Linux Security

Authors Paul Cobbaut

License GFDL-1.3-no-invariants-or-later

Plaintext

Linux Security
Paul Cobbaut
Linux Security
Paul Cobbaut
Paul Cobbaut

Publication date 2015-05-24 CEST

Abstract

This book is meant to be used in an instructor-led training. For self-study, the intent is to read
this book next to a working Linux computer so you can immediately do every subject, practicing
each command.

This book is aimed at novice Linux system administrators (and might be interesting and useful
for home users that want to know a bit more about their Linux system). However, this book
is not meant as an introduction to Linux desktop applications like text editors, browsers, mail
clients, multimedia or office applications.

More information and free .pdf available at http://linux-training.be .

Feel free to contact the author:

• Paul Cobbaut: paul.cobbaut@gmail.com, http://www.linkedin.com/in/cobbaut

Contributors to the Linux Training project are:

• Serge van Ginderachter: serge@ginsys.be, build scripts; infrastructure setup; minor stuff

• Hendrik De Vloed: hendrik.devloed@ugent.be, buildheader.pl script

We'd also like to thank our reviewers:

• Wouter Verhelst: wouter@grep.be, http://grep.be

• Geert Goossens: mail.goossens.geert@gmail.com, http://www.linkedin.com/in/
geertgoossens

• Elie De Brauwer: elie@de-brauwer.be, http://www.de-brauwer.be

• Christophe Vandeplas: christophe@vandeplas.com, http://christophe.vandeplas.com

• Bert Desmet: bert@devnox.be, http://bdesmet.be

• Rich Yonts: richyonts@gmail.com,

Permission is granted to copy, distribute and/or modify this document under the terms of the
GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled 'GNU Free Documentation
License'.
Table of Contents
I. local user management ................................................................................................................................. 1
1. introduction to users ........................................................................................................................ 4
1.1. whoami .................................................................................................................................. 5
1.2. who ........................................................................................................................................ 5
1.3. who am i ................................................................................................................................ 5
1.4. w ............................................................................................................................................ 5
1.5. id ............................................................................................................................................ 5
1.6. su to another user .................................................................................................................. 6
1.7. su to root ............................................................................................................................... 6
1.8. su as root ............................................................................................................................... 6
1.9. su - $username ...................................................................................................................... 6
1.10. su - ....................................................................................................................................... 6
1.11. run a program as another user ............................................................................................ 7
1.12. visudo ................................................................................................................................... 7
1.13. sudo su - .............................................................................................................................. 8
1.14. sudo logging ........................................................................................................................ 8
1.15. practice: introduction to users ............................................................................................. 9
1.16. solution: introduction to users ........................................................................................... 10
2. user management ........................................................................................................................... 12
2.1. user management ................................................................................................................. 13
2.2. /etc/passwd ........................................................................................................................... 13
2.3. root ....................................................................................................................................... 13
2.4. useradd ................................................................................................................................. 14
2.5. /etc/default/useradd .............................................................................................................. 14
2.6. userdel .................................................................................................................................. 14
2.7. usermod ................................................................................................................................ 14
2.8. creating home directories .................................................................................................... 15
2.9. /etc/skel/ ............................................................................................................................... 15
2.10. deleting home directories .................................................................................................. 15
2.11. login shell .......................................................................................................................... 16
2.12. chsh .................................................................................................................................... 16
2.13. practice: user management ................................................................................................ 17
2.14. solution: user management ................................................................................................ 18
3. user passwords ............................................................................................................................... 20
3.1. passwd .................................................................................................................................. 21
3.2. shadow file .......................................................................................................................... 21
3.3. encryption with passwd ....................................................................................................... 22
3.4. encryption with openssl ....................................................................................................... 22
3.5. encryption with crypt .......................................................................................................... 23
3.6. /etc/login.defs ....................................................................................................................... 24
3.7. chage .................................................................................................................................... 24
3.8. disabling a password ........................................................................................................... 25
3.9. editing local files ................................................................................................................. 25
3.10. practice: user passwords .................................................................................................... 26
3.11. solution: user passwords ................................................................................................... 27
4. user profiles .................................................................................................................................... 29
4.1. system profile ...................................................................................................................... 30
4.2. ~/.bash_profile ..................................................................................................................... 30
4.3. ~/.bash_login ........................................................................................................................ 31
4.4. ~/.profile .............................................................................................................................. 31
4.5. ~/.bashrc ............................................................................................................................... 31
4.6. ~/.bash_logout ...................................................................................................................... 32
4.7. Debian overview .................................................................................................................. 33
4.8. RHEL5 overview ................................................................................................................. 33
4.9. practice: user profiles .......................................................................................................... 34

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Linux Security

4.10. solution: user profiles ........................................................................................................ 35
5. groups .............................................................................................................................................. 36
5.1. groupadd .............................................................................................................................. 37
5.2. group file ............................................................................................................................. 37
5.3. groups .................................................................................................................................. 37
5.4. usermod ................................................................................................................................ 38
5.5. groupmod ............................................................................................................................. 38
5.6. groupdel ............................................................................................................................... 38
5.7. gpasswd ................................................................................................................................ 39
5.8. newgrp ................................................................................................................................. 40
5.9. vigr ....................................................................................................................................... 40
5.10. practice: groups ................................................................................................................. 41
5.11. solution: groups ................................................................................................................. 42
II. file security ............................................................................................................................................... 43
6. standard file permissions .............................................................................................................. 45
6.1. file ownership ...................................................................................................................... 46
6.2. list of special files ............................................................................................................... 48
6.3. permissions .......................................................................................................................... 49
6.4. practice: standard file permissions ...................................................................................... 54
6.5. solution: standard file permissions ...................................................................................... 55
7. advanced file permissions .............................................................................................................. 57
7.1. sticky bit on directory ......................................................................................................... 58
7.2. setgid bit on directory ......................................................................................................... 58
7.3. setgid and setuid on regular files ........................................................................................ 59
7.4. setuid on sudo ..................................................................................................................... 59
7.5. practice: sticky, setuid and setgid bits ................................................................................ 60
7.6. solution: sticky, setuid and setgid bits ................................................................................ 61
8. access control lists .......................................................................................................................... 63
8.1. acl in /etc/fstab ..................................................................................................................... 64
8.2. getfacl .................................................................................................................................. 64
8.3. setfacl ................................................................................................................................... 64
8.4. remove an acl entry ............................................................................................................. 65
8.5. remove the complete acl ..................................................................................................... 65
8.6. the acl mask ......................................................................................................................... 65
8.7. eiciel ..................................................................................................................................... 66
9. file links ........................................................................................................................................... 67
9.1. inodes ................................................................................................................................... 68
9.2. about directories .................................................................................................................. 69
9.3. hard links ............................................................................................................................. 70
9.4. symbolic links ...................................................................................................................... 71
9.5. removing links ..................................................................................................................... 71
9.6. practice : links ..................................................................................................................... 72
9.7. solution : links ..................................................................................................................... 73
III. iptables firewall ....................................................................................................................................... 74
10. introduction to routers ................................................................................................................ 76
10.1. router or firewall ............................................................................................................... 77
10.2. packet forwarding .............................................................................................................. 77
10.3. packet filtering ................................................................................................................... 77
10.4. stateful ................................................................................................................................ 77
10.5. nat (network address translation) ...................................................................................... 78
10.6. pat (port address translation) ............................................................................................. 78
10.7. snat (source nat) ................................................................................................................ 78
10.8. masquerading ..................................................................................................................... 78
10.9. dnat (destination nat) ......................................................................................................... 78
10.10. port forwarding ................................................................................................................ 78
10.11. /proc/sys/net/ipv4/ip_forward .......................................................................................... 79
10.12. /etc/sysctl.conf .................................................................................................................. 79
10.13. sysctl ................................................................................................................................ 79

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Linux Security

10.14. practice: packet forwarding ............................................................................................. 80
10.15. solution: packet forwarding ............................................................................................. 82
11. iptables firewall ............................................................................................................................ 85
11.1. iptables tables .................................................................................................................... 86
11.2. starting and stopping iptables ............................................................................................ 86
11.3. the filter table .................................................................................................................... 87
11.4. practice: packet filtering .................................................................................................... 92
11.5. solution: packet filtering ................................................................................................... 93
11.6. network address translation ............................................................................................... 94
IV. selinux ...................................................................................................................................................... 97
12. introduction to SELinux .............................................................................................................. 99
12.1. selinux modes .................................................................................................................. 100
12.2. logging ............................................................................................................................. 100
12.3. activating selinux ............................................................................................................. 100
12.4. getenforce ......................................................................................................................... 101
12.5. setenforce ......................................................................................................................... 101
12.6. sestatus ............................................................................................................................. 102
12.7. policy ............................................................................................................................... 102
12.8. /etc/selinux/config ............................................................................................................ 102
12.9. DAC or MAC .................................................................................................................. 103
12.10. ls -Z ............................................................................................................................... 103
12.11. -Z .................................................................................................................................... 103
12.12. /selinux ........................................................................................................................... 104
12.13. identity ........................................................................................................................... 104
12.14. role ................................................................................................................................. 104
12.15. type (or domain) ............................................................................................................ 105
12.16. security context .............................................................................................................. 106
12.17. transition ........................................................................................................................ 106
12.18. extended attributes ......................................................................................................... 107
12.19. process security context ................................................................................................ 107
12.20. chcon .............................................................................................................................. 107
12.21. an example ..................................................................................................................... 108
12.22. setroubleshoot ................................................................................................................ 110
12.23. booleans ......................................................................................................................... 112
V. Appendix ................................................................................................................................................. 113
A. License .......................................................................................................................................... 115
Index ............................................................................................................................................................. 122

v
List of Tables
4.1. Debian User Environment ...................................................................................................................... 33
4.2. Red Hat User Environment .................................................................................................................... 33
6.1. Unix special files .................................................................................................................................... 48
6.2. standard Unix file permissions ............................................................................................................... 49
6.3. Unix file permissions position ............................................................................................................... 49
6.4. Octal permissions ................................................................................................................................... 52
10.1. Packet Forwarding Exercise ................................................................................................................. 80
10.2. Packet Forwarding Solution ................................................................................................................. 82

vi
Part I. local user management
Table of Contents
1. introduction to users ................................................................................................................................. 4
1.1. whoami ............................................................................................................................................ 5
1.2. who .................................................................................................................................................. 5
1.3. who am i ......................................................................................................................................... 5
1.4. w ...................................................................................................................................................... 5
1.5. id ...................................................................................................................................................... 5
1.6. su to another user ........................................................................................................................... 6
1.7. su to root ......................................................................................................................................... 6
1.8. su as root ......................................................................................................................................... 6
1.9. su - $username ................................................................................................................................ 6
1.10. su - ................................................................................................................................................. 6
1.11. run a program as another user ...................................................................................................... 7
1.12. visudo ............................................................................................................................................ 7
1.13. sudo su - ........................................................................................................................................ 8
1.14. sudo logging .................................................................................................................................. 8
1.15. practice: introduction to users ...................................................................................................... 9
1.16. solution: introduction to users .................................................................................................... 10
2. user management ..................................................................................................................................... 12
2.1. user management ........................................................................................................................... 13
2.2. /etc/passwd ..................................................................................................................................... 13
2.3. root ................................................................................................................................................ 13
2.4. useradd ........................................................................................................................................... 14
2.5. /etc/default/useradd ........................................................................................................................ 14
2.6. userdel ........................................................................................................................................... 14
2.7. usermod ......................................................................................................................................... 14
2.8. creating home directories .............................................................................................................. 15
2.9. /etc/skel/ ......................................................................................................................................... 15
2.10. deleting home directories ............................................................................................................ 15
2.11. login shell .................................................................................................................................... 16
2.12. chsh .............................................................................................................................................. 16
2.13. practice: user management .......................................................................................................... 17
2.14. solution: user management ......................................................................................................... 18
3. user passwords ......................................................................................................................................... 20
3.1. passwd ........................................................................................................................................... 21
3.2. shadow file .................................................................................................................................... 21
3.3. encryption with passwd ................................................................................................................ 22
3.4. encryption with openssl ................................................................................................................ 22
3.5. encryption with crypt .................................................................................................................... 23
3.6. /etc/login.defs ................................................................................................................................. 24
3.7. chage .............................................................................................................................................. 24
3.8. disabling a password ..................................................................................................................... 25
3.9. editing local files .......................................................................................................................... 25
3.10. practice: user passwords ............................................................................................................. 26
3.11. solution: user passwords ............................................................................................................. 27
4. user profiles .............................................................................................................................................. 29
4.1. system profile ................................................................................................................................ 30
4.2. ~/.bash_profile ............................................................................................................................... 30
4.3. ~/.bash_login ................................................................................................................................. 31
4.4. ~/.profile ........................................................................................................................................ 31
4.5. ~/.bashrc ........................................................................................................................................ 31
4.6. ~/.bash_logout ............................................................................................................................... 32
4.7. Debian overview ........................................................................................................................... 33
4.8. RHEL5 overview .......................................................................................................................... 33
4.9. practice: user profiles .................................................................................................................... 34
4.10. solution: user profiles ................................................................................................................. 35

2
local user management

5. groups ........................................................................................................................................................ 36
5.1. groupadd ........................................................................................................................................ 37
5.2. group file ....................................................................................................................................... 37
5.3. groups ............................................................................................................................................ 37
5.4. usermod ......................................................................................................................................... 38
5.5. groupmod ....................................................................................................................................... 38
5.6. groupdel ......................................................................................................................................... 38
5.7. gpasswd ......................................................................................................................................... 39
5.8. newgrp ........................................................................................................................................... 40
5.9. vigr ................................................................................................................................................ 40
5.10. practice: groups ........................................................................................................................... 41
5.11. solution: groups ........................................................................................................................... 42

3
Chapter 1. introduction to users
This little chapter will teach you how to identify your user account on a Unix computer using
commands like who am i, id, and more.

In a second part you will learn how to become another user with the su command.

And you will learn how to run a program as another user with sudo.

4
introduction to users

1.1. whoami
The whoami command tells you your username.
[paul@centos7 ~]$ whoami
paul
[paul@centos7 ~]$

1.2. who
The who command will give you information about who is logged on the system.
[paul@centos7 ~]$ who
root pts/0 2014-10-10 23:07 (10.104.33.101)
paul pts/1 2014-10-10 23:30 (10.104.33.101)
laura pts/2 2014-10-10 23:34 (10.104.33.96)
tania pts/3 2014-10-10 23:39 (10.104.33.91)
[paul@centos7 ~]$

1.3. who am i
With who am i the who command will display only the line pointing to your current session.
[paul@centos7 ~]$ who am i
paul pts/1 2014-10-10 23:30 (10.104.33.101)
[paul@centos7 ~]$

1.4. w
The w command shows you who is logged on and what they are doing.
[paul@centos7 ~]$ w
23:34:07 up 31 min, 2 users, load average: 0.00, 0.01, 0.02
USER TTY LOGIN@ IDLE JCPU PCPU WHAT
root pts/0 23:07 15.00s 0.01s 0.01s top
paul pts/1 23:30 7.00s 0.00s 0.00s w
[paul@centos7 ~]$

1.5. id
The id command will give you your user id, primary group id, and a list of the groups that
you belong to.
paul@debian7:~$ id
uid=1000(paul) gid=1000(paul) groups=1000(paul)

On RHEL/CentOS you will also get SELinux context information with this command.
[root@centos7 ~]# id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r\
:unconfined_t:s0-s0:c0.c1023

5
introduction to users

1.6. su to another user
The su command allows a user to run a shell as another user.
laura@debian7:~$ su tania
Password:
tania@debian7:/home/laura$

1.7. su to root
Yes you can also su to become root, when you know the root password.
laura@debian7:~$ su root
Password:
root@debian7:/home/laura#

1.8. su as root
You need to know the password of the user you want to substitute to, unless your are logged
in as root. The root user can become any existing user without knowing that user's password.
root@debian7:~# id
uid=0(root) gid=0(root) groups=0(root)
root@debian7:~# su - valentina
valentina@debian7:~$

1.9. su - $username
By default, the su command maintains the same shell environment. To become another user
and also get the target user's environment, issue the su - command followed by the target
username.
root@debian7:~# su laura
laura@debian7:/root$ exit
exit
root@debian7:~# su - laura
laura@debian7:~$ pwd
/home/laura

1.10. su -
When no username is provided to su or su -, the command will assume root is the target.
tania@debian7:~$ su -
Password:
root@debian7:~#

6
introduction to users

1.11. run a program as another user
The sudo program allows a user to start a program with the credentials of another user.
Before this works, the system administrator has to set up the /etc/sudoers file. This can be
useful to delegate administrative tasks to another user (without giving the root password).

