A Detailed Documentation on How to Set up Ceph Kerberos Authentication

This document provides details on the Kerberos authorization protocol. This is the 1st draft and we will try to keep it updated along with code changes that might take place.

Several free implementations of this protocol are available (MIT, Heimdal, MS…), covering a wide range of operating systems. The Massachusetts Institute of Technology (MIT), where Kerberos was originally developed, continues to develop their Kerberos package and it is the implementation we chose to work with. MIT Kerberos.

Please, provide feedback to Daniel Oliveira (doliveira@suse.com)

Last update: Dec 3, 2018


Background

Before we get into Kerberos details, let us define a few terms so we can understand what to expect from it, what it can and can’t do:

Directory Services

A directory service is a customizable information store that functions as a single point from which users can locate resources and services distributed throughout the network. This customizable information store also gives administrators a single point for managing its objects and their attributes. Although this information store appears as a single point to the users of the network, it is actually most often stored in a distributed form. A directory service consists of at least one Directory Server and a Directory Client and are implemented based on X.500 standards.

OpenLDAP, 389 Directory Server, MS Active Directory, NetIQ eDirectory are some good examples.

A directory service is often characterized as a write-once-read-many-times service, meaning the data that would normally be stored in an directory service would not be expected to change on every access.

The database that forms a directory service is not designed for transactional data.


LDAP (Lightweight Directory Access Protocol v3)

LDAP is a set of LDAP Protocol Exchanges (not an implementation of a server) that defines the method by which data is accessed. LDAPv3 is a standard defined by the IETF in RFC 2251 and describes how data is represented in the Directory Service (the Data Model or DIT).

Finally, it defines how data is loaded into (imported) and saved from (exported) a directory service (using LDIF). LDAP does not define how data is stored or manipulated. Data Store is an ‘automagic’ process as far as the standard is concerned and is generally handled by back-end modules.

No Directory Service implementation has all the features of LDAP v3 protocol implemented. All Directory Server implementations have their different problems and/or anomalies, and features that may not return results as another Directory Server implementation would.


Authentication

Authentication is about validating credentials (like User Name/ID and password) to verify the identity. The system determines whether one is what they say they are using their credentials.

Usually, authentication is done by a username and password, and sometimes in conjunction with (single, two, or multi) factors of authentication, which refers to the various ways to be authenticated.


Authorization

Authorization occurs after the identity is successfully authenticated by the system, which ultimately gives one full permission to access the resources such as information, files, databases, and so forth, almost anything. It determines the ability to access the system and up to what extent (what kind of permissions/rights are given and to where/what).


Auditing

Auditing takes the results from both authentication and authorization and records them into an audit log. The audit log records records all actions taking by/during the authentication and authorization for later review by the administrators. While authentication and authorization are preventive systems (in which unauthorized access is prevented), auditing is a reactive system (in which it gives detailed log of how/when/where someone accessed the environment).


Kerberos (KRB v5)

Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key cryptography (symmetric key). A free implementation of this protocol is available from the MIT. However, Kerberos is available in many commercial products as well.

It was designed to provide secure authentication to services over an insecure network. Kerberos uses tickets to authenticate a user, or service application and never transmits passwords over the network in the clear. So both client and server can prove their identity without sending any unencrypted secrets over the network.

Kerberos can be used for single sign-on (SSO). The idea behind SSO is simple, we want to login just once and be able to use any service that we are entitled to, without having to login on each of those services.


Simple Authentication and Security Layer (SASL)

SASL (RFC 4422) is a framework that helps developers to implement different authentication mechanisms (implementing a series of challenges and responses), allowing both clients and servers to negotiate a mutually acceptable mechanism for each connection, instead of hard-coding them.

Examples of SASL mechanisms:

  • ANONYMOUS (RFC 4505)

    • For guest access, meaning unauthenticated

  • CRAM-MD5 (RFC 2195)

    • Simple challenge-response scheme based on HMAC-MD5. It does not establish any security layer. Less secure than DIGEST-MD5 and GSSAPI.

  • DIGEST-MD5 (RFC 2831)

    • HTTP Digest compatible (partially) challenge-response scheme based upon MD5, offering a data security layer. It is preferred over PLAIN text passwords, protecting against plain text attacks. It is a mandatory authentication method for LDAPv3 servers.

  • EXTERNAL (RFCs 4422, 5246, 4301, 2119)

    • Where authentication is implicit in the context (i.e; for protocols using IPsec or TLS [TLS/SSL to performing certificate- based authentication] already). This method uses public keys for strong authentication.

  • GS2 (RFC 5801)

    • Family of mechanisms supports arbitrary GSS-API mechanisms in SASL

  • NTLM (MS Proprietary)

    • MS Windows NT LAN Manager authentication mechanism

  • OAuth 1.0/2.0 (RFCs 5849, 6749, 7628)

    • Authentication protocol for delegated resource access

  • OTP (RFC 2444)

    • One-time password mechanism (obsoletes the SKEY mechanism)

  • PLAIN (RFC 4616)

    • Simple Cleartext password mechanism (RFC 4616). This is not a preferred mechanism for most applications because of its relative lack of strength.

