Kerberos

Kerberos is a network authentication system. See krb5 documentation.

Related articles

Installation

Install the krb5 package on your clients and server.

It is highly recommended to use a time synchronization daemon to keep client/server clocks in sync.

If hostname resolution has not been configured, you can manually add your clients and server to the hosts(5) file of each machine. Note that the FQDN (myclient.example.com) must be the first hostname after the IP address in the hosts file.

Server configuration

Domain creation

Edit /etc/krb5.conf to configure your domain:

/etc/krb5.conf
[libdefaults]
    default_realm = EXAMPLE.COM

[realms]
    EXAMPLE.COM = {
        admin_server = $ADDRESS
        # use "kdc = ..." if the kerberos SRV records aren't in DNS (see Advanced section)
        kdc = $ADDRESS
        # This breaks krb4 compatibility but increases security
        default_principal_flags = +preauth
    }

[domain_realm]
    example.com  = EXAMPLE.COM
    .example.com = EXAMPLE.COM

[logging]
    kdc          = SYSLOG:NOTICE
    admin_server = SYSLOG:NOTICE
    default      = SYSLOG:NOTICE

Where $ADDRESS is IP address or domain name, where kerberos lives, like or kerberos.example.com.

This file's format is described in the MIT Kerberos documentation

Create the database:

# kdb5_util -r EXAMPLE.COM create -s
Loading random data                                                             
Initializing database '/var/lib/krb5kdc/principal' for realm 'EXAMPLE.COM',                  
master key name 'K/M@EXAMPLE.COM'
You will be prompted for the database Master Password.                          
It is important that you NOT FORGET this password.                              
Enter KDC database master key: ***
Re-enter KDC database master key to verify: ***

Finally, start/enable and .

Add principals

Start the Kerberos administration tool, using local authentication

Add a user principal to the Kerberos database:

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

Add the KDC principal to the Kerberos database:

kadmin.local: addprinc -randkey host/kerberos.example.com
WARNING: no policy specified for host/kerberos.example.com@EXAMPLE.COM; defaulting to no policy
Principal "host/kerberos.example.com@EXAMPLE.COM" created.

Finally, Add the KDC principal to the server's keytab:

Quit the Kerberos administration tool:

kadmin.local: quit

You should now be able to get a Kerberos ticket:

Firewall

Add ALLOW rules to your firewall for any applicable ports/protocols:

  • 88, TCP and UDP for Kerberos v5
  • 749, TCP and UDP for kadmin if you plan to configure it
  • 750, TCP and UDP for Kerberos v4 if you need backwards compatibility

DNS records

This is not necessary if you specify the kerberos and kadmin server in each machine's krb5.conf Do not forget reverse DNS.

Client configuration

Edit the client's /etc/krb5.conf to match your server's configuration. You can copy this file from the server, or just set the required realm information.

Testing

You should now be able to get a Kerberos ticket on the client:

Configuring kadmin

You will need /etc/krb5.conf configured on the kadmin client, and the server's firewall configured for kadmin.

Configuring kadmin ACL

Create a principal for administration:

Add the user to the kadmin ACL file: This file's format is described in the MIT Kerberos documentation

Configure kdc.conf: This file's format is described in the MIT Kerberos documentation

Restart and .

You can now use kadmin as your own user, authenticating with kerberos:

$ kadmin
Authenticating as principal myuser/admin@EXAMPLE.COM with password.
Password for myuser/admin@EXAMPLE.COM: ***
kadmin:

Service principals and keytabs

First, ensure you have configured krb5.conf on all involved machines.

A kerberos principal has three components, formatted as `primary/instance@REALM`. For user principals, the primary is your username and the instance is omitted or is a role (eg. "admin"): `myuser@EXAMPLE.COM` or `myuser/admin@EXAMPLE.COM`. For hosts, the primary is "host" and the instance is the server FQDN: `host/myserver.example.com@EXAMPLE.COM`. For services, the primary is the service abbreviation and the instance is the FQDN: `nfs/myserver.example.com@EXAMPLE.COM`. The realm can often be omitted, the local computer's default realm is usually assumed.

With remote kadmin

This is the easier method, but requires you to have configured kadmin.