The screenshot below shows the usage of sudo. User paul received the right to run useradd
with the credentials of root. This allows paul to create new users on the system without
becoming root and without knowing the root password.

First the command fails for paul.
paul@debian7:~$ /usr/sbin/useradd -m valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.

But with sudo it works.
paul@debian7:~$ sudo /usr/sbin/useradd -m valentina
[sudo] password for paul:
paul@debian7:~$

1.12. visudo
Check the man page of visudo before playing with the /etc/sudoers file. Editing the sudoers
is out of scope for this fundamentals book.
paul@rhel65:~$ apropos visudo
visudo (8) - edit the sudoers file
paul@rhel65:~$

7
introduction to users

1.13. sudo su -
On some Linux systems like Ubuntu and Xubuntu, the root user does not have a password
set. This means that it is not possible to login as root (extra security). To perform tasks as
root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are
members of the admin group can use sudo to run all commands as root.
root@laika:~# grep admin /etc/sudoers
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

The end result of this is that the user can type sudo su - and become root without having to
enter the root password. The sudo command does require you to enter your own password.
Thus the password prompt in the screenshot below is for sudo, not for su.
paul@laika:~$ sudo su -
Password:
root@laika:~#

1.14. sudo logging
Using sudo without authorization will result in a severe warning:
paul@rhel65:~$ sudo su -

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.

[sudo] password for paul:
paul is not in the sudoers file. This incident will be reported.
paul@rhel65:~$

The root user can see this in the /var/log/secure on Red Hat and in /var/log/auth.log on
Debian).
root@rhel65:~# tail /var/log/secure | grep sudo | tr -s ' '
Apr 13 16:03:42 rhel65 sudo: paul : user NOT in sudoers ; TTY=pts/0 ; PWD=\
/home/paul ; USER=root ; COMMAND=/bin/su -
root@rhel65:~#

8
introduction to users

1.15. practice: introduction to users
1. Run a command that displays only your currently logged on user name.

2. Display a list of all logged on users.

3. Display a list of all logged on users including the command they are running at this very
moment.

4. Display your user name and your unique user identification (userid).

5. Use su to switch to another user account (unless you are root, you will need the password
of the other account). And get back to the previous account.

6. Now use su - to switch to another user and notice the difference.

Note that su - gets you into the home directory of Tania.

7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)

8. Now try the same, but with sudo before your command.

9
introduction to users

1.16. solution: introduction to users
1. Run a command that displays only your currently logged on user name.
laura@debian7:~$ whoami
laura
laura@debian7:~$ echo $USER
laura

2. Display a list of all logged on users.
laura@debian7:~$ who
laura pts/0 2014-10-13 07:22 (10.104.33.101)
laura@debian7:~$

3. Display a list of all logged on users including the command they are running at this very
moment.
laura@debian7:~$ w
07:47:02 up 16 min, 2 users, load average: 0.00, 0.00, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
root pts/0 10.104.33.101 07:30 6.00s 0.04s 0.00s w
root pts/1 10.104.33.101 07:46 6.00s 0.01s 0.00s sleep 42
laura@debian7:~$

4. Display your user name and your unique user identification (userid).
laura@debian7:~$ id
uid=1005(laura) gid=1007(laura) groups=1007(laura)
laura@debian7:~$

5. Use su to switch to another user account (unless you are root, you will need the password
of the other account). And get back to the previous account.
laura@debian7:~$ su tania
Password:
tania@debian7:/home/laura$ id
uid=1006(tania) gid=1008(tania) groups=1008(tania)
tania@debian7:/home/laura$ exit
laura@debian7:~$

6. Now use su - to switch to another user and notice the difference.
laura@debian7:~$ su - tania
Password:
tania@debian7:~$ pwd
/home/tania
tania@debian7:~$ logout
laura@debian7:~$

Note that su - gets you into the home directory of Tania.

10
introduction to users

7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)
laura@debian7:~$ useradd valentina
-su: useradd: command not found
laura@debian7:~$ /usr/sbin/useradd valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.

It is possible that useradd is located in /sbin/useradd on your computer.

8. Now try the same, but with sudo before your command.
laura@debian7:~$ sudo /usr/sbin/useradd valentina
[sudo] password for laura:
laura is not in the sudoers file. This incident will be reported.
laura@debian7:~$

Notice that laura has no permission to use the sudo on this system.

11
Chapter 2. user management
This chapter will teach you how to use useradd, usermod and userdel to create, modify
and remove user accounts.

You will need root access on a Linux computer to complete this chapter.

12
user management

2.1. user management
User management on Linux can be done in three complementary ways. You can use the
graphical tools provided by your distribution. These tools have a look and feel that depends
on the distribution. If you are a novice Linux user on your home system, then use the
graphical tool that is provided by your distribution. This will make sure that you do not run
into problems.

Another option is to use command line tools like useradd, usermod, gpasswd, passwd and
others. Server administrators are likely to use these tools, since they are familiar and very
similar across many different distributions. This chapter will focus on these command line
tools.

A third and rather extremist way is to edit the local configuration files directly using vi (or
vipw/vigr). Do not attempt this as a novice on production systems!

2.2. /etc/passwd
The local user database on Linux (and on most Unixes) is /etc/passwd.
[root@RHEL5 ~]# tail /etc/passwd
inge:x:518:524:art dealer:/home/inge:/bin/ksh
ann:x:519:525:flute player:/home/ann:/bin/bash
frederik:x:520:526:rubius poet:/home/frederik:/bin/bash
steven:x:521:527:roman emperor:/home/steven:/bin/bash
pascale:x:522:528:artist:/home/pascale:/bin/ksh
geert:x:524:530:kernel developer:/home/geert:/bin/bash
wim:x:525:531:master damuti:/home/wim:/bin/bash
sandra:x:526:532:radish stresser:/home/sandra:/bin/bash
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh

As you can see, this file contains seven columns separated by a colon. The columns contain
the username, an x, the user id, the primary group id, a description, the name of the home
directory, and the login shell.

More information can be found by typing man 5 passwd.
[root@RHEL5 ~]# man 5 passwd

2.3. root
The root user also called the superuser is the most powerful account on your Linux system.
This user can do almost anything, including the creation of other users. The root user always
has userid 0 (regardless of the name of the account).
[root@RHEL5 ~]# head -1 /etc/passwd
root:x:0:0:root:/root:/bin/bash

13
user management

2.4. useradd
You can add users with the useradd command. The example below shows how to add a
user named yanina (last parameter) and at the same time forcing the creation of the home
directory (-m), setting the name of the home directory (-d), and setting a description (-c).
[root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina
[root@RHEL5 ~]# tail -1 /etc/passwd
yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash

The user named yanina received userid 529 and primary group id 529.

2.5. /etc/default/useradd
Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/useradd
that contains some default user options. Besides using cat to display this file, you can also
use useradd -D.
[root@RHEL4 ~]# useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel

2.6. userdel
You can delete the user yanina with userdel. The -r option of userdel will also remove the
home directory.
[root@RHEL5 ~]# userdel -r yanina

2.7. usermod
You can modify the properties of a user with the usermod command. This example uses
usermod to change the description of the user harry.
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:harry potter:/home/harry:/bin/bash
[root@RHEL4 ~]# usermod -c 'wizard' harry
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:wizard:/home/harry:/bin/bash

14
user management

2.8. creating home directories
The easiest way to create a home directory is to supply the -m option with useradd (it is
likely set as a default option on Linux).

A less easy way is to create a home directory manually with mkdir which also requires
setting the owner and the permissions on the directory with chmod and chown (both
commands are discussed in detail in another chapter).
[root@RHEL5 ~]# mkdir /home/laura
[root@RHEL5 ~]# chown laura:laura /home/laura
[root@RHEL5 ~]# chmod 700 /home/laura
[root@RHEL5 ~]# ls -ld /home/laura/
drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/

2.9. /etc/skel/
When using useradd the -m option, the /etc/skel/ directory is copied to the newly created
home directory. The /etc/skel/ directory contains some (usually hidden) files that contain
profile settings and default values for applications. In this way /etc/skel/ serves as a default
home directory and as a default user profile.
[root@RHEL5 ~]# ls -la /etc/skel/
total 48
drwxr-xr-x 2 root root 4096 Apr 1 00:11 .
drwxr-xr-x 97 root root 12288 Jun 24 15:36 ..
-rw-r--r-- 1 root root 24 Jul 12 2006 .bash_logout
-rw-r--r-- 1 root root 176 Jul 12 2006 .bash_profile
-rw-r--r-- 1 root root 124 Jul 12 2006 .bashrc

2.10. deleting home directories
The -r option of userdel will make sure that the home directory is deleted together with the
user account.
[root@RHEL5 ~]# ls -ld /home/wim/
drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/
[root@RHEL5 ~]# userdel -r wim
[root@RHEL5 ~]# ls -ld /home/wim/
ls: /home/wim/: No such file or directory

15
user management

2.11. login shell
The /etc/passwd file specifies the login shell for the user. In the screenshot below you can
see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh
shell.
[root@RHEL5 ~]# tail -2 /etc/passwd
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh

You can use the usermod command to change the shell for a user.
[root@RHEL5 ~]# usermod -s /bin/bash laura
[root@RHEL5 ~]# tail -1 /etc/passwd
laura:x:528:534:art dealer:/home/laura:/bin/bash

2.12. chsh
Users can change their login shell with the chsh command. First, user harry obtains a list of
available shells (he could also have done a cat /etc/shells) and then changes his login shell
to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash.
[laura@centos7 ~]$ chsh -l
/bin/sh
/bin/bash
/sbin/nologin
/usr/bin/sh
/usr/bin/bash
/usr/sbin/nologin
/bin/ksh
/bin/tcsh
/bin/csh
[laura@centos7 ~]$

Note that the -l option does not exist on Debian and that the above screenshot assumes that
ksh and csh shells are installed.

The screenshot below shows how laura can change her default shell (active on next login).
[laura@centos7 ~]$ chsh -s /bin/ksh
Changing shell for laura.
Password:
Shell changed.

16
user management

2.13. practice: user management
1. Create a user account named serena, including a home directory and a description (or
comment) that reads Serena Williams. Do all this in one single command.

2. Create a user named venus, including home directory, bash shell, a description that reads
Venus Williams all in one single command.

3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group.

4. Verify that their home directory was created.

5. Create a user named einstime with /bin/date as his default logon shell.

7. What happens when you log on with the einstime user ? Can you think of a useful real
world example for changing a user's login shell to an application ?

8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.

9. Verify this setup by creating (and deleting) a test user account.

10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.

17
user management

2.14. solution: user management
1. Create a user account named serena, including a home directory and a description (or
comment) that reads Serena Williams. Do all this in one single command.
root@debian7:~# useradd -m -c 'Serena Williams' serena

2. Create a user named venus, including home directory, bash shell, a description that reads
Venus Williams all in one single command.
root@debian7:~# useradd -m -c "Venus Williams" -s /bin/bash venus

3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group.
root@debian7:~# tail -2 /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
venus:x:1009:1011:Venus Williams:/home/venus:/bin/bash
root@debian7:~# tail -2 /etc/shadow
serena:!:16358:0:99999:7:::
venus:!:16358:0:99999:7:::
root@debian7:~# tail -2 /etc/group
serena:x:1010:
venus:x:1011:

4. Verify that their home directory was created.
root@debian7:~# ls -lrt /home | tail -2
drwxr-xr-x 2 serena serena 4096 Oct 15 10:50 serena
drwxr-xr-x 2 venus venus 4096 Oct 15 10:59 venus
root@debian7:~#

5. Create a user named einstime with /bin/date as his default logon shell.
root@debian7:~# useradd -s /bin/date einstime

Or even better:
root@debian7:~# useradd -s $(which date) einstime

7. What happens when you log on with the einstime user ? Can you think of a useful real
world example for changing a user's login shell to an application ?
root@debian7:~# su - einstime
Wed Oct 15 11:05:56 UTC 2014 # You get the output of the date command
root@debian7:~#

It can be useful when users need to access only one application on the server. Just logging
in opens the application for them, and closing the application automatically logs them out.

18
user management

8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.
root@debian7:~# echo Hello > /etc/skel/welcome.txt

9. Verify this setup by creating (and deleting) a test user account.
root@debian7:~# useradd -m test
root@debian7:~# ls -l /home/test
total 4
-rw-r--r-- 1 test test 6 Oct 15 11:16 welcome.txt
root@debian7:~# userdel -r test
root@debian7:~#

10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
root@debian7:~# usermod -s /bin/bash serena
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/bash
root@debian7:~#

19
Chapter 3. user passwords
This chapter will tell you more about passwords for local users.

Three methods for setting passwords are explained; using the passwd command, using
openssel passwd, and using the crypt function in a C program.

The chapter will also discuss password settings and disabling, suspending or locking
accounts.

20
user passwords

3.1. passwd
Passwords of users can be set with the passwd command. Users will have to provide their
old password before twice entering the new one.
[tania@centos7 ~]$ passwd
Changing password for user tania.
Changing password for tania.
(current) UNIX password:
New password:
BAD PASSWORD: The password is shorter than 8 characters
New password:
BAD PASSWORD: The password is a palindrome
New password:
BAD PASSWORD: The password is too similar to the old one
passwd: Have exhausted maximum number of retries for service

As you can see, the passwd tool will do some basic verification to prevent users from using
too simple passwords. The root user does not have to follow these rules (there will be
a warning though). The root user also does not have to provide the old password before
entering the new password twice.
root@debian7:~# passwd tania
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

3.2. shadow file
User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only
and can only be read by root. We will see in the file permissions section how it is possible
for users to change their password. For now, you will have to know that users can change
their password with the /usr/bin/passwd command.
[root@centos7 ~]# tail -4 /etc/shadow
paul:$6$ikp2Xta5BT.Tml.p$2TZjNnOYNNQKpwLJqoGJbVsZG5/Fti8ovBRd.VzRbiDSl7TEq\
IaSMH.TeBKnTS/SjlMruW8qffC0JNORW.BTW1:16338:0:99999:7:::
tania:$6$8Z/zovxj$9qvoqT8i9KIrmN.k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu.z4DlpvMREH\
eQpQzvRnqFdKkVj17H5ST.c79HDZw0:16356:0:99999:7:::
laura:$6$glDuTY5e$/NYYWLxfHgZFWeoujaXSMcR.Mz.lGOxtcxFocFVJNb98nbTPhWFXfKWG\
SyYh1WCv6763Wq54.w24Yr3uAZBOm/:16356:0:99999:7:::
valentina:$6$jrZa6PVI$1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh.v4iqUk9MVreui1lv7\
GxHOUDSKA0N55ZRNhGHa6T2ouFnVno/0o1:16356:0:99999:7:::
[root@centos7 ~]#

The /etc/shadow file contains nine colon separated columns. The nine fields contain (from
left to right) the user name, the encrypted password (note that only inge and laura have an
encrypted password), the day the password was last changed (day 1 is January 1, 1970),
number of days the password must be left unchanged, password expiry day, warning number
of days before password expiry, number of days after expiry before disabling the account,
and the day the account was disabled (again, since 1970). The last field has no meaning yet.

