come straight from this Debian repository.
• While Kali’s focus can be quickly summarized with “penetration testing and security audit-
ing”, there are several use cases including system administrators wishing to monitor their
networks, forensic analysis, embedded device installations, wireless monitoring, installa-
tion on mobile platforms, and more.
• Kali’s menus make it easy to get to tools for various tasks and activities including: vulnera-
bility analysis, web application analysis, database assessment, password attacks, wireless at-
tacks, reverse engineering, exploitation tools, sniffing and spoofing, post exploitation tools,
forensics, reporting tools, and social engineering tools.
• Kali Linux has many advanced features including: use as a live (non-installed) system, a ro-
bust and safe forensics mode, a custom Linux kernel, ability to completely customize the
system, a trusted and secure base operating system, ARM installation capability, secure de-
fault network policies, and a curated set of applications.
In the next chapter (chapter 2, “Getting Started with Kali Linux” [page 14]), we will jump in and
try out Kali Linux thanks to its live mode.
28
https://bugs.kali.org/
29
https://www.kali.org
30
https://www.debian.org/releases/testing/
Chapter 1 — About Kali Linux 11
Keywords
Download
ISO image
Live boot
Chapter
Getting Started with 2
Kali Linux
Contents
Downloading a Kali ISO Image 14 Booting a Kali ISO Image in Live Mode 24 Summary 44
Unlike some other operating systems, Kali Linux makes getting started easy, thanks to the fact that
a live disk image is available, meaning that you can boot the downloaded image without following
any prior installation procedure. This means you can use the same image for testing, for use as
a bootable USB or DVD-ROM image in a forensics case, or for installing as a permanent operating
system on physical or virtual hardware.
Because of this simplicity, it is easy to forget that certain precautions must be taken. Kali users
are often the target of those with ill intentions, whether state sponsored groups, elements of orga-
nized crime, or individual hackers. The open-source nature of Kali Linux makes it relatively easy
to build and distribute fake versions, so it is essential that you get into the habit of downloading
from original sources and verifying the integrity and the authenticity of your download. This is
especially relevant to security professionals who often have access to sensitive networks and are
entrusted with client data.
2.1. Downloading a Kali ISO Image
2.1.1. Where to Download
The only official source of Kali Linux ISO images is the Downloads section of the Kali website. Due
to its popularity, numerous sites offer Kali images for download, but they should not be considered
trustworthy and indeed may be infected with malware or otherwise cause irreparable damage to
your system.
è https://www.kali.org/downloads/
The website is available over HTTPS, making it difficult to impersonate. Being able to carry out
a man-in-the-middle attack is not sufficient as the attacker would also need a www.kali.org cer-
tificate signed by a Transport Layer Security (TLS) certificate authority that is trusted by the vic-
tim’s browser. Because certificate authorities exist precisely to prevent this type of problem, they
deliver certificates only to people whose identities have been verified and who have provided
evidence that they control the corresponding website.
cdimage.kali.org The links found on the download page point to the cdimage.kali.org domain, which
redirects to a mirror close to you, improving your transfer speed while reducing the
burden on Kali’s central servers.
A list of available mirrors can be found here:
è https://cdimage.kali.org/README.mirrorlist
2.1.2. What to Download
The official download page shows a short list of ISO images, as shown in Figure 2.1, “List of Images
Offered for Download” [page 15].
14 Kali Linux Revealed
Figure 2.1 List of Images Offered for Download
Chapter 2 — Getting Started with Kali Linux 15
All disk images labeled 64-bit or 32-bit refer to images suitable for CPUs, found in most modern
desktop and laptop computers. If you are downloading for use on a fairly modern machine, it most
likely contains a 64-bit processor. If you are unsure, rest assured that all 64-bit processors can run
32-bit instructions. You can always download and run the 32-bit image. The reverse is not true,
however. Refer to the sidebar for more detailed information.
If you are planning to install Kali on an ARM-based device, you must refer to Offensive Security’s
download page1 for the list of available devices.
Is My CPU 64-bit or Under Microsoft Windows, you can find this information by running the System Infor-
32-bit? mation application (found in the Accessories > System Tools folder). On the System
Summary screen, you can inspect the System Type field: it will contain ”x64-based
PC” for a 64-bit CPU or “x86-based PC” for a 32-bit CPU.
Under OS X/macOS, there is no standard application showing this information but
you can still infer it from the output of the uname -m command run on the terminal.
It will return x86_64 on a system with a 64-bit kernel (which can only run on a 64-bit
CPU), systems with a 32-bit kernel, it will return i386 or something similar (i486,
i586, or i686), and on systems with an arm64 kernel, it will return arm64. Any 32-
bit kernel can run on a 64-bit CPU, but since Apple controls the hardware and the
software, it is unlikely you will find this configuration.
Under Linux, you can inspect the flags field in the /proc/cpuinfo virtual file. If it
contains the lm attribute, then your CPU is a 64-bit; otherwise, it is a 32-bit. The
following command line will tell you what kind of CPU you have:
$ grep -qP ’^flags\s*:.*\blm\b’ /proc/cpuinfo && echo 64-bit
å || echo 32-bit
64-bit
Now that you know whether you need a 64-bit or 32-bit image, there is only one step left: selecting
the kind of image. The available images differ in how they go about installation. The Installer
and NetInstaller images, specialized for a straightforward install featuring selectable installation
options, do not come with the ability to run the live system. The Live image, however, comes
with the ability to run the live system or start the installation process, although it does lack the
selectable options featured in the installation images. The selectable options include choices in
desktop environments as well as which collection of packages to install. We will be using the live
image throughout this book.
Once you have decided on the image you need, you can download the image by clicking on the
title in the respective row. Alternatively, you can download the image from the BitTorrent peer-
to-peer network by clicking on ”Torrent,” provided that you have a BitTorrent client associated
with the .torrent extension.
While your chosen ISO image is downloading, you should take note of the checksum written in
the sha256sum column. Once you have downloaded your image, use this checksum to verify that
1
https://www.offensive-security.com/kali-linux-arm-images/
16 Kali Linux Revealed
the downloaded image matches the one the Kali development team put online (see section 2.1.3,
“Verifying Integrity and Authenticity” [page 17]).
2.1.3. Verifying Integrity and Authenticity
Security professionals must verify the integrity of their tools to not only protect their data and
networks but also those of their clients. While the Kali download page and links are TLS-protected,
Kali relies on a network of external mirrors to distribute the image means that you should not
blindly trust what you download. The mirror you were directed to may have been compromised,
or you might be the victim of an attack yourself.
To alleviate this, the Kali project always provides checksums of the images it distributes. But to
make such a check effective, you must be sure that the checksum you grabbed is effectively the
checksum published by the Kali Linux developers. You have different ways to ascertain this.
Relying on the TLS-Protected Website
When you retrieve the checksum from the TLS-protected download webpage, its origin is indi-
rectly guaranteed by the X.509 certificate security model: the content you see comes from a web
site that is effectively under the control of the person who requested the TLS certificate.
Now you should generate the checksum of your downloaded image and ensure that it matches
what you recorded from the Kali website:
$ sha256sum kali-linux-2020.3-live-amd64.iso
1a0b2ea83f48861dd3f3babd5a2892a14b30a7234c8c9b5013a6507d1401874f kali-linux-2020.3-live-amd64.iso
If your generated checksum matches the one on the Kali Linux download page, you have the cor-
rect file. If the checksums differ, there is a problem, although this does not always indicate a com-
promise or an attack; downloads occasionally get corrupted as they traverse the Internet. Try
your download again, from another official Kali mirror, if possible (see “cdimage.kali.org” [page
14] for more information about available mirrors).
Relying on PGP’s Web of Trust
If you don’t trust HTTPS for authentication, you are a bit paranoid but rightfully so. There are
many examples of badly managed certificate authorities that issued rogue certificates, which
ended up being misused. You may also be the victim of a “friendly” man-in-the-middle at-
tack implemented on many corporate networks, using a custom, browser-implanted trust store
that presents fake certificates to encrypted websites, allowing corporate auditors to monitor en-
crypted traffic.
Chapter 2 — Getting Started with Kali Linux 17
For cases like this, we also provide a GnuPG key that we use to sign the checksums of the images
we provide. The key’s identifiers and its fingerprints are shown here:
pub rsa4096 2012-03-05 [SC] [expires: 2023-01-16]
44C6 513A 8E4F B3D3 0875 F758 ED44 4FF0 7D8D 0BF6
uid Kali Linux Repository <devel@kali.org>
sub rsa4096 2012-03-05 [E] [expires: 2023-01-16]
This key is part of a global web of trust because it has been signed at least by me (Raphaël Hertzog)
and I am part of the web of trust due to my heavy GnuPG usage as a Debian developer.
The PGP/GPG security model is very unique. Anyone can generate any key with any identity, but
you would only trust that key if it has been signed by another key that you already trust. When you
sign a key, you certify that you met the holder of the key and that you know that the associated
identity is correct. And you define the initial set of keys that you trust, which obviously includes
your own key.
This model has its own limitations so you can opt to download Kali’s public key over HTTPS (or
from a keyserver) and just decide that you trust it because its fingerprint matches what we an-
nounced in multiple places, including just above in this book:
$ wget -q -O - https://archive.kali.org/archive-key.asc | gpg --import
[ or ]
$ gpg --keyserver hkp://keys.gnupg.net --recv-key 44C6513A8E4FB3D30875F758ED444FF07D8D0BF6
gpg: key ED444FF07D8D0BF6: public key ”Kali Linux Repository <devel@kali.org>” imported
gpg: Total number processed: 1
gpg: imported: 1
[...]
$ gpg --fingerprint 44C6513A8E4FB3D30875F758ED444FF07D8D0BF6
[...]
44C6 513A 8E4F B3D3 0875 F758 ED44 4FF0 7D8D 0BF6
[...]
After you have retrieved the key, you can use it to verify the checksums of the distributed im-
ages. Let’s download the file with the checksums (SHA256SUMS) and the associated signature file
(SHA256SUMS.gpg) and verify the signature:
$ wget http://cdimage.kali.org/current/SHA256SUMS
[...]
$ wget http://cdimage.kali.org/current/SHA256SUMS.gpg
[...]
$ gpg --verify SHA256SUMS.gpg SHA256SUMS
gpg: Signature made Tue 18 Aug 2020 10:31:15 AM EDT
gpg: using RSA key 44C6513A8E4FB3D30875F758ED444FF07D8D0BF6
gpg: Good signature from ”Kali Linux Repository <devel@kali.org>”
If you get that “Good signature” message, you can trust the content of the SHA256SUMS file and use
it to verify the files you downloaded. Otherwise, there is a problem. You should review whether
you downloaded the files from a legitimate Kali Linux mirror.
18 Kali Linux Revealed
Note that you can use the following command line to verify that the downloaded file has the same
checksum that is listed in SHA256SUMS, provided that the downloaded ISO file is in the same direc-
tory:
$ grep kali-linux-2020.3-live-amd64.iso SHA256SUMS | sha256sum -c
kali-linux-2020.3-live-amd64.iso: OK
If you don’t get OK in response, then the file you have downloaded is different from the one re-
leased by the Kali team. It cannot be trusted and should not be used.
2.1.4. Copying the Image on a DVD-ROM or USB Key
Unless you want to run Kali Linux in a virtual machine, the ISO image is of limited use in and of
itself. You must burn it on a DVD-ROM or copy it onto a USB key to be able to boot your machine
into Kali Linux. We have chosen the Kali live image as we wish to boot from a USB allowing us to
either use a live environment or install Kali Linux’s default configuration.
We won’t cover how to burn the ISO image onto a DVD-ROM, as the process varies widely by plat-
form and environment, but in most cases, right clicking on the .iso file will present a contextual
menu item that executes a DVD-ROM burning application. Try it out!
Warning
In this section, you will learn how to overwrite an arbitrary disk with a Kali
Linux ISO image. Always double-check the target disk before launching
the operation as a single mistake would likely cause complete data loss and
possibly damage your setup beyond repair.
Creating a Bootable Kali USB Drive on Windows
As a prerequisite, you should download and install Win32 Disk Imager:
è https://sourceforge.net/projects/win32diskimager/
Plug your USB key into your Microsoft Windows PC and note the drive designator associated to it
(for example, “E:\”).
Launch Win32 Disk Imager and choose the Kali Linux ISO file that you want to copy on the USB key.
Verify that the letter of the device selected corresponds with that assigned to the USB key. Once
you are certain that you have selected the correct drive, click the Write button and confirm that
Chapter 2 — Getting Started with Kali Linux 19
you want to overwrite the contents of the USB key as shown in Figure 2.2, “Win32 Disk Imager in
action” [page 20].
Figure 2.2 Win32 Disk Imager in action
Once the copy is completed, safely eject the USB drive from your Microsoft Windows system. You
can now use the USB device to boot Kali Linux.
20 Kali Linux Revealed
Creating a Bootable Kali USB Drive on Linux
Creating a bootable Kali Linux USB key in a Linux environment is easy. The GNOME desktop envi-
ronment, which is installed by default in many Linux distributions, comes with a Disks utility (in
the gnome-disk-utility package). That program shows a list of disks, which refreshes dynamically
when you plug or unplug a disk. When you select your USB key in the list of disks, detailed infor-
mation will appear and will help you confirm that you selected the correct disk. Note that you can
find its device name in the title bar as shown in Figure 2.3, “GNOME Disks” [page 21].
Figure 2.3 GNOME Disks
Click on the menu button and select Restore Disk Image... in the displayed pop-up menu. Select
the ISO image that you formerly downloaded and click on Start Restoring... as shown in Figure 2.4,
“Restore Disk Image Dialog” [page 21].
Figure 2.4 Restore Disk Image Dialog
Chapter 2 — Getting Started with Kali Linux 21
Enjoy a cup of coffee while it finishes copying the image on the USB key (Figure 2.5, “Progression
of the Image Restoration” [page 22]).
Figure 2.5 Progression of the Image Restoration
Create the Bootable USB Even though the graphical process is fairly straightforward, the operation is just as
Drive from the Command easy for command line users.
Line When you insert your USB key, the Linux kernel will detect it and assign it a name,
which is printed in the kernel logs. You can find its name by inspecting the logs
returned by dmesg.
$ dmesg
[...]
[ 2596.727036] usb 1-2.1: new high-speed USB device number 7 using uhci_hcd
[ 2597.023023] usb 1-2.1: New USB device found, idVendor=0781, idProduct=5575,
å bcdDevice= 1.26
[ 2597.023025] usb 1-2.1: New USB device strings: Mfr=1, Product=2,
å SerialNumber=3
[ 2597.023026] usb 1-2.1: Product: Cruzer Glide
[ 2597.023026] usb 1-2.1: Manufacturer: SanDisk
[ 2597.023026] usb 1-2.1: SerialNumber: 200533495211C0824E58
[ 2597.025989] usb-storage 1-2.1:1.0: USB Mass Storage device detected
[ 2597.026064] scsi host3: usb-storage 1-2.1:1.0
[ 2598.055632] scsi 3:0:0:0: Direct-Access SanDisk Cruzer Glide 1.26
å PQ: 0 ANSI: 5
[ 2598.058596] sd 3:0:0:0: Attached scsi generic sg2 type 0
[ 2598.063036] sd 3:0:0:0: [sdb] 31266816 512-byte logical blocks: (16.0 GB
å /14.9 GiB)
[ 2598.067356] sd 3:0:0:0: [sdb] Write Protect is off
[ 2598.067361] sd 3:0:0:0: [sdb] Mode Sense: 43 00 00 00
[ 2598.074276] sd 3:0:0:0: [sdb] Write cache: disabled, read cache: enabled,
å doesn’t support DPO or FUA
[ 2598.095976] sdb: sdb1
[ 2598.108225] sd 3:0:0:0: [sdb] Attached SCSI removable disk
Now that you know that the USB key is available as /dev/sdb, you can proceed to
copy the image with the dd command:
# dd if=kali-linux-2020.3-live-amd64.iso of=/dev/sdb
6129688+0 records in
6129688+0 records out
3138400256 bytes (3.1 GB, 2.9 GiB) copied, 678.758 s, 4.6 MB/s
22 Kali Linux Revealed
Note that you need root permissions for this operation to succeed and you should
also ensure that the USB key is unused. That is, you should make sure that none
of its partitions are mounted. The command also assumes that it is run while in the
directory hosting the ISO image, otherwise the full path will need to be provided.
For reference, if stands for “input file” and of for “output file.” The dd command reads
data from the input file and writes it back to the output file. It does not show any
progress information so you must be patient while it is doing its work (It is not unusual
for the command to take more than half an hour!). Look at the write activity LED on
the USB key if you want to double check that the command is working. The statistics
shown above are displayed only when the command has completed. On OS X/macOS,
you can also press CTRL+T during the operation to get statistical information about
the copy including how much data has been copied.
Creating a Bootable Kali USB Drive on OS X/macOS
OS X/macOS is based on UNIX, so the process of creating a bootable Kali Linux USB drive is similar
to the Linux procedure. Once you have downloaded and verified your chosen Kali ISO file, use dd
to copy it over to your USB stick.
To identify the device name of the USB key, run diskutil list to list the disks available on
your system. Next, insert your USB key and run the diskutil list command again. The second
output should list an additional disk. You can determine the device name of the USB key by com-
paring the output from both commands. Look for a new line identifying your USB disk and note
the /dev/diskX where X represents the disk ID.
You should make sure that the USB key is not mounted, which can be accomplished with an explicit
unmount command (assuming /dev/disk6 is the device name of the USB key):
$ diskutil unmount /dev/disk6
Now proceed to execute the dd command. This time, add a supplementary parameter — bs for
block size. It defines the size of the block that is read from the input file and then written to the
output file. We will also utilize the raw disk path (signified by the r before disk) which will allow
faster write speeds.
# dd if=kali-linux-2020.3-live-amd64.iso of=/dev/rdisk2 bs=4m
748+1 records in
748+1 records out
3138400256 bytes transferred in 713.156461 secs (4400718 bytes/sec)
That’s it. Your USB key is now ready and you can boot from it or use it to install Kali Linux.
Booting an Alternate Disk To boot from an alternate drive on an OS X/macOS system, bring up the boot menu by
on OS X/macOS pressing and holding the Option key immediately after powering on the device and
selecting the drive you want to use.
For more information, see Apple’s knowledge base2 .
Chapter 2 — Getting Started with Kali Linux 23
2.2. Booting a Kali ISO Image in Live Mode
2.2.1. On a Real Computer
As a prerequisite, you need either a USB key prepared (as detailed in the previous section) or a
DVD-ROM burned with a Kali Linux ISO image.
The BIOS/UEFI is responsible for the early boot process and can be configured through a piece of
software called Setup. In particular, it allows users to choose which boot device is preferred. In
this case, you want to select either the DVD-ROM drive or USB drive, depending on which device
you have created. Depending on your BIOS/UEFI, you may have a one time boot menu option,
allowing to temporarily change the boot order.
Starting Setup usually involves pressing a particular key very soon after the computer is powered
on. This key is often Delete or Escape, and sometimes F2, F8, F10 or F12. Most of the time, the
choice is briefly flashed onscreen when the computer powers on, before the operating system
loads.
Once the BIOS/UEFI has been properly configured to boot from your device, booting Kali Linux
is simply a matter of inserting the DVD-ROM or plugging in the USB drive and powering on the
computer.
Disable Secure Boot While the Kali Linux images can be booted in UEFI mode, they do not support secure
boot. You should disable that feature in your machine’s Setup.
2.2.2. In a Virtual Machine
Virtual machines have multiple benefits for Kali Linux users. They are especially useful if you want
to try out Kali Linux but aren’t ready to commit to installing it permanently on your machine or if
you have a powerful system and want to run multiple operating systems simultaneously. This is a
popular choice for many penetration testers and security professionals who need to use the wide
range of tools available in Kali Linux but still want to have full access to their primary operating
system. This also provides them with the ability to archive or securely delete the virtual machine
and any client data it may contain rather than reinstalling their entire operating system.
The snapshot features of virtualization software also make it easy to experiment with potentially
dangerous operations, such as malware analysis, while allowing for an easy way out by restoring
a previous snapshot.
There are many virtualization tools available for all major operating systems, including VirtualBox®,
VMware Workstation®, Xen, KVM, and Hyper-V to name a few. Ultimately, you will use the one that
best suits you but we will cover the two most frequently-used in a desktop context: VirtualBox® and
2
https://support.apple.com/en-gb/guide/mac-help/mchlp1034/mac
24 Kali Linux Revealed
VMware Workstation®, both running on Microsoft Windows 10. If you don’t have corporate policy
constraints or personal preference, our recommendation is that you try out VirtualBox first, as it
is free, works well, is (mostly) open-source, and is available for most operating systems.
For the next sections, we will assume that you have already installed the appropriate virtualization
tool and are familiar with its operation.
Preliminary Remarks
To fully benefit from virtualization, you should have a CPU with the appropriate virtualization
features and they should not be disabled by the BIOS/UEFI. Double check for any “Intel® Virtual-
ization Technology” and/or “Intel® VT-d Feature” options in the machine’s Setup screens.
You should also have a 64-bit host operating system, such as amd64 architecture for Debian-based
Linux distributions, x86_64 architecture for RedHat-based Linux distributions, and 64-bit for Mi-
crosoft Windows.
If you lack any of the prerequisites, either the virtualization tool will not work properly or it will
be restricted to running only 32-bit guest operating systems.
Since virtualization tools hook into the host operating system and hardware at a low level, there
are often incompatibilities between them. Do not expect these tools to run well at the same time.
Also, Microsoft Windows users beware that professional editions or higher come with Hyper-V
installed and enabled, which might interfere with your virtualization tool of choice. To turn it off,
execute “Turn Windows features on or off” from Windows Settings.
VirtualBox
After the initial installation, VirtualBox’s main screen looks something like Figure 2.6, “Virtual-
Box’s Start Screen” [page 26].
Chapter 2 — Getting Started with Kali Linux 25
Figure 2.6 VirtualBox’s Start Screen
Click on New (Figure 2.7, “Name and Operating System” [page 27]) to start a wizard that will guide
you through the multiple steps required to input all the parameters of the new virtual machine.
26 Kali Linux Revealed
Figure 2.7 Name and Operating System
In the first step, shown in Figure 2.7, “Name and Operating System” [page 27], you must assign a
name to your new virtual machine. We will use “Kali Linux.” You must also indicate what kind
of operating system will be used. Since Kali Linux is based on Debian GNU/Linux, select ”Linux”
for the type and ”Debian (32-bit)” or ”Debian (64-bit)” for the version. Although any other Linux
version will most likely work, this will help distinguish between the various virtual machines that
you might have installed.
Chapter 2 — Getting Started with Kali Linux 27
Figure 2.8 Memory Size
In the second step, you must decide how much memory to allocate to the virtual machine. While
the recommended size of 1024 MB is acceptable for a Debian virtual machine acting as a server, it
is definitely not enough to run a Kali desktop system, especially not for a Kali Linux live system,
as the live system uses memory to store changes made to the file system. We recommend increas-
ing the value to 1500 MB (Figure 2.8, “Memory Size” [page 28]) and highly recommend that you
allocate no less than 2048 MB of RAM. For more information, see section 4.1, “Minimal Installation
Requirements” [page 66]
28 Kali Linux Revealed
Figure 2.9 Hard disk
In the third step (shown in Figure 2.9, “Hard disk” [page 29]), you are prompted to choose a physi-
cal or virtual hard disk for your new virtual machine. Although a hard disk is not required to run
Kali Linux as a live system, add one for when we demonstrate the installation procedure later, in
chapter 4, “Installing Kali Linux” [page 66].
Chapter 2 — Getting Started with Kali Linux 29
Figure 2.10 Hard Disk File Type
The content of the hard disk of the virtual machine is stored on the host machine as a file. Virtu-
alBox is able to store the contents of the hard disk using multiple formats (shown in Figure 2.10,
“Hard Disk File Type” [page 30]): the default (VDI) corresponds to VirtualBox’s native format;
VMDK is the format used by VMware. Keep the default value, because you don’t have any reason
to change it. The ability to use multiple formats is interesting mainly when you want to move a
virtual machine from one virtualization tool to another.
30 Kali Linux Revealed
Figure 2.11 Storage on Physical Hard Disk
The explanation text in Figure 2.11, “Storage on Physical Hard Disk” [page 31] clearly describes
the advantages and drawbacks of dynamic and fixed disk allocation. In this example, we accept
the default selection (Dynamically allocated), since we are using a laptop with SSD disks. We don’t
want to waste space and won’t need the extra bit of performance as the machine is already quite
fast to begin with.
Chapter 2 — Getting Started with Kali Linux 31
Figure 2.12 File Location and Size
The default hard disk size of 20 GB shown in Figure 2.12, “File Location and Size” [page 32] is
enough for a standard installation of Kali Linux, so we will not change it. For more information
about Kali’s requirements see section 4.1, “Minimal Installation Requirements” [page 66]. You can
also tweak the name and the location of the disk image. This can be handy when you don’t have
enough space on your hard disk, allowing you to store the disk image on an external drive.
32 Kali Linux Revealed
Figure 2.13 The New Virtual Machine Appears in the List
The virtual machine has been created but you can’t really run it yet, because there is no operating
system installed. You also have some settings to tweak. Click on Settings on the VM Manager
screen and let’s review some of the most useful settings.
Chapter 2 — Getting Started with Kali Linux 33
Figure 2.14 Storage Settings
In the Storage screen (Figure 2.14, “Storage Settings” [page 34]), you should associate the Kali
Linux ISO image with the virtual CD/DVD-ROM reader. First, select the CD-ROM drive in the Stor-
age Tree list and then click on the small CD-ROM icon on the right to display a contextual menu
where you can Choose Virtual Optical Disk File….
34 Kali Linux Revealed
Figure 2.15 System Settings: Motherboard
In the System screen (Figure 2.15, “System Settings: Motherboard” [page 35]), you will find a Moth-
erboard tab. Make sure that the boot order indicates that the system will first try to boot from
any optical device before trying a hard disk. This is also the tab where you can alter the amount
of memory allocated to the virtual machine, should the need arise.
Chapter 2 — Getting Started with Kali Linux 35
Figure 2.16 System Settings: Processor
In the same screen but on the “Processor” tab (Figure 2.16, “System Settings: Processor” [page
36]), you can adjust the number of processors assigned to the virtual machine. Most importantly,
if you use a 32-bit image, enable PAE/NX or the Kali image will not boot since the default kernel
variant used by Kali for i386 (aptly named “686-pae”) is compiled in a way that requires Physical
Address Extension (PAE) support in the CPU.
There are many other parameters that can be configured, like the network setup (defining how
the traffic on the network card is handled), but the above changes are sufficient to be able to boot
a working Kali Linux live system. Finally, click Boot and the virtual machine should boot properly,
as shown in Figure 2.17, “Kali Linux Boot Screen in VirtualBox” [page 37]. If not, carefully review
all settings and try again.
36 Kali Linux Revealed
Figure 2.17 Kali Linux Boot Screen in VirtualBox
VMware Workstation
VMware Workstation is very similar to VirtualBox in terms of features and user interface, because
they are both designed primarily for desktop usage, but the setup process for a new virtual ma-
chine is a bit different. We will be using VMware Workstation Pro edition.
Chapter 2 — Getting Started with Kali Linux 37
Figure 2.18 VMware Start Screen
The initial screen, shown in Figure 2.18, “VMware Start Screen” [page 38], displays a big Create a
New Virtual Machine button that starts a wizard to guide you through the creation of your virtual
machine.
Figure 2.19 New virtual Machine Wizard
38 Kali Linux Revealed
In the first step, you must decide whether you want to be presented with advanced settings dur-
ing the setup process. In this example, there are no special requirements so choose the typical
installation, as shown in Figure 2.19, “New virtual Machine Wizard” [page 38].
Figure 2.20 Guest Operating System Installation
The wizard assumes that you want to install the operating system immediately and asks you to
select the ISO image containing the installation program (Figure 2.20, “Guest Operating System
Installation” [page 39]). Select “Installer disc image file (iso)” and click on Browse to select the
image file.
Chapter 2 — Getting Started with Kali Linux 39
Figure 2.21 Select a Guest Operating System
When the operating system cannot be detected from the selected ISO image, the wizard asks you
which guest OS type you intend to run. You should select “Linux” for the OS and “Debian 10.x
64-bit” for the version, as shown in Figure 2.21, “Select a Guest Operating System” [page 40]. We
select ”Debian 10.x” due to Kali Linux being constantly updated to the newest version of Debian.
40 Kali Linux Revealed
Figure 2.22 Name the Virtual Machine
Choose ”Kali Linux” as the name of the new virtual machine (Figure 2.22, “Name the Virtual Ma-
chine” [page 41]). As with VirtualBox, you also have the option to store the virtual machine files
in an alternate location.
Figure 2.23 Specify Disk Capacity
Chapter 2 — Getting Started with Kali Linux 41
The default hard disk size of 20 GB (Figure 2.23, “Specify Disk Capacity” [page 41]) is usually suf-
ficient but you can adjust it here depending on your expected needs. As opposed to VirtualBox,
which can use a single file of varying size, VMware has the ability to store the disk’s content over
multiple files. In both cases, the goal is to conserve the host’s disk space.
Figure 2.24 Ready to Create Virtual Machine
VMware Workstation is now configured to create the new virtual machine. It displays a summary
of the choices made so that you can double-check everything before creating the machine. Notice
that the wizard opted to allocate 2048 MB of RAM to the virtual machine, which is sufficient for
our needs. If the allocated value is lower, that is not enough so click on Customize Hardware... (Fig-
ure 2.24, “Ready to Create Virtual Machine” [page 42]) and tweak the Memory setting, as shown
in Figure 2.25, “Configure Hardware Window” [page 43].
42 Kali Linux Revealed
Figure 2.25 Configure Hardware Window
After a last click on Finish (Figure 2.24, “Ready to Create Virtual Machine” [page 42]), the virtual
machine is now configured and can be started by clicking ”Power on this virtual machine” as
shown in Figure 2.26, “Kali Linux Virtual Machine Ready” [page 44].
Chapter 2 — Getting Started with Kali Linux 43
Figure 2.26 Kali Linux Virtual Machine Ready
2.3. Summary
In this chapter, you learned about the various Kali Linux ISO images, learned how to verify and
download them, and learned how to create bootable USB disks from them on various operating sys-
tems. We also discussed how to boot the USB disks and reviewed how to configure the BIOS/UEFI
and startup settings on various hardware platforms so that the USB disks will boot.
Summary Tips:
• www.kali.org is the only official download site for Kali ISOs. Do not download them from
any other site, because those downloads could contain malware.
• Always validate the sha256sum of your downloads with the sha256sum command to ensure
the integrity of your ISO download. If it doesn’t match, try the download again or use a
different source.
• You must write the Kali Linux ISO image to a bootable media if you want to boot it on a
physical machine. Use Win32 Disk Imager on Microsoft Windows, the Disks utility on Linux
using GNOME, or the dd command on Mac OS X/macOS/Linux. Be very careful when writing
the image. Selecting the wrong disk could permanently damage data on your machine.
• Configure the BIOS/UEFI Setup screens on a PC or hold the Option key on OS X/macOS to
allow the machine to boot from the USB drive.
• Virtual machine programs like VirtualBox and VMware Workstation Pro are especially useful
if you want to try out Kali Linux but aren’t ready to commit to installing it permanently on
44 Kali Linux Revealed
your machine or if you have a powerful system and want to run multiple operating systems
simultaneously.
Now that you have a working installation of Kali Linux, it is time to delve into some Linux funda-
mentals that are required for basic and advanced operation of Kali. If you are a moderate to ad-
vanced Linux user, consider skimming the next chapter (chapter 3, “Linux Fundamentals” [page
48]).
Chapter 2 — Getting Started with Kali Linux 45
Keywords
Linux kernel
User space
Command line
bash
Filesystem Hierarchy
Unix commands
Chapter
Linux Fundamentals 3
Contents
What Is Linux and What Is It Doing? 48 The Command Line 51 The File System 54
Useful Commands 56 Summary 62
Before you can master Kali Linux, you must be at ease with a generic Linux system. Linux profi-
ciency will serve you well, because a large percentage of web, email, and other Internet services
run on Linux servers.
In this section, we strive to cover the basics of Linux, but we assume that you already know about
computer systems in general, including components such as the CPU, RAM, motherboard, and
hard disk, as well as device controllers and their associated connectors.
3.1. What Is Linux and What Is It Doing?
The term ”Linux” is often used to refer to the entire operating system, but in reality, Linux is
the operating system kernel, which is started by the boot loader, which is itself started by the
BIOS/UEFI. The kernel assumes a role similar to that of a conductor in an orchestra—it ensures
coordination between hardware and software. This role includes managing hardware, processes,
users, permissions, and the file system. The kernel provides a common base to all other programs
on the system and typically runs in ring zero, also known as kernel space.
The User Space We use the term user space to lump together everything that happens outside of the
kernel.
Among the programs running in user space are many core utilities from the GNU
project1 , most of which are meant to be run from the command line. You can use
them in scripts to automate many tasks. Refer to section 3.4, “Useful Commands”
[page 56] for more information about the most important commands.
Let’s quickly review the various tasks handled by the Linux kernel.
3.1.1. Driving Hardware
The kernel is tasked, first and foremost, with controlling the computer’s hardware components.
It detects and configures them when the computer powers on, or when a device is inserted or re-
moved (for example, a USB device). It also makes them available to higher-level software, through
a simplified programming interface, so applications can take advantage of devices without having
to address details such as which extension slot an option board is plugged into. The programming
interface also provides an abstraction layer; this allows video-conferencing software, for exam-
ple, to use a webcam regardless of its maker and model. The software can use the Video for Linux
(V4L) interface and the kernel will translate function calls of the interface into actual hardware
commands needed by the specific webcam in use.
The kernel exports data about detected hardware through the /proc/ and /sys/ virtual file sys-
tems. Applications often access devices by way of files created within /dev/. Specific files rep-
1
https://www.gnu.org/
48 Kali Linux Revealed
resent disk drives (for instance, /dev/sda), partitions (/dev/sda1), mice (/dev/input/mouse0),
keyboards (/dev/input/event0), sound cards (/dev/snd/*), serial ports (/dev/ttyS*), and other
components.
There are two types of device files: block and character. The former has characteristics of a block of
data: It has a finite size, and you can access bytes at any position in the block. The latter behaves
like a flow of characters. You can read and write characters, but you cannot seek to a given position
and change arbitrary bytes. To find out the type of a given device file, inspect the first letter in
the output of ls -l. It is either b, for block devices, or c, for character devices:
$ ls -l /dev/sda /dev/ttyS0
brw-rw---- 1 root disk 8, 0 Mar 21 08:44 /dev/sda
crw-rw---- 1 root dialout 4, 64 Mar 30 08:59 /dev/ttyS0
As you might expect, disk drives and partitions use block devices, whereas mouse, keyboard, and
serial ports use character devices. In both cases, the programming interface includes device-
specific commands that can be invoked through the ioctl system call.
3.1.2. Unifying File Systems
File systems are a prominent aspect of the kernel. Unix-like systems merge all the file stores into a
single hierarchy, which allows users and applications to access data by knowing its location within
that hierarchy.
The starting point of this hierarchical tree is called the root, represented by the “/” character.
This directory can contain named subdirectories. For instance, the home subdirectory of / is called
/home/. This subdirectory can, in turn, contain other subdirectories, and so on. Each directory can
also contain files, where the data will be stored. Thus, /home/kali/Desktop/hello.txt refers
to a file named hello.txt stored in the Desktop subdirectory of the kali subdirectory of the home
directory, present in the root. The kernel translates between this naming system and the storage
location on a disk.
Unlike other systems, Linux possesses only one such hierarchy, and it can integrate data from
several disks. One of these disks becomes the root, and the others are mounted on directories in
the hierarchy (the Linux command is called mount). These other disks are then available under the
mount points. This allows storing users’ home directories (traditionally stored within /home/) on
a separate hard disk, which will contain the kali directory (along with home directories of other
users). Once you mount the disk on /home/, these directories become accessible at their usual
locations, and paths such as /home/kali/Desktop/hello.txt keep working.
There are many file system formats, corresponding to many ways of physically storing data on
disks. The most widely known are ext2, ext3, and ext4, but others exist. For instance, VFAT is the
filesystem that was historically used by DOS and Microsoft Windows operating systems. Linux’s
support for VFAT allows hard disks to be accessible under Kali as well as under Microsoft Windows.
In any case, you must prepare a file system on a disk before you can mount it and this operation is
Chapter 3 — Linux Fundamentals 49
known as formatting. Commands such as mkfs.ext4 (where mkfs stands for MaKe FileSystem) han-
dle formatting. These commands require, as a parameter, a device file representing the partition
to be formatted (for instance, /dev/sda1, the first partition on the first drive). This operation is
destructive and should be run only once, unless you want to wipe a filesystem and start fresh.
There are also network filesystems such as NFS, which do not store data on a local disk. Instead,
data is transmitted through the network to a server that stores and retrieves them on demand.
Thanks to the file system abstraction, you don’t have to worry about how this disk is connected,
since the files remain accessible in their usual hierarchical way.
3.1.3. Managing Processes
A process is a running instance of a program, which requires memory to store both the program
itself and its operating data. The kernel is in charge of creating and tracking processes. When
a program runs, the kernel first sets aside some memory, loads the executable code from the file
system into it, and then starts the code running. It keeps information about this process, the most
visible of which is an identification number known as the process identifier (PID).
Like most modern operating systems, those with Unix-like kernels, including Linux, are capable
of multi-tasking. In other words, they allow the system to run many processes at the same time.
There is actually only one running process at any one time, but the kernel divides CPU time into
small slices and runs each process in turn. Since these time slices are very short (in the millisecond
range), they create the appearance of processes running in parallel, although they are active only
during their time interval and are idle the rest of the time. The kernel’s job is to adjust its schedul-
ing mechanisms to keep that appearance, while maximizing global system performance. If the
time slices are too long, the application may not appear as responsive as desired. Too short, and
the system loses time by switching tasks too frequently. These decisions can be refined with pro-
cess priorities, where high-priority processes will run for longer periods and with more frequent
time slices than low-priority processes.
Multi-Processor Systems The limitation described above, of only one process running at a time, doesn’t always
(and Variants) apply: the actual restriction is that there can be only one running process per pro-
cessor core. Multi-processor, multi-core, or hyper-threaded systems allow several
processes to run in parallel. The same time-slicing system is used, though, to handle
cases where there are more active processes than available processor cores. This is not
unusual: a basic system, even a mostly idle one, almost always has tens of running
processes.
The kernel allows several independent instances of the same program to run, but each is allowed
to access only its own time slices and memory. Their data thus remains independent.
50 Kali Linux Revealed
3.1.4. Rights Management
Unix-like systems support multiple users and groups and allow control of permissions. Most of
the time, a process is identified by the user who started it. That process is only permitted to
take actions permitted for its owner. For instance, opening a file requires the kernel to check
the process identity against access permissions (for more details on this particular example, see
section 3.4.4, “Managing Rights” [page 57]).
3.2. The Command Line
By “command line”, we mean a text-based interface that allows you to enter commands, execute
them, and view the results. You can run a terminal (a textual screen within the graphical desktop,
or the text console itself outside of any graphical interface) and a command interpreter inside it
(the shell).
3.2.1. How To Get a Command Line
When your system is working properly, the easiest way to access the command line is to run a
terminal in your graphical desktop session.
Figure 3.1 Starting QTerminal
Chapter 3 — Linux Fundamentals 51
For instance, on a default Kali Linux system, QTerminal can be started from the list of favorite
applications. You can also type “terminal” while in the applications menu (the one that gets acti-
vated when you move the mouse to the top-left corner) and click on the correct application icon
that appears (Figure 3.1, “Starting QTerminal” [page 51]).
In the event that your graphical interface is broken, you can still get a command line on virtual
consoles (up to six of them can be accessible through the six key combinations of CTRL+ALT+F1
through CTRL+ALT+F6 — the CTRL key can be omitted if you are already in text mode, outside of
Xorg or Wayland’s graphical interface). You get a very basic login screen where you enter your
login and password before being granted access to the command line with its shell:
Kali GNU/Linux Rolling kali tty3
kali login: kali
Password:
Linux kali 5.7.0-kali1-amd64 #1 SMP Debian 5.7.6-1kali2 (2020-07-01) X86_64
The programs included with the Kali GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Kali GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
kali@kali:~$
The program handling your input and executing your commands is called a shell (or a command-
line interpreter). The default shell provided in Kali Linux is Bash (it stands for Bourne Again SHell).
The trailing “$“ or “#” character indicates that the shell is awaiting your input. It also indicates
whether Bash recognizes you as a normal user (the former case with the dollar, $) or as a super
user (the latter case with the hash, #).
3.2.2. Command Line Basics: Browsing the Directory Tree and Managing Files
This section only provides a brief overview of the covered commands, all of which have many
options not described here, so please refer to the abundant documentation available in their re-
spective manual pages. In penetration tests, you will most often receive shell access to a system
after a successful exploit, rather than a graphical user interface. Proficiency with the command
line is essential for your success as a security professional.
Once a session is open, the pwd command (which stands for print working directory) displays your
current location in the filesystem. The current directory is changed with the cd directory com-
mand (cd is for change directory). When you don’t specify the target directory, you are taken to
your home directory. When you use cd - (dash), you go back to the former working directory
(the one in use before the last cd call). The parent directory is always called .. (two dots), whereas
52 Kali Linux Revealed
the current directory is also known as . (one dot). The ls command allows listing the contents of
a directory. If you don’t provide parameters, ls operates on the current directory.
$ pwd
/home/kali
$ cd Desktop
$ pwd
/home/kali/Desktop
$ cd .
$ pwd
/home/kali/Desktop
$ cd ..
$ pwd
/home/kali
$ ls
Desktop Downloads Pictures Templates
Documents Music Public Videos
You can create a new directory with mkdir directory, and remove an existing (empty) directory
with rmdir directory. The mv command allows moving and renaming files and directories; remov-
ing a file is achieved with rm file, and copying a file is done with cp source-file target-file.
$ mkdir test
$ ls
Desktop Downloads Pictures Templates Videos
Documents Music Public test
$ mv test new
$ ls
Desktop Downloads new Public Videos
Documents Music Pictures Templates
$ rmdir new
$ ls
Desktop Downloads Pictures Templates Videos
Documents Music Public
The shell executes each command by running the first program of the given name that it finds
in a directory listed in the PATH environment variable. Most often, these programs are in /bin,
/sbin, /usr/bin, or /usr/sbin. For example, the ls command is found in /bin/ls; the which
command reports the location of a given executable. Sometimes the command is directly handled
by the shell, in which case, it is called a shell built-in command (cd and pwd are among those); the
type command lets you query the type of each command.
$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
$ which ls
/bin/ls
$ type rm
Chapter 3 — Linux Fundamentals 53
rm is /bin/rm
$ type cd
cd is a shell builtin
Note the usage of the echo command, which simply displays a string on the terminal. In this case,
it is used to print the contents of an environment variable since the shell automatically substitutes
variables with their values before executing the command line.
Environment Variables Environment variables allow storage of global settings for the shell or various other
programs. They are contextual but inheritable. For example, each process has its
own set of environment variables (they are contextual). Shells, like login shells, can
declare variables, which will be passed down to other programs they execute (they
are inheritable).
These variables can be defined system-wide in /etc/profile or per-user in ~/
.profile but variables that are not specific to command line interpreters are better
put in /etc/environment, since those variables will be injected into all user sessions
thanks to a Pluggable Authentication Module (PAM) – even when no shell is executed.
3.3. The File System
3.3.1. The Filesystem Hierarchy Standard
As with other Linux distributions, Kali Linux is organized to be consistent with the Filesystem Hi-
erarchy Standard (FHS), allowing users of other Linux distributions to easily find their way around
Kali. The FHS defines the purpose of each directory. The top-level directories are described as
follows.
• /bin/: basic programs
• /boot/: Kali Linux kernel and other files required for its early boot process
• /dev/: device files
• /etc/: configuration files
• /home/: user’s personal files
• /lib/: basic libraries
• /media/: mount points for removable devices (CD/DVD-ROM, USB keys, and so on)
• /mnt/: temporary mount point
• /opt/: extra applications provided by third parties
• /root/: administrator’s (root’s) personal files
• /run/: volatile runtime data that does not persist across reboots (not yet included in the
FHS)
54 Kali Linux Revealed
• /sbin/: system programs
• /srv/: data used by servers hosted on this system
• /tmp/: temporary files (this directory is often emptied at boot)
• /usr/: applications (this directory is further subdivided into bin, sbin, lib according to
the same logic as in the root directory) Furthermore, /usr/share/ contains architecture-
independent data. The /usr/local/ directory is meant to be used by the administrator for
installing applications manually without overwriting files handled by the packaging system
(dpkg).
• /var/: variable data handled by services. This includes log files, queues, spools, and caches.
• /proc/ and /sys/ are specific to the Linux kernel (and not part of the FHS). They are used
by the kernel for exporting data to user space.
3.3.2. The User’s Home Directory
The contents of a user’s home directory are not standardized but there are still a few noteworthy
conventions. One is that a user’s home directory is often referred to by a tilde (“~”). That is useful
to know because command interpreters automatically replace a tilde with the correct directory
(which is stored in the HOME environment variable, and whose usual value is /home/user/).
Traditionally, application configuration files are often stored directly under your home directory,
but the filenames usually start with a dot (for instance, the mutt email client stores its configura-
tion in ~/.muttrc). Note that filenames that start with a dot are hidden by default; the ls com-
mand only lists them when the -a option is used and graphical file managers need to be explicitly
configured to display hidden files.
Some programs also use multiple configuration files organized in one directory (for instance, ~/
.ssh/). Some applications (such as the Firefox web browser) also use their directory to store a
cache of downloaded data. This means that those directories can end up consuming a lot of disk
space.
These configuration files stored directly in your home directory, often collectively referred to
as dotfiles, have long proliferated to the point that these directories can be quite cluttered with
them. Fortunately, an effort led collectively under the FreeDesktop.org2 umbrella has resulted
in the XDG Base Directory Specification, a convention that aims at cleaning up these files and
directories. This specification states that configuration files should be stored under ~/.config,
cache files under ~/.cache, and application data files under ~/.local (or subdirectories thereof).
This convention is slowly gaining traction.
Graphical desktops usually have shortcuts to display the contents of the ~/Desktop/ directory (or
whatever the appropriate translation is for systems not configured in English).
2
https://www.freedesktop.org/
Chapter 3 — Linux Fundamentals 55
Finally, the email system sometimes stores incoming emails into a ~/Mail/ directory.
3.4. Useful Commands
3.4.1. Displaying and Modifying Text Files
The cat file command (intended to concatenate files to the standard output device) reads a file
and displays its contents on the terminal. If the file is too big to fit on a screen, you can use a pager
such as less (or more) to display it page by page.
The editor command starts a text editor (such as Vi or Nano) and allows creating, modifying,
and reading text files. The simplest files can sometimes be created directly from the command
interpreter thanks to redirection: command > file creates a file named file containing the out-
put of the given command. command >> file is similar except that it appends the output of the
command to the file rather than overwriting it.
$ echo ”Kali rules!” > kali-rules.txt
$ cat kali-rules.txt
Kali rules!
$ echo ”Kali is the best!” >> kali-rules.txt
$ cat kali-rules.txt
Kali rules!
Kali is the best!
3.4.2. Searching for Files and within Files
The find directory criteria command searches for files in the hierarchy under directory ac-
cording to several criteria. The most commonly used criterion is -name filename, which allows
searching for a file by name. You can also use common wildcards such as “*” in the filename
search.
$ find /etc -name hosts
/etc/hosts
/etc/avahi/hosts
$ find /etc -name ”hosts*”
/etc/hosts
/etc/hosts.allow
/etc/hosts.deny
/etc/avahi/hosts
The grep expression files command searches the contents of the files and extracts lines
matching the regular expression. Adding the -r option enables a recursive search on all files con-
tained in the directory. This allows you to look for a file when you only know a part of its contents.
56 Kali Linux Revealed
3.4.3. Managing Processes
The ps aux command lists the processes currently running and helps to identify them by showing
their PID. Once you know the PID of a process, the kill -signal pid command allows you to send
it a signal (if you own the process). Several signals exist; most commonly used are TERM (a request
to terminate gracefully) and KILL (a forced kill).
The command interpreter can also run programs in the background if the command is followed
by “&”. By using the ampersand, you resume control of the shell immediately even though the
command is still running (hidden from view as a background process). The jobs command lists
the processes running in the background; running fg %job-number (for foreground) restores a
job to the foreground. When a command is running in the foreground (either because it was
started normally, or brought back to the foreground with fg), the Control+Z key combination
pauses the process and resumes control of the command line. The process can then be restarted
in the background with bg %job-number (for background).
3.4.4. Managing Rights
Linux is a multi-user system so it is necessary to provide a permissions system to control the set of
authorized operations on files and directories, which includes all the system resources and devices
(on a Unix system, any device is represented by a file or directory). This principle is common to
all Unix-like systems.
Each file or directory has specific permissions for three categories of users:
• Its owner (symbolized by u, as in User)
• Its owner group (symbolized by g, as in Group), representing all the members of the group
• The others (symbolized by o, as in Other)
Three types of rights can be combined:
• Reading (symbolized by r, as in Read);
• Writing (or modifying, symbolized by w, as in Write);
• Executing (symbolized by x, as in eXecute).
In the case of a file, these rights are easily understood: read access allows reading the content
(including copying), write access allows changing it, and execute access allows running it (which
will only work if it is a program).
setuidand setgid Two particular rights are relevant to executable files: setuid and setgid (symbolized
executables with the letter “s”). Note that we frequently speak of bit, since each of these boolean
values can be represented by a 0 or a 1. These two rights allow any user to execute
the program with the rights of the owner or the group, respectively. This mechanism
Chapter 3 — Linux Fundamentals 57
grants access to features requiring higher level permissions than those you would
usually have.
Since a setuid root program is systematically run under the super-user identity, it is
very important to ensure it is secure and reliable. Any user who manages to subvert
a setuid root program to call a command of their choice could then impersonate the
root user and have all rights on the system. Penetration testers regularly search for
these types of files when they gain access to a system as a way of escalating their
privileges.
A directory is handled differently from a file. Read access gives the right to consult the list of its
contents (files and directories); write access allows creating or deleting files; and execute access
allows crossing through the directory to access its contents (for example, with the cd command).
Being able to cross through a directory without being able to read it gives the user permission to
access the entries therein that are known by name, but not to find them without knowing their
exact name.
SECURITY The setgid bit also applies to directories. Any newly-created item in such directories
setgid directory and sticky is automatically assigned the owner group of the parent directory, instead of inherit-
bit ing the creator’s main group as usual. Because of this, you don’t have to change your
main group (with the newgrp command) when working in a file tree shared between
several users of the same dedicated group.
The sticky bit (symbolized by the letter “t”) is a permission that is only useful in
directories. It is especially used for temporary directories where everybody has write
access (such as /tmp/): it restricts deletion of files so that only their owner or the
owner of the parent directory can delete them. Lacking this, everyone could delete
other users’ files in /tmp/.
Three commands control the permissions associated with a file:
• chown user file changes the owner of the file
• chgrp group file alters the owner group
• chmod rights file changes the permissions for the file
TIP Frequently you want to change the group of a file at the same time that you change
Changing the user and the owner. The chown command has a special syntax for that: chown user:group
group file
There are two ways of representing rights. Among them, the symbolic representation is probably
the easiest to understand and remember. It involves the letter symbols mentioned above. You
can define rights for each category of users (u/g/o), by setting them explicitly (with =), by adding
(+), or subtracting (-). Thus the u=rwx,g+rw,o-r formula gives the owner read, write, and execute
rights, adds read and write rights for the owner group, and removes read rights for other users.
58 Kali Linux Revealed
Rights not altered by the addition or subtraction in such a command remain unmodified. The
letter a, for all, covers all three categories of users, so that a=rx grants all three categories the
same rights (read and execute, but not write).
The (octal) numeric representation associates each right with a value: 4 for read, 2 for write, and
1 for execute. We associate each combination of rights with the sum of the three figures, and a
value is assigned to each category of users, in the usual order (owner, group, others).
For instance, the chmod 754 file command will set the following rights: read, write and execute
for the owner (since 7 = 4 + 2 + 1); read and execute for the group (since 5 = 4 + 1); read-only for
others. The 0 means no rights; thus chmod 600 file allows for read and write permissions for the
owner, and no rights for anyone else. The most frequent right combinations are 755 for executable
files and directories, and 644 for data files.
To represent special rights, you can prefix a fourth digit to this number according to the same
principle, where the setuid, setgid, and sticky bits are 4, 2, and 1, respectively. The command
chmod 4754 will associate the setuid bit with the previously described rights.
Note that the use of octal notation only allows you to set all the rights at once on a file; you cannot
use it to add a new right, such as read access for the group owner, since you must take into account
the existing rights and compute the new corresponding numerical value.
The octal representation is also used with the umask command, which is used to restrict permis-
sions on newly created files. When an application creates a file, it assigns indicative permissions,
knowing that the system automatically removes the rights defined with umask. Enter umask in a
shell; you will see a mask such as 0022. This is simply an octal representation of the rights to be
systematically removed (in this case, the write rights for the group and other users).
If you give it a new octal value, the umask command modifies the mask. Used in a shell initial-
ization file (for example, ~/.bash_profile), it will effectively change the default mask for your
work sessions.
TIP Sometimes we have to change rights for an entire file tree. All the commands above
Recursive operation have a -R option to operate recursively in sub-directories.
The distinction between directories and files sometimes causes problems with recur-
sive operations. That is why the “X” letter has been introduced in the symbolic repre-
sentation of rights. It represents a right to execute which applies only to directories
(and not to files lacking this right). Thus, chmod -R a+X directory will only add
execute rights for all categories of users (a) for all of the sub-directories and files for
which at least one category of user (even if their sole owner) already has execute
rights.
Chapter 3 — Linux Fundamentals 59
3.4.5. Getting System Information and Logs
The free command displays information on memory; (df (for disk free) reports on the available
disk space on each of the disks mounted in the file system. Its -h option (for human readable)
converts the sizes into a more legible unit (usually mebibytes or gibibytes). In a similar fashion,
the free command supports the -m and -g options, and displays its data either in mebibytes or in
gibibytes, respectively.
$ free
total used free shared buff/cache available
Mem: 2052944 661232 621208 10520 770504 1359916
Swap: 0 0 0
$ df
Filesystem 1K-blocks Used Available Use% Mounted on
udev 1014584 0 1014584 0% /dev
tmpfs 205296 8940 196356 5% /run
/dev/vda1 30830588 11168116 18073328 39% /
tmpfs 1026472 456 1026016 1% /dev/shm
tmpfs 5120 0 5120 0% /run/lock
tmpfs 1026472 0 1026472 0% /sys/fs/cgroup
tmpfs 205296 36 205260 1% /run/user/132
tmpfs 205296 24 205272 1% /run/user/0
The id command displays the identity of the user running the session along with the list of groups
they belong to. Since access to some files or devices may be limited to group members, checking
available group membership may be useful.
$ id
uid=1000(kali) gid=1000(kali) groups=1000(kali),27(sudo)
The uname -a command returns a single line documenting the kernel name (Linux), the hostname,
the kernel release, the kernel version, the machine type (an architecture string such as x86_64),
and the name of the operating system (GNU/Linux). The output of this command should usually
be included in bug reports as it clearly defines the kernel in use and the hardware platform you
are running on.
$ uname -a
Linux kali 5.8.0-kali3-amd64 #1 SMP Debian 5.8.14-1kali1 (2020-10-13) x86_64 GNU/Linux
All these commands provide run-time information, but often you need to consult logs to under-
stand what happened on your computer. In particular, the kernel emits messages that it stores in
a ring buffer whenever something interesting happens (such as a new USB device being inserted,
a failing hard disk operation, or initial hardware detection on boot). You can retrieve the kernel
logs with the dmesg command.
Systemd’s journal also stores multiple logs (stdout/stderr output of services, syslog messages, ker-
nel logs) and makes it easy to query them with journalctl. Without any arguments, it just dumps
60 Kali Linux Revealed
all the available logs in a chronological way. With the -r option, it will reverse the order so that
newer messages are shown first. With the -f option, it will continuously print new log entries
as they are appended to its database. The -u option can limit the messages to those emitted by a
specific systemd unit (ex: journalctl -u ssh.service).
3.4.6. Discovering the Hardware
The kernel exports many details about detected hardware through the /proc/ and /sys/ virtual
filesystems. Several tools summarize those details. Among them, lspci (in the pciutils package)
lists PCI devices, lsusb (in the usbutils package) lists USB devices, and lspcmcia (in the pcmciau-
tils package) lists PCMCIA cards. These tools are very useful for identifying the exact model of
a device. This identification also allows more precise searches on the web, which in turn, lead
to more relevant documents. Note that the pciutils and usbutils packages are already installed on
the base Kali system but pcmciautils must be installed with apt update followed by apt install
pcmciautils. We will discuss more about package installation and management in section 8.2,
“Basic Package Interaction” [page 181]
Example 3.1 Example of information provided by lspci and lsusb
$ lspci
[...]