  • SCRAM (RFCs 5802, 7677)

    • Modern challenge-response scheme based mechanism with channel binding support


Generic Security Services Application Program Interface (GSSAPI)

GSSAPI (RFCs 2078, 2743, 2744, 4121, 4752) is widely used by protocol implementers as a way to implement Kerberos v5 support in their applications. It provides a generic interface and message format that can encapsulate authentication exchanges from any authentication method that has a GSSAPI-compliant library.

It does not define a protocol, authentication, or security mechanism itself; it instead makes it easier for application programmers to support multiple authentication mechanisms by providing a uniform, generic API for security services. It is a set of functions that include both an API and a methodology for approaching authentication, aiming to insulate application protocols from the specifics of security protocols as much as possible.

Microsoft Windows Kerberos implementation does not include GSSAPI support but instead includes a Microsoft-specific API, the Security Support Provider Interface (SSPI). In Windows, an SSPI client can communicate with a GSSAPI server.

Most applications that support GSSAPI also support Kerberos v5.


Simple and Protected GSSAPI Negotiation Mechanism (SPNEGO)

As we can see, GSSAPI solves the problem of providing a single API to different authentication mechanisms. However, it does not solve the problem of negotiating which mechanism to use. In fact for GSSAPI to work, the two applications communicating with each other must know in advance what authentication mechanism they plan to use, which usually is not a problem if only one mechanism is supported (meaning Kerberos v5).

However, if there are multiple mechanisms to choose from, a method is needed to securely negotiate an authentication mechanism that is mutually supported between both client and server; which is where SPNEGO (RFC 2478, 4178) makes a difference.

SPNEGO provides a framework for two parties that are engaged in authentication to select from a set of possible authentication mechanisms, in a manner that preserves the opaque nature of the security protocols to the application protocol that uses it.

It is a security protocol that uses a GSSAPI authentication mechanism and negotiates among several available authentication mechanisms in an implementation, selecting one for use to satisfy the authentication needs of the application protocol.

It is a meta protocol that travels entirely in other application protocols; it is never used directly without an application protocol.


Why is this important and why do we care? Like, at all?

Having this background information in mind, we can easily describe things like:

1. Ceph Kerberos authentication is based totally on MIT Kerberos implementation using GSSAPI.

2. At the moment we are still using Kerberos default backend database, however we plan on adding LDAP as a backend which would provide us with authentication with GSSAPI (KRB5) and authorization with LDAP (LDAPv3), via SASL mechanism.


Before We Start

We assume the environment already has some external services up and running properly:

  • Kerberos needs to be properly configured, which also means (for both every server and KDC):

    • Time Synchronization (either using NTP or chrony).

      • Not only Kerberos, but also Ceph depends and relies on time synchronization.

    • DNS resolution

      • Both (forward and reverse) zones, with fully qualified domain name (fqdn) (hostname + domain.name)

      • KDC discover can be set up to use DNS (srv resources) as service location protocol (RFCs 2052, 2782), as well as host or domain to the appropriate realm (txt record).

      • Even though these DNS entries/settings are not required to run a Kerberos realm, they certainly help to eliminate the need for manual configuration on all clients.

      • This is extremely important, once most of the Kerberos issues are usually related to name resolution. Kerberos is very picky when checking on systems names and host lookups.

  • Whenever possible, in order to avoid a single point of failure, set up a backup, secondary, or slave, for every piece/part in the infrastructure (ntp, dns, and kdc servers).

Also, the following Kerberos terminology is important:

  • Ticket

    • Tickets or Credentials, are a set of information that can be used to verify the client’s identity. Kerberos tickets may be stored in a file, or they may exist only in memory.

    • The first ticket obtained is a ticket-granting ticket (TGT), which allows the clients to obtain additional tickets. These additional tickets give the client permission for specific services. The requesting and granting of these additional tickets happens transparently.

      • The TGT, which expires at a specified time, permits the client to obtain additional tickets, which give permission for specific services. The requesting and granting of these additional tickets is user-transparent.

  • Key Distribution Center (KDC).

    • The KDC creates a ticket-granting ticket (TGT) for the client, encrypts it using the client’s password as the key, and sends the encrypted TGT back to the client. The client then attempts to decrypt the TGT, using its password. If the client successfully decrypts the TGT (i.e., if the client gave the correct password), it keeps the decrypted TGT, which indicates proof of the client’s identity.

    • The KDC is comprised of three components:

      • Kerberos database, which stores all the information about the principals and the realm they belong to, among other things.

      • Authentication service (AS)

      • Ticket-granting service (TGS)

  • Client

    • Either a user, host or a service who sends a request for a ticket.