All the passwords in the screenshot above are hashes of hunter2.

21
user passwords

3.3. encryption with passwd
Passwords are stored in an encrypted format. This encryption is done by the crypt function.
The easiest (and recommended) way to add a user with a password to the system is to add
the user with the useradd -m user command, and then set the user's password with passwd.
[root@RHEL4 ~]# useradd -m xavier
[root@RHEL4 ~]# passwd xavier
Changing password for user xavier.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[root@RHEL4 ~]#

3.4. encryption with openssl
Another way to create users with a password is to use the -p option of useradd, but that
option requires an encrypted password. You can generate this encrypted password with the
openssl passwd command.

The openssl passwd command will generate several distinct hashes for the same password,
for this it uses a salt.
paul@rhel65:~$ openssl passwd hunter2
86jcUNlnGDFpY
paul@rhel65:~$ openssl passwd hunter2
Yj7mDO9OAnvq6
paul@rhel65:~$ openssl passwd hunter2
YqDcJeGoDbzKA
paul@rhel65:~$

This salt can be chosen and is visible as the first two characters of the hash.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$

This example shows how to create a user with password.
root@rhel65:~# useradd -m -p $(openssl passwd hunter2) mohamed

Note that this command puts the password in your command history!

22
user passwords

3.5. encryption with crypt
A third option is to create your own C program using the crypt function, and compile this
into a command.
paul@rhel65:~$ cat MyCrypt.c
#include <stdio.h>
#define __USE_XOPEN
#include <unistd.h>

int main(int argc, char** argv)
{
if(argc==3)
{
printf("%s\n", crypt(argv[1],argv[2]));
}
else
{
printf("Usage: MyCrypt $password $salt\n" );
}
return 0;
}

This little program can be compiled with gcc like this.
paul@rhel65:~$ gcc MyCrypt.c -o MyCrypt -lcrypt

To use it, we need to give two parameters to MyCrypt. The first is the unencrypted password,
the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096
different ways. This variation prevents two users with the same password from having the
same entry in /etc/shadow.
paul@rhel65:~$ ./MyCrypt hunter2 42
42ZrbtP1Ze8G.
paul@rhel65:~$ ./MyCrypt hunter2 33
33d6taYSiEUXI

Did you notice that the first two characters of the password are the salt?

The standard output of the crypt function is using the DES algorithm which is old and can
be cracked in minutes. A better method is to use md5 passwords which can be recognized
by a salt starting with $1$.
paul@rhel65:~$ ./MyCrypt hunter2 '$1$42'
$1$42$7l6Y3xT5282XmZrtDOF9f0
paul@rhel65:~$ ./MyCrypt hunter2 '$6$42'
$6$42$OqFFAVnI3gTSYG0yI9TZWX9cpyQzwIop7HwpG1LLEsNBiMr4w6OvLX1KDa./UpwXfrFk1i...

The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between
the second and third $, so never use the password as the salt!
paul@rhel65:~$ ./MyCrypt hunter2 '$1$hunter2'
$1$hunter2$YVxrxDmidq7Xf8Gdt6qM2.

23
user passwords

3.6. /etc/login.defs
The /etc/login.defs file contains some default settings for user passwords like password
aging and length settings. (You will also find the numerical limits of user ids and group ids
and whether or not a home directory should be created by default).
root@rhel65:~# grep ^PASS /etc/login.defs
PASS_MAX_DAYS 99999
PASS_MIN_DAYS 0
PASS_MIN_LEN 5
PASS_WARN_AGE 7

Debian also has this file.
root@debian7:~# grep PASS /etc/login.defs
# PASS_MAX_DAYS Maximum number of days a password may be used.
# PASS_MIN_DAYS Minimum number of days allowed between password changes.
# PASS_WARN_AGE Number of days warning given before a password expires.
PASS_MAX_DAYS 99999
PASS_MIN_DAYS 0
PASS_WARN_AGE 7
#PASS_CHANGE_TRIES
#PASS_ALWAYS_WARN
#PASS_MIN_LEN
#PASS_MAX_LEN
# NO_PASSWORD_CONSOLE
root@debian7:~#

3.7. chage
The chage command can be used to set an expiration date for a user account (-E), set a
minimum (-m) and maximum (-M) password age, a password expiration date, and set the
number of warning days before the password expiration date. Much of this functionality is
also available from the passwd command. The -l option of chage will list these settings for
a user.
root@rhel65:~# chage -l paul
Last password change : Mar 27, 2014
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7
root@rhel65:~#

24
user passwords

3.8. disabling a password
Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field
in /etc/passwd starts with an exclamation mark, then the password can not be used.

Using this feature is often called locking, disabling, or suspending a user account. Besides
vi (or vipw) you can also accomplish this with usermod.

The first command in the next screenshot will show the hashed password of laura in /etc/
shadow. The next command disables the password of laura, making it impossible for Laura
to authenticate using this password.
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV
root@debian7:~# usermod -L laura

As you can see below, the password hash is simply preceded with an exclamation mark.
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:!$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJ
root@debian7:~#

The root user (and users with sudo rights on su) still will be able to su into the laura account
(because the password is not needed here). Also note that laura will still be able to login
if she has set up passwordless ssh!
root@debian7:~# su - laura
laura@debian7:~$

You can unlock the account again with usermod -U.
root@debian7:~# usermod -U laura
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV

Watch out for tiny differences in the command line options of passwd, usermod, and
useradd on different Linux distributions. Verify the local files when using features like
"disabling, suspending, or locking" on user accounts and their passwords.

3.9. editing local files
If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these
commands for password management, then use vipw instead of vi(m) directly. The vipw
tool will do proper locking of the file.
[root@RHEL5 ~]# vipw /etc/passwd
vipw: the password file is busy (/etc/ptmp present)

25
user passwords

3.10. practice: user passwords
1. Set the password for serena to hunter2.

2. Also set a password for venus and then lock the venus user account with usermod. Verify
the locking in /etc/shadow before and after you lock it.

3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
before and after disabling.

4. What is the difference between locking a user account and disabling a user account's
password like we just did with usermod -L and passwd -d?

5. Try changing the password of serena to serena as serena.

6. Make sure serena has to change her password in 10 days.

7. Make sure every new user needs to change their password every 10 days.

8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
venus to serena. Can serena now log on with hunter2 as a password ?

9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?

10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
etc/shells.

11. Which useradd option allows you to name a home directory ?

12. How can you see whether the password of user serena is locked or unlocked ? Give a
solution with grep and a solution with passwd.

26
user passwords

3.11. solution: user passwords
1. Set the password for serena to hunter2.
root@debian7:~# passwd serena
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

2. Also set a password for venus and then lock the venus user account with usermod. Verify
the locking in /etc/shadow before and after you lock it.
root@debian7:~# passwd venus
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@debian7:~# grep venus /etc/shadow | cut -c1-70
venus:$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM/
root@debian7:~# usermod -L venus
root@debian7:~# grep venus /etc/shadow | cut -c1-70
venus:!$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM

Note that usermod -L precedes the password hash with an exclamation mark (!).

3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
before and after disabling.
root@debian7:~# grep serena /etc/shadow | cut -c1-70
serena:$6$Es/omrPE$F2Ypu8kpLrfKdW0v/UIwA5jrYyBD2nwZ/dt.i/IypRgiPZSdB/B
root@debian7:~# passwd -d serena
passwd: password expiry information changed.
root@debian7:~# grep serena /etc/shadow
serena::16358:0:99999:7:::
root@debian7:~#

4. What is the difference between locking a user account and disabling a user account's
password like we just did with usermod -L and passwd -d?

Locking will prevent the user from logging on to the system with his password by putting
a ! in front of the password in /etc/shadow.

Disabling with passwd will erase the password from /etc/shadow.

5. Try changing the password of serena to serena as serena.
log on as serena, then execute: passwd serena... it should fail!

6. Make sure serena has to change her password in 10 days.
chage -M 10 serena

7. Make sure every new user needs to change their password every 10 days.
vi /etc/login.defs (and change PASS_MAX_DAYS to 10)

27
user passwords

8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
venus to serena. Can serena now log on with hunter2 as a password ?
Yes.

9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?
vipw will give a warning when someone else is already using that file (with vipw).

10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
etc/shells.
chsh -l
cat /etc/shells

11. Which useradd option allows you to name a home directory ?
-d

12. How can you see whether the password of user serena is locked or unlocked ? Give a
solution with grep and a solution with passwd.
grep serena /etc/shadow

passwd -S serena

28
Chapter 4. user profiles
Logged on users have a number of preset (and customized) aliases, variables, and functions,
but where do they come from ? The shell uses a number of startup files that are executed
(or rather sourced) whenever the shell is invoked. What follows is an overview of startup
scripts.

29
user profiles

4.1. system profile
Both the bash and the ksh shell will verify the existence of /etc/profile and source it if it
exists.

When reading this script, you will notice (both on Debian and on Red Hat Enterprise Linux)
that it builds the PATH environment variable (among others). The script might also change
the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc

This screenshot uses grep to show PATH manipulation in /etc/profile on Debian.
root@debian7:~# grep PATH /etc/profile
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games"
export PATH
root@debian7:~#

This screenshot uses grep to show PATH manipulation in /etc/profile on RHEL7/CentOS7.
[root@centos7 ~]# grep PATH /etc/profile
case ":${PATH}:" in
PATH=$PATH:$1
PATH=$1:$PATH
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL
[root@centos7 ~]#

The root user can use this script to set aliases, functions, and variables for every user on
the system.

4.2. ~/.bash_profile
When this file exists in the home directory, then bash will source it. On Debian Linux 5/6/7
this file does not exist by default.

RHEL7/CentOS7 uses a small ~/.bash_profile where it checks for the existence of
~/.bashrc and then sources it. It also adds $HOME/bin to the $PATH variable.
[root@rhel7 ~]# cat /home/paul/.bash_profile
# .bash_profile

# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi

# User specific environment and startup programs

PATH=$PATH:$HOME/.local/bin:$HOME/bin

export PATH
[root@rhel7 ~]#

30
user profiles

4.3. ~/.bash_login
When .bash_profile does not exist, then bash will check for ~/.bash_login and source it.

Neither Debian nor Red Hat have this file by default.

4.4. ~/.profile
When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence
of ~/.profile and execute it. This file does not exist by default on Red Hat.

On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH
variable.
root@debian7:~# tail -11 /home/paul/.profile
if [ -n "$BASH_VERSION" ]; then
# include .bashrc if it exists
if [ -f "$HOME/.bashrc" ]; then
. "$HOME/.bashrc"
fi
fi

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
PATH="$HOME/bin:$PATH"
fi

RHEL/CentOS does not have this file by default.

4.5. ~/.bashrc
The ~/.bashrc script is often sourced by other scripts. Let us take a look at what it does
by default.

Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and sourcing it. It also leaves
room for custom aliases and functions.
[root@rhel7 ~]# cat /home/paul/.bashrc
# .bashrc

# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi

# Uncomment the following line if you don't like systemctl's auto-paging feature:
# export SYSTEMD_PAGER=

# User specific aliases and functions

On Debian this script is quite a bit longer and configures $PS1, some history variables and
a number af active and inactive aliases.
root@debian7:~# wc -l /home/paul/.bashrc
110 /home/paul/.bashrc

31
user profiles

4.6. ~/.bash_logout
When exiting bash, it can execute ~/.bash_logout.

Debian use this opportunity to clear the console screen.
serena@deb503:~$ cat .bash_logout
# ~/.bash_logout: executed by bash(1) when login shell exits.

# when leaving the console clear the screen to increase privacy

if [ "$SHLVL" = 1 ]; then
[ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q
fi

Red Hat Enterprise Linux 5 will simple call the /usr/bin/clear command in this script.
[serena@rhel53 ~]$ cat .bash_logout
# ~/.bash_logout

/usr/bin/clear

Red Hat Enterprise Linux 6 and 7 create this file, but leave it empty (except for a comment).
paul@rhel65:~$ cat .bash_logout
# ~/.bash_logout

32
user profiles

4.7. Debian overview
Below is a table overview of when Debian is running any of these bash startup scripts.

Table 4.1. Debian User Environment
script su su - ssh gdm
~./bashrc no yes yes yes
~/.profile no yes yes yes
/etc/profile no yes yes yes
/etc/bash.bashrc yes no no yes

4.8. RHEL5 overview
Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash
startup scripts.

Table 4.2. Red Hat User Environment
script su su - ssh gdm
~./bashrc yes yes yes yes
~/.bash_profile no yes yes yes
/etc/profile no yes yes yes
/etc/bashrc yes yes yes yes

33
user profiles

4.9. practice: user profiles
1. Make a list of all the profile files on your system.

2. Read the contents of each of these, often they source extra scripts.

3. Put a unique variable, alias and function in each of those files.

4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrl-
alt-F1, ...) and verify which of your custom variables, aliases and function are present in
your environment.

5. Do you also know the order in which they are executed?

6. When an application depends on a setting in $HOME/.profile, does it matter whether
$HOME/.bash_profile exists or not ?

34
user profiles

4.10. solution: user profiles
1. Make a list of all the profile files on your system.
ls -a ~ ; ls -l /etc/pro* /etc/bash*

2. Read the contents of each of these, often they source extra scripts.

3. Put a unique variable, alias and function in each of those files.

5. Do you also know the order in which they are executed?
same name aliases, functions and variables will overwrite each other

6. When an application depends on a setting in $HOME/.profile, does it matter whether
$HOME/.bash_profile exists or not ?
Yes it does matter. (man bash /INVOCATION)

35
Chapter 5. groups
Users can be listed in groups. Groups allow you to set permissions on the group level instead
of having to set permissions for every individual user.

Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users
are advised to use this graphical tool. More experienced users can use command line tools to
manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI
tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant
files directly with vi or vigr.

36
groups

5.1. groupadd
Groups can be created with the groupadd command. The example below shows the creation
of five (empty) groups.
root@laika:~# groupadd tennis
root@laika:~# groupadd football
root@laika:~# groupadd snooker
root@laika:~# groupadd formula1
root@laika:~# groupadd salsa

5.2. group file
Users can be a member of several groups. Group membership is defined by the /etc/group
file.
root@laika:~# tail -5 /etc/group
tennis:x:1006:
football:x:1007:
snooker:x:1008:
formula1:x:1009:
salsa:x:1010:
root@laika:~#

The first field is the group's name. The second field is the group's (encrypted) password (can
be empty). The third field is the group identification or GID. The fourth field is the list of
members, these groups have no members.

5.3. groups
A user can type the groups command to see a list of groups where the user belongs to.
[harry@RHEL4b ~]$ groups
harry sports
[harry@RHEL4b ~]$

37
groups

5.4. usermod
Group membership can be modified with the useradd or usermod command.
root@laika:~# usermod -a -G tennis inge
root@laika:~# usermod -a -G tennis katrien
root@laika:~# usermod -a -G salsa katrien
root@laika:~# usermod -a -G snooker sandra
root@laika:~# usermod -a -G formula1 annelies
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
snooker:x:1008:sandra
formula1:x:1009:annelies
salsa:x:1010:katrien
root@laika:~#

Be careful when using usermod to add users to groups. By default, the usermod command
will remove the user from every group of which he is a member if the group is not listed in
the command! Using the -a (append) switch prevents this behaviour.