00:02.1 Display controller: Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller (rev 03)
00:1c.0 PCI bridge: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) PCI Express Port 1 (rev 03)
00:1d.0 USB Controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB UHCI #1 (rev 03)
[...]
01:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5751 Gigabit Ethernet PCI Express (rev 01)
02:03.0 Network controller: Intel Corporation PRO/Wireless 2200BG Network Connection (rev 05)
$ lsusb
Bus 005 Device 004: ID 413c:a005 Dell Computer Corp.
Bus 005 Device 008: ID 413c:9001 Dell Computer Corp.
Bus 005 Device 007: ID 045e:00dd Microsoft Corp.
Bus 005 Device 006: ID 046d:c03d Logitech, Inc.
[...]
Bus 002 Device 004: ID 413c:8103 Dell Computer Corp. Wireless 350 Bluetooth
These programs have a -v option that lists much more detailed (but usually unnecessary) informa-
tion. Finally, the lsdev command (in the procinfo package) lists communication resources used by
devices.
The lshw program is a combination of the above programs and displays a long description of the
hardware discovered in a hierarchical manner. You should attach its full output to any report
about hardware support problems.
Chapter 3 — Linux Fundamentals 61
3.5. Summary
In this section, we took a whirlwind tour of the Linux landscape. We discussed the kernel and user
space, reviewed many common Linux shell commands, discussed processes and how to manage
them, reviewed user and group security concepts, discussed the FHS, and toured some of the most
common directories and files found on Kali Linux.
Summary Tips:
• Linux is often used to refer to the entire operating system but in reality Linux itself is the
operating system kernel that is started by the boot loader, which is itself started by the
BIOS/UEFI.
• User space refers to everything that happens outside of the kernel. Among the programs
running in user space, there are many core utilities from the GNU project3 , most of which
are meant to be run from the command line (a text-based interface that allows you to enter
commands, execute them, and view the results). A shell executes your commands within
that interface.
• Common commands include: pwd (print working directory), cd (change directory), ls (list
file or directory contents), mkdir (make directory), rmdir (remove directory), mv, rm, and
cp (move, remove, or copy file or directory respectively), cat (concatenate or show file),
less/more (show files a page at a time), editor (start a text editor), find (locate a file or
directory), free (display memory information), df (show disk free space), id (display the
identity of the user along with the list of groups they belong to), dmesg (review kernel logs),
and journalctl (show all available logs).
• You can inspect the hardware on a Kali system with several commands: lspci (list PCI de-
vices), lsusb (list USB devices), and lspcmcia lists PCMCIA cards.
• A process is a running instance of a program, which requires memory to store both the pro-
gram itself and its operating data. You can manage processes with commands like: ps (show
processes), kill (kill processes), bg (send process to background), fg (bring background
process to foreground), and jobs (show background processes).
• Unix-like systems are multi-user. They support multiple users and groups and allow con-
trol over actions, based on permissions. You can manage file and directory rights with sev-
eral commands, including: chmod (change permissions), chown (change owner), and chgrp
(change group).
• As with other professional Linux distributions, Kali Linux is organized to be consistent with
the Filesystem Hierarchy Standard (FHS), allowing users coming from other Linux distributions
to easily find their way around Kali.
• Traditionally, application configuration files are stored under your home directory, in hid-
den files or directories starting with a period (or dot).
3
https://www.gnu.org/
62 Kali Linux Revealed
Now that you have a handle on Linux fundamentals, let’s get Kali Linux set up and running.
Chapter 3 — Linux Fundamentals 63
Keywords
Installation
Unattended
installation
ARM devices
Troubleshooting
Chapter
Installing Kali Linux 4
Contents
Minimal Installation Requirements 66 Step by Step Installation on a Hard Drive 66 Unattended Installations 95
ARM Installations 98 Troubleshooting Installations 99 Summary 104
In this chapter, we will focus on the Kali Linux installation process. First, we will discuss the min-
imum installation requirements (section 4.1, “Minimal Installation Requirements” [page 66]) to
ensure that your real or virtual system is well-configured to handle the type of installation that
you will pursue. Then we will go through each step of the installation process (section 4.2, “Step
by Step Installation on a Hard Drive” [page 66]) for a plain installation, as well as for a more se-
cure installation involving a fully encrypted file system. We will also discuss preseeding, which
allows unattended installations (section 4.3, “Unattended Installations” [page 95]) by providing
predetermined answers to installation questions. We will also show you how to install Kali Linux
on various ARM devices (section 4.4, “ARM Installations” [page 98]), which expands Kali’s capabili-
ties far beyond the desktop. Finally, we will show you what to do in the rare case of an installation
failure (section 4.5, “Troubleshooting Installations” [page 99]), so you can work through the issue
and successfully finish a tough install.
4.1. Minimal Installation Requirements
The installation requirements for Kali Linux vary depending on what you would like to install. On
the low end, you can set up Kali as a basic Secure Shell (SSH) server with no desktop, using as little
as 128 MB of RAM (512 MB recommended) and 2 GB of disk space. On the higher end, if you opt to
install the default Xfce desktop and the kali-linux-default metapackage, you should really aim for
at least 2048 MB of RAM and 20 GB of disk space.
Besides the RAM and hard disk requirements, your computer needs to have a CPU supported by
at least one of the amd64, i386, or arm64 architectures.
4.2. Step by Step Installation on a Hard Drive
In this section, we assume that you have a bootable USB drive or DVD (see section 2.1.4, “Copying
the Image on a DVD-ROM or USB Key” [page 19] for details on how to prepare such a drive) and
that you booted from it to start the installation process.
4.2.1. Plain Installation
First, we will take a look at a standard Kali installation, with an unencrypted file system.
Booting and Starting the Installer
Once the BIOS/UEFI has begun booting from the USB drive or DVD-ROM, the isolinux boot loader
menu appears, as shown in Figure 4.1, “Boot Screen” [page 67]. At this stage, the Linux kernel is
not yet loaded; this menu allows you to choose the kernel to boot and enter optional parameters
to be transferred to it in the process.
66 Kali Linux Revealed
Using the arrow keys todo a standard installation, either choose Graphical Install or Install (for
classic text-mode), then press the Enter key to initiate the remainder of the installation process.
Each menu entry hides a specific boot command line, which can be configured as needed by press-
ing the Tab key before validating the entry and booting.
Figure 4.1 Boot Screen
Once booted, the installation program guides you step-by-step through the process. We will take
a look at each of these steps in detail. We will cover installation from a standard Kali Linux live
image; installations from a mini.iso may look slightly different. We will also address graphical
mode installation, but the only difference from classic text-mode installation is the appearance.
The versions pose identical questions and present identical options.
Selecting the Language
As shown in Figure 4.2, “Selecting the Language” [page 68], the installation program begins in
English but the first step allows you to choose the language that will be used for the rest of the
Chapter 4 — Installing Kali Linux 67
installation process. This language choice is also used to define more relevant default choices in
subsequent stages (notably the keyboard layout).
Navigating with the Some steps in the installation process require you to enter information. These screens
Keyboard have several areas that may gain focus (text entry area, checkboxes, list of choices, OK
and Cancel buttons), and the Tab key allows you to move from one to another.
In graphical installation mode, you can use the mouse as you would normally on an
installed graphical desktop.
Figure 4.2 Selecting the Language
Selecting the Country
The second step (Figure 4.3, “Selecting the Country” [page 69]) consists of choosing your country.
Combined with the language, this information enables the installation program to offer the most
appropriate keyboard layout. This will also influence the configuration of the time zone. In the
United States, a standard QWERTY keyboard is suggested and the installer presents a choice of
appropriate time zones.
68 Kali Linux Revealed
Figure 4.3 Selecting the Country
Selecting the Keyboard Layout
The proposed American English keyboard corresponds to the usual QWERTY layout as shown in
Figure 4.4, “Choice of Keyboard” [page 70].
Chapter 4 — Installing Kali Linux 69
Figure 4.4 Choice of Keyboard
Detecting Hardware
In the vast majority of cases, the hardware detection step is completely automatic. The installer
detects your hardware and tries to identify the boot device used in order to access its content. It
loads the modules corresponding to the various hardware components detected and then mounts
the boot device in order to read it. The previous steps were completely contained in the boot image
included on the boot device, a file of limited size and loaded into memory by the bootloader when
booting from the boot device.
Loading Components
With the contents of the boot device now available, the installer loads all the files necessary to
continue with its work. This includes additional drivers for the remaining hardware (especially
the network card), as well as all the components of the installation program.
70 Kali Linux Revealed
Detecting Network Hardware
In this step, the installer will try to automatically identify the network card and load the corre-
sponding module. If automatic detection fails, you can manually select the module to load. If all
else fails, you can load a specific module from a removable device. This last solution is usually
only needed if the appropriate driver is not included in the standard Linux kernel, but available
elsewhere, such as the manufacturer’s website.
This step must absolutely be successful for network installations (such as those done when booting
from a mini.iso), since the Debian packages must be loaded from the network.
Configuring the Network
In order to automate the process as much as possible, the installer attempts an automatic net-
work configuration using Dynamic Host Configuration Protocol (DHCP) (for IPv4 and IPv6) and
ICMPv6’s Neighbor Discovery Protocol (for IPv6), as shown in Figure 4.5, “Network Autoconfigu-
ration” [page 71].
Figure 4.5 Network Autoconfiguration
Chapter 4 — Installing Kali Linux 71
If the automatic configuration fails, the installer offers more choices: try again with a normal
DHCP configuration, attempt DHCP configuration by declaring the name of the machine, or set up
a static network configuration.
This last option requires an IP address, a subnet mask, an IP address for a potential gateway, a
machine name, and a domain name.
Configuration without If the local network is equipped with a DHCP server that you do not wish to use
DHCP because you prefer to define a static IP address for the machine during installation,
you can add the netcfg/use_dhcp=false option when booting. You just need to
edit the desired menu entry by pressing the Tab key and adding the desired option
before pressing the Enter key.
User Creation
The installer prompts to create a new user (Figure 4.6, “Create User” [page 73]) since it automati-
cally creates a user account in the ”sudo” group. This means that the user has full administrative
privileges through the sudo command. This is helpful due to the fact that multiple items are only
available through administrative privileges.
The installer also asks for a username for the account as well as a password. The installer will
request confirmation of the password to prevent any input error.
72 Kali Linux Revealed
Figure 4.6 Create User
The Default User The default user’s password should be long (eight characters or more) and impossi-
Password ble to guess, since attackers target Internet-connected computers and servers with
automated tools, attempting to log in with obvious passwords. Sometimes attackers
leverage dictionary attacks, using many combinations of words and numbers as pass-
words. Avoid using the names of children or parents and dates of birth, because these
are easily guessed.
These remarks are equally applicable to other user passwords but the consequences
of a compromised account are less drastic for users without sudo privileges.
If you are lacking inspiration, don’t hesitate to use a password generator, such as
pwgen (found in the package of the same name, which is already included in the base
Kali installation).
Chapter 4 — Installing Kali Linux 73
Configuring the Clock
If the network is available, the system’s internal clock will be updated from a network time proto-
col (NTP) server. This is beneficial because it ensures timestamps on logs will be correct from the
first boot.
If your country spans multiple timezones, you will be asked to select the timezone that you want
to use, as shown in Figure 4.7, “Timezone Selection” [page 74].
Figure 4.7 Timezone Selection
Detecting Disks and Other Devices
This step automatically detects the hard drives on which Kali may be installed, each of which will
be presented in the next step: partitioning.
74 Kali Linux Revealed
Partitioning
Partitioning is an indispensable step in installation, which consists of dividing the available space
on the hard drives into discrete sections (partitions) according to the intended function of the
computer and those partitions. Partitioning also involves choosing the file systems to be used. All
of these decisions will have an influence on performance, data security, and server administration.
The partitioning step is traditionally difficult for new users. However, the Linux file systems and
partitions, including virtual memory (or swap partitions) must be defined as they form the foun-
dation of the system. This task can become complicated if you have already installed another
operating system on the machine and you want the two to coexist. In this case, you must make
sure not to alter its partitions, or if need be, resize them without causing damage.
To accommodate more common (and simpler) partition schemes, most users will prefer the Guided
mode that recommends partition configurations and provides suggestions each step of the way.
More advanced users will appreciate the Manual mode, which allows for more advanced configu-
rations. Each mode shares certain capabilities.
Guided Partitioning The first screen in the partitioning tool (Figure 4.8, “Choice of Partitioning
Mode” [page 76]) presents entry points for the guided and manual partitioning modes. “Guided -
use entire disk” is the simplest and most common partition scheme, which will allocate an entire
disk to Kali Linux.
The next two selections use Logical Volume Manager (LVM) to set up logical (instead of physical),
optionally encrypted, partitions. We will discuss LVM and encryption later in this chapter with
section 4.2.2, “Installation on a Fully Encrypted File System” [page 88]).
Finally, the last choice initiates manual partitioning, which allows for more advanced partitioning
schemes, such as installing Kali Linux alongside other operating systems. We will discuss manual
mode in the next section (“Beware: The Boot Loader and Dual Boot” [page 87])
In this example, we will allocate an entire hard disk to Kali, so we select “Guided - use entire disk”
to proceed to the next step.
Chapter 4 — Installing Kali Linux 75
Figure 4.8 Choice of Partitioning Mode
The next screen (shown in Figure 4.9, “Disk to Use for Guided Partitioning” [page 77]) allows you
to choose the disk where Kali will be installed by selecting the corresponding entry (for example,
“SCSI3 (0,0,0) (sda) - 21.5 GB VMware, VMware Virtual S”). Once selected, guided partitioning will
continue. This option will erase all of the data on this disk, so choose wisely.
76 Kali Linux Revealed
Figure 4.9 Disk to Use for Guided Partitioning
Next, the guided partitioning tool offers three partitioning methods, which correspond to differ-
ent usages, as shown in Figure 4.10, “Guided Partition Allocation” [page 78].
Chapter 4 — Installing Kali Linux 77
Figure 4.10 Guided Partition Allocation
The first method is called “All files in one partition.” The entire Linux system tree is stored in a
single file system, corresponding to the root (”/”) directory. This simple and robust partitioning
scheme works perfectly well for personal or single-user systems. Despite the name, two partitions
will actually be created: the first will house the complete system, the second the virtual memory
(or “swap”).
The second method, “Separate /home/ partition,” is similar, but splits the file hierarchy in two:
one partition contains the Linux system (/), and the second contains “home directories” (meaning
user data, in files and subdirectories available under /home/). One benefit to this method is that
it is easy to preserve the users’ data if you have to reinstall the system.
The last partitioning method, called “Separate /home, /var, and /tmp partitions,” is appropriate
for servers and multi-user systems. It divides the file tree into many partitions: in addition to
the root (/) and user accounts (/home/) partitions, it also has partitions for server software data
(/var/), and temporary files (/tmp/). One benefit to this method is that end users cannot lock up
the server by consuming all available hard drive space (they can only fill up /tmp/ and /home/).
At the same time, service data (especially logs) can no longer clog up the rest of the system.
78 Kali Linux Revealed
After choosing the type of partition, the installer presents a summary of your selections on the
screen as a partition map (Figure 4.11, “Validating Partitioning” [page 79]). You can modify each
partition individually by selecting a partition. For example, you could choose another file system
if the standard (ext4) isn’t appropriate. In most cases, however, the proposed partitioning is rea-
sonable and you can accept it by selecting “Finish partitioning and write changes to disk.” It may
go without saying, but choose wisely as this will erase the contents of the selected disk.
Figure 4.11 Validating Partitioning
Manual Partitioning Selecting Manual at the main “Partition disks” screen (Figure 4.8, “Choice
of Partitioning Mode” [page 76]) permits greater flexibility, allowing you to choose more advanced
configurations and specifically dictate the purpose and size of each partition. For example, this
mode allows you to install Kali alongside other operating systems, enable a software-based redun-
dant array of independent disks (RAID) to protect data from hard disk failures, and safely resize
existing partitions without losing data, among other things.
If you are a less experienced user working on a system with existing data, please be very careful
with this setup method as it is very easy to make mistakes that could lead to data loss.
Chapter 4 — Installing Kali Linux 79
Shrinking a Windows To install Kali Linux alongside an existing operating system (Microsoft Windows or
Partition other), you will need available, unused hard drive space for the partitions dedicated to
Kali. In most cases, this means shrinking an existing partition and reusing the freed
space.
If you are using the manual partitioning mode, the installer can shrink a Microsoft
Windows partition quite easily. You only need to choose the Microsoft Windows par-
tition and enter its new size (this works the same with both FAT and NTFS partitions).
The first screen in the manual installer is actually the same as the one shown in Figure 4.11, “Vali-
dating Partitioning” [page 79], except that it doesn’t include any new partitions to create. It is up
to you to add those.
First, you will see an option to enter “Guided partitioning” followed by several configuration op-
tions. Next, the installer will show the available disks, their partitions, and any possible free space
that has not yet been partitioned. You can select each displayed element and press the Enter key
to interact with it, as usual.
If the disk is entirely new, you might have to create a partition table. You can do this by selecting
the disk. Once done, you should see free space available within the disk.
To make use of this free space, you should select it and the installer will offer you two ways to
create partitions in that space.
80 Kali Linux Revealed
Figure 4.12 Creating Partitions in the Free Space
The first entry will create a single partition with the characteristics (including the size) of your
choice. The second entry will use all the free space and will create multiple partitions in it with
the help of the guided partitioning wizard (see section 4.2.1.12.1, “Guided Partitioning” [page 75]).
This option is particularly interesting when you want to install Kali alongside another operating
system but when you don’t want to micro-manage the partition layout. The last entry will show
the cylinder/head/sector numbers of the start and of the end of the free space.
When you select to “Create a new partition,” you will enter into the meat of the manual parti-
tioning sequence. After selecting this option, you will be prompted for a partition size. If the disk
uses an MSDOS partition table, you will be given the option to create a primary or logical partition.
(Things to know: You can only have four primary partitions but many more logical partitions. The
partition containing /boot, and thus the kernel, must be a primary one, logical partitions reside
in an extended partition, which consumes one of the four primary partitions.) Then you should
see the generic partition configuration screen:
Chapter 4 — Installing Kali Linux 81
Figure 4.13 Partition Configuration Screen
To summarize this step of manual partitioning, let’s take a look at what you can do with the new
partition. You can:
• Format it and include it in the file tree by choosing a mount point. The mount point is the
directory that will house the contents of the file system on the selected partition. Thus,
a partition mounted at /home/ is traditionally intended to contain user data, while “/” is
known as the root of the file tree, and therefore the root of the partition that will actually
host the Kali system.
• Use it as a swap partition. When the Linux kernel lacks sufficient free memory, it will store in-
active parts of RAM in a special swap partition on the hard disk. The virtual memory subsys-
tem makes this transparent to applications. To simulate the additional memory, Microsoft
Windows uses a swap (paging) file that is directly contained in a file system. Conversely,
Linux uses a partition dedicated to this purpose, hence the term swap partition.
• Make it into a “physical volume for encryption” to protect the confidentiality of data on
certain partitions. This case is automated in the guided partitioning. See section 4.2.2, “In-
stallation on a Fully Encrypted File System” [page 88] for more information.
82 Kali Linux Revealed
• Make it a “physical volume for LVM” (not covered in this book). Note that this feature is
used by the guided partitioning when you set up encrypted partitions.
• Use it as a RAID device (not covered in this book).
• Choose not to use the partition, and leave it unchanged.
When finished, you can either back out of manual partitioning by selecting “Undo changes to
partitions” or write your changes to the disk by selecting “Finish partitioning and write changes
to disk” from the manual installer screen (Figure 4.11, “Validating Partitioning” [page 79]).
Copying the Live Image
This next step, which doesn’t require any user interaction, copies the contents of the live image
to the target file system, as shown in Figure 4.14, “Copying the Data from the Live Image” [page
83].
Figure 4.14 Copying the Data from the Live Image
Chapter 4 — Installing Kali Linux 83
Configuring the Package Manager (apt)
If you want to use a local mirror instead of http.kali.org, you can pass its name on the kernel
command line (at boot-time) with a syntax like this: mirror/http/hostname=my.own.mirror .
The installer program proposes to use an HTTP proxy as shown in Figure 4.15, “Use an HTTP Proxy”
[page 84]. An HTTP proxy is a server that forwards HTTP requests for network users. It sometimes
helps to speed up downloads by keeping a copy of files that have been transferred through it (we
then speak of a caching proxy). In some cases, it is the only means of accessing an external web
server; in such cases the installer will only be able to download the Debian packages if you properly
fill in this field during installation. If you do not provide a proxy address, the installer will attempt
to connect directly to the Internet.
Figure 4.15 Use an HTTP Proxy
Next, the Packages.xz and Sources.xz files will be automatically downloaded to update the list
of packages recognized by APT.
84 Kali Linux Revealed
Installing Metapackages
If you are installing the system from either the installer or netinstaller images, you will now be
given the option to pick which packages you would like to install. Keep in mind that this will
require Internet access if you are using the netinstaller image. You are given the choice of Desktop
Environment and which tool selection you would like to be included. You can also change these
selections at any time after Kali Linux has been installed, even if you use the live image to install
Kali.
Figure 4.16 Installing Metapackages
Installing the GRUB Boot Loader
The boot loader is the first program started by the BIOS. This program loads the Linux kernel into
memory and then executes it. The boot loader often offers a menu that allows you to choose the
kernel to load or the operating system to boot.
Chapter 4 — Installing Kali Linux 85
Due to its technical superiority, GRUB is the default boot loader installed by Kali: it works with
most file systems and therefore doesn’t require an update after each installation of a new kernel,
since it reads its configuration during boot and finds the exact position of the new kernel.
You should install GRUB to the Master Boot Record (MBR) unless you already have another Linux
system installed that knows how to boot Kali Linux. As noted in Figure 4.17, “Install the GRUB Boot
Loader on a Hard Disk” [page 86], modifying the MBR will make unrecognized operating systems
that depend on it unbootable until you fix GRUB’s configuration.
Figure 4.17 Install the GRUB Boot Loader on a Hard Disk
In this step (Figure 4.18, “Device for Boot Loader Installation” [page 87]), you must select which
device GRUB will be installed on. This should be your current boot drive.
86 Kali Linux Revealed
Figure 4.18 Device for Boot Loader Installation
By default, the boot menu proposed by GRUB shows all the installed Linux kernels, as well as any
other operating systems that were detected. This is why you should accept the offer to install it
in the Master Boot Record. Keeping older kernel versions preserves the ability to boot the system
if the most recently installed kernel is defective or poorly adapted to the hardware. We thus
recommend that you keep a few older kernel versions installed.
Beware: The Boot Loader This phase in the installation process detects the operating systems that are already
and Dual Boot installed on the computer and will automatically add corresponding entries in the
boot menu. However, not all installation programs do this.
In particular, if you install (or reinstall) Microsoft Windows thereafter, the boot loader
will be erased. Kali will still be on the hard drive, but will no longer be accessi-
ble from the boot menu. You would then have to start the Kali installer with the
rescue/enable=true parameter on the kernel command line to reinstall the boot
loader. This operation is described in detail in the Debian installation manual.
è https://www.debian.org/releases/stable/amd64/ch08s06.en.html
Chapter 4 — Installing Kali Linux 87
Finishing the Installation and Rebooting
Now that installation is complete, the program asks you to unplug your USB drive or remove the
DVD-ROM from the reader so that your computer can boot into your new Kali system after the
installer restarts the system (Figure 4.19, “Installation Complete” [page 88]).
Finally, the installer will do some cleanup work, like removing packages that are specific to creat-
ing the live environment. It will also detect if Kali Linux has been installed in a Virtual Machine
(VM) and automatically install any guest tools to help integrate between the host and Kali guest.
Figure 4.19 Installation Complete
4.2.2. Installation on a Fully Encrypted File System
To guarantee the confidentiality of your data, you can set up encrypted partitions. This will pro-
tect your data if your laptop or hard drive is lost or stolen. The partitioning tool can help you in
this process, both in guided and manual mode.
88 Kali Linux Revealed
The guided partitioning mode will combine the use of two technologies: Linux Unified Key Setup
(LUKS) for encrypting partitions and Logical Volume Management (LVM) for managing storage
dynamically. Both features can also be set up and configured through manual partitioning mode.
Introduction to LVM
Let’s discuss LVM first. Using LVM terminology, a virtual partition is a logical volume, which is
part of a volume group, or an association of several physical volumes. Physical volumes are real
partitions (or virtual partitions exported by other abstractions, such as a software RAID device or
an encrypted partition).
With its lack of distinction between “physical” and “logical” partitions, LVM allows you to create
“virtual” partitions that span several disks. The benefits are twofold: the size of the partitions is
no longer limited by individual disks but by their cumulative volume, and you can resize existing
partitions at any time, such as after adding an additional disk.
This technique works in a very simple way: each volume, whether physical or logical, is split into
blocks of the same size, which LVM correlates. The addition of a new disk will cause the creation
of a new physical volume providing new blocks that can be associated to any volume group. All of
the partitions in the volume group can then take full advantage of the additional allocated space.
Introduction to LUKS
To protect your data, you can add an encryption layer underneath your file system of choice. Linux
(and more particularly the dm-crypt driver) uses the device mapper to create the virtual partition
(whose contents are protected) based on an underlying partition that will store the data in an
encrypted form (thanks to LUKS). LUKS standardizes the storage of the encrypted data as well as
meta-information that indicates the encryption algorithms used.
We will be setting up LUKS as part for our ”nuke” feature later in “Adding a Nuke Password for
Extra Safety” [page 250].
Encrypted Swap Partition When an encrypted partition is used, the encryption key is stored in memory (RAM),
and when hibernating, a laptop will copy the key, along with other contents of RAM,
to the hard disk’s swap partition. Since anyone with access to the swap file (including
a technician or a thief) could extract the key and decrypt your data, the swap file must
be protected with encryption.
Because of this, the installer will warn you if you try to use an encrypted partition
alongside an unencrypted swap partition.
Chapter 4 — Installing Kali Linux 89
Setting Up Encrypted Partitions
The installation process for encrypted LVM is the same as a standard installation except for the
partitioning step (Figure 4.20, “Guided Partitioning with Encrypted LVM” [page 90]) where you
will instead select “Guided - use entire disk and set up encrypted LVM.” The net result will be a
system that cannot be booted or accessed until the encryption passphrase is provided. This will
encrypt and protect the data on your disk.
Figure 4.20 Guided Partitioning with Encrypted LVM
The guided partitioning installer will automatically assign a physical partition for the storage of
encrypted data, as shown in Figure 4.21, “Confirm Changes to the Partition Table” [page 91]. At
this point, the installer will confirm the changes before they are written on the disk.
90 Kali Linux Revealed
Figure 4.21 Confirm Changes to the Partition Table
This new partition is then initialized with random data, as shown in Figure 4.22, “Erasing Data on
Encrypted Partition” [page 92]. This makes the areas that contain data indistinguishable from the
unused areas, making it more difficult to detect, and subsequently attack, the encrypted data.
Chapter 4 — Installing Kali Linux 91
Figure 4.22 Erasing Data on Encrypted Partition
Next, the installer asks you to enter an encryption passphrase (Figure 4.23, “Enter Your Encryp-
tion Passphrase” [page 93]). In order to view the contents of the encrypted partition, you will
need to enter this passphrase every time you reboot the system. Note the warning in the installer:
your encrypted system will only be as strong as this passphrase.
92 Kali Linux Revealed
Figure 4.23 Enter Your Encryption Passphrase
The partitioning tool now has access to a new virtual partition whose contents are stored en-
crypted in the underlying physical partition. Since LVM uses this new partition as a physical
volume, it can protect several partitions (or LVM logical volumes) with the same encryption key,
including the swap partition (see sidebar “Encrypted Swap Partition” [page 89]). Here, LVM is not
used to make it easy to extend the storage size, but just for the convenience of the indirection
allowing to split a single encrypted partition into multiple logical volumes.
End of the Guided Partitioning with Encrypted LVM
Next, the resulting partitioning scheme is displayed (Figure 4.24, “Validating Partitioning for En-
crypted LVM Installation” [page 94]) so you can tweak settings as needed.
Chapter 4 — Installing Kali Linux 93
Figure 4.24 Validating Partitioning for Encrypted LVM Installation
Finally, after validating the partition setup, the tool asks for confirmation to write the changes on
the disks, as shown in Figure 4.25, “Confirm Partitions to be Formatted” [page 95].
94 Kali Linux Revealed
Figure 4.25 Confirm Partitions to be Formatted
Finally, the installation process continues as usual as documented in section 4.2.1.14, “Configuring
the Package Manager (apt)” [page 84].
4.3. Unattended Installations
The Debian and Kali installers are very modular: at the basic level, they are just executing many
scripts (packaged in tiny packages called udeb—for µdeb or micro-deb) one after another. Each
script relies on debconf (see “The debconf Tool” [page 220]), which interacts with you, the user,
and stores installation parameters. Because of this, the installer can also be automated through
debconf preseeding, a function that allows you to provide unattended answers to installation ques-
tions.
Chapter 4 — Installing Kali Linux 95
4.3.1. Preseeding Answers
There are multiple ways to preseed answers to the installer. Each method has its own advantages
and disadvantages. Depending on when the preseeding happens, the questions that can be pre-
seeded vary.
With Boot Parameters
You can preseed any installer question with boot parameters that end up in the kernel command-
line, accessible through /proc/cmdline. Some bootloaders will let you edit these parameters
interactively (which is practical for testing purposes), but if you want to make the changes persis-
tent, you will have to modify the bootloader configuration.
You can directly use the full identifier of the debconf questions (such as debian-
installer/language=en) or you can use abbreviations for the most common questions (like
language=en or hostname=kali). See the full list1 of aliases in the Debian installation manual.
There is no restriction on which questions you can preseed since boot parameters are available
from the start of the installation process and they are processed very early. However, the number
of boot parameters is limited to 32 and a number of those are already used by default. It is also
important to realize that changing the boot loader configuration can be non-trivial at times.
In section 9.3, “Building Custom Kali Live ISO Images” [page 241] you will also learn how to modify
the isolinux configuration when you generate your own Kali ISO image.
With a Preseed File in the Initrd
You can add a file named preseed.cfg at the root of the installer’s initrd (this is the initrd which
is used to start the installer). Usually, this requires rebuilding the debian-installer source package
to generate new versions of the initrd. However, live-build offers a convenient way to do this,
which is detailed in section 9.3, “Building Custom Kali Live ISO Images” [page 241].
This method also does not have any restrictions on the questions that you can preseed as the
preseed file is available immediately after boot. In Kali, we already make use of this feature to
customize the behavior of the official Debian installer.
With a Preseed File in the Boot Media
You can add a preseed file on the boot media (DVD or USB key); preseeding then happens as soon as
the media is mounted, which means right after the questions about language and keyboard layout.
The preseed/file boot parameter can be used to indicate the location of the preseeding file (for
1
https://www.debian.org/releases/stable/amd64/apbs02#preseed-aliases
96 Kali Linux Revealed
instance, /cdrom/preseed.cfg when installing from a DVD-ROM, or /hd-media/preseed.cfg
when installing from a USB key).
You may not preseed answers to language and country options as the preseeding file is loaded later
in the process, once the hardware drivers have been loaded. On the positive side, live-build
makes it easy to put a supplementary file in the generated ISO images (see section 9.3, “Building
Custom Kali Live ISO Images” [page 241]).
With a Preseed File Loaded from the Network
You can make a preseed file available on the network through a web server and tell the installer
to download that preseed file by adding the boot parameter preseed/url=http://server /preseed.cfg
(or by using the url alias).
However, when using this method, remember that the network must first be configured. This
means that network-related debconf questions (in particular hostname and domain name) and
all the preceding questions (like language and country) cannot be preseeded with this method.
This method is most often used in combination with boot parameters preseeding those specific
questions.
This preseeding method is the most flexible one as you can change the installation configuration
without changing the installation media.
Delaying the Language, To overcome the limitation of not being able to preseed the language, country, and
Country, Keyboard keyboard questions, you can add the boot parameter auto-install/enable=true
Questions (or auto=true). With this option the questions will be asked later in the process,
after the network has been configured and thus after download of the preseed file.
The downside is that the first steps (notably network configuration) will always hap-
pen in English and if there are errors the user will have to work through English
screens (with a keyboard configured in QWERTY).
4.3.2. Creating a Preseed File
A preseed file is a plain text file in which each line contains the answer to one Debconf question.
A line is split across four fields separated by white space (spaces or tabs). For instance, d-i mirror/-
suite string kali-rolling:
• The first field indicates the owner of the question. For example, “d-i” is used for questions
relevant to the installer. You may also see a package name, for questions coming from De-
bian packages (as in this example: atftpd atftpd/use_inetd boolean false).
• The second field is an identifier for the question.
• The third field lists the type of question.
Chapter 4 — Installing Kali Linux 97
• The fourth and final field contains the value for the expected answer. Note that it must be
separated from the third field with a single space; additional space characters are consid-
ered part of the value.
The simplest way to write a preseed file is to install a system by hand. Then the
debconf-get-selections --installer command will provide the answers you provided to the
installer. You can obtain answers directed to other packages with debconf-get-selections.
However, a cleaner solution is to write the preseed file by hand, starting from an example and
then going through the documentation. With this approach, only questions where the default
answer needs to be overridden can be preseeded. Provide the priority=critical boot parameter to
instruct Debconf to only ask critical questions, and to use the default answer for others.
Installation Guide The Debian installation guide, available online, includes detailed documentation on
Appendix the use of a preseed file in an appendix. It also includes a detailed and commented
sample file, which can serve as a base for local customizations.
è https://www.debian.org/releases/stable/amd64/apb
è https://www.debian.org/releases/stable/example-preseed.txt
Note however, that the above links document the stable version of Debian and that
Kali uses the testing version so you may encounter slight differences. You can also
consult the installation manual hosted on the Debian-installer project’s website. It
may be more up-to-date.
4.4. ARM Installations
Kali Linux runs on a wide variety of ARM-based devices (laptops, embedded computers, and devel-
oper boards, for example) but you cannot use the traditional Kali installer on these devices since
they often have specific requirements in terms of kernel or boot loader configuration.
To make those devices more accessible to Kali users, Offensive Security developed scripts to build
disk images2 that are ready for use with various ARM devices. They provide these images pre-
generated, ready to download on their website:
è https://www.offensive-security.com/kali-linux-arm-images/
Since these images are available, your task of installing Kali on an ARM device is greatly simplified.
Here are the basic steps:
1. Download the image for your ARM device and ensure that the checksum matches the one
provided on the website (see section 2.1.3, “Verifying Integrity and Authenticity” [page 17]
for explanations on how to do that). Note that the images are usually xz-compressed; make
sure to uncompress them with unxz.
2
https://gitlab.com/kalilinux/build-scripts/kali-arm
98 Kali Linux Revealed
2. Depending on the storage expansion slot available on your specific ARM device, acquire
an SD card, micro SD card, or embedded multi-media controller (eMMC) module that has a
capacity of at least 8 GB.
3. Copy the downloaded image to the storage device with dd. This is similar to the process of
copying an ISO image onto a USB key (see section 2.1.4, “Copying the Image on a DVD-ROM
or USB Key” [page 19]).
# dd if=kali-image.img of=/dev/something bs=512k
4. Plug the SD-card/eMMC into your ARM device.
5. Boot your ARM device and log into it (user ”kali”, password ”kali”). If you don’t have a screen
connected, then you will have to discover the IP address that has been assigned via DHCP
and connect to that address over SSH. Some DHCP servers have tools or web interfaces to
show the current leases. If you don’t have this as an option, use a sniffer to look for DHCP
lease traffic, or either a ICMP/ARP scan of the local network.
6. Change the default user password and generate new SSH host keys, especially if the device
will be permanently running on a public network! These steps are relatively straightfor-
ward, see “Generating New SSH Host Keys ” [page 115].
7. Enjoy your new ARM device running Kali Linux!
Special Cases and More These instructions are generic and while they work for most devices, there are always
Detailed Documentation exceptions. For example, Chromebooks require developer mode and other devices
require a special keypress in order to boot from external media.
Since ARM devices are added relatively frequently and their specifications are so dy-
namic, we won’t cover specific installation instructions for various ARM devices here.
Instead, refer to the dedicated ”Kali on ARM” section of the Kali documentation web-
site for information about each ARM device supported by Offensive Security:
è https://www.kali.org/docs/arm/
4.5. Troubleshooting Installations
The installer is quite reliable, but you may encounter bugs or face external problems such as: net-
work problems, bad mirrors, and insufficient disk space. Because of this, it is quite useful to be
able to troubleshoot problems that appear in the installation process.
When the installer fails, it will show you a rather unhelpful screen such as the one shown in Fig-
ure 4.26, “Installation Step Failed” [page 100].
Chapter 4 — Installing Kali Linux 99
Figure 4.26 Installation Step Failed
At this point, it is good to know that the installer makes use of multiple virtual consoles: the main
screen that you see is running either on the fifth console (for the graphical installer, CTRL+ALT+F5)
or on the first console (for the classic text-mode installer, CTRL+ALT+F1). In both cases, the fourth
console ( CTRL+ALT+F4 ) displays logs of what is happening and you can usually see a more useful
error message there, such as the one in Figure 4.27, “The Log Screen of the Installer” [page 101],
which reveals that the installer has run out of disk space.
100 Kali Linux Revealed
Figure 4.27 The Log Screen of the Installer
The second and third consoles ( CTRL+ALT+F2 and CTRL+ALT+F3, respectively) host shells that you
can use to investigate the current situation in more detail. Most of the command line tools are
provided by BusyBox so the feature set is rather limited, but it is enough to figure out most of the
problems that you are likely to encounter.
What Can be Done in the You can inspect and modify the debconf database with debconf-get and
Installer Shell debconf-set. These commands are especially convenient for testing preseeding val-
ues.
You can inspect any file (such as the full installation log available in /var/log/
syslog) with cat or less. You can edit any file with nano, including all files be-
ing installed onto the system. The root file system will be mounted on /target once
the partitioning step of the installation process has completed.
Once network access has been configured, you can use wget and nc (netcat) to retrieve
and export data over the network.
Once you click Continue from the main installer failure screen ( Figure 4.26, “Installation Step
Failed” [page 100]), you will be returned to a screen that you will normally never see (the Main
Chapter 4 — Installing Kali Linux 101
Menu shown in Figure 4.28, “Main Menu of the Installer” [page 102]), which allows you to launch
one installation step after another. If you managed to fix the problem through the shell access
(congratulations!) then you can retry the step that failed.
Figure 4.28 Main Menu of the Installer
If you are unable to resolve the problem, you might want to file a bug report. The report must
then include the installer logs, which you can retrieve with the main menu’s “Save debug logs”
function. It offers multiple ways to export the logs, as shown in Figure 4.29, “Save Debug Logs
(1/2)” [page 103].
102 Kali Linux Revealed
Figure 4.29 Save Debug Logs (1/2)
The most convenient method, and the one that we recommend, is to let the installer start a web
server hosting the log files (Figure 4.30, “Save Debug Logs (2/2)” [page 104]). You can then launch
a browser from another computer on the same network and download all the log files and screen-
shots that you have taken with the Screenshot button available on each screen.
Chapter 4 — Installing Kali Linux 103
Figure 4.30 Save Debug Logs (2/2)
4.6. Summary
In this chapter, we focused on the Kali Linux installation process. We discussed Kali Linux’s mini-
mum installation requirements, the installation process for standard and fully encrypted file sys-
tems, preseeding, which allows unattended installations, how to install Kali Linux on various ARM
devices, and what to do in the rare case of an installation failure.
Summary Tips:
• The installation requirements for Kali Linux vary from a basic SSH server with no desktop,
as little as 128 MB RAM (512 MB recommended) and 2 GB disk space, to the higher-end kali-
linux-default metapackage, with at least 2048 MB of RAM and 20 GB of disk space. In addition,
your machine must have a CPU supported by at least one of the amd64, i386, or arm64 ar-
chitectures.
• Kali can easily be installed as the primary operating system, alongside other operating sys-
tems through partitioning and boot loader modification, or as a virtual machine.
104 Kali Linux Revealed
• To guarantee the confidentiality of your data, you can set up encrypted partitions. This will
protect your data if your laptop or hard drive is lost or stolen.
• The installer can also be automated through debconf preseeding, a function that allows you
to provide unattended answers to installation questions.
• A preseed file is a plain text file in which each line contains the answer to one Debconf
question. A line is split across four fields separated by white space (spaces or tabs). You can
preseed answers to the installer with boot parameters, with a preseed file in initrd, with a
preseed file on the boot media, or with a preseed file from the network.
• Kali Linux runs on a wide variety of ARM-based devices such as laptops, embedded comput-
ers, and developer boards. ARM installation is fairly straightforward. Download the proper
image, copy it to an SD card, USB drive, or embedded multi-media controller (eMMC) mod-
ule, plug it in, boot the ARM device, find your device on the network, log in, and change the
SSH password and SSH host keys.
• You can debug failed installations with virtual consoles (accessible with the CTRL+ALT and
function keys), debconf-get and debconf-set commands, reading the /var/log/syslog
log file, or by submitting a bug report with log files retrieved with the installer’s “Save debug
logs” function.
Now that we have discussed Linux fundamentals and Kali Linux installation, let’s discuss configu-
ration so you can begin to tailor Kali to suit your needs.
Chapter 4 — Installing Kali Linux 105
Keywords
Network
Users and groups
Services
Apache
PostgreSQL
SSH
Chapter
Configuring Kali Linux 5
Contents
Configuring the Network 108 Managing Unix Users and Unix Groups 112 Configuring Services 114
Managing Services 121 Summary 123
In this chapter, we will take a look at various ways you can configure Kali Linux. First, in sec-
tion 5.1, “Configuring the Network” [page 108], we will show you how to configure your network
settings using a graphical environment and the command line. In section 5.2, “Managing Unix
Users and Unix Groups” [page 112], we will talk about users and groups, showing you how to cre-
ate and modify user accounts, set passwords, disable accounts, and manage groups. Finally, we
will discuss services in section 5.3, “Configuring Services” [page 114] and explain how to set up
and maintain generic services and also focus on three very important and specific services: SSH,
PostgreSQL, and Apache.
5.1. Configuring the Network
5.1.1. On the Desktop with NetworkManager
In a typical desktop installation, you’ll have NetworkManager already installed and it can be con-
trolled and configured through Xfce’s system settings and through the top-right menu as shown
in Figure 5.1, “Network Configuration Screen” [page 108].
Figure 5.1 Network Configuration Screen
The default network configuration relies on DHCP to obtain an IP address, DNS server, and gate-
way, but you can use the gear icon in the lower-left corner to alter the configuration in many ways
(for example: set the MAC address, switch to a static setup, enable or disable IPv6, and add addi-
tional routes). You can create profiles to save multiple wired network configurations and easily
108 Kali Linux Revealed
switch between them. For wireless networks, their settings are automatically tied to their public
identifier (SSID).
NetworkManager also handles connections by mobile broadband (Wireless Wide Area Network
WWAN) and by modems using point-to-point protocol over ethernet (PPPoE). Last but not least,
it provides integration with many types of virtual private networks (VPN) through dedicated plu-
gins: SSH, OpenVPN, Cisco’s VPNC, PPTP, Strongswan. Check out the network-manager-* packages;
most of them are not installed by default.
5.1.2. On the Command Line with Ifupdown
Alternatively, when you prefer not to use (or don’t have access to) a graphical desktop, you can
configure the network with the already-installed ifupdown package, which includes the ifup and
ifdown tools. These tools read definitions from the /etc/network/interfaces configuration file
and are at the heart of the /etc/init.d/networking init script that configures the network at
boot time.
Using sudo to access The sudo (super user do) command allows privileged users to run commands with
Administrative Privileges administrative permissions. This gives full access to items that may be restricted to
only the root user, such as programs in /sbin/ or access to network options that are
useful for common penetration testing tools.
The command takes one argument, being the subsequent command that will be run
with administrative permissions. One useful use case when dealing with services is to
elevate to the root user account. To elevate to root user, we will use the command su
(substitute user) to create a shell under the root user. The substitute user command
takes a user account as an argument. Additionally, the su command has a useful flag (
--login, or -l, or -) to use the substituted user’s login environment. Multiple commands
in the following chapter require the use of sudo, as such we will elevate to the root
user to seamlessly execute these commands.
$ sudo su --login
[sudo] password for kali:
root@kali:~#
Each network device managed by ifupdown can be deconfigured at any time with ifdown
network-device. You can then modify /etc/network/interfaces and bring the network back
up (with the new configuration) with ifup network-device.
Let’s take a look at what we can put in ifupdown’s configuration file. There are two main directives:
auto network-device, which tells ifupdown to automatically configure the network interface once it
is available, and iface network-device inet/inet6 type to configure a given interface. For example,
a plain DHCP configuration looks like this:
auto lo
Chapter 5 — Configuring Kali Linux 109
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
Note that the special configuration for the loopback device should always be present in this file.
For a fixed IP address configuration, you have to provide more details such as the IP address, the
network, and the IP of the gateway:
auto eth0
iface eth0 inet static
address 192.168.0.3
netmask 255.255.255.0
broadcast 192.168.0.255
network 192.168.0.0
gateway 192.168.0.1
For wireless interfaces, you must have the wpasupplicant package (included in Kali by default),
which provides many wpa-* options that can be used in /etc/network/interfaces. Have a look
at /usr/share/doc/wpasupplicant/README.Debian.gz for examples and explanations. The
most common options are wpa-ssid (which defines the name of the wireless network to join) and
wpa-psk (which defines the passphrase or the key protecting the network).
iface wlan0 inet dhcp
wpa-ssid MyNetWork
wpa-psk plaintextsecret
5.1.3. On the Command Line with systemd-networkd
While ifupdown is the historical tool used by Debian and Kali, and while it is still the default for
minimal installations, there is a newer tool worth considering: systemd-networkd. Its integration
with the systemd init system makes it a very attractive choice. It is not specific to Debian-based
distributions (contrary to ifupdown) and has been designed to be very small, efficient, and rela-
tively easy to configure if you understand the syntax of systemd unit files. This is an especially
attractive choice if you consider NetworkManager bloated and hard to configure.
You configure systemd-networkd by placing .network files into the /etc/systemd/network/
directory. Alternatively, you can use /lib/systemd/network/ for packaged files or /run/
systemd/network/ for files generated at run-time. The format of those files is documented in
systemd.network(5) (see section 6.1.1, “Manual Pages” [page 128]). The [Match] section indi-
cates the network interfaces the configuration applies to. You can specify the interface in many
ways, including by media access control (MAC) address or device type. The [Network] section
defines the network configuration.
110 Kali Linux Revealed
Example 5.1 Static Configuration in /etc/systemd/network/50-static.network
[Match]
Name=enp2s0
[Network]
Address=192.168.0.15/24
Gateway=192.168.0.1
DNS=8.8.8.8
Example 5.2 DHCP-based Configuration in /etc/systemd/network/80-dhcp.network
[Match]
Name=en*
[Network]
DHCP=yes
Note that system-networkd is disabled by default, so if you want to use it, you should enable it.
It also depends on systemd-resolved for proper integration of DNS resolution, which in turn
requires you to replace /etc/resolv.conf with a symlink to /run/systemd/resolve/resolv.
conf, which is managed by systemd-resolved.
# systemctl enable systemd-networkd
# systemctl enable systemd-resolved
# systemctl start systemd-networkd
# systemctl start systemd-resolved
# ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf
Although systemd-networkd suffers from some limitations, like the lack of integrated support
for wireless networks, you can rely on a pre-existing external wpa_supplicant configuration for
wireless support. However, it is particularly useful in containers and virtual machines, and was
originally developed for environments in which a container’s network configuration depended on
its host’s network configuration. In this scenario, systemd-networkd makes it easier to manage
both sides in a consistent manner while still supporting all sorts of virtual network devices that
you might need in this type of scenario (see systemd.netdev(5) in section 6.1.1, “Manual Pages”
[page 128]).
Chapter 5 — Configuring Kali Linux 111
5.2. Managing Unix Users and Unix Groups
The database of Unix users and groups consists of the textual files /etc/passwd (list of users),
/etc/shadow (encrypted passwords of users), /etc/group (list of groups), and /etc/gshadow
(encrypted passwords of groups). Their formats are documented in passwd(5), shadow(5),
group(5), and gshadow(5) respectively (see section 6.1.1, “Manual Pages” [page 128]). While
these files can be manually edited with tools like vipw and vigr, there are higher level tools to
perform the most common operations.
Using getent to Consult The getent (get entries) command checks the system databases (including those of
the User Database users and groups) using the appropriate library functions, which in turn call the name
service switch (NSS) modules configured in the /etc/nsswitch.conf file. The com-
mand takes one or two arguments: the name of the database to check, and a possible
search key. Thus, the command getent passwd kaliuser1 will return the informa-
tion from the user database regarding the user kaliuser1.
# getent passwd kaliuser1
kaliuser1:x:1001:1001:Kali User
å ,4444,123-867-5309,321-867-5309:/home/kaliuser1:/bin/
å bash
5.2.1. Creating User Accounts
Although Kali is most often run while privileged with sudo permissions, you may often need to
create non-privileged user accounts for various reasons, particularly if you are using Kali as a
primary operating system. The most typical way to add a user is with the adduser command,
which takes a required argument: the username for the new user that you would like to create.
The adduser command asks a few questions before creating the account but its usage is fairly
straightforward. Its configuration file, /etc/adduser.conf, includes many interesting settings.
You can, for example, define the range of user identifiers (UIDs) that can be used, dictate whether
or not users share a common group or not, define default shells, and more.
The creation of an account triggers the population of the user’s home directory with the contents
of the /etc/skel/ template. This provides the user with a set of standard directories and config-
uration files.
In some cases, it will be useful to add a user to a group (other than their default main group) in
order to grant additional permissions. For example, a user who is included in the docker group
has full access to docker commands and services. This can be achieved with a command such as
adduser user group.
112 Kali Linux Revealed
5.2.2. Modifying an Existing Account or Password
The following commands allow modification of the information stored in specific fields of the user
databases:
• passwd—permits a regular user to change their password, which in turn, updates the /etc/
shadow file.
• chfn—(CHange Full Name), reserved for the super-user (root), modifies the GECOS, or ”gen-
eral information” field.
• chsh—(CHange SHell) changes the user’s login shell. However, available choices will be lim-
ited to those listed in /etc/shells; the administrator, on the other hand, is not bound by
this restriction and can set the shell to any program chosen.
• chage—(CHange AGE) allows the administrator to change the password expiration settings
by passing the user name as an argument or list current settings using the -l user option.
Alternatively, you can also force the expiration of a password using the passwd -e user
command, which forces the user to change their password the next time they log in.
5.2.3. Disabling an Account
You may find yourself needing to disable an account (lock out a user) as a disciplinary measure, for
the purposes of an investigation, or simply in the event of a prolonged or definitive absence of a
user. A disabled account means the user cannot login or gain access to the machine. The account
remains intact on the machine and no files or data are deleted; it is simply inaccessible. This is
accomplished by using the command passwd -l user (lock). Re-enabling the account is done in
similar fashion, with the -u option (unlock).
5.2.4. Managing Unix Groups
The addgroup and delgroup commands add or delete a group, respectively. The groupmod com-
mand modifies a group’s information (its gid or identifier). The command gpasswdgroup changes
the password for the group, while the gpasswd -r group command deletes it.
Working with Several Each user may be a member of many groups. A user’s main group is, by default,
Groups created during initial user configuration. By default, each file that a user creates
belongs to the user, as well as to the user’s main group. This is not always desirable;
for example, when the user needs to work in a directory shared by a group other
than their main group. In this case, the user needs to change groups using one of the
following commands: newgrp, which starts a new shell, or sg, which simply executes a
command using the supplied alternate group. These commands also allow the user to
join a group to which they do not currently belong. If the group is password protected,
they will need to supply the appropriate password before the command is executed.
Chapter 5 — Configuring Kali Linux 113
Alternatively, the user can set the setgid bit on the directory, which causes files cre-
ated in that directory to automatically belong to the correct group. For more details,
see sidebar “setgid directory and sticky bit ” [page 58].
The id command displays the current state of a user, with their personal identifier
(uid variable), current main group (gid variable), and the list of groups to which they
belong (groups variable).
5.3. Configuring Services
In this section we will take a look at services (sometimes called daemons), or programs that run as
a background process and perform various functions for the system. We will start by discussing
configuration files and will proceed to explain how some important services (such as SSH, Post-
greSQL, and Apache) function and how they can be configured.
Kali Linux’s policy is to have any network services disabled by default, which is a different behavior
to other Linux operating systems. For more information see section 1.5, “Kali Linux Policies” [page
10].
5.3.1. Configuring a Specific Program
When you want to configure an unknown package, you must proceed in stages. First, you should
read what the package maintainer has documented. The /usr/share/doc/package/README.
Debian file is a good place to start. This file will often contain information about the package,
including pointers that may refer you to other documentation. You will often save yourself a
lot of time, and avoid a lot of frustration, by reading this file first since it often details the most
common errors and solutions to most common problems.
Next, you should look at the software’s official documentation. Refer to section 6.1, “Documen-
tation Sources” [page 128] for tips on how to find various documentation sources. The dpkg -L
package command gives a list of files included in the package; you can therefore quickly identify
the available documentation (as well as the configuration files, located in /etc/). Also, dpkg -s
package displays the package meta-data and shows any possible recommended or suggested pack-
ages; in there, you can find documentation or perhaps a utility that will ease the configuration of
the software.
Finally, the configuration files are often self-documented by many explanatory comments detail-
ing the various possible values for each configuration setting. In some cases, you can get software
up and running by uncommenting a single line in the configuration file. In other cases, examples
of configuration files are provided in the /usr/share/doc/package/examples/ directory. They
may serve as a basis for your own configuration file.
114 Kali Linux Revealed
5.3.2. Configuring SSH for Remote Logins
SSH allows you to remotely log into a machine, transfer files, or execute commands. It is an indus-
try standard tool (ssh) and service (sshd) for connecting to machines remotely.
While the openssh-server package is installed by default, the SSH service is disabled by default and
thus is not started at boot time. You can manually start the SSH service with systemctl start
ssh or configure it to start at boot time with systemctl enable ssh.
The SSH service has a relatively sane default configuration, but given its powerful capabilities and
sensitive nature, it is good to know what you can do with its configuration file, /etc/ssh/sshd_
config. All the options are documented in sshd_config(5) (see section 6.1.1, “Manual Pages”
[page 128])
The default configuration allows password-based logins. If this is not wanted, you can disable this
by setting PasswordAuthentication to no. Doing so will mean that and SSH key will need to be
generated. The SSH service listens by default on port 22 but you can change this with the Port
directive.
To apply the new settings, you should run systemctl reload ssh.
Generating New SSH Host Each SSH server has its own cryptographic keys; they are named “SSH host keys”
Keys and are stored in /etc/ssh/ssh_host_*. They must be kept private if you want
confidentiality and they should not be shared by multiple machines.
When you install your system by copying a full disk image (instead of using debian-
installer such as ARM images), the image might contain pre-generated SSH host keys
that you should thus replace with newly-generated keys. The image probably also
comes with a default user password that you want to reset at the same time. You can
do all this with the following commands:
# passwd
[...]
# rm /etc/ssh/ssh_host_*
# dpkg-reconfigure openssh-server
# systemct restart ssh
5.3.3. Configuring PostgreSQL Databases
PostgreSQL is a database server. It is rarely useful on its own but is used by many other services to
store data. Those services will generally access the database server over the network and usually
require authentication credentials to be able to connect. Setting up those services thus requires
creating PostgreSQL databases and user accounts with appropriate privileges on the database. To
Chapter 5 — Configuring Kali Linux 115
be able to do that, we need the service to be running, so let’s start it with systemctl start
postgresql.
Multiple PostgreSQL The PostgreSQL packaging allows for multiple versions of the database server to be
versions supported co-installed. It is also possible to handle multiple clusters (a cluster is a collection of
databases served by the same postmaster). To achieve this, the configuration files
are stored in /etc/postgresql/version/cluster-name/.
In order for clusters to run side-by-side, each new cluster gets assigned the next avail-
able port number (usually 5433 for the second cluster). The postgresql.service file
is an empty shell, making it easy to act on all clusters together as each cluster has its
own unit (postgresql@version- cluster.service).
Connection Type and Client Authentication
By default, PostgreSQL listens for incoming connections in two ways: on TCP port 5432 of the local-
host interface and on file-based socket /var/run/postgresql/.s.PGSQL.5432. This can be con-
figured in postgresql.conf with various directives: listen_addresses for the addresses to listen
to, port for the TCP port, and unix_socket_directories to define the directory where the file-based
sockets are created.
Depending on how they connect, clients are authenticated in different ways. The pg_hba.conf
configuration file defines who is allowed to connect on each socket and how they are authenti-
cated. By default, connections on the file-based socket use the Unix user account as the name of
the PostgreSQL user, and it assumes that no further authentication is required. On the TCP con-
nection, PostgreSQL requires the user to authenticate with a username and a password (though
not a Unix username/password but rather one managed by PostgreSQL itself).
The postgres user is special and has full administrative privileges over all databases. We will use
this identity to create new users and new databases.