  • Principal

    • It is a unique identity to which Kerberos can assign tickets. Principals can have an arbitrary number of components. Each component is separated by a component separator, generally /. The last component is the realm, separated from the rest of the principal by the realm separator, generally @.

    • If there is no realm component in the principal, then it will be assumed that the principal is in the default realm for the context in which it is being used.

    • Usually, a principal is divided into three parts:

      • The primary, the instance, and the realm

      • The format of a typical Kerberos V5 principal is primary/instance@REALM.

      • The primary is the first part of the principal. In the case of a user, it’s the same as the username. For a host, the primary is the word host. For Ceph, will use ceph as a primary name which makes it easier to organize and identify Ceph related principals.

      • The instance is an optional string that qualifies the primary. The instance is separated from the primary by a slash /. In the case of a user, the instance is usually null, but a user might also have an additional principal, with an instance called admin, which one uses to administrate a database.

        The principal johndoe@MYDOMAIN.COM is completely separate from the principal johndoe/admin@MYDOMAIN.COM, with a separate password, and separate permissions. In the case of a host, the instance is the fully qualified hostname, i.e., osd1.MYDOMAIN.COM.

      • The realm is the Kerberos realm. Usually, the Kerberos realm is the domain name, in upper-case letters. For example, the machine osd1.MYDOMAIN.COM would be in the realm MYDOMAIN.COM.

  • Keytab

    • A keytab file stores the actual encryption key that can be used in lieu of a password challenge for a given principal. Creating keytab files are useful for noninteractive principals, such as Service Principal Names, which are often associated with long-running processes like Ceph daemons. A keytab file does not have to be a “1:1 mapping” to a single principal. Multiple different principal keys can be stored in a single keytab file:

      • The keytab file allows a user/service to authenticate without knowledge of the password. Due to this, keytabs should be protected with appropriate controls to prevent unauthorized users from authenticating with it.

      • The default client keytab file is /etc/krb5.keytab


The ‘Ceph side’ of the things

In order to configure connections (from Ceph nodes) to the KDC:

  1. Login to the Kerberos client (Ceph server nodes) and confirm it is properly configured, by checking and editing /etc/krb5.conf file properly:

    /etc/krb5.conf
[libdefaults]
    dns_canonicalize_hostname = false
    rdns = false
    forwardable = true
    dns_lookup_realm = true
    dns_lookup_kdc = true
    allow_weak_crypto = false
    default_realm = MYDOMAIN.COM
    default_ccache_name = KEYRING:persistent:%{uid}
[realms]
    MYDOMAIN.COM = {
        kdc = kerberos.mydomain.com
        admin_server = kerberos.mydomain.com
        ...
    }
...

  2. Login to the KDC Server and confirm it is properly configured to authenticate to the Kerberos realm in question:

    1. Kerberos related DNS RRs:

      /var/lib/named/master/mydomain.com
kerberos                IN A        192.168.10.21
kerberos-slave          IN A        192.168.10.22
_kerberos               IN TXT      "MYDOMAIN.COM"
_kerberos._udp          IN SRV      1 0 88 kerberos
_kerberos._tcp          IN SRV      1 0 88 kerberos
_kerberos._udp          IN SRV      20 0 88 kerberos-slave
_kerberos-master._udp   IN SRV      0 0 88 kerberos
_kerberos-adm._tcp      IN SRV      0 0 749 kerberos
_kpasswd._udp           IN SRV      0 0 464 kerberos
...

    2. KDC configuration file:

      /var/lib/kerberos/krb5kdc/kdc.conf
[kdcdefaults]
        kdc_ports = 750,88
[realms]
        MYDOMAIN.COM = {
            acl_file = /var/lib/kerberos/krb5kdc/kadm5.acl
            admin_keytab = FILE:/var/lib/kerberos/krb5kdc/kadm5.keytab
            default_principal_flags = +postdateable +forwardable +renewable +proxiable
                                                    +dup-skey -preauth -hwauth +service
                                                    +tgt-based +allow-tickets -pwchange
                                                    -pwservice
            dict_file = /var/lib/kerberos/krb5kdc/kadm5.dict
            key_stash_file = /var/lib/kerberos/krb5kdc/.k5.MYDOMAIN.COM
            kdc_ports = 750,88
            max_life = 0d 10h 0m 0s
            max_renewable_life = 7d 0h 0m 0s
        }
...

  3. Still on the KDC Server, run the Kerberos administration utility; kadmin.local so we can list all the principals already created.

    kadmin.local:  listprincs
K/M@MYDOMAIN.COM
krbtgt/MYDOMAIN.COM@MYDOMAIN.COM
kadmin/admin@MYDOMAIN.COM
kadmin/changepw@MYDOMAIN.COM
kadmin/history@MYDOMAIN.COM
kadmin/kerberos.mydomain.com@MYDOMAIN.COM
root/admin@MYDOMAIN.COM
...