5.5. groupmod
You can change the group name with the groupmod command.
root@laika:~# groupmod -n darts snooker
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
formula1:x:1009:annelies
salsa:x:1010:katrien
darts:x:1008:sandra

5.6. groupdel
You can permanently remove a group with the groupdel command.
root@laika:~# groupdel tennis
root@laika:~#

38
groups

5.7. gpasswd
You can delegate control of group membership to another user with the gpasswd command.
In the example below we delegate permissions to add and remove group members to serena
for the sports group. Then we su to serena and add harry to the sports group.
[root@RHEL4b ~]# gpasswd -A serena sports
[root@RHEL4b ~]# su - serena
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry)
[serena@RHEL4b ~]$ gpasswd -a harry sports
Adding user harry to group sports
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry),522(sports)
[serena@RHEL4b ~]$ tail -1 /etc/group
sports:x:522:serena,venus,harry
[serena@RHEL4b ~]$

Group administrators do not have to be a member of the group. They can remove themselves
from a group, but this does not influence their ability to add or remove members.
[serena@RHEL4b ~]$ gpasswd -d serena sports
Removing user serena from group sports
[serena@RHEL4b ~]$ exit

Information about group administrators is kept in the /etc/gshadow file.
[root@RHEL4b ~]# tail -1 /etc/gshadow
sports:!:serena:venus,harry
[root@RHEL4b ~]#

To remove all group administrators from a group, use the gpasswd command to set an empty
administrators list.
[root@RHEL4b ~]# gpasswd -A "" sports

39
groups

5.8. newgrp
You can start a child shell with a new temporary primary group using the newgrp
command.
root@rhel65:~# mkdir prigroup
root@rhel65:~# cd prigroup/
root@rhel65:~/prigroup# touch standard.txt
root@rhel65:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt
root@rhel65:~/prigroup# echo $SHLVL
1
root@rhel65:~/prigroup# newgrp tennis
root@rhel65:~/prigroup# echo $SHLVL
2
root@rhel65:~/prigroup# touch newgrp.txt
root@rhel65:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root tennis 0 Apr 13 17:49 newgrp.txt
-rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt
root@rhel65:~/prigroup# exit
exit
root@rhel65:~/prigroup#

5.9. vigr
Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since
it will do proper locking of the file. Only experienced senior administrators should use vi
or vigr to manage groups.

40
groups

5.10. practice: groups
1. Create the groups tennis, football and sports.

2. In one command, make venus a member of tennis and sports.

3. Rename the football group to foot.

4. Use vi to add serena to the tennis group.

5. Use the id command to verify that serena is a member of tennis.

6. Make someone responsible for managing group membership of foot and sports. Test that
it works.

41
groups

5.11. solution: groups
1. Create the groups tennis, football and sports.
groupadd tennis ; groupadd football ; groupadd sports

2. In one command, make venus a member of tennis and sports.
usermod -a -G tennis,sports venus

3. Rename the football group to foot.
groupmod -n foot football

4. Use vi to add serena to the tennis group.
vi /etc/group

5. Use the id command to verify that serena is a member of tennis.
id (and after logoff logon serena should be member)

6. Make someone responsible for managing group membership of foot and sports. Test that
it works.
gpasswd -A (to make manager)

gpasswd -a (to add member)

42
Part II. file security
Table of Contents
6. standard file permissions ........................................................................................................................ 45
6.1. file ownership ................................................................................................................................ 46
6.2. list of special files ......................................................................................................................... 48
6.3. permissions .................................................................................................................................... 49
6.4. practice: standard file permissions ............................................................................................... 54
6.5. solution: standard file permissions ............................................................................................... 55
7. advanced file permissions ....................................................................................................................... 57
7.1. sticky bit on directory ................................................................................................................... 58
7.2. setgid bit on directory ................................................................................................................... 58
7.3. setgid and setuid on regular files ................................................................................................. 59
7.4. setuid on sudo ............................................................................................................................... 59
7.5. practice: sticky, setuid and setgid bits .......................................................................................... 60
7.6. solution: sticky, setuid and setgid bits ......................................................................................... 61
8. access control lists ................................................................................................................................... 63
8.1. acl in /etc/fstab .............................................................................................................................. 64
8.2. getfacl ............................................................................................................................................ 64
8.3. setfacl ............................................................................................................................................. 64
8.4. remove an acl entry ...................................................................................................................... 65
8.5. remove the complete acl ............................................................................................................... 65
8.6. the acl mask .................................................................................................................................. 65
8.7. eiciel .............................................................................................................................................. 66
9. file links ..................................................................................................................................................... 67
9.1. inodes ............................................................................................................................................. 68
9.2. about directories ............................................................................................................................ 69
9.3. hard links ....................................................................................................................................... 70
9.4. symbolic links ............................................................................................................................... 71
9.5. removing links .............................................................................................................................. 71
9.6. practice : links ............................................................................................................................... 72
9.7. solution : links ............................................................................................................................... 73

44
Chapter 6. standard file permissions
This chapter contains details about basic file security through file ownership and file
permissions.

45
standard file permissions

6.1. file ownership
6.1.1. user owner and group owner
The users and groups of a system can be locally managed in /etc/passwd and /etc/group,
or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files.
Actually, every file has a user owner and a group owner, as can be seen in the following
screenshot.
paul@rhel65:~/owners$ ls -lh
total 636K
-rw-r--r--. 1 paul snooker 1.1K Apr 8 18:47 data.odt
-rw-r--r--. 1 paul paul 626K Apr 8 18:46 file1
-rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2
-rw-rw-r--. 1 root root 0 Apr 8 18:47 stuff.txt
paul@rhel65:~/owners$

User paul owns three files; file1 has paul as user owner and has the group paul as group
owner, data.odt is group owned by the group snooker, file2 by the group tennis.

The last file is called stuff.txt and is owned by the root user and the root group.

6.1.2. listing user accounts
You can use the following command to list all local user accounts.
paul@debian7~$ cut -d: -f1 /etc/passwd | column
root ntp sam bert naomi
daemon mysql tom rino matthias2
bin paul wouter antonio bram
sys maarten robrecht simon fabrice
sync kevin bilal sven chimene
games yuri dimitri wouter2 messagebus
man william ahmed tarik roger
lp yves dylan jan frank
mail kris robin ian toon
news hamid matthias ivan rinus
uucp vladimir ben azeddine eddy
proxy abiy mike eric bram2
www-data david kevin2 kamel keith
backup chahid kenzo ischa jesse
list stef aaron bart frederick
irc joeri lorenzo omer hans
gnats glenn jens kurt dries
nobody yannick ruben steve steve2
libuuid christof jelle constantin tomas
Debian-exim george stefaan sam2 johan
statd joost marc bjorn tom2
sshd arno thomas ronald

46
standard file permissions

6.1.3. chgrp
You can change the group owner of a file using the chgrp command.
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners# chgrp snooker file2
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root snooker 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners#

6.1.4. chown
The user owner of a file can be changed with chown command.
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown paul FileForPaul
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul

You can also use chown to change both the user owner and the group owner.
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown root:project42 FileForPaul
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul

47
standard file permissions

6.2. list of special files
When you use ls -l, for each file you can see ten characters before the user and group owner.
The first character tells us the type of file. Regular files get a -, directories get a d, symbolic
links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets
an s.

Table 6.1. Unix special files
first character file type
- normal file
d directory
l symbolic link
p named pipe
b block device
c character device
s socket

Below a screenshot of a character device (the console) and a block device (the hard disk).
paul@debian6lt~$ ls -ld /dev/console /dev/sda
crw------- 1 root root 5, 1 Mar 15 12:45 /dev/console
brw-rw---- 1 root disk 8, 0 Mar 15 12:45 /dev/sda

And here you can see a directory, a regular file and a symbolic link.
paul@debian6lt~$ ls -ld /etc /etc/hosts /etc/motd
drwxr-xr-x 128 root root 12288 Mar 15 18:34 /etc
-rw-r--r-- 1 root root 372 Dec 10 17:36 /etc/hosts
lrwxrwxrwx 1 root root 13 Dec 5 10:36 /etc/motd -> /var/run/motd

48
standard file permissions

6.3. permissions
6.3.1. rwx
The nine characters following the file type denote the permissions in three triplets. A
permission can be r for read access, w for write access, and x for execute. You need the r
permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a
directory. You need the w permission to create files in or remove files from a directory.

Table 6.2. standard Unix file permissions
permission on a file on a directory
r (read) read file contents (cat) read directory contents (ls)
w (write) change file contents (vi) create files in (touch)
x (execute) execute the file enter the directory (cd)

6.3.2. three sets of rwx
We already know that the output of ls -l starts with ten characters for each file. This
screenshot shows a regular file (because the first character is a - ).
paul@RHELv4u4:~/test$ ls -l proc42.bash
-rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash

Below is a table describing the function of all ten characters.

Table 6.3. Unix file permissions position
position characters function
1 - this is a regular file
2-4 rwx permissions for the user owner
5-7 r-x permissions for the group owner
8-10 r-- permissions for others

When you are the user owner of a file, then the user owner permissions apply to you. The
rest of the permissions have no influence on your access to the file.

When you belong to the group that is the group owner of a file, then the group owner
permissions apply to you. The rest of the permissions have no influence on your access to
the file.

When you are not the user owner of a file and you do not belong to the group owner, then
the others permissions apply to you. The rest of the permissions have no influence on your
access to the file.

49
standard file permissions

6.3.3. permission examples
Some example combinations on files and directories are seen in this screenshot. The name
of the file explains the permissions.
paul@laika:~/perms$ ls -lh
total 12K
drwxr-xr-x 2 paul paul 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:21 EveryoneFullControl.txt
-r--r----- 1 paul paul 0 2007-02-07 22:21 OnlyOwnersRead.txt
-rwxrwx--- 1 paul paul 0 2007-02-07 22:21 OwnersAll_RestNothing.txt
dr-xr-x--- 2 paul paul 4.0K 2007-02-07 22:25 UserAndGroupEnter
dr-x------ 2 paul paul 4.0K 2007-02-07 22:25 OnlyUserEnter
paul@laika:~/perms$

To summarise, the first rwx triplet represents the permissions for the user owner. The
second triplet corresponds to the group owner; it specifies permissions for all members
of that group. The third triplet defines permissions for all other users that are not the user
owner and are not a member of the group owner.

50
standard file permissions

6.3.4. setting permissions (chmod)
Permissions can be changed with chmod. The first example gives the user owner execute
permissions.
paul@laika:~/perms$ ls -l permissions.txt
-rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod u+x permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

This example removes the group owners read permission.
paul@laika:~/perms$ chmod g-r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

This example removes the others read permission.
paul@laika:~/perms$ chmod o-r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt

This example gives all of them the write permission.
paul@laika:~/perms$ chmod a+w permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt

You don't even have to type the a.
paul@laika:~/perms$ chmod +x permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt

You can also set explicit permissions.
paul@laika:~/perms$ chmod u=rw permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt

Feel free to make any kind of combination.
paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

Even fishy combinations are accepted by chmod.
paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

51
standard file permissions

6.3.5. setting octal permissions
Most Unix administrators will use the old school octal system to talk about and set
permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1.

Table 6.4. Octal permissions
binary octal permission
000 0 ---
001 1 --x
010 2 -w-
011 3 -wx
100 4 r--
101 5 r-x
110 6 rw-
111 7 rwx

This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod
command will accept these numbers.
paul@laika:~/perms$ chmod 777 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 664 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 750 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt

52
standard file permissions

6.3.6. umask
When creating a file or directory, a set of default permissions are applied. These default
permissions are determined by the umask. The umask specifies permissions that you do
not want set on by default. You can display the umask with the umask command.
[Harry@RHEL4b ~]$ umask
0002
[Harry@RHEL4b ~]$ touch test
[Harry@RHEL4b ~]$ ls -l test
-rw-rw-r-- 1 Harry Harry 0 Jul 24 06:03 test
[Harry@RHEL4b ~]$

As you can also see, the file is also not executable by default. This is a general security
feature among Unixes; newly created files are never executable by default. You have to
explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the
umask has no meaning--a umask of 0022 is the same as 0033.

6.3.7. mkdir -m
When creating directories with mkdir you can use the -m option to set the mode. This
screenshot explains.
paul@debian5~$ mkdir -m 700 MyDir
paul@debian5~$ mkdir -m 777 Public
paul@debian5~$ ls -dl MyDir/ Public/
drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/
drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/

6.3.8. cp -p
To preserve permissions and time stamps from source files, use cp -p.
paul@laika:~/perms$ cp file* cp
paul@laika:~/perms$ cp -p file* cpp
paul@laika:~/perms$ ll *
-rwx------ 1 paul paul 0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42

cp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:34 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42

cpp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42

53
standard file permissions

6.4. practice: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.

2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?

3. As root, create a file in the users ~/permissions directory.

4. As normal user, look at who owns this file created by root.

5. Change the ownership of all files in ~/permissions to yourself.

6. Make sure you have all rights to these files, and others can only read.

7. With chmod, is 770 the same as rwxrwx--- ?

8. With chmod, is 664 the same as r-xr-xr-- ?

9. With chmod, is 400 the same as r-------- ?

10. With chmod, is 734 the same as rwxr-xr-- ?

11a. Display the umask in octal and in symbolic form.

11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.

12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.

13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.

13b. Can root read this file ? Can root write to this file with vi ?

14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.

54
standard file permissions

6.5. solution: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
mkdir ~/permissions ; touch ~/permissions/myfile.txt

2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
cp /etc/hosts ~/permissions/

The copy is owned by you.

3. As root, create a file in the users ~/permissions directory.
(become root)# touch /home/username/permissions/rootfile

4. As normal user, look at who owns this file created by root.
ls -l ~/permissions

The file created by root is owned by root.

5. Change the ownership of all files in ~/permissions to yourself.
chown user ~/permissions/*

You cannot become owner of the file that belongs to root.

6. Make sure you have all rights to these files, and others can only read.
chmod 644 (on files)

chmod 755 (on directories)

7. With chmod, is 770 the same as rwxrwx--- ?

yes

8. With chmod, is 664 the same as r-xr-xr-- ?

9. With chmod, is 400 the same as r-------- ?

yes

10. With chmod, is 734 the same as rwxr-xr-- ?

11a. Display the umask in octal and in symbolic form.
umask ; umask -S

11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
umask -S u=rwx,go=

55
standard file permissions

12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.
(become root)

# echo hello > /home/username/root.txt

# chmod 744 /home/username/root.txt

(become user)

vi ~/root.txt

13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.
echo hello > file ; chmod 744 file

Yes, others can read this file

13b. Can root read this file ? Can root write to this file with vi ?

Yes, root can read and write to this file. Permissions do not apply to root.

14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.
mkdir /home/project42 ; groupadd project42

chgrp project42 /home/project42 ; chmod 775 /home/project42

You can not yet do the last part of this exercise...