Creating Users and Databases
The createuser command adds a new user and dropuser removes one. Likewise, the createdb
command adds a new database and dropdb removes one. Each of these commands have their own
manual pages but we will discuss some of the options here. Each command acts on the default
cluster (running on port 5432) but you can pass --port=port to modify users and databases of an
alternate cluster.
These commands must connect to the PostgreSQL server to do their job and they must be authenti-
cated as a user with sufficient privileges to be able to execute the specified operation. The easiest
way to achieve this is to use the postgres Unix account and connect over the file-based socket:
# su - postgres
116 Kali Linux Revealed
postgres@kali:~$ createuser -P king_phisher
Enter password for new role:
Enter it again:
postgres@kali:~$ createdb -T template0 -E UTF-8 -O king_phisher king_phisher
postgres@kali:~$ exit
In the example above, the -P option asks createuser to query for a password once it creates the
new king_phisher user. Looking at the createdb command, the -O defines the user owning the
new database (which will thus have full rights to create tables and grant permissions and so on).
We also want to be able to use Unicode strings, so we add the -E UTF-8 option to set the encoding,
which in turn requires us to use the -T option to pick another database template.
We can now test that we can connect to the database over the socket listening on localhost (-h
localhost) as the king_phisher user (-U king_phisher):
# psql -h localhost -U king_phisher king_phisher
Password for user king_phisher:
psql (9.5.2)
SSL connection (protocol: TLSv1.2, cipher: ECDHE-RSA-AES256-GCM-SHA384, bits: 256,
å compression: off)
Type ”help” for help.
king_phisher=>
As you can see, the connection was successful.
Managing PostgreSQL Clusters
First, it is worth noting that the concept of “PostgreSQL cluster” is a Debian-specific addition1 and
that you will not find any reference to this term in the official PostgreSQL documentation2 . From
the point of view of the PostgreSQL tools, such a cluster is just an instance of a database server
running on a specific port.
That said, Debian’s postgresql-common package provides multiple tools to manage such
clusters: pg_createcluster, pg_dropcluster, pg_ctlcluster, pg_upgradecluster,
pg_renamecluster, and pg_lsclusters. We won’t cover all those tools here, but you can
refer to their respective manual pages for more information.
What you must know is that when a new major version of PostgreSQL gets installed on your system,
it will create a new cluster that will run on the next port (usually 5433) and you will keep using
the old version until you migrate your databases from the old cluster to the new one.
You can retrieve a list of all the clusters and their status with pg_lsclusters. More impor-
tantly, you can automate the migration of your cluster to the latest PostgreSQL version with
1
https://wiki.debian.org/PostgreSql
2
https://www.postgresql.org/docs/
Chapter 5 — Configuring Kali Linux 117
pg_upgradecluster old-version cluster-name. For this to succeed, you might have to first
remove the (empty) cluster created for the new version (with pg_dropcluster new-version
cluster-name). The old cluster is not dropped in the process, but it also won’t be started au-
tomatically. You can drop it once you have checked that the upgraded cluster works fine.
5.3.4. Configuring Apache
A typical Kali Linux installation includes the Apache web server, provided by the apache2 package.
Being a network service, it is disabled by default. You can manually start it with systemctl start
apache2.
With more and more applications being distributed as web applications, it is important to have
some knowledge of Apache in order to host those applications, whether for local usage or for
making them available over the network.
Apache is a modular server and many features are implemented by external modules that the
main program loads during its initialization. The default configuration only enables the most
common modules, but enabling new modules is easily done by running a2enmod module. Use
a2dismod module to disable a module. These programs actually only create (or delete) symbolic
links in /etc/apache2/mods-enabled/, pointing at the actual files (stored in /etc/apache2/
mods-available/).
There are many modules available, but two are worth initial consideration: PHP and SSL (used for
TLS). Web applications written with PHP are executed by the Apache web server with the help of
the dedicated module provided by the libapache-mod-php package, and its installation automati-
cally enables the module.
Apache 2.4 includes the SSL module required for Hypertext Transfer Protocol Secure (HTTPS)
out of the box. It first needs to be enabled with a2enmod ssl, then the required directives
must be added to the configuration files. A configuration example is provided in /etc/apache2/
sites-available/default-ssl.conf. See https://httpd.apache.org/docs/2.4/mod/mod_
ssl.html for more information.
The full list of standard Apache modules can be found online at https://httpd.apache.org/
docs/2.4/mod/index.html.
With its default configuration, the web server listens on port 80 (as configured in /etc/apache2/
ports.conf), and serves pages from the /var/www/html/ directory by default (as configured in
/etc/apache2/sites-enabled/000-default.conf).
Configuring Virtual Hosts
A virtual host is an extra identity for the web server. The same Apache process can serve multiple
websites (say www.kali.org and www.offensive-security.com) because the HTTP requests embed
118 Kali Linux Revealed
both the name of the website requested and the URL localpart (this feature is known as name-based
virtual hosts).
The default configuration for Apache 2 enables name-based virtual hosts. In addition, a default
virtual host is defined in the /etc/apache2/sites-enabled/000-default.conf file; this virtual host will
be used if no host matching the request sent by the client is found.
Important
Requests concerning unknown virtual hosts will always be served by the
first defined virtual host, which is why the package ships a 000-default.
conf configuration file, which is sorted first among all other files that you
might create.
Each extra virtual host is then described by a file stored in /etc/apache2/sites-available/.
The file is usually named after the hostname of the website followed by a .conf suffix (for example:
www.kali.org.conf). You can then enable the new virtual host with a2ensite www.kali.org.
Here is a minimal virtualhost configuration for a website whose files are stored in /srv/www.kali.
org/www/ (defined with the DocumentRoot option):
<VirtualHost *:80>
ServerName www.kali.org
ServerAlias kali.org
DocumentRoot /srv/www.kali.org/www
</VirtualHost>
You might also consider adding CustomLog and ErrorLog directives to configure Apache to output
logs in files dedicated to the virtual host.
Common Directives
This section briefly reviews some of the commonly-used Apache configuration directives.
The main configuration file usually includes several Directory blocks; they allow specifying dif-
ferent behaviors for the server depending on the location of the file being served. Such a block
commonly includes Options and AllowOverride directives:
<Directory /var/www>
Options Includes FollowSymLinks
AllowOverride All
DirectoryIndex index.php index.html index.htm
</Directory>
Chapter 5 — Configuring Kali Linux 119
The DirectoryIndex directive contains a list of files to try when the client request matches a direc-
tory. The first existing file in the list is used and sent as a response.
The Options directive is followed by a list of options to enable. The None value disables all options;
correspondingly, All enables them all except MultiViews. Available options include:
• ExecCGI—indicates that CGI scripts can be executed.
• FollowSymLinks—tells the server that symbolic links can be followed, and that the response
should contain the contents of the target of such links.
• SymLinksIfOwnerMatch—also tells the server to follow symbolic links, but only when the
link and its target have the same owner.
• Includes—enables Server Side Includes (SSI). These are directives embedded in HTML pages
and executed on the fly for each request.
• Indexes—tells the server to list the contents of a directory if the HTTP request sent by the
client points to a directory without an index file (that is, when no files mentioned by the
DirectoryIndex directive exist in this directory).
• MultiViews—enables content negotiation; this can be used by the server to return a web
page matching the preferred language as configured in the browser.
Requiring Authentication In some circumstances, access to part of a website needs to be re-
stricted, so only legitimate users who provide a username and a password are granted access to
the contents.
The .htaccess file contains Apache configuration directives enforced each time a request con-
cerns an element from the directory where the .htaccess file is stored. These directives are
recursive, expanding the scope to all subdirectories.
Most of the directives that can occur in a Directory block are also legal in an .htaccess file. The
AllowOverride directive lists all the options that can be enabled or disabled by way of .htaccess.
A common use of this option is to restrict ExecCGI, so that the administrator chooses which users
are allowed to run programs under the web server’s identity (the www-data user).
Example 5.3 .htaccess File Requiring Authentication
Require valid-user
AuthName ”Private directory”
AuthType Basic
AuthUserFile /etc/apache2/authfiles/htpasswd-private
120 Kali Linux Revealed
Basic Authentication The authentication system used in the above example (Basic) has minimal security
Offers No Security as the password is sent in clear text (it is only encoded as base64, which is a simple
encoding rather than an encryption method). It should also be noted that the docu-
ments protected by this mechanism also go over the network in the clear. If security
is important, the entire HTTP session should be encrypted with Transport Layer Se-
curity (TLS).
The /etc/apache2/authfiles/htpasswd-private file contains a list of users and passwords; it
is commonly manipulated with the htpasswd command. For example, the following command is
used to add a user or change their password:
# htpasswd /etc/apache2/authfiles/htpasswd-private user
New password:
Re-type new password:
Adding password for user user
Restricting Access The Require directive controls access restrictions for a directory (and its
subdirectories, recursively).
It can be used to restrict access based on many criteria; we will stop at describing access restriction
based on the IP address of the client but it can be made much more powerful than that, especially
when several Require directives are combined within a RequireAll block.
For instance, you could restrict access to the local network with the following directive:
Require ip 192.168.0.0/16
5.4. Managing Services
Kali uses systemd as its init system, which is not only responsible for the boot sequence, but also
permanently acts as a full featured service manager, starting and monitoring services.
systemd can be queried and controlled with systemctl. Without any argument, it runs the
systemctl list-units command, which outputs a list of the active units. If you run systemctl
status, the output shows a hierarchical overview of the running services. Comparing both out-
puts, you immediately see that there are multiple kinds of units and that services are only one
among them.
Each service is represented by a service unit, which is described by a service file usually shipped in
/lib/systemd/system/ (or /run/systemd/system/, or /etc/systemd/system/; they are listed
by increasing order of importance, and the last one wins). Each is possibly modified by other
service-name.service.d/*.conf files in the same set of directories. Those unit files are plain
text files whose format is inspired by the well-known “*.ini” files of Microsoft Windows, with key
Chapter 5 — Configuring Kali Linux 121
= value pairs grouped between [section] headers. Here we see a sample service file for /lib/
systemd/system/ssh.service:
[Unit]
Description=OpenBSD Secure Shell server
After=network.target auditd.service
ConditionPathExists=!/etc/ssh/sshd_not_to_be_run
[Service]
EnvironmentFile=-/etc/default/ssh
ExecStart=/usr/sbin/sshd -D $SSHD_OPTS
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartPreventExitStatus=255
Type=notify
[Install]
WantedBy=multi-user.target
Alias=sshd.service
Target units are another part of systemd’s design. They represent a desired state that you want to
attain in terms of activated units (which means a running service in the case of service units). They
exist mainly as a way to group dependencies on other units. When the system starts, it enables the
units required to reach the default.target (which is a symlink to graphical.target, and which
in turn depends on multi-user.target). So all the dependencies of those targets get activated
during boot.
Such dependencies are expressed with the Wants directive on the target unit. But you don’t have
to edit the target unit to add new dependencies, you can also create a symlink pointing to the
dependent unit in the /etc/systemd/system/target-name.target.wants/ directory. And this is
exactly what systemctl enable foo.service does. When you enable a service, you tell systemd
to add a dependency on the targets listed in the WantedBy entry of the [Install] section of the
service unit file. Conversely, systemctl disable foo.service drops the same symlink and thus
the dependency.
The enable and disable commands do not change anything regarding the current status of
the services. They only influence what will happen at next boot. If you want to run the ser-
vice immediately, you should execute systemctl start foo.service. Conversely, you can
stop it with systemctl stop foo.service. You can also inspect the current status of a service
with systemctl status foo.service, which usefully includes the latest lines of the associated
log. After having changed the configuration of a service, you may wish to reload it or restart
it: those operations are done with systemctl reload foo.service and systemctl restart
foo.service respectively.
# systemctl status postgresql
122 Kali Linux Revealed
● postgresql.service - PostgreSQL RDBMS
Loaded: loaded (/lib/systemd/system/postgresql.service; disabled; vendor preset:
å disabled)
Active: inactive (dead)
# ls -al /etc/systemd/system/multi-user.target.wants/postgresql.service
ls: cannot access ’/etc/systemd/system/multi-user.target.wants/postgresql.service’: No
å such file or directory
# systemctl enable postgresql
[...]
# ls -al /etc/systemd/system/multi-user.target.wants/postgresql.service
lrwxrwxrwx 1 root root 38 Jan 26 19:09 /etc/systemd/system/multi-user.target.wants/
å postgresql.service -> /lib/systemd/system/postgresql.service
# systemctl status postgresql
● postgresql.service - PostgreSQL RDBMS
Loaded: loaded (/lib/systemd/system/postgresql.service; enabled; vendor preset:
å disabled)
Active: inactive (dead)
# systemctl start postgresql
# systemctl status postgresql
● postgresql.service - PostgreSQL RDBMS
Loaded: loaded (/lib/systemd/system/postgresql.service; enabled; vendor preset:
å disabled)
Active: active (exited) since Tue 2021-01-26 19:10:31 EST; 5s ago
Process: 1495 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
Main PID: 1495 (code=exited, status=0/SUCCESS)
Jan 26 19:10:31 kali-rolling systemd[1]: Starting PostgreSQL RDBMS...
Jan 26 19:10:31 kali-rolling systemd[1]: Started PostgreSQL RDBMS.
5.5. Summary
In this chapter, we learned how to configure Kali Linux. We configured network settings, talked
about users and groups, and discussed how to create and modify user accounts, set passwords,
disable accounts, and manage groups. Finally, we discussed services and explained how to set up
and maintain generic services, specifically SSH, PostgreSQL, and Apache.
Summary Tips:
• In a typical desktop installation, you will have NetworkManager already installed and it can be
controlled and configured through Xfce’s system settings and through the top-right menu.
• You can configure the network from the command line with the ifup and ifdown tools,
which read their instructions from the /etc/network/interfaces configuration file. An
even newer tool, systemd-networkd works with the systemd init system.
Chapter 5 — Configuring Kali Linux 123
• By default, the database of Unix users and groups consists of the textual files /etc/passwd
(list of users), /etc/shadow (encrypted passwords of users), /etc/group (list of groups), and
/etc/gshadow (encrypted passwords of groups).
• You can use the getent command to consult the user database and other system databases.
• The adduser command asks a few questions before creating the account, but is a straight-
forward way to create a new user account.
• Several commands can be used to modify specific fields in the user database including:
passwd (change password), chfn (change full name and the GECOS, or general informa-
tion field), chsh (change login shell), chage (change password age), and passwd -e user
(forces the user to change their password the next time they log in).
• Each user can be a member of one or multiple groups. Several commands can be used to
modify group identity: newgrp changes the current group ID, sg executes a command using
the supplied alternate group, the setgid bit can be placed on a directory, causing files created
in that directory to automatically belong to the correct group. In addition, the id command
displays the current state of a user including a list of their group membership.
• You can manually start SSH with systemctl start ssh or permanently enable it with
systemctl enable ssh.
• PostgreSQL is a database server. It is rarely useful on its own but is used by many other
services to store data.
• A typical Kali Linux installation includes the Apache web server, provided by the apache2
package. Being a network service, it is disabled by default. You can manually start it with
systemctl start apache2.
• With its default configuration, Apache listens on port 80 (as configured in /etc/apache2/
ports.conf), and serves pages from the /var/www/html/ directory by default (as config-
ured in /etc/apache2/sites-enabled/000-default.conf).
Now that we have tackled Linux fundamentals and Kali Linux installation and configuration, let’s
discuss how to troubleshoot Kali and teach you some tools and tricks to get you back up and run-
ning when you run into problems.
124 Kali Linux Revealed
Keywords
Documentation
Forums
IRC channel
Bug report
Chapter
Helping Yourself and 6
Getting Help
Contents
Documentation Sources 128 Kali Linux Communities 132 Filing a Good Bug Report 134 Summary 151
No matter how many years of experience you have, there is no doubt that—sooner or later—you
will encounter a problem. Solving that problem is then often a matter of understanding it and
then taking advantage of various resources to find a solution or work-around.
In this chapter, we will discuss the various information sources available and discuss the best
strategies for finding the help you need or the solution to a problem you might be facing. We will
also take you on a tour of some of the Kali Linux community resources available, including forums
and Internet Relay Chat (IRC) channel. Lastly, we will introduce bug reporting and show you how
to take advantage of bug filing systems to troubleshoot problems and lay out strategies to help
you file your own bug report so that undocumented issues can be handled quickly and effectively.
6.1. Documentation Sources
Before you can understand what is really going on when there is a problem, you need to know the
theoretical role played by each program involved in the problem. One of the best ways to do this
is to review the program’s documentation. Let’s begin by discussing where, exactly, you can find
documentation since it is often scattered.
How to Avoid RTFM This acronym stands for “read the f***ing manual,” but can also be expanded in a
Answers friendlier variant, “read the fine manual.” This phrase is sometimes used in (terse)
responses to questions from newbies. It is rather abrupt, and betrays a certain an-
noyance at a question asked by someone who has not even bothered to read the
documentation. Some say that this classic response is better than no response at all
since this at least hints that the answer lies within the documentation.
When you are posting questions, don’t necessarily be offended by the occasional
RTFM response, but do what you can to at least show that you have taken the time
to do some research before posting the question; mention the sources that you have
consulted and describe the various steps that you have personally taken to find infor-
mation. This will go a long way to show that you are not lazy and are truly seeking
knowledge. Following Eric Raymond’s guidelines is a good way to avoid the most
common mistakes and get useful answers.
è http://catb.org/~esr/faqs/smart-questions.html
6.1.1. Manual Pages
Manual (man) pages, while relatively terse in style, contain a great deal of essential information.
To view a manual page, simply type man manual-page. The manual page usually coincides with
the command name. For example, to learn about the possible options for the cp command, you
would type man cp at the command prompt.
Manual pages not only document programs accessible from the command line, but also config-
uration files, system calls, C library functions, and so forth. Sometimes names can collide. For
128 Kali Linux Revealed
example, the shell’s read command has the same name as the read system call. This is why man-
ual pages are organized in the following numbered sections:
1. Commands that can be executed from the command line
2. System calls (functions provided by the kernel)
3. Library functions (provided by system libraries)
4. Devices (on Unix-like systems, these are special files, usually placed in the /dev/ directory)
5. Configuration files (formats and conventions)
6. Games
7. Sets of macros and standards
8. System administration commands
9. Kernel routines
You can specify the section of the manual page that you are looking for: to view the documentation
for the read system call, you would type man 2 read. When no section is explicitly specified, the
first section that has a manual page with the requested name will be shown. Thus, man shadow
returns shadow(5) because there are no manual pages for shadow in sections 1–4.
Of course, if you do not know the names of the commands, the manual is not going to be of much
use to you. Enter the apropos command, which searches manual pages (or more specifically their
short descriptions) for any keywords that you provide. The apropos command then returns a
list of manual pages whose summary mentions the requested keywords along with the one-line
summary from the manual page. If you choose your keywords well, you will find the name of the
command that you need.
Example 6.1 Finding cp with apropos
$ apropos ”copy file”
cp (1) - copy files and directories
cpio (1) - copy files to and from archives
gvfs-copy (1) - Copy files
gvfs-move (1) - Copy files
hcopy (1) - copy files from or to an HFS volume
install (1) - copy files and set attributes
ntfscp (8) - copy file to an NTFS volume.
Browsing Documentation Many manual pages have a “See Also” section, usually near the end of the document,
by Following Links which refers to other manual pages relevant to similar commands, or to external doc-
umentation. You can use this section to find relevant documentation even when the
first choice is not optimal.
Chapter 6 — Helping Yourself and Getting Help 129
In addition to man, you can use konqueror (in KDE) and yelp (in GNOME) to search manual pages
as well.
6.1.2. Info Documents
The GNU project has written manuals for most of its programs in the info format; this is why many
manual pages refer to the corresponding info documentation. This format offers some advantages
but the default program to view these documents (also called info) is slightly more complex. You
would be well advised to use pinfo instead (from the pinfo package). To install it, simply run apt
update followed by apt install pinfo (see section 8.2.2.2, “Installing Packages with APT” [page
183]).
The info documentation has a hierarchical structure and if you invoke pinfo without parameters,
it will display a list of the nodes available at the first level. Usually, nodes bear the name of the
corresponding commands.
You can use the arrow keys to navigate between nodes. Alternatively, you could also use a graph-
ical browser (which is a lot more user-friendly) such as konqueror or yelp.
As far as language translations are concerned, the info system is always in English and is not suit-
able for translation, unlike the man page system. However, when you ask the pinfo program to
display a non-existing info page, it will fall back on the man page by the same name (if it exists),
which might be translated.
6.1.3. Package-Specific Documentation
Each package includes its own documentation and even the least documented programs generally
have a README file containing some interesting and/or important information. This documenta-
tion is installed in the /usr/share/doc/package/ directory (where package represents the name
of the package). If the documentation is particularly large, it may not be included in the program’s
main package, but might be offloaded to a dedicated package which is usually named package-doc.
The main package generally recommends the documentation package so that you can easily find
it.
The /usr/share/doc/package/ directory also contains some files provided by Debian, which com-
plete the documentation by specifying the package’s particularities or improvements compared
to a traditional installation of the software. The README.Debian file also indicates all of the adap-
tations that were made to comply with the Debian Policy1 . The changelog.Debian.gz file allows
the user to follow the modifications made to the package over time; it is very useful to try to
understand what has changed between two installed versions that do not have the same behav-
ior. Finally, there is sometimes a NEWS.Debian.gz file that documents the major changes in the
program that may directly concern the administrator.
1
https://www.debian.org/doc/debian-policy/
130 Kali Linux Revealed
6.1.4. Websites
In many cases, you can find websites that are used to distribute free software programs and to
bring together the community of its developers and users. These sites are loaded with relevant
information in various forms such as official documentation, frequently asked questions (FAQ),
and mailing list archives. In most cases, the FAQ or mailing list archives address problems that
you have encountered. As you search for information online, it is immensely valuable to master
search syntax. One quick tip: try restricting a search to a specific domain, like the one dedicated
to the program that is giving you trouble. If the search returns too many pages or if the results
do not match what you seek, you can add the keyword kali or debian to limit results and target
relevant information.
From the Error to a If the software returns a very specific error message, enter it into a search engine
Solution (between double quotes, ”, in order to search for the complete phrase, rather than the
individual keywords). In most cases, the first links returned will contain the answer
that you need.
In other cases, you will get very general errors, such as “Permission denied”. In this
case, it is best to check the permissions of the elements involved (files, user ID, groups,
etc.). In short, don’t get in the habit of always using a search engine to find a solution
to your problem. You will find it is much too easy to forget to use common sense.
If you do not know the address of the software’s website, there are various means of locating
it. First, look for a Homepage field in the package’s meta-information (apt show package). Al-
ternatively, the package description may contain a link to the program’s official website. If
no URL is indicated, the package maintainer may have included a URL in the /usr/share/
doc/package/copyright file. Finally, you may be able to use a Internet search engine (such as
Google, Bing, DuckDuckGo, etc.) to find the software’s website.
6.1.5. Kali Documentation at kali.org/docs/
The Kali project maintains a collection of useful documentation at https://www.kali.org/docs/.
While this book covers a large part of what you should know about Kali Linux, the documentation
there might still be useful as it contains step-by-step instructions (much like how-tos) on many
topics.
è https://www.kali.org/docs/
Let’s review the various topics covered there:
• Introduction: documentation describing what Kali Linux is and all of its features
• Installation: various documents describing Kali Linux installation, including how to install
it side-by-side with other operating systems
Chapter 6 — Helping Yourself and Getting Help 131
• Virtualization: various documents describing how to create Kali Linux virtual machines
through various software
• USB: documentation describing how to create a Kali Linux bootable live USB
• Kali Linux on ARM: many recipes about running Kali Linux on various ARM-based devices
• Containers: documentation describing how to use Kali Linux in containers such as Docker
or LXC
• WSL: various documents containing information on potential ways to use Kali Linux through
the Windows Subsystem for Linux
• Cloud: documentation describing how to create cloud instances of Kali Linux
• Kali NetHunter: documentation on everything involving the mobile port of Kali
• General Use: various documents on the typical use cases of Kali Linux and answers to various
questions that may be asked
• Tools: documentation on the tools contained within Kali Linux
• Troubleshooting: various documents that contain information that may be beneficial when
troubleshooting issues
• Development: documentation describing many aspects of the Kali Linux creation process
• Community: documentation describing how to get involved in the Kali Linux community
• Policy: explanations about what makes Kali Linux special when compared to other Linux
distributions
6.2. Kali Linux Communities
There are many Kali Linux communities around the world using many different tools to commu-
nicate (forums, real-time chat and social networks, for example). In this section, we will only
present two official Kali Linux communities.
6.2.1. Forums on forums.kali.org
The official community forums for the Kali Linux project are located at forums.kali.org2 . Like
every web-based forum, you must create an account to be able to post and the system remembers
what posts you have already seen, making it easy to follow conversations on a regular basis.
Before posting, you should read the forum rules:
è https://www.kali.org/docs/community/kali-linux-community-forums/
2
https://forums.kali.org/
132 Kali Linux Revealed
We won’t copy them here but it is worth noting that you are not allowed to speak about illegal ac-
tivities such as breaking into other people’s networks. You must be respectful of other community
members so as to create a welcoming community. Advertising is banned and off-topic discussions
are to be avoided. There are enough categories to cover everything that you would like to discuss
about Kali Linux.
6.2.2. #kali-linux IRC Channel on Freenode
IRC is a real-time chat system. Discussions happen in chat rooms that are called channels and are
usually centered around a particular topic or community. The Kali Linux project uses the #kali-
linux channel on the Freenode3 network (you can use chat.freenode.net as IRC server, on port 6667
for a TLS-encrypted connection or port 6666 for a clear-text connection).
To join the discussions on IRC, you have to use an IRC client such as hexchat (in graphical mode)
or irssi (in console mode). There is also a web-based client available on webchat.freenode.net4 .
While it is really easy to join the conversation, you should be aware that IRC channels have their
own rules and that there are channel operators (their nickname is prefixed with @ when using
the HexChat5 IRC client) who can enforce the rules: they can kick you out of the channel (or even
ban you if you continue to disobey the rules). The #kali-linux channel is no exception. The rules
have been documented here:
è https://www.kali.org/docs/community/kali-linux-irc-channel/
To summarize the rules: you have to be friendly, tolerant, and reasonable. You should avoid off-
topic discussions. In particular, discussions about illegal activities, pirated software, politics, and
religions are forbidden. Keep in mind that your IP address will be available to others by default.
If you want to ask for help, follow the recommendations listed in “How to Avoid RTFM Answers”
[page 128]: do your research first and share the results. When you are asked for supplementary
information, please provide it accurately (if you must provide some verbose output, don’t paste it
in the channel directly, instead use a service like Pastebin6 and post only the Pastebin URL).
Do not expect an immediate answer. Even though IRC is a real-time communication platform,
participants log in from all over the world, so time zones and work schedules vary. It may take
a few minutes or hours for someone to respond to your question. However, when others include
your nickname in a reply, your nickname will be highlighted and most IRC clients will notify you,
so leave your client connected and be patient.
3
https://freenode.net/
4
https://webchat.freenode.net/
5
https://hexchat.github.io/
6
https://pastebin.com/
Chapter 6 — Helping Yourself and Getting Help 133
6.3. Filing a Good Bug Report
If all of your efforts to resolve a problem fail, it is possible that the problem is due to a bug in the
program. In this case, the problem may have resulted in a bug report. You can search for bug
reports to find a solution to your problem but let’s take a look at the procedure of reporting a bug
to Kali, Debian, or directly to the upstream developers so you understand the process should you
need to submit your own report.
The goal of a bug report is to provide enough information so that the developers or maintainers of
the (supposedly) faulty program can reproduce the problem, debug its behavior, and develop a fix.
This means that your bug report must contain appropriate information and must be directed to
the correct person or project team. The report must also be well-written and thorough, ensuring
a faster response.
The exact procedure for the bug report will vary depending on where you will submit the report
(Kali, Debian, or upstream developers) but there are some generic recommendations that apply
to all cases. In this chapter we will discuss those recommendations.
6.3.1. Generic Recommendations
Let’s discuss some general recommendations and guidelines that will help you submit a bug report
that is clear, comprehensive, and improves the chances that the bug will be addressed by the
developers in a timely fashion.
How to Communicate
Write Your Report in English The Free Software community is international and unless you
know your interlocutor, you should be using plain English. If you are a native speaker of English,
use simple sentences and avoid constructions that might be hard to understand for people with
limited English skills. Even though most developers are highly intelligent, not all of them have
strong English language skills. It is best never to assume.
Be Respectful of the Developers’ Work Remember that most Free Software developers (includ-
ing those behind Kali Linux) are benevolent and are spending their limited free time to work on
the software that you are freely using. Many are doing this out of altruism. Thus, when you file
a bug report, be respectful (even if the bug looks like an obvious mistake by the developer) and
don’t assume that they owe you a fix. Thank them for their contribution instead.
If you know how to modify and recompile the software, offer to assist the developers in testing
any patches that they submit to you. This will show them that you are willing to invest your own
time as well.
134 Kali Linux Revealed
Be Reactive and Ready to Provide More Information In some cases, the developer will come
back to you with requests for more information or requests for you to try to re-create the problem
perhaps by using different options or using an updated package. You should try to respond to
those queries as quickly as possible. The quicker you submit your response, the higher the chance
that they will be able to solve it quickly while the initial analysis is still fresh in their mind.
While you should aim to respond quickly, you should also not go too fast: the data submitted must
be correct and it must contain everything that the developers requested. They may be annoyed
if they have to request something a second time.
What to Put in the Bug Report
Instructions to Reproduce the Problem To be able to reproduce the issue, the developers need
to know what you are using, where you got it from, and how you installed it.
You should provide precise, step-by-step instructions describing how to reproduce the problem.
If you need to use some data to reproduce the problem, attach the corresponding file to the bug
report. Try to come up with the minimal set of instructions needed to reproduce the bug.
Give Some Context and Set Your Expectations Explain what you were trying to do and how you
expected the program to behave.
In some cases, the bug is only triggered because you were using the program in a way that it was
not designed to operate by the developers. By explaining what you were trying to achieve, you
will allow the developers to clearly see when this is the case.
In some other cases, the behavior that you describe as a bug might actually be the normal behavior.
Be explicit about what you expected the program to do. This will clarify the situation for the
developers. They may either improve the behavior or improve the documentation, but at least
they know that the behavior of their program is confusing some users!
Be Specific Include the versions numbers of the software that you use, possibly with the version
numbers of their dependencies. When you refer to something that you downloaded, include its
complete URL.
When you get an error message, quote it exactly as you saw it. If possible, include a copy of your
screen output or a screenshot. Include a copy of any relevant log file, ensuring that you remove
any sensitive data first.
Mention Possible Fixes or Workarounds Before filing the bug report, you probably tried to re-
solve the problem. Explain what you tried and what results you received. Be very clear about
what is a fact and what was just a hypothesis on your part.
If you did an Internet search and found some explanations about a similar problem, you can men-
tion them, in particular when you found other similar bug reports in bug trackers.
Chapter 6 — Helping Yourself and Getting Help 135
If you found a way of achieving the desired result without triggering the bug, please document
that as well. This will help other users who are hit by the same issue.
Long Bug Reports Are Fine A two-line bug report is insufficient; providing all the information
needed usually requires several paragraphs (or sometimes pages) of text.
Supply all the information you can. Try to stick to what is relevant, but if you are uncertain, too
much is better than too little.
If your bug report is really long, take some time to structure the content and provide a short
summary at the start.
Miscellaneous Tips
Avoid Filing Duplicate Bug Reports In the Free Software world, all bug trackers are public. Open
issues can be browsed and they even have a search feature. Thus, before filing a new bug report,
try to determine if your problem has already been reported by someone else.
If you find an existing bug report, subscribe to it and possibly add supplementary information. Do
not post comments to bump, such as “Me too” or “+1”; they serve no purpose. But you can indicate
that you are available for further tests if the original submitter did not offer this.
If you have not found any report of your problem, go ahead and file it. If you have found related
tickets, be sure to mention them.
Ensure You Use the Latest Version It is very frustrating for developers to receive bug reports for
problems that they have already solved or problems that they can’t reproduce with the version
that they are using (developers almost always use the latest version of their product). Even when
older versions are maintained by the developers, the support is often limited to security fixes and
major problems. Are you sure that your bug is one of those?
That is why, before filing a bug report, you should make sure that you are using the latest version of
the problematic system and application and that you can reproduce the problem in that situation.
If Kali Linux does not offer the latest version of the application, you have alternative solutions:
you can try a manual installation of the latest version in a throw-away virtual machine, or you
can review the upstream Changelog (or any history logs in their chosen version control system)
to see that there hasn’t been any change that could fix the problem that you are seeing (and then
file the bug even though you did not try the latest version).
Do Not Mix Multiple Issues in a Single Bug Report File one bug report per issue. That way,
the subsequent discussions do not get too messy and each bug can be fixed according to its own
schedule. If you don’t do that, either the single bug needs to be repurposed multiple times and
can only be closed when all issues have been fixed, or the developers must file the supplementary
reports that you should have created in the first place.
136 Kali Linux Revealed
6.3.2. Where to File a Bug Report
To be able to decide where to file the bug report, you must have a good understanding of the
problem and you must have identified in which piece of software the problem lies.
Ideally, you track the problem down to a file on your system and then you can use dpkg to find
out which package owns that file and where that package comes from. Let’s assume that you
found a bug in a graphical application. After looking at the list of running processes (the out-
put of ps auxf), you discovered that the application was started with the /usr/bin/cherrytree
executable:
$ dpkg -S /usr/bin/cherrytree
cherrytree: /usr/bin/cherrytree
$ dpkg -s cherrytree | grep ^Version:
Version: 0.38.8-0kali1
You learn that /usr/bin/cherrytree is provided by the cherrytree package, which is in version
0.38.8-0kali1. The fact that the version string contains kali indicates to you that the package comes
from Kali Linux (or is modified by Kali Linux). Any package that does not have kali in its version
string (or in its package name) comes straight from Debian (Debian Testing in general).
Double Check Before If you find a bug in a package imported straight from Debian, it should ideally be
Filing Bugs against reported and fixed on the Debian side. However, before doing this, ensure that the
Debian problem is reproducible on a plain Debian system since Kali may have caused the
problem by modifying other packages or dependencies.
The easiest way to accomplish this is to setup a virtual machine running Debian Test-
ing. You can find an installation ISO for Debian Testing on the Debian Installer web-
site:
è https://www.debian.org/devel/debian-installer/
If you can confirm the problem in the virtual machine, then you can submit the bug to
Debian by running reportbug within the virtual machine and following the instruc-
tions provided.
Most bug reports about the behavior of applications should be directed to their upstream projects
except when facing an integration problem: in that case, the bug is a mistake in the way the
software gets packaged and integrated into Debian or Kali. For example, if an application offers
compile-time options that the package does not enable or the application does not work because of
a missing library (thus putting into light a missing dependency in the package meta-information),
you may be facing an integration problem. When you don’t know what kind of problem you face,
it is usually best to file the issue on both sides and to cross-reference them.
Identifying the upstream project and finding where to file the bug report is usually easy. You just
have to browse the upstream website, which is referenced in the Homepage field of the packaging
meta-data:
Chapter 6 — Helping Yourself and Getting Help 137
$ dpkg -s wpscan | grep ^Homepage:
Homepage: http://www.wpscan.org
6.3.3. How to File a Bug Report
Filing a Bug Report in Kali
Kali uses a web-based bug tracker at https://bugs.kali.org/ where you can consult all the bug
reports anonymously, but if you would like to comment or file a new bug report, you will need to
register an account.
Signing Up for a Bug Tracker Account To begin, simply click Signup for new account on the bug
tracker website, as shown in Figure 6.1, “Kali Bug Tracker Start Page” [page 138].
Figure 6.1 Kali Bug Tracker Start Page
Next, provide a username, e-mail address, and response to the CAPTCHA challenge. Then click
the Signup button to proceed (Figure 6.2, “Signup Page” [page 139]).
138 Kali Linux Revealed
Figure 6.2 Signup Page
If successful, the next page (Figure 6.3, “Signup Confirmation Page” [page 139]) will notify you that
the account registration has been processed, and the bug tracker system will send a confirmation
email to the address you provided. You will need to click the link in the email in order to activate
your account.
Once your account has been activated, click Proceed to continue to the bug tracker login page.
Figure 6.3 Signup Confirmation Page
Chapter 6 — Helping Yourself and Getting Help 139
Creating the Report To begin your report, log into your account and click the Report Issue link
on the landing page. You will be presented a form with many fields to fill, as shown in Figure 6.4,
“Form to report a bug” [page 140].
Figure 6.4 Form to report a bug
Here is a rundown of all the fields on the form:
Category (mandatory) This field describes the category of the bug you are submitting. Reports
that can be attributed to a specific package should be filed in the Kali Package Bug or Kali
140 Kali Linux Revealed
Package Improvement categories. Other reports should use the General Bug or Feature Re-
quests categories. The remaining categories are for specific use cases: Tool Upgrade can be
used to notify the Kali developers of the availability of a new version of a software packaged
in Kali. New Tool Requests can be used to suggest new tools to package and integrate in the
Kali distribution. Kali Websites & Docs can be used to report bugs or updates relating to the
various Kali websites. Queued Tool Addition is reserved for the Kali tool team, for when a
tool submission has been agreed-upon to be added into Kali.
Reproducibility This field documents whether the problem is reproducible in a predictable way
or if it happens only somewhat randomly.
Severity and Priority Those fields are best left unmodified as they are mainly for the developers.
They can use them to sort the list of issues according to the severity of the problem and to
the priority at which it must be handled.
Product Version This field should indicate what version of Kali Linux you are running (or the
one which is the closest to what you are running). Think twice before reporting an issue on
an old release that is no longer supported.
Assign To and Target Version Those fields are also best left unmodified as they are again,
mainly for the developers. They can use them to indicate which developer is handling the
issue and when the issue should be resolved.
Summary (mandatory) This is essentially the title of your bug report and it is the first thing that
people will see. Make sure that it conveys the reason why you are filing the report. Avoid
generic descriptions like “X doesn’t work” and opt instead for “X fails with error Y under
condition Z.”
Description (mandatory) This is the body of your report. Here you should enter all of the in-
formation you collected about the problem that you are experiencing. Don’t forget all the
recommendations given in the former section.
Steps to Reproduce In this field, list all the detailed instructions explaining how to trigger the
problem.
Additional Information In this section, you can provide any additional information you believe
is relevant to the issue. If you have a fix or workaround for the issue, please provide it in
this section.
Attach Tags This is a section better left unmodified. Tags are used by the developers to allow
easier access to similar bug reports.
Upload File Not everything can be explained with plain text. This field lets you attach arbitrary
files to your reports: screenshots to show the error, sample documents triggering the prob-
lem, log files, etc.
Chapter 6 — Helping Yourself and Getting Help 141
View Status Leave that field set to “public” so that everybody can see your bug report. Use “pri-
vate” only for security-related reports containing information about undisclosed security
vulnerabilities.
Filing a Bug Report in Debian
Debian uses a (mostly) email-based bug tracking system known as Debbugs. To open a new
bug report, you will send an email (with a special syntax) to submit@bugs.debian.org. This
will allocate a bug number XXXXXX and inform you that you can send additional informa-
tion by mailing XXXXXX @bugs.debian.org. Each bug is associated to a Debian package. You
can browse all the bugs of a given package (including the bug that you are thinking of re-
porting) at https://bugs.debian.org/package. You can check the history of a given bug at
https://bugs.debian.org/XXXXXX .
Setting Up Reportbug While you can open a new bug with a simple e-mail, we recommend
using reportbug because it will help you draft a solid bug report with all the required information.
Ideally, you should run it from a Debian system (for example, in the virtual machine where you
reproduced the problem).
The first run of reportbug starts a configuration script. First, select a skill level. You should
choose Novice or Standard; we use the latter because it offers more fine-grained control. Next,
select an interface and enter your personal details. Finally, select a user interface. The configura-
tion script will allow you to use a local mail transport agent, an SMTP server, or as a last resort, a
Debian SMTP server.
Welcome to reportbug! Since it looks like this is the first time you have
used reportbug, we are configuring its behavior. These settings will be
saved to the file ”/home/kali/.reportbugrc”, which you will be free to edit
further.
Please choose the default operating mode for reportbug.
1 novice Offer simple prompts, bypassing technical questions.
2 standard Offer more extensive prompts, including asking about things
that a moderately sophisticated user would be expected to
know about Debian.
3 advanced Like standard, but assumes you know a bit more about Debian,
å including ”incoming”.
4 expert Bypass most handholding measures and preliminary triage
routines. This mode should not be used by people unfamiliar
with Debian’s policies and operating procedures.
142 Kali Linux Revealed
Select mode: [novice] standard
Please choose the default interface for reportbug.
1 text A text-oriented console user interface
2 gtk2 A graphical (GTK+) user interface.
3 urwid A menu-based console user interface
Select interface: text
Will reportbug often have direct Internet access? (You should answer
yes to this question unless you know what you are doing and plan to
check whether duplicate reports have been filed via some other channel.)
[Y|n|q|?]? Y
What real name should be used for sending bug reports?
[kali]> Raphaël Hertzog
Which of your email addresses should be used when sending bug reports?
(Note that this address will be visible in the bug tracking system, so you
may want to use a webmail address or another address with good spam
filtering capabilities.)
[kali@localhost.localdomain]> buxy@kali.org
Do you have a ”mail transport agent” (MTA) like Exim, Postfix or SSMTP
configured on this computer to send mail to the Internet? [y|N|q|?]? N
Please enter the name of your SMTP host. Usually it’s called something
like ”mail.example.org” or ”smtp.example.org”. If you need to use a
different port than default, use the <host>:<port> alternative
format. Just press ENTER if you don’t have one or don’t know, and so a
Debian SMTP host will be used.
>
Please enter the name of your proxy server. It should only use this
parameter if you are behind a firewall. The PROXY argument should be
formatted as a valid HTTP URL, including (if necessary) a port number; for
example, http://192.168.1.1:3128/. Just press ENTER if you don’t have one
or don’t know.
>
Default preferences file written. To reconfigure, re-run reportbug with
the ”--configure” option.
Using Reportbug With the setup phase completed, the actual bug report can begin. You will be
prompted for a package name, although you can also provide the package name directly on the
command line with reportbug package).
Please enter the name of the package in which you have found a problem, or
type ’other’ to report a more general problem. If you don’t know what
package the bug is in, please contact debian-user@lists.debian.org for
assistance.
Chapter 6 — Helping Yourself and Getting Help 143
> wireshark
Contrary to the advice given above, if you don’t know against which package to file the bug, you
should get in touch with a Kali support forum (described in section 6.2, “Kali Linux Communities”
[page 132]). In the next step, reportbug downloads the list of bugs filed against the given package
and lets you browse them to see if you can find yours.
*** Welcome to reportbug. Use ? for help at prompts. ***
Note: bug reports are publicly archived (including the email address of the submitter).
Detected character set: UTF-8
Please change your locale if this is incorrect.
Using ’”Raphaël Hertzog” <buxy@kali.org>’ as your from address.
Getting status for wireshark...
Verifying package integrity...
Checking for newer versions at madison...
Will send report to Debian (per lsb_release).
Querying Debian BTS for reports on wireshark (source)...
35 bug reports found:
Bugs with severity important
1) #478200 tshark: seems to ignore read filters when writing to...
2) #776206 mergecap: Fails to create output file > 2GB
3) #780089 wireshark: ”On gnome wireshark has not title bar. Does...
Bugs with severity normal
4) #151017 ethereal: ”Protocol Hierarchy Statistics” give misleading...
5) #275839 doesn’t correctly dissect ESMTP pipelining
[...]
35) #815122 wireshark: add OID 1.3.6.1.4.1.11129.2.4.2
(24-35/35) Is the bug you found listed above [y|N|b|m|r|q|s|f|e|?]? ?
y - Problem already reported; optionally add extra information.
N - (default) Problem not listed above; possibly check more.
b - Open the complete bugs list in a web browser.
m - Get more information about a bug (you can also enter a number
without selecting ”m” first).
r - Redisplay the last bugs shown.
q - I’m bored; quit please.
s - Skip remaining problems; file a new report immediately.
f - Filter bug list using a pattern.
e - Open the report using an e-mail client.
? - Display this help.
(24-35/35) Is the bug you found listed above [y|N|b|m|r|q|s|f|e|?]? n
Maintainer for wireshark is ’Balint Reczey <balint@balintreczey.hu>’.
Looking up dependencies of wireshark...
If you find your bug already filed, you can choose to send supplementary information, otherwise,
you are invited to file a new bug report:
144 Kali Linux Revealed
Briefly describe the problem (max. 100 characters allowed). This will be the bug email
å subject, so keep the summary as concise as possible, for example: ”fails to send
å email”
or ”does not start with -q option specified” (enter Ctrl+c to exit reportbug without
å reporting a bug).
> does not dissect protocol foobar
Rewriting subject to ’wireshark: does not dissect protocol foobar’
After providing a one-line summary of your problem, you must rate its severity along an extended
scale:
How would you rate the severity of this problem or report?
1 critical makes unrelated software on the system (or the whole system) break,
å or causes serious data loss, or introduces a security hole on systems where you
å install
the package.
2 grave makes the package in question unusable by most or all users, or
å causes data loss, or introduces a security hole allowing access to the accounts
å of users who
use the package.
3 serious is a severe violation of Debian policy (that is, the problem is a
å violation of a ’must’ or ’required’ directive); may or may not affect the
å usability of the
package. Note that non-severe policy violations may be ’normal,’ ’
å minor,’ or ’wishlist’ bugs. (Package maintainers may also
å designate other bugs as
’serious’ and thus release-critical; however, end users should not do
å so.). For the canonical list of issues deserving a serious
å severity you can refer to
this webpage: http://release.debian.org/testing/rc_policy.txt .
4 important a bug which has a major effect on the usability of a package, without
å rendering it completely unusable to everyone.
5 does-not-build a bug that stops the package from being built from source. (This is a
å ’virtual severity’.)
6 normal a bug that does not undermine the usability of the whole package; for
å example, a problem with a particular option or menu item.
7 minor things like spelling mistakes and other minor cosmetic errors that do
å not affect the core functionality of the package.
8 wishlist suggestions and requests for new features.
Please select a severity level: [normal]
If you are unsure, just keep the default severity of normal.
You can also tag your report with a few keywords:
Chapter 6 — Helping Yourself and Getting Help 145
Do any of the following apply to this report?
1 a11y This bug is relevant to the accessibility of the package.
2 d-i This bug is relevant to the development of debian-installer.
3 ftbfs The package fails to build from source.
4 ipv6 This bug affects support for Internet Protocol version 6.
5 l10n This bug reports a localization/internationalization issue.
6 lfs This bug affects support for large files (over 2 gigabytes).
7 newcomer This bug has a known solution but the maintainer requests someone else
å implement it.
8 patch You are including a patch to fix this problem.
9 upstream This bug applies to the upstream part of the package.
10 none
Please select tags: (one at a time) [none]
Most tags are rather esoteric, but if your report includes a fix, you should select the patch tag.
Once this is completed, reportbug opens a text editor with a template that you should edit (Ex-
ample 6.2, “Template generated by reportbug” [page 146]). It contains a few questions that you
should delete and answer, as well as some information about your system that has been automati-
cally collected. Notice how the first few lines are structured. They should not be modified as they
will be parsed by the bug tracker to assign the report to the correct package.
Example 6.2 Template generated by reportbug
Subject: wireshark: does not dissect protocol foobar
Package: wireshark
Version: 3.2.5-1
Severity: normal
Dear Maintainer,
*** Reporter, please consider answering these questions, where appropriate ***
* What led up to the situation?
* What exactly did you do (or not do) that was effective (or
ineffective)?
* What was the outcome of this action?
* What outcome did you expect instead?
*** End of the template - remove these template lines ***
146 Kali Linux Revealed
-- System Information:
Distributor ID: Kali
Description: Kali GNU/Linux Rolling
Release: 2020.3
Codename: kali-rolling
Architecture: x86_64
Kernel: Linux 5.7.0-kali1-amd64 (SMP w/4 CPU threads)
Kernel taint flags: TAINT_UNSIGNED_MODULE
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled
Versions of packages wireshark depends on:
ii wireshark-qt 3.2.5-1
wireshark recommends no packages.
wireshark suggests no packages.
-- no debconf information
Once you save the report and close the text editor, you return to reportbug, which provides many
other options and offers to send the resulting report.
Spawning sensible-editor...
Report will be sent to ”Debian Bug Tracking System” <submit@bugs.debian.org>
Submit this report on wireshark (e to edit) [Y|n|a|c|e|i|l|m|p|q|d|t|s|?]? ?
Y - (default) Submit the bug report via email.
n - Don’t submit the bug report; instead, save it in a temporary file (exits reportbug).
a - Attach a file.
c - Change editor and re-edit.
e - Re-edit the bug report.
i - Include a text file.
l - Pipe the message through the pager.
m - Choose a mailer to edit the report.
p - print message to stdout.
q - Save it in a temporary file and quit.
d - Detach an attachment file.
t - Add tags.
s - Add a X-Debbugs-CC recipient (a CC but after BTS processing).
? - Display this help.
Submit this report on wireshark (e to edit) [Y|n|a|c|e|i|l|m|p|q|d|t|s|?]? Y
Saving a backup of the report at /tmp/reportbug-wireshark-backup-20210328-19073-87oJWJ
Connecting to reportbug.debian.org via SMTP...
Bug report submitted to: ”Debian Bug Tracking System” <submit@bugs.debian.org>
Copies will be sent after processing to:
buxy@kali.org
Chapter 6 — Helping Yourself and Getting Help 147
If you want to provide additional information, please wait to receive the
bug tracking number via email; you may then send any extra information to
n@bugs.debian.org (e.g. 999999@bugs.debian.org), where n is the bug
number. Normally you will receive an acknowledgement via email including
the bug report number within an hour; if you haven’t received a
confirmation, then the bug reporting process failed at some point
(reportbug or MTA failure, BTS maintenance, etc.).
Filing a Bug Report in another Free Software Project
There is a large diversity of free software projects, using different workflows and tools. This di-
versity also applies to the bug trackers in use. While many projects are hosted on GitHub and use
GitHub issues to track their bugs, there are also many others hosting their own trackers, based
on Bugzilla, Trac, Redmine, Flyspray, and others. Most of them are web-based and require you to
register an account to submit a new ticket.
We will not cover all the trackers here. It is up to you to learn the specifics of various trackers for
other free software projects, but since GitHub7 is relatively popular, we will take a brief look at it
here. As with other trackers, you must first create an account and sign in. Next, click the Issues
tab, as shown in Figure 6.5, “Main page of a GitHub project” [page 149].
7
https://github.com/
148 Kali Linux Revealed
Figure 6.5 Main page of a GitHub project
You can then browse (and search) the list of open issues. Once you are confident that your bug is
not yet filed, you can click on the New issue button (Figure 6.6, “Issues page of a GitHub project”
[page 150]).
Chapter 6 — Helping Yourself and Getting Help 149
Figure 6.6 Issues page of a GitHub project
You are now on a page where you must describe your problem (Figure 6.7, “GitHub form to file
a new issue” [page 150]). GitHub has a template feature allowing the owner of the repository to
define their own custom issue template, however it appears that this repository does not have one
setup. However, the bug reporting mechanism is fairly straight-forward, allowing you to attach
files, apply formatting to text, and much more. Of course, for best results, be sure to follow our
guidelines for creating a detailed and well-described report.
Figure 6.7 GitHub form to file a new issue
150 Kali Linux Revealed
6.4. Summary
In this section, we discussed various methods to help you find documentation and information
about programs and how to find help with problems you may encounter. We took a look at manual
and info pages and the apropos and info commands. We discussed bug trackers, provided some
tips on how to search for and submit good bug reports, and provided some tips to help you figure
out who owns the program or project in question.
Summary Tips:
• Before you can understand what is really going on when there is a problem, you need to
know the theoretical role played by each program involved in the problem. One of the best
ways to do this is to review the program’s documentation.
• To view a manual page, simply type man manual-page, filling in the name of the command
after an optional section number.
• The apropos command returns a list of manual pages whose summary mentions the re-
quested keywords, along with the one-line summary from the manual page.
• The GNU project has written manuals for most of its programs in the info format. This is
why many manual pages refer to corresponding info documentation.
• Each package includes its own documentation and even the least documented programs
generally have a README file containing some interesting and/or important information.
This documentation is installed in the /usr/share/doc/package/ directory.
• In most cases, the FAQ or mailing list archives of a program’s official website may address
problems that you have encountered.
• The Kali project maintains a collection of useful documentation at https://www.kali.org/
docs/.
• The Kali Linux project uses the #kali-linux channel on the Freenode8 IRC network. You can
use chat.freenode.net as IRC server, on port 6667 for a TLS-encrypted connection or port
6666 for a clear-text connection. To join the discussions on IRC, you have to use an IRC
client such as hexchat (in graphical mode) or irssi (in console mode). There is also a web-
based client available on webchat.freenode.net9 .
• The official community forums for the Kali Linux project are located at forums.kali.org10 .
• If you uncover a bug in a program, you can search bug reports or file your own. Be sure
to follow the guidelines that we have outlined to ensure your report is clear, comprehen-
sive, and improves the chances that the bug will be addressed by the developers in a timely
fashion.
8
https://freenode.net/
9
https://webchat.freenode.net/
10
https://forums.kali.org/
Chapter 6 — Helping Yourself and Getting Help 151
• Some bug reports should be filed to Kali, while others may be filed on the Debian side. A
command like dpkg -s package-name | grep ^Version: will reveal the version number
and will be tagged as ”kali” if it is a Kali-modified package.
• Identifying an upstream project and finding where to file the bug report is usually easy. Sim-
ply browse the upstream website that is referenced in the Homepage field of the packaging
meta-data.
• Kali uses a web-based bug tracker at https://bugs.kali.org/ where you can consult all
the bug reports anonymously, but if you would like to comment or file a new bug report,
you will need to register an account.
• Debian uses a (mostly) email-based bug tracking system known as Debbugs. To open a new
bug report, you can send an email (with a special syntax) to submit@bugs.debian.org or you
can use the reportbug command, which will guide you through the process.
• While many projects are hosted on GitHub and use GitHub issues to track their bugs, there
are also many others hosting their own trackers. You may have to research the basics of
third-party bug trackers if you need to post to them.
Now that you have the basic tools for navigating Linux, installing and configuring Kali, and trou-
bleshooting your system and getting help, it is time to look at locking down Kali so that you can
protect your installation as well as your client’s data.
152 Kali Linux Revealed
Keywords
Security policy
Firewall
iptables
Monitoring
Logging
Chapter
Securing and 7
Monitoring Kali Linux
Contents
Defining a Security Policy 156 Possible Security Measures 158 Securing Network Services 159
Firewall or Packet Filtering 159 Monitoring and Logging 167 Summary 171
As you begin to use Kali Linux for increasingly sensitive and higher-profile work, you will likely
need to take the security of your installation more seriously. In this chapter, we will first discuss
security policies, highlighting various points to consider when defining such a policy, and outlin-
ing some of the threats to your system and to you as a security professional. We will also discuss
security measures for desktop and laptop systems and focus on firewalls and packet filtering. Fi-
nally, we will discuss monitoring tools and strategies and show you how to best implement them
to detect potential threats to your system.
7.1. Defining a Security Policy
It is impractical to discuss security in broad strokes since the idea represents a vast range of con-
cepts, tools, and procedures, none of which apply universally. Choosing among them requires a
precise idea of what your goals are. Securing a system starts with answering a few questions. Rush-
ing headlong into implementing an arbitrary set of tools runs the risk of focusing on the wrong
aspects of security.
It is usually best to determine a specific goal. A good approach to help with that determination
starts with the following questions:
• What are you trying to protect? The security policy will be different depending on whether
you want to protect computers or data. In the latter case, you also need to know which data.
• What are you trying to protect against? Is it leakage of confidential data? Accidental data
loss? Revenue loss caused by disruption of service?
• Also, who are you trying to protect against? Security measures will be quite different for
guarding against a typo by a regular user of the system versus protecting against a deter-
mined external attacker group.
The term ”risk” is customarily used to refer collectively to these three factors: what to protect,
what should be prevented, and who might make this happen. Modeling the risk requires answers
to these three questions. From this risk model, a security policy can be constructed and the policy
can be implemented with concrete actions.
Permanent Questioning Bruce Schneier, a world expert in security matters (not only computer security), tries
to counter one of security’s most important myths with a motto: “Security is a process,
not a product.” Assets to be protected change over time and so do threats and the
means available to potential attackers. Even if a security policy has initially been
perfectly designed and implemented, you should never rest on your laurels. The risk
components evolve and the response to that risk must evolve accordingly.
Extra constraints are also worth taking into account as they can restrict the range of available
policies. How far are you willing to go to secure a system? This question has a major impact on
which policy to implement. Too often, the answer is only defined in terms of monetary costs,
156 Kali Linux Revealed
but other elements should also be considered, such as the amount of inconvenience imposed on
system users or performance degradation.
Once the risk has been modeled, you can start thinking about designing an actual security policy.
There are extremes that can come into play when deciding the level of security protections to
adopt. On one hand, it can be extremely simple to provide basic system security.