  4. Add a principal for each Ceph cluster node we want to be authenticated by Kerberos:

    1. Adding principals:

      kadmin.local:  addprinc -randkey ceph/ceph-mon1
Principal "ceph/ceph-mon1@MYDOMAIN.COM" created.
kadmin.local:  addprinc -randkey ceph/ceph-osd1
Principal "ceph/ceph-osd1@MYDOMAIN.COM" created.
kadmin.local:  addprinc -randkey ceph/ceph-osd2
Principal "ceph/ceph-osd2@MYDOMAIN.COM" created.
kadmin.local:  addprinc -randkey ceph/ceph-osd3
Principal "ceph/ceph-osd3@MYDOMAIN.COM" created.
kadmin.local:  addprinc -randkey ceph/ceph-osd4
Principal "ceph/ceph-osd4@MYDOMAIN.COM" created.
kadmin.local:  listprincs
K/M@MYDOMAIN.COM
krbtgt/MYDOMAIN.COM@MYDOMAIN.COM
kadmin/admin@MYDOMAIN.COM
kadmin/changepw@MYDOMAIN.COM
kadmin/history@MYDOMAIN.COM
kadmin/kerberos.mydomain.com@MYDOMAIN.COM
root/admin@MYDOMAIN.COM
ceph/ceph-mon1@MYDOMAIN.COM
ceph/ceph-osd1@MYDOMAIN.COM
ceph/ceph-osd2@MYDOMAIN.COM
ceph/ceph-osd3@MYDOMAIN.COM
ceph/ceph-osd4@MYDOMAIN.COM
...

    2. This follows the same idea if we are creating a user principal

      kadmin.local:  addprinc johndoe
WARNING: no policy specified for johndoe@MYDOMAIN.COM; defaulting to no policy
Enter password for principal "johndoe@MYDOMAIN.COM":
Re-enter password for principal "johndoe@MYDOMAIN.COM":
Principal "johndoe@MYDOMAIN.COM" created.
...

  5. Create a keytab file for each Ceph cluster node:

    As the default client keytab file is /etc/krb5.keytab, we will want to use a different file name, so we especify which keytab file to create and which principal to export keys from:

    kadmin.local:  ktadd -k /etc/gss_client_mon1.ktab ceph/ceph-mon1
Entry for principal ceph/ceph-mon1 with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_mon1.ktab.
Entry for principal ceph/ceph-mon1 with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_mon1.ktab.
Entry for principal ceph/ceph-mon1 with kvno 2, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/gss_client_mon1.ktab.
Entry for principal ceph/ceph-mon1 with kvno 2, encryption type arcfour-hmac added to keytab WRFILE:/etc/gss_client_mon1.ktab.
kadmin.local:  ktadd -k /etc/gss_client_osd1.ktab ceph/ceph-osd1
Entry for principal ceph/ceph-osd1 with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd1.ktab.
Entry for principal ceph/ceph-osd1 with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd1.ktab.
Entry for principal ceph/ceph-osd1 with kvno 2, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/gss_client_osd1.ktab.
Entry for principal ceph/ceph-osd1 with kvno 2, encryption type arcfour-hmac added to keytab WRFILE:/etc/gss_client_osd1.ktab.
kadmin.local:  ktadd -k /etc/gss_client_osd2.ktab ceph/ceph-osd2
Entry for principal ceph/ceph-osd2 with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd2.ktab.
Entry for principal ceph/ceph-osd2 with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd2.ktab.
Entry for principal ceph/ceph-osd2 with kvno 2, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/gss_client_osd2.ktab.
Entry for principal ceph/ceph-osd2 with kvno 2, encryption type arcfour-hmac added to keytab WRFILE:/etc/gss_client_osd2.ktab.
kadmin.local:  ktadd -k /etc/gss_client_osd3.ktab ceph/ceph-osd3
Entry for principal ceph/ceph-osd3 with kvno 3, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd3.ktab.
Entry for principal ceph/ceph-osd3 with kvno 3, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd3.ktab.
Entry for principal ceph/ceph-osd3 with kvno 3, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/gss_client_osd3.ktab.
Entry for principal ceph/ceph-osd3 with kvno 3, encryption type arcfour-hmac added to keytab WRFILE:/etc/gss_client_osd3.ktab.
kadmin.local:  ktadd -k /etc/gss_client_osd4.ktab ceph/ceph-osd4
Entry for principal ceph/ceph-osd4 with kvno 4, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd4.ktab.
Entry for principal ceph/ceph-osd4 with kvno 4, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/gss_client_osd4.ktab.
Entry for principal ceph/ceph-osd4 with kvno 4, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/gss_client_osd4.ktab.
Entry for principal ceph/ceph-osd4 with kvno 4, encryption type arcfour-hmac added to keytab WRFILE:/etc/gss_client_osd4.ktab.