56
Chapter 7. advanced file permissions

57
advanced file permissions

7.1. sticky bit on directory
You can set the sticky bit on a directory to prevent users from removing files that they do
not own as a user owner. The sticky bit is displayed at the same location as the x permission
for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there
is no x for others).
root@RHELv4u4:~# mkdir /project55
root@RHELv4u4:~# ls -ld /project55
drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55
root@RHELv4u4:~# chmod +t /project55/
root@RHELv4u4:~# ls -ld /project55
drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55
root@RHELv4u4:~#

The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets.
root@RHELv4u4:~# chmod 1775 /project55/
root@RHELv4u4:~# ls -ld /project55
drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55
root@RHELv4u4:~#

You will typically find the sticky bit on the /tmp directory.
root@barry:~# ls -ld /tmp
drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp

7.2. setgid bit on directory
setgid can be used on directories to make sure that all files inside the directory are owned
by the group owner of the directory. The setgid bit is displayed at the same location as the x
permission for group owner. The setgid bit is represented by an s (meaning x is also there)
or a S (when there is no x for the group owner). As this example shows, even though root
does not belong to the group proj55, the files created by root in /project55 will belong to
proj55 since the setgid is set.
root@RHELv4u4:~# groupadd proj55
root@RHELv4u4:~# chown root:proj55 /project55/
root@RHELv4u4:~# chmod 2775 /project55/
root@RHELv4u4:~# touch /project55/fromroot.txt
root@RHELv4u4:~# ls -ld /project55/
drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/
root@RHELv4u4:~# ls -l /project55/
total 4
-rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt
root@RHELv4u4:~#

You can use the find command to find all setgid directories.
paul@laika:~$ find / -type d -perm -2000 2> /dev/null
/var/log/mysql
/var/log/news
/var/local
...

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advanced file permissions

7.3. setgid and setuid on regular files
These two permissions cause an executable file to be executed with the permissions of the
file owner instead of the executing owner. This means that if any user executes a program
that belongs to the root user, and the setuid bit is set on that program, then the program
runs as root. This can be dangerous, but sometimes this is good for security.

Take the example of passwords; they are stored in /etc/shadow which is only readable by
root. (The root user never needs permissions anyway.)

root@RHELv4u4:~# ls -l /etc/shadow
-r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow

Changing your password requires an update of this file, so how can normal non-root users
do this? Let's take a look at the permissions on the /usr/bin/passwd.

root@RHELv4u4:~# ls -l /usr/bin/passwd
-r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd

When running the passwd program, you are executing it with root credentials.

You can use the find command to find all setuid programs.

paul@laika:~$ find /usr/bin -type f -perm -04000
/usr/bin/arping
/usr/bin/kgrantpty
/usr/bin/newgrp
/usr/bin/chfn
/usr/bin/sudo
/usr/bin/fping6
/usr/bin/passwd
/usr/bin/gpasswd
...

In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will
result in these programs to run with the credentials of their group owner.

7.4. setuid on sudo
The sudo binary has the setuid bit set, so any user can run it with the effective userid of root.
paul@rhel65:~$ ls -l $(which sudo)
---s--x--x. 1 root root 123832 Oct 7 2013 /usr/bin/sudo
paul@rhel65:~$

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advanced file permissions

7.5. practice: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.

1b. Members of the sports group should be able to create files in this directory.

1c. All files created in this directory should be group-owned by the sports group.

1d. Users should be able to delete only their own user-owned files.

1e. Test that this works!

2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
password as a normal user. Reset the permissions back and try again.

3. If time permits (or if you are waiting for other students to finish this practice), read about
file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
test that it works.

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advanced file permissions

7.6. solution: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.
groupadd sports

mkdir /home/sports

chown root:sports /home/sports

1b. Members of the sports group should be able to create files in this directory.
chmod 770 /home/sports

1c. All files created in this directory should be group-owned by the sports group.
chmod 2770 /home/sports

1d. Users should be able to delete only their own user-owned files.
chmod +t /home/sports

1e. Test that this works!

Log in with different users (group members and others and root), create files and watch the
permissions. Try changing and deleting files...

2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
password as a normal user. Reset the permissions back and try again.

root@deb503:~# ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
root@deb503:~# chmod 755 /usr/bin/passwd
root@deb503:~# ls -l /usr/bin/passwd
-rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd

A normal user cannot change password now.

root@deb503:~# chmod 4755 /usr/bin/passwd
root@deb503:~# ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd

paul@laika:~$ sudo su -
[sudo] password for paul:
root@laika:~# mkdir attr
root@laika:~# cd attr/
root@laika:~/attr# touch file42
root@laika:~/attr# lsattr
------------------ ./file42
root@laika:~/attr# chattr +i file42

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advanced file permissions

root@laika:~/attr# lsattr
----i------------- ./file42
root@laika:~/attr# rm -rf file42
rm: cannot remove `file42': Operation not permitted
root@laika:~/attr# chattr -i file42
root@laika:~/attr# rm -rf file42
root@laika:~/attr#

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Chapter 8. access control lists
Standard Unix permissions might not be enough for some organisations. This chapter
introduces access control lists or acl's to further protect files and directories.

63
access control lists

8.1. acl in /etc/fstab
File systems that support access control lists, or acls, have to be mounted with the acl
option listed in /etc/fstab. In the example below, you can see that the root file system has
acl support, whereas /home/data does not.
root@laika:~# tail -4 /etc/fstab
/dev/sda1 / ext3 acl,relatime 0 1
/dev/sdb2 /home/data auto noacl,defaults 0 0
pasha:/home/r /home/pasha nfs defaults 0 0
wolf:/srv/data /home/wolf nfs defaults 0 0

8.2. getfacl
Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl
of file33 with getfacl.
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rw-
group::r--
mask::rwx
other::r--

8.3. setfacl
Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how
to change the acl of file33 with setfacl.

First we add user sandra with octal permission 7 to the acl.
paul@laika:~/test$ setfacl -m u:sandra:7 file33

Then we add the group tennis with octal permission 6 to the acl of the same file.
paul@laika:~/test$ setfacl -m g:tennis:6 file33

The result is visible with getfacl.
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rw-
user:sandra:rwx
group::r--
group:tennis:rw-
mask::rwx
other::r--

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access control lists

8.4. remove an acl entry
The -x option of the setfacl command will remove an acl entry from the targeted file.
paul@laika:~/test$ setfacl -m u:sandra:7 file33
paul@laika:~/test$ getfacl file33 | grep sandra
user:sandra:rwx
paul@laika:~/test$ setfacl -x sandra file33
paul@laika:~/test$ getfacl file33 | grep sandra

Note that omitting the u or g when defining the acl for an account will default it to a user
account.

8.5. remove the complete acl
The -b option of the setfacl command will remove the acl from the targeted file.
paul@laika:~/test$ setfacl -b file33
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rw-
group::r--
other::r--

8.6. the acl mask
The acl mask defines the maximum effective permissions for any entry in the acl. This
mask is calculated every time you execute the setfacl or chmod commands.

You can prevent the calculation by using the --no-mask switch.

paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rw-
user:sandra:rwx #effective:rw-
group::r--
mask::rw-
other::r--

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access control lists

8.7. eiciel
Desktop users might want to use eiciel to manage acls with a graphical tool.

You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to
manage acls.

paul@laika:~$ sudo aptitude install eiciel nautilus-actions

66
Chapter 9. file links
An average computer using Linux has a file system with many hard links and symbolic
links.

To understand links in a file system, you first have to understand what an inode is.

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file links

9.1. inodes
9.1.1. inode contents
An inode is a data structure that contains metadata about a file. When the file system stores
a new file on the hard disk, it stores not only the contents (data) of the file, but also extra
properties like the name of the file, the creation date, its permissions, the owner of the file,
and more. All this information (except the name of the file and the contents of the file) is
stored in the inode of the file.

The ls -l command will display some of the inode contents, as seen in this screenshot.
root@rhel53 ~# ls -ld /home/project42/
drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/

9.1.2. inode table
The inode table contains all of the inodes and is created when you create the file system
(with mkfs). You can use the df -i command to see how many inodes are used and free on
mounted file systems.
root@rhel53 ~# df -i
Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/mapper/VolGroup00-LogVol00
4947968 115326 4832642 3% /
/dev/hda1 26104 45 26059 1% /boot
tmpfs 64417 1 64416 1% /dev/shm
/dev/sda1 262144 2207 259937 1% /home/project42
/dev/sdb1 74400 5519 68881 8% /home/project33
/dev/sdb5 0 0 0 - /home/sales
/dev/sdb6 100744 11 100733 1% /home/research

In the df -i screenshot above you can see the inode usage for several mounted file systems.
You don't see numbers for /dev/sdb5 because it is a fat file system.

9.1.3. inode number
Each inode has a unique number (the inode number). You can see the inode numbers with
the ls -li command.
paul@RHELv4u4:~/test$ touch file1
paul@RHELv4u4:~/test$ touch file2
paul@RHELv4u4:~/test$ touch file3
paul@RHELv4u4:~/test$ ls -li
total 12
817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1
817267 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file2
817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
paul@RHELv4u4:~/test$

These three files were created one after the other and got three different inodes (the first
column). All the information you see with this ls command resides in the inode, except for
the filename (which is contained in the directory).

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file links

9.1.4. inode and file contents
Let's put some data in one of the files.
paul@RHELv4u4:~/test$ ls -li
total 16
817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1
817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2
817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
paul@RHELv4u4:~/test$ cat file2
It is winter now and it is very cold.
We do not like the cold, we prefer hot summer nights.
paul@RHELv4u4:~/test$

The data that is displayed by the cat command is not in the inode, but somewhere else on
the disk. The inode contains a pointer to that data.

9.2. about directories
9.2.1. a directory is a table
A directory is a special kind of file that contains a table which maps filenames to inodes.
Listing our current directory with ls -ali will display the contents of the directory file.
paul@RHELv4u4:~/test$ ls -ali
total 32
817262 drwxrwxr-x 2 paul paul 4096 Feb 5 15:42 .
800768 drwx------ 16 paul paul 4096 Feb 5 15:42 ..
817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1
817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2
817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
paul@RHELv4u4:~/test$

9.2.2. . and ..
You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself,
and the dotdot .. is a mapping to the parent directory. The three other names are mappings
to different inodes.

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file links

9.3. hard links
9.3.1. creating hard links
When we create a hard link to a file with ln, an extra entry is added in the directory. A new
file name is mapped to an existing inode.
paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2
paul@RHELv4u4:~/test$ ls -li
total 24
817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1
817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2
817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2
paul@RHELv4u4:~/test$

Both files have the same inode, so they will always have the same permissions and the same
owner. Both files will have the same content. Actually, both files are equal now, meaning
you can safely remove the original file, the hardlinked file will remain. The inode contains
a counter, counting the number of hard links to itself. When the counter drops to zero, then
the inode is emptied.

9.3.2. finding hard links
You can use the find command to look for files with a certain inode. The screenshot below
shows how to search for all filenames that point to inode 817270. Remember that an inode
number is unique to its partition.
paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null
/home/paul/test/file2
/home/paul/test/hardlink_to_file2

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file links

9.4. symbolic links
Symbolic links (sometimes called soft links) do not link to inodes, but create a name to
name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic
link gets an inode of its own.
paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2
paul@RHELv4u4:~/test$ ls -li
total 32
817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1
817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2
817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2
817267 lrwxrwxrwx 1 paul paul 5 Feb 5 16:55 symlink_to_file2 -> file2
paul@RHELv4u4:~/test$

Permissions on a symbolic link have no meaning, since the permissions of the target apply.
Hard links are limited to their own partition (because they point to an inode), symbolic links
can link anywhere (other file systems, even networked).

9.5. removing links
Links can be removed with rm.
paul@laika:~$ touch data.txt
paul@laika:~$ ln -s data.txt sl_data.txt
paul@laika:~$ ln data.txt hl_data.txt
paul@laika:~$ rm sl_data.txt
paul@laika:~$ rm hl_data.txt

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file links

9.6. practice : links
1. Create two files named winter.txt and summer.txt, put some text in them.

2. Create a hard link to winter.txt named hlwinter.txt.

3. Display the inode numbers of these three files, the hard links should have the same inode.

4. Use the find command to list the two hardlinked files

5. Everything about a file is in the inode, except two things : name them!

6. Create a symbolic link to summer.txt called slsummer.txt.

7. Find all files with inode number 2. What does this information tell you ?

8. Look at the directories /etc/init.d/ /etc/rc2.d/ /etc/rc3.d/ ... do you see the links ?

9. Look in /lib with ls -l...

10. Use find to look in your home directory for regular files that do not(!) have one hard link.

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file links

9.7. solution : links
1. Create two files named winter.txt and summer.txt, put some text in them.
echo cold > winter.txt ; echo hot > summer.txt

2. Create a hard link to winter.txt named hlwinter.txt.
ln winter.txt hlwinter.txt

3. Display the inode numbers of these three files, the hard links should have the same inode.
ls -li winter.txt summer.txt hlwinter.txt

4. Use the find command to list the two hardlinked files
find . -inum xyz #replace xyz with the inode number

5. Everything about a file is in the inode, except two things : name them!

The name of the file is in a directory, and the contents is somewhere on the disk.

6. Create a symbolic link to summer.txt called slsummer.txt.
ln -s summer.txt slsummer.txt

7. Find all files with inode number 2. What does this information tell you ?

It tells you there is more than one inode table (one for every formatted partition + virtual
file systems)

8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ?
ls -l /etc/init.d

ls -l /etc/rc2.d

ls -l /etc/rc3.d

9. Look in /lib with ls -l...
ls -l /lib

10. Use find to look in your home directory for regular files that do not(!) have one hard link.
find ~ ! -links 1 -type f

73
Part III. iptables firewall
Table of Contents
10. introduction to routers .......................................................................................................................... 76
10.1. router or firewall ......................................................................................................................... 77
10.2. packet forwarding ....................................................................................................................... 77
10.3. packet filtering ............................................................................................................................ 77
10.4. stateful ......................................................................................................................................... 77
10.5. nat (network address translation) ................................................................................................ 78
10.6. pat (port address translation) ...................................................................................................... 78
10.7. snat (source nat) .......................................................................................................................... 78
10.8. masquerading ............................................................................................................................... 78
10.9. dnat (destination nat) .................................................................................................................. 78
10.10. port forwarding ......................................................................................................................... 78
10.11. /proc/sys/net/ipv4/ip_forward .................................................................................................... 79
10.12. /etc/sysctl.conf ........................................................................................................................... 79
10.13. sysctl .......................................................................................................................................... 79
10.14. practice: packet forwarding ...................................................................................................... 80
10.15. solution: packet forwarding ...................................................................................................... 82
11. iptables firewall ...................................................................................................................................... 85
11.1. iptables tables .............................................................................................................................. 86
11.2. starting and stopping iptables ..................................................................................................... 86
11.3. the filter table .............................................................................................................................. 87
11.4. practice: packet filtering ............................................................................................................. 92
11.5. solution: packet filtering ............................................................................................................. 93
11.6. network address translation ........................................................................................................ 94

75
Chapter 10. introduction to routers
What follows is a very brief introduction to using Linux as a router.

76
introduction to routers

10.1. router or firewall
A router is a device that connects two networks. A firewall is a device that besides acting
as a router, also contains (and implements) rules to determine whether packets are allowed
to travel from one network to another. A firewall can be configured to block access based
on networks, hosts, protocols and ports. Firewalls can also change the contents of packets
while forwarding them.

10.2. packet forwarding
Packet forwarding means allowing packets to go from one network to another. When a
multihomed host is connected to two different networks, and it allows packets to travel from
one network to another through its two network interfaces, it is said to have enabled packet
forwarding.

10.3. packet filtering
Packet filtering is very similar to packet forwarding, but every packet is individually tested
against rules that decide on allowing or dropping the packet. The rules are stored by iptables.

10.4. stateful
A stateful firewall is an advancement over stateless firewalls that inspect every individual
packet. A stateful firewall will keep a table of active connections, and is knowledgeable
enough to recognise when new connections are part of an active session. Linux iptables is
a stateful firewall.