For instance, if the system to be protected only comprises a second-hand computer, the sole use of
which is to add a few numbers at the end of the day, deciding not to do anything special to protect
it would be quite reasonable. The intrinsic value of the system is low and the value of the data
are zero since they are not stored on the computer. A potential attacker infiltrating this system
would only gain a calculator. The cost of securing such a system would probably be greater than
the cost of a breach.
At the other end of the spectrum, you might want to protect the confidentiality of secret data in
the most comprehensive way possible, trumping any other consideration. In this case, an appro-
priate response would be the total destruction of the data (securely erasing the files, shredding of
the hard disks to bits, then dissolving these bits in acid, and so on). If there is an additional require-
ment that data must be kept in store for future use (although not necessarily readily available),
and if cost still isn’t a factor, then a starting point would be storing the data on iridium–platinum
alloy plates stored in bomb-proof bunkers under various mountains in the world, each of which
being (of course) both entirely secret and guarded by entire armies.
Extreme though these examples may seem, they would nevertheless be an adequate response to
certain defined risks, insofar as they are the outcome of a thought process that takes into account
the goals to reach and the constraints to fulfill. When coming from a reasoned decision, no security
policy is more, or less, respectable than any other.
Coming back to a more typical case, an information system can be segmented into consistent and
mostly independent subsystems. Each subsystem will have its own requirements and constraints,
and so the risk assessment and the design of the security policy should be undertaken separately
for each. A good principle to keep in mind is that a small attack surface is easier to defend than a
large one. The network organization should also be designed accordingly: the sensitive services
should be concentrated on a small number of machines, and these machines should only be ac-
cessible via a minimal number of routes or check-points. The logic is straightforward: it is easier
to secure these checkpoints than to secure all the sensitive machines against the entirety of the
outside world. It is at this point that the usefulness of network filtering (including by firewalls)
becomes apparent. This filtering can be implemented with dedicated hardware but a simpler and
more flexible solution is to use a software firewall such as the one integrated in the Linux kernel.
Chapter 7 — Securing and Monitoring Kali Linux 157
7.2. Possible Security Measures
As the previous section explained, there is no single response to the question of how to secure Kali
Linux. It all depends on how you use it and what you are trying to protect.
7.2.1. On a Server
If you run Kali Linux on a publicly accessible server, you most likely want to secure network ser-
vices by changing any default passwords that might be configured (see section 7.3, “Securing Net-
work Services” [page 159]) and possibly also by restricting their access with a firewall (see sec-
tion 7.4, “Firewall or Packet Filtering” [page 159]).
If you hand out user accounts either directly on the server or on one of the services, you want to
ensure that you set strong passwords (they should resist brute-force attacks). At the same time,
you might want to setup fail2ban, which will make it much harder to brute-force passwords over
the network (by filtering away IP addresses that exceed a limit of failed login attempts). Install
fail2ban with apt update followed by apt install fail2ban.
If you run web services, you probably want to host them over HTTPS to prevent network interme-
diaries from sniffing your traffic (which might include authentication cookies).
7.2.2. On a Laptop
The laptop of a penetration tester is not subject to the same risks as a public server: for instance,
you are less likely to be subject to random scans from script kiddies and even when you are, you
probably won’t have any network services enabled.
Real risk often arises when you travel from one customer to the next. For example, your laptop
could be stolen while traveling or seized by customs. That is why you most likely want to use full
disk encryption (see section 4.2.2, “Installation on a Fully Encrypted File System” [page 88]) and
possibly also setup the “nuke” feature (see “Adding a Nuke Password for Extra Safety” [page 250]):
the data that you have collected during your engagements are confidential and require the utmost
protection.
You may also need firewall rules (see section 7.4, “Firewall or Packet Filtering” [page 159]) but not
for the same purpose as on the server. You might want to forbid all outbound traffic except the
traffic generated by your VPN access. This is meant as a safety net, so that when the VPN is down,
you immediately notice it (instead of falling back to the local network access). That way, you
do not divulge the IP addresses of your customers when you browse the web or do other online
activities. In addition, if you are performing a local internal engagement, it is best to remain in
control of all of your activity to reduce the noise you create on the network, which can alert the
customer and their defense systems.
158 Kali Linux Revealed
7.3. Securing Network Services
In general, it is a good idea to disable services that you do not use. Kali makes it easy to do this
since network services are disabled by default.
As long as services remain disabled, they do not pose any security threat. However, you must be
careful when you enable them because:
• there is no firewall by default, so if they listen on all network interfaces, they are effectively
publicly available.
• some services have no authentication credentials and let you set them on first use; others
have default (and thus widely known) credentials preset. Make sure to (re)set any password
to something that only you know.
• many services run as root with full administrator privileges, so the consequences of unau-
thorized access or a security breach are therefore usually severe.
Default Credentials We won’t list here all tools that come with default credentials, instead you should
check the README.Debian file of the respective packages, as well as kali.org/docs/1
and tools.kali.org2 to see if the service needs some special care to be secured.
SSH Service If you run in live mode, the password of the kali account is ”kali.” Thus you should
not enable SSH before changing the password of the kali account, or before having
tweaked its configuration to disallow password-based logins.
You may also want to generate new host SSH keys, if you installed Kali by a pre-
generated image. This is covered in “Generating New SSH Host Keys ” [page 115].
7.4. Firewall or Packet Filtering
A firewall is a piece of computer equipment with hardware, software, or both that parses the in-
coming or outgoing network packets (coming to or leaving from a local network) and only lets
through those matching certain predefined conditions.
A filtering network gateway is a type of firewall that protects an entire network. It is usually
installed on a dedicated machine configured as a gateway for the network so that it can parse all
packets that pass in and out of the network. Alternatively, a local firewall is a software service that
runs on one particular machine in order to filter or limit access to some services on that machine,
or possibly to prevent outgoing connections by rogue software that a user could, willingly or not,
have installed.
1
https://www.kali.org/docs/introduction/default-credentials/
2
https://tools.kali.org/
Chapter 7 — Securing and Monitoring Kali Linux 159
The Linux kernel embeds the netfilter firewall. There is no turn-key solution for configuring any
firewall since network and user requirements differ. However, you can control netfilter from user
space with the iptables and ip6tables commands. The difference between these two commands
is that the former works for IPv4 networks, whereas the latter works on IPv6. Since both network
protocol stacks will probably be around for many years, both tools will need to be used in parallel.
You can also use the excellent GUI-based fwbuilder tool, which provides a graphical representa-
tion of the filtering rules.
However you decide to configure it, netfilter is Linux’s firewall implementation, so let’s take a
closer look at how it works.
7.4.1. Netfilter Behavior
Netfilter uses four distinct tables, which store rules regulating three kinds of operations on packets:
• filter concerns filtering rules (accepting, refusing, or ignoring a packet);
• nat (Network Address Translation) concerns translation of source or destination addresses
and ports of packets;
• mangle concerns other changes to the IP packets (including the ToS—Type of Service—field
and options);
• raw allows other manual modifications on packets before they reach the connection track-
ing system.
Each table contains lists of rules called chains. The firewall uses standard chains to handle packets
based on predefined circumstances. The administrator can create other chains, which will only
be used when referred by one of the standard chains (either directly or indirectly).
The filter table has three standard chains:
• INPUT: concerns packets whose destination is the firewall itself;
• OUTPUT: concerns packets emitted by the firewall;
• FORWARD: concerns packets passing through the firewall (which is neither their source
nor their destination).
The nat table also has three standard chains:
• PREROUTING: to modify packets as soon as they arrive;
• POSTROUTING: to modify packets when they are ready to go on their way;
• OUTPUT: to modify packets generated by the firewall itself.
These chains are illustrated in Figure 7.1, “How Netfilter Chains are Called” [page 161].
160 Kali Linux Revealed
Figure 7.1 How Netfilter Chains are Called
Each chain is a list of rules; each rule is a set of conditions and an action to perform when the
conditions are met. When processing a packet, the firewall scans the appropriate chain, one rule
after another, and when the conditions for one rule are met, it jumps (hence the -j option in the
commands with section 7.4.2.2, “Rules” [page 163]) to the specified action to continue processing.
The most common behaviors are standardized and dedicated actions exist for them. Taking one
of these standard actions interrupts the processing of the chain, since the packets fate is already
sealed (barring an exception mentioned below). Listed below are the Netfilter actions.
• ACCEPT: allow the packet to go on its way.
• REJECT: reject the packet with an Internet control message protocol (ICMP) error packet
(the --reject-with type option of iptables determines the type of error to send).
• DROP: delete (ignore) the packet.
• LOG: log (via syslogd) a message with a description of the packet. Note that this action
does not interrupt processing, and the execution of the chain continues at the next rule,
which is why logging refused packets requires both a LOG and a REJECT/DROP rule. Common
parameters associated with logging include:
– --log-level, with default value warning, indicates the syslog severity level.
– --log-prefix allows specifying a text prefix to differentiate between logged messages.
– --log-tcp-sequence, --log-tcp-options, and --log-ip-options indicate extra data to be in-
tegrated into the message: respectively, the TCP sequence number, TCP options, and
IP options.
• ULOG: log a message via ulogd, which can be better adapted and more efficient than
syslogd for handling large numbers of messages; note that this action, like LOG, also re-
turns processing to the next rule in the calling chain.
• chain_name: jump to the given chain and evaluate its rules.
Chapter 7 — Securing and Monitoring Kali Linux 161
• RETURN: interrupt processing of the current chain and return to the calling chain; in case
the current chain is a standard one, there’s no calling chain, so the default action (defined
with the -P option to iptables) is executed instead.
• SNAT (only in the nat table): apply Source Network Address Translation (SNAT). Extra options
describe the exact changes to apply, including the --to-source address:port option, which
defines the new source IP address and/or port.
• DNAT (only in the nat table): apply Destination Network Address Translation (DNAT). Extra op-
tions describe the exact changes to apply, including the --to-destination address:port option,
which defines the new destination IP address and/or port.
• MASQUERADE (only in the nat table): apply masquerading (a special case of Source NAT).
• REDIRECT (only in the nat table): transparently redirect a packet to a given port of the
firewall itself; this can be used to set up a transparent web proxy that works with no con-
figuration on the client side, since the client thinks it connects to the recipient whereas the
communications actually go through the proxy. The --to-ports port(s) option indicates the
port, or port range, where the packets should be redirected.
Other actions, particularly those concerning the mangle table, are outside the scope of this text.
The iptables(8) and ip6tables(8) manual pages have a comprehensive list.
What is ICMP? Internet Control Message Protocol (ICMP) is the protocol used to transmit ancillary
information on communications. It tests network connectivity with the ping com-
mand, which sends an ICMP echo request message, which the recipient is meant to
answer with an ICMP echo reply message. It signals a firewall rejecting a packet, indi-
cates an overflow in a receive buffer, proposes a better route for the next packets in the
connection, and so on. This protocol is defined by several RFC documents. RFC777
and RFC792 were the first, but many others extended and/or revised the protocol.
è http://www.faqs.org/rfcs/rfc777.html
è http://www.faqs.org/rfcs/rfc792.html
For reference, a receive buffer is a small memory zone storing data between the time
it arrives from the network and the time the kernel handles it. If this zone is full, new
data cannot be received and ICMP signals the problem so that the emitter can slow
down its transfer rate (which should ideally reach an equilibrium after some time).
Note that although an IPv4 network can work without ICMP, ICMPv6 is strictly re-
quired for an IPv6 network, since it combines several functions that were, in the IPv4
world, spread across ICMPv4, Internet Group Membership Protocol (IGMP), and Ad-
dress Resolution Protocol (ARP). ICMPv6 is defined in RFC4443.
è http://www.faqs.org/rfcs/rfc4443.html
162 Kali Linux Revealed
7.4.2. Syntax of iptables and ip6tables
The iptables and ip6tables commands are used to manipulate tables, chains, and rules. Their
-t table option indicates which table to operate on (by default, filter).
Commands
The major options for interacting with chains are listed below:
• -L chain lists the rules in the chain. This is commonly used with the -n option to disable
name resolution (for example, iptables -n -L INPUT will display the rules related to in-
coming packets).
• -N chain creates a new chain. You can create new chains for a number of purposes, including
testing a new network service or fending off a network attack.
• -X chain deletes an empty and unused chain (for example, iptables -X ddos-attack).
• -A chain rule adds a rule at the end of the given chain. Remember that rules are processed
from top to bottom so be sure to keep this in mind when adding rules.
• -I chain rule_num rule inserts a rule before the rule number rule_num. As with the -A option,
keep the processing order in mind when inserting new rules into a chain.
• -D chain rule_num (or -D chain rule) deletes a rule in a chain; the first syntax identifies the
rule to be deleted by its number (iptables -L --line-numbers will display these num-
bers), while the latter identifies it by its contents.
• -F chain flushes a chain (deletes all its rules). For example, to delete all of the rules related
to outgoing packets, you would run iptables -F OUTPUT. If no chain is mentioned, all the
rules in the table are deleted.
• -P chain action defines the default action, or “policy” for a given chain; note that only stan-
dard chains can have such a policy. To drop all incoming traffic by default, you would run
iptables -P INPUT DROP.
Rules
Each rule is expressed as conditions -j action action_options. If several conditions are described
in the same rule, then the criterion is the conjunction (logical AND) of the conditions, which is at
least as restrictive as each individual condition.
The -p protocol condition matches the protocol field of the IP packet. The most common values
are tcp, udp, icmp, and icmpv6. This condition can be complemented with conditions on the TCP
ports, with clauses such as --source-port port and --destination-port port .
Chapter 7 — Securing and Monitoring Kali Linux 163
Negating Conditions Prefixing a condition with an exclamation mark negates the condition. For example,
negating a condition on the -p option matches “any packet with a different protocol
than the one specified.” This negation mechanism can be applied to all other condi-
tions as well.
The -s address or -s network/mask condition matches the source address of the packet. Corre-
spondingly, -d address or -d network/mask matches the destination address.
The -i interface condition selects packets coming from the given network interface. -o interface
selects packets going out on a specific interface.
The --state state condition matches the state of a packet in a connection (this requires the
ipt_conntrack kernel module, for connection tracking). The NEW state describes a packet start-
ing a new connection, ESTABLISHED matches packets belonging to an already existing connec-
tion, and RELATED matches packets initiating a new connection related to an existing one (which
is useful for the ftp-data connections in the “active” mode of the FTP protocol).
There are many available options for iptables and ip6tables and mastering them all requires
a great deal of study and experience. However, one of the options you will use most often is the
one to block malicious network traffic from a host or range of hosts. For example, to silently block
incoming traffic from the IP address 10.0.1.5 and the 31.13.74.0/24 class C subnet:
# iptables -A INPUT -s 10.0.1.5 -j DROP
# iptables -A INPUT -s 31.13.74.0/24 -j DROP
# iptables -n -L INPUT
Chain INPUT (policy ACCEPT)
target prot opt source destination
DROP all -- 10.0.1.5 0.0.0.0/0
DROP all -- 31.13.74.0/24 0.0.0.0/0
Another commonly-used iptables command is to permit network traffic for a specific service or
port. To allow users to connect to SSH, HTTP, and IMAP, you could run the following commands:
# iptables -A INPUT -m state --state NEW -p tcp --dport 22 -j ACCEPT
# iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
# iptables -A INPUT -m state --state NEW -p tcp --dport 143 -j ACCEPT
# iptables -n -L INPUT
Chain INPUT (policy ACCEPT)
target prot opt source destination
DROP all -- 10.0.1.5 0.0.0.0/0
DROP all -- 31.13.74.0/24 0.0.0.0/0
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:80
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:143
It is considered to be good computer hygiene to clean up old and unnecessary rules. The easiest
way to delete iptables rules is to reference the rules by line number, which you can retrieve with
164 Kali Linux Revealed
the --line-numbers option. Be wary though: dropping a rule will renumber all the rules appearing
further down in the chain.
# iptables -n -L INPUT --line-numbers
Chain INPUT (policy ACCEPT)
num target prot opt source destination
1 DROP all -- 10.0.1.5 0.0.0.0/0
2 DROP all -- 31.13.74.0/24 0.0.0.0/0
3 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22
4 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:80
5 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:143
# iptables -D INPUT 2
# iptables -D INPUT 1
# iptables -n -L INPUT --line-numbers
Chain INPUT (policy ACCEPT)
num target prot opt source destination
1 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22
2 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:80
3 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:143
There are more specific conditions, depending on the generic conditions described above. For
more information refer to manual pages for iptables(8) and ip6tables(8)
7.4.3. Creating Rules
Each rule creation requires one invocation of iptables or ip6tables. Typing these commands
manually can be tedious, so the calls are usually stored in a script so that the system is automati-
cally configured the same way every time the machine boots. This script can be written by hand
but it can also be interesting to prepare it with a high-level tool such as fwbuilder.
# apt install fwbuilder
The principle is simple. In the first step, describe all the elements that will be involved in the
actual rules:
• The firewall itself, with its network interfaces
• The networks, with their corresponding IP ranges
• The servers
• The ports belonging to the services hosted on the servers
Next, create the rules with simple drag-and-drop actions on the objects as shown in Figure 7.2,
“Fwbuilder’s Main Window” [page 166]. A few contextual menus can change the condition (negat-
ing it, for instance). Then the action needs to be chosen and configured.
As far as IPv6 is concerned, you can either create two distinct rulesets for IPv4 and IPv6, or create
only one and let fwbuilder translate the rules according to the addresses assigned to the objects.
Chapter 7 — Securing and Monitoring Kali Linux 165
Figure 7.2 Fwbuilder’s Main Window
fwbuilder will generate a script configuring the firewall according to the rules that you have
defined. Its modular architecture gives it the ability to generate scripts targeting different systems
including iptables for Linux, ipf for FreeBSD, and pf for OpenBSD.
7.4.4. Installing the Rules at Each Boot
In order to implement the firewall rules each time the machine is booted, you will need to register
the configuration script in an up directive of the /etc/network/interfaces file. In the following
example, the script is stored under /usr/local/etc/arrakis.fw (arrakis being the hostname of
the machine).
auto eth0
iface eth0 inet static
address 192.168.0.1
network 192.168.0.0
netmask 255.255.255.0
broadcast 192.168.0.255
up /usr/local/etc/arrakis.fw
166 Kali Linux Revealed
This example assumes that you are using ifupdown to configure the network interfaces. If you
are using something else (like NetworkManager or systemd-networkd), then refer to their respective
documentation to find out ways to execute a script after the interface has been brought up.
7.5. Monitoring and Logging
Data confidentiality and protection is an important aspect of security but it is equally important
to ensure availability of services. As an administrator and security practitioner, you must ensure
that everything works as expected, and it is your responsibility to detect anomalous behavior and
service degradation in a timely manner. Monitoring and logging software plays a key role in this
aspect of security, providing insight into what is happening on the system and the network.
In this section, we will review some tools that can be used to monitor several aspects of a Kali
system.
7.5.1. Monitoring Logs with logcheck
The logcheck program monitors log files every hour by default and sends unusual log messages
in emails to the administrator for further analysis.
The list of monitored files is stored in /etc/logcheck/logcheck.logfiles. The default values
work fine if the /etc/rsyslog.conf file has not been completely overhauled.
logcheck can report in various levels of detail: paranoid, server, and workstation. paranoid is very
verbose and should probably be restricted to specific servers such as firewalls. server is the default
mode and is recommended for most servers. workstation is obviously designed for workstations
and is extremely terse, filtering out more messages than the other options.
In all three cases, logcheck should probably be customized to exclude some extra messages (de-
pending on installed services), unless you really want to receive hourly batches of long unin-
teresting emails. Since the message selection mechanism is rather complex, /usr/share/doc/
logcheck-database/README.logcheck-database.gz is a required—if challenging—read.
The applied rules can be split into several types:
• those that qualify a message as a cracking attempt (stored in a file in the /etc/logcheck/
cracking.d/ directory);
• ignored cracking attempts (/etc/logcheck/cracking.ignore.d/);
• those classifying a message as a security alert (/etc/logcheck/violations.d/);
• ignored security alerts (/etc/logcheck/violations.ignore.d/);
• finally, those applying to the remaining messages (considered as system events).
Chapter 7 — Securing and Monitoring Kali Linux 167
ignore.d files are used to (obviously) ignore messages. For example, a message tagged as a cracking
attempt or a security alert (following a rule stored in a /etc/logcheck/violations.d/myfile
file) can only be ignored by a rule in a /etc/logcheck/violations.ignore.d/myfile or /etc/
logcheck/violations.ignore.d/myfile- extension file.
A system event is always signaled unless a rule in one of the /etc/logcheck/ignore.d.
{paranoid,server,workstation}/ directories states the event should be ignored. Of course, the
only directories taken into account are those corresponding to verbosity levels equal or greater
than the selected operation mode.
Curly Brackets {} in a The use of braces in a bash command, has many different functions. In the example
Command above, we are using it for a shorthand for repeating parts of the command. Bash then
will expand out the command before executing it.
In the example below, it will have have the same outcome, three files created in our
home directory.
# touch /home/kali/file1.txt /home/kali/file2.txt /home/kali/
å file3.txt
# touch /home/kali/file{1,2,3}.txt
7.5.2. Monitoring Activity in Real Time
top is an interactive tool that displays a list of currently running processes. The default sorting
is based on the current amount of processor use and can be obtained with the P key. Other sort
orders include a sort by occupied memory (M key), by total processor time (T key), and by process
identifier (N key). The k key kills a process by entering its process identifier. The r key changes
the priority of a process.
When the system seems to be overloaded, top is a great tool to see which processes are compet-
ing for processor time or consuming too much memory. In particular, it is often interesting to
check if the processes consuming resources match the real services that the machine is known to
host. An unknown process running as the ”www-data” user should really stand out and be inves-
tigated since it’s probably an instance of software installed and executed on the system through
a vulnerability in a web application.
top is a very flexible tool and its manual page gives details on how to customize its display and
adapt it to your personal needs and habits.
The xfce4-taskmanager graphical tool is similar to top and it provides roughly the same features.
For GNOME users there is gnome-system-monitor and for KDE users there is ksysguard which are
both similar as well.
168 Kali Linux Revealed
7.5.3. Detecting Changes
Once a system is installed and configured, most system files should stay relatively static until
the system is upgraded. Therefore, it is a good idea to monitor changes in system files since any
unexpected change could be cause for alarm and should be investigated. This section presents a
few of the most common tools used to monitor system files, detect changes, and optionally notify
you as the administrator of the system.
Auditing Packages with dpkg --verify
dpkg --verify (or dpkg -V) is an interesting tool since it displays the system files that have
been modified (potentially by an attacker), but this output should be taken with a grain of salt. To
do its job, dpkg relies on checksums stored in its own database which is stored on the hard disk
(found in /var/lib/dpkg/info/package.md5sums). A thorough attacker will therefore modify
these files so they contain the new checksums for the subverted files, or an advanced attacker
will compromise the package on your Debian mirror. To protect against this class of attack, use
APT’s digital signature verification system (see section 8.3.6, “Validating Package Authenticity”
[page 208]) to properly verify the packages.
What Is a File As a reminder: a fingerprint is a value, often a number (although in hexadecimal
Fingerprint? notation), that contains a kind of signature for the contents of a file. This signature
is calculated with an algorithm (MD5, SHA1, SHA256 being well-known examples)
that more or less guarantees that even the tiniest change in the file contents will
result in a change of the fingerprint; this is known as the “avalanche effect”. A simple
numerical fingerprint then serves as a litmus test to check whether the contents of a
file have been altered. These algorithms are not reversible; in other words, for most of
them, knowing a fingerprint doesn’t allow finding the corresponding contents. Recent
mathematical advances seem to weaken the absoluteness of these principles but their
use is not called into question so far, since creating different contents yielding the
same fingerprint still seems to be quite a difficult task.
Running dpkg -V will verify all installed packages and will print out a line for each file that fails
verification. Each character denotes a test on some specific meta-data. Unfortunately, dpkg does
not store the meta-data needed for most tests and will thus output question marks for them. Cur-
rently only the checksum test can yield a 5 on the third character (when it fails).
# dpkg -V
??5?????? /lib/systemd/system/ssh.service
??5?????? c /etc/libvirt/qemu/networks/default.xml
??5?????? c /etc/lvm/lvm.conf
??5?????? c /etc/salt/roster
In the example above, dpkg reports a change to SSH’s service file that the administrator made to
the packaged file instead of using an appropriate /etc/systemd/system/ssh.service override
Chapter 7 — Securing and Monitoring Kali Linux 169
(which would be stored below /etc like any configuration change should be). It also lists multiple
configuration files (identified by the “c” letter on the second field) that had been legitimately
modified.
Monitoring Files: AIDE
The Advanced Intrusion Detection Environment (AIDE) tool checks file integrity and detects any
change against a previously-recorded image of the valid system. The image is stored as a database
(/var/lib/aide/aide.db) containing the relevant information on all files of the system (finger-
prints, permissions, timestamps, and so on).
You can install AIDE by running apt update followed by apt install aide. You will first initial-
ize the database with aideinit; it will then run daily (via the /etc/cron.daily/aide script) to
check that nothing relevant changed. When changes are detected, AIDE records them in log files
(/var/log/aide/*.log) and sends its findings to the administrator by email.
Protecting the Database Since AIDE uses a local database to compare the states of the files, the validity of
its results is directly linked to the validity of the database. If an attacker gets root
permissions on a compromised system, they will be able to replace the database and
cover their tracks. One way to prevent this subversion is to store the reference data
on read-only storage media.
You can use options in /etc/default/aide to tweak the behavior of the aide package. The
AIDE configuration proper is stored in /etc/aide/aide.conf and /etc/aide/aide.conf.d/ (ac-
tually, these files are only used by update-aide.conf to generate /var/lib/aide/aide.conf.
autogenerated). The configuration indicates which properties of which files need to be checked.
For instance, the contents of log files changes routinely, and such changes can be ignored as long
as the permissions of these files stay the same, but both contents and permissions of executable
programs must be constant. Although not very complex, the configuration syntax is not fully
intuitive and we recommend reading the aide.conf(5) manual page for more details.
A new version of the database is generated daily in /var/lib/aide/aide.db.new; if all recorded
changes were legitimate, it can be used to replace the reference database.
Tripwire is very similar to AIDE; even the configuration file syntax is almost the same. The main
addition provided by tripwire is a mechanism to sign the configuration file so that an attacker
cannot make it point at a different version of the reference database.
Samhain also offers similar features as well as some functions to help detect rootkits (see “The
checksecurity and chkrootkit/rkhunter packages” [page 171] below). It can also be deployed globally
on a network and record its traces on a central server (with a signature).
170 Kali Linux Revealed
The checksecurity and checksecurity consists of several small scripts that perform basic checks on the system
chkrootkit/rkhunter (searching for empty passwords, new setuid files, and so on) and warn you if these
packages conditions are detected. Despite its explicit name, you should not rely solely on it to
make sure a Linux system is secure.
The chkrootkit and rkhunter packages detect certain rootkits potentially installed on
the system. As a reminder, these are pieces of software designed to hide the compro-
mise of a system while discreetly keeping control of the machine. The tests are not
100 percent reliable but they can usually draw your attention to potential problems.
7.6. Summary
In this chapter, we took a look at the concept of security policies, highlighting various points to
consider when defining such a policy and outlining some of the threats to your system and to you
personally, as a security professional. We discussed desktop and laptop security measures as well
as firewalls and packet filtering. Finally, we reviewed monitoring tools and strategies and showed
how to best implement them to detect potential threats to your system.
Summary Tips:
• Take time to define a comprehensive security policy.
• Real risk often arises when you travel from one customer to the next. For example, your lap-
top could be stolen while traveling or seized by customs. Prepare for these unfortunate pos-
sibilities by using full disk encryption (see section 4.2.2, “Installation on a Fully Encrypted
File System” [page 88]) and consider the nuke feature (see “Adding a Nuke Password for
Extra Safety” [page 250]) to protect your clients data.
• Disable services that you do not use. Kali makes it easy to do this since all external network
services are disabled by default.
• If you are running Kali on a publicly accessible server, change any default passwords for
services that might be configured (see section 7.3, “Securing Network Services” [page 159])
and restrict their access with a firewall (see section 7.4, “Firewall or Packet Filtering” [page
159]) prior to launching them.
• Use fail2ban to detect and block password-guessing attacks and remote brute force password
attacks.
• If you run web services, host them over HTTPS to prevent network intermediaries from
sniffing your traffic (which might include authentication cookies).
• The Linux kernel embeds the netfilter firewall. There is no turn-key solution for configuring
any firewall, since network and user requirements differ. However, you can control netfilter
from user space with the iptables and ip6tables commands.
• Implement firewall rules (see section 7.4, “Firewall or Packet Filtering” [page 159]) to forbid
all outbound traffic except the traffic generated by your VPN access. This is meant as a safety
Chapter 7 — Securing and Monitoring Kali Linux 171
net, so that when the VPN is down you immediately notice it (instead of falling back to the
local network access).
• top is an interactive tool that displays a list of currently running processes.
• The logcheck program monitors log files every hour by default and sends unusual log mes-
sages in emails to the administrator for further analysis.
• dpkg --verify (or dpkg -V) displays the system files that have been modified (potentially
by an attacker), but relies on checksums, which may be subverted by a clever attacker.
• The Advanced Intrusion Detection Environment (AIDE) tool checks file integrity and detects
any changes against a previously-recorded image of the valid system.
• Tripwire is very similar to AIDE but uses a mechanism to sign the configuration file, so that
an attacker cannot make it point at a different version of the reference database.
• Consider the use of rkhunter, checksecurity, and chkrootkit to help detect rootkits on
your system.
In the next chapter, we are going to dig into Debian fundamentals (chapter 8, “Debian Package
Management” [page 176]) and package management. You will quickly understand the power be-
hind Kali’s Debian roots and learn how the developers have harnessed that power. Be warned,
the next chapter is fairly dense, but it is critical that you understand Debian basics and package
management if you are going to be a Kali power user.
172 Kali Linux Revealed
Keywords
dpkg
apt
sources.list
Upgrades
Package repositories
Chapter
Debian Package 8
Management
Contents
Introduction to APT 176 Basic Package Interaction 181 Advanced APT Configuration and Usage 200
APT Package Reference: Digging Deeper into the Debian Package System 210 Summary 222
After the basics of Linux, it is time to learn the package management system of a Debian-based
distribution. In such distributions, including Kali, the Debian package is the canonical way to
make software available to end-users. Understanding the package management system will give
you a great deal of insight into how Kali is structured, enable you to more effectively troubleshoot
issues, and help you quickly locate help and documentation for the wide array of tools and utilities
included in Kali Linux.
In this chapter, we will introduce the Debian package management system and introduce dpkg and
the APT suite of tools. One of the primary strengths of Kali Linux lies in the flexibility of its package
management system, which leverages these tools to provide near-seamless installation, upgrades,
removal, and manipulation of application software, and even of the base operating system itself.
It is critical that you understand how this system works to get the most out of Kali and streamline
your efforts. The days of painful compilations, disastrous upgrades, debugging gcc, make, and
configure problems are long gone, however, the number of available applications has exploded
and you need to understand the tools designed to take advantage of them. This is also a critical
skill because there are a number of security tools that, due to licensing or other issues, cannot be
included in Kali but have Debian packages available for download. It is important that you know
how to process and install these packages and how they impact the system, especially when things
do not go as expected.
We will begin with some basic overviews of APT, describe the structure and contents of binary and
source packages, take a look at some basic tools and scenarios, and then dig deeper to help you
wring every ounce of utility from this spectacular package system and suite of tools.
8.1. Introduction to APT
Let’s begin with some basic definitions, an overview, and some history about Debian packages,
starting with dpkg and APT.
8.1.1. Relationship between APT and dpkg
A Debian package is a compressed archive of a software application. A binary package (a .deb file)
contains files that can be directly used (such as programs or documentation), while a source pack-
age contains the source code for the software and the instructions required for building a binary
package. A Debian package contains the application’s files as well as other metadata including the
names of the dependencies the application needs, as well as scripts that enable the execution of
commands at different stages in the package’s lifecycle (installation, upgrades, and removal).
The dpkg tool was designed to process and install .deb packages, but if it encountered an unsat-
isfied dependency (like a missing library) that would prevent the package from installing, dpkg
would simply list the missing dependency, because it had no awareness or built-in logic to find or
process the packages that might satisfy those dependencies. The Advanced Package Tool (APT),
176 Kali Linux Revealed
including apt and apt-get, were designed to address these shortcomings and could automatically
resolve these issues. We will talk about both dpkg and the APT tools in this chapter.
The base command for handling Debian packages on the system is dpkg, which performs installa-
tion or analysis of .deb packages and their contents. However, dpkg has only a partial view of the
Debian universe: it knows what is installed on the system and whatever you provide on the com-
mand line, but knows nothing of the other available packages. As such, it will fail if a dependency
is not met. APT addresses the limitations.
APT is a set of tools that help manage Debian packages, or applications on your Debian system. You
can use APT to install and remove applications, update packages, and even upgrade your entire
system. The magic of APT lies in the fact that it is a complete package management system that
will not only install or remove a package, but will consider the requirements and dependencies of
the packaged application (and even their requirements and dependencies) and attempt to satisfy
them automatically. APT relies on dpkg but APT differs from dpkg, as the former installs the latest
package from an online source and works to resolve dependencies while dpkg installs a package
located on your local system and does not automatically resolve dependencies.
If you have been around long enough to remember compiling programs with gcc (even with the
help of utilities such as make and configure), you likely remember that it was a painful process,
especially if the application had several dependencies. By deciphering the various warnings and
error messages, you may have been able to determine which part of the code was failing and
most often that failure was due to a missing library or other dependency. You would then track
down that missing library or dependency, correct it, and try again. Then, if you were lucky, the
compile would complete, but often the build would fail again, complaining about another broken
dependency.
APT was designed to help alleviate that problem, collate program requirements and dependencies,
and resolve them. This functionality works out-of-the-box on Kali Linux, but it isn’t foolproof. It
is important that you understand how Debian and Kali’s packaging system works because you will
need to install packages, update software, or troubleshoot problems with packages. You will use
APT in your day-to-day work with Kali Linux and in this chapter, we will introduce you to APT
and show you how to install, remove, upgrade, and manage packages, and even show you how to
move packages between different Linux distributions. We will also talk about graphical tools that
leverage APT, show you how to validate the authenticity of packages, and delve into the concept
of a rolling distribution, a technique that brings daily updates to your Kali system.
Before we dig in and show you how to use dpkg and APT to install and manage packages, it is
important that we delve into some of the inner workings of APT and discuss some terminology
surrounding it.
Package Source and The word source can be ambiguous. A source package—a package containing the
Source Package source code of a program—should not be confused with a package source—a repos-
itory (website, FTP server, CD/DVD-ROM, local directory, etc.) that contains pack-
ages.
Chapter 8 — Debian Package Management 177
APT retrieves its packages from a repository, a package storage system or simply, ”package
source”. The /etc/apt/sources.list file lists the different repositories (or sources) that pub-
lish Debian packages.
8.1.2. Understanding the sources.list File
The sources.list file is the key configuration file for defining package sources, and it is impor-
tant to understand how it is laid out and how to configure it since APT will not function without a
properly defined list of package sources. Let’s discuss its syntax, take a look at the various repos-
itories that are used by Kali Linux, and discuss mirrors and mirror redirection, then you will be
ready to put APT to use.
Each active line of the /etc/apt/sources.list file (and of the /etc/apt/sources.list.d/*.
list files) contains the description of a source, made of three parts separated by spaces. Com-
mented lines begin with a # character:
#deb cdrom:[Debian GNU/Linux 2020.3 _Kali-rolling_ - Official Snapshot amd64 LIVE/
å INSTALL Binary 20200728-20:25]/ kali-last-snapshot contrib main non-free
deb http://http.kali.org/kali kali-rolling main non-free contrib
Let’s take a look at the syntax of this file. The first field indicates the source type:
• deb for binary packages,
• deb-src for source packages.
The second field gives the base URL of the source: this can consist of a Kali mirror or any other
package archive set up by a third party. The URL can start with file:// to indicate a local source
installed in the system’s file hierarchy, with http:// to indicate a source accessible from a web
server, or with ftp:// for a source available on an FTP server. The URL can also start with cdrom:
for CD/DVD-ROM/Blu-ray disc-based installations, although this is less frequent since network-
based installation methods are more and more common.
The cdrom entries describe the CD/DVD-ROM you have. Contrary to other entries, a CD/DVD-ROM
is not always available, since it has to be inserted into the drive and usually only one disc can be
read at a time. For those reasons, these sources are managed in a slightly different way and need to
be added with the apt-cdrom program, usually executed with the add parameter. The latter will
then request the disc to be inserted in the drive and will browse its contents looking for Packages
files. It will use these files to update its database of available packages (this operation is usually
done by the apt update command). After that, APT will request the disc if it needs a package
stored on it.
The syntax of the last field depends on the structure of the repository. In the simplest cases, you
can easily indicate a subdirectory (with a required trailing slash) of the desired source (this is often
a simple “./”, which refers to the absence of a subdirectory—the packages are then directly at the
178 Kali Linux Revealed
specified URL). But in the most common case, the repositories will be structured like a Kali mirror,
with multiple distributions each having multiple components. In those cases, name the chosen
distribution, then the components (or sections) to enable. Let’s take a moment to introduce these
sections.
Debian and Kali use three sections to differentiate packages according to the licenses chosen by
the authors of each work. A difference between Debian and Kali is that, Debian only has main
enabled by default, whereas Kali has all three enabled by default.
main contains all packages that fully comply with the Debian Free Software Guidelines1 .
The non-free archive is different because it contains software that does not (entirely) conform to
these principles but which can nevertheless be distributed without restrictions.
contrib (contributions) is a set of open source software that cannot function without some non-
free elements. These elements may include software from the non-free section or non-free files
such as game ROMs, BIOS of consoles, etc. contrib also includes free software whose compilation
requires proprietary elements, such as VirtualBox, which requires a non-free compiler to build
some of its files.
Now, let’s take a look at the standard Kali Linux package sources, or repositories.
8.1.3. Kali Repositories
A standard sources.list file for a system running Kali Linux refers to one repository (kali-rolling)
and the three previously mentioned components: main, contrib, and non-free:
# Main Kali repository
deb http://http.kali.org/kali kali-rolling main contrib non-free
Let’s take a look at the Kali repositories.
The Kali-Rolling Repository
This is the main repository for end-users. It should always contain installable and recent packages.
It is managed by a tool that merges Debian Testing and the Kali-specific packages in a way that
ensures that each package’s dependencies can be satisfied within kali-rolling. In other words,
barring any bug in maintainer scripts, all packages should be installable.
Since Debian Testing evolves daily, so does Kali Rolling. The Kali-specific packages are also regu-
larly updated as we monitor the upstream releases of the most important packages.
1
https://www.debian.org/social_contract#guidelines
Chapter 8 — Debian Package Management 179
The Kali-Dev Repository
This repository is not for public use. It is a space where Kali developers resolve dependency prob-
lems arising from the merge of the Kali-specific packages into Debian Testing.
It is also the place where updated packages land first, so if you need an update that was released
recently and that has not yet reached kali-rolling, you might be able to grab it from this repository.
This is not recommended for regular users.
The Kali Linux Mirrors
The sources.list extracts above refer to http.kali.org: this is a server running MirrorBrain2 ,
which will redirect your HTTP requests to an official mirror close to you. MirrorBrain monitors
each mirror to ensure that they are working and up-to-date; it will always redirect you to a good
mirror.
Debugging a Mirror If you have a problem with the mirror (for instance because apt update fails), you
Redirection can use curl -sI to see where you are being redirected:
$ curl -sI http://http.kali.org/README
HTTP/1.1 302 Found
Date: Wed, 27 Jan 2021 05:04:09 GMT
Server: Apache/2.4.10 (Debian)
X-MirrorBrain-Mirror: kali.download
X-MirrorBrain-Realm: country
Link: <http://http.kali.org/README.meta4>; rel=describedby;
å type=”application/metalink4+xml”
Link: <http://kali.download/kali/README>; rel=duplicate; pri
å =1; geo=us
Link: <http://mirrors.ocf.berkeley.edu/kali/README>; rel=
å duplicate; pri=2; geo=us
Link: <http://mirror.anquan.cl/kali/README>; rel=duplicate;
å pri=3; geo=cl
Link: <http://ftp.free.fr/pub/kali/README>; rel=duplicate;
å pri=4; geo=fr
Link: <http://archive-4.kali.org/kali/README>; rel=duplicate;
å pri=5; geo=fr
Location: http://kali.download/kali/README
Content-Type: text/html; charset=iso-8859-1
If the problem persists, you can edit /etc/apt/sources.list and hardcode the
name of another known working mirror in place of (or before) the http.kali.org
entry.
2
http://mirrorbrain.org/
180 Kali Linux Revealed
We also have a second MirrorBrain instance: where http.kali.org hosts the package repositories,
cdimage.kali.org hosts the released ISO images.
è http://cdimage.kali.org/
If you want to request a list of official Kali Linux Mirrors, you can add .mirrorlist to any valid URL
pointing to http.kali.org or cdimage.kali.org.
è http://http.kali.org/README.mirrorlist
è http://cdimage.kali.org/README.mirrorlist
These lists are not exhaustive due to some MirrorBrain limitations (most notably mirrors re-
stricted to some countries do not appear in the list unless you are in the given country). But
they contain the best mirrors: they are well maintained and have large amounts of bandwidth
available.
8.2. Basic Package Interaction
Armed with a basic understanding of the APT landscape, let’s take a look at some basic package
interactions including the initialization of APT; installation, removal, and purging of packages;
and upgrading of the Kali Linux system. Then let’s venture from the command line to take a look
at some graphical APT tools.
8.2.1. Initializing APT
APT is a vast project and tool set, whose original plans included a graphical interface. From a client
perspective, it is centered around the command-line tool apt-get as well as apt, which was later
developed to overcome design flaws of apt-get.
There are graphical alternatives developed by third parties, including synaptic and aptitude,
which we will discuss later. We tend to prefer apt, which we use in the examples that follow. We
will, however, detail some of the major syntax differences between tools, as they arise.
When working with APT, you should first download the list of currently available packages with
apt update. Depending on the speed of your connection, this can take some time because various
packages’ list, sources’ list and translation files have grown in size alongside Kali development. Of
course, CD/DVD-ROM installation sets install much more quickly, because they are local to your
machine.
Chapter 8 — Debian Package Management 181
8.2.2. Installing Packages
Thanks to the thoughtful design of the Debian package system, you can install packages, with or
without their dependencies, fairly easily. Let’s take a look at package installation with dpkg and
apt.
Installing Packages with dpkg
dpkg is the core tool that you will use (either directly or indirectly through APT) when you need
to install a package. It is also a go-to choice if you are operating offline, since it doesn’t require an
Internet connection. Remember, dpkg will not install any dependencies that the package might
require. To install a package with dpkg, simply provide the -i or --install option and the path to
the .deb. This implies that you have previously downloaded (or obtained in some other way) the
.deb file of the package to install.
# dpkg -i man-db_2.9.3-2_amd64.deb
(Reading database ... 309317 files and directories currently installed.)
Preparing to unpack man-db_2.9.3-2_amd64.deb ...
Unpacking man-db (2.9.3-2) over (2.9.3-2) ...
Setting up man-db (2.9.3-2) ...
Updating database of manual pages ...
man-db.service is a disabled or a static unit not running, not starting it.
Processing triggers for kali-menu (2020.4.0) ...
Processing triggers for mime-support (3.64) ...
We can see the different steps performed by dpkg and can see at what point any error may have
occurred. The -i or --install option performs two steps automatically: it unpacks the package and
runs the configuration scripts. You can perform these two steps independently (as apt does behind
the scenes) with the --unpack and --configure options, respectively:
# dpkg --unpack man-db_2.9.3-2_amd64.deb
(Reading database ... 309317 files and directories currently installed.)
Preparing to unpack man-db_2.9.3-2_amd64.deb ...
Unpacking man-db (2.9.3-2) over (2.9.3-2) ...
Processing triggers for kali-menu (2020.4.0) ...
Processing triggers for mime-support (3.64) ...
# dpkg --configure man-db
Setting up man-db (2.9.3-2) ...
Updating database of manual pages ...
Note that the “Processing triggers” lines refer to code that is automatically executed whenever a
package adds, removes, or modifies files in some monitored directories. For instance, the mime-
support package monitors /usr/lib/mime/packages and executes the update-mime command
182 Kali Linux Revealed
whenever something changes in that directory (like /usr/lib/mime/packages/man-db in the
specific case of man-db).
Sometimes dpkg will fail to install a package and return an error. However, you can order
dpkg to ignore this and only issue a warning with various --force-* options. Issuing the dpkg
--force-help command will display a complete list of these options. For example, you can use
dpkg to forcibly install zsh:
$ dpkg -i --force-overwrite zsh_5.8-5+b1_amd64.deb
A frequent error, which you are bound to encounter sooner or later, is a file collision. When a
package contains a file that is already installed by another package, dpkg will refuse to install it.
The following types of messages will then appear:
Unpacking libgdm (from .../libgdm_3.8.3-2_amd64.deb) ...
dpkg: error processing /var/cache/apt/archives/libgdm_3.8.3-2_amd64.deb (--unpack):
å trying to overwrite ’/usr/bin/gdmflexiserver’, which is also in package gdm3
å 3.4.1-9
In this case, if you think that replacing this file is not a significant risk to the stability of your
system (which is usually the case), you can use --force-overwrite to overwrite the file.
While there are many available --force-* options, only --force-overwrite is likely to be used regu-
larly. These options exist for exceptional situations, and it is better to leave them alone as much as
possible in order to respect the rules imposed by the packaging mechanism. Do not forget, these
rules ensure the consistency and stability of your system.
Installing Packages with APT
Although APT is much more advanced than dpkg and does a lot more behind the scenes, you will
find that interacting with packages is quite simple. You can add a package to the system with a
simple apt install package. APT will automatically install the necessary dependencies:
# apt install kali-tools-gpu
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
oclgausscrack truecrack
The following NEW packages will be installed:
Need to get 2,602 kB of archives.
After this operation, 2,898 kB of additional disk space will be used.
Get:1 http://kali.download/kali kali-rolling/main amd64 oclgausscrack amd64 1.3-1kali3
å [30.7 kB]
Get:2 http://kali.download/kali kali-rolling/main amd64 truecrack amd64 3.6+git20150326
å -0kali1 [2,558 kB]
Chapter 8 — Debian Package Management 183
Get:3 http://kali.download/kali kali-rolling/main amd64 kali-tools-gpu amd64 2021.1.0
å [12.6 kB]
Fetched 2,602 kB in 1s (2,645 kB/s)
Selecting previously unselected package oclgausscrack.
(Reading database ... 108127 files and directories currently installed.)
Preparing to unpack .../oclgausscrack_1.3-1kali3_amd64.deb ...
Unpacking oclgausscrack (1.3-1kali3) ...
Selecting previously unselected package truecrack.
Preparing to unpack .../truecrack_3.6+git20150326-0kali1_amd64.deb ...
Unpacking truecrack (3.6+git20150326-0kali1) ...
Selecting previously unselected package kali-tools-gpu.
Preparing to unpack .../kali-tools-gpu_2021.1.0_amd64.deb ...
Unpacking kali-tools-gpu (2021.1.0) ...
Setting up oclgausscrack (1.3-1kali3) ...
Setting up truecrack (3.6+git20150326-0kali1) ...
Setting up kali-tools-gpu (2021.1.0) ...
Processing triggers for man-db (2.9.3-2) ...
Processing triggers for kali-menu (2020.4.0) ...
You can also use apt-get install package, or aptitude install package. For simple pack-
age installation, they do essentially the same thing. As you will see later, the differences are more
meaningful for upgrades or when dependencies resolution do not have any perfect solution.
If sources.list lists several distributions, you can specify the package version with apt install
package=version, but indicating its distribution of origin (kali-rolling or kali-dev) with apt
install package/distribution is usually preferred.
# apt install zsh=5.7.1-1
# apt install zsh/kali-dev
As with dpkg, you can also instruct apt to forcibly install a package and overwrite files with --force-
overwrite, but the syntax is a bit strange since you are passing the argument through to dpkg:
# apt -o Dpkg::Options::=”--force-overwrite” install zsh
8.2.3. Upgrading Kali Linux
As a rolling distribution, Kali Linux has spectacular upgrade capabilities. In this section, we will
take a look at how simple it is to upgrade Kali, and we will discuss strategies for planning your
updates.
We recommend regular upgrades, because they will install the latest security updates. To upgrade,
use apt update followed by either apt upgrade, apt-get upgrade, or aptitude safe-upgrade.
These commands look for installed packages that can be upgraded without removing any packages.
In other words, the goal is to ensure the least intrusive upgrade possible. The apt-get command
184 Kali Linux Revealed
line tool is slightly more demanding than aptitude or apt because it will refuse to install packages
that were not installed beforehand.
The apt tool will generally select the most recent version number.
To tell apt to use a specific distribution when searching for upgraded packages, you need to use
the -t or --target-release option, followed by the name of the distribution you want (for example:
apt -t kali-rolling upgrade). To avoid specifying this option every time you use apt, you
can add APT::Default-Release ”kali-rolling”; in the file /etc/apt/apt.conf.d/local.
For more important upgrades, such as major version upgrades, use apt full-upgrade. With this
instruction, apt will complete the upgrade even if it has to remove some obsolete packages or
install new dependencies. This is also the command that you should use for regular upgrades of
your Kali Rolling system. It is so simple that it hardly needs explanation: APT’s reputation is based
on this great functionality.
Unlike apt and aptitude, apt-get doesn’t know the full-upgrade command. Instead, you
should use apt-get dist-upgrade (distribution upgrade), a well-known command that apt and
aptitude also accept for backwards compatibility.
Be Aware of Important To anticipate some of these problems, you can install the apt-listchanges package,
Changes which displays information about possible problems at the beginning of a pack-
age upgrade. This information is compiled by the package maintainers and put in
/usr/share/doc/package/NEWS.Debian files for your benefit. Reading these files
(possibly through apt-listchanges) should help you avoid nasty surprises.
Since becoming a rolling distribution, Kali can receive upgrades several times a day. However,
that might not be the best strategy. So, how often should you upgrade Kali Linux? There is no
hard rule but there are some guidelines that can help you. You should upgrade:
• When you are aware of a security issue that is fixed in an update.
• When you suspect that an updated version might fix a bug that you are experiencing.
• Before reporting a bug to make sure it is still present in the latest version that you have
available.
• Often enough to get the security fixes that you have not heard about.
There are also cases where it is best to not upgrade. For example, it might not be a good idea to
upgrade:
• If you can’t afford any breakage (for example, because you go offline, or because you are
about to give a presentation with your computer); it is best to do the upgrade later, when
you have enough time to troubleshoot any issue introduced in the process.
• If a disruptive change happened recently (or is still ongoing) and you fear that all issues
have not yet been discovered. For example, when a new GNOME version is released, not all
Chapter 8 — Debian Package Management 185
packages are updated at the same time and you are likely to have a mix of packages with
the old version and the new version. Most of the time this is fine and it helps everybody to
release those updates progressively, but there are always exceptions and some applications
might be broken due to such discrepancies.
• If the apt full-upgrade output tells you that it will remove packages that you consider
important for your work. In those cases, you want to review the situation and try to under-
stand why apt wants to remove them. Maybe the packages are currently broken and in this
case you might want to wait until fixed versions are available, or they have been obsoleted
and you should identify their replacements and then proceed with the full upgrade anyway.
In general, we recommend that you upgrade Kali at least once a week. You can certainly upgrade
daily but it doesn’t make sense to do it more often than that. Even if our mirrors are synchronized
four times a day, the updates coming from Debian usually land only once a day.
8.2.4. Removing and Purging Packages
Removing a package is even simpler than installing one. Let’s take a look at how to remove a
package with dpkg and apt.
To remove a package with dpkg, supply the -r or --remove option, followed by the name of a pack-
age. This removal is not, however, complete: all of the configuration files, maintainer scripts, log
files (system logs), data generated by the service (such as the content of an LDAP server directory
or the content of a database for an SQL server), and most other user data handled by the pack-
age remain intact. The remove option makes it easy to uninstall a program and later re-install it
with the same configuration. Also remember that dependencies are not removed. Consider this
example:
# dpkg --remove kali-tools-gpu
(Reading database ... 108172 files and directories currently installed.)
Removing kali-tools-gpu (2021.1.0) ...
You can also remove packages from the system with apt remove package. APT will automatically
delete the packages that depend on the package that is being removed. Like the dpkg example,
configuration files and user data will not be removed.
Through the addition of suffixes to package names, you can use apt (or apt-get and aptitude)
to install certain packages and remove others on the same command line. With an apt install
command, add “-” to the names of the packages you wish to remove. With an apt remove com-
mand, add “+” to the names of the packages you wish to install.
The next example shows two different ways to install package1 and to remove package2.
# apt install package1 package2-
[...]
# apt remove package1+ package2
186 Kali Linux Revealed
[...]
This can also be used to exclude packages that would otherwise be installed, for example due to
a Recommends (discussed later in section 8.4.1.3, “Recommends, Suggests, and Enhances Fields”
[page 214]). In general, the dependency solver will use that information as a hint to look for alter-
native solutions.
To remove all data associated with a package, you can purge the package with the dpkg -P
package, or apt purge package commands. This will completely remove the package and all
user data, and in the case of apt, will delete dependencies as well.
# dpkg -r debian-cd
(Reading database ... 333606 files and directories currently installed.)
Removing debian-cd (3.1.32) ...
# dpkg -P debian-cd
(Reading database ... 332950 files and directories currently installed.)
Removing debian-cd (3.1.32) ...
Purging configuration files for debian-cd (3.1.32) ...
Warning! Given the definitive nature of purge, do not execute it lightly. You will lose everything
associated with that package.
8.2.5. Inspecting Packages
Next, let’s take a look at some of the tools that can be used to inspect Debian packages. We will
learn of dpkg, apt, and apt-cache commands that can be used to query and visualize the package
database.
Querying dpkg’s Database and Inspecting .deb Files
We will begin with several dpkg options that query the internal dpkg database. This database
resides on the filesystem at /var/lib/dpkg and contains multiple sections including configura-
tion scripts (/var/lib/dpkg/info), a list of files the package installed (/var/lib/dpkg/info/*.
list), and the status of each package that has been installed (/var/lib/dpkg/status). You can
use dpkg to interact with the files in this database. Note that most options are available in a long
version (one or more relevant words, preceded by a double dash) and a short version (a single
letter, often the initial of one word from the long version, and preceded by a single dash). This
convention is so common that it is a POSIX standard.
First, let’s take a look at --listfiles package (or -L), which lists the files that were installed by the
specified package:
$ dpkg -L base-passwd
/.
Chapter 8 — Debian Package Management 187
/usr
/usr/sbin
/usr/sbin/update-passwd
/usr/share
/usr/share/lintian
/usr/share/lintian/overrides
/usr/share/lintian/overrides/base-passwd
/usr/share/doc-base
/usr/share/doc-base/users-and-groups
/usr/share/base-passwd
/usr/share/base-passwd/group.master
/usr/share/base-passwd/passwd.master
/usr/share/man
/usr/share/man/pl
/usr/share/man/pl/man8
/usr/share/man/pl/man8/update-passwd.8.gz
[...]
/usr/share/doc
/usr/share/doc/base-passwd
/usr/share/doc/base-passwd/users-and-groups.txt.gz
/usr/share/doc/base-passwd/changelog.gz
/usr/share/doc/base-passwd/copyright
/usr/share/doc/base-passwd/README
/usr/share/doc/base-passwd/users-and-groups.html
Next, dpkg --search file (or -S), finds any packages containing the file or path passed in the
argument. For example, to find the package containing /bin/date:
$ dpkg -S /bin/date
coreutils: /bin/date
The dpkg --status package (or -s) command displays the headers of an installed package. For
example, to search the headers for the coreutils package:
$ dpkg -s coreutils
Package: coreutils
Status: install ok installed
Priority: required
Section: utils
Installed-Size: 13855
Maintainer: Michael Stone <mstone@debian.org>
Architecture: amd64
Multi-Arch: foreign
Version: 8.23-3
Replaces: mktemp, realpath, timeout
188 Kali Linux Revealed
Pre-Depends: libacl1 (>= 2.2.51-8), libattr1 (>= 1:2.4.46-8), libc6 (>= 2.17),
å libselinux1 (>= 2.1.13)
Conflicts: timeout
Description: GNU core utilities
This package contains the basic file, shell and text manipulation
utilities which are expected to exist on every operating system.
.
Specifically, this package includes:
arch base64 basename cat chcon chgrp chmod chown chroot cksum comm cp
csplit cut date dd df dir dircolors dirname du echo env expand expr
factor false flock fmt fold groups head hostid id install join link ln
logname ls md5sum mkdir mkfifo mknod mktemp mv nice nl nohup nproc numfmt
od paste pathchk pinky pr printenv printf ptx pwd readlink realpath rm
rmdir runcon sha*sum seq shred sleep sort split stat stty sum sync tac
tail tee test timeout touch tr true truncate tsort tty uname unexpand
uniq unlink users vdir wc who whoami yes
Homepage: http://gnu.org/software/coreutils
The dpkg --list (or -l) command displays the list of packages known to the system and their
installation status. You can also use grep on the output to search for certain fields, or provide
wildcards (such as b*) to search for packages that match a particular partial search string. This
will show a summary of the packages. For example, to show a summary list of all packages that
start with ’b’:
$ dpkg -l ’b*’
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-=========================-==========================-============-=====================
ii backdoor-factory 3.4.2+dfsg-4 all Patch 32/64 bits ELF & win32/64 binaries with shellcode
un backupninja <none> <none> (no description available)
un backuppc <none> <none> (no description available)
un balsa <none> <none> (no description available)
un base <none> <none> (no description available)
[...]