# ls -1 /etc/gss_client_*
/etc/gss_client_mon1.ktab
/etc/gss_client_osd1.ktab
/etc/gss_client_osd2.ktab
/etc/gss_client_osd3.ktab
/etc/gss_client_osd4.ktab

    We can also check these newly created keytab client files by:

    # klist -kte /etc/gss_client_mon1.ktab
Keytab name: FILE:/etc/gss_client_mon1.ktab
KVNO Timestamp           Principal
---- ------------------- ------------------------------------------------------
   2 10/8/2018 14:35:30 ceph/ceph-mon1@MYDOMAIN.COM (aes256-cts-hmac-sha1-96)
   2 10/8/2018 14:35:31 ceph/ceph-mon1@MYDOMAIN.COM (aes128-cts-hmac-sha1-96)
   2 10/8/2018 14:35:31 ceph/ceph-mon1@MYDOMAIN.COM (des3-cbc-sha1)
   2 10/8/2018 14:35:31 ceph/ceph-mon1@MYDOMAIN.COM (arcfour-hmac)
...

  6. A new set parameter was added in Ceph, gss_ktab_client_file which points to the keytab file related to the Ceph node (or principal) in question.

    By default it points to /var/lib/ceph/$name/gss_client_$name.ktab. So, in the case of a Ceph server osd1.mydomain.com, the location and name of the keytab file should be: /var/lib/ceph/osd1/gss_client_osd1.ktab

    Therefore, we need to scp each of these newly created keytab files from the KDC to their respective Ceph cluster nodes (i.e): # for node in mon1 osd1 osd2 osd3 osd4; do scp /etc/gss_client_$node*.ktab root@ceph-$node:/var/lib/ceph/$node/; done

    Or whatever other way one feels comfortable with, as long as each keytab client file gets copied over to the proper location.

    At this point, even without using any keytab client file we should be already able to authenticate a user principal:

    # kdestroy -A && kinit -f johndoe && klist -f
Password for johndoe@MYDOMAIN.COM:
Ticket cache: KEYRING:persistent:0:0
Default principal: johndoe@MYDOMAIN.COM

Valid starting       Expires              Service principal
10/10/2018 15:32:01  10/11/2018 07:32:01  krbtgt/MYDOMAIN.COM@MYDOMAIN.COM
    renew until 10/11/2018 15:32:01, Flags: FRI
...

    Given that the keytab client file is/should already be copied and available at the Kerberos client (Ceph cluster node), we should be able to athenticate using it before going forward:

    # kdestroy -A && kinit -k -t /etc/gss_client_mon1.ktab -f 'ceph/ceph-mon1@MYDOMAIN.COM' && klist -f
Ticket cache: KEYRING:persistent:0:0
Default principal: ceph/ceph-mon1@MYDOMAIN.COM

Valid starting       Expires              Service principal
10/10/2018 15:54:25  10/11/2018 07:54:25  krbtgt/MYDOMAIN.COM@MYDOMAIN.COM
    renew until 10/11/2018 15:54:25, Flags: FRI
...

  7. The default client keytab is used, if it is present and readable, to automatically obtain initial credentials for GSSAPI client applications. The principal name of the first entry in the client keytab is used by default when obtaining initial credentials:

    1. The KRB5_CLIENT_KTNAME environment variable.

    2. The default_client_keytab_name profile variable in [libdefaults].

    3. The hardcoded default, DEFCKTNAME.

    So, what we do is to internally, set the environment variable KRB5_CLIENT_KTNAME to the same location as gss_ktab_client_file, so /var/lib/ceph/osd1/gss_client_osd1.ktab, and change the ceph.conf file to add the new authentication method.

    /etc/ceph/ceph.conf
[global]
    ...
    auth_cluster_required = gss
    auth_service_required = gss
    auth_client_required = gss
    gss_ktab_client_file = /{$my_new_location}/{$my_new_ktab_client_file.keytab}
    ...

  8. With that the GSSAPIs will then be able to read the keytab file and using the process of name and service resolution (provided by the DNS), able to request a TGT as follows:

    1. User/Client sends principal identity and credentials to the KDC Server (TGT request).

    2. KDC checks its internal database for the principal in question.

    3. a TGT is created and wrapped by the KDC, using the principal’s key (TGT + Key).

    4. The newly created TGT, is decrypted and stored in the credentials cache.

    5. At this point, Kerberos/GSSAPI aware applications (and/or services) are able to check the list of active TGT in the keytab file.



** For Ceph Developers Only **

We certainly could have used straight native KRB5 APIs (instead of GSSAPIs), but we wanted a more portable option as regards network security, which is the hallmark of the GSS (Generic Security Standard) -API. It does not actually provide security services itself.

Rather, it is a framework that provides security services to callers in a generic way.

+---------------------------------+
|        Application              |
+---------------------------------+
| Protocol (RPC, Etc. [Optional]) |
+---------------------------------+
|         GSS-API                 |
+---------------------------------+
|   Security Mechs (Krb v5, Etc)  |
+---------------------------------+

The GSS-API does two main things:

  1. It creates a security context in which data can be passed between applications. A context can be thought of as a sort of “state of trust” between two applications.