Open kadmin as root (so we can write the keytab) on the client, authenticating with your admin principal:

Add a principal for any services you will be using, eg. "host" for SSH authentication or "nfs" for NFS:

kadmin: addprinc -randkey host/kbclient.example.com
WARNING: no policy specified for host/kbclient.example.com@EXAMPLE.COM; defaulting to no policy
Principal "host/kbclient.example.com@EXAMPLE.COM" created.

Save each key to the local keytab:

Without remote kadmin

Start kadmin on the Kerberos server, using either unix or kerberos authentication:

Add a principal for any services you will be using, eg. "host" for SSH authentication or "nfs" for NFS:

Save each key to a new keytab to be transferred to the client:

Finally, copy from the server to the client using SCP or similar, then put it in place with correct permissions:

Finally, delete kbclient.keytab from the server and client.

Cross-Realm Trust

Set up a second server as shown above, then create the cross-realm principal on both KDCs. Cross-realm principals must be created with strong passwords, not , and the same password must be used on both KDCs. The principal must have the same key version number (kvno) in both KDCs.

To grant EXAMPLE.COM principals access to EXAMPLE.ORG resources, you would use the following principal:

kadmin# addprinc krbtgt/EXAMPLE.ORG@EXAMPLE.COM

The section of krb5.conf can be used to further control cross-realm trust relationships.

SSH authentication

Use the instructions in Service principals and keytabs to create a principal for the "host" service for both client and server, then put the client's keys in the client's keytab and the server's keys in the server's keytab.

Modify your SSH server configuration to enable GSSAPI authentication:

And modify your client configuration to send GSSAPI requests:

Get a ticket-granting ticket on the client before using ssh:

$ kinit myuser@EXAMPLE.COM
Password for myuser@EXAMPLE.COM: ***

Pass the -v option to ssh to watch what's happening:

And you should now see a host ticket on the client:

Authorize other principals

To allow a different kerberos principal to authenticate to a user account, add the principal name to the target account's file. For example, to allow to SSH to alice's account:

NFS security

First, configure your NFS server. Also see NFS/Troubleshooting. Configuring a time synchronization daemon on both the clients and the server is strongly recommended. Clock drift will cause this to break, and the error message will not be helpful.

Use the instructions in #Service principals and keytabs to create a principal for the "nfs" service for both client and server, then put the client's keys in the client's keytab and the server's keys in the server's keytab.

NFS server

Add a Kerberos export option. If necessary, multiple options can be specified using a colon as a delimiter with the preferred setting first, e.g. .

  • sec=krb5p uses kerberos for authentication, integrity, and encryption.
  • uses kerberos for authentication and integrity checking, but still transmits data unencrypted.
  • uses kerberos for authentication only, and transmits the data unauthenticated and unencrypted.
  • is the default and does not provide any cryptographic security.

And reload the exports:

# exportfs -arv

NFS client

Mount the exported directory:

# mount nfsserver:/srv/export /mnt/

You can add -vv for verbose information, and may need and or your chosen security option.

Check that it worked with the command:

Browsers

Some browsers have support for Kerberos protocol but disable it by default. Here are the instructions how to enable it:

Chromium

Chromium needs to be run with a command line parameter that specifies a list of sites where Kerberos authentication is allowed. The easiest way is to add persistent flag to the configuration file:

Firefox

To configure Firefox with trusted sites visit about:config and set property to FOO.COM (Note: for Firefox there is no "*."; for Chrome, there is).

Troubleshooting

Cannot set GSSAPI authentication names

 
Cannot set GSSAPI authentication names, aborting

Your realm is missing either the or principal.

For clients, invalid arguments/options may happen on first setup if rpc-gssd is not loaded. Loading it is usually acomplished by enabling and starting , but after first setup this target will need a restart.

SSH authentication fails while connecting to a server requiring GSSAPI with KeyExchange

If any of the following errors are encountered:

$ ssh -v -o GSSAPIDelegateCredentials=yes -o GSSAPIAuthentication=yes <user>@<IP address>
Unable to negotiate with <IP address> port 22: no matching key exchange method found. Their offer: gss-group14-sha1-...
$ ssh -v -o GSSAPIDelegateCredentials=yes -o GSSAPIKeyExchange=yes -o GSSAPIAuthentication=yes <user>@<IP address>
command-line: line 0: Bad configuration option: gssapikeyexchange

it means that package is not configured with GSSAPI patch for OpenSSH. You can install or follow this method.

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See also
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