The dpkg --contents file.deb (or -c) command lists all the files in a particular .deb file:
$ dpkg -c /var/cache/apt/archives/gpg_2.2.20-1_amd64.deb
drwxr-xr-x root/root 0 2020-03-23 15:05 ./
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/bin/
-rwxr-xr-x root/root 1062832 2020-03-23 15:05 ./usr/bin/gpg
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/doc/
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/doc/gpg/
-rw-r--r-- root/root 677 2020-03-23 14:45 ./usr/share/doc/gpg/NEWS.Debian.gz
-rw-r--r-- root/root 24353 2020-03-23 15:05 ./usr/share/doc/gpg/changelog.Debian.gz
-rw-r--r-- root/root 384091 2020-03-20 11:38 ./usr/share/doc/gpg/changelog.gz
Chapter 8 — Debian Package Management 189
-rw-r--r-- root/root 10555 2020-03-23 15:04 ./usr/share/doc/gpg/copyright
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/man/
drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/man/man1/
-rw-r--r-- root/root 46139 2020-03-23 15:05 ./usr/share/man/man1/gpg.1.gz
The dpkg --info file.deb (or -I) command displays the headers of the specified .deb file:
$ dpkg -I /var/cache/apt/archives/gpg_2.2.20-1_amd64.deb
new Debian package, version 2.0.
size 894224 bytes: control archive=1160 bytes.
1219 bytes, 25 lines control
374 bytes, 6 lines md5sums
Package: gpg
Source: gnupg2
Version: 2.2.20-1
Architecture: amd64
Maintainer: Debian GnuPG Maintainers <pkg-gnupg-maint@lists.alioth.debian.org>
Installed-Size: 1505
Depends: gpgconf (= 2.2.20-1), libassuan0 (>= 2.5.0), libbz2-1.0, libc6 (>= 2.25),
å libgcrypt20 (>= 1.8.0), libgpg-error0 (>= 1.35), libreadline8 (>= 6.0),
å libsqlite3-0 (>= 3.7.15), zlib1g (>= 1:1.1.4)
Recommends: gnupg (= 2.2.20-1)
Breaks: gnupg (<< 2.1.21-4)
Replaces: gnupg (<< 2.1.21-4)
Section: utils
Priority: optional
Multi-Arch: foreign
Homepage: https://www.gnupg.org/
Description: GNU Privacy Guard -- minimalist public key operations
GnuPG is GNU’s tool for secure communication and data storage.
It can be used to encrypt data and to create digital signatures.
It includes an advanced key management facility and is compliant
with the proposed OpenPGP Internet standard as described in RFC4880.
[...]
You can also use dpkg to compare package version numbers with the --compare-versions option,
which is often called by external programs, including configuration scripts executed by dpkg itself.
This option requires three parameters: a version number, a comparison operator, and a second
version number. The different possible operators are: lt (strictly less than), le (less than or equal
to), eq (equal), ne (not equal), ge (greater than or equal to), and gt (strictly greater than). If the
comparison is correct, dpkg returns 0 (success); if not, it gives a non-zero return value (indicating
failure). Consider these comparisons:
$ dpkg --compare-versions 1.2-3 gt 1.1-4
$ echo $?
0
190 Kali Linux Revealed
$ dpkg --compare-versions 1.2-3 lt 1.1-4
$ echo $?
1
$ dpkg --compare-versions 2.6.0pre3-1 lt 2.6.0-1
$ echo $?
1
Note the unexpected failure of the last comparison: for dpkg, the string ”pre” (usually denoting
a pre-release) has no particular meaning, and dpkg simply interprets it as a string, in which case
”2.6.0pre3-1” is alphabetically greater than ”2.6.0-1”. When we want a package’s version number
to indicate that it is a pre-release, we use the tilde character, “~”:
$ dpkg --compare-versions 2.6.0~pre3-1 lt 2.6.0-1
$ echo $?
0
Querying the Database of Available Packages with apt-cache and apt
The apt-cache command can display much of the information stored in APT’s internal database.
This information is a sort of cache since it is gathered from the different sources listed in the
sources.list file. This happens during the apt update operation.
VOCABULARY A cache is a temporary storage system used to speed up frequent data access when
Cache the usual access method is expensive (performance-wise). This concept can be applied
in numerous situations and at different scales, from the core of microprocessors up to
high-end storage systems.
In the case of APT, the reference Packages files are those located on Debian mirrors.
That said, it would be very ineffective to push every search through the online package
databases. That is why APT stores a copy of those files (in /var/lib/apt/lists/)
and searches are done within those local files. Similarly, /var/cache/apt/archives/
contains a cached copy of already downloaded packages to avoid downloading them
again if you need to reinstall them.
To avoid excessive disk usage when you upgrade frequently, you should regularly sort
through the /var/cache/apt/archives/ directory. Two commands can be used for
this: apt clean (or apt-get clean) entirely empties the directory; apt autoclean
(apt-get autoclean) only removes packages that can no longer be downloaded be-
cause they have disappeared from the mirror and are therefore useless. Note that the
configuration parameter APT::Clean-Installed can be used to prevent the removal
of .deb files that are currently installed. Also, note that apt drops the downloaded
files once they have been installed, so this matters mainly when you use other tools.
The apt-cache command can do keyword-based package searches with apt-cache search
keyword. It can also display the headers of the package’s available versions with apt-cache show
package. This command provides the package’s description, its dependencies, and the name of its
Chapter 8 — Debian Package Management 191
maintainer. This feature is particularly useful in determining the packages that are installed via
metapackages, such as kali-tools-wireless, kali-tools-web, and kali-tools-gpu. Note that apt search,
apt show, aptitude search, and aptitude show work in the same way.
An Alternative: axi-cache apt-cache search is a very rudimentary tool, basically implementing grep on pack-
age’s descriptions. It often returns too many results or none at all, when too many
keywords are included.
axi-cache search term, on the other hand, provides better results, sorted by rele-
vancy. It uses the Xapian search engine and is part of the apt-xapian-index package,
which indexes all package information (and more, like the .desktop files from all De-
bian packages). It knows about tags and returns results in a matter of milliseconds.
$ axi-cache search forensics graphical
7 results found.
Results 1-7:
100% autopsy - graphical interface to SleuthKit
94% forensics-all-gui - Debian Forensics Environment - GUI
å components (metapackage)
87% forensics-extra-gui - Forensics Environment - extra GUI
å components (metapackage)
86% forensics-colorize - show differences between files using
å color graphics
44% gpart - Guess PC disk partition table, find lost
å partitions
39% testdisk - Partition scanner and disk recovery tool, and
å PhotoRec file recovery tool
8% texlive-publishers - TeX Live: Publisher styles, theses,
å etc.
More terms: tools experts autopsy picture ethical pentesters
å hackers
More tags: admin::forensics security::forensics admin::
å recovery interface::commandline admin::boot scope::
å utility role::program
Some features are more rarely used. For instance, apt-cache policy displays the priorities
of package sources as well as those of individual packages. Another example is apt-cache
dumpavail, which displays the headers of all available versions of all packages. apt-cache
pkgnames displays the list of all the packages that appear at least once in the cache.
8.2.6. Troubleshooting
Sooner or later, you will run into a problem when interacting with a package. In this section, we
will outline some basic troubleshooting steps that you can take and provide some tools that will
lead you closer to a potential solution.
192 Kali Linux Revealed
Handling Problems after an Upgrade
In spite of the Debian or Kali maintainers’ best efforts, a system upgrade isn’t always as smooth
as we would hope. New software versions may be incompatible with previous ones (for instance,
their default behavior or their data format may have changed), or bugs may slip through the cracks
despite the testing performed by package maintainers and Debian Unstable users.
Leveraging Bug Reports You might sometimes find that a new version of software doesn’t work
at all. This generally happens if the application isn’t particularly popular and hasn’t been tested
enough. The first thing to do is to have a look at the Kali bug tracker3 and at the Debian bug
tracking system4 at https://bugs.debian.org/package , and check whether the problem has already
been reported. If it hasn’t, you should report it yourself (see section 6.3, “Filing a Good Bug Report”
[page 134] for detailed instructions). If it is already known, the bug report and the associated
messages are usually an excellent source of information related to the bug. In some cases, a patch
already exists and has been made available in the bug report itself; you can then recompile a fixed
version of the broken package locally (see section 9.1, “Modifying Kali Packages” [page 228]). In
other cases, users may have found a workaround for the problem and shared their insights about
it in their replies to the report; those instructions may help you work around the problem until a
fix or patch is released. In a best-case scenario, the package may have already been fixed and you
may find details in the bug report.
Downgrading to a Working Version When the problem is a clear regression (where the former
version worked), you can try to downgrade the package. In this case, you will need a copy of the
old version. If you have access to the old version in one of the repositories configured in APT, you
can use a simple one-liner command to downgrade (see section 8.2.2.2, “Installing Packages with
APT” [page 183]). But with Kali’s rolling release, you will usually only find a single version of each
package at any one time.
You can still try to find the old .deb file and install it manually with dpkg. Old .deb files can be
found in multiple places:
• In APT’s cache in /var/cache/apt/archives/
• In the pool directory on your usual Kali mirror (removed and obsolete packages are kept
for three to four days to avoid problems with users not having the latest package indices)
• In https://snapshot.debian.org/ if the affected package was provided by Debian and
not by Kali; this service keeps historical versions of all Debian packages
3
https://bugs.kali.org/
4
https://bugs.debian.org/
Chapter 8 — Debian Package Management 193
Dealing with Broken Maintainer Scripts Sometimes the upgrade gets interrupted because one
of the package maintainer scripts fails (usually, it is the postinst). In those cases, you can try to
diagnose the problem, and possibly work around it, by editing the problematic script.
Here we rely on the fact that maintainer scripts are stored in /var/lib/dpkg/info/ and that we
can review and modify them.
Since maintainer scripts are usually simple shell scripts, it is possible to add a set -x line just after
the shebang line and arrange them to be rerun (with dpkg --configure -a for postinst) to see
precisely what is happening and where it is failing. This output can also nicely complement any
bug report that you might file.
With this newly gained knowledge, you can either fix the underlying problem or transform the
failing command into a working one (for example by adding || true at the end of the line).
Note that this tip does not work for a failing preinst since that script is executed even before
the package gets installed so it is not yet in its final location. It does work for postrm and prerm
although you will need to execute a package removal (respectively upgrade) to trigger them.
The dpkg Log File
The dpkg tool keeps a log of all of its actions in /var/log/dpkg.log. This log is extremely verbose,
since it details all the stages of each package. In addition to offering a way to track dpkg’s behav-
ior, it helps to keep a history of the development of the system: you can find the exact moment
when each package has been installed or updated, and this information can be extremely useful
in understanding a recent change in behavior. Additionally, with all versions being recorded, it is
easy to cross-check the information with the changelog.Debian.gz for packages in question, or
even with online bug reports.
# tail /var/log/dpkg.log
2021-01-06 23:16:37 status installed kali-tools-gpu:amd64 2021.1.0
2021-01-06 23:16:37 remove kali-tools-gpu:amd64 2021.1.0 <none>
2021-01-06 23:16:37 status half-configured kali-tools-gpu:amd64 2021.1.0
2021-01-06 23:16:37 status half-installed kali-tools-gpu:amd64 2021.1.0
2021-01-06 23:16:37 status config-files kali-tools-gpu:amd64 2021.1.0
2021-01-06 23:16:37 status not-installed kali-tools-gpu:amd64 <none>
Reinstalling Packages with apt --reinstall and aptitude reinstall
When you mistakenly damage your system by removing or modifying certain files, the easiest way
to restore them is to reinstall the affected package. Unfortunately, the packaging system finds that
the package is already installed and politely refuses to reinstall it. To avoid this, use the --reinstall
option of the apt and apt-get commands. The following command reinstalls postfix even if it is
already present:
194 Kali Linux Revealed
# apt --reinstall install postfix
The aptitude command line is slightly different but achieves the same result with aptitude
reinstall postfix. The dpkg command does not prevent re-installation, but it is rarely called
directly.
Do Not Use apt Using apt --reinstall to restore packages modified during an attack will certainly
--reinstall to Recover not recover the system as it was.
from an Attack After an attack, you can’t rely on anything: dpkg and apt might have been replaced by
malicious programs, not reinstalling the files as you would like them to. The attacker
might also have altered or created files outside the control of dpkg.
Remember that you can specify a specific distribution with apt as well, which means you can roll
back to an older version of a package (if for instance you know that it works well), provided that
it is still available in one of the sources referenced by the sources.list file:
# apt install w3af/kali-dev
Leveraging --force-* to Repair Broken Dependencies
If you are not careful, the use of a --force-* option or some other malfunction can lead to a system
where the APT family of commands will refuse to function. In effect, some of these options allow
installation of a package when a dependency is not met, or when there is a conflict. The result is
an inconsistent system from the point of view of dependencies, and the APT commands will refuse
to execute any action except those that will bring the system back to a consistent state (this often
consists of installing the missing dependency or removing a problematic package). This usually
results in a message like this one, obtained after installing a new version of rdesktop while ignoring
its dependency on a newer version of libc6:
# apt full-upgrade
[...]
You might want to run ’apt-get -f install’ to correct these.
The following packages have unmet dependencies:
rdesktop: Depends: libc6 (>= 2.14) but 2.3.6.ds1-13etch7 is installed
E: Unmet dependencies. Try using -f.
If you are a courageous administrator who is certain of the correctness of your analysis, you may
choose to ignore a dependency or conflict and use the corresponding --force-* option. In this case,
if you want to be able to continue to use apt or aptitude, you must edit /var/lib/dpkg/status
to delete or modify the dependency, or conflict, that you have chosen to override.
This manipulation is an ugly hack and should never be used, except in the most extreme case of
necessity. Quite frequently, a more fitting solution is to recompile the package that is causing
Chapter 8 — Debian Package Management 195
the problem or use a new version (potentially corrected) from a repository providing backports
(backports are newer versions especially recompiled to work in an older environment).
8.2.7. Frontends: aptitude and synaptic
APT is a C++ program whose code mainly resides in the libapt-pkg shared library. Thanks to this
shared library, it opened the door for the creation of user interfaces (front-ends), since the shared
library code can easily be reused. Historically, apt-get was only designed as a test front-end for
libapt-pkg but its success tends to obscure this fact.
Over time, despite the popularity of command line interfaces like apt and apt-get, various graph-
ical interfaces were developed. We will take a look at two of those interfaces in this section:
aptitude and synaptic.
Aptitude
Aptitude, shown in Figure 8.1, “The aptitude package manager” [page 196], is an interactive pro-
gram that can be used in semi-graphical mode on the console. You can browse the list of installed
and available packages, look up all the information, and select packages to install or remove. The
program is designed specifically to be used by administrators so its default behavior is much more
intelligent than APT’s, and its interface much easier to understand.
Figure 8.1 The aptitude package manager
196 Kali Linux Revealed
When you run aptitude, you are shown a list of packages sorted by state (installed, not-installed,
or installed but not available on the mirrors), while other sections display tasks, virtual packages,
and new packages that appeared recently on mirrors. To facilitate thematic browsing, other views
are available.
In all cases, aptitude displays a list combining categories and packages on the screen. Categories
are organized through a tree structure, whose branches can respectively be unfolded or folded
with the Enter, [, and ] keys. The + key should be used to mark a package for installation, - to mark
it for removal, and _ to purge it. Note that these keys can also be used for categories, in which case
the corresponding actions will be applied to all the packages of the category. The u key updates
the lists of available packages and Shift+u prepares a global system upgrade. The g key switches
to a summary view of the requested changes (and typing g again will apply the changes), and q
quits the current view. If you are in the initial view, this will close aptitude.
aptitude’s Documentation This section does not cover the finer details of using aptitude, it rather focuses on
giving you a user survival kit. aptitude is rather well documented and we advise
you to use its complete manual available in the aptitude-doc-en package (see /usr/
share/doc/aptitude/html/en/index.html) or at
è https://www.debian.org/doc/manuals/aptitude/
.
To search for a package, you can type / followed by a search pattern. This pattern matches the
name of the package but can also be applied to the description (if preceded by ~d), to the section
(with ~s), or to other characteristics detailed in the documentation. The same patterns can filter
the list of displayed packages: type the l key (as in limit) and enter the pattern.
Managing the automatic flag of Debian packages (see section 8.3.4, “Tracking Automatically In-
stalled Packages” [page 205]) is a breeze with aptitude. It is possible to browse the list of installed
packages and mark packages as automatic with Shift+m or you can remove the mark with the m
key. Automatic packages are displayed with an “A” in the list of packages. This feature also offers
a simple way to visualize the packages in use on a machine, without all the libraries and depen-
dencies that you don’t really care about. The related pattern that can be used with l (to activate
the filter mode) is ~i!~M. It specifies that you only want to see installed packages (~i) not marked
as automatic (!~M).
Using aptitude on the Most of Aptitude’s features are accessible via the interactive interface as well as via the
Command-Line Interface command-line. These command-lines will seem familiar to regular users of apt-get
and apt-cache.
The advanced features of aptitude are also available on the command-line. You can
use the same package search patterns as in the interactive version. For example, if
you want to clean up the list of manually installed packages, and if you know that
none of the locally installed programs require any particular libraries or Perl modules,
you can mark the corresponding packages as automatic with a single command:
Chapter 8 — Debian Package Management 197
# aptitude markauto ’~slibs|~sperl’
Here, you can clearly see the power of the search pattern system of aptitude, which
enables the instant selection of all the packages in the libs and perl sections.
Beware, if some packages are marked as automatic and if no other package depends
on them, they will be removed immediately (after a confirmation request).
Managing Recommendations, Suggestions, and Tasks Another interesting feature of
aptitude is the fact that it respects recommendations between packages while still giving users
the choice not to install them on a case-by-case basis. For example, the gnome package recom-
mends gdebi (among others). When you select the former for installation, the latter will also be
selected (and marked as automatic if not already installed on the system). Typing g will make it
obvious: gdebi appears on the summary screen of pending actions in the list of packages installed
automatically to satisfy dependencies. However, you can decide not to install it by deselecting it
before confirming the operations.
Note that this recommendation tracking feature does not apply to upgrades. For instance, if a new
version of gnome recommends a package that it did not recommend formerly, the package won’t be
marked for installation. However, it will be listed on the upgrade screen so that the administrator
can still select it for installation.
Suggestions between packages are also taken into account, but in a manner adapted to their spe-
cific status. For example, since gnome suggests dia-gnome, the latter will be displayed on the sum-
mary screen of pending actions (in the section of packages suggested by other packages). This
way, it is visible and the administrator can decide whether to take the suggestion into account or
not. Since it is only a suggestion and not a dependency or a recommendation, the package will
not be selected automatically—its selection requires manual intervention (thus, the package will
not be marked as automatic).
In the same spirit, remember that aptitude makes intelligent use of the concept of tasks. Since
tasks are displayed as categories in the screens of packages lists, you can either select a full task
for installation or removal or browse the list of packages included in the task to select a smaller
subset.
Better Solver Algorithms To conclude this section, let’s note that aptitude has more elaborate
algorithms compared to apt when it comes to resolving difficult situations. When a set of actions
is requested and when these combined actions would lead to an incoherent system, aptitude
evaluates several possible scenarios and presents them in order of decreasing relevance. However,
these algorithms are not foolproof. Fortunately, there is always the possibility to manually select
the actions to perform. When the currently selected actions lead to contradictions, the upper part
of the screen indicates a number of broken packages (you can directly navigate to those packages
198 Kali Linux Revealed
by pressing b). Then you can manually build a solution. In particular, you can get access to the
different available versions by selecting the package with Enter. If the selection of one of these
versions solves the problem, you should not hesitate to use the function. When the number of
broken packages gets down to zero, you can safely go to the summary screen of pending actions
for a last check before you apply them.
Aptitude’s Log Like dpkg, aptitude keeps a trace of executed actions in its logfile (/var/log/
aptitude). However, since both commands work at a very different level, you cannot
find the same information in their respective logfiles. While dpkg logs all the opera-
tions executed on individual packages step by step, aptitude gives a broader view of
high-level operations like a system-wide upgrade.
Beware, this logfile only contains a summary of operations performed by aptitude.
If other front-ends (or even dpkg itself) are occasionally used, then aptitude’s log
will only contain a partial view of the operations, so you can’t rely on it to build a
trustworthy history of the system.
Synaptic
Synaptic is a graphical package manager that features a clean and efficient graphical interface
(shown in Figure 8.2, “synaptic Package Manager” [page 200]) based on GTK+. Its many ready-to-
use filters give fast access to newly available packages, installed packages, upgradable packages,
obsolete packages, and so on. If you browse through these lists, you can select the operations to
be done on the packages (install, upgrade, remove, purge); these operations are not performed
immediately, but put into a task list. A single click on a button then validates the operations and
they are performed in one go.
Chapter 8 — Debian Package Management 199
Figure 8.2 synaptic Package Manager
8.3. Advanced APT Configuration and Usage
Now it is time to dive into some more advanced topics. First, we will take a look at advanced
configuration of APT, which will allow you to set more permanent options that will apply to APT
tools. We will then show how package priorities can be manipulated, which opens the door for
advanced fine-tuned, customized updates and upgrades. We will also show how to handle multiple
distributions so that you can start experimenting with packages coming from other distributions.
Next, we will take a look at how to track automatically installed packages, a capability that enables
you to manage packages that are installed through dependencies. We will also explain how multi-
arch support opens the door for running packages built for various hardware architectures. Last
but not least, we will discuss the cryptographic protocols and utilities in place that will let you
validate each package’s authenticity.
8.3.1. Configuring APT
Before we dive into the configuration of APT, let’s take a moment to discuss the configuration
mechanism of a Debian-based system. Historically, configuration was handled by dedicated con-
figuration files. However, in modern Linux systems like Debian and Kali, configuration directories
with the .d suffix are becoming more commonly used. Each directory represents a configuration
200 Kali Linux Revealed
file that is split into multiple files. In this sense, all of the files in /etc/apt/apt.conf.d/ are in-
structions for the configuration of APT. APT processes the files in alphabetical order, so that the
later files can modify configuration elements defined in the earlier files.
This structure brings some flexibility to administrators and package maintainers, allowing them
to make software configuration changes through file additions without having to change an ex-
isting file. This is especially helpful for package maintainers because they can use this approach
to adapt the configuration of other software to ensure that it perfectly co-exists with theirs, with-
out breaking the Debian policy that explicitly forbids modifying configuration files of other pack-
ages. Because of the .d configuration mechanism, you don’t have to manually follow multiple
package configuration instructions typically found in the package’s /usr/share/doc/package
/README.Debian file, since the installer can drop in configuration files.
Beware of Configuration While APT has native support of its /etc/apt/apt.conf.d directory, this is not al-
Files Generated from .d ways the case. For some applications (like exim, for example), the .d directory is a
Directories Debian-specific addition used as input to dynamically generate the canonical configu-
ration file used by the application. In those cases, the packages provide an “update-*”
command (for example: update-exim4.conf) that will concatenate the files from the
.d directory and overwrite the main configuration file.
In those cases, you must not manually edit the main configuration file as your changes
will be lost on the next execution of the update-* command, and you must also not
forget to run the former command after having edited a file out of the .d directory
(or your changes will not be used).
Armed with an understanding of the .d configuration mechanism, let’s talk about how you
can leverage it to configure APT. As we have discussed, you can alter APT’s behavior through
command-line arguments to dpkg like this example, which performs a forced overwrite install of
zsh:
# apt -o Dpkg::Options::=”--force-overwrite” install zsh
Obviously this is very cumbersome, especially if you use options frequently, but you can also use
the .d directory configuration structure to configure certain aspects of APT by adding directives
to a file in the /etc/apt/apt.conf.d/ directory. For example, this (and any other) directive can
easily be added to a file in /etc/apt/apt.conf.d/. The name of this file is somewhat arbitrary,
but a common convention is to use either local or 99local:
$ cat /etc/apt/apt.conf.d/99local
Dpkg::Options {
”--force-overwrite”;
}
There are many other helpful configuration options and we certainly can’t cover them all, but
one we will touch on involves network connectivity. For example, if you can only access the web
Chapter 8 — Debian Package Management 201
through a proxy, add a line like Acquire::http::proxy ”http://yourproxy :3128”. For an FTP proxy,
use Acquire::ftp::proxy ”ftp://yourproxy ”.
To discover more configuration options, read the apt.conf(5) manual page with the man
apt.conf command (for details on manual pages, see section 6.1.1, “Manual Pages” [page 128]).
8.3.2. Managing Package Priorities
One of the most important aspects in the configuration of APT is the management of the priori-
ties associated with each package source. For instance, you might want to extend your Kali Linux
system with one or two newer packages from Debian Unstable or Debian Experimental. It is pos-
sible to assign a priority to each available package (the same package can have several priorities
depending on its version or the distribution providing it). These priorities will influence APT’s
behavior: for each package, it will always select the version with the highest priority (except if
this version is older than the installed one and its priority is less than 1000).
APT defines several default priorities. Each installed package version has a priority of 100. A non-
installed version has a priority of 500 by default but it can jump to 990 if it is part of the target
release (defined with the -t command-line option or the APT::Default-Release configuration direc-
tive).
You can modify the priorities by adding entries in the /etc/apt/preferences file with the names
of the affected packages, their version, their origin and their new priority.
APT will never install an older version of a package (that is, a package whose version number is
lower than the one of the currently installed package) except when its priority is higher than 1000.
APT will always install the highest priority package that follows this constraint. If two packages
have the same priority, APT installs the newest one (whose version number is the highest). If
two packages of same version have the same priority but differ in their content, APT installs the
version that is not installed (this rule has been created to cover the case of a package update
without the increment of the revision number, which is usually required).
In more concrete terms, a package whose priority is less than 0 will never be installed. A package
with a priority ranging between 0 and 100 will only be installed if no other version of the package is
already installed. With a priority between 100 and 500, the package will only be installed if there
is no other newer version installed or available in another distribution. A package of priority
between 501 and 990 will only be installed if there is no newer version installed or available in the
target distribution. With a priority between 990 and 1000, the package will be installed except if
the installed version is newer. A priority greater than 1000 will always lead to the installation of
the package even if it forces APT to downgrade to an older version.
When APT checks /etc/apt/preferences, it first takes into account the most specific entries
(often those specifying the concerned package), then the more generic ones (including for exam-
ple all the packages of a distribution). If several generic entries exist, the first match is used. The
available selection criteria include the package’s name and the source providing it. Every package
202 Kali Linux Revealed
source is identified by the information contained in a Release file that APT downloads together
with the Packages files. These files specify the origin, usually “Kali” for the packages from Kali’s
official mirrors and “Debian” for the packages from Debian’s official mirrors, but the origin can
also be a person’s or an organization’s name for third-party repositories. The Release file also
provides the name of the distribution together with its version. Let’s have a look at its syntax
through some realistic case studies of this mechanism.
Priority of Debian If you listed Debian experimental in your sources.list file, the corresponding pack-
Experimental ages will almost never be installed because their default APT priority is 1. This
is of course a specific case, designed to keep users from installing experimental
packages by mistake. The packages can only be installed by typing apt install
package/experimental, assuming of course that you are aware of the risks and po-
tential headaches of life on the edge. It is still possible (though not recommended)
to treat packages of experimental like those of other distributions by giving them a
priority of 500. This is done with a specific entry in /etc/apt/preferences:
Package: *
Pin: release a=experimental
Pin-Priority: 500
Let’s suppose that you only want to use packages from Kali and that you only want Debian pack-
ages installed when explicitly requested. You could write the following entries in the /etc/apt/
preferences file (or in any file in /etc/apt/preferences.d/):
Package: *
Pin: release o=Kali
Pin-Priority: 900
Package: *
Pin: release o=Debian
Pin-Priority: -10
In the last two examples, you have seen a=experimental, which defines the name of the selected
distribution and o=Kali and o=Debian, which limit the scope to packages whose origin are Kali and
Debian, respectively.
Let’s now assume that you have a server with several local programs depending on the version 5.22
of Perl and that you want to ensure that upgrades will not install another version of it. You could
use this entry:
Package: perl
Pin: version 5.22*
Pin-Priority: 1001
The reference documentation for this configuration file is available in the manual page
apt_preferences(5), which you can display with man apt_preferences.
Chapter 8 — Debian Package Management 203
Adding Comments in There is no official syntax for comments in /etc/apt/preferences, but some textual
/etc/apt/preferences descriptions can be provided by prepending one or more Explanation fields into each
entry:
Explanation: The package xserver-xorg-video-intel provided
Explanation: in experimental can be used safely
Package: xserver-xorg-video-intel
Pin: release a=experimental
Pin-Priority: 500
8.3.3. Working with Several Distributions
Given that apt is such a marvelous tool, you will likely want to dive in and start experimenting
with packages coming from other distributions. For example, after installing a Kali Rolling system,
you might want to try out a software package available in Kali Dev, Debian Unstable, or Debian
Experimental without diverging too much from the system’s initial state.
Even if you will occasionally encounter problems while mixing packages from different distribu-
tions, apt manages such coexistence very well and limits risks very effectively (provided that the
package dependencies are accurate). First, list all distributions used in /etc/apt/sources.list
and define your reference distribution with the APT::Default-Release parameter (see section 8.2.3,
“Upgrading Kali Linux” [page 184]).
Let’s suppose that Kali Rolling is your reference distribution but that Kali Dev and Debian
Unstable are also listed in your sources.list file. In this case, you can use apt install
package/unstable to install a package from Debian Unstable. If the installation fails due to some
unsatisfiable dependencies, let it solve those dependencies within Unstable by adding the -t un-
stable parameter.
In this situation, upgrades (upgrade and full-upgrade) are done within Kali Rolling except for
packages already upgraded to another distribution: those will follow updates available in the other
distributions. We will explain this behavior with the help of the default priorities set by APT below.
Do not hesitate to use apt-cache policy (see sidebar “Using apt-cache policy” [page 204]) to
verify the given priorities.
Everything relies on the fact that APT only considers packages of higher or equal version than the
installed package (assuming that /etc/apt/preferences has not been used to force priorities
higher than 1000 for some packages).
Using apt-cache policy To gain a better understanding of the mechanism of priority, do not hesitate to exe-
cute apt-cache policy to display the default priority associated with each package
source. You can also use apt-cache policy package to display the priorities of all
available versions of a given package.
204 Kali Linux Revealed
Let’s assume that you have installed version 1 of a first package from Kali Rolling and that version 2
and 3 are available respectively in Kali Dev and Debian Unstable. The installed version has a priority
of 100 but the version available in Kali Rolling (the very same) has a priority of 990 (because it is
part of the target release). Packages in Kali Dev and Debian Unstable have a priority of 500 (the
default priority of a non-installed version). The winner is thus version 1 with a priority of 990.
The package stays in Kali Rolling.
Let’s take the example of another package whose version 2 has been installed from Kali Dev. Ver-
sion 1 is available in Kali Rolling and version 3 in Debian Unstable. Version 1 (of priority 990—thus
lower than 1000) is discarded because it is lower than the installed version. This only leaves ver-
sion 2 and 3, both of priority 500. Faced with this alternative, APT selects the newest version, the
one from Debian Unstable. If you don’t want a package installed from Kali Dev to migrate to Debian
Unstable, you have to assign a priority lower than 500 (490 for example) to packages coming from
Debian Unstable. You can modify /etc/apt/preferences to this effect:
Package: *
Pin: release a=unstable
Pin-Priority: 490
8.3.4. Tracking Automatically Installed Packages
One of the essential functionalities of apt is the tracking of packages installed only through de-
pendencies. These packages are called automatic and often include libraries.
With this information, when packages are removed, the package managers can compute a list of
automatic packages that are no longer needed (because there are no manually installed packages
depending on them). The command apt autoremove will get rid of those packages. Aptitude does
not have this command because it removes them automatically as soon as they are identified. In
all cases, the tools display a clear message listing the affected packages.
It is a good habit to mark as automatic any package that you don’t need directly so that they
are automatically removed when they aren’t necessary anymore. You can use apt-mark auto
package to mark the given package as automatic, whereas apt-mark manual package does the
opposite. aptitude markauto and aptitude unmarkauto work in the same way, although they
offer more features for marking many packages at once (see section 8.2.7.1, “Aptitude” [page 196]).
The console-based interactive interface of aptitude also makes it easy to review the automatic
flag on many packages.
You might want to know why an automatically installed package is present on the system. To get
this information from the command line, you can use aptitude why package (apt and apt-get
have no similar feature):
$ aptitude why python-debian
i aptitude Recommends apt-xapian-index
i A apt-xapian-index Depends python-debian (>= 0.1.15)
Chapter 8 — Debian Package Management 205
8.3.5. Leveraging Multi-Arch Support
All Debian packages have an Architecture field in their control information. This field can contain
either “all” (for packages that are architecture-independent) or the name of the architecture that
it targets (like amd64, or armhf). In the latter case, by default, dpkg will only install the package
if its architecture matches the host’s architecture as returned by dpkg --print-architecture.
This restriction ensures that you do not end up with binaries compiled for an incorrect architec-
ture. Everything would be perfect except that (some) computers can run binaries for multiple
architectures, either natively (an amd64 system can run i386 binaries) or through emulators.
Enabling Multi-Arch
Multi-arch support for dpkg allows users to define foreign architectures that can be installed on
the current system. This is easily done with dpkg --add-architecture, as in the example below
where the i386 architecture needs to be added to the amd64 system in order to run Microsoft Win-
dows applications using Wine5 . There is a corresponding dpkg --remove-architecture to drop
support of a foreign architecture, but it can only be used when no packages of this architecture
remain installed.
# dpkg --print-architecture
amd64
# wine
it looks like wine32 is missing, you should install it.
multiarch needs to be enabled first. as root, please
execute ”dpkg --add-architecture i386 & apt-get update &
apt-get install wine32”
Usage: wine PROGRAM [ARGUMENTS...] Run the specified program
wine --help Display this help and exit
wine --version Output version information and exit
# dpkg --add-architecture i386
# dpkg --print-foreign-architectures
i386
# apt update
[...]
# apt install wine32
[...]
Setting up libwine:i386 (1.8.6-5) ...
Setting up vdpau-driver-all:i386 (1.1.1-6) ...
Setting up wine32:i386 (1.8.6-5) ...
5
https://www.winehq.org/
206 Kali Linux Revealed
Setting up libasound2-plugins:i386 (1.1.1-1) ...
Processing triggers for libc-bin (2.24-9)
# wine
Usage: wine PROGRAM [ARGUMENTS...] Run the specified program
wine --help Display this help and exit
wine --version Output version information and exit
# dpkg --remove-architecture i386
dpkg: error: cannot remove architecture ’i386’ currently in use by the database
# dpkg --print-foreign-architectures
i386
APT will automatically detect when dpkg has been configured to support foreign architectures
and will start downloading the corresponding Packages files during its update process.
Foreign packages can then be installed with apt install package:architecture.
Using Proprietary i386 There are multiple use cases for multi-arch, but the most popular one is the possibility
Binaries on amd64 to execute 32-bit binaries (i386) on 64-bit systems (amd64), in particular since several
popular proprietary applications (like Skype) are only provided in 32-bit versions.
Multi-Arch Related Changes
To make multi-arch actually useful and usable, libraries had to be repackaged and moved to an
architecture-specific directory so that multiple copies (targeting different architectures) can be
installed alongside one another. Such updated packages contain the Multi-Arch: same header field
to tell the packaging system that the various architectures of the package can be safely co-installed
(and that those packages can only satisfy dependencies of packages of the same architecture).
$ dpkg -s libwine
dpkg-query: error: --status needs a valid package name but ’libwine’ is not: ambiguous
å package name ’libwine’ with more than one installed instance
Use --help for help about querying packages.
$ dpkg -s libwine:amd64 libwine:i386 | grep ^Multi
Multi-Arch: same
Multi-Arch: same
$ dpkg -L libgcc1:amd64 | grep .so
[...]
/usr/lib/x86_64-linux-gnu/wine/libwine.so.1
$ dpkg -S /usr/share/doc/libwine/copyright
libwine:amd64, libwine:i386: /usr/share/doc/libwine/copyright
It is worth noting that Multi-Arch: same packages must have their names qualified with their
architecture to be unambiguously identifiable. These packages may also share files with other
Chapter 8 — Debian Package Management 207
instances of the same package; dpkg ensures that all packages have bit-for-bit identical files when
they are shared. Also, all instances of a package must have the same version, therefore they must
be upgraded together.
Multi-Arch support also brings some interesting challenges in the way dependencies are handled.
Satisfying a dependency requires either a package marked Multi-Arch: foreign or a package whose
architecture matches the one of the package declaring the dependency (in this dependency res-
olution process, architecture-independent packages are assumed to be of the same architecture
as the host). A dependency can also be weakened to allow any architecture to fulfill it, with the
package:any syntax, but foreign packages can only satisfy such a dependency if they are marked
Multi-Arch: allowed.
8.3.6. Validating Package Authenticity
System upgrades are very sensitive operations and you really want to ensure that you only install
official packages from the Kali repositories. If the Kali mirror you are using has been compro-
mised, a computer cracker could try to add malicious code to an otherwise legitimate package.
Such a package, if installed, could do anything the cracker designed it to do including disclose
passwords or confidential information. To circumvent this risk, Kali provides a tamper-proof seal
to guarantee—at install time—that a package really comes from its official maintainer and hasn’t
been modified by a third party.
The seal works with a chain of cryptographic hashes and a signature. The signed file is the Release
file, provided by the Kali mirrors. It contains a list of the Packages files (including their com-
pressed forms, Packages.gz and Packages.xz, and the incremental versions), along with their
MD5, SHA1, and SHA256 hashes, which ensures that the files haven’t been tampered with. These
Packages files contain a list of the Debian packages available on the mirror along with their hashes,
which ensures in turn that the contents of the packages themselves haven’t been altered either.
The trusted keys are managed with the apt-key command found in the apt package. This program
maintains a keyring of GnuPG public keys, which are used to verify signatures in the Release.gpg
files available on the mirrors. It can be used to add new keys manually (when non-official mirrors
are needed). Generally however, only the official Kali keys are needed. These keys are automati-
cally kept up-to-date by the kali-archive-keyring package (which puts the corresponding keyrings
in /etc/apt/trusted.gpg.d). However, the first installation of this particular package requires
caution: even if the package is signed like any other, the signature cannot be verified externally.
Cautious administrators should therefore check the fingerprints of imported keys before trusting
them to install new packages:
# apt-key fingerprint
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
/etc/apt/trusted.gpg.d/debian-archive-buster-automatic.gpg
----------------------------------------------------------
pub rsa4096 2019-04-14 [SC] [expires: 2027-04-12]
80D1 5823 B7FD 1561 F9F7 BCDD DC30 D7C2 3CBB ABEE
uid [ unknown] Debian Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>
208 Kali Linux Revealed
sub rsa4096 2019-04-14 [S] [expires: 2027-04-12]
/etc/apt/trusted.gpg.d/debian-archive-buster-security-automatic.gpg
-------------------------------------------------------------------
pub rsa4096 2019-04-14 [SC] [expires: 2027-04-12]
5E61 B217 265D A980 7A23 C5FF 4DFA B270 CAA9 6DFA
uid [ unknown] Debian Security Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>
sub rsa4096 2019-04-14 [S] [expires: 2027-04-12]
/etc/apt/trusted.gpg.d/debian-archive-buster-stable.gpg
-------------------------------------------------------
pub rsa4096 2019-02-05 [SC] [expires: 2027-02-03]
6D33 866E DD8F FA41 C014 3AED DCC9 EFBF 77E1 1517
uid [ unknown] Debian Stable Release Key (10/buster) <debian-release@lists.debian.org>
/etc/apt/trusted.gpg.d/debian-archive-jessie-automatic.gpg
----------------------------------------------------------
pub rsa4096 2014-11-21 [SC] [expires: 2022-11-19]
126C 0D24 BD8A 2942 CC7D F8AC 7638 D044 2B90 D010
uid [ unknown] Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>
/etc/apt/trusted.gpg.d/debian-archive-jessie-security-automatic.gpg
-------------------------------------------------------------------
pub rsa4096 2014-11-21 [SC] [expires: 2022-11-19]
D211 6914 1CEC D440 F2EB 8DDA 9D6D 8F6B C857 C906
uid [ unknown] Debian Security Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>
/etc/apt/trusted.gpg.d/debian-archive-jessie-stable.gpg
-------------------------------------------------------
pub rsa4096 2013-08-17 [SC] [expires: 2021-08-15]
75DD C3C4 A499 F1A1 8CB5 F3C8 CBF8 D6FD 518E 17E1
uid [ unknown] Jessie Stable Release Key <debian-release@lists.debian.org>
/etc/apt/trusted.gpg.d/debian-archive-stretch-automatic.gpg
-----------------------------------------------------------
pub rsa4096 2017-05-22 [SC] [expires: 2025-05-20]
E1CF 20DD FFE4 B89E 8026 58F1 E0B1 1894 F66A EC98
uid [ unknown] Debian Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>
sub rsa4096 2017-05-22 [S] [expires: 2025-05-20]
/etc/apt/trusted.gpg.d/debian-archive-stretch-security-automatic.gpg
--------------------------------------------------------------------
pub rsa4096 2017-05-22 [SC] [expires: 2025-05-20]
6ED6 F5CB 5FA6 FB2F 460A E88E EDA0 D238 8AE2 2BA9
uid [ unknown] Debian Security Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>
sub rsa4096 2017-05-22 [S] [expires: 2025-05-20]
/etc/apt/trusted.gpg.d/debian-archive-stretch-stable.gpg
--------------------------------------------------------
pub rsa4096 2017-05-20 [SC] [expires: 2025-05-18]
067E 3C45 6BAE 240A CEE8 8F6F EF0F 382A 1A7B 6500
uid [ unknown] Debian Stable Release Key (9/stretch) <debian-release@lists.debian.org>
/etc/apt/trusted.gpg.d/kali-archive-keyring.gpg
-----------------------------------------------
pub rsa4096 2012-03-05 [SC] [expires: 2023-01-16]
44C6 513A 8E4F B3D3 0875 F758 ED44 4FF0 7D8D 0BF6
uid [ unknown] Kali Linux Repository <devel@kali.org>
sub rsa4096 2012-03-05 [E] [expires: 2023-01-16]
When a third-party package source is added to the sources.list file, APT needs to be told to
trust the corresponding GPG authentication key (otherwise it will keep complaining that it can’t
ensure the authenticity of the packages coming from that repository). The first step is of course
Chapter 8 — Debian Package Management 209
to get the public key. More often than not, the key will be provided as a small text file, which we
will call key.asc in the following examples.
To add the key to the trusted keyring, the administrator can run apt-key add < key.asc. An-
other way is to use the synaptic graphical interface: its Authentication tab in the Settings
→ Repositories menu provides the ability to import a key from the key.asc file.
For people who prefer a dedicated application and more details on the trusted keys, it is possible to
use gui-apt-key (in the package of the same name), a small graphical user interface that manages
the trusted keyring.
Once the appropriate keys are in the keyring, APT will check the signatures before any risky op-
eration, so that front-ends will display a warning if asked to install a package whose authenticity
can’t be ascertained.
8.4. APT Package Reference: Digging Deeper into the Debian Package Sys-
tem
Now it is time to dive really deep into Debian and Kali’s package system. At this point, we are
going to move beyond tools and syntax and focus more on the nuts and bolts of the packaging
system. This behind-the-scenes view will help you understand how APT works at its foundation
and will give you insight into how to seriously streamline and customize your Kali system. You
may not necessarily memorize all the material in this section, but the walk-through and reference
material will serve you well as you grow in your mastery of the Kali Linux system.
So far, you have interacted with APT’s package data through the various tools designed to interface
with it. Next, we will dig deeper and take a look inside the packages and look at the internal meta-
information (or information about other information) used by the package management tools.
This combination of a file archive and of meta-information is directly visible in the structure of a
.deb file, which is simply an ar archive, concatenating three files:
$ ar t /var/cache/apt/archives/apt_1.4~beta1_amd64.deb
debian-binary
control.tar.gz
data.tar.xz
The debian-binary file contains a single version number describing the format of the archive:
$ ar p /var/cache/apt/archives/apt_1.4~beta1_amd64.deb debian-binary
2.0
The control.tar.gz archive contains meta-information:
$ ar p /var/cache/apt/archives/apt_1.4~beta1_amd64.deb control.tar.gz | tar -tzf -
./
210 Kali Linux Revealed
./conffiles
./control
./md5sums
./postinst
./postrm
./preinst
./prerm
./shlibs
./triggers
And finally, the data.tar.xz archive (the compression format might vary) contains the actual
files to be installed on the file system:
$ ar p /var/cache/apt/archives/apt_1.4~beta1_amd64.deb data.tar.xz | tar -tJf -
./
./etc/
./etc/apt/
./etc/apt/apt.conf.d/
./etc/apt/apt.conf.d/01autoremove
./etc/apt/preferences.d/
./etc/apt/sources.list.d/
./etc/apt/trusted.gpg.d/
./etc/cron.daily/
./etc/cron.daily/apt-compat
./etc/kernel/
./etc/kernel/postinst.d/
./etc/kernel/postinst.d/apt-auto-removal
./etc/logrotate.d/
./etc/logrotate.d/apt
./lib/
./lib/systemd/
[...]
Note that in this example, you are viewing a .deb package in APT’s archive cache and that your
archive may contain files with different version numbers than what is shown.
In this section, we will introduce this meta-information contained in each package and show you
how to leverage it.
8.4.1. The control File
We will begin by looking at the control file, which is contained in the control.tar.gz archive.
The control file contains the most vital information about the package. It uses a structure similar
to email headers and can be viewed with the dpkg -I command. For example, the control file
for apt looks like this:
Chapter 8 — Debian Package Management 211
$ dpkg -I apt_1.4~beta1_amd64.deb control
Package: apt
Version: 1.4~beta1
Architecture: amd64
Maintainer: APT Development Team <deity@lists.debian.org>
Installed-Size: 3478
Depends: adduser, gpgv | gpgv2 | gpgv1, debian-archive-keyring, init-system-helpers (>=
å 1.18~), libapt-pkg5.0 (>= 1.3~rc2), libc6 (>= 2.15), libgcc1 (>= 1:3.0),
å libstdc++6 (>= 5.2)
Recommends: gnupg | gnupg2 | gnupg1
Suggests: apt-doc, aptitude | synaptic | wajig, dpkg-dev (>= 1.17.2), powermgmt-base,
å python-apt
Breaks: apt-utils (<< 1.3~exp2~)
Replaces: apt-utils (<< 1.3~exp2~)
Section: admin
Priority: important
Description: commandline package manager
This package provides commandline tools for searching and
managing as well as querying information about packages
as a low-level access to all features of the libapt-pkg library.
.
These include:
* apt-get for retrieval of packages and information about them
from authenticated sources and for installation, upgrade and
removal of packages together with their dependencies
* apt-cache for querying available information about installed
as well as installable packages
* apt-cdrom to use removable media as a source for packages
* apt-config as an interface to the configuration settings
* apt-key as an interface to manage authentication keys
In this section, we will walk you through the control file and explain the various fields. Each of
these will give you a better understanding of the packaging system, give you more fine-tuned
configuration control, and provide you with insight needed to troubleshoot problems that may
occur.
Dependencies: the Depends Field
The package dependencies are defined in the Depends field in the package header. This is a list of
conditions to be met for the package to work correctly—this information is used by tools such as
apt in order to install the required libraries, in appropriate versions fulfilling the dependencies of
the package to be installed. For each dependency, you can restrict the range of versions that meet
that condition. In other words, it is possible to express the fact that you need the package libc6
212 Kali Linux Revealed
in a version equal to or greater than “2.15” (written “libc6 (>= 2.15)”). Version comparison
operators are as follows:
• <<: less than;
• <=: less than or equal to;
• =: equal to (note that “2.6.1” is not equal to “2.6.1-1”);
• >=: greater than or equal to;
• >>: greater than.
In a list of conditions to be met, the comma serves as a separator, interpreted as a logical “AND.”
In conditions, the vertical bar (“|”) expresses a logical “OR” (it is an inclusive “OR,” not an exclu-
sive “either/or”). Carrying greater priority than “AND,” you can use it as many times as necessary.
Thus, the dependency “(A OR B) AND C” is written A | B, C. In contrast, the expression “A OR (B
AND C)” should be written as “(A OR B) AND (A OR C)”, since the Depends field does not tolerate
parentheses that change the order of priorities between the logical operators “OR” and “AND”.
It would thus be written A | B, A | C. See https://www.debian.org/doc/debian-policy/
ch-relationships.html for more information.
The dependencies system is a good mechanism for guaranteeing the operation of a program but it
has another use with metapackages. These are empty packages that only describe dependencies.
They facilitate the installation of a consistent group of programs preselected by the metapackage
maintainer; as such, apt install metapackage will automatically install all of these programs
using the metapackage’s dependencies. The gnome, kali-tools-wireless, and kali-linux-large packages
are examples of metapackages. For more information on Kali’s metapackages, see https://tools.
kali.org/kali-metapackages
Pre-Depends, a More Demanding Depends
Pre-dependencies, which are listed in the Pre-Depends field in the package headers, complete the
normal dependencies; their syntax is identical. A normal dependency indicates that the pack-
age in question must be unpacked and configured before configuration of the package declaring
the dependency. A pre-dependency stipulates that the package in question must be unpacked
and configured before execution of the pre-installation script of the package declaring the pre-
dependency, that is before its installation.
A pre-dependency is very demanding for apt because it adds a strict constraint on the ordering of
the packages to install. As such, pre-dependencies are discouraged unless absolutely necessary. It
is even recommended to consult other developers on debian-devel@lists.debian.org before adding
a pre-dependency as it is generally possible to find another solution as a work-around.
Chapter 8 — Debian Package Management 213
Recommends, Suggests, and Enhances Fields
The Recommends and Suggests fields describe dependencies that are not compulsory. The rec-
ommended dependencies, the most important, considerably improve the functionality offered by
the package but are not indispensable to its operation. The suggested dependencies, of secondary
importance, indicate that certain packages may complement and increase their respective utility,
but it is perfectly reasonable to install one without the others.
You should always install the recommended packages unless you know exactly why you do not
need them. Conversely, it is not necessary to install suggested packages unless you know why you
need them.
The Enhances field also describes a suggestion, but in a different context. It is indeed located in
the suggested package, and not in the package that benefits from the suggestion. Its interest lies
in that it is possible to add a suggestion without having to modify the package that is concerned.
Thus, all add-ons, plug-ins, and other extensions of a program can then appear in the list of sug-
gestions related to the software. Although it has existed for several years, this last field is still
largely ignored by programs such as apt or synaptic. The original goal was to let a package like
xul-ext-adblock-plus (a Firefox extension) declare Enhances: firefox, firefox-esr and thus appear in
the list of suggested packages associated to firefox and firefox-esr.
Conflicts: the Conflicts Field
The Conflicts field indicates when a package cannot be installed simultaneously with another. The
most common reasons for this are that both packages include a file of the same name, provide the
same service on the same transmission control protocol (TCP) port, or would hinder each other’s
operation.
If it triggers a conflict with an already installed package, dpkg will refuse to install a package,
except if the new package specifies that it will replace the installed package, in which case dpkg
will choose to replace the old package with the new one. APT always follows your instructions: if
you choose to install a new package, it will automatically offer to uninstall the package that poses
a problem.
Incompatibilities: the Breaks Field
The Breaks field has an effect similar to that of the Conflicts field, but with a special meaning. It
signals that the installation of a package will break another package (or particular versions of it).
In general, this incompatibility between two packages is transitory and the Breaks relationship
specifically refers to the incompatible versions.
214 Kali Linux Revealed
When a package breaks an already installed package, dpkg will refuse to install it, and apt will try
to resolve the problem by updating the package that would be broken to a newer version (which
is assumed to be fixed and, thus, compatible again).
This type of situation may occur in the case of updates without backwards compatibility: this is
the case if the new version no longer functions with the older version and causes a malfunction in
another program without making special provisions. The Breaks field helps prevent these types
of problems.
Provided Items: the Provides Field
This field introduces the very interesting concept of a virtual package. It has many roles, but two are
of particular importance. The first role consists in using a virtual package to associate a generic
service with it (the package provides the service). The second indicates that a package completely
replaces another and that for this purpose, it can also satisfy the dependencies that the other
would satisfy. It is thus possible to create a substitution package without having to use the same
package name.
metapackage and Virtual It is essential to clearly distinguish metapackages from virtual packages. The former
Package are real packages (including real .deb files), whose only purpose is to express depen-
dencies.
Virtual packages, however, do not exist physically; they are only a means of identify-
ing real packages based on common, logical criteria (for example, service provided, or
compatibility with a standard program or a pre-existing package).
Providing a Service Let’s discuss the first case in greater detail with an example: all mail
servers, such as postfix or sendmail are said to provide the mail-transport-agent virtual package.
Thus, any package that needs this service to be functional (e.g. a mailing list manager, such as
smartlist or sympa) simply states in its dependencies that it requires a mail-transport-agent instead
of specifying a large yet incomplete list of possible solutions. Furthermore, it is useless to install
two mail servers on the same machine, which is why each of these packages declares a conflict
with the mail-transport-agent virtual package. A conflict between a package and itself is ignored
by the system, but this technique will prohibit the installation of two mail servers side by side.
Interchangeability with Another Package The Provides field is also interesting when the con-
tent of a package is included in a larger package. For example, the libdigest-md5-perl Perl module
was an optional module in Perl 5.6, and has been integrated as standard in Perl 5.8. As such, the
package perl has since version 5.8 declared Provides: libdigest-md5-perl so that the dependencies
on this package are met if the system has Perl 5.8 (or newer). The libdigest-md5-perl package itself
was deleted, since it no longer had any purpose when old Perl versions were removed.
Chapter 8 — Debian Package Management 215
Figure 8.3 Use of a Provides Field in Order to Not Break Dependencies
This feature is very useful, since it is never possible to anticipate the vagaries of development
and it is necessary to be able to adjust to renaming, and other automatic replacement, of obsolete
software.
Replacing Files: The Replaces Field
The Replaces field indicates that the package contains files that are also present in another pack-
age, but that the package is legitimately entitled to replace them. Without this specification, dpkg
fails, stating that it cannot overwrite the files of another package (technically, it is possible to
force it to do so with the --force-overwrite option, but that is not considered standard operation).
This allows identification of potential problems and requires the maintainer to study the matter
prior to choosing whether to add such a field.
The use of this field is justified when package names change or when a package is included in
another. This also happens when the maintainer decides to distribute files differently among
various binary packages produced from the same source package: a replaced file no longer belongs
to the old package, but only to the new one.
If all of the files in an installed package have been replaced, the package is considered to be re-
moved. Finally, this field also encourages dpkg to remove the replaced package where there is a
conflict.
216 Kali Linux Revealed
8.4.2. Configuration Scripts
In addition to the control file, the control.tar.gz archive for each Debian package may contain
a number of scripts (postinst, postrm, preinst, prerm) called by dpkg at different stages in the
processing of a package. We can use dpkg -I to show these files as they reside in a .deb package
archive:
$ dpkg -I /var/cache/apt/archives/zsh_5.3-1_amd64.deb | head
new debian package, version 2.0.
size 814486 bytes: control archive=2557 bytes.
838 bytes, 20 lines control
3327 bytes, 43 lines md5sums
969 bytes, 41 lines * postinst #!/bin/sh
348 bytes, 20 lines * postrm #!/bin/sh
175 bytes, 5 lines * preinst #!/bin/sh
175 bytes, 5 lines * prerm #!/bin/sh
Package: zsh
Version: 5.3-1
$ dpkg -I zsh_5.3-1_amd64.deb preinst
#!/bin/sh
set -e
# Automatically added by dh_installdeb
dpkg-maintscript-helper symlink_to_dir /usr/share/doc/zsh zsh-common 5.0.7-3 -- ”$@”
# End automatically added section
The Debian Policy6 describes each of these files in detail, specifying the scripts called and the
arguments they receive. These sequences may be complicated, since if one of the scripts fails,
dpkg will try to return to a satisfactory state by canceling the installation or removal in progress
(insofar as it is possible).
The dpkg Database You can traverse the dpkg database on the filesystem at /var/lib/dpkg/. This di-
rectory contains a running record of all the packages that have been installed on
the system. All of the configuration scripts for installed packages are stored in the
/var/lib/dpkg/info/ directory, in the form of a file prefixed with the package’s
name:
$ ls /var/lib/dpkg/info/zsh.*
/var/lib/dpkg/info/zsh.list
/var/lib/dpkg/info/zsh.md5sums
/var/lib/dpkg/info/zsh.postinst
/var/lib/dpkg/info/zsh.postrm
/var/lib/dpkg/info/zsh.preinst
/var/lib/dpkg/info/zsh.prerm
This directory also includes a file with the .list extension for each package, contain-
ing the list of files that belong to that package:
6
https://www.debian.org/doc/debian-policy/ch-maintainerscripts.html
Chapter 8 — Debian Package Management 217
$ head /var/lib/dpkg/info/zsh.list
/.