    Applications that share a context know who each other are and thus can permit data transfers between them as long as the context lasts.

  2. It applies one or more types of protection, known as “security services”, to the data to be transmitted.

GSS-API provides several types of portability for applications:

  1. Mechanism independence. GSS-API provides a generic interface to the mechanisms for which it has been implemented. By specifying a default security mechanism, an application does not need to know which mechanism it is using (for example, Kerberos v5), or even what type of mechanism it uses. As an example, when an application forwards a user’s credential to a server, it does not need to know if that credential has a Kerberos format or the format used by some other mechanism, nor how the credentials are stored by the mechanism and accessed by the application. (If necessary, an application can specify a particular mechanism to use)

  2. Protocol independence. The GSS-API is independent of any communications protocol or protocol suite. It can be used with applications that use, for example, sockets, RCP, or TCP/IP. RPCSEC_GSS “RPCSEC_GSS Layer” is an additional layer that smoothly integrates GSS-API with RPC.

  3. Platform independence. The GSS-API is completely oblivious to the type of operating system on which an application is running.

  4. Quality of Protection independence. Quality of Protection (QOP) is the name given to the type of algorithm used in encrypting data or generating cryptographic tags; the GSS-API allows a programmer to ignore QOP, using a default provided by the GSS-API. (On the other hand, an application can specify the QOP if necessary.)

The basic security offered by the GSS-API is authentication. Authentication is the verification of an identity: if you are authenticated, it means that you are recognized to be who you say you are.

The GSS-API provides for two additional security services, if supported by the underlying mechanisms:

  1. Integrity: It’s not always sufficient to know that an application sending you data is who it claims to be. The data itself could have become corrupted or compromised.

    The GSS-API provides for data to be accompanied by a cryptographic tag, known as an Message Integrity Code (MIC), to prove that the data that arrives at your doorstep is the same as the data that the sender transmitted. This verification of the data’s validity is known as “integrity”.

  2. Confidentiality: Both authentication and integrity, however, leave the data itself alone, so if it’s somehow intercepted, others can read it.

    The GSS-API therefore allows data to be encrypted, if underlying mechanisms support it. This encryption of data is known as “confidentiality”.


Mechanisms Available With GSS-API:

The current implementation of the GSS-API works only with the Kerberos v5 security mechanism.

Mechanism Name          Object Identifier       Shared Library  Kernel Module
----------------------  ----------------------  --------------  --------------
diffie_hellman_640_0    1.3.6.4.1.42.2.26.2.4   dh640-0.so.1
diffie_hellman_1024_0   1.3.6.4.1.42.2.26.2.5   dh1024-0.so.1
SPNEGO                  1.3.6.1.5.5.2
iakerb                  1.3.6.1.5.2.5
SCRAM-SHA-1             1.3.6.1.5.5.14
SCRAM-SHA-256           1.3.6.1.5.5.18
GSS-EAP (arc)           1.3.6.1.5.5.15.1.1.*
kerberos_v5             1.2.840.113554.1.2.2    gl/mech_krb5.so gl_kmech_krb5

Therefore:
    Kerberos Version 5 GSS-API Mechanism
    OID {1.2.840.113554.1.2.2}

    Kerberos Version 5 GSS-API Mechanism
    Simple and Protected GSS-API Negotiation Mechanism
    OID {1.3.6.1.5.5.2}

There are two different formats:

  1. The first, { 1 2 3 4 }, is officially mandated by the GSS-API specs. gss_str_to_oid() expects this first format.

  2. The second, 1.2.3.4, is more widely used but is not an official standard format.

Although the GSS-API makes protecting data simple, it does not do certain things, in order to maximize its generic nature. These include:

  1. Provide security credentials for a user or application. These must be provided by the underlying security mechanism(s). The GSS-API does allow applications to acquire credentials, either automatically or explicitly.

  2. Transfer data between applications. It is the application’s responsibility to handle the transfer of all data between peers, whether it is security-related or “plain” data.

  3. Distinguish between different types of transmitted data (for example, to know or determine that a data packet is plain data and not GSS-API related).

  4. Indicate status due to remote (asynchronous) errors.

  5. Automatically protect information sent between processes of a multiprocess program.

  6. Allocate string buffers (“Strings and Similar Data”) to be passed to GSS-API functions.

  7. Deallocate GSS-API data spaces. These must be explicitly deallocated with functions such as gss_release_buffer() and gss_delete_name().


These are the basic steps in using the GSS-API:

  1. Each application, sender and recipient, acquires credentials explicitly, if credentials have not been acquired automatically.

  2. The sender initiates a security context and the recipient accepts it.

  3. The sender applies security protection to the message (data) it wants to transmit. This means that it either encrypts the message or stamps it with an identification tag. The sender transmits the protected message. (The sender can choose not to apply either security protection, in which case the message has only the default GSS-API security service associated with it. That is authentication, in which the recipient knows that the sender is who it claims to be.)