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introduction to routers

10.5. nat (network address translation)
A nat device is a router that is also changing the source and/or target ip-address in packets.
It is typically used to connect multiple computers in a private address range (rfc 1918) with
the (public) internet. A nat can hide private addresses from the internet.

It is important to understand that people and vendors do not always use the right term when
referring to a certain type of nat. Be sure you talk about the same thing. We can distuinguish
several types of nat.

10.6. pat (port address translation)
nat often includes pat. A pat device is a router that is also changing the source and/or target
tcp/udp port in packets. pat is Cisco terminology and is used by snat, dnat, masquerading
and port forwarding in Linux. RFC 3022 calls it NAPT and defines the nat/pat combo as
"traditional nat". A device sold to you as a nat-device will probably do nat and pat.

10.7. snat (source nat)
A snat device is changing the source ip-address when a packet passes our nat. snat
configuration with iptables includes a fixed target source address.

10.8. masquerading
Masquerading is a form of snat that will hide the (private) source ip-addresses of your
private network using a public ip-address. Masquerading is common on dynamic internet
interfaces (broadband modem/routers). Masquerade configuration with iptables uses a
dynamic target source address.

10.9. dnat (destination nat)
A dnat device is changing the destination ip-address when a packet passes our nat.

10.10. port forwarding
When static dnat is set up in a way that allows outside connections to enter our private
network, then we call it port forwarding.

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introduction to routers

10.11. /proc/sys/net/ipv4/ip_forward
Whether a host is forwarding packets is defined in /proc/sys/net/ipv4/ip_forward. The
following screenshot shows how to enable packet forwarding on Linux.

root@router~# echo 1 > /proc/sys/net/ipv4/ip_forward

The next command shows how to disable packet forwarding.

root@router~# echo 0 > /proc/sys/net/ipv4/ip_forward

Use cat to check if packet forwarding is enabled.

root@router~# cat /proc/sys/net/ipv4/ip_forward

10.12. /etc/sysctl.conf
By default, most Linux computers are not configured for automatic packet forwarding.
To enable packet forwarding whenever the system starts, change the net.ipv4.ip_forward
variable in /etc/sysctl.conf to the value 1.
root@router~# grep ip_forward /etc/sysctl.conf
net.ipv4.ip_forward = 0

10.13. sysctl
For more information, take a look at the man page of sysctl.
root@debian6~# man sysctl
root@debian6~# sysctl -a 2>/dev/null | grep ip_forward
net.ipv4.ip_forward = 0

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introduction to routers

10.14. practice: packet forwarding
0. You have the option to select (or create) an internal network when adding a network card
in VirtualBox or VMWare. Use this option to create two internal networks. I named them
leftnet and rightnet, but you can choose any other name.

1. Set up two Linux machines, one on leftnet, the other on rightnet. Make sure they both
get an ip-address in the correct subnet. These two machines will be 'left' and 'right' from
the 'router'.

Table 10.1. Packet Forwarding Exercise
leftnet computer the router rightnet computer
MAC
IP

3. How can you verify whether the router will allow packet forwarding by default or not ?
Test that you can ping from the router to the two other machines, and from those two
machines to the router. Use arp -a to make sure you are connected with the correct mac
addresses.

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introduction to routers

4. Ping from the leftnet computer to the rightnet computer. Enable and/or disable packet
forwarding on the router and verify what happens to the ping between the two networks. If
you do not succeed in pinging between the two networks (on different subnets), then use a
sniffer like wireshark or tcpdump to discover the problem.

5. Use wireshark or tcpdump -xx to answer the following questions. Does the source MAC
change when a packet passes through the filter ? And the destination MAC ? What about
source and destination IP-addresses ?

6. Remember the third network card on the router ? Connect this card to a LAN with internet
connection. On many LAN's the command dhclient eth0 just works (replace eth0 with the
correct interface).
root@router~# dhclient eth0

You now have a setup similar to this picture. What needs to be done to give internet access
to leftnet and rightnet.

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introduction to routers

10.15. solution: packet forwarding

1. Set up two Linux machines, one on leftnet, the other on rightnet. Make sure they both
get an ip-address in the correct subnet. These two machines will be 'left' and 'right' from
the 'router'.

The ip configuration on your computers should be similar to the following two screenshots.
Both machines must be in a different subnet (here 192.168.60.0/24 and 192.168.70.0/24). I
created a little script on both machines to configure the interfaces.
root@left~# cat leftnet.sh
pkill dhclient
ifconfig eth0 192.168.60.8 netmask 255.255.255.0

root@right~# cat rightnet.sh
pkill dhclient
ifconfig eth0 192.168.70.9 netmask 255.255.255.0

2. Set up a third Linux computer with three network cards, one on leftnet, the other on
rightnet. This computer will be the 'router'. Complete the table below with the relevant
names, ip-addresses and mac-addresses.
root@router~# cat router.sh
ifconfig eth1 192.168.60.1 netmask 255.255.255.0
ifconfig eth2 192.168.70.1 netmask 255.255.255.0
#echo 1 > /proc/sys/net/ipv4/ip_forward

Your setup may use different ip and mac addresses than the ones in the table below.

Table 10.2. Packet Forwarding Solution
leftnet computer the router rightnet computer
08:00:27:f6:ab:b9 08:00:27:43:1f:5a 08:00:27:be:4a:6b 08:00:27:14:8b:17
192.168.60.8 192.168.60.1 192.168.70.1 192.168.70.9

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This can be done with "grep ip_forward /etc/sysctl.conf" (1 is enabled, 0 is disabled) or
with sysctl -a | grep ip_for.
root@router~# grep ip_for /etc/sysctl.conf
net.ipv4.ip_forward = 0

Did you forget to add a default gateway to the LAN machines ? Use route add default
gw 'ip-address'.
root@left~# route add default gw 192.168.60.1

root@right~# route add default gw 192.168.70.1

You should be able to ping when packet forwarding is enabled (and both default gateways
are properly configured). The ping will not work when packet forwarding is disabled or
when gateways are not configured correctly.

Both MAC addresses are changed when passing the router. Use tcpdump -xx like this:
root@router~# tcpdump -xx -i eth1

root@router~# tcpdump -xx -i eth2

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introduction to routers

You now have a setup similar to this picture. What needs to be done to give internet access
to leftnet and rightnet.

The clients on leftnet and rightnet need a working dns server. We use one of Google's
dns servers here.
echo nameserver 8.8.8.8 > /etc/resolv.conf

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Chapter 11. iptables firewall
This chapter introduces some simple firewall rules and how to configure them with iptables.

iptables is an application that allows a user to configure the firewall functionality built into
the Linux kernel.

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11.1. iptables tables
By default there are three tables in the kernel that contain sets of rules.

The filter table is used for packet filtering.
root@debian6~# iptables -t filter -L
Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

The nat table is used for address translation.
root@debian6~# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target prot opt source destination

Chain POSTROUTING (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

The mangle table can be used for special-purpose processing of packets.

Series of rules in each table are called a chain. We will discuss chains and the nat table
later in this chapter.

11.2. starting and stopping iptables
The following screenshot shows how to stop and start iptables on Red Hat/Fedora/CentOS
and compatible distributions.
[root@centos6 ~]# service iptables stop
[root@centos6 ~]# service iptables start
iptables: Applying firewall rules [ ok ]
[root@centos6 ~]#

Debian and *buntu distributions do not have this script, but allow for an uninstall.
root@debian6~# aptitude purge iptables

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11.3. the filter table
11.3.1. about packet filtering
Packet filtering is a bit more than packet forwarding. While packet forwarding uses only
a routing table to make decisions, packet filtering also uses a list of rules. The kernel will
inspect packets and decide based on these rules what to do with each packet.

11.3.2. filter table
The filter table in iptables has three chains (sets of rules). The INPUT chain is used for any
packet coming into the system. The OUTPUT chain is for any packet leaving the system.
And the FORWARD chain is for packets that are forwarded (routed) through the system.

The screenshot below shows how to list the filter table and all its rules.
[root@RHEL5 ~]# iptables -t filter -nL
Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination
[root@RHEL5 ~]#

As you can see, all three chains in the filter table are set to ACCEPT everything. ACCEPT
is the default behaviour.

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11.3.3. setting default rules
The default for the default rule is indeed to ACCEPT everything. This is not the most secure
firewall.

A more secure setup would be to DROP everything. A package that is dropped will not
continue in any chain, and no warning or error will be sent anywhere.

The below commands lock down a computer. Do not execute these commands inside a
remote ssh shell.
root@debianpaul~# iptables -P INPUT DROP
root@debianpaul~# iptables -P OUTPUT DROP
root@debianpaul~# iptables -P FORWARD DROP
root@debianpaul~# iptables -L
Chain INPUT (policy DROP)
target prot opt source destination

Chain FORWARD (policy DROP)
target prot opt source destination

Chain OUTPUT (policy DROP)
target prot opt source destination

11.3.4. changing policy rules
To start, let's set the default policy for all three chains to drop everything. Note that you
might lose your connection when typing this over ssh ;-).
[root@RHEL5 ~]# iptables -P INPUT DROP
[root@RHEL5 ~]# iptables -P FORWARD DROP
[root@RHEL5 ~]# iptables -P OUTPUT DROP

Next, we allow the server to use its own loopback device (this allows the server to access
its services running on localhost). We first append a rule to the INPUT chain to allow
(ACCEPT) traffic from the lo (loopback) interface, then we do the same to allow packets to
leave the system through the loopback interface.
[root@RHEL5 ~]# iptables -A INPUT -i lo -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o lo -j ACCEPT

Looking at the filter table again (omitting -t filter because it is the default table).
[root@RHEL5 ~]# iptables -nL
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0

Chain FORWARD (policy DROP)
target prot opt source destination

Chain OUTPUT (policy DROP)
target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0

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11.3.5. Allowing ssh over eth0
This example show how to add two rules to allow ssh access to your system from outside.
[root@RHEL5 ~]# iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT

The filter table will look something like this screenshot (note that -v is added for more
verbose output).
[root@RHEL5 ~]# iptables -nvL
Chain INPUT (policy DROP 7 packets, 609 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22

Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination

Chain OUTPUT (policy DROP 3 packets, 228 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22
[root@RHEL5 ~]#

11.3.6. Allowing access from a subnet
This example shows how to allow access from any computer in the 10.1.1.0/24 network, but
only through eth1. There is no port (application) limitation here.
[root@RHEL5 ~]# iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT

Together with the previous examples, the policy is expanding.
[root@RHEL5 ~]# iptables -nvL
Chain INPUT (policy DROP 7 packets, 609 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
0 0 ACCEPT tcp -- eth1 * 10.1.1.0/24 0.0.0.0/0

Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination

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11.3.7. iptables save
Use iptables save to automatically implement these rules when the firewall is (re)started.
[root@RHEL5 ~]# /etc/init.d/iptables save
Saving firewall rules to /etc/sysconfig/iptables: [ OK ]
[root@RHEL5 ~]#

11.3.8. scripting example
You can write a simple script for these rules. Below is an example script that implements
the firewall rules that you saw before in this chapter.
#!/bin/bash
# first cleanup everything
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X

# default drop
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

# allow loopback device
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# allow ssh over eth0 from outside to system
iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT

# allow any traffic from 10.1.1.0/24 to system
iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT
iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT

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11.3.9. Allowing ICMP(ping)
When you enable iptables, you will get an 'Operation not permitted' message when trying
to ping other hosts.
[root@RHEL5 ~# ping 192.168.187.130
PING 192.168.187.130 (192.168.187.130) 56(84) bytes of data.
ping: sendmsg: Operation not permitted
ping: sendmsg: Operation not permitted

The screenshot below shows you how to setup iptables to allow a ping from or to your
machine.
[root@RHEL5 ~]# iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT

The previous two lines do not allow other computers to route ping messages through your
router, because it only handles INPUT and OUTPUT. For routing of ping, you will need
to enable it on the FORWARD chain. The following command enables routing of icmp
messages between networks.
[root@RHEL5 ~]# iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT

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iptables firewall

11.4. practice: packet filtering
1. Make sure you can ssh to your router-system when iptables is active.

2. Make sure you can ping to your router-system when iptables is active.

3. Define one of your networks as 'internal' and the other as 'external'. Configure the router
to allow visits to a website (http) to go from the internal network to the external network
(but not in the other direction).

4. Make sure the internal network can ssh to the external, but not the other way around.

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11.5. solution: packet filtering
A possible solution, where leftnet is the internal and rightnet is the external network.
#!/bin/bash

# first cleanup everything
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X

# default drop
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

# allow loopback device
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# question 1: allow ssh over eth0
iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT

# question 2: Allow icmp(ping) anywhere
iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT
iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT

# question 3: allow http from internal(leftnet) to external(rightnet)
iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 80 -j ACCEPT
iptables -A FORWARD -i eth2 -o eth1 -p tcp --sport 80 -j ACCEPT

# question 4: allow ssh from internal(leftnet) to external(rightnet)
iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 22 -j ACCEPT
iptables -A FORWARD -i eth2 -o eth1 -p tcp --sport 22 -j ACCEPT

# allow http from external(rightnet) to internal(leftnet)
# iptables -A FORWARD -i eth2 -o eth1 -p tcp --dport 80 -j ACCEPT
# iptables -A FORWARD -i eth1 -o eth2 -p tcp --sport 80 -j ACCEPT

# allow rpcinfo over eth0 from outside to system
# iptables -A INPUT -i eth2 -p tcp --dport 111 -j ACCEPT
# iptables -A OUTPUT -o eth2 -p tcp --sport 111 -j ACCEPT

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11.6. network address translation
11.6.1. about NAT
A NAT device is a router that is also changing the source and/or target ip-address in packets.
It is typically used to connect multiple computers in a private address range with the (public)
internet. A NAT can hide private addresses from the internet.

NAT was developed to mitigate the use of real ip addresses, to allow private address ranges
to reach the internet and back, and to not disclose details about internal networks to the
outside.

The nat table in iptables adds two new chains. PREROUTING allows altering of packets
before they reach the INPUT chain. POSTROUTING allows altering packets after they exit
the OUTPUT chain.

Use iptables -t nat -nvL to look at the NAT table. The screenshot below shows an empty
NAT table.
[root@RHEL5 ~]# iptables -t nat -nL
Chain PREROUTING (policy ACCEPT)
target prot opt source destination

Chain POSTROUTING (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination
[root@RHEL5 ~]#

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iptables firewall

11.6.2. SNAT (Source NAT)
The goal of source nat is to change the source address inside a packet before it leaves the
system (e.g. to the internet). The destination will return the packet to the NAT-device. This
means our NAT-device will need to keep a table in memory of all the packets it changed, so
it can deliver the packet to the original source (e.g. in the private network).

Because SNAT is about packets leaving the system, it uses the POSTROUTING chain.

Here is an example SNAT rule. The rule says that packets coming from 10.1.1.0/24 network
and exiting via eth1 will get the source ip-address set to 11.12.13.14. (Note that this is a
one line command!)
iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j SNAT \
--to-source 11.12.13.14

Of course there must exist a proper iptables filter setup to allow the packet to traverse from
one network to the other.

11.6.3. SNAT example setup
This example script uses a typical nat setup. The internal (eth0) network has access via
SNAT to external (eth1) webservers (port 80).
#!/bin/bash
#
# iptables script for simple classic nat websurfing
# eth0 is internal network, eth1 is internet
#
echo 0 > /proc/sys/net/ipv4/ip_forward
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -o eth1 -s 10.1.1.0/24 -p tcp \
--dport 80 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -d 10.1.1.0/24 -p tcp \
--sport 80 -j ACCEPT
iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j SNAT \
--to-source 11.12.13.14
echo 1 > /proc/sys/net/ipv4/ip_forward

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11.6.4. IP masquerading
IP masquerading is very similar to SNAT, but is meant for dynamic interfaces. Typical
example are broadband 'router/modems' connected to the internet and receiving a different
ip-address from the isp, each time they are cold-booted.