/bin
/bin/zsh
/bin/zsh5
/usr
/usr/lib
/usr/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu/zsh
/usr/lib/x86_64-linux-gnu/zsh/5.2
/usr/lib/x86_64-linux-gnu/zsh/5.2/zsh
[...]
The /var/lib/dpkg/status file contains a series of data blocks (in the format of
the famous mail headers request for comment, RFC 2822) describing the status of
each package. The information from the control file of the installed packages is also
replicated there.
$ more /var/lib/dpkg/status
Package: gnome-characters
Status: install ok installed
Priority: optional
Section: gnome
Installed-Size: 1785
Maintainer: Debian GNOME Maintainers <pkg-gnome-
å maintainers@lists.alioth.debian.org>
Architecture: amd64
Version: 3.20.1-1
[...]
Let’s discuss the configuration files and see how they interact. In general, the preinst script is
executed prior to installation of the package, while the postinst follows it. Likewise, prerm is
invoked before removal of a package and postrm afterwards. An update of a package is equivalent
to removal of the previous version and installation of the new one. It is not possible to describe in
detail all the possible scenarios here but we will discuss the most common two: an installation/up-
date and a removal.
These sequences can be quite confusing, but a visual representation may help. Manoj Srivastava
made some diagrams7 explaining how the configuration scripts are called by dpkg. Similar dia-
grams have also been developed by the Debian Women project8 ; they are a bit simpler to under-
stand, but less complete.
7
https://people.debian.org/~srivasta/MaintainerScripts.html
8
https://wiki.debian.org/MaintainerScripts
218 Kali Linux Revealed
Caution The sequences described in this section call configuration scripts by specific names,
Symbolic Names of the such as old-prerm or new-postinst. They are, respectively, the prerm script con-
Scripts tained in the old version of the package (installed before the update) and the postinst
script contained in the new version (installed by the update).
Installation and Upgrade Script Sequence
Here is what happens during an installation (or an update):
1. For an update, dpkg calls the old-prerm upgrade new-version.
2. Still for an update, dpkg then executes new-preinst upgrade old-version; for a first in-
stallation, it executes new-preinst install. It may add the old version in the last parame-
ter if the package has already been installed and removed (but not purged, the configuration
files having been retained).
3. The new package files are then unpacked. If a file already exists, it is replaced, but a backup
copy is made and temporarily stored.
4. For an update, dpkg executes old-postrm upgrade new-version.
5. dpkg updates all of the internal data (file list, configuration scripts, etc.) and removes the
backups of the replaced files. This is the point of no return: dpkg no longer has access to all
of the elements necessary to return to the previous state.
6. dpkg will update the configuration files, prompting you to decide if it is unable to automat-
ically manage this task. The details of this procedure are discussed in section 8.4.3, “Check-
sums, Conffiles” [page 220].
7. Finally,dpkg configures the package by executing new-postinst configure
last-version-configured.
Package Removal
Here is what happens during a package removal.
1. dpkg calls prerm remove.
2. dpkg removes all of the package’s files, with the exception of the configuration files and
configuration scripts.
3. dpkg executes postrm remove. All of the configuration scripts, except postrm, are removed.
If you have not used the purge option, the process stops here.
4. For a complete purge of the package (command issued with dpkg --purge or dpkg -P),
the configuration files are also deleted, as well as a certain number of copies (*.dpkg-tmp,
*.dpkg-old, *.dpkg-new) and temporary files; dpkg then executes postrm purge.
Chapter 8 — Debian Package Management 219
In some cases, a package might use debconf to require configuration information from you: the
four scripts detailed above are then complemented by a config script designed to acquire that in-
formation. During installation, this script defines in detail what questions debconf will ask. The
responses are recorded in the debconf database for future reference. The script is generally exe-
cuted by apt prior to installing packages one by one in order to group all the questions together at
the beginning of the process. The pre- and post-installation scripts can then use this information
to operate according to your wishes.
The debconf Tool The debconf tool was created to resolve a recurring problem in Debian. All Debian
packages unable to function without a minimum of configuration used to ask ques-
tions with calls to the echo and read commands in postinst shell scripts (and other
similar scripts). This forced the installer to babysit large installations or updates in
order to respond to various configuration queries as they arose. These manual inter-
actions have now been almost entirely dispensed with, thanks to debconf.
The debconf tool has many interesting features: It requires the developer to specify
user interaction; it allows localization of all the displayed strings (all translations are
stored in the templates file describing the interactions); it provides different fron-
tends for questions (text mode, graphical mode, non-interactive); and it allows cre-
ation of a central database of responses to share the same configuration with several
computers. The most important feature is that all of the questions can be presented in
a row, all at once, prior to starting a long installation or update process. Now, you can
go about your business while the system handles the installation on its own, without
having to stay there staring at the screen, waiting for questions to pop up.
8.4.3. Checksums, Conffiles
In addition to the maintainer scripts and control data already mentioned in the previous sections,
the control.tar.gz archive of a Debian package may contain other interesting files:
# ar p /var/cache/apt/archives/bash_4.4-2_amd64.deb control.tar.gz | tar -tzf -
./
./conffiles
./control
./md5sums
./postinst
./postrm
./preinst
./prerm
The first—md5sums—contains the MD5 checksums for all of the package’s files. Its main advantage
is that it allows dpkg --verify to check if these files have been modified since their installation.
Note that when this file doesn’t exist, dpkg will generate it dynamically at installation time (and
store it in the dpkg database just like other control files).
220 Kali Linux Revealed
conffiles lists package files that must be handled as configuration files. Configuration files can
be modified by the administrator, and dpkg will try to preserve those changes during a package
update.
In effect, in this situation, dpkg behaves as intelligently as possible: if the standard configuration
file has not changed between the two versions, it does nothing. If, however, the file has changed,
it will try to update this file. Two cases are possible: either the administrator has not touched this
configuration file, in which case dpkg automatically installs the new version; or the file has been
modified, in which case dpkg asks the administrator which version they wish to use (the old one
with modifications, or the new one provided with the package). To assist in making this decision,
dpkg offers to display a diff that shows the difference between the two versions. If you choose to
retain the old version, the new one will be stored in the same location in a file with the .dpkg-dist
suffix. If you choose the new version, the old one is retained in a file with the .dpkg-old suffix.
Another available action consists of momentarily interrupting dpkg to edit the file and attempt to
reinstate the relevant modifications (previously identified with diff).
dpkg handles configuration file updates, but, while doing so, regularly interrupts its work to ask
for input from the administrator. This can be time consuming and inconvenient. Fortunately,
you can instruct dpkg to respond to these prompts automatically. The --force-confold option
retains the old version of the file, while --force-confnew will use the new version. These choices
are respected, even if the file has not been changed by the administrator, which only rarely has
the desired effect. Adding the --force-confdef option tells dpkg to decide by itself when pos-
sible (in other words, when the original configuration file has not been touched), and only uses
--force-confnew or --force-confold for other cases.
These options apply to dpkg, but most of the time the administrator will work directly with the
aptitude or apt programs. It is, thus, necessary to know the syntax used to indicate the options
to pass to the dpkg command (their command line interfaces are very similar).
# apt -o DPkg::options::=”--force-confdef” -o DPkg::options::=”--force-confold” full-
å upgrade
These options can be stored directly in apt’s configuration. To do so, simply write the following
line in the /etc/apt/apt.conf.d/local file:
DPkg::options { ”--force-confdef”; ”--force-confold”; }
Including this option in the configuration file means that it will also be used in a graphical inter-
face such as aptitude.
Conversely, you can also force dpkg to ask configuration file questions. The --force-confask
option instructs dpkg to display the questions about the configuration files, even in cases where
they would not normally be necessary. Thus, when reinstalling a package with this option, dpkg
will ask the questions again for all of the configuration files modified by the administrator. This is
very convenient, especially for reinstalling the original configuration file if it has been deleted and
Chapter 8 — Debian Package Management 221
no other copy is available: a normal re-installation won’t work, because dpkg considers removal
as a form of legitimate modification, and, thus, doesn’t install the desired configuration file.
8.5. Summary
In this section, we learned more about the Debian package system, discussed the Advanced Pack-
age Tool (APT) and dpkg, learned about basic package interaction, advanced APT configuration
and usage, and dug deeper into the Debian package system with a brief reference of the .deb file
format. We looked at the control file, configuration scripts, checksums, and the conffiles file.
Summary Tips:
A Debian package is a compressed archive of a software application. It contains the application’s
files as well as other metadata including the names of the dependencies that the application needs
plus any scripts that enable the execution of commands at different stages in the package’s lifecy-
cle (installation, removal, upgrades).
The dpkg tool, contrary to apt and apt-get (of the APT family), has no knowledge of all the avail-
able packages that could be used to fulfill package dependencies. Thus, to manage Debian pack-
ages, you will likely use the latter tools as they are able to automatically resolve dependency issues.
You can use APT to install and remove applications, update packages, and even upgrade your
entire system. Here are the key points that you should know about APT and its configuration:
• The sources.list file is the key configuration file for defining package sources (or reposi-
tories that contain packages).
• Debian and Kali use three sections to differentiate packages according to the licenses chosen
by the authors of each work: main contains all packages that fully comply with the Debian
Free Software Guidelines9 ; non-free contains software that does not (entirely) conform to
the Free Software Guidelines but can nevertheless be distributed without restrictions; and
contrib (contributions) includes open source software that cannot function without some
non-free elements.
• Kali maintains several repositories including: kali-rolling, which is the main repository for
end-users and should always contain installable and recent packages; and kali-dev, which is
used by Kali developers and is not for public use.
• When working with APT, you should first download the list of currently-available packages
with apt update.
• You can add a package to the system with a simple apt install package. APT will auto-
matically install the necessary dependencies.
• To remove a package use apt remove package. It will also remove the reverse dependen-
cies of the package (i.e. packages that depend on the package to be removed).
9
https://www.debian.org/social_contract#guidelines
222 Kali Linux Revealed
• To remove all data associated with a package, you can “purge” the package with the apt
purge package command. Unlike a removal, this will not only remove the package but
also its configuration files and sometimes the associated user data.
We recommend regular upgrades to install the latest security updates. To upgrade, use apt
update followed by either apt upgrade, apt-get upgrade, or aptitude safe-upgrade. These
commands look for installed packages that can be upgraded without removing any packages.
For more important upgrades, such as major version upgrades, use apt full-upgrade. With this
instruction, apt will complete the upgrade even if it has to remove some obsolete packages or
install new dependencies. This is also the command that you should use for regular upgrades of
your Kali Rolling system. Review the pros and cons of updates we outlined in this chapter.
Several tools can be used to inspect Debian packages:
• dpkg --listfiles package (or -L) lists the files that were installed by the specified package.
• dpkg --search file (or -S) finds any packages containing the file or path passed in the
argument.
• dpkg --list (or -l) displays the list of packages known to the system and their installation
status.
• dpkg --contents file.deb (or -c) lists all the files in a particular .deb file.
• dpkg --info file.deb (or -I) displays the headers of the specified .deb file.
• The various apt-cache subcommands display much of the information stored in APT’s in-
ternal database.
To avoid excessive disk usage, you should regularly sort through /var/cache/apt/archives/.
Two commands can be used for this: apt clean (or apt-get clean) entirely empties the direc-
tory; apt autoclean (apt-get autoclean) only removes packages that can no longer be down-
loaded because they have disappeared from the mirror and are therefore useless.
aptitude is an interactive program that can be used in semi-graphical mode on the console. It is
an extremely robust program that can help you install and troubleshoot packages.
synaptic is a graphical package manager that features a clean and efficient graphical interface.
As an advanced user, you can create files in /etc/apt/apt.conf.d/ to configure certain aspects
of APT. You can also manage package priorities, track automatically installed packages, work with
several distributions or architectures at once, use cryptographic signatures to validate packages,
and upgrade files using the techniques outlined in this chapter.
In spite of the Debian or Kali maintainers’ best efforts, a system upgrade isn’t always as smooth as
we would hope. When this happens, you can look at the Kali bug tracker10 and at the Debian bug
tracking system11 at https://bugs.debian.org/package to check whether the problem has already
10
https://bugs.kali.org/
11
https://bugs.debian.org/
Chapter 8 — Debian Package Management 223
been reported. You can also try to downgrade the package or to debug and repair a failed package
maintainer script.
224 Kali Linux Revealed
Keywords
Custom packages
Custom kernel
Custom images
live-build
Persistence
Chapter
Advanced Usage 9
Contents
Modifying Kali Packages 228 Recompiling the Linux Kernel 237 Building Custom Kali Live ISO Images 241
Adding Persistence to the Live ISO with a USB Key 246 Summary 251
Kali has been built as a highly modular and customizable penetration testing platform and allows
for some fairly advanced customization and usage. Customizations can happen at multiple lev-
els, beginning at the source code level. The sources of all Kali packages are publicly available. In
this chapter, we will show how you can retrieve packages, modify them, and build your own cus-
tomized packages out of them. The Linux kernel is somewhat of a special case and as such, it is
covered in a dedicated section (section 9.2, “Recompiling the Linux Kernel” [page 237]), where we
will discuss where to find sources, how to configure the kernel build, and finally how to compile
it and how to build the associated kernel packages.
The second level of customization is in the process of building live ISO images. We will show how
the live-build tool offers plenty of hooks and configuration options to customize the resulting
ISO image, including the possibility to use custom Debian packages in place of the packages avail-
able on mirrors.
We will also discuss how you can create a persistent live ISO built onto a USB key that will preserve
files and operating system changes between reboots.
9.1. Modifying Kali Packages
Modifying Kali packages is usually a task for Kali contributors and developers: they update pack-
ages with new upstream versions, they tweak the default configuration for a better integration in
the distribution, or they fix bugs reported by users. But you might have specific needs not fulfilled
by the official packages and knowing how to build a modified package can thus be very valuable.
You might wonder why you need to bother with the package at all. After all, if you have to modify
a piece of software, you can always grab its source code (usually with git) and run the modified
version directly from the source checkout. This is fine when it is possible and when you use your
home directory for this purpose, but if your application requires a system-wide setup (for example,
with a make install step) then it will pollute your file system with files unknown to dpkg and will
soon create problems that cannot be caught by package dependencies. Furthermore, with proper
packages you will be able to share your changes and deploy them on multiple computers much
more easily or revert the changes after having discovered that they were not working as well as
you hoped.
So when would you want to modify a package? Let’s take a look at a few examples. First, we
will assume that you are a heavy user of Social-Engineer Toolkit (SET) and you noticed a new
upstream release but the Kali developers are all busy for a conference and you want to try it out
immediately. You want to update the package yourself. In another case, we will assume that you
are struggling to get your MIFARE NFC card working and you want to rebuild “libfreefare” to
enable debug messages in order to have actionable data to provide in a bug report that you are
currently preparing. In a last case, we will assume that the “pyrit” program fails with a cryptic
error message. After a web search, you find a commit that you expect to fix your problem in the
upstream GitHub repository and you want to rebuild the package with this fix applied.
228 Kali Linux Revealed
We will go through all of those samples in the following sections. We will try to generalize the
explanations so that you can better apply the instructions to other cases but it is impossible to
cover all situations that you might encounter. If you hit problems, apply your best judgment to
find a solution or go seek help on the most appropriate forums (see chapter 6, “Helping Yourself
and Getting Help” [page 128]).
Whatever change you want to make, the general process is always the same: grab the source pack-
age, extract it, make your changes, then build the package. But for each step, there are often
multiple tools that can handle the task. We picked the most relevant and most popular tools, but
our review is not exhaustive.
9.1.1. Getting the Sources
Rebuilding a Kali package starts with getting its source code. A source package is composed of
multiple files: the main file is the *.dsc (Debian Source Control) file as it lists the other accompa-
nying files, which can be *.tar.gz,bz2,xz, sometimes *.diff.gz, or *.debian.tar.gz,bz2,xz
files.
The source packages are stored on Kali mirrors that are available over HTTP. You could use your
web browser to download all the required files but the easiest way to accomplish this is to use
the apt source source_package_name command. This command requires a deb-src line in the
/etc/apt/sources.list file and up-to-date index files (accomplished by running apt update).
By default, Kali doesn’t add the required line as few Kali users actually need to retrieve source
packages but you can easily add it (see sample file in section 8.1.3, “Kali Repositories” [page 179]
and the associated explanations in section 8.1.2, “Understanding the sources.list File” [page
178]).
$ apt source libfreefare
Reading package lists... Done
NOTICE: ’libfreefare’ packaging is maintained in the ’Git’ version control system at:
git://anonscm.debian.org/collab-maint/libnfc.git
Please use:
git clone git://anonscm.debian.org/collab-maint/libnfc.git
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 119 kB of source archives.
Get:1 http://kali.download/kali kali-rolling/main libfreefare 0.4.0-2.1 (dsc) [2,144 B]
Get:2 http://kali.download/kali kali-rolling/main libfreefare 0.4.0-2.1 (tar) [113 kB]
Get:3 http://kali.download/kali kali-rolling/main libfreefare 0.4.0-2.1 (diff) [3,732 B]
Fetched 119 kB in 1s (100 kB/s)
dpkg-source: info: extracting libfreefare in libfreefare-0.4.0
dpkg-source: info: unpacking libfreefare_0.4.0.orig.tar.gz
dpkg-source: info: unpacking libfreefare_0.4.0-2.1.debian.tar.xz
$ cd libfreefare-0.4.0
$ ls
AUTHORS cmake configure.ac COPYING examples libfreefare m4 NEWS test
ChangeLog CMakeLists.txt contrib debian HACKING libfreefare.pc.in Makefile.am README TODO
$ ls debian
changelog compat control copyright libfreefare0.install libfreefare-bin.install libfreefare-dev.install libfreefare-doc.install README.Source rules source watch
In this example, while we received the source package from a Kali mirror, the package is the
same as in Debian since the version string doesn’t contain “kali.” This means that no kali-specific
changes have been applied.
If you need a specific version of the source package, which is currently not available in the repos-
itories listed in /etc/apt/sources.list, then the easiest way to download it is to find out the
URL of its .dsc file by looking it up on https://pkg.kali.org/ and then handing that URL over
to dget (from the devscripts package).
Chapter 9 — Advanced Usage 229
After having looked up the URL of the libreefare source package available in kali-dev, you can
download it with dget. It will first download the .dsc file, then parse it to know what other files
are referenced, and then download those from the same location:
$ dget http://http.kali.org/pool/main/libf/libfreefare/libfreefare_0.4.0+0~
å git1439352548.ffde4d-1.dsc
dget: retrieving http://http.kali.org/pool/main/libf/libfreefare/libfreefare_0.4.0+0~
å git1439352548.ffde4d-1.dsc
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 362 100 362 0 0 1117 0 --:--:-- --:--:-- --:--:-- 1120
100 1935 100 1935 0 0 3252 0 --:--:-- --:--:-- --:--:-- 3252
dget: retrieving http://http.kali.org/pool/main/libf/libfreefare/libfreefare_0.4.0+0~
å git1439352548.ffde4d.orig.tar.gz
[...]
libfreefare_0.4.0+0~git1439352548.ffde4d-1.dsc:
dscverify: libfreefare_0.4.0+0~git1439352548.ffde4d-1.dsc failed signature check:
gpg: WARNING: no command supplied. Trying to guess what you mean ...
gpg: Signature made Wed 12 Aug 2015 12:14:03 AM EDT
gpg: using RSA key 43EF73F4BD8096DA
gpg: Can’t check signature: No public key
Validation FAILED!!
$ dpkg-source -x libfreefare_0.4.0+0~git1439352548.ffde4d-1.dsc
gpgv: Signature made Wed 12 Aug 2015 12:14:03 AM EDT
gpgv: using RSA key 43EF73F4BD8096DA
gpgv: Can’t check signature: No public key
dpkg-source: warning: failed to verify signature on ./libfreefare_0.4.0+0~git1439352548
å .ffde4d-1.dsc
dpkg-source: info: extracting libfreefare in libfreefare-0.4.0+0~git1439352548.ffde4d
dpkg-source: info: unpacking libfreefare_0.4.0+0~git1439352548.ffde4d.orig.tar.gz
dpkg-source: info: unpacking libfreefare_0.4.0+0~git1439352548.ffde4d-1.debian.tar.xz
It is worth noting that dget did not automatically extract the source package because it could
not verify the PGP signature on the source package. Thus we did that step manually with
dpkg-source -x dsc-file. You can also force the source package extraction by passing the -
-allow-unauthenticated or -u option. Inversely, you can use --download-only to skip the source
package extraction step.
Retrieving Sources from You might have noticed that the apt source invocation tells you about a possible Git
Git repository used to maintain the package. It might point to a Debian Git repository or
to a Kali Git repository.
All Kali-specific packages are maintained in Git repositories hosted on gitlab.com/-
kalilinux/packages1 . You can retrieve the sources from those repositories with git
clone https://gitlab.com/kalilinux/packages/source-package.git.
230 Kali Linux Revealed
Contrary to what you get with apt source, the obtained tree will not have patches
automatically applied. Have a look at debian/patches/ to learn about the possible
changes made by Kali.
$ git clone https://gitlab.com/kalilinux/packages/kali-meta.
å git
Cloning into ’kali-meta’...
remote: Counting objects: 760, done.
remote: Compressing objects: 100% (614/614), done.
remote: Total 760 (delta 279), reused 0 (delta 0)
Receiving objects: 100% (760/760), 141.01 KiB | 0 bytes/s,
å done.
Resolving deltas: 100% (279/279), done.
Checking connectivity... done.
$ cd kali-meta
$ ls
debian
$ ls debian
changelog compat control copyright rules source
You can use the Git repositories as another way to retrieve the sources and thus
(mostly) follow the other instructions from this section. But when Kali developers
work with those repositories, they use another packaging workflow and use tools
from the git-buildpackage package that we will not cover here. You can learn more
about those tools here:
è https://honk.sigxcpu.org/piki/projects/git-buildpackage/
9.1.2. Installing Build Dependencies
Now that you have the sources, you still need to install build dependencies. They will be necessary
to build the desired binary packages but are also likely required for partial builds that you might
want to run to test the changes while you make them.
Each source package declares its build dependencies in the Build-Depends field of the debian/
control file. Let’s instruct apt to install those (assuming that you are in a directory containing
an unpacked source package):
$ sudo apt build-dep ./
Note, using directory ’./’ to get the build dependencies
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
autoconf automake autopoint autotools-dev debhelper dh-autoreconf
1
https://gitlab.com/kalilinux/packages
Chapter 9 — Advanced Usage 231
dh-strip-nondeterminism gettext intltool-debian libarchive-zip-perl
libfile-stripnondeterminism-perl libtool po-debconf
0 upgraded, 13 newly installed, 0 to remove and 0 not upgraded.
Need to get 4 456 kB of archives.
After this operation, 14,6 MB of additional disk space will be used.
Do you want to continue? [Y/n]
[...]
In this sample, all build dependencies can be satisfied with packages available to APT. This might
not always be the case as the tool building kali-rolling does not ensure installability of build de-
pendencies (only dependencies of binary packages are taken into account). In practice, binary
dependencies and build dependencies are often tightly coupled and most packages will have their
build dependencies satisfiable.
9.1.3. Making Changes
We can’t cover all the possible changes that you might want to make to a given package in this
section. This would amount to teaching you all the nitty gritty2 details of Debian packaging. How-
ever, we will cover the three common use cases presented earlier and we will explain some of the
unavoidable parts (like maintaining the changelog file).
The first thing to do is to change the package version number so that the rebuilt packages can be
distinguished from the original packages provided by Debian or Kali. To achieve this, we usually
add a suffix identifying the entity (person or company) applying the changes. Since buxy is my
IRC nickname, I will use it as a suffix. Such a change is best effected with the dch command (Debian
CHangelog) from the devscripts package, with a command such as dch --local buxy. This invokes
a text editor (sensible-editor, which runs the editor assigned in the VISUAL or EDITOR environ-
ment variables, or /usr/bin/editor otherwise), which allows you to document the differences
introduced by this rebuild. This editor shows that dch really did change the debian/changelog
file:
$ head -n 1 debian/changelog
libfreefare (0.4.0-2) unstable; urgency=low
$ dch --local buxy
[...]
$ head debian/changelog
libfreefare (0.4.0-2buxy1) UNRELEASED; urgency=medium
* Enable --with-debug configure option.
-- Raphael Hertzog <buxy@kali.org> Fri, 22 Jan 2021 10:36:00 -0400
libfreefare (0.4.0-2) unstable; urgency=low
2
https://www.debian.org/doc/manuals/maint-guide/
232 Kali Linux Revealed
* Update debian/copyrtight.
Fix license to LGPL3+.
If you do such changes regularly, you might want to set the DEBFULLNAME and DEBEMAIL en-
vironment variables to your full name and your email, respectively. Their values will be used
by many packaging tools, including dch, which will embed them on the trailer line shown above
(starting with “ -- ”).
Applying a Patch
In one of our use cases, we have downloaded the pyrit source package and we want to apply a patch
that we found in the upstream Git repository. This is a common operation and it should always be
simple. Unfortunately, patches can be handled in different ways depending on the source package
format and on the Git packaging workflow in use (when Git is used to maintain the package).
With an Unpacked Source Package You have run apt source pyrit and you have a pyrit-0.
4.0 directory. You can apply your patch directly with patch -p1 < patch-file:
$ apt source pyrit
[...]
$ cd pyrit-0.4.0
$ wget https://github.com/JPaulMora/Pyrit/commit/14
å ec997174b8e8fd20d22b6a97c57e19633f12a0.patch -O /tmp/pyrit-patch
[...]
$ patch -p1 </tmp/pyrit-patch
patching file cpyrit/pckttools.py
Hunk #1 succeeded at 53 (offset -1 lines).
$ dch --local buxy ”Apply patch to work with scapy 2.3”
At this point, you have manually patched the source code and you can already build binary pack-
ages of your modified version (see section 9.1.4, “Starting the Build” [page 236]). But if you
try to build an updated source package, it will fail, complaining about “unexpected upstream
changes.” This is because pyrit (like a majority of the source packages) uses the source format (see
debian/source/format file) known as 3.0 (quilt), where changes to the upstream code must be
recorded in separate patches stored in debian/patches/ and where the debian/patches/series
file indicates the order in which patches must be applied. You can register your changes in a new
patch by running dpkg-source --commit:
$ dpkg-source --commit
dpkg-source: info: local changes detected, the modified files are:
pyrit-0.4.0/cpyrit/pckttools.py
Enter the desired patch name: fix-for-scapy-2.3.patch
Chapter 9 — Advanced Usage 233
dpkg-source: info: local changes have been recorded in a new patch: pyrit-0.4.0/debian/
å patches/fix-for-scapy-2.3.patch
$ tail -n 1 debian/patches/series
fix-for-scapy-2.3.patch
Quilt Patch Series This patch management convention has been popularized by a tool named quilt
and the “3.0 (quilt)” source package format is thus compatible with this tool—with
the small deviation that it uses debian/patches instead of patches. This tool is
available in the package of the same name and you can find a nice tutorial here:
è https://raphaelhertzog.com/2012/08/08/
how-to-use-quilt-to-manage-patches-in-debian-packages/
If the source package uses the 1.0 or 3.0 (native) source format, then there is no requirement to
register your upstream changes in a patch. They are automatically bundled in the resulting source
package.
With a Git Repository If you have used Git to retrieve the source package, the situation is even
more complicated. There are multiple Git workflows and associated tools, and obviously not all
Debian packages are using the same workflows and tools. The distinction already explained about
source format is still relevant but you must also check whether patches are pre-applied in the
source tree or whether they are only stored in debian/patches (in this case, they are then applied
at build time).
The most popular tool is git-buildpackage. It is what we use to manage all repositories on git-
lab.com/kalilinux/packages. When you use it, patches are not pre-applied in the source tree but
they are stored in debian/patches. You can manually add patches in that directory and list them
in debian/patches/series but users of git-buildpackage tend to use gbp pq to edit the entire
patch series as a single branch that you can extend or rebase to your liking. Check the manual
pages for gbp-pq(1) to learn how to invoke it.
git-dpm (with associated command of the same name) is another git packaging tool that you can
find in use. It records metadata in debian/.git-dpm and keeps patches applied in the source tree
by merging a constantly-rebased branch that it builds out of the content of debian/patches.
Tweaking Build Options
You usually have to tweak build options when you want to enable an optional feature or behavior
that is not activated in the official package, or when you want to customize parameters that are
set at build time through a ./configure option or through variables set in the build environment.
In those cases, the changes are usually limited to debian/rules, which drives the steps in the
package build process. In the simplest cases, the lines concerning the initial configuration (./con-
figure …) or the actual build ($(MAKE) … or make …) are easy to spot. If these commands are not
234 Kali Linux Revealed
explicitly called, they are probably a side effect of another explicit command, in which case, please
refer to their documentation to learn more about how to change the default behavior. With pack-
ages using dh, you might need to add an override for the dh_auto_configure or dh_auto_build
commands (see their respective manual pages for explanations on how to achieve this).
To make those explanations more concrete, let’s apply them to our sample use case. You decided
to modify libfreefare to pass the --enable-debug option to the ./configure script so that you
could get a more verbose output from your near field communication (NFC) tools and file a better
bug report about your non-recognized Mifare NFC card. Since the package uses dh to drive the
build process, you add (or in this case modify) the override_dh_auto_configure target. Here is the
corresponding extract of libfreefare’s debian/rules file:
override_dh_auto_configure:
dh_auto_configure -- --without-cutter --disable-silent-rules --enable-debug
Packaging a New Upstream Version
Let’s take a look at an example at this point, as we discuss packaging upstream versions. Let’s say
you are a SET power-user and you noticed a new upstream release (7.4.5) that is not yet available
in Kali (which only has version 7.4.4). You want to build an updated package and try it out. This is a
minor version bump and you thus don’t expect the update to require any change at the packaging
level.
To update the source package, you extract the new source tarball next to the current source pack-
age and you copy the debian directory from the current source package to the new one. Then
you bump the version in debian/changelog.
$ apt source set
Reading package lists... Done
NOTICE: ’set’ packaging is maintained in the ’Git’ version control system at:
https://gitlab.com/kalilinux/packages/set.git
Please use:
git clone https://gitlab.com/kalilinux/packages/set.git
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 42.3 MB of source archives.
[...]
dpkg-source: warning: failed to verify signature on ./set_7.4.4-0kali1.dsc
dpkg-source: info: extracting set in set-7.4.4
dpkg-source: info: unpacking set_7.4.4.orig.tar.gz
dpkg-source: info: unpacking set_7.4.4-0kali1.debian.tar.xz
dpkg-source: info: applying edit-config-file
dpkg-source: info: applying fix-path-interpreter.patch
$ wget https://github.com/trustedsec/social-engineer-toolkit/archive/7.4.5.tar.gz -O
å set_7.4.5.orig.tar.gz
[...]
Chapter 9 — Advanced Usage 235
$ tar xvf set_7.4.5.orig.tar.gz
[...]
social-engineer-toolkit-7.4.5/src/wireless/wifiattack.py
$ cp -a set-7.4.4/debian social-engineer-toolkit-7.4.5/debian
$ cd social-engineer-toolkit-7.4.5
$ dch -v 7.4.5-0buxy1 ”New upstream release”
That’s it. You can now build the updated package.
Depending on the kind of changes that the new upstream version introduces, you may also need
to change build dependencies and run-time dependencies, and install new files. Those are much
more involved operations that are not covered by this book.
9.1.4. Starting the Build
When all the needed changes have been applied to the sources, you can start generating the actual
binary package or .deb file. The whole process is managed by the dpkg-buildpackage command
and it looks like this:
$ dpkg-buildpackage -us -uc -b
dpkg-buildpackage: source package libfreefare
dpkg-buildpackage: source version 0.4.0-2buxy1
dpkg-buildpackage: source distribution UNRELEASED
dpkg-buildpackage: source changed by Raphael Hertzog <buxy@kali.org>
dpkg-buildpackage: host architecture amd64
[...]
dh_builddeb
dpkg-deb: building package ’libfreefare0-dbgsym’ in ’../libfreefare0-dbgsym_0.4.0-2buxy1_amd64.deb’.
dpkg-deb: building package ’libfreefare0’ in ’../libfreefare0_0.4.0-2buxy1_amd64.deb’.
dpkg-deb: building package ’libfreefare-dev’ in ’../libfreefare-dev_0.4.0-2buxy1_amd64.deb’.
dpkg-deb: building package ’libfreefare-bin-dbgsym’ in ’../libfreefare-bin-dbgsym_0.4.0-2buxy1_amd64.deb’.
dpkg-deb: building package ’libfreefare-bin’ in ’../libfreefare-bin_0.4.0-2buxy1_amd64.deb’.
dpkg-deb: building package ’libfreefare-doc’ in ’../libfreefare-doc_0.4.0-2buxy1_all.deb’.
dpkg-genchanges -b >../libfreefare_0.4.0-2buxy1_amd64.changes
dpkg-genchanges: binary-only upload (no source code included)
dpkg-source --after-build libfreefare-0.4.0
dpkg-buildpackage: binary-only upload (no source included)
The -us -uc options disable signatures on some of the generated files (.dsc, .changes) because this
operation will fail if you do not have a GnuPG key associated with the identity you have put in the
changelog file. The -b option asks for a “binary-only build.” In this case, the source package (.dsc)
will not be created, only the binary (.deb) packages will. Use this option to avoid failures during
the source package build: if you haven’t properly recorded your changes in the patch management
system, it might complain and interrupt the build process.
As suggested by dpkg-deb’s messages, the generated binary packages are now available in the
parent directory (the one that hosts the directory of the source package). You can install them
with dpkg -i or apt install.
$ sudo apt install ../libfreefare0_0.4.0-2buxy1_amd64.deb \
../libfreefare-bin_0.4.0-2buxy1_amd64.deb
Reading package lists... Done
236 Kali Linux Revealed
Building dependency tree
Reading state information... Done
Note, selecting ’libfreefare0’ instead of ’../libfreefare0_0.4.0-2buxy1_amd64.deb’
Note, selecting ’libfreefare-bin’ instead of ’../libfreefare-bin_0.4.0-2buxy1_amd64.deb’
The following packages will be upgraded:
libfreefare-bin libfreefare0
2 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/69,4 kB of archives.
After this operation, 2 048 B of additional disk space will be used.
[...]
We prefer apt install over dpkg -i as it will deal with missing dependencies gracefully. But
not so long ago, you had to use dpkg as apt was not able to deal with .deb files outside of any
repository.
dpkg-buildpackage More often than not, Debian developers use a higher-level program such as debuild;
wrappers this runs dpkg-buildpackage as usual, but it also adds an invocation of a program
(lintian) that runs many checks to validate the generated package against the De-
bian policy3 . This script also cleans up the environment so that local environment
variables do not pollute the package build. The debuild command is one of the tools
in the devscripts suite, which share some consistency and configuration to make the
maintainers’ task easier.
9.2. Recompiling the Linux Kernel
The kernels provided by Kali include the largest possible number of features, as well as the max-
imum number of drivers, in order to cover the broadest spectrum of existing hardware configu-
rations. This is why some users prefer to recompile the kernel in order to include only what they
specifically need. There are two reasons for this choice. First, it is a way to optimize memory
consumption since all kernel code, even if it is never used, occupies physical memory. Because
the statically compiled portions of the kernel are never moved to swap space, an overall decrease
in system performance will result from having drivers and features built in that are never used.
Second, reducing the number of drivers and kernel features reduces the risk of security problems
since only a fraction of the available kernel code is being run.
3
https://www.debian.org/doc/debian-policy/
Chapter 9 — Advanced Usage 237
Important
If you choose to compile your own kernel, you must accept the conse-
quences: Kali cannot ensure security updates for your custom kernel. By
keeping the kernel provided by Kali, you benefit from updates prepared by
the Debian Project.
Recompilation of the kernel is also necessary if you want to use certain features that are only
available as patches (and not included in the standard kernel version).
The Debian Kernel The Debian kernel team maintains the Debian Kernel Handbook (also available in the
Handbook debian-kernel-handbook package) with comprehensive documentation about most
kernel-related tasks and about how official Debian kernel packages are handled. This
is the first place you should look into if you need more information than what is
provided in this section.
è https://kernel-team.pages.debian.net/kernel-handbook/
9.2.1. Introduction and Prerequisites
Unsurprisingly, Debian and Kali manage the kernel in the form of a package, which is not how ker-
nels have traditionally been compiled and installed. Since the kernel remains under the control of
the packaging system, it can then be removed cleanly, or deployed on several machines. Further-
more, the scripts associated with these packages automate the interaction with the bootloader
and the initrd generator.
The upstream Linux sources contain everything needed to build a Debian package of the kernel
but you still need to install the build-essential package to ensure that you have the tools required to
build a Debian package. Furthermore, the configuration step for the kernel requires the libncurses5-
dev package. Finally, the fakeroot package will enable creation of the Debian package without need-
ing administrative privileges.
# apt install build-essential libncurses5-dev fakeroot
9.2.2. Getting the Sources
Since the Linux kernel sources are available as a package, you can retrieve them by installing the
linux-source-version package. The apt-cache search ^linux-source command should list the
latest kernel version packaged by Kali. Note that the source code contained in these packages
238 Kali Linux Revealed
does not correspond precisely with that published by Linus Torvalds and the kernel developers4 ;
like all distributions, Debian and Kali apply a number of patches, which might (or might not) find
their way into the upstream version of Linux. These modifications include backports of fixes/fea-
tures/drivers from newer kernel versions, new features not yet (entirely) merged in the upstream
Linux tree, and sometimes even Debian or Kali-specific changes.
The remainder of this section focuses on the 4.9 version of the Linux kernel, but the examples can,
of course, be adapted to the particular version of the kernel that you want.
In this example, we assume that the linux-source-4.9 binary package has been installed. Note that
we install a binary package containing the upstream sources but do not retrieve the Kali source
package named linux.
# apt install linux-source-4.9
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
bc libreadline7
Suggested packages:
libncurses-dev | ncurses-dev libqt4-dev
The following NEW packages will be installed:
bc libreadline7 linux-source-4.9
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 95.4 MB of archives.
After this operation, 95.8 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
[...]
# ls /usr/src
linux-config-4.9 linux-patch-4.9-rt.patch.xz linux-source-4.9.tar.xz
Notice that the package contains /usr/src/linux-source-4.9.tar.xz, a compressed archive of
the kernel sources. You must extract these files in a new directory (not directly under /usr/src/,
since there is no need for special permissions to compile a Linux kernel). Instead, ~/kernel/ is
more appropriate.
$ mkdir ~/kernel; cd ~/kernel
$ tar -xaf /usr/src/linux-source-4.9.tar.xz
9.2.3. Configuring the Kernel
The next step consists of configuring the kernel according to your needs. The exact procedure
depends on the goals.
4
https://www.kernel.org/
Chapter 9 — Advanced Usage 239
The kernel build depends on a kernel configuration file. In most cases, you will most likely keep
as close as possible to that proposed by Kali, which, like all Linux distributions, is installed in the
/boot directory. In this case, rather than reconfiguring everything from scratch, it is sufficient to
make a copy of the /boot/config- version file. The version should be the same as that version of
the kernel currently used, which can be found with the uname -r command. Place the copy into
a .config file in the directory containing the kernel sources.
$ cp /boot/config-4.9.0-kali1-amd64 ~/kernel/linux-source-4.9/.config
Alternatively, since the kernel provides default configurations in arch/arch/configs/*_
defconfig, you can put your selected configuration in place with a command like make
x86_64_defconfig (in the case of a 64-bit PC) or make i386_defconfig (in the case of a 32-bit
PC).
Unless you need to change the configuration, you can stop here and skip to section 9.2.4, “Com-
piling and Building the Package” [page 241]. If you need to make changes or if you decide to
reconfigure everything from scratch, you must take the time to configure your kernel. There are
various dedicated interfaces in the kernel source directory that can be used by calling the make
target command, where target is one of the values described below.
make menuconfig compiles and launches a text-mode kernel configuration interface (this is
where the libncurses5-dev package is required), which allows navigating the many available ker-
nel options in a hierarchical structure. Pressing the Space key changes the value of the selected
option, and Enter validates the button selected at the bottom of the screen; Select returns to the
selected sub-menu; Exit closes the current screen and moves back up in the hierarchy; Help will
display more detailed information on the role of the selected option. The arrow keys allow mov-
ing within the list of options and buttons. To exit the configuration program, choose Exit from
the main menu. The program then offers to save the changes that you have made; accept if you
are satisfied with your choices.
Other interfaces have similar features but they work within more modern graphical interfaces,
such as make xconfig, which uses a Qt graphical interface, and make gconfig, which uses GTK+.
The former requires libqt4-dev, while the latter depends on libglade2-dev and libgtk2.0-dev.
Dealing with Outdated When you provide a .config file that has been generated with another (usually older)
.config Files kernel version, you will have to update it. You can do so with make oldconfig, which
will interactively ask you the questions corresponding to the new configuration op-
tions. If you want to use the default answer to all those questions, you can use make
olddefconfig. With make oldnoconfig, it will assume a negative answer to all ques-
tions.
240 Kali Linux Revealed
9.2.4. Compiling and Building the Package
Clean Up Before If you have already compiled a kernel in the directory and wish to rebuild everything
Rebuilding from scratch (for example because you substantially changed the kernel configura-
tion), you will have to run make clean to remove the compiled files. make distclean
removes even more generated files, including your .config file, so make sure to back
it up first.
Once the kernel configuration is ready, a simple make deb-pkg will generate up to five Debian
packages in standard .deb format: linux-image-version, which contains the kernel image and the
associated modules; linux-headers-version, which contains the header files required to build ex-
ternal modules; linux-firmware-image-version, which contains the firmware files needed by some
drivers (this package might be missing when you build from the kernel sources provided by De-
bian or Kali); linux-image-version-dbg, which contains the debugging symbols for the kernel image
and its modules; and linux-libc-dev, which contains headers relevant to some user-space libraries
like GNU’s C library (glibc).
The version is defined by the concatenation of the upstream version (as defined by the variables
VERSION, PATCHLEVEL, SUBLEVEL, and EXTRAVERSION in the Makefile), of the LOCALVER-
SION configuration parameter, and of the LOCALVERSION environment variable. The package
version reuses the same version string with an appended revision that is regularly incremented
(and stored in .version), except if you override it with the KDEB_PKGVERSION environment
variable.
$ make deb-pkg LOCALVERSION=-custom KDEB_PKGVERSION=$(make kernelversion)-1
[...]
$ ls ../*.deb
../linux-headers-4.9.0-kali1-custom_4.9.2-1_amd64.deb
../linux-image-4.9.0-kali1-custom_4.9.2-1_amd64.deb
../linux-image-4.9.0-kali1-custom-dbg_4.9.2-1_amd64.deb
../linux-libc-dev_4.9.2-1_amd64.deb
To actually use the built kernel, the only step left is to install the required packages with dpkg -i
file.deb. The “linux-image” package is required; you only have to install the “linux-headers”
package if you have some external kernel modules to build, which is the case if you have some
“*-dkms” packages installed (check with dpkg -l ”*-dkms” | grep ^ii). The other packages
are generally not needed (unless you know why you need them)!
9.3. Building Custom Kali Live ISO Images
Kali Linux has a ton of functionality and flexibility right out of the box. Once Kali is installed, you
can perform all sorts of amazing feats with a little guidance, creativity, patience, and practice.
Chapter 9 — Advanced Usage 241
However, you can also customize a Kali build so that it contains specific files or packages (to scale
up or scale down performance and features) and can perform certain functions automatically. For
example, the Kali ISO of Doom5 and the Kali Evil Wireless Access Point6 are both excellent projects
that rely on a custom-built implementation of Kali Linux. Let’s take a look at the process of rolling
a custom Kali Linux ISO image.
Official Kali ISO images are built with live-build7 , which is a set of scripts that allows for the com-
plete automation and customization of all facets of ISO image creation. The live-build suite uses
an entire directory structure as input for its configuration. We store this configuration and some
associated helper scripts in a live-build-config Git repository. We will use this repository as a basis
for building customized images.
Before going further, you must know that the commands shown in this section are meant to be
run on an up-to-date Kali Linux system. They are very likely to fail if run on a non-Kali system or
if the system is out of date.
9.3.1. Installing Pre-Requisites
The first step is to install the packages needed and to retrieve the Git repository with the Kali
live-build configuration:
# apt install curl git live-build
[...]
# git clone https://gitlab.com/kalilinux/build-scripts/live-build-config.git
[...]
# cd live-build-config
# ls
auto bin build_all.sh build.sh kali-config README.md simple-cdd
At this point, you can already create an updated (but unmodified) Kali live ISO image just by run-
ning ./build.sh --verbose. The build will take a long time to complete as it will download all
the packages to include. When finished, you will find the freshly created ISO image in the new
images directory.
9.3.2. Building Live Images with Different Desktop Environments
The build.sh live-build wrapper that we provide is responsible for setting up the config direc-
tory that live-build expects to find. It can put in place different configurations depending on
its --variant option.
5
https://www.offensive-security.com/kali-linux/kali-linux-iso-of-doom/
6
https://www.offensive-security.com/kali-linux/kali-linux-evil-wireless-access-point/
7
https://live-team.pages.debian.net/live-manual/html/live-manual/index.en.html
242 Kali Linux Revealed
The wrapper creates the config directory by combining files from kali-config/common and
kali-config/variant- X, where X is the name of a variant given with the --variant parameter.
When the option is not explicitly given, it uses default as the name of the variant.
The kali-config directory contains directories for the most common desktop environments:
• e17 for Enlightenment;
• gnome for GNOME;
• i3wm for the corresponding window manager;
• kde for KDE;
• lxde for LXDE;
• mate for the Mate Desktop Environment;
• xfce for Xfce.
You can easily create a Kali live image using KDE as desktop environment with this single com-
mand:
# ./build.sh --variant kde --verbose
This concept of variant allows for some high-level pre-defined customizations but if you take the
time to read through the Debian Live System Manual8 , you will discover many other ways to cus-
tomize the images, just by changing the content of the appropriate sub-directory of kali-config.
The following sections will provide some examples.
9.3.3. Changing the Set of Installed Packages
Once launched, live-build installs all the packages listed in package-lists/*.list.chroot
files. The default configuration that we provide includes a package-lists/kali.list.chroot
file, which lists kali-linux-default (the main metapackage pulling all the Kali packages to include).
You can comment out this package and put another metapackage of your choice or include a pre-
cise set of other packages. You can also combine both approaches by starting with a metapackage
and adding supplementary packages of your choice.
With package-lists, you can only include packages that are already available in the official Kali
repository. But if you have custom packages, you can include them in the live image by placing the
.deb files in a packages.chroot directory (for example kali-config/config-gnome/packages.
chroot if you build the GNOME variant).
metapackages are empty packages whose sole purpose is to have many dependencies on other
packages. They make it easier to install sets of packages that you often want to install together.
The kali-meta source package builds all the metapackages provided by Kali Linux:
8
https://live-team.pages.debian.net/live-manual/html/live-manual.en.html
Chapter 9 — Advanced Usage 243
• kali-linux-core: the base system (it is pulled by all the other metapackages)
• kali-linux-headless: the default Kali Linux installation command line tools
• kali-linux-default: the default Kali Linux installation, both command line and graphical
• kali-linux-large: wider range set of tools, which are not as commonly used
• kali-linux-everything: metapackage of all the metapackages and other packages (almost
everything that Kali provides so it is really huge!)
• kali-tools-top10: the ten most popular tools
• kali-tools-web: web applications assessment tools
• kali-tools-passwords: password cracking tools
• kali-tools-wireless: The collection of 802.11, Bluetooth, RFID and SDR wireless assessment
and analysis tools
• kali-tools-forensics: forensic tools (finding evidence of what happened)
• kali-tools-802-11: wireless assessment and analysis tools
• kali-tools-bluetooth: Bluetooth focused tools
• kali-tools-crypto-stego: cryptography and steganography tools
• kali-tools-crypto-fuzzing: fuzzing attack tools
• kali-tools-gpu: GPU-powered tools (tools making use of the computing power available in
your graphical card)
• kali-tools-hardware: tools designed to attacking hardware
• kali-tools-rfid: Radio Frequency IDentification (RFID) tools
• kali-tools-sdr: Software Defined Radio (SDR) tools
• kali-tools-voip: Voice Over IP tools
• kali-tools-windows-resources: Pre-compiled Microsoft Windows binaries
You can leverage these metapackages when you create custom package lists for live-build. The
full list of available metapackages and the tools they include can be found at https://tools.kali.
org/kali-metapackages
Debconf Preseeding of You can provide Debconf preseed files (see section 4.3.2, “Creating a Preseed File”
Installed Packages [page 97] for explanations) as preseed/*.cfg files. They will be used to configure
the packages installed in the live file system.
244 Kali Linux Revealed
9.3.4. Using Hooks to Tweak the Contents of the Image
live-build offers hooks that can be executed at different steps of the build process. Chroot
hooks are executable scripts that you install as hooks/live/*.chroot files in your config tree
and that are executed within the chroot. While chroot is the command that lets you temporar-
ily changes the operating system’s root directory to a directory of your choice, it is also used
by extension to designate a directory hosting a full (alternate) file system tree. This is the case
here with live-build, where the chroot directory is the directory where the live file system is
being prepared. Since applications started in a chroot can’t see outside of that directory, the
same goes with the chroot hooks: you can only use and modify anything available in that ch-
root environment. We rely on those hooks to perform multiple Kali specific customizations (see
kali-config/common/hooks/live/kali-hacks.chroot).
Binary hooks (hooks/live/*.binary) are executed in the context of the build process (and not
chrooted anywhere) at the end of the process. You can modify the content of the ISO image built
but not of the live file system since at this point, it has already been generated. We use this feature
in Kali to make some changes to the default isolinux configuration generated by live-build. For
example, see kali-config/common/hooks/live/persistence.binary where we add the boot
menu entries enabling persistence.
9.3.5. Adding Files in the ISO Image or in the Live Filesystem
Another very common customization is to add files either in the live file system or in the ISO
image.
You can add files to the live file system by putting them at their expected location be-
low the includes.chroot config directory. For example, we provide kali-config/common/
includes.chroot/usr/lib/live/config/0031-kali-password, which ends up as /usr/lib/
live/config/0031-kali-password in the live file system.
Live-Boot Hooks Scripts installed as /lib/live/config/XXXX- name are executed by the init script
of the live-boot package. They reconfigure many aspects of the system to be suited
for a live system. You can add scripts of your own to customize your live system at
run-time: it’s notably used to implement a custom boot parameter for example.
You can add files to the ISO image by putting them at their expected location below the includes.
binary config directory. For example, we provide kali-config/common/includes.binary/
isolinux/splash.png to override the background image used by the Isolinux bootloader (which
is stored in /isolinux/splash.png in the filesystem of the ISO image).
Chapter 9 — Advanced Usage 245
9.4. Adding Persistence to the Live ISO with a USB Key
9.4.1. The Persistence Feature: Explanations
Next, we will discuss the steps required to add persistence to a Kali USB key. The nature of a live
system is to be ephemeral. All data stored on the live system and all the changes made are lost
when you reboot. To remedy this, you can use a feature of live-boot called persistence, which is
enabled when the boot parameters include the persistence keyword.
Since modifying the boot menu is a non-trivial task, the Kali live image includes two menu entries
by default that enable persistence: Live USB Persistence and Live USB Encrypted Persistence, as
shown in Figure 9.1, “Persistence Menu Entries” [page 246].
Figure 9.1 Persistence Menu Entries
When this feature is enabled, live-boot will scan all partitions looking for file systems labeled persis-
tence (which can be overridden with the persistence-label=value boot parameter) and the installer
will set up persistence of the directories which are listed in the persistence.conf file found in
that partition (one directory per line). The special value “/ union” enables full persistence of all
directories with a union mount, an overlay that stores only the changes when compared to the
underlying file system. The data of the persisted directories are stored in the file system that
contains the corresponding persistence.conf file.
246 Kali Linux Revealed
9.4.2. Setting Up Unencrypted Persistence on a USB Key
In this section, we assume that you have prepared a Kali live USB key by following the instructions
at section 2.1.4, “Copying the Image on a DVD-ROM or USB Key” [page 19] and that you have used
a USB key big enough to hold the ISO image (roughly 4 GB) and the data of the directories that you
want to persist. We also assume that the USB key is recognized by Linux as /dev/sdb and that it
only contains the two partitions that are part of the default ISO image (/dev/sdb1 and /dev/sdb2).
Be very careful when performing this procedure. You can easily destroy important data if you
re-partition the wrong drive.
To add a new partition, you must know the size of the image that you copied so that you can make
the new partition start after the live image. Then use parted to actually create the partition. The
commands below analyze the ISO image named kali-linux-2020.3-live-amd64.iso, which is
assumed to be present on the USB key as well:
# parted /dev/sdb print
Model: SanDisk Cruzer Glide (scsi)
Disk /dev/sdb: 16.0GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
1 32.8kB 3516MB 3516MB primary boot, hidden
2 3516MB 3517MB 754kB primary
# start=$(du --block-size=1MB kali-linux-2020.3-live-amd64.iso | awk ’{print $1}’)
# echo ”Size of image is $start MB”
Size of image is 3518 MB
# parted -a optimal /dev/sdb mkpart primary ”${start}MB” 100%
Information: You may need to update /etc/fstab.
# parted /dev/sdb print
Model: SanDisk Cruzer Glide (scsi)
Disk /dev/sdb: 16.0GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
1 32.8kB 3516MB 3516MB primary boot, hidden
2 3516MB 3517MB 754kB primary
3 3518MB 16.0GB 12.5GB primary
With the new /dev/sdb3 partition in place, format it with an ext4 filesystem labelled “persistence”
with the help of the mkfs.ext4 command (and its -L option to set the label). The partition is then
mounted on the /mnt directory and you add the required persistence.conf configuration file. As
Chapter 9 — Advanced Usage 247
always, use caution when formatting any disk. You could lose valuable information if you format
the wrong disk or partition.
# mkfs.ext4 -L persistence /dev/sdb3
mke2fs 1.45.6 (20-Mar-2020)
Creating filesystem with 3049472 4k blocks and 762528 inodes
Filesystem UUID: cedc7589-a002-4aae-8963-f5197d6d9d4a
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208
Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done
# mount /dev/sdb3 /mnt
# echo ”/ union” >/mnt/persistence.conf
# ls -l /mnt
total 20
drwx------ 2 root root 16384 May 10 13:31 lost+found
-rw-r--r-- 1 root root 8 May 10 13:34 persistence.conf
# umount /mnt
The USB key is now ready and can be booted with the “Live USB Persistence” boot menu entry.
9.4.3. Setting Up Encrypted Persistence on a USB Key
live-boot is also able to handle persistence file systems on encrypted partitions. You can thus
protect the data of your persistent directories by creating a LUKS encrypted partition holding the
persistence data.
The initial steps are the same up to the creation of the partition but instead of formatting it with
an ext4 file system, use cryptsetup to initialize it as a LUKS container. Then open that container
and setup the ext4 file system in the same way as in the non-encrypted setup, but instead of using
the /dev/sdb3 partition, use the virtual partition created by cryptsetup. This virtual partition
represents the decrypted content of the encrypted partition, which is available in /dev/mapper
under the name that you assigned it. In the example below, we will use the name kali_persistence.
Again, ensure that you are using the correct drive and partition.
# cryptsetup --verbose --verify-passphrase luksFormat /dev/sdb3
WARNING!
========
This will overwrite data on /dev/sdb3 irrevocably.
Are you sure? (Type uppercase yes): YES
Enter passphrase for /dev/sdb3:
248 Kali Linux Revealed
Verify passphrase:
Key slot 0 created.
Command successful.
# cryptsetup luksOpen /dev/sdb3 kali_persistence
Enter passphrase for /dev/sdb3:
# mkfs.ext4 -L persistence /dev/mapper/kali_persistence
mke2fs 1.45.6 (20-Mar-2020)
Creating filesystem with 3045376 4k blocks and 761856 inodes
Filesystem UUID: 22adeb99-e32d-43e7-b9df-f04389dd0ba4
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208
Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done
# mount /dev/mapper/kali_persistence /mnt
# echo ”/ union” >/mnt/persistence.conf
# umount /mnt
# cryptsetup luksClose /dev/mapper/kali_persistence
9.4.4. Using Multiple Persistence Stores
If you have multiple use-cases for your Kali live system, you can use multiple filesystems with dif-
ferent labels and indicate on the boot command line which (set of) filesystems should be used for
the persistence feature: this is done with the help of the persistence-label=label boot parameter.
Let’s assume that you are a professional pen-tester. When you work for a customer, you use an
encrypted persistence partition to protect the confidentiality of your data in case the USB key is
stolen or compromised. At the same time, you want to be able to showcase Kali and some pro-
motional material stored in an unencrypted partition of the same USB key. Since you don’t want
to manually edit the boot parameters on each boot, you want to build a custom live image with
dedicated boot menu entries.