  4. The recipient decrypts the message (if needed) and verifies it (if appropriate).

  5. (Optional) The recipient returns an identification tag to the sender for confirmation.

  6. Both applications destroy the shared security context. If necessary, they can also deallocate any “leftover” GSS-API data.

Applications that use the GSS-API should include the file gssapi.h.

Good References:

** Kerberos Server Setup **

First and foremost, this is not a recommendation for a production environment. We are not covering Master/Slave replication cluster or anything production environment related (ntp/chrony, dns, pam/nss, sssd, etc).

Also, on the server side there might be different dependencies and/or configuration steps needed, depending on which backend database will be used. LDAP as a backend database is a good example of that.

On the client side there are different steps depending on which client backend configuration will be used. For example PAM/NSS or SSSD (along with LDAP for identity service, [and Kerberos for authentication service]) which is the best suited option for joining MS Active Directory domains, and doing User Logon Management.

By no means we intend to cover every possible scenario/combination here. These steps are for a simple get a (MIT) Kerberos Server up and running.

Please, note that rpm packages might have slightly different names, as well as the locations for the binaries and/or configuration files, depending on which Linux distro we are referring to.

Finally, keep in mind that some Linux distros will have their own wizards, which can perform the basic needed configuration:

SUSE:
    Kerberos server:
        yast2 auth-server

    Kerberos client:
        pam/nss: yast2 ldapkrb
        sssd: yast2 auth-client

However, we are going through the manual configuration.

In order to get a new MIT KDC Server running:

  1. Install the KDC server by:

    1. Install the needed packages:

      SUSE: zypper install krb5 krb5-server krb5-client
    Additionally:
        for development: krb5-devel
        if using 'sssd': sssd-krb5 sssd-krb5-common

REDHAT: yum install krb5-server krb5-libs krb5-workstation
    Additionally: 'Needs to be checked'

    2. Edit the KDC Server configuration file:

      /var/lib/kerberos/krb5kdc/kdc.conf
[kdcdefaults]
        kdc_ports = 750,88
[realms]
        MYDOMAIN.COM = {
            acl_file = /var/lib/kerberos/krb5kdc/kadm5.acl
            admin_keytab = FILE:/var/lib/kerberos/krb5kdc/kadm5.keytab
            default_principal_flags = +postdateable +forwardable +renewable +proxiable
                                                    +dup-skey -preauth -hwauth +service
                                                    +tgt-based +allow-tickets -pwchange
                                                    -pwservice
            dict_file = /var/lib/kerberos/krb5kdc/kadm5.dict
            key_stash_file = /var/lib/kerberos/krb5kdc/.k5.MYDOMAIN.COM
            kdc_ports = 750,88
            max_life = 0d 10h 0m 0s
            max_renewable_life = 7d 0h 0m 0s
        }
...

    3. Edit the Kerberos Client configuration file:

      /etc/krb5.conf
[libdefaults]
    dns_canonicalize_hostname = false
    rdns = false
    forwardable = true
    dns_lookup_realm = true     //--> if using DNS/DNSMasq
    dns_lookup_kdc = true       //--> if using DNS/DNSMasq
    allow_weak_crypto = false
    default_realm = MYDOMAIN.COM
    default_ccache_name = KEYRING:persistent:%{uid}

[realms]
    MYDOMAIN.COM = {
        kdc = kerberos.mydomain.com
        admin_server = kerberos.mydomain.com
        ...
    }
...

  2. Create the Kerberos database:

    SUSE: kdb5_util create -s

REDHAT: kdb5_util create -s

  3. Enable and Start both ‘KDC and KDC admin’ servers:

    SUSE: systemctl enable/start krb5kdc
      systemctl enable/start kadmind

REDHAT: systemctl enable/start krb5kdc
        systemctl enable/start kadmin
  4. Create a Kerberos Administrator

    Kerberos principals can be created either locally on the KDC server itself or through the network, using an ‘admin principal’. On the KDC server, using kadmin.local:

    1. List the existing principals:

      kadmin.local:  listprincs
K/M@MYDOMAIN.COM
krbtgt/MYDOMAIN.COM@MYDOMAIN.COM
kadmin/admin@MYDOMAIN.COM
kadmin/changepw@MYDOMAIN.COM
kadmin/history@MYDOMAIN.COM
kadmin/kerberos.mydomain.com@MYDOMAIN.COM
root/admin@MYDOMAIN.COM
...

    b. In case we don’t have a built-in ‘admin principal’, we then create one (whatever principal name, we are using root, once by default kinit tries to authenticate using the same system login user name, unless a principal is passed as an argument kinit principal):

    # kadmin.local -q "addprinc root/admin"
Authenticating as principal root/admin@MYDOMAIN.COM with password.
WARNING: no policy specified for root/admin@MYDOMAIN.COM; defaulting to no policy
Enter password for principal "root/admin@MYDOMAIN.COM":

