How to set up a secondary KDC

Note:
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Once you have one Key Distribution Center (KDC) on your network, it is good practice to have a secondary KDC in case the primary becomes unavailable.

Also, if you have Kerberos clients that are on different networks (possibly separated by routers using NAT), it is wise to place a secondary KDC in each of those networks.

Note:
The native replication mechanism explained here relies on a cron job; it essentially dumps the DB on the primary and loads it back up on the secondary. You may want to take a look at using the kldap backend, which can use the OpenLDAP replication mechanism. This is explained further below.

Install the required packages

First, install the packages, and when asked for the Kerberos and Admin server names enter the name of the Primary KDC:

sudo apt install krb5-kdc krb5-admin-server

Once you have installed the packages, create the host principals for both KDCs. From a terminal prompt, enter:

$ kadmin -q "addprinc -randkey host/kdc01.example.com"
$ kadmin -q "addprinc -randkey host/kdc02.example.com"

Note:
The kadmin command defaults to using a principal like username/admin@EXAMPLE.COM, where username is your current shell user. If you need to override that, use -p <principal-you-want>.

Extract the key file for the kdc02 principal, which is the server we are on:

$ sudo kadmin -p ubuntu/admin -q "ktadd host/kdc02.example.com"

Next, there needs to be a kpropd.acl file on each KDC that lists all KDCs for the realm. For example, on both the primary and secondary KDC, create /etc/krb5kdc/kpropd.acl:

host/kdc01.example.com@EXAMPLE.COM
host/kdc02.example.com@EXAMPLE.COM

Note:
It’s customary to allow both KDCs because one may want to switch their roles if one goes bad. For such an eventuality, both are already listed here.

Create an empty database on the secondary KDC:

$ sudo kdb5_util create -s

Now install kpropd daemon, which listens for connections from the kprop utility from the primary KDC:

$ sudo apt install krb5-kpropd

The service will be running immediately after installation.

From a terminal on the primary KDC, create a dump file of the principal database:

$ sudo kdb5_util dump /var/lib/krb5kdc/dump

Still on the Primary KDC, extract its key:

$ sudo kadmin.local -q "ktadd host/kdc01.example.com"

On the primary KDC, run the kprop utility to push the database dump made before to the secondary KDC:

$ sudo kprop -r EXAMPLE.COM -f /var/lib/krb5kdc/dump kdc02.example.com
Database propagation to kdc02.example.com: SUCCEEDED

Note the SUCCEEDED message, which signals that the propagation worked. If there is an error message, check /var/log/syslog on the secondary KDC for more information.

You may also want to create a cron job to periodically update the database on the secondary KDC. For example, the following will push the database every hour:

# m h  dom mon dow   command
0 * * * * root /usr/sbin/kdb5_util dump /var/lib/krb5kdc/dump && /usr/sbin/kprop -r EXAMPLE.COM -f /var/lib/krb5kdc/dump kdc02.example.com

Finally, start the krb5-kdc daemon on the secondary KDC:

$ sudo systemctl start krb5-kdc.service

Note:
The secondary KDC does not run an admin server, since it’s a read-only copy.

From now on, you can specify both KDC servers in /etc/krb5.conf for the EXAMPLE.COM realm, in any host participating in this realm (including kdc01 and kdc02), but remember that there can only be one admin server and that’s the one running on kdc01:

[realms]
    EXAMPLE.COM = {
            kdc = kdc01.example.com
            kdc = kdc02.example.com
            admin_server = kdc01.example.com
    }

The secondary KDC should now be able to issue tickets for the realm. You can test this by stopping the krb5-kdc daemon on the primary KDC, then using kinit to request a ticket. If all goes well you should receive a ticket from the secondary KDC. Otherwise, check /var/log/syslog and /var/log/auth.log on the secondary KDC.

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