The first step is to build the custom live ISO (following section 9.3, “Building Custom Kali Live
ISO Images” [page 241] and in particular section 9.3.4, “Using Hooks to Tweak the Contents of
the Image” [page 245]). The main customization is to modify kali-config/common/hooks/live/
persistence-menu.binary to make it look like this (note the persistence-label parameters):
#!/bin/sh
if [ ! -d isolinux ]; then
cd binary
fi
Chapter 9 — Advanced Usage 249
cat >>isolinux/live.cfg <<END
label live-demo
menu label ^Live USB with Demo Data
linux /live/vmlinuz
initrd /live/initrd.img
append boot=live username=root hostname=kali persistence-label=demo persistence
label live-work
menu label ^Live USB with Work Data
linux /live/vmlinuz
initrd /live/initrd.img
append boot=live username=root hostname=kali persistence-label=work persistence-
å encryption=luks persistence
END
Next, we will build our custom ISO and copy it to the USB key. Then we will create and initialize the
two partitions and files systems that will be used for persistence. The first partition is unencrypted
(labeled “demo”), and the second is encrypted (labeled “work”). Assuming /dev/sdb is our USB
key and the size of our custom ISO image is 3000 MB, it would look like this:
# parted /dev/sdb mkpart primary 3000 MB 55%
# parted /dev/sdb mkpart primary 55% 100%
# mkfs.ext4 -L demo /dev/sdb3
[...]
# mount /dev/sdb3 /mnt
# echo ”/ union” >/mnt/persistence.conf
# umount /mnt
# cryptsetup --verbose --verify-passphrase luksFormat /dev/sdb4
[...]
# cryptsetup luksOpen /dev/sdb4 kali_persistence
[...]
# mkfs.ext4 -L work /dev/mapper/kali_persistence
[...]
# mount /dev/mapper/kali_persistence /mnt
# echo ”/ union” >/mnt/persistence.conf
# umount /mnt
# cryptsetup luksClose /dev/mapper/kali_persistence
And that’s all. You can now boot the USB key and select from the new boot menu entries as needed!
Adding a Nuke Password Kali provides a cryptsetup-nuke-password package that modifies cryptsetup’s early
for Extra Safety boot scripts to implement a new feature: you can set a nuke password which—when
used—will destroy all keys used to manage the encrypted partition.
250 Kali Linux Revealed
This can be useful when you travel a lot and need a quick way to ensure your data
cannot be recovered. When booting, just type the nuke password instead of the real
one and it will then be impossible for anyone (including you) to access your data.
Before using that feature, it is thus wise to make a backup copy of your encryption
keys and keep them at some secure place.
You can configure a nuke password with this command (assuming that you have the
package installed):
# dpkg-reconfigure cryptsetup-nuke-password
More information about this feature can be found in the following tutorial:
è https://www.kali.org/tutorials/nuke-kali-linux-luks/
9.5. Summary
In this chapter, we learned about modifying Kali source packages, which are the basic building
blocks of all applications shipped in Kali. We also discovered how to customize and install the Kali
kernel. Then we discussed the live-build environment and discussed how to build a customized
Kali Linux ISO. We also demonstrated how to create both encrypted and unencrypted Kali USB
installs.
9.5.1. Summary Tips for Modifying Kali Packages
Modifying Kali packages is usually a task for Kali contributors and developers, but you might have
specific needs not fulfilled by the official packages and knowing how to build a modified package
can be very valuable, especially if you want to share your changes, deploy them internally, or
cleanly roll the software back to a previous state.
When you need to modify a piece of software, it might be tempting to download the source, make
the changes, and use the modified software. However, if your application requires a system-wide
setup (e.g. with a make install step), then it will pollute your file system with files unknown to
dpkg and will soon create problems that cannot be caught by package dependencies. In addition,
this type of software modification is more tedious to share.
When creating a modified package, the general process is always the same: grab the source pack-
age, extract it, make your changes, and then build the package. For each step, there are often
multiple tools that can handle each task.
To start rebuilding a Kali package, first download the source package, which is composed of a
*.dsc (Debian Source Control) file and of additional files referenced from that control file.
Chapter 9 — Advanced Usage 251
Source packages are stored on HTTP-accessible mirrors. The most efficient way to obtain them is
with apt source source-package-name, which requires that you add a deb-src line to the /etc/
apt/sources.list file and update the index files with apt update.
Additionally, you can use dget (from the devscripts package) to download a .dsc file directly
together with its accompanying files. For Kali-specific packages whose sources are hosted in a
Git repository on gitlab.com/kalilinux/packages9 , you can retrieve the sources with git clone
https://gitlab.com/kalilinux/packages/source-package.git.
After downloading sources, install the packages listed in the source package’s build dependencies
with sudo apt build-dep ./. This command must be run from the package’s source directory.
Updates to a source package consist of a combination of some of the following steps:
• The required first step is changing the version number to distinguish your package from
the original with dch --local version-identifier, or modify other package details with
dch.
• Applying a patch with patch -p1 <patch-file or modifying quilt’s patch series.
• Tweaking build options, usually found in the package’s debian/rules file, or other files in
the debian/ directory.
After modifying a source package, you can build the binary package with dpkg-buildpackage
-us -uc -b from the source directory, which will generate an unsigned binary package. The pack-
age can then be installed with dpkg -i package-name_version_arch.deb.
9.5.2. Summary Tips for Recompiling the Linux Kernel
As an advanced user, you may wish to recompile the Kali kernel. You may want to slim down the
standard Kali kernel, which is loaded with many features and drivers, add non-standard drivers
or features, or apply kernel patches. Beware though: a misconfigured kernel may destabilize your
system and you must be prepared to accept that Kali cannot ensure security updates for your
custom kernel.
For most kernel modifications, you will need to install a few packages with apt install
build-essential libncurses5-dev fakeroot.
The command apt-cache search ^linux-source should list the latest kernel version packaged
by Kali, and apt install linux-source-version-number installs a compressed archive of the
kernel source into /usr/src.
The source files should be extracted with tar -xaf into a directory other than /usr/src (such as
~/kernel).
When the time comes to configure your kernel, keep these points in mind:
9
https://gitlab.com/kalilinux/packages
252 Kali Linux Revealed
• Unless you are an advanced user, you should first populate a kernel configuration file.
The preferred method is to borrow Kali’s standard configuration by copying /boot/
config- version-string to ~/kernel/linux-source- version-number/.config. Alterna-
tively, you can use make architecture_defconfig to get a reasonable configuration for
the given architecture.
• The text-based make menuconfig kernel configuration tool will read the .config file and
present you all the configuration items in a huge menu that you can navigate. Selecting an
item shows you its documentation, its possible values, and permits you to enter a new value.
When run from your kernel source directory, make clean will remove previously-compiled files
and make deb-pkg will generate up to five Debian packages. The linux-image-version .deb file
contains the kernel image and the associated modules.
To actually use the built kernel, install the required packages with dpkg -i file.deb. The “linux-
image” package is required; you only have to install the “linux-headers” package if you have some
external kernel modules to build, which is the case if you have some “*-dkms” packages installed
(check with dpkg -l ”*-dkms” | grep ^ii). The other packages are generally not needed (un-
less you know why you need them!).
9.5.3. Summary Tips for Building Custom Kali Live ISO Images
Official Kali ISO images are built with live-build10 , which is a set of scripts that allows for the
complete automation and customization of all facets of ISO image creation.
Your Kali system must be completely up-to-date before using live-build.
The Kali live-build configuration can be retrieved from Kali’s Git repositories
with two commands: apt install curl git live-build followed by git clone
https://gitlab.com/kalilinux/build-scripts/live-build-config.git
To generate an updated but unmodified Kali live ISO image, simply run ./build.sh --verbose.
The build will take a long time to complete as it will download all the packages to include. When
finished, you will find the new ISO image in the images directory. If you add --variant variant
to the command line, it will build the given variant of the Kali ISO image. The various variants are
defined by their configuration directories kali-config/variant-*. The main image is the Xfce
variant.
There are several ways to customize your ISO by modifying live-build’s configuration directory:
• Packages can be added to (or removed from) a live ISO by modifying package-lists/*.
list.chroot files.
• Custom packages can be included in the live image by placing the .deb files in a packages.
chroot directory. Their installation can be preseeded with preseed/*.cfg files.
10
https://live-team.pages.debian.net/live-manual/html/live-manual/index.en.html
Chapter 9 — Advanced Usage 253
• You can add files to the live filesystem by putting them at their expected location below the
includes.chroot config directory.
• You can execute scripts during the live system’s chroot setup process by installing them as
hooks/live/*.chroot files. You can also execute scripts at boot time of the generated live
image: you must arrange for them to be installed in /usr/lib/live/config/XXXX- name, for
example by relying on the includes.chroot config directory.
• The Debian Live Systems Manual11 is an excellent reference for live-build configuration and
testing.
Setting up encrypted and unencrypted persistence on a USB key: it’s fairly simple to create a
standard Kali Live USB installation. Although the process may seem syntactically complex, it is
relatively straight-forward to add both encrypted and unencrypted persistence to your portable
installation to significantly extend its functionality.
In the next chapter (chapter 10, “Kali Linux in the Enterprise” [page 258]), we will discuss how Kali
scales to the enterprise. We will discuss configuration management and show you how to extend
and customize Kali Linux in a way that is easy to deploy whether you have a pair of machines, or
several thousand.
11
https://live-team.pages.debian.net/live-manual/html/live-manual/index.en.html
254 Kali Linux Revealed
Keywords
PXE installation
Configuration
management
Saltstack
Forking Kali packages
Configuration
packages
Package repository
Chapter
Kali Linux in the 10
Enterprise
Contents
Installing Kali Linux Over the Network (PXE Boot) 258 Leveraging Configuration Management 261
Extending and Customizing Kali Linux 268 Summary 279
So far, we have seen that Kali is an extremely capable and secure Debian derivative provid-
ing industrial-strength security and encryption features, advanced package management, multi-
platform capability, and (what it is most-known for) an arsenal of world-class tools for the security
professional. What might not be obvious is how Kali scales beyond the desktop to medium or large
scale deployments and even to the enterprise level. In this chapter, we will show you how well
Kali can scale beyond the desktop, providing centralized management and enterprise-level con-
trol over multiple Kali Linux installations. In short, after reading this chapter you will be able to
quickly deploy highly secure Kali systems preconfigured for your specific needs and keep them
synchronized thanks to Kali’s (semi-automatic) installation of package updates.
This level of scale requires several steps, including initiating a PXE network boot, use of an ad-
vanced configuration management tool (SaltStack), the ability to fork and customize packages,
and the deployment of a package repository. We will cover each step in detail, show you how to
get the “heavy lifting” out of the way, and deploy, manage, and maintain multitudes of custom
Kali Linux installations with relative ease. As if that were not enough, we will throw in a crowd of
minions to assist you in running your empire.
10.1. Installing Kali Linux Over the Network (PXE Boot)
As we have seen in previous chapters, the basic Kali Linux installation process is straightforward
once you know your way around. But if you have to install Kali on multiple machines, the standard
setup can be quite tedious. Thankfully, you can start the Kali installation procedure by booting a
computer over the network. This allows you to install Kali quickly and easily on many machines
at a time.
First, you will need to boot your target machine from the network. This is facilitated by the Pre-
boot eXecution Environment (PXE), a client/server interface designed to boot any networked ma-
chine from the network even if it does not have an operating system installed. Setting up PXE
network boot requires that you configure at least a Trivial File Transfer Protocol (TFTP) server
and a DHCP/BOOTP server. You will also need a web server if you want to host a debconf preseed-
ing file that will be automatically used in the installation process.
Fortunately, dnsmasq handles both DHCP and TFTP so that you can rely on a single service to set
up everything you need. And the Apache web server is installed (but not enabled) by default on
Kali systems.
Separate DHCP and TFTP For more complex setups, dnsmasq’s feature set might be too limited or you might
services want to enable PXE booting on your main network that already runs a DHCP service.
In both cases, you will then have to configure separate DHCP and TFTP services.
The Debian installation manual covers the setup of isc-dhcp-server and tftpd-hpa for
PXE booting.
è https://www.debian.org/releases/stable/amd64/ch04s05
258 Kali Linux Revealed
In order to set up dnsmasq, you must first configure it through /etc/dnsmasq.conf. A basic con-
figuration consists of only a few key lines:
# Network interface to handle
interface=eth0
# DHCP options
# IP range to allocate
dhcp-range=192.168.101.100,192.168.101.200,12h
# Gateway to announce to clients
dhcp-option=option:router,192.168.101.1
# DNS servers to announce to clients
dhcp-option=option:dns-server,8.8.8.8,8.8.4.4
# Boot file to announce to clients
dhcp-boot=pxelinux.0
# TFTP options
enable-tftp
# Directory hosting files to serve
tftp-root=/tftpboot/
With /etc/dnsmasq.conf configured, you will need to place the installation boot files in the
/tftpboot/ directory. Kali Linux provides a file archive dedicated to this purpose that can be
directly unpacked into /tftpboot/. Simply select between 32-bit (i386) and 64-bit (amd64) and
standard or graphical (gtk) install methods for your target machine and choose the appropriate
archive:
è http://http.kali.org/dists/kali-rolling/main/installer-amd64/current/images/
netboot/gtk/netboot.tar.gz
è http://http.kali.org/dists/kali-rolling/main/installer-amd64/current/images/
netboot/netboot.tar.gz
è http://http.kali.org/dists/kali-rolling/main/installer-i386/current/images/
netboot/gtk/netboot.tar.gz
è http://http.kali.org/dists/kali-rolling/main/installer-i386/current/images/
netboot/netboot.tar.gz
Once you have selected the archive, create /tftpboot/, download the archive, and unpack it into
that directory:
# mkdir /tftpboot
# cd /tftpboot
# wget http://http.kali.org/dists/kali-rolling/main/installer-amd64/current/images/
å netboot/netboot.tar.gz
# tar xf netboot.tar.gz
# ls -l
total 25896
drwxrwxr-x 3 root root 4096 May 6 04:43 debian-installer
Chapter 10 — Kali Linux in the Enterprise 259
lrwxrwxrwx 1 root root 47 May 6 04:43 ldlinux.c32 -> debian-installer/amd64/boot
å -screens/ldlinux.c32
-rw-r--r-- 1 root root 26507247 May 6 04:43 netboot.tar.gz
lrwxrwxrwx 1 root root 33 May 6 04:43 pxelinux.0 -> debian-installer/amd64/
å pxelinux.0
lrwxrwxrwx 1 root root 35 May 6 04:43 pxelinux.cfg -> debian-installer/amd64/
å pxelinux.cfg
-rw-rw-r-- 1 root root 71 May 6 04:43 version.info
The unpacked files include the pxelinux bootloader, which uses the same configuration files as
syslinux and isolinux. Because of this, you can tweak the boot files in debian-installer/amd64/
boot-screens/ as you would when generating custom Kali Linux live ISO images.
For example, assuming that you have picked the textual installer, you can add boot parameters
to preseed the language, country, keymap, hostname, and domain name values. You can also
point the installer to an external preseed URL and configure the timeout so that the boot happens
automatically if no key is pressed within 5 seconds. To accomplish this, you would first modify
the debian-installer/amd64/txt.cfg file:
label install
menu label ^Install
kernel debian-installer/amd64/linux
append vga=788 initrd=debian-installer/amd64/initrd.gz --- quiet language=en
å country=US keymap=us hostname=kali domain= url=http://192.168.101.1/
å preseed.cfg
Then, you would modify the debian-installer/amd64/syslinux.cfg file to adjust the timeout:
# D-I config version 2.0
# search path for the c32 support libraries (libcom32, libutil etc.)
path debian-installer/amd64/boot-screens/
include debian-installer/amd64/boot-screens/menu.cfg
default debian-installer/amd64/boot-screens/vesamenu.c32
prompt 0
timeout 50
Armed with the ability to boot any machine from the network via PXE, you can take ad-
vantage of all the features outlined in section 4.3, “Unattended Installations” [page 95], en-
abling you to do full booting, preseeding, and unattended installation on multiple comput-
ers without physical boot media. Also, don’t forget the flexibility of the boot parameter pre-
seed/url=http://server /preseed.cfg (nor the use of the url alias), which allows you to set a network-
based preseed file.
260 Kali Linux Revealed
10.2. Leveraging Configuration Management
With the ability to install Kali on multiple computers very quickly, you will need some help in
managing those machines post-installation. You can leverage configuration management tools
to manage machines or configure replacement computers to any desired state.
Kali Linux contains many popular configuration management tools that you might want to use
(Ansible, chef, Puppet, SaltStack, etc.) but in this section, we will only cover SaltStack.
è https://www.saltstack.com/
10.2.1. Setting Up SaltStack
SaltStack is a centralized configuration management service: a salt master manages many salt min-
ions. You should install the salt-master package on a server that is reachable by all the hosts that
you want to manage and salt-minion on the hosts that you wish to manage. Each minion must be
told where to find their master. Simply edit /etc/salt/minion and set the master key to the DNS
name (or IP address) of the Salt master. Note that Salt uses YAML as format for its configuration
files.
minion# vim /etc/salt/minion
minion# grep ^master /etc/salt/minion
master: 192.168.122.105
Each minion has a unique identifier stored in /etc/salt/minion_id, which defaults to its host-
name. This minion identifier will be used in the configuration rules and as such, it is important to
set it properly before the minion opens its connection to the master:
minion# echo kali-scratch >/etc/salt/minion_id
minion# systemctl enable salt-minion
minion# systemctl start salt-minion
When the salt-minion service is running, it will try to connect to the Salt master to exchange some
cryptographic keys. On the master side, you have to accept the key that the minion is using to
identify itself to let the connection proceed. Subsequent connections will be automatic:
master# systemctl enable salt-master
master# systemctl start salt-master
master# salt-key --list all
Accepted Keys:
Denied Keys:
Unaccepted Keys:
kali-scratch
Rejected Keys:
master# salt-key --accept kali-scratch
The following keys are going to be accepted:
Chapter 10 — Kali Linux in the Enterprise 261
Unaccepted Keys:
kali-scratch
Proceed? [n/Y] y
Key for minion kali-scratch accepted.
10.2.2. Executing Commands on Minions
As soon as minions are connected, you can execute commands on them from the master:
master# salt ’*’ test.ping
kali-scratch:
True
kali-master:
True
This command asks all minions (the ’*’ is a wildcard targeting all minions) to execute the ping
function from the test execution module. This function returns a True value on success and is a
simple way to ensure that the connection is working between the master and the various minions.
You can also target a specific minion by giving its identifier in the first parameter, or possibly
a subset of minions by using a less-generic wildcard (such as ’*-scratch’ or ’kali-*’). Here is an
example of how to execute an arbitrary shell command on the kali-scratch minion:
master# salt kali-scratch cmd.shell ’uptime; uname -a’
kali-scratch:
05:25:48 up 44 min, 2 users, load average: 0.00, 0.01, 0.05
Linux kali-scratch 4.5.0-kali1-amd64 #1 SMP Debian 4.5.3-2kali1 (2020-05-09) x86_64
å GNU/Linux
Salt Module Reference There are many execution modules available for all sorts of use cases. We won’t cover
them all here, but the full list is available at https://docs.saltproject.io/en/
latest/ref/modules/all/index.html. You can also obtain a description of all the
execution modules and their available functions on a given minion with the salt
minion sys.doc command. Running this command returns a very long list of func-
tions, but you can filter the list by passing the name of a function or module prefixed
by its parent module as a parameter:
master# salt kali-scratch sys.doc disk.usage
disk.usage:
Return usage information for volumes mounted on this
å minion
One of the most useful modules is pkg, which is a package manager abstraction relying on the
appropriate package manager for the system (apt-get for Debian and its derivatives like Kali).
262 Kali Linux Revealed
The pkg.refresh_db command updates the package list (that is, it performs apt-get update)
while pkg.upgrade installs all the available updates (it performs apt-get upgrade or apt-get
dist-upgrade, depending on the options received). The pkg.list_upgrades command lists the
pending upgrade operations (that would be performed by the pkg.upgrade dist_upgrade=True
command).
The service module is an abstraction of the service manager (systemd in the case of Kali),
which lets you perform all the usual systemctl operations: service.enable, service.disable,
service.start, service.stop, service.restart, and service.reload:
master# salt ’*’ service.enable ssh
kali-scratch:
True
kali-master:
True
master# salt ’*’ service.start ssh
kali-master:
True
kali-scratch:
True
master# salt ’*’ pkg.refresh_db
kali-scratch:
----------
kali-master:
----------
master# salt ’*’ pkg.upgrade dist_upgrade=True
kali-scratch:
----------
changes:
----------
base-files:
----------
new:
1:2020.2.1
old:
1:2020.2.0
[...]
zaproxy:
----------
new:
2.5.0-0kali1
old:
2.4.3-0kali3
comment:
result:
True
Chapter 10 — Kali Linux in the Enterprise 263
As a more concrete sample, you could easily set up a distributed Nmap scan with dnmap. After
having installed the package on all the minions, you start the server in a first terminal:
server# salt ’*’ pkg.install dnmap
[...]
server# vim dnmap.txt
server# dnmap_server -f dnmap.txt
Assuming that the server IP is 1.2.3.4, you can next tell all minions to start a client process that
connects to the server:
server# salt ’*’ cmd.run_bg template=jinja ’dnmap_client -s 1.2.3.4 -a {{ grains.id }}’
kali-scratch:
----------
pid:
17137
[...]
Note that the example uses cmd.run_bg to run the dnmap_client command in the background.
Don’t wait until it finishes, since it is a long-running process. Unfortunately, it doesn’t kill itself
properly when you interrupt the server so you might have to clean it up:
server# salt ’*’ cmd.shell ’pkill -f dnmap_client’
10.2.3. Salt States and Other Features
While remote execution is an important building block, it is only a tiny fraction of what SaltStack
can do.
When setting up a new machine, you often run many commands and tests to determine the details
of the system prior to installation. These operations can be formalized in re-usable configuration
templates called state files. The operations described in state files can then be performed with a
single state.apply salt command.
To save some time, you can rely on many ready-to-use state files that have been created by the
community and which are distributed in “Salt formulas”:
è https://docs.saltproject.io/en/latest/topics/development/conventions/
formulas.html
There are many other features that can be combined:
• Scheduled execution of actions
• Defining actions in response to events triggered by minions
• Collecting data out of minions
264 Kali Linux Revealed
• Orchestration of a sequence of operations across multiple minions
• Applying states over SSH without installing the salt-minion service
• Provisioning systems on cloud infrastructures and bringing them under management
• And more
SaltStack is quite vast and we can’t possibly cover all the features here. In fact, there are books
dedicated entirely to SaltStack and the online documentation is very extensive as well. Check it
out if you want to learn more about its features:
è https://docs.saltproject.io/en/latest/
If you manage a significant number of machines, you would be well advised to learn more about
SaltStack as you can save a significant amount of time when deploying new machines and you will
be able to maintain a coherent configuration throughout your network.
To give you a taste of what it looks like to work with state files, we will cover a simple example:
how to enable the APT repository and install a package that you create in section 10.3.3, “Creating
a Package Repository for APT” [page 275] and section 10.3.2, “Creating Configuration Packages”
[page 269]. You will also register a SSH key in root’s account so that you can login remotely in
case of problems.
By default, state files are stored in /srv/salt on the master; they are YAML structured files with
a .sls extension. Just like for running commands, applying a state relies on many state modules:
è https://docs.saltproject.io/en/latest/topics/tutorials/starting_states.html
è https://docs.saltproject.io/en/latest/ref/states/all/
Your /srv/salt/offsec.sls file will call three of those modules:
offsec_repository:
pkgrepo.managed:
- name: deb http://pkgrepo.offsec.com offsec-internal main
- file: /etc/apt/sources.list.d/offsec.list
- key_url: salt://offsec-apt-key.asc
- require_in:
- pkg: offsec-defaults
offsec-defaults:
pkg.installed
ssh_key_for_root:
ssh_auth.present:
- user: root
- name: ssh-rsa AAAAB3NzaC1yc2...89C4N rhertzog@kali
The offsec_repository state relies on the pkgrepo state module. The example uses the managed
function in that state module to register a package repository. With the key_url attribute, you
Chapter 10 — Kali Linux in the Enterprise 265
let salt know that the (ASCII armored) GPG key required to verify the repository’s signature can
be fetched from /srv/salt/offsec-apt-key.asc on the salt master. The require_in attribute
ensures that this state is processed before the offsec-defaults, since the latter needs the repository
correctly configured to be able to install the package.
The offsec-defaults state installs the package of the same name. This shows that the name of the
key is often an important value for states, although it can always be overridden with a name at-
tribute (as done for the former state). For simple-cases like this one, this is both readable and
concise.
The last state (ssh_key_for_root) adds the SSH key given in the name attribute to /root/.ssh/
authorized_keys (the target user is set in the user attribute). Note that we have shortened the
key for readability here, but you should put the full key in the name attribute.
This state file can next be applied to a given minion:
server# salt kali-scratch state.apply offsec
kali-scratch:
----------
ID: offsec_repository
Function: pkgrepo.managed
Name: deb http://pkgrepo.offsec.com offsec-internal main
Result: True
Comment: Configured package repo ’deb http://pkgrepo.offsec.com offsec-internal
å main’
Started: 06:00:15.767794
Duration: 4707.35 ms
Changes:
----------
repo:
deb http://pkgrepo.offsec.com offsec-internal main
----------
ID: offsec-defaults
Function: pkg.installed
Result: True
Comment: The following packages were installed/updated: offsec-defaults
Started: 06:00:21.325184
Duration: 19246.041 ms
Changes:
----------
offsec-defaults:
----------
new:
1.0
old:
----------
ID: ssh_key_for_root
Function: ssh_auth.present
266 Kali Linux Revealed
Name: ssh-rsa AAAAB3NzaC1yc2...89C4N rhertzog@kali
Result: True
Comment: The authorized host key AAAAB3NzaC1yc2...89C4N for user root was added
Started: 06:00:40.582539
Duration: 62.103 ms
Changes:
----------
AAAAB3NzaC1yc2...89C4N:
New
Summary for kali-scratch
------------
Succeeded: 3 (changed=3)
Failed: 0
------------
Total states run: 3
Total run time: 24.015 s
It can also be permanently associated to the minion by recording it in the /srv/salt/top.sls
file, which is used by the state.highstate command to apply all relevant states in a single pass:
server# cat /srv/salt/top.sls
base:
kali-scratch:
- offsec
server# salt kali-scratch state.highstate
kali-scratch:
----------
ID: offsec_repository
Function: pkgrepo.managed
Name: deb http://pkgrepo.offsec.com offsec-internal main
Result: True
Comment: Package repo ’deb http://pkgrepo.offsec.com offsec-internal main’ already
å configured
Started: 06:06:20.650053
Duration: 62.805 ms
Changes:
----------
ID: offsec-defaults
Function: pkg.installed
Result: True
Comment: Package offsec-defaults is already installed
Started: 06:06:21.436193
Duration: 385.092 ms
Changes:
----------
ID: ssh_key_for_root
Chapter 10 — Kali Linux in the Enterprise 267
Function: ssh_auth.present
Name: ssh-rsa AAAAB3NzaC1yc2...89C4N rhertzog@kali
Result: True
Comment: The authorized host key AAAAB3NzaC1yc2...89C4N is already present for
å user root
Started: 06:06:21.821811
Duration: 1.936 ms
Changes:
Summary for kali-scratch
------------
Succeeded: 3
Failed: 0
------------
Total states run: 3
Total run time: 449.833 ms
10.3. Extending and Customizing Kali Linux
Sometimes you need to modify Kali Linux to make it fit your local needs. The best way to achieve
this is to maintain your own package repository hosting the modified versions of the Kali packages
that you had to fork, as well as supplementary packages providing custom configuration and extra
software (not provided by Kali Linux).
10.3.1. Forking Kali Packages
Please refer to section 9.1, “Modifying Kali Packages” [page 228] for explanations about this topic.
All packages can be forked if you have a good reason but you must be aware that forking a package
has a cost, since you have to update it every time that Kali publishes an update. Here are some
reasons why you might want to fork a package:
• To add a patch to fix a bug or add a new feature. Although in most cases, you will want to
submit that patch to the upstream developers so that the bug is fixed or the feature is added
at the source.
• To compile it with different options (assuming that there are good reasons why Kali did not
compile it with those options; otherwise it might be best to discuss this with Kali developers1
to see if they can enable the desired options).
By contrast, here are some bad reasons to fork a package along with suggestions of how to handle
your problem:
1
https://bugs.kali.org/
268 Kali Linux Revealed
• To modify a configuration file. You have multiple, better options like using configuration
management to automatically install a modified configuration file or installing a configu-
ration package that will put a file in a configuration directory (when available) or that will
divert the original configuration file.
• To update to a newer upstream version. Again, it is better to work with developers to update
the package directly in Debian or Kali. With the rolling release model, updates are rather
quick to reach end users.
Among all the available packages, there are some that are building blocks of Kali Linux and that
could be interesting to fork in some situations:
• kali-meta: this source package builds all the kali-linux-* and kali-tools-* meta packages and
notably kali-linux-default, which defines what packages are installed in the default Kali Linux
ISO image.
• desktop-base: This source package contains various miscellaneous files that are used by de-
fault in desktop installations. Consider forking this package if you would like to show your
organization’s brand in the default background or change the theme of the desktop.
• kali-menu: this package defines the structure of the Kali menu and provides .desktop files
for all applications that should be listed in the Kali menu.
10.3.2. Creating Configuration Packages
Now that we have touched on PXE booting and discussed configuration management with Salt-
Stack as well as package forking, it is time to wrap these processes up into a practical example and
extend the scenario by creating a custom configuration package to deploy a custom configuration
to multiple machines semi-automatically.
In this example, you will create a custom package that sets up and utilizes your own package repos-
itory and GnuPG signing key, distributes a SaltStack configuration, pushes a custom background,
and provides default desktop settings in a unified way to all your Kali installations.
This may seem like a daunting task (especially if you glance through the Debian New Maintainer
Guide2 ) but fortunately for us, a configuration package is mainly a sophisticated file archive and
turning it into a package is rather easy.
Looking into a Sample If you want to look into a real package that is basically a configuration package, con-
Package sider the kali-defaults package. It is not as simple as the sample in this section but
it has all the relevant characteristics and even uses some advanced techniques (like
dpkg-divert) to replace files already provided by other packages.
The offsec-defaults package will contain a few files:
2
https://www.debian.org/doc/manuals/maint-guide/
Chapter 10 — Kali Linux in the Enterprise 269
• /etc/apt/sources.list.d/offsec.list: a sources.list entry for APT, enabling the
company’s internal package repository
• /etc/apt/trusted.gpg.d/offsec.gpg: the GnuPG key used to sign the company’s inter-
nal package repository
• /etc/salt/minion.d/offsec.conf: a SaltStack configuration file to indicate where to find
the Salt master
• /usr/share/images/offsec/background.png: a nice background image with the Offen-
sive Security logo
• /usr/share/glib-2.0/schemas/90_offsec-defaults.gschema.override: a file provid-
ing alternate default settings for the GNOME desktop
First, create an offsec-defaults-1.0 directory and put all the files in that directory. Then run
dh_make --native (from the dh-make package) to add Debian packaging instructions, which will
be stored in a debian sub-directory:
$ mkdir offsec-defaults-1.0; cd offsec-defaults-1.0
$ dh_make --native
Type of package: (single, indep, library, python)
[s/i/l/p]? i
Email-Address : buxy‘
License : gpl3
Package Name : offsec-defaults
Maintainer Name : Raphaël Hertzog
Version : 1.0
Package Type : indep
Date : Thu, 16 Jun 2020 18:04:21 +0200
Are the details correct? [Y/n/q] y
Currently there is not top level Makefile. This may require additional tuning
Done. Please edit the files in the debian/ subdirectory now.
First, you are prompted for a package type. In the example, we selected indep, which indicates
that this source package will generate a single binary package that can be shared across all archi-
tectures (Architecture: all). single acts as a counterpart, and produces a single binary package that
is dependent on the target architecture (Architecture: any). In this case, indep is more relevant,
since the package only contains text files and no binary programs, so that it can be used similarly
on computers of all architectures. The library type is useful for shared libraries, since they need to
follow strict packaging rules. In a similar fashion, python should be restricted to Python modules.
Maintainer’s Name and Most of the programs involved in package maintenance will look for your name and
Email Address email address in the DEBFULLNAME and DEBEMAIL or EMAIL environment variables.
Defining them, once and for all, prevents re-typing them multiple times. If your usual
shell is Bash, it is a simple matter of adding the following two lines in your ~/.bashrc
file. For example:
270 Kali Linux Revealed
export EMAIL=”buxy@kali.org”
export DEBFULLNAME=”Raphael Hertzog”
The dh_make command created a debian subdirectory containing many files. Some are required,
in particular rules, control, changelog, and copyright. Files with the .ex extension are ex-
ample files that can be used by modifying them and removing the extension. When they are not
needed, we recommend removing them. The compat file should be kept, since it is required for
the correct functioning of the debhelper suite of programs (all beginning with the dh_ prefix) used
at various stages of the package build process.
The copyright file must contain information about the authors of the documents included in the
package, and the related license. If the default license selected by dh_make does not suit you, then
you must edit this file. Here is the modified version of the copyright file:
Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
Upstream-Name: offsec-defaults
Files: *
Copyright: 2020 Offensive Security
License: GPL-3.0+
License: GPL-3.0+
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
.
This package is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
.
On Debian systems, the complete text of the GNU General
Public License version 3 can be found in ”/usr/share/common-licenses/GPL-3”.
The default changelog file is generally appropriate; replacing the “Initial release” with a more
verbose explanation should be enough:
offsec-defaults (1.0) unstable; urgency=medium
* Add salt minion’s configuration file.
* Add an APT’s sources.list entry and an APT’s trusted GPG key.
Chapter 10 — Kali Linux in the Enterprise 271
* Override the gsettings schema defining the background picture.
-- Raphaël Hertzog <buxy@kali.org> Thu, 16 Jun 2020 18:04:21 +0200
In the example, we will make changes to the control file. We will change the Section field to misc
and remove the Homepage, Vcs-Git, and Vcs-Browser fields. Lastly, we will fill in the Description
field:
Source: offsec-defaults
Section: misc
Priority: optional
Maintainer: Raphaël Hertzog <buxy@kali.org>
Build-Depends: debhelper (>= 9)
Standards-Version: 3.9.8
Package: offsec-defaults
Architecture: all
Depends: ${misc:Depends}
Description: Default settings for Offensive Security
This package contains multiple files to configure computers
owned by Offensive Security.
.
It notably modifies:
- APT’s configuration
- salt-minion’s configuration
- the default desktop settings
The rules file usually contains a set of rules used to configure, build, and install the software
in a dedicated subdirectory (named after the generated binary package). The contents of this
subdirectory are then archived within the Debian package as if it were the root of the filesystem.
In this case, files will be installed in the debian/offsec-defaults/ subdirectory. For example,
to end up with a package installing /etc/apt/sources.list.d/offsec.list, install the file in
debian/offsec-defaults/etc/apt/sources.list.d/offsec.list. The rules file is used as a
Makefile, with a few standard targets (including clean and binary, used respectively to clean the
source directory and generate the binary package).
What is a Makefile file? You may have noticed the message concerning the missing Makefile at the end of
the dh_make output and the mention of its similarity to the rules file. A Makefile is
a script file used by the make program; it describes rules for how to build a set of files
from each other in a tree of dependencies. For instance, a program can be built from
a set of source files. The Makefile file describes these rules in the following format:
target: source1 source2 ...
command1
command2
272 Kali Linux Revealed
The interpretation of such a rule is as follows: if one of the source* files is more
recent than the target file, then the target needs to be generated, using command1
and command2.
Note that the command lines must start with a tab character; also note that when
a command line starts with a dash character (-), failure of the command does not
interrupt the whole process.
Although this file is the heart of the process, it contains only the bare minimum for running a
standard set of commands provided by the debhelper tool. Such is the case for files generated by
dh_make. To install most of your files, we recommend configuring the behavior of the dh_install
command by creating the following debian/offsec-defaults.install file:
apt/offsec.list etc/apt/sources.list.d/
apt/offsec.gpg etc/apt/trusted.gpg.d/
salt/offsec.conf etc/salt/minion.d/
images/background.png usr/share/images/offsec/
You could also use this to install the gsettings override file but debhelper provides a dedicated
tool for this (dh_installgsettings) so you can rely on it. First, put your settings in debian/
offsec-defaults.gsettings-override:
[org.gnome.desktop.background]
picture-options=’zoom’
picture-uri=’file:///usr/share/images/offsec/background.png’
Next, override the dh_installgsettings call in debian/rules to increase the priority to the
level expected for an organization override (which is 90 according to the manual page):
#!/usr/bin/make -f
%:
dh $@
override_dh_installgsettings:
dh_installgsettings --priority=90
At this point, the source package is ready. All that is left to do is to generate the binary package
with the same method used previously for rebuilding packages: run the dpkg-buildpackage -us
-uc command from within the offsec-defaults-1.0 directory:
$ dpkg-buildpackage -us -uc
dpkg-buildpackage: info: source package offsec-defaults
dpkg-buildpackage: info: source version 1.0
dpkg-buildpackage: info: source distribution unstable
dpkg-buildpackage: info: source changed by Raphaël Hertzog <buxy@kali.org>
dpkg-buildpackage: info: host architecture amd64
Chapter 10 — Kali Linux in the Enterprise 273
dpkg-source --before-build offsec-defaults-1.0
fakeroot debian/rules clean
dh clean
dh_testdir
dh_auto_clean
dh_clean
dpkg-source -b offsec-defaults-1.0
dpkg-source: info: using source format ’3.0 (native)’
dpkg-source: info: building offsec-defaults in offsec-defaults_1.0.tar.xz
dpkg-source: info: building offsec-defaults in offsec-defaults_1.0.dsc
debian/rules build
dh build
dh_testdir
dh_update_autotools_config
dh_auto_configure
dh_auto_build
dh_auto_test
fakeroot debian/rules binary
dh binary
dh_testroot
dh_prep
dh_auto_install
dh_install
dh_installdocs
dh_installchangelogs
debian/rules override_dh_installgsettings
make[1]: Entering directory ’/home/kali/offsec-defaults-1.0’
dh_installgsettings --priority=90
make[1]: Leaving directory ’/home/kali/offsec-defaults-1.0’
dh_perl
dh_link
dh_strip_nondeterminism
dh_compress
dh_fixperms
dh_installdeb
dh_gencontrol
dh_md5sums
dh_builddeb
dpkg-deb: building package ’offsec-defaults’ in ’../offsec-defaults_1.0_all.deb’.
dpkg-genchanges >../offsec-defaults_1.0_amd64.changes
dpkg-genchanges: info: including full source code in upload
dpkg-source --after-build offsec-defaults-1.0
dpkg-buildpackage: info: full upload; Debian-native package (full source is included)
274 Kali Linux Revealed
10.3.3. Creating a Package Repository for APT
Now that you have a custom package, you can distribute it through an APT package repository.
Use reprepro to create the desired repository and to fill it. This tool is rather powerful and its
manual page is certainly worth reading.
A package repository is typically hosted on a server. To properly separate it from other services
running on the server, it is best to create a user dedicated to this service. In the dedicated user
account, you will be able to host the repository files and also the GnuPG key that will be used to
sign the package repository:
# apt install reprepro gnupg
[...]
# adduser --system --group pkgrepo
Adding system user ‘pkgrepo’ (UID 136) ...
Adding new group ‘pkgrepo’ (GID 142) ...
Adding new user ‘pkgrepo’ (UID 136) with group ‘pkgrepo’ ...
Creating home directory ‘/home/pkgrepo’ ...
# chown pkgrepo $(tty)
# su - -s /bin/bash pkgrepo
pkgrepo@kali:~$ gpg --gen-key
gpg (GnuPG) 2.1.11; Copyright (C) 2020 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
gpg: directory ’/home/pkgrepo/.gnupg’ created
gpg: new configuration file ’/home/pkgrepo/.gnupg/dirmngr.conf’ created
gpg: new configuration file ’/home/pkgrepo/.gnupg/gpg.conf’ created
gpg: keybox ’/home/pkgrepo/.gnupg/pubring.kbx’ created
Note: Use ”gpg --full-gen-key” for a full featured key generation dialog.
GnuPG needs to construct a user ID to identify your key.
Real name: Offensive Security Repository Signing Key
Email address: repoadmin@offsec.com
You selected this USER-ID:
”Offensive Security Repository Signing Key <repoadmin@offsec.com>”
Change (N)ame, (E)mail, or (O)kay/(Q)uit? o
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
[...]
gpg: /home/pkgrepo/.gnupg/trustdb.gpg: trustdb created
gpg: key B4EF2D0D marked as ultimately trusted
gpg: directory ’/home/pkgrepo/.gnupg/openpgp-revocs.d’ created
Chapter 10 — Kali Linux in the Enterprise 275
gpg: revocation certificate stored as ’/home/pkgrepo/.gnupg/openpgp-revocs.d/
å F8FE22F74F1B714E38DA6181B27F74F7B4EF2D0D.rev’
public and secret key created and signed.
gpg: checking the trustdb
gpg: marginals needed: 3 completes needed: 1 trust model: PGP
gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u
pub rsa2048/B4EF2D0D 2020-06-17 [S]
Key fingerprint = F8FE 22F7 4F1B 714E 38DA 6181 B27F 74F7 B4EF 2D0D
uid [ultimate] Offensive Security Repository Signing Key <repoadmin@offsec.com>
sub rsa2048/38035F38 2020-06-17 []
Note that when you are prompted for a passphrase, you should enter an empty value (and confirm
that you don’t want to protect your private key) as you want to be able to sign the repository non-
interactively. Note also that gpg requires write access to the terminal to be able to securely prompt
for a passphrase: that is why you changed the ownership of the virtual terminal (which is owned
by root since you initially connected as that user) before starting a shell as pkgrepo.
Now you can start setting up the repository. A dedicated directory is necessary for reprepro and
inside that directory you have to create a conf/distributions file documenting which distribu-
tions are available in the package repository:
pkgrepo@kali:~$ mkdir -p reprepro/conf
pkgrepo@kali:~$ cd reprepro
pkgrepo@kali:~/reprepro$ cat >conf/distributions <<END
Codename: offsec-internal
AlsoAcceptFor: unstable
Origin: Offensive Security
Description: Offsec’s Internal packages
Architectures: source amd64 i386
Components: main
SignWith: F8FE22F74F1B714E38DA6181B27F74F7B4EF2D0D
END
The required fields are Codename, which gives the name of the distribution, Architectures, which
indicates which architectures will be available in the distribution (and accepted on the input side),
and Components, which indicates the various components available in the distribution (com-
ponents are a sort of sub-section of the distribution, which can be enabled separately in APT’s
sources.list). The Origin and Description fields are purely informative and they are copied as-is
in the Release file. The SignWith field asks reprepro to sign the repository with the GnuPG key
whose identifier is listed (put the full fingerprint here to ensure you use the correct key, and not
another one colliding on the short identifier). The AlsoAcceptFor setting is not required but makes
it possible to process .changes files whose Distribution field has a value listed here (without this,
it would only accept the distribution’s codename in that field).
With this basic setup in place, you can let reprepro generate an empty repository:
276 Kali Linux Revealed
pkgrepo@kali:~/reprepro$ reprepro export
Exporting indices...
pkgrepo@kali:~/reprepro$ find .
.
./db
./db/version
./db/references.db
./db/contents.cache.db
./db/checksums.db
./db/packages.db
./db/release.caches.db
./conf
./conf/distributions
./dists
./dists/offsec-internal
./dists/offsec-internal/Release.gpg
./dists/offsec-internal/Release
./dists/offsec-internal/main
./dists/offsec-internal/main/source
./dists/offsec-internal/main/source/Release
./dists/offsec-internal/main/source/Sources.gz
./dists/offsec-internal/main/binary-amd64
./dists/offsec-internal/main/binary-amd64/Packages
./dists/offsec-internal/main/binary-amd64/Release
./dists/offsec-internal/main/binary-amd64/Packages.gz
./dists/offsec-internal/main/binary-i386
./dists/offsec-internal/main/binary-i386/Packages
./dists/offsec-internal/main/binary-i386/Release
./dists/offsec-internal/main/binary-i386/Packages.gz
./dists/offsec-internal/InRelease
As you can see, reprepro created the repository meta-information in a dists sub-directory. It
also initialized an internal database in a db sub-directory.
It is now time to add your first package. First, copy the files generated by the build
of the offsec-defaults package (offsec-defaults_1.0.dsc, offsec-defaults_1.0.tar.xz,
offsec-defaults_1.0_all.deb, and offsec-defaults_1.0_amd64.changes) into /tmp on the
server hosting the package repository and ask reprepro to include the package:
pkgrepo@kali:~/reprepro$ reprepro include offsec-internal /tmp/offsec-defaults_1.0
å _amd64.changes
Exporting indices...
pkgrepo@kali:~/reprepro$ find pool
pool
pool/main
pool/main/o
Chapter 10 — Kali Linux in the Enterprise 277
pool/main/o/offsec-defaults
pool/main/o/offsec-defaults/offsec-defaults_1.0.dsc
pool/main/o/offsec-defaults/offsec-defaults_1.0.tar.xz
pool/main/o/offsec-defaults/offsec-defaults_1.0_all.deb
As you can see, it added the files into its own package pool in a pool sub-directory.
The dists and pool directories are the two directories that you need to make (publicly) available
over HTTP to finish the setup of your APT repository. They contain all the files that APT will want
to download.
Assuming that you want to host this on a virtual host named pkgrepo.offsec.com, you could cre-
ate the following Apache configuration file, save it to /etc/apache2/sites-available/pkgrepo.
offsec.com.conf, and enable it with a2ensite pkgrepo.offsec.com):
<VirtualHost *:80>
ServerName pkgrepo.offsec.com
ServerAdmin repoadmin@offsec.com
ErrorLog /var/log/apache2/pkgrepo.offsec.com-error.log
CustomLog /var/log/apache2/pkgrepo.offsec.com-access.log ”%h %l %u %t \”%r\” %>s %O”
DocumentRoot /home/pkgrepo/reprepro
<Directory ”/home/pkgrepo/reprepro”>
Options Indexes FollowSymLinks MultiViews
Require all granted
AllowOverride All
</Directory>
</VirtualHost>
And the corresponding sources.list entry to add on machines that need packages from this
repository would look like this:
deb http://pkgrepo.offsec.com offsec-internal main
# Enable next line if you want access to source packages too
# deb-src http://pkgrepo.offsec.com offsec-internal main
Your package is now published and should be available to your networked hosts.
Although this has been a lengthy setup, the “heavy lifting” is now completed. You can boot your
networked machines via PXE, install a customized version of Kali Linux without interaction thanks
to a network-delivered preseed, configure SaltStack to manage your configurations (and control
minions!), create forked custom packages, and distribute those packages through your own pack-
age repository. This provides centralized management and enterprise-level control over multiple
Kali Linux installations. In short, you can now quickly deploy highly secure Kali systems precon-
figured for your specific needs and keep them synchronized thanks to Kali’s (semi-automatic) in-
stallation of all package updates.
278 Kali Linux Revealed
10.4. Summary
Kali Linux scales beyond the desktop to medium or large scale deployments and even to the en-
terprise level. In this chapter, we covered how to centralize management of multiple Kali instal-
lations with SaltStack, allowing you to quickly deploy highly secure Kali systems preconfigured
for your specific needs. We also revealed how you can keep them synchronized thanks to Kali’s
(semi-automatic) installation of package updates.
We discussed package forking, which allows you to create your own customized distributable
source packages.
In summary, let’s review the major steps required to establish Salt masters and minions, which
allow you remote control and configuration of remote hosts.
Summary Tips:
• Boot machine from the network with PXE, with at least a TFTP file server, a DHCP/BOOTP
server (and a web server for debconf preseeding). dnsmasq handles both DHCP and TFTP,
and the apache2 web server comes pre-installed (but disabled) on Kali.
• The Debian installation manual covers the setup of isc-dhcp-server and tftpd-hpa for PXE boot-
ing:
è https://www.debian.org/releases/stable/amd64/ch04s05
• dnsmasq is configured through /etc/dnsmasq.conf. A basic configuration consists of only
a few key lines:
# Network interface to handle
interface=eth0
# DHCP options
# IP range to allocate
dhcp-range=192.168.101.100,192.168.101.200,12h
# Gateway to announce to clients
dhcp-option=option:router,192.168.101.1
# DNS servers to announce to clients
dhcp-option=option:dns-server,8.8.8.8,8.8.4.4
# Boot file to announce to clients
dhcp-boot=pxelinux.0
# TFTP options
enable-tftp
# Directory hosting files to serve
tftp-root=/tftpboot/
• Unpack 32-bit (i386), 64-bit (amd64), standard or graphical (gtk) installation boot files from
the Kali archive into /tftpboot/. The archives can be found here:
è http://http.kali.org/dists/kali-rolling/main/installer-amd64/current/
images/netboot/gtk/netboot.tar.gz
Chapter 10 — Kali Linux in the Enterprise 279
è http://http.kali.org/dists/kali-rolling/main/installer-amd64/current/
images/netboot/netboot.tar.gz
è http://http.kali.org/dists/kali-rolling/main/installer-i386/current/
images/netboot/gtk/netboot.tar.gz
è http://http.kali.org/dists/kali-rolling/main/installer-i386/current/
images/netboot/netboot.tar.gz
# mkdir /tftpboot
# cd /tftpboot
# wget http://http.kali.org/dists/kali-rolling/main/installer-amd64/current/
å images/netboot/netboot.tar.gz
# tar xf netboot.tar.gz
• Optionally modify txt.cfg to preseed parameters or custom timeouts. See section 4.3,
“Unattended Installations” [page 95]. Next, you can leverage configuration management
tools to manage machines or configure remote computers to any desired state.
• SaltStack is a centralized configuration management service: a Salt master manages many
Salt minions. Install the salt-master package on a reachable server and salt-minion on man-
aged hosts.
• Edit the /etc/salt/minion YAML-formatted config file and set the master key to the DNS
name (or IP address) of the Salt master.
• Set minion’s unique identifier in /etc/salt/minion_id:
minion# echo kali-scratch >/etc/salt/minion_id
minion# systemctl enable salt-minion
minion# systemctl start salt-minion
• Key exchange will follow. On the master, accept minion’s identification key. Subsequent
connections will be automatic:
master# systemctl enable salt-master
master# systemctl start salt-master
master# salt-key --list all
Accepted Keys:
Denied Keys:
Unaccepted Keys:
kali-scratch
Rejected Keys:
master# salt-key --accept kali-scratch
The following keys are going to be accepted:
Unaccepted Keys:
kali-scratch
Proceed? [n/Y] y
Key for minion kali-scratch accepted.
280 Kali Linux Revealed
• Once minions are connected, you can execute commands on them from the master. Exam-
ples:
master# salt ’*’ test.ping
kali-scratch:
True
kali-master:
True
master# salt kali-scratch cmd.shell ’uptime; uname -a’
master# salt kali-scratch sys.doc’
master# salt ’*’ service.enable ssh
[...]
master# salt ’*’ service.start ssh
[...]
master# salt ’*’ pkg.refresh_db
[...]
master# salt ’*’ pkg.upgrade dist_upgrade=True
server# salt ’*’ cmd.shell ’pkill -f dnmap_client’
• The full list of execution modules can be found at https://docs.saltproject.io/en/
latest/ref/modules/all/index.html.
• Use Salt state files (re-usable configuration templates) to schedule actions, collect data, or-
chestrate sequences of operations on multiple minions, provision cloud systems and bring
them under management, and more. Save time with pre-defined Salt formulas:
è https://docs.saltproject.io/en/latest/topics/development/conventions/
formulas.html
• When it comes time to fork a package, first decide if it is a task that you need to tackle. There
are significant advantages and disadvantages. Review them carefully. The kali-meta, desktop-
base, and kali-menu packages are interesting, probable choices. The process of forking a
package can be daunting and is difficult to summarize.
Now that we have covered all the bases in terms of installation, configuration, customization,
and deployment of Kali Linux, let’s turn towards the role of Kali Linux in the field of Information
Security.
Chapter 10 — Kali Linux in the Enterprise 281
Keywords
Types of assessments
Vulnerability
assessment
Compliance
penetration test
Traditional
penetration test
Application
assessment
Types of attacks
Denial of service
Memory corruption
Web vulnerabilities
Password attacks
Client-side attacks
Chapter
Introduction to 11
Security
Assessments
Contents
Kali Linux in an Assessment 285 Types of Assessments 287 Formalization of the Assessment 297
Types of Attacks 298 Summary 301
We have covered many Kali Linux-specific features up to this point so you should have a strong
understanding of what makes Kali special and how to accomplish a number of complex tasks.
Before putting Kali to use however, there are a few concepts relating to security assessments that
you should understand. In this chapter, we will introduce these concepts to get you started and
provide references that will help if you need to use Kali to perform a security assessment.
To start with, it is worth taking some time to explore exactly what ”security” means when dealing
with information systems. When attempting to secure an information system, you focus on three
primary attributes of the system:
• Confidentiality: can actors who should not have access to the system or information access
the system or information?
• Integrity: can the data or the system be modified in some way that is not intended?
• Availability: are the data or the system accessible when and how it is intended to be?
Together, these concepts make up the CIA (Confidentiality, Integrity, Availability) triad and in
large part, are the primary items that you will focus on when securing a system as part of standard
deployment, maintenance, or assessment.
It is also important to note that in some cases, you may be far more concerned with one aspect
of the CIA triad than others. For instance, if you have a personal journal that contains your most
secret thoughts, the confidentiality of the journal may be far more important to you than the
integrity or the availability. In other words, you may not be as concerned about whether some-
one can write to the journal (as opposed to reading it) or whether or not the journal is always
accessible. On the other hand, if you are securing a system that tracks medical prescriptions, the
integrity of the data will be most critical. While it is important to prevent other people from read-
ing what medications someone uses and it is important that you can access this list of medications,
if someone were able to change the contents of the system (altering the integrity), it could lead to
life-threatening results.
When you are securing a system and an issue is discovered, you will have to consider which of these
three concepts, or which combination of them, the issue falls into. This helps you understand the
problem in a more comprehensive manner and allows you to categorize the issues and respond
accordingly. It is possible to identify vulnerabilities that impact a single, or multiple items from
the CIA triad. To use a web application with a SQL injection vulnerability as an example:
• Confidentiality: a SQL injection vulnerability that allows an attacker to extract the full con-
tents of the web application, allowing them to have full access to read all the data, but no
ability to change the information or disable access to the database.
• Integrity: a SQL injection vulnerability that allows an attacker to change the existing infor-
mation in the database. The attacker can’t read the data or prevent others from accessing
the database.
284 Kali Linux Revealed
• Availability: a SQL injection vulnerability that initiates a long-running query, consuming a
large amount of resources on the server. This query, when initiated multiple times, leads to
a denial of service (DoS) situation. The attacker has no ability to access or change data but
can prevent legitimate users from accessing the web application.
• Multiple: a SQL injection vulnerability leads to full interactive shell access to the host op-
erating system running the web application. With this access, the attacker can breach the
confidentiality of the system by accessing data as they please, compromise the integrity of
the system by altering data, and if they so choose, destroy the web application, leading to a
compromise of the availability of the system.
The concepts behind the CIA triad are not overly complicated, and realistically are items that you
are working with intuitively, even if you don’t recognize it. However, it is important to mindfully
interact with the concept as it can help you recognize where to direct your efforts. This conceptual
foundation will assist you with the identification of the critical components of your systems and
the amount of effort and resources worth investing in correcting identified problems.
Another concept that we will address in detail is risk, and how it is made up of threats and vulnera-
bilities. These concepts are not too complex, but they are easy to get wrong. We will cover these
concepts in detail later on, but at a high level, it is best to think of risk as what you are trying to
prevent from happening, threat as who would do it to you, and vulnerability as what allows them to
do it. Controls can be put in place to address the threat or vulnerability, with the goal of mitigating
the risk.
For example, when visiting some parts of the world, you may be at substantial risk of catching
malaria. This is because the threat of mosquitoes is very high in some areas, and you are almost
certainly not immune to malaria. Fortunately, you can control the vulnerability with medication
and attempt to control the threat with the use of bug repellent and mosquito nets. With controls
in place addressing both the threat and the vulnerability, you can help ensure the risk does not
actualize.
11.1. Kali Linux in an Assessment
When preparing to use Kali Linux in the field, you must first ensure you have a clean, working
installation. A common mistake that many novice security professionals make is using a single
installation across multiple assessments. This is a problem for two primary reasons:
• Over the course of an assessment, you will often manually install, tweak, or otherwise
change your system. These one-off changes may get you up and running quickly or solve
a particular problem, but they are difficult to keep track of; they make your system more
difficult to maintain; and they complicate future configurations.
• Each security assessment is unique. Leaving behind notes, code, and other changes can lead
to confusion, or worse — cross-contamination of client data.
Chapter 11 — Introduction to Security Assessments 285
That is why starting with a clean Kali installation is highly recommended and why having a pre-
customized version of Kali Linux that is ready for automated installation quickly pays off. Be sure
to refer back to section 9.3, “Building Custom Kali Live ISO Images” [page 241] and section 4.3,
“Unattended Installations” [page 95] on how to do this, since the more you automate today, the
less time you waste tomorrow.