    1. Confirm the newly created ‘admin principal’ has the needed permissions in the KDC ACL (if ACLs are changed, kadmind needs to be restarted):

      SUSE: /var/lib/kerberos/krb5kdc/kadm5.acl
REDHAT: /var/kerberos/krb5kdc/kadm5.acl

###############################################################################
#Kerberos_principal      permissions     [target_principal]      [restrictions]
###############################################################################
#
*/admin@MYDOMAIN.COM     *

    2. Create a simple ‘user principal’ (same steps as by The ‘Ceph side’ of the things; 4a):

      kadmin.local:  addprinc johndoe
WARNING: no policy specified for johndoe@MYDOMAIN.COM; defaulting to no policy
Enter password for principal "johndoe@MYDOMAIN.COM":
Re-enter password for principal "johndoe@MYDOMAIN.COM":
Principal "johndoe@MYDOMAIN.COM" created.

    3. Confirm the newly created ‘user principal’ is able to authenticate (same steps as by The ‘Ceph side’ of the things; 6):

      # kdestroy -A && kinit -f johndoe && klist -f
Password for johndoe@MYDOMAIN.COM:
Ticket cache: KEYRING:persistent:0:0
Default principal: johndoe@MYDOMAIN.COM

Valid starting       Expires              Service principal
11/16/2018 13:11:16  11/16/2018 23:11:16  krbtgt/MYDOMAIN.COM@MYDOMAIN.COM
        renew until 11/17/2018 13:11:16, Flags: FRI
...

  5. At this point, we should have a simple (MIT) Kerberos Server up and running:

    1. Considering we will want to work with keytab files, for both ‘user and service’ principals, refer to The ‘Ceph side’ of the things starting at step 4.

    2. Make sure you are comfortable with following and their manpages:

      krb5.conf       -> Krb client config file
kdc.conf        -> KDC server config file

krb5kdc         -> KDC server daemon
kadmind         -> KDC administration daemon

kadmin          -> Krb administration tool
kdb5_util       -> Krb low-level database administration tool

kinit           -> Obtain and cache Kerberos ticket-granting ticket tool
klist           -> List cached Kerberos tickets tool
kdestroy        -> Destroy Kerberos tickets tool
  6. Name Resolution

    As mentioned earlier, Kerberos relies heavly on name resolution. Most of the Kerberos issues are usually related to name resolution, since Kerberos is very picky on both systems names and host lookups.

    1. As described in The ‘Ceph side’ of the things; step 2a, DNS RRs greatly improves service location and host/domain resolution, by using (srv resources) and (txt record) respectively (as per Before We Start; DNS resolution).

      /var/lib/named/master/mydomain.com
kerberos                IN A        192.168.10.21
kerberos-slave          IN A        192.168.10.22
_kerberos               IN TXT      "MYDOMAIN.COM"
_kerberos._udp          IN SRV      1 0 88 kerberos
_kerberos._tcp          IN SRV      1 0 88 kerberos
_kerberos._udp          IN SRV      20 0 88 kerberos-slave
_kerberos-master._udp   IN SRV      0 0 88 kerberos
_kerberos-adm._tcp      IN SRV      0 0 749 kerberos
_kpasswd._udp           IN SRV      0 0 464 kerberos
...

    2. For a small network or development environment, where a DNS server is not available, we have the option to use DNSMasq, an ease-to-configure lightweight DNS server (along with some other capabilities).

      These records can be added to /etc/dnsmasq.conf (in addition to the needed ‘host records’):

      /etc/dnsmasq.conf
...
txt-record=_kerberos.mydomain.com,"MYDOMAIN.COM"
srv-host=_kerberos._udp.mydomain.com,kerberos.mydomain.com,88,1
srv-host=_kerberos._udp.mydomain.com,kerberos-2.mydomain.com,88,20
srv-host=_kerberos-master._udp.mydomain.com,kerberos.mydomain.com,88,0
srv-host=_kerberos-adm._tcp.mydomain.com,kerberos.mydomain.com,749,0
srv-host=_kpasswd._udp.mydomain.com,kerberos.mydomain.com,464,0
srv-host=_kerberos._tcp.mydomain.com,kerberos.mydomain.com,88,1
...

    3. After ‘b)’ is all set, and dnsmasq service up and running, we can test it using:

      # nslookup kerberos
Server:     192.168.10.1
Address:    192.168.10.1#53

Name:   kerberos.mydomain.com
Address: 192.168.10.21

# host -t SRV _kerberos._tcp.mydomain.com
_kerberos._tcp.mydomain.com has SRV record 1 0 88 kerberos.mydomain.com.

# host -t SRV {each srv-host record}
# host -t TXT _kerberos.mydomain.com
_kerberos.mydomain.com descriptive text "MYDOMAIN.COM"
...

    1. As long as name resolution is working properly, either dnsmasq or named, Kerberos should be able to find the needed service records.