The only change needed to convert the SNAT script to a masquerading is one line.
iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j MASQUERADE

11.6.5. DNAT (Destination NAT)
DNAT is typically used to allow packets from the internet to be redirected to an internal
server (in your DMZ) and in a private address range that is inaccessible directly form the
internet.

This example script allows internet users to reach your internal (192.168.1.99) server via
ssh (port 22).
#!/bin/bash
#
# iptables script for DNAT
# eth0 is internal network, eth1 is internet
#
echo 0 > /proc/sys/net/ipv4/ip_forward
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -o eth1 -s 10.1.1.0/24 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -p tcp --dport 22 -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 22 \
-j DNAT --to-destination 10.1.1.99
echo 1 > /proc/sys/net/ipv4/ip_forward

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Part IV. selinux
Table of Contents
12. introduction to SELinux ....................................................................................................................... 99
12.1. selinux modes ............................................................................................................................ 100
12.2. logging ....................................................................................................................................... 100
12.3. activating selinux ...................................................................................................................... 100
12.4. getenforce .................................................................................................................................. 101
12.5. setenforce ................................................................................................................................... 101
12.6. sestatus ....................................................................................................................................... 102
12.7. policy ......................................................................................................................................... 102
12.8. /etc/selinux/config ...................................................................................................................... 102
12.9. DAC or MAC ........................................................................................................................... 103
12.10. ls -Z ......................................................................................................................................... 103
12.11. -Z ............................................................................................................................................. 103
12.12. /selinux ..................................................................................................................................... 104
12.13. identity ..................................................................................................................................... 104
12.14. role ........................................................................................................................................... 104
12.15. type (or domain) ..................................................................................................................... 105
12.16. security context ....................................................................................................................... 106
12.17. transition .................................................................................................................................. 106
12.18. extended attributes .................................................................................................................. 107
12.19. process security context .......................................................................................................... 107
12.20. chcon ....................................................................................................................................... 107
12.21. an example .............................................................................................................................. 108
12.22. setroubleshoot .......................................................................................................................... 110
12.23. booleans ................................................................................................................................... 112

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Chapter 12. introduction to SELinux
Security Enhanced Linux or SELinux is a set of modifications developed by the United
States National Security Agency (NSA) to provide a variety of security policies for Linux.
SELinux was released as open source at the end of 2000. Since kernel version 2.6 it is an
integrated part of Linux.

SELinux offers security! SELinux can control what kind of access users have to files and
processes. Even when a file received chmod 777, SELinux can still prevent applications
from accessing it (Unix file permissions are checked first!). SELinux does this by placing
users in roles that represent a security context. Administrators have very strict control on
access permissions granted to roles.

SELinux is present in the latest versions of Red Hat Enterprise Linux, Debian, CentOS,
Fedora, and many other distributions..

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introduction to SELinux

12.1. selinux modes
selinux knows three modes: enforcing, permissive and disabled. The enforcing mode will
enforce policies, and may deny access based on selinux rules. The permissive mode will not
enforce policies, but can still log actions that would have been denied in enforcing mode.
The disabled mode disables selinux.

12.2. logging
Verify that syslog is running and activated on boot to enable logging of deny messages in
/var/log/messages.
[root@rhel55 ~]# chkconfig --list syslog
syslog 0:off 1:off 2:on 3:on 4:on 5:on 6:off

Verify that auditd is running and activated on boot to enable logging of easier to read
messages in /var/log/audit/audit.log.
[root@rhel55 ~]# chkconfig --list auditd
auditd 0:off 1:off 2:on 3:on 4:on 5:on 6:off

If not activated, then run chkconfig --levels 2345 auditd on and service auditd start.
[root@rhel55 ~]# service auditd status
auditd (pid 1660) is running...
[root@rhel55 ~]# service syslog status
syslogd (pid 1688) is running...
klogd (pid 1691) is running...

The /var/log/messages log file will tell you that selinux is disabled.
root@deb503:~# grep -i selinux /var/log/messages
Jun 25 15:59:34 deb503 kernel: [ 0.084083] SELinux: Disabled at boot.

Or that it is enabled.
root@deb503:~# grep SELinux /var/log/messages | grep -i Init
Jun 25 15:09:52 deb503 kernel: [ 0.084094] SELinux: Initializing.

12.3. activating selinux
On RHEL you can use the GUI tool to activate selinux, on Debian there is the selinux-
activate command. Activation requires a reboot.
root@deb503:~# selinux-activate
Activating SE Linux
Searching for GRUB installation directory ... found: /boot/grub
Searching for default file ... found: /boot/grub/default
Testing for an existing GRUB menu.lst file ... found: /boot/grub/menu.lst
Searching for splash image ... none found, skipping ...
Found kernel: /boot/vmlinuz-2.6.26-2-686
Updating /boot/grub/menu.lst ... done

SE Linux is activated. You may need to reboot now.

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12.4. getenforce
Use getenforce to verify whether selinux is enforced, disabled or permissive.
[root@rhel55 ~]# getenforce
Permissive

The /selinux/enforce file contains 1 when enforcing, and 0 when permissive mode is active.
root@fedora13 ~# cat /selinux/enforce
1root@fedora13 ~#

12.5. setenforce
You can use setenforce to switch between the Permissive or the Enforcing state once
selinux is activated..
[root@rhel55 ~]# setenforce Enforcing
[root@rhel55 ~]# getenforce
Enforcing
[root@rhel55 ~]# setenforce Permissive
[root@rhel55 ~]# getenforce
Permissive

Or you could just use 0 and 1 as argument.
[root@centos65 ~]# setenforce 1
[root@centos65 ~]# getenforce
Enforcing
[root@centos65 ~]# setenforce 0
[root@centos65 ~]# getenforce
Permissive
[root@centos65 ~]#

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12.6. sestatus
You can see the current selinux status and policy with the sestatus command.
[root@rhel55 ~]# sestatus
SELinux status: enabled
SELinuxfs mount: /selinux
Current mode: permissive
Mode from config file: permissive
Policy version: 21
Policy from config file: targeted

12.7. policy
Most Red Hat server will have the targeted policy. Only NSA/FBI/CIA/DOD/HLS use the
mls policy.

The targted policy will protect hundreds of processes, but lets other processes run
'unconfined' (= they can do anything).

12.8. /etc/selinux/config
The main configuration file for selinux is /etc/selinux/config. When in permissive mode,
the file looks like this.

The targeted policy is selected in /etc/selinux/config.
[root@centos65 ~]# cat /etc/selinux/config
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - SELinux is fully disabled.
SELINUX=permissive
# SELINUXTYPE= type of policy in use. Possible values are:
# targeted - Only targeted network daemons are protected.
# strict - Full SELinux protection.
SELINUXTYPE=targeted

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12.9. DAC or MAC
Standard Unix permissions use Discretionary Access Control to set permissions on files.
This means that a user that owns a file, can make it world readable by typing chmod 777
$file.

With selinux the kernel will enforce Mandatory Access Control which strictly controls
what processes or threads can do with files (superseding DAC). Processes are confined by
the kernel to the minimum access they require.

SELinux MAC is about labeling and type enforcing! Files, processes, etc are all labeled with
an SELinux context. For files, these are extended attributes, for processes this is managed
by the kernel.

The format of the labels is as follows:
user:role:type:(level)

We only use the type label in the targeted policy.

12.10. ls -Z
To see the DAC permissions on a file, use ls -l to display user and group owner and
permissions.

For MAC permissions there is new -Z option added to ls. The output shows that file in /root
have a XXXtype of admin_home_t.
[root@centos65 ~]# ls -Z
-rw-------. root root system_u:object_r:admin_home_t:s0 anaconda-ks.cfg
-rw-r--r--. root root system_u:object_r:admin_home_t:s0 install.log
-rw-r--r--. root root system_u:object_r:admin_home_t:s0 install.log.syslog

[root@centos65 ~]# useradd -m -s /bin/bash pol
[root@centos65 ~]# ls -Z /home/pol/.bashrc
-rw-r--r--. pol pol unconfined_u:object_r:user_home_t:s0 /home/pol/.bashrc

12.11. -Z
There are also some other tools with the -Z switch:
mkdir -Z
cp -Z
ps -Z
netstat -Z
...

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12.12. /selinux
When selinux is active, there is a new virtual file system named /selinux. (You can compare
it to /proc and /dev.)
[root@centos65 ~]# ls -l /selinux/
total 0
-rw-rw-rw-. 1 root root 0 Apr 12 19:40 access
dr-xr-xr-x. 2 root root 0 Apr 12 19:40 avc
dr-xr-xr-x. 2 root root 0 Apr 12 19:40 booleans
-rw-r--r--. 1 root root 0 Apr 12 19:40 checkreqprot
dr-xr-xr-x. 83 root root 0 Apr 12 19:40 class
--w-------. 1 root root 0 Apr 12 19:40 commit_pending_bools
-rw-rw-rw-. 1 root root 0 Apr 12 19:40 context
-rw-rw-rw-. 1 root root 0 Apr 12 19:40 create
-r--r--r--. 1 root root 0 Apr 12 19:40 deny_unknown
--w-------. 1 root root 0 Apr 12 19:40 disable
-rw-r--r--. 1 root root 0 Apr 12 19:40 enforce
dr-xr-xr-x. 2 root root 0 Apr 12 19:40 initial_contexts
-rw-------. 1 root root 0 Apr 12 19:40 load
-rw-rw-rw-. 1 root root 0 Apr 12 19:40 member
-r--r--r--. 1 root root 0 Apr 12 19:40 mls
crw-rw-rw-. 1 root root 1, 3 Apr 12 19:40 null
-r--------. 1 root root 0 Apr 12 19:40 policy
dr-xr-xr-x. 2 root root 0 Apr 12 19:40 policy_capabilities
-r--r--r--. 1 root root 0 Apr 12 19:40 policyvers
-r--r--r--. 1 root root 0 Apr 12 19:40 reject_unknown
-rw-rw-rw-. 1 root root 0 Apr 12 19:40 relabel
-r--r--r--. 1 root root 0 Apr 12 19:40 status
-rw-rw-rw-. 1 root root 0 Apr 12 19:40 user

Although some files in /selinux appear wih size 0, they often contain a boolean value. Check
/selinux/enforce to see if selinux is running in enforced mode.
[root@RHEL5 ~]# ls -l /selinux/enforce
-rw-r--r-- 1 root root 0 Apr 29 08:21 /selinux/enforce
[root@RHEL5 ~]# echo $(cat /selinux/enforce)
1

12.13. identity
The SELinux Identity of a user is distinct from the user ID. An identity is part of a security
context, and (via domains) determines what you can do. The screenshot shows user root
having identity user_u.
[root@rhel55 ~]# id -Z
user_u:system_r:unconfined_t

12.14. role
The selinux role defines the domains that can be used. A role is denied to enter a domain,
unless the role is explicitely authorized to do so.

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introduction to SELinux

12.15. type (or domain)
The selinux context is the security context of a process. An selinux type determines what a
process can do. The screenshot shows init running in type init_t and the mingetty's running
in type getty_t.
[root@centos65 ~]# ps fax -Z | grep /sbin/init
system_u:system_r:init_t:s0 1 ? Ss 0:00 /sbin/init
[root@centos65 ~]# ps fax -Z | grep getty_t
system_u:system_r:getty_t:s0 1307 tty1 Ss+ 0:00 /sbin/mingetty /dev/tty1
system_u:system_r:getty_t:s0 1309 tty2 Ss+ 0:00 /sbin/mingetty /dev/tty2
system_u:system_r:getty_t:s0 1311 tty3 Ss+ 0:00 /sbin/mingetty /dev/tty3
system_u:system_r:getty_t:s0 1313 tty4 Ss+ 0:00 /sbin/mingetty /dev/tty4
system_u:system_r:getty_t:s0 1320 tty5 Ss+ 0:00 /sbin/mingetty /dev/tty5
system_u:system_r:getty_t:s0 1322 tty6 Ss+ 0:00 /sbin/mingetty /dev/tty6

The selinux type is similar to an selinux domain, but refers to directories and files instead
of processes.

Hundreds of binaries also have a type:
[root@centos65 sbin]# ls -lZ useradd usermod userdel httpd postcat postfix
-rwxr-xr-x. root root system_u:object_r:httpd_exec_t:s0 httpd
-rwxr-xr-x. root root system_u:object_r:postfix_master_exec_t:s0 postcat
-rwxr-xr-x. root root system_u:object_r:postfix_master_exec_t:s0 postfix
-rwxr-x---. root root system_u:object_r:useradd_exec_t:s0 useradd
-rwxr-x---. root root system_u:object_r:useradd_exec_t:s0 userdel
-rwxr-x---. root root system_u:object_r:useradd_exec_t:s0 usermod

Ports also have a context.
[root@centos65 sbin]# netstat -nptlZ | tr -s ' ' | cut -d' ' -f6-

Foreign Address State PID/Program name Security Context
LISTEN 1096/rpcbind system_u:system_r:rpcbind_t:s0
LISTEN 1208/sshd system_u:system_r:sshd_t:s0-s0:c0.c1023
LISTEN 1284/master system_u:system_r:postfix_master_t:s0
LISTEN 1114/rpc.statd system_u:system_r:rpcd_t:s0
LISTEN 1096/rpcbind system_u:system_r:rpcbind_t:s0
LISTEN 1666/httpd unconfined_u:system_r:httpd_t:s0
LISTEN 1208/sshd system_u:system_r:sshd_t:s0-s0:c0.c1023
LISTEN 1114/rpc.statd system_u:system_r:rpcd_t:s0
LISTEN 1284/master system_u:system_r:postfix_master_t:s0

You can also get a list of ports that are managed by SELinux:
[root@centos65 ~]# semanage port -l | tail
xfs_port_t tcp 7100
xserver_port_t tcp 6000-6150
zabbix_agent_port_t tcp 10050
zabbix_port_t tcp 10051
zarafa_port_t tcp 236, 237
zebra_port_t tcp 2600-2604, 2606
zebra_port_t udp 2600-2604, 2606
zented_port_t tcp 1229
zented_port_t udp 1229
zope_port_t tcp 8021

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introduction to SELinux

12.16. security context
The combination of identity, role and domain or type make up the selinux security context.
The id will show you your security context in the form identity:role:domain.
[paul@RHEL5 ~]$ id | cut -d' ' -f4
context=user_u:system_r:unconfined_t

The ls -Z command shows the security context for a file in the form identity:role:type.
[paul@RHEL5 ~]$ ls -Z test
-rw-rw-r-- paul paul user_u:object_r:user_home_t test

The security context for processes visible in /proc defines both the type (of the file in /proc)
and the domain (of the running process). Let's take a look at the init process and /proc/1/ .

The init process runs in domain init_t.
[root@RHEL5 ~]# ps -ZC init
LABEL PID TTY TIME CMD
system_u:system_r:init_t 1 ? 00:00:01 init

The /proc/1/ directory, which identifies the init process, has type init_t.
[root@RHEL5 ~]# ls -Zd /proc/1/
dr-xr-xr-x root root system_u:system_r:init_t /proc/1/

It is not a coincidence that the domain of the init process and the type of /proc/1/ are both
init_t.

Don't try to use chcon on /proc! It will not work.

12.17. transition
An selinux transition (aka an selinux labelling) determines the security context that will be
assigned. A transition of process domains is used when you execute a process. A transition
of file type happens when you create a file.