Everyone has different requirements when it comes to how they like Kali Linux configured when
they are in the field, but there are some universal recommendations that you really want to follow.
First, consider using an encrypted installation as documented in section 4.2.2, “Installation on a
Fully Encrypted File System” [page 88]. This will protect your data on the physical machine, which
is a life-saver if your laptop is ever stolen.
For extra safety during travel, you might want to nuke the decryption key (see “Adding a Nuke
Password for Extra Safety” [page 250]) after having sent an (encrypted) copy of the key to a co-
worker in the office. That way, your data are secure until you get back to the office where you can
restore the laptop with the decryption key.
Another item that you should double-check is the list of packages that you have installed. Consider
what tools you might need for the work you are setting out to accomplish. For example, if you are
embarking on a wireless security assessment, you may consider installing the kali-tools-wireless
metapackage, which contains all of the wireless assessment tools available in Kali Linux, or if a web
application assessment is coming up, you can install all of the available web application testing
tools with the kali-tools-web metapackage. It is best to assume that you will not have easy access
to the Internet while conducting a security assessment, so be sure to prepare as much as possible
in advance.
For the same reason, you might want to review your network settings (see section 5.1, “Config-
uring the Network” [page 108] and section 7.3, “Securing Network Services” [page 159]). Double-
check your DHCP settings and review the services that are listening on your assigned IP address.
These settings might make a critical impact to your success. You can’t assess what you can’t see
and excessive listening services might flag your system and get you shut down before you get
started.
If your role involves investigating network intrusions, paying close attention to your network set-
tings is even more important and you need to avoid altering the impacted systems. A customized
version of Kali with the kali-tools-forensics metapackage booted up in forensics mode will not auto-
matically mount disks or use a swap partition. In this way, you can help maintain the integrity of
the system under analysis while making use of the many forensics tools available in Kali Linux.
It is critical that you properly prepare your Kali Linux installation for the job. You will find that
a clean, efficient, and effective Kali environment will always make everything that follows much
smoother.
286 Kali Linux Revealed
11.2. Types of Assessments
Now that you have ensured that your Kali environment is ready, the next step is defining exactly
what sort of assessment you are conducting. At the highest level, we may describe four types of
assessments: a vulnerability assessment, a compliance test, a traditional penetration test, and an applica-
tion assessment. An engagement may involve various elements of each type of assessment but it is
worth describing them in some detail and explaining their relevance to your Kali Linux build and
environment.
Before delving into the different types of assessments, it is important to first note the difference
between a vulnerability and an exploit.
A vulnerability is defined as a flaw that, when taken advantage of, will compromise the confiden-
tiality, integrity, or availability of an information system. There are many different types of vul-
nerabilities that can be encountered, including:
• File Inclusion: File inclusion vulnerabilities1 in web applications allow you to include the
contents of a local or remote file into the computation of a program. For example, a web
application may have a ”Message of the day” function that reads the contents of a file and
includes it in the web page to display it to the user. When this type of feature is programmed
incorrectly, it can allow an attacker to modify their web request to force the site to include
the contents of a file of their choosing.
• SQL Injection: A SQL injection2 attack is one where the input validation routines for the
program are bypassed, allowing an attacker to provide SQL commands for the targeted pro-
gram to execute. This is a form of command execution that can lead to potential security
issues.
• Buffer Overflow: A buffer overflow3 is a vulnerability that bypasses input validation rou-
tines to write data into a buffer’s adjacent memory. In some cases, that adjacent memory
location may be critical to the operation of the targeted program and control of code exe-
cution can be obtained through careful manipulation of the overwritten memory data.
• Race Conditions: A race condition4 is a vulnerability that takes advantage of timing depen-
dencies in a program. In some cases, the workflow of a program depends on a specific
sequence of events to occur. If you can alter this sequence of events, that may lead to a
vulnerability.
An exploit, on the other hand, is software that, when used, takes advantage of a specific vulner-
ability, although not all vulnerabilities are exploitable. Since an exploit must change a running
process, forcing it to make an unintended action, exploit creation can be complex. Furthermore,
there are a number of anti-exploit technologies in modern computing platforms that have been
1
https://en.wikipedia.org/wiki/File_inclusion_vulnerability
2
https://en.wikipedia.org/wiki/SQL_injection
3
https://en.wikipedia.org/wiki/Buffer_overflow
4
https://en.wikipedia.org/wiki/Race_condition
Chapter 11 — Introduction to Security Assessments 287
designed to make it harder to exploit vulnerabilities, such as Data Execution Prevention5 (DEP) and
Address Space Layout Randomization6 (ASLR). However, just because there is no publicly-known
exploit for a specific vulnerability, that does not mean that one does not exist (or that one can not
be created). For example, many organizations sell commercialized exploits that are never made
public, so all vulnerabilities must be treated as potentially exploitable.
11.2.1. Vulnerability Assessment
A vulnerability is considered a weakness that could be used in some manner to compromise the
confidentiality, integrity, or availability of an information system. In a vulnerability assessment,
your objective is to create a simple inventory of discovered vulnerabilities within the target envi-
ronment. This concept of a target environment is extremely important. You must be sure to stay
within the scope of your client’s target network and required objectives. Creeping outside the
scope of an assessment can cause an interruption of service, a breach of trust with your client, or
legal action against you and your employer.
Due to its relative simplicity, a vulnerability test is often completed in more mature environments
on a regular basis as part of demonstrating their due diligence. In most cases, an automated tool,
such as the ones in the Vulnerability Analysis7 and Web Applications8 categories of the Kali tools
site and Kali desktop applications menu, is used to discover live systems in a target environment,
identify listening services, and enumerate them to discover as much information as possible such
as the server software, version, platform, and so on.
This information is then checked for known signatures of potential issues or vulnerabilities. These
signatures are made up of data point combinations that are intended to represent known issues.
Multiple data points are used, because the more data points you use, the more accurate the iden-
tification. A very large number of potential data points exist, including but not limited to:
• Operating System Version: It is not uncommon for software to be vulnerable on one op-
erating system version but not on another. Because of this, the scanner will attempt to
determine, as accurately as possible, what operating system version is hosting the targeted
application.
• Patch Level: Many times, patches for an operating system will be released that do not in-
crease the version information, but still change the way a vulnerability will respond, or
even eliminate the vulnerability entirely.
• Processor Architecture: Many software applications are available for multiple processor
architectures such as Intel x86, Intel x64, multiple versions of ARM, UltraSPARC, and so on.
5
https://en.wikipedia.org/wiki/Executable_space_protection#Windows
6
https://en.wikipedia.org/wiki/Address_space_layout_randomization
7
https://tools.kali.org/category/vulnerability-analysis
8
https://tools.kali.org/category/web-applications
288 Kali Linux Revealed
In some cases, a vulnerability will only exist on a specific architecture, so knowing this bit
of information can be critical for an accurate signature.
• Software Version: The version of the targeted software is one of the basic items that needs
to be captured to identify a vulnerability.
These, and many other data points, will be used to make up a signature as part of a vulnerability
scan. As expected, the more data points that match, the more accurate the signature will be. When
dealing with signature matches, you can have a few different potential results:
• True Positive: The signature is matched and it captures a true vulnerability. These results
are the ones you will need to follow up on and correct, as these are the items that malicious
individuals can take advantage of to hurt your organization (or your client’s).
• False Positive: The signature is matched; however the detected issue is not a true vulnera-
bility. In an assessment, these are often considered noise and can be quite frustrating. You
never want to dismiss a true positive as a false positive without more extensive validation.
• True Negative: The signature is not matched and there is no vulnerability. This is the ideal
scenario, verifying that a vulnerability does not exist on a target.
• False Negative: The signature is not matched but there is an existing vulnerability. As bad
as a false positive is, a false negative is much worse. In this case, a problem exists but the
scanner did not detect it, so you have no indication of its existence.
As you can imagine, the accuracy of the signatures is extremely important for accurate results.
The more data that is provided, the greater the chance there is to have accurate results from an
automated signature-based scan, which is why authenticated scans are often so popular.
With an authenticated scan, the scanning software will use provided credentials to authenticate
to the target. This provides a deeper level of visibility into a target than would otherwise be possi-
ble. For instance, on a normal scan you may only detect information about the system that can be
derived from listening services and the functionality they provide. This can be quite a bit of infor-
mation sometimes but it can’t compete with the level and depth of data that will be obtained if you
authenticate to the system and comprehensively review all installed software, applied patches,
running processes, and so on. This breadth of data is useful for detecting vulnerabilities that oth-
erwise may not have been discovered.
A well-conducted vulnerability assessment presents a snapshot of potential problems in an organi-
zation and provides metrics to measure change over time. This is a fairly lightweight assessment,
but even still, many organizations will regularly perform automated vulnerability scans in off-
hours to avoid potential problems during the day when service availability and bandwidth are
most critical.
As previously mentioned, a vulnerability scan will have to check many different data points in
order to get an accurate result. All of these different checks can create load on the target system
as well as consume bandwidth. Unfortunately, it is difficult to know exactly how many resources
will be consumed on the target as it depends on the number of open services and the types of
Chapter 11 — Introduction to Security Assessments 289
checks that would be associated with those services. This is the cost of doing a scan; it is going to
occupy system resources. Having a general idea of the resources that will be consumed and how
much load the target system can take is important when running these tools.
Scanning Threads Most vulnerability scanners include an option to set threads per scan, which equates
to the number of concurrent checks that occur at one time. Increasing this number
will have a direct impact on the load on the assessment platform as well as the net-
works and targets you are interacting with. This is important to keep in mind as you
use these scanners. It is tempting to increase the threads in order to complete scans
faster but remember the substantial load increase associated with doing so.
When a vulnerability scan is finished, the discovered issues are typically linked back to industry
standard identifiers such as CVE number9 , EDB-ID10 , and vendor advisories. This information,
along with the vulnerabilities CVSS score11 , is used to determine a risk rating. Along with false
negatives (and false positives), these arbitrary risk ratings are common issues that need to be
considered when analyzing the scan results.
Since automated tools use a database of signatures to detect vulnerabilities, any slight deviation
from a known signature can alter the result and likewise the validity of the perceived vulnerabil-
ity. A false positive incorrectly flags a vulnerability that does not exist, while a false negative is
effectively blind to a vulnerability and does not report it. Because of this, a scanner is often said
to only be as good as its signature rule base. For this reason, many vendors provide multiple sig-
nature sets: one that might be free to home users and another fairly expensive set that is more
comprehensive, which is generally sold to corporate customers.
The other issue that is often encountered with vulnerability scans is the validity of the suggested
risk ratings. These risk ratings are defined on a generic basis, considering many different factors
such as privilege level, type of software, and pre- or post-authentication. Depending on your
environment, these ratings may or may not be applicable so they should not be accepted blindly.
Only those well-versed in the systems and the vulnerabilities can properly validate risk ratings.
While there is no universally defined agreement on risk ratings, NIST Special publication 800-3012
is recommended as a baseline for evaluation of risk ratings and their accuracy in your environ-
ment. NIST SP 800-30 defines the true risk of a discovered vulnerability as a combination of the
likelihood of occurrence and the potential impact.
9
https://cve.mitre.org/
10
https://www.exploit-db.com/
11
https://www.first.org/cvss/
12
https://csrc.nist.gov/publications/sp#800-30
290 Kali Linux Revealed
Likelihood of Occurrence
According to the National Institute of Standards and Technology (NIST), the likelihood of occur-
rence is based on the probability that a particular threat is capable of exploiting a particular vul-
nerability, with possible ratings of Low, Medium, or High.
• High: the potential adversary is highly skilled and motivated and the measures that have
been put in place to protect against the vulnerability are insufficient.
• Medium: the potential adversary is motivated and skilled but the measures put in place to
protect against the vulnerability may impede their success.
• Low: the potential adversary is unskilled or lacks motivation and there are measures in
place to protect against the vulnerability that are partially or completely effective.
Impact
The level of impact is determined by evaluating the amount of harm that could occur if the vul-
nerability in question were exploited or otherwise taken advantage of.
• High: taking advantage of the vulnerability could result in very significant financial losses,
serious harm to the mission or reputation of the organization, or even serious injury, in-
cluding loss of life.
• Medium: taking advantage of the vulnerability could lead to financial losses, harm to the
mission or reputation of the organization, or human injury.
• Low: taking advantage of the vulnerability could result in some degree of financial loss or
impact to the mission and reputation of the organization.
Overall Risk
Once the likelihood of occurrence and impact have been determined, you can then determine the
overall risk rating, which is defined as a function of the two ratings. The overall risk can be rated
Low, Medium, or High, which provides guidance to those responsible for securing and maintaining
the systems in question.
• High: There is a strong requirement for additional measures to be implemented to protect
against the vulnerability. In some cases, the system may be allowed to continue operating
but a plan must be designed and implemented as soon as possible.
• Medium: There is a requirement for additional measures to be implemented to protect
against the vulnerability. A plan to implement the required measures must be done in a
timely manner.
Chapter 11 — Introduction to Security Assessments 291
• Low: The owner of the system will determine whether to implement additional measures
to protect against the vulnerability or they can opt to accept the risk instead and leave the
system unchanged.
In Summary
With so many factors making up the true risk of a discovered vulnerability, the pre-defined risk
ratings from tool output should only be used as a starting point to determine the true risk to the
overall organization.
Competently-created reports from a vulnerability assessment, when analyzed by a professional,
can provide an initial foundation for other assessments, such as compliance penetration tests. As
such, it is important to understand how to get the best results possible from this initial assessment.
Kali makes an excellent platform for conducting a vulnerability assessment and does not need
any special configuration. In the Kali Applications menu, you will find numerous tools for vul-
nerability assessments in the Information Gathering, Vulnerability Analysis, and Web Application
Analysis categories. Several sites, including the aforementioned Kali Linux Tools Listing13 , The
Kali Linux Official Documentation14 site, and the free Metasploit Unleashed15 course provide ex-
cellent resources for using Kali Linux during a vulnerability assessment.
11.2.2. Compliance Penetration Test
The next type of assessment in order of complexity is a compliance- based penetration test. These
are the most common penetration tests as they are government- and industry-mandated require-
ments based on a compliance framework the entire organization operates under.
While there are many industry-specific compliance frameworks, the most common would likely
be Payment Card Industry Data Security Standard16 (PCI DSS), a framework dictated by payment
card companies that retailers processing card-based payments must comply with. However, a
number of other standards exist such as the Defense Information Systems Agency Security Techni-
cal Implementation Guides17 (DISA STIG), Federal Risk and Authorization Management Program18
(FedRAMP), Federal Information Security Management Act19 (FISMA), and others. In some cases,
a corporate client may request an assessment, or ask to see the results of the most recent assess-
ment for various reasons. Whether ad-hoc or mandated, these sorts of assessments are collectively
13
https://tools.kali.org/tools-listing
14
https://www.kali.org/docs/
15
https://www.offensive-security.com/metasploit-unleashed/
16
https://www.pcisecuritystandards.org/documents/Penetration_Testing_Guidance_March_2015.pdf
17
https://public.cyber.mil/stigs/
18
https://www.fedramp.gov/about-us/about/
19
https://csrc.nist.gov/projects/risk-management
292 Kali Linux Revealed
called compliance-based penetration tests, or simply “compliance assessments” or “compliance
checks”.
A compliance test often begins with a vulnerability assessment. In the case of PCI compliance
auditing20 , a vulnerability assessment, when performed properly, can satisfy several of the base
requirements, including: “2. Do not use vendor-supplied defaults for system passwords and other
security parameters” (for example, with tools from the Password Attacks menu category), “11.
Regularly test security systems and processes” (with tools from the Database Assessment cate-
gory) and others. Some requirements, such as “9. Restrict physical access to cardholder data” and
“12. Maintain a policy that addresses information security for all personnel” don’t seem to lend
themselves to traditional tool-based vulnerability assessment and require additional creativity
and testing.
Despite the fact that it might not seem straight-forward to use Kali Linux for some elements of
a compliance test, the fact is that Kali is a perfect fit in this environment, not just because of
the wide range of security-related tools, but because of the open-source Debian environment it
is built on, allowing for the installation of a wide range of tools. Searching the package manager
with carefully chosen keywords from whichever compliance framework you are using is almost
certain to turn up multiple results. As it stands, many organizations use Kali Linux as the standard
platform for these exact sorts of assessments.
11.2.3. Traditional Penetration Test
A traditional penetration test has become a difficult item to define, with many working from dif-
ferent definitions, depending on the space they operate in. Part of this market confusion is driven
by the fact that the term “Penetration Test” has become more commonly used for the previously
mentioned compliance-based penetration test (or even a vulnerability assessment) where, by de-
sign, you are not delving too deep into the assessment because that would go beyond the minimum
requirements.
For the purposes of this section, we will side-step that debate and use this category to cover as-
sessments that go beyond the minimum requirements; assessments that are designed to actually
improve the overall security of the organization.
As opposed to the previously-discussed assessment types, penetration tests don’t often start with
a scope definition, but instead a goal such as, “simulate what would happen if an internal user is
compromised” or, “identify what would happen if the organization came under focused attack by
an external malicious party.” A key differentiator of these sorts of assessments is that they don’t
just find and validate vulnerabilities, but instead leverage identified issues to uncover the worst-
case scenario. Instead of relying solely on heavy vulnerability scanning toolsets, you must follow
up with validation of the findings through the use of exploits or tests to eliminate false positives
and do your best to detect hidden vulnerabilities or false negatives. This often involves exploiting
20
https://www.pcisecuritystandards.org/documents/PCIDSS_QRGv3_2.pdf
Chapter 11 — Introduction to Security Assessments 293
vulnerabilities discovered initially, exploring the level of access the exploit provides, and using
this increased access as leverage for additional attacks against the target.
This requires critical review of the target environment along with manual searching, creativity,
and outside-the-box thinking to discover other avenues of potential vulnerability and ultimately
using other tools and tests outside those found by the heavier vulnerability scanners. Once this is
completed, it is often necessary to start the whole process over again multiple times to do a full
and complete job.
Even with this approach, you will often find that many assessments are composed of different
phases. Kali makes it easy to find programs for each phase by way of the Kali Menu:
• Information Gathering: In this phase, you focus on learning as much as possible about the
target environment. Typically, this activity is non-invasive and will appear similar to stan-
dard user activity. These actions will make up the foundation of the rest of the assessment
and therefore need to be as complete as possible. Kali’s Information Gathering category has
dozens of tools to uncover as much information as possible about the environment being
assessed.
• Vulnerability Discovery: This will often be called ”active information gathering”, where you
don’t attack but engage in non-standard user behavior in an attempt to identify potential
vulnerabilities in the target environment. This is where the previously-discussed vulnera-
bility scanning will most often take place. The programs listed in the Vulnerability Analysis,
Web Application Analysis, Database Assessment, and Reverse Engineering categories will be
useful for this phase.
• Exploitation: With the potential vulnerabilities discovered, in this phase you try to exploit
them to get a foothold into the target. Tools to assist you in this phase can be found in the
Web Application Analysis, Database Assessment, Password Attacks, and Exploitation Tools
categories.
• Pivoting and Exfiltration: Once the initial foothold is established, further steps have to be
completed. These are often escalating privileges to a level adequate to accomplish your
goals as an attacker, pivoting into other systems that may not have been previously acces-
sible to you, and exfiltrating sensitive information from the targeted systems. Refer to the
Password Attacks, Exploitation Tools, Sniffing & Spoofing, and Post Exploitation categories
to help with this phase.
• Reporting: Once the active portion of the assessment is completed, you then have to docu-
ment and report on the activities that were conducted. This phase is often not as technical
as the previous phases, however it is highly important to ensure your client gets full value
from the work completed. The Reporting Tools category contains a number of tools that
have proven useful in the reporting phase.
In most cases, these assessments will be very unique in their design as every organization will
operate with different threats and assets to protect. Kali Linux makes a very versatile base for
294 Kali Linux Revealed
these sorts of assessments and this is where you can really take advantage of the many Kali Linux
customization features. Many organizations that conduct these sorts of assessments will maintain
highly customized versions of Kali Linux for internal use to speed up deployment of systems before
a new assessment.
Customizations that organizations make to their Kali Linux installations will often include:
• Pre-installation of commercial packages with licensing information. For instance, you may
have a package such as a commercial vulnerability scanner that you would like to use. To
avoid having to install this package with each build, you can do it once and have it show up
in every Kali deployment you do.
• Pre-configured connect-back Virtual Private Networks (VPN). These are very useful in leave-
behind devices that allow you to conduct ”remote internal” assessments. In most cases,
these systems will connect back to an assessor-controlled system, creating a tunnel that
the assessor can use to access internal systems. The Kali Linux ISO of Doom21 is an example
of this exact type of customization.
• Pre-installed internally-developed software and tools. Many organizations will have private
toolsets, so setting these up once in a customized Kali install saves time.
• Pre-configured OS configurations such as host mappings, desktop wallpaper, proxy settings,
etc. Many Kali users have specific settings22 they like to have tweaked just so. If you are
going to do a re-deployment of Kali on a regular basis, capturing these changes makes a lot
of sense.
11.2.4. Application Assessment
While most assessments have a broad scope, an application assessment is a specialty that is nar-
rowly focused on a single application. These sorts of assessments are becoming more common
due to the complexity of mission-critical applications that organizations use, many of which are
built in-house. An application assessment is usually added on to a broader assessment, as required.
Applications that may be assessed in this manner include, but are not limited to:
• Web applications: The most common externally-facing attack surface, web applications
make great targets simply because they are accessible. Often, standard assessments will
find basic problems in web applications, however a more focused review is often worth the
time to identify issues relating to the workflow of the application. The kali-tools-web meta-
package has a number of tools to help with these assessments.
• Compiled desktop applications: Server software is not the only target; desktop applications
also make up a wonderful attack surface. In years past, many desktop applications such as
PDF readers or web-based video programs were highly targeted, forcing them to mature.
21
https://www.offensive-security.com/kali-linux/kali-rolling-iso-of-doom/
22
https://www.offensive-security.com/kali-linux/kali-linux-recipes/
Chapter 11 — Introduction to Security Assessments 295
However, there are still a wide number of desktop applications that are a wealth of vulner-
abilities when properly reviewed.
• Mobile applications: As mobile devices become more popular, mobile applications will be-
come that much more of a standard attack surface in many assessments. This is a fast mov-
ing target and methodologies are still maturing in this area, leading to new developments
practically every week. Tools related to the analysis of mobile applications can be found in
the Reverse Engineering menu category.
Application assessments can be conducted in a variety of different ways. As a simple example, an
application-specific automated tool can be run against the application in an attempt to identify
potential issues. These tools will use application-specific logic in an attempt to identify unknown
issues rather than just depending on a set of known signatures. These tools must have a built-in
understanding of the application’s behavior. A common example of this would be a web applica-
tion vulnerability scanner such as Burp Suite23 , directed against an application that first identifies
various input fields and then sends common SQL injection attacks to these fields while monitoring
the application’s response for indications of a successful attack.
In a more complex scenario, an application assessment can be conducted interactively in either a
black box or white box manner.
• Black Box Assessment: The tool (or assessor) interacts with the application with no special
knowledge or access beyond that of a standard user. For instance, in the case of a web ap-
plication, the assessor may only have access to the functions and features that are available
to a user that has not logged into the system. Any user accounts used would be ones where
a general user can self-register the account. This would prevent the attacker from being
able to review any functionality that is only available to users that need to be created by an
administrator.
• White Box Assessment: The tool (or assessor) will often have full access to the source code,
administrative access to the platform running the application, and so on. This ensures that
a full and comprehensive review of all application functionality is completed, regardless of
where that functionality lives in the application. The trade-off with this is that the assess-
ment is in no way a simulation of actual malicious activity.
There are obviously shades of grey in between. Typically, the deciding factor is the goal of the
assessment. If the goal is to identify what would happen in the event that the application came
under a focused external attack, a black box assessment would likely be best. If the goal is to
identify and eliminate as many security issues as possible in a relatively short time period, a white
box approach may be more efficient.
In other cases, a hybrid approach may be taken where the assessor does not have full access to the
application source code of the platform running the application, but user accounts are provisioned
by an administrator to allow access to as much application functionality as possible.
23
https://portswigger.net/burp
296 Kali Linux Revealed
Kali is an ideal platform for all manner of application assessments. On a default installation, a
range of different application-specific scanners are available. For more advanced assessments, a
range of tools, source editors, and scripting environments exist. You may find the Web Applica-
tion24 and Reverse Engineering25 sections of the Kali Tools26 website helpful.
11.3. Formalization of the Assessment
With your Kali environment ready and the type of assessment defined, you are almost ready to
start working. Your last step is to formalize the work to be done. This is critically important, as
this defines what the expectations for the work will be, and grants you permission to conduct what
might otherwise be illegal activity. We will cover this at a high level, but this is a very complex
and important step so you will likely want to check with your organization’s legal representative
for assistance.
As part of the formalization process, you will need to define the rules of engagement for the work.
This covers items such as:
• What systems are you allowed to interact with? It is important to ensure you don’t acciden-
tally interfere with anything that is critical to business operations.
• What time of day and over what attack window is the assessment allowed to occur? Some
organizations like to limit the times that the assessment work can be conducted.
• When you discover a potential vulnerability, are you allowed to exploit it? If not, what is
the approval process? There are some organizations that take a very controlled approach
to each exploitation attempt, whereas others would like a more realistic approach. It is best
to define these expectations clearly before work begins.
• If a significant issue is discovered, how should it be handled? Sometimes, organizations
want to be informed right away, otherwise it is typically addressed at the end of the assess-
ment.
• In case of emergency, who should you contact? It is always important to know who to con-
tact when a problem of any sort occurs.
• Who will know about the activity? How will it be communicated to them? In some cases,
organizations will want to test their incident response and detection performance as part of
the assessment. It is always a good idea to know this beforehand, so you know if you should
take any degree of stealth in the approach to the assessment.
• What are the expectations at the end of the assessment? How will results be communicated?
Know what all parties expect at the end of the assessment. Defining the deliverable is the
best way to keep everyone happy after the work is completed.
24
https://tools.kali.org/category/web-applications
25
https://tools.kali.org/category/reverse-engineering
26
https://tools.kali.org/
Chapter 11 — Introduction to Security Assessments 297
While not complete, this listing gives you an idea of the details that should be covered. However,
you should realize that there is no substitute for good legal representation. Once these items are
defined, you need to acquire proper authorization to perform the assessment, since much of the
activity that you will do in the course of an assessment may not be legal without proper authority
from someone with the authority to give that permission.
With all that in place, there is still one last step you will want to take before starting work: valida-
tion. Never trust the scope that you are provided—always validate it. Use multiple information
sources to confirm that the systems within scope are in fact owned by the client and that they are
operated by the client as well. With the prevalence of cloud services, an organization may forget
that they don’t actually own the systems providing them service. You may find that you have to
obtain special permission from a cloud service provider before starting work. In addition, always
validate IP address blocks. Don’t count on an organization’s assumption that they own entire IP
blocks, even if they sign off on them as viable targets. For example, we have seen examples of or-
ganizations that request an assessment of an entire class C network range when, in fact, they only
owned a subset of those addresses. By attacking the entire class C address space, we would have
ended up attacking the organization’s network neighbors. The OSINT Analysis sub-category of
the Information Gathering menu contains a number of tools that can assist you with this validation
process.
11.4. Types of Attacks
Once the work is taking place, what are some of the specific sorts of attacks that you will be con-
ducting? Each type of vulnerability27 has its own associated exploitation techniques. This section
will cover the various classes of vulnerabilities that you will interact with most often.
No matter what category of vulnerability you are looking at, Kali makes these tools and exploits
easy to find. The Kali menu on your graphical user interface is divided up into categories to help
make the right tool easier to find. In addition, the Kali Tools website28 has comprehensive listings
of the various tools available in Kali, organized by category and tagged for easy browsing. Each
entry contains detailed information about the tool as well as example usage.
11.4.1. Denial of Service
Denial of service attacks leverage a vulnerability to create a loss of service, often by crashing the
vulnerable process. The Stress Testing category of the Kali Linux menu contains a number of tools
for this purpose.
When many people hear the term “denial of service attack”, they immediately think of resource
consumption attacks that are sent out from multiple sources at once against a single target. These
27
https://www.cvedetails.com/vulnerabilities-by-types.php
28
https://tools.kali.org/tools-listing
298 Kali Linux Revealed
would be a distributed denial of services attack, or DDoS. These sorts of attacks are rarely part of a
professional security assessment.
Instead, a singular denial of service attack is most often the result of an improper attempt to
exploit a vulnerability. If an exploit writer releases partially functional, or proof-of-concept (PoC)
code and it is used in the field, this could create a denial of service condition. Even a properly-
coded exploit may only work under very specific circumstances but cause a denial of service under
lesser circumstances. It may seem that the solution is to only use safe and tested exploit code,
or to write your own. Even with this solution, there are no guarantees and this severely limits
the assessor, causing undue constraints, which results in a lesser assessment. Instead, the key is
compromise. Avoid PoC code and untested exploits in the field and always make sure a lawyer has
you covered for other mishaps.
Typically, denial of service attacks are not launched intentionally. Most automated vulnerability
tools will declare denial of service vulnerabilities as lower risk due to the fact that while you can
remove a service from operation, that service can’t be exploited for code execution. However, it
is important to remember that not all exploits are released publicly and a denial of service vulner-
ability may mask a deeper, more serious threat. A code execution exploit for a known denial of
service may exist but not be public. The point is, pay attention to denial of service vulnerabilities
and encourage your customer to get them patched regardless of their (often low) threat rating.
11.4.2. Memory Corruption
A memory corruption happens when a location within the memory space of a process is acciden-
tally modified due to programming mistakes. Memory corruption bugs usually lead to unpre-
dictable program behavior, however in many cases, these bugs allow process memory manipula-
tion in such a way that the program execution flow can be controlled, allowing attacker-defined
activity.
These attacks are typically referred to as buffer overflows, although this term is an over-
simplification. The most common types of memory corruption are vastly different from one an-
other and have their own tactics and techniques required for successful exploitation.
• Stack Buffer Overflow: When a program writes more data to a buffer on the stack than there
is space available for it, adjacent memory can be corrupted, often causing the program to
crash.
• Heap Corruption: Heap memory is allocated at run- time and usually contains data from the
running program. Heap corruptions occur by manipulating the data to overwrite through
the linked list of heap memory pointers.
• Integer Overflow: These overflows occur when an application tries to create a numeric value
that can’t be contained within its allocated storage space.
• Format String: When a program accepts user input and formats it without checking it, mem-
ory locations can be revealed or overwritten, depending on the format tokens that are used.
Chapter 11 — Introduction to Security Assessments 299
11.4.3. Web Vulnerabilities
Due to the fact that modern web sites are no longer static pages, but instead dynamically generated
for the user, the average website is quite complex. Web vulnerabilities take advantage of this
complexity in an effort to attack either the back end page generation logic or the presentation to
the visitor of the site.
These sorts of attacks are extremely common, as many organizations have reached the point
where they have very few externally facing services. Two of the most prevalent web application
attack types29 are SQL injection and cross-site scripting (XSS).
• SQL injection: These attacks take advantage of improperly-programmed applications that
do not properly sanitize user input, leading to the ability to extract information from the
database or even the complete takeover of the server.
• Cross-site scripting: As with SQL injection, XSS attacks result from improper sanitization
of user input, allowing attackers to manipulate the user or site into executing code in the
context of their own browser session.
Complex, rich, and complicated web applications are very common, presenting a welcome attack
surface for malicious parties. You will find a large number of useful tools in the Web Application
Analysis menu category and the kali-tools-web metapackage.
11.4.4. Password Attacks
Password attacks are attacks against the authentication system of a service. These attacks are
often broken into online password attacks and offline password attacks, which you will find re-
flected in the Password Attacks menu category. In an online password attack, multiple passwords
are attempted against a running system. In an offline password attack, the hashed or encrypted
values of the passwords are obtained and the attacker attempts to obtain the clear text values.
The protection against this sort of attack is the fact that it is computationally expensive to work
through this process, limiting the number of attempts per second you can generate. However,
workarounds for this do exist, such as using graphic processor units (GPUs) to accelerate the num-
ber of attempts that can be made. The kali-tools-gpu metapackage contains a number of tools that
tap into this power.
Most commonly, password attacks target vendor-supplied default passwords. As these are well-
known values, attackers will scan for these default accounts, hoping to get lucky. Other common
attacks include custom dictionary attacks where a wordlist is created that has been tailored to
the target environment and then an online password attack against common, default, or known
accounts is conducted where each word is attempted in sequence.
29
https://owasp.org/www-project-top-ten/
300 Kali Linux Revealed
In an assessment, it is very important to understand the potential consequences of this sort of
attack. First, they are often very noisy due to the repeated authentication attempts. Secondly,
these attacks can often result in an account lock out situation after too many invalid attempts are
performed against a single account. Finally, the performance of these attacks is often quite slow,
resulting in difficulty when attempting to use a comprehensive wordlist.
11.4.5. Client-Side Attacks
Most attacks are conducted against servers, but as services have become harder to attack, easier
targets have been selected. Client-side attacks are a result of this, where an attacker will target
the various applications installed on the workstation of an employee within a target organization.
The Social Engineering Tools menu category has a number of excellent applications that can help
conduct these types of attacks.
This sort of attack were commonly exploited by Flash, Acrobat Reader, and Java in the early 2000s.
Currently HTML Application (HTA) is the popular method. In the above cases, attackers would try
to solicit a target to visit a malicious web page. These pages would contain specialized code that
would trigger either vulnerabilities in these client-side applications or trick the user, resulting in
the ability to run malicious code on the targets system.
Client-side attacks are incredibly difficult to prevent, requiring a great deal of user education,
constant application updates, and network controls to effectively mitigate the risk.
11.5. Summary
In this chapter, we took a brief look at Kali’s role in the field of information security. We discussed
the importance of a clean, working installation and the use of encryption before heading out to
the field in order to protect your client’s information, and the importance of legal representation
to protect you and your client’s interests.
The components of the CIA (confidentiality, integrity, availability) triad are the primary items
that you will focus on when securing a system as part of standard deployment, maintenance, or
assessment. This conceptual foundation will assist you with the identification of the critical com-
ponents of your systems and the amount of effort and resources worth investing into correcting
identified problems.
We discussed several types of vulnerabilities including file inclusion, SQL injection, buffer over-
flows, and race conditions.
The accuracy of the signatures is extremely important to get useful vulnerability assessment re-
sults. The more data that are provided, the higher chance there is to have accurate results from
an automated signature-based scan, which is why authenticated scans are often so popular.
Chapter 11 — Introduction to Security Assessments 301
Since automated tools use a database of signatures to detect vulnerabilities, any slight deviation
from a known signature can alter the result and likewise the validity of the perceived vulnerability.
We also discussed the four types of assessments: the vulnerability assessment, compliance test, tradi-
tional penetration test, and the application assessment. Even though each type of assessment leverages
a core set of tools, many of the tools and techniques overlap.
The vulnerability assessment is relatively simple in comparison to the other assessment types and
often consists of an automated inventory of discovered issues within a target environment. In
this section, we discussed that a vulnerability is a flaw that, when exploited, will compromise the
confidentiality, integrity, or availability of an information system. Since it is signature-based, this
type of assessment relies on accurate signatures and can present false positives and negatives. You
will find the core tools for this type of assessment in the Vulnerability Analysis and Exploitation
Tools menu categories of Kali Linux.
Compliance tests are based on government- and industry-mandated requirements (such as PCI
DSS, DISA STIG, and FISMA), which are in turn based on a compliance framework. This test usually
begins with a vulnerability assessment.
A traditional penetration test is a thorough security assessment that is designed to improve the
overall security posture of an organization based on certain real-world threats. This type of test
involves several steps (mirrored by the Kali Linux menu structure) and culminates in exploitation
of vulnerabilities and pivoting access to other machines and networks within the target scope.
Application assessments (usually white- or black-box) focus on a single application and use spe-
cialized tools such as those found in the Web Application Analysis, Database Assessment, Reverse
Engineering, and Exploitation Tools menu categories.
Several types of attacks were discussed including: Denial of Service, which breaks the behavior of
an application and makes it inaccessible; memory corruption, which leads to manipulation of pro-
cess memory, often allowing an attacker code execution; web attacks, which attack web services
using techniques like SQL injection and XSS attacks; and password attacks, which often leverage
password lists to attack service credentials.
302 Kali Linux Revealed
Keywords
Constant changes
Certifications
Trainings
Chapter
Conclusion: The 12
Road Ahead
Contents
Keeping Up with Changes 306 Showing Off Your Newly Gained Knowledge 306 Going Further 307
Congratulations! Hopefully you should now be more familiar with your Kali Linux system and you
should not be afraid of using it for any experiment that you can think of. You have discovered its
most interesting features but you also know its limits and various ways to work around those
limitations.
If you have not put all features into practice, keep this book around for reference purposes and
refresh your memory when you are about to try a new feature. Remember that there is nothing
better than practice (and perseverance) to develop new skills. Try Harder1 , as Offensive Security
says.
12.1. Keeping Up with Changes
With a constantly-changing distribution like kali-rolling, some parts of the book will necessarily
become obsolete. We will do our best to keep it up to date (at least for the online version2 ) but for
most parts we tried to provide generic explanations that should be useful for a long time to come.
That said, you should be ready to embrace changes and to find out solutions to any problem that
might pop up. With the better understanding of Kali Linux and its relationship to Debian, you
can rely on both the Debian and Kali communities and their numerous resources (bug trackers,
forums, mailing lists, etc.) when you are getting stuck.
Don’t be afraid to file bugs (see section 6.3, “Filing a Good Bug Report” [page 134])! If you are
like me, by the time you have completed the steps involved in filing a good bug report (and it
takes some time), you will have solved the problem or at least found a good work-around. And by
actually filing the bug, you will be helping others who are affected by the issue.
12.2. Showing Off Your Newly Gained Knowledge
Are you proud of your new Kali Linux skills? Would you like to ensure that you remember the
really important things? If you answer yes to one of those questions, then you should consider
applying for the Kali Linux Certified Professional program.
It is a comprehensive certification that will ensure that you know how to deploy and use Kali Linux
in many realistic use cases. It is a nice addition to your resume and it also proves that you are ready
to go further.
1
https://www.offensive-security.com/offsec/say-try-harder/
2
https://kali.training/
306 Kali Linux Revealed
12.3. Going Further
This book taught you a lot of things that any Kali Linux user should know, but we made some hard
choices to keep it short, and there are many topics that were not covered.
12.3.1. Towards System Administration
If you want to learn more about system administration, then we can only recommend that you
check out the Debian Administrator’s Handbook:
è https://debian-handbook.info/get/
You will find there, many supplementary chapters covering common Unix services that we have
entirely skipped in this book. And even for chapters that have been reused in the Kali book, you
will find plenty of supplementary tips, notably on the packaging system (which is also covered
more extensively at its lowest level).
The Debian book obviously presents more deeply the Debian community and the way it is orga-
nized. While this knowledge is not vital, it is really useful when you have to interact with Debian
contributors, for example through bug reports.
12.3.2. Towards Penetration Testing
You probably noticed by now that this book did not teach you penetration testing. But the things
you learned are still important. You are now ready to fully exploit the power of Kali Linux, the
best penetration testing framework. And you have the basic Linux skills required to participate
in Offensive Security’s training.
If you feel that you are not yet ready for a paid course, you can start by following the Metasploit
Unleashed3 free online training. Metasploit is a very popular penetration testing tool and you
have to know it if you are serious about your plans to learn penetration testing.
The next logical step would then be to follow the Penetration Testing with Kali Linux4 online
course leading the path to the famous “Offensive Security Certified Professional” certification.
This online course can be followed at your own pace but the certification is actually a difficult,
24h long, real-word, hands-on penetration test which takes place in an isolated VPN network.
Are you up to the challenge?
3
https://www.offensive-security.com/metasploit-unleashed/
4
https://www.offensive-security.com/courses-and-certifications/
Chapter 12 — Conclusion: The Road Ahead 307
Index
_ add a user to a group, 112
.config, 239 addgroup, 113
.d, 200 adduser, 112
.htaccess, 120 Advanced Package Tool, 177
/dev, 48 aide (Debian package), 170
/etc/apt/apt.conf.d/, 200 AllowOverride, Apache directive, 119, 120
/etc/apt/preferences, 202 analysis
/etc/apt/sources.list, 178 vulnerability, 6
/etc/apt/trusted.gpg.d/, 208 web application, 6
/etc/group, 112 ansible, 261
/etc/gshadow, 112 Apache, 118
/etc/network/interfaces, 109 directives, 119
/etc/passwd, 112 Apache directives, 121
/etc/salt/minion, 261 application assessments, 295
/etc/shadow, 112 applications
/etc/ssh/sshd_config, 115 collection, 10
/proc, 48 menu, 6
/sys, 48 applying a patch, 233
/var/lib/dpkg/, 217 apropos, 128
/var/www/html/, 118 APT, 177
32-bit CPU, 16 configuration, 200
64-bit CPU, 16 header display, 191
initial configuration, 84
A interfaces, 196
a2dismod, 118 package search, 191
a2enmod, 118 pinning, 202
a2ensite, 119 preferences, 202
ACCEPT, 161 apt, 181
account apt build-dep, 231
creation, 112 apt dist-upgrade, 185
disable, 113 apt full-upgrade, 185
modification, 113 apt install, 183
activity, monitoring, 168 apt purge, 186
apt remove, 186 assessment
apt search, 191 application, 295
apt show, 191 black box, 296
apt source, 229 formalization, 297
apt update, 181 vulnerability, 288
apt upgrade, 184 white box, 296
apt-cache, 191 attacks
apt-cache dumpavail, 192 client side, 301
apt-cache pkgnames, 192 database, 6
apt-cache policy, 192 denial of service, 298
apt-cache search, 191 memory corruption, 299
apt-cache show, 191 password, 7, 300
apt-cdrom, 178 types of, 298
apt-get, 181 web, 300
apt-get dist-upgrade, 185 wireless, 7
apt-get install, 183 auditing, security, 5
apt-get purge, 187 authentication
apt-get remove, 186 package authentication, 208
apt-get update, 181 AuthName, Apache directive, 120
apt-get upgrade, 184 AuthType, Apache directive, 120
apt-key, 208 AuthUserFile, Apache directive, 120
apt-mark auto, 205 automatic installation, 95
automatically installed packages, 205
apt-mark manual, 205
avalanche effect, 169
apt-xapian-index, 192
axi-cache, 192
apt.conf.d/, 200
aptitude, 181, 196 B
aptitude dist-upgrade, 185 background process, 57
aptitude full-upgrade, 185 BackTrack, XXI, 2
aptitude install, 183 bash curly brackets, 168
aptitude markauto, 205 bash trick, 168
aptitude purge, 187 bg, 57
aptitude remove, 186 BIOS, 24
aptitude safe-upgrade, 184 block device file, 49
aptitude search, 191 boot preseed, 96
aptitude show, 191 boot screen, 67
aptitude unmarkauto, 205 bootable USB key, 19
aptitude update, 181 bootloader, 85
aptitude why, 205 BOOTP, 258
architecture Breaks, header field, 214
multi-arch support, 206 broken dependency, 195
ARM installations, 98 Bruce Schneier, 156
Index 309
brute-force attacks, 300 compilation
buffer of a kernel, 237
overflow, 299 compliance penetration test, 292
receive buffer, 162 component (of a repository), 179
bug report, 134 conffiles, 220
bugs.kali.org, 138 confidentiality
build dependencies, installation, 231 files, 88
build options, 234 config, debconf script, 220
Build-Depends, 231 configuration
building creating configuration packages, 269
a custom live ISO image, 241 files, 220
a package, 236 initial configuration of APT, 84
management, 261
C
network
cache, proxy, 84
DHCP, 71
cat, 56
static, 71
cd, 52
of the kernel, 239
cdimage.kali.org, 14, 181
program configuration, 114
cdrom preseed, 96
conflicts, 214
certification, 306
chage, 113 Conflicts, header field, 214
chain, 160 contrib, section, 179
changelog file, 271 control, 211
changelog.Debian.gz, 130 control file, 272
character device file, 49 control sum, 169
checksecurity, 171 control.tar.gz, 217
checksums, 220 copying, ISO image, 19
chef, 261 copyright, 131
chfn, 113 copyright file, 271
chgrp, 58 country selection, 68
chmod, 58 cp, 53
choice createdb, 116
of country, 68 createuser, 116
of language, 67 creation
chown, 58 of a PostgreSQL database, 116
chroot, 245 of a PostgreSQL user, 116
chsh, 113 of groups, 113
client side attacks, 301 of user accounts, 112
cluster, PostgreSQL cluster, 116, 117 credentials, default, 159
command line, 51 cross-site scripting (XSS), 300
communities, 132 cryptsetup, 248
comparison of versions, 190 nuke password, 250
310 Kali Linux Revealed
curly braces, 168 dh_install, 273
customization of live ISO image, 241 DHCP, 258
dictionary attacks, 300
D directives, Apache, 119, 121
database assessment, 6 DirectoryIndex, Apache directive, 120
database server, 115 disable an account, 113
dch, 232 disk preseed, 96
dd, 22 Disks (program), 21
debconf, 220 diskutil, 23
debconf-get, 101 distribution, Linux, 2
debconf-get-selections, 98 dm-crypt, 89
debconf-set, 101 dmesg, 60
DEBEMAIL, 270 DNAT, 161
DEBFULLNAME, 270 dnsmasq, 258
Debian documentation, 128, 130
relationship with Kali Linux, 4 download
Debian Administrator’s Handbook, 307 ISO image, 14
Debian Free Software Guidelines, 5 the sources, 229
Debian GNU/Linux, 2 dpkg, 176
Debian Policy, 5 database, 217
debian-archive-keyring, 208 dpkg --verify, 169
debian-kernel-handbook, 238 internal operation, 219
debian/changelog, 232, 271 dpkg-buildpackage, 236
debian/control, 272 dpkg-deb, 236
debian/copyright, 271 dpkg-source --commit, 233
debian/patches, 230 drive, USB drive, 19
debian/rules, 234, 273 DROP, 161
debuild, 237 dropdb, 116
default passwords, 159 dropuser, 116
default.target, 122 dual boot, 87
deletion of a group, 113
delgroup, 113 E
denial of service, 298 echo, 54
dependency, 212 editor, 56
Depends, header field, 212 encrypted partition, 88
desktop environment, 3 encrypted persistence, 248
choice during build of live ISO, 242 engineering
desktop-base, 269 reverse, 7
detecting changes on the filesystem, 169 social engineering, 7
device file, 49 Enhances, header field, 214
df, 60 environment
dh-make, 270 environment variable, 54
Index 311
ExecCGI, Apache directive, 120 Info, 130
execution modules, salt, 262 gpasswd, 113
execution, right, 57 GPG key, 17
experimental, 203 graphical.target, 122
Explanation, 204 grep, 56
exploitation tools, 7 group
add a user, 112
F change, 113
fail2ban, 158 creation, 113
features, 8 deletion, 113
fg, 57 of volumes, 89
file owner, 57
confidentiality, 88 groupmod, 113
configuration files, 220 GRUB, 85
file system, 49 gui-apt-key, 210
filesystem guided partitioning, 76
hierarchy, 54
filtering rule, 160, 163 H
find, 56 hardware discovery, 61
fingerprint, 169 heap corruption, 299
firewall, 159 history of Kali Linux, 2
FollowSymLinks, Apache directive, 120 HOME, 55
forensics, 7 home directory, 55
mode, 8 host, virtual host, 118
formalization of the assessment, 297 htpasswd, 121
format disk, 49 HTTP proxy, 84
forums, 132 HTTP server, 118
forums.kali.org, 132 http.kali.org, 180
FORWARD, 160 HTTPS, 118
free, 60 Hyper-V, 24
Freenode, 133
fwbuilder, 166 I
ICMP, 162
G id, 60, 113
get the sources, 229 ifupdown, 109
getent, 112 impersonation, 7
git clone, 230 Includes, Apache directive, 120
GitHub issues, 148 incompatibilities, 214
GNOME, 3 Indexes, Apache directive, 120
gnome-disk-utility, 21 info, 130
gnome-system-monitor, 168 information gathering, 6
GNU initrd preseed, 96
312 Kali Linux Revealed
INPUT, 160 kali-dev, 4, 180
installation, 66 kali-linux-* metapackages, 243
automatic, 95 kali-menu, 269
of build dependencies, 231 kali-meta, 269
on ARM devices, 98 kali-rolling, 4, 179
package installation, 182, 183 kali.org/docs/, 131
troubleshooting, 99 KDE, 3
unattended, 95 kernel, 48
installer preseeding, 96 compilation, 237
integer overflow, 299 configuration, 239
Internet Control Message Protocol, 162 logs, 60
ip6tables, 159, 163 sources, 238
iptables, 159, 163 key
IRC channel, 133 APT’s authentication keys, 209
isc-dhcp-server, 258 USB key, 19
ISO image keyboard layout, 69
authentication, 17 kill, 57
booting, 24 konqueror, 130
copying, 19 KVM, 24
custom build, 241
download, 14 L
mirrors, 14 language selection, 67
variants, 16 layout, keyboard, 69
less, 56
J libapache-mod-php, 118
journal, 60 Linux, 48
journalctl, 60 distribution, 2
kernel, 2, 9
K kernel sources, 238
Kali Linux live ISO image, 14
communities, 132 custom build, 241
documentation, 131 live-boot, 245
download, 14 live-build, 241
features, 8 adding files, 245
getting started, 14 debconf preseeding, 244
history, 2 hooks, 245
metapackages, 243 packages to install, 243
policies, 10 loader
relationship with Debian, 4 bootloader, 85
repositories, 179 LOG, 161
kali-archive-keyring, 208 logcheck, 167
kali-defaults, 269 logging, 167
Index 313
Logical Volume Manager, 89 mirrors, 14, 180
login, remote login, 115 mkdir, 53
logs mkfs, 49
aptitude, 199 modification of a package, 228
dpkg, 194 modification, right, 57
journal, 60 monitoring, 167
kernel, 60 activity, 168
monitoring, 167 files, 170
ls, 52 log files, 167
lsdev, 61 more, 56
lshw, 61 mount, 49
lspci, 61 mount point, 82
lspcmcia, 61 Multi-Arch, 206
lsusb, 61 multi-user.target, 122
LUKS, 89 MultiViews, Apache directive, 120
LVM, 89 mv, 53
M N
machine, virtual machine, 24 netfilter, 159
main, section, 179 network configuration, 71, 108
make deb-pkg, 241 with ifupdown, 109
Makefile, 272 with NetworkManager, 108
man, 128 with systemd-network, 110
management network installation, 258
configuration management, 261 network preseed, 97
of services, 121 network services, 10
manual pages, 128 securing, 159
manually installed packages, 205 NetworkManager, 108
mask newgrp, 58, 113
rights mask, 59 NEWS.Debian.gz, 130
MASQUERADE, 161 non-free, section, 179
master boot record, 87 nuke password, 250
master, salt master, 261
MD5, 169 O
md5sums, 220 octal representation of rights, 59
memory corruption, 299 Offensive Security, 2
menu, Kali Linux’s applications menu, 6 openssh-server, 115
metapackage, 213, 215 Options, Apache directive, 119
kali-linux-*, 243 OUTPUT, 160
metapackages, 85 overflow, buffer, 299
Metasploit Unleashed, 307 overlay filesystem, 246
minion, salt minion, 261 owner
314 Kali Linux Revealed
group, 57 filter, 159
user, 57 IP, 159
PAE (Physical Address Extension), 36
P parted, 247
package partition
authenticity check, 208 encrypted, 88
binary package, 176 swap partition, 82
build, 236 partitioning, 75
configuration, 269 guided partitioning, 75
conflict, 214
manual partitioning, 79
content inspection, 189
passwd, 113
Debian package, 176
password, 113
dependency, 212
attacks, 300
file list, 187
default passwords, 159
header list, 190
policy, 158
incompatibility, 214
password attacks, 7
info, 190
patch, 233
installation, 182, 183
patch application, 233
making changes, 232
PATH, 53
meta-information, 210, 211
modification, 228 PCI, 292
priority, 202 penetration test
purge, 187 compliance, 292
removal, 183, 186 traditional, 293
replacement, 216 penetration testing, 5
repository, 275 penetration testing course, 307
seal, 208 permissions, 57
search, 188, 191 persistence, 246
signature, 208 encrypted, 248
source of, 178 multiple stores, 249
source package, 176 pg_createcluster, 117
status, 188 pg_ctlcluster, 117
unpacking, 182 pg_dropcluster, 117
version comparison, 190 pg_hba.conf, 116
virtual package, 215 pg_lsclusters, 117
package tracker, 5 pg_renamecluster, 117
Packages.xz, 178 pg_upgradecluster, 117
packaging PGP key, 17
build options, 234 PHP, 118
configuration packages, 269 PID, process identifier, 50
new upstream version, 235 Pin, 204
packet Pin-Priority, 204
Index 315
pinfo, 130 reinstallation, 194
ping, 162 REJECT, 161
pinning, APT pinning, 202 Release.gpg, 208
point, mount point, 82 remote login, 115
post exploitation, 7 removal of a package, 183
PostgreSQL, 115 removing a package, 186
postinst, 217 replacement, 216
postrm, 217 Replaces, header field, 216
POSTROUTING, 160 report a bug, 134
pre-dependency, 213 reportbug, 143
Pre-Depends, header field, 213 reporting tools, 7
preferences, 202 repository of packages, 275
preinst, 217 reprepro, 275
prerm, 217 Require, Apache directive, 121
PREROUTING, 160 requirements, minimal installation require-
preseed file, 97 ments, 66
preseeding debian-installer, 96 rescue mode of installer, 87
priority resize a partition, 80
package priority, 202 retrieve the sources, 229
program reverse engineering, 7
configuration, 114 rights, 57
Provides, header field, 215 mask, 59
proxy, 84 octal representation, 59
proxy cache, 84 risk model, 156
ps, 57 risk ratings, 290
puppet, 261 rkhunter, 171
purge of a package, 187 rm, 53
purging a package, 187 rmdir, 53
pwd, 52 Rolling, Kali Rolling, 3
PXE boot, 258 root password, 159
RTFM, 128
Q rules file, 273
QCOW, 30
QEMU, 24 S
salt execution modules, 262
R salt formulas, 264
read, right, 57 salt state modules, 265
README.Debian, 130 salt states, 264
receive buffer, 162 salt-key, 261
Recommends, header field, 214 saltstack, 261
REDIRECT, 161 samhain, 170
redirection, 56 scanning threads, 290
316 Kali Linux Revealed
Schneier, Bruce, 156 sources.list, 178
search of packages, 191 Sources.xz, 178
section spoofing, 7
contrib, 179 SQL injection, 300
main, 179 SSH, 115
non-free, 179 ssh service, 159
secure boot, 24 SSL, 118
secure ssh, 159 state modules, salt, 265
securing, 156 sticky bit, 58
a laptop, 158 sudo, 109
a server, 158 Suggests, header field, 214
network services, 159 swap, 82
security swap partition, 82
assessments, 284 SymLinksIfOwnerMatch, Apache directive, 120
auditing, 5 synaptic, 196, 199
policy, 156 system administration, 307
service file, systemd service file, 122 system, file system, 49
services management, 121 systemctl, 121
setgid directory, 58 systemd, 121
setgid, right, 58 systemd-network, 110
setuid, right, 58 systemd-resolved, 111
Setup, 24
T
sg, 113
target, systemd target, 122
SHA1, 169
TFTP, 258
SHA256SUMS, 17
tftpd-hpa, 258
shell, 52
threat model, 156
shrink a partition, 80
TLS, 118
signal, 57 top, 168
signature tracker
package signature, 208 package tracker, 5
SNAT, 161 traditional penetration test, 293
sniffing, 7 training, 306
social engineering tools, 7 tripwire, 170
source troubleshooting installations, 99
of packages, 178 trust, web of trust, 18
of the Linux kernel, 238 trusted key, 209
package, 176
retrieval, 229 U
source package UEFI, 24
build, 236 ULOG, 161
making changes, 232 umask, 59
Index 317
uname, 60 web, 300
unattended installation, 95
union mount, 246 W
unit, systemd unit, 121 WantedBy, systemd directive, 122
unpacking Wants, systemd directive, 122
binary package, 182 web access restriction, 121
upgrade web application analysis, 6
handling problems after an upgrade, 193 web attacks, 300
system upgrade, 184 web authentication, 120
upstream version, packaging a new one, 235 web of trust, 18
USB key, 19 web server, 118
user Win32 Disk Imager, 19
owner, 57 wireless attacks, 7
user creation, 72 write, right, 57
user space, 48
X
V XDG, 55
variable, environment, 54 Xen, 24
variants of live ISO image, 242 Xfce, 3
VDI, 30
Y
version, comparison, 190
yelp, 130
vigr, 112
vipw, 112
virtual host, 118
virtual machine, 24
virtual memory, 82
virtual package, 215
VirtualBox, 24
VMware, 24
volume
group, 89
logical volume, 89
physical volume, 89
vulnerability
analysis, 6
assessments, 288
client side, 301
denial of service, 298
memory corruption, 299
password, 300
scans, 290
types of, 298
318 Kali Linux Revealed