An example of file type transition.
[pol@centos65 ~]$ touch test /tmp/test
[pol@centos65 ~]$ ls -Z test
-rw-rw-r--. pol pol unconfined_u:object_r:user_home_t:s0 test
[pol@centos65 ~]$ ls -Z /tmp/test
-rw-rw-r--. pol pol unconfined_u:object_r:user_tmp_t:s0 /tmp/test

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introduction to SELinux

12.18. extended attributes
Extended attributes are used by selinux to store security contexts. These attributes can be
viewed with ls when selinux is running.
[root@RHEL5 home]# ls --context
drwx------ paul paul system_u:object_r:user_home_dir_t paul
drwxr-xr-x root root user_u:object_r:user_home_dir_t project42
drwxr-xr-x root root user_u:object_r:user_home_dir_t project55
[root@RHEL5 home]# ls -Z
drwx------ paul paul system_u:object_r:user_home_dir_t paul
drwxr-xr-x root root user_u:object_r:user_home_dir_t project42
drwxr-xr-x root root user_u:object_r:user_home_dir_t project55
[root@RHEL5 home]#

When selinux is not running, then getfattr is the tool to use.
[root@RHEL5 etc]# getfattr -m . -d hosts
# file: hosts
security.selinux="system_u:object_r:etc_t:s0\000"

12.19. process security context
A new option is added to ps to see the selinux security context of processes.
[root@RHEL5 etc]# ps -ZC mingetty
LABEL PID TTY TIME CMD
system_u:system_r:getty_t 2941 tty1 00:00:00 mingetty
system_u:system_r:getty_t 2942 tty2 00:00:00 mingetty

12.20. chcon
Use chcon to change the selinux security context.

This example shows how to use chcon to change the type of a file.
[root@rhel55 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r-- root root user_u:object_r:httpd_sys_content_t /var/www/html/test4\
2.txt
[root@rhel55 ~]# chcon -t samba_share_t /var/www/html/test42.txt
[root@rhel55 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r-- root root user_u:object_r:samba_share_t /var/www/html/test42.txt

Be sure to read man chcon.

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introduction to SELinux

12.21. an example
The Apache2 webserver is by default targeted with SELinux. The next screenshot shows
that any file created in /var/www/html will by default get the httpd_sys_content_t type.
[root@centos65 ~]# touch /var/www/html/test42.txt
[root@centos65 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r--. root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/h\
tml/test42.txt

Files created elsewhere do not get this type.
[root@centos65 ~]# touch /root/test42.txt
[root@centos65 ~]# ls -Z /root/test42.txt
-rw-r--r--. root root unconfined_u:object_r:admin_home_t:s0 /root/test42.txt

Make sure Apache2 runs.
[root@centos65 ~]# service httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]

Will this work ? Yes it does.
[root@centos65 ~]# wget http://localhost/test42.txt
--2014-04-12 20:56:47-- http://localhost/test42.txt
Resolving localhost... ::1, 127.0.0.1
Connecting to localhost|::1|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: “test42.txt”
...

Why does this work ? Because Apache2 runs in the httpd_t domain and the files in /var/
www/html have the httpd_sys_content_t type.
[root@centos65 ~]# ps -ZC httpd | head -4
LABEL PID TTY TIME CMD
unconfined_u:system_r:httpd_t:s0 1666 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t:s0 1668 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t:s0 1669 ? 00:00:00 httpd

108
introduction to SELinux

So let's set SELinux to enforcing and change the type of this file.
[root@centos65 ~]# chcon -t samba_share_t /var/www/html/test42.txt
[root@centos65 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r--. root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/t\
est42.txt
[root@centos65 ~]# setenforce 1
[root@centos65 ~]# getenforce
Enforcing

There are two possibilities now: either it works, or it fails. It works when selinux is in
permissive mode, it fails when in enforcing mode.
[root@centos65 ~]# wget http://localhost/test42.txt
--2014-04-12 21:05:02-- http://localhost/test42.txt
Resolving localhost... ::1, 127.0.0.1
Connecting to localhost|::1|:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2014-04-12 21:05:02 ERROR 403: Forbidden.

The log file gives you a cryptic message...
[root@centos65 ~]# tail -3 /var/log/audit/audit.log
type=SYSCALL msg=audit(1398200702.803:64): arch=c000003e syscall=4 succ\
ess=no exit=-13 a0=7f5fbc334d70 a1=7fff553b4f10 a2=7fff553b4f10 a3=0 it\
ems=0 ppid=1666 pid=1673 auid=500 uid=48 gid=48 euid=48 suid=48 fsuid=4\
8 egid=48 sgid=48 fsgid=48 tty=(none) ses=1 comm="httpd" exe="/usr/sbin\
/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
type=AVC msg=audit(1398200702.804:65): avc: denied { getattr } for p\
id=1673 comm="httpd" path="/var/www/html/test42.txt" dev=dm-0 ino=26324\
1 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:objec\
t_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1398200702.804:65): arch=c000003e syscall=6 succ\
ess=no exit=-13 a0=7f5fbc334e40 a1=7fff553b4f10 a2=7fff553b4f10 a3=1 it\
ems=0 ppid=1666 pid=1673 auid=500 uid=48 gid=48 euid=48 suid=48 fsuid=4\
8 egid=48 sgid=48 fsgid=48 tty=(none) ses=1 comm="httpd" exe="/usr/sbin\
/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)

And /var/log/messages mentions nothing of the failed download.

109
introduction to SELinux

12.22. setroubleshoot
The log file above was not very helpful, but these two packages can make your life much
easier.
[root@centos65 ~]# yum -y install setroubleshoot setroubleshoot-server

You need to reboot for this to work...

So we reboot, restart the httpd server, reactive SELinux Enforce, and do the wget again...
and it fails (because of SELinux).
[root@centos65 ~]# service httpd restart
Stopping httpd: [FAILED]
Starting httpd: [ OK ]
[root@centos65 ~]# getenforce
Permissive
[root@centos65 ~]# setenforce 1
[root@centos65 ~]# getenforce
Enforcing
[root@centos65 ~]# wget http://localhost/test42.txt
--2014-04-12 21:44:13-- http://localhost/test42.txt
Resolving localhost... ::1, 127.0.0.1
Connecting to localhost|::1|:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2014-04-12 21:44:13 ERROR 403: Forbidden.

The /var/log/audit/ is still not out best friend, but take a look at /var/log/messages.
[root@centos65 ~]# tail -2 /var/log/messages
Apr 12 21:44:16 centos65 setroubleshoot: SELinux is preventing /usr/sbin/h\
ttpd from getattr access on the file /var/www/html/test42.txt. For complete \
SELinux messages. run sealert -l b2a84386-54c1-4344-96fb-dcf969776696
Apr 12 21:44:16 centos65 setroubleshoot: SELinux is preventing /usr/sbin/h\
ttpd from getattr access on the file /var/www/html/test42.txt. For complete \
SELinux messages. run sealert -l b2a84386-54c1-4344-96fb-dcf969776696

So we run the command it suggests...
[root@centos65 ~]# sealert -l b2a84386-54c1-4344-96fb-dcf969776696
SELinux is preventing /usr/sbin/httpd from getattr access on the file /va\
r/www/html/test42.txt.

***** Plugin restorecon (92.2 confidence) suggests *********************

If you want to fix the label.
/var/www/html/test42.txt default label should be httpd_sys_content_t.
Then you can run restorecon.
Do
# /sbin/restorecon -v /var/www/html/test42.txt
...

110
introduction to SELinux

We follow the friendly advice and try again to download our file:
[root@centos65 ~]# /sbin/restorecon -v /var/www/html/test42.txt
/sbin/restorecon reset /var/www/html/test42.txt context unconfined_u:objec\
t_r:samba_share_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
[root@centos65 ~]# wget http://localhost/test42.txt
--2014-04-12 21:54:03-- http://localhost/test42.txt
Resolving localhost... ::1, 127.0.0.1
Connecting to localhost|::1|:80... connected.
HTTP request sent, awaiting response... 200 OK

It works!

111
introduction to SELinux

12.23. booleans
Booleans are on/off switches
[root@centos65 ~]# getsebool -a | head
abrt_anon_write --> off
abrt_handle_event --> off
allow_console_login --> on
allow_cvs_read_shadow --> off
allow_daemons_dump_core --> on
allow_daemons_use_tcp_wrapper --> off
allow_daemons_use_tty --> on
allow_domain_fd_use --> on
allow_execheap --> off
allow_execmem --> on

You can set and read individual booleans.
[root@centos65 ~]# setsebool httpd_read_user_content=1
[root@centos65 ~]# getsebool httpd_read_user_content
httpd_read_user_content --> on
[root@centos65 ~]# setsebool httpd_enable_homedirs=1
[root@centos65 ~]# getsebool httpd_enable_homedirs
httpd_enable_homedirs --> on

You can set these booleans permanent.
[root@centos65 ~]# setsebool -P httpd_enable_homedirs=1
[root@centos65 ~]# setsebool -P httpd_read_user_content=1

The above commands regenerate the complete /etc/selinux/targeted directory!
[root@centos65 ~]# cat /etc/selinux/targeted/modules/active/booleans.local
# This file is auto-generated by libsemanage
# Do not edit directly.

httpd_enable_homedirs=1
httpd_read_user_content=1

112
Part V. Appendix
Table of Contents
A. License .................................................................................................................................................... 115

114
Appendix A. License
GNU Free Documentation License

Version 1.3, 3 November 2008

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.

This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense. It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals; it
can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein. The "Document", below, refers
to any such manual or work. Any member of the public is a licensee,
and is addressed as "you". You accept the license if you copy, modify
or distribute the work in a way requiring permission under copyright
law.

A "Modified Version" of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A "Secondary Section" is a named appendix or a front-matter section of
the Document that deals exclusively with the relationship of the
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subject (or to related matters) and contains nothing that could fall
directly within that overall subject. (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The "Invariant Sections" are certain Secondary Sections whose titles

115
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are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License. If a
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allowed to be designated as Invariant. The Document may contain zero
Invariant Sections. If the Document does not identify any Invariant
Sections then there are none.

The "Cover Texts" are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License. A Front-Cover Text may be
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A "Transparent" copy of the Document means a machine-readable copy,
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The "Title Page" means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page. For works in
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the Document to the public.

A section "Entitled XYZ" means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
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specific section name mentioned below, such as "Acknowledgements",
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of such a section when you modify the Document means that it remains a
section "Entitled XYZ" according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either

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commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no
other conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
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You may add other material on the covers in addition. Copying with
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copying in other respects.

If the required texts for either cover are too voluminous to fit
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reasonably) on the actual cover, and continue the rest onto adjacent
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If you publish or distribute Opaque copies of the Document numbering
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It is requested, but not required, that you contact the authors of the
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4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

* A. Use in the Title Page (and on the covers, if any) a title
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version if the original publisher of that version gives permission.

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* B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in the
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of the Document (all of its principal authors, if it has fewer than
five), unless they release you from this requirement.
* C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.
* D. Preserve all the copyright notices of the Document.
* E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.
* F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified Version under
the terms of this License, in the form shown in the Addendum below.
* G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's license
notice.
* H. Include an unaltered copy of this License.
* I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new authors,
and publisher of the Modified Version as given on the Title Page. If
there is no section Entitled "History" in the Document, create one
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* J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and likewise
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version it refers to gives permission.
* K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
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* L. Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
* M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.
* N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant Section.
* O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,

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you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled "History"
in the various original documents, forming one section Entitled
"History"; likewise combine any sections Entitled "Acknowledgements",
and any sections Entitled "Dedications". You must delete all sections
Entitled "Endorsements".

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the rules
of this License for verbatim copying of each of the documents in all
other respects.

You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert a
copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an "aggregate" if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

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8. TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt otherwise
to copy, modify, sublicense, or distribute it is void, and will
automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
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Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the
GNU Free Documentation License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
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number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation. If the Document specifies

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that a proxy can decide which future versions of this License can be
used, that proxy's public statement of acceptance of a version
permanently authorizes you to choose that version for the Document.

11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server. A
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means any set of copyrightable works thus published on the MMC site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or in
part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.

121
D
Index df -i, 68
dhclient, 81
Symbols directory, 69
/bin/bash, 30 DNAT, 78
/bin/ksh, 30
/etc/bashrc, 31 E
/etc/default/useradd, 14 eiciel, 66
/etc/fstab, 64
/etc/group, 37, 46 F
/etc/gshadow, 39 file ownership, 46
/etc/inputrc, 30 filter table (iptables), 86
/etc/login.defs, 24 find(1), 58, 59, 70
/etc/passwd, 13, 16, 25, 25, 46 firewall, 77
/etc/profile, 30
/etc/selinux/config, 102
/etc/shadow, 21, 23, 59 G
/etc/shells, 16 gcc(1), 23
/etc/skel, 15 getenforce, 101
/etc/sudoers, 7, 8 getfacl, 64
/etc/sysctl.conf, 79 getfattr(1), 107
/proc/sys/net/ipv4/ip_forward, 79 GID, 37
/selinux, 104 gpasswd, 39
/selinux/enforce, 104 groupadd(1), 37
/tmp, 58 groupdel(1), 38
/usr/bin/getfacl, 64 groupmod(1), 38
/usr/bin/passwd, 59 groups, 36
/usr/bin/setfacl, 64 groups(1), 37
/var/log/audit/audit.log, 100
.. (directory), 69 H
. (directory), 69 hard link, 70
.bash_login, 31
.bash_logout, 32 I
.bash_profile, 30
id, 5
.bashrc, 30, 31
id(1), 106
777, 52
identity(selinux), 104
inode, 67, 70
A inode table, 68
access control list, 64 iptables, 85, 86
acl, 66 iptables save, 90
acls, 64
auditd, 100
K
Korn Shell, 16
C
chage, 24
chain (iptables), 86
L
chcon(1), 106, 107 ln, 71
chgrp(1), 47 ln(1), 70
chkconfig, 100 ls, 49, 68, 103
chmod, 15, 52, 103 ls(1), 68, 69, 107
chmod(1), 51 ls -l, 48
chmod +x, 53
chown, 15 M
chown(1), 47 mac address, 80
chsh(1), 16 mangle table (iptables), 86
context type(selinux), 105 masquerading, 78
crypt, 22 md5, 23

122
Index

mkdir, 15 U
mkdir(1), 53
umask(1), 53
mkfs, 68
useradd, 14, 15, 22
useradd(1), 15
N useradd -D, 14
NAPT, 78 userdel(1), 14
NAT, 78 usermod, 25, 25, 38
nat table (iptables), 86 usermod(1), 14

O V
octal permissions, 52 vi, 40
openssl, 22 vigr(1), 40
owner, 49 vipw, 25
virtualbox, 80
P visudo, 7
packet filtering, 77, 87 vmware, 80
packet forwarding, 77
passwd, 21, 21, 22, 24 W
passwd(1), 59 w, 5
PAT, 78 who, 5
ping, 80, 81 whoami, 5
port forwarding, 78 who am i, 5
primary group, 14 wireshark, 81
ps(1), 107

R
rm(1), 71
role(selinux), 104
root, 6, 7, 8, 13
router, 77

S
salt (encryption), 23
SELinux, 99
selinux, 102
selinux-activate, 100
sestatus, 102
setenforce, 101
setfacl, 64
setgid, 58, 58
setuid, 59, 59, 59
shell, 29
SNAT, 78
soft link, 71
stateful firewall, 77
sticky bit, 58
su, 6, 6, 25, 39
sudo, 7, 8, 25
sudo su -, 8
superuser, 13
symbolic link, 71
sysctl, 79

T
tcpdump, 81
transition(selinux), 106
type(selinux), 105

123