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Kerberos howtos

by Alejandro Liu
in 2013

tagged authentication, backups, boot, configuration, directory, domain, network, password, service

Updated: 2021-12-25

Kerberos is a network authentication protocol which works on the basis of "tickets" to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. (Source Kerberos_(protocol) )

Backups

Create backup:

kdb5_util dump _dump_file_

Restore from dump file:

kdb5_util load _dump_file_

Master/Slave replication

Initial set-up:

(master)# kdb5_util dump _dump_file_
(master)# kprop -d -f _dump_file_ _slave_

In crontab on master:

krb5_util dump _dump_file_
kprop -f _dump_file_ _slave_

kadmin command

From command line:

kadmin.local -q 'cmd'
  • listprincs - list principals
  • ank principal - new principal (input: password)
  • delprinc principal - delete principal (input: yes/no)
  • ank -randkey host/fqdn@REALM - create a service key
  • ktadd -k filename host/fqdn@REALM export key to keytab file.

Save keytab in /etc/krb5.keytab and

chown root:root /etc/krb5.keytab
chmod 400 /etc/krb5.keytab

Logging

To turn logging on, add this section to /etc/krb5.conf (adapt the file paths to your likings):

 [logging]
     default = FILE:/var/log/krb5.log
     kdc = FILE:/var/log/krb5kdc.log
     admin_server = FILE:/var/log/kadmind.log

Merging (or editing) a keytab file

Merging or editing keytabs is done through the ktutil command. Suppose we have two keytabs, keytab1 and keytab2, each having their own set of keys, and we would like to merge the two keytabs in one (or create a new keytab containing specific keys). The operation is done through the ktutil shell, with rkt and write_kt commands, and optionally delent if you want to delete some entities. Example:

 # ktutil

Read content of keytab1:

 ktutil: rkt keytab1
 ktutil: list
 slot KVNO Principal
 ---- ---- -------------------------------------------------------------
 1 3 <principal and key of keytab1>
 2 3 <principal and key of keytab1>

Now, we will read the content of keytab2:

 ktutil: rkt keytab2
 ktutil: list
 slot KVNO Principal
 ---- ---- -----------------------------------------------------------
 1 3 <principal and key of keytab1>
 2 3 <principal and key of keytab1>
 3 2 <principal and key of keytab2>
 4 2 <principal and key of keytab2>

Save this content in a temporary keytab:

 ktutil: write_kt /tmp/krb5.keytab

This utility is used to duplicate and tweak keytab entries (as its name implies), and remove the need of exporting the keys out of the KDC twice or more (simultaneously avoiding KVNO's increment).

OpenWRT recipes

Packages

Server

  • krb5-server
    • krb5-libs (dependency of krb5-server)

Client

  • krb5-client

Configuration

Create the file /etc/krb5.conf with the following credentials. Example:

[libdefaults]
    default_realm = YOURDOMAIN.ORG
    dns_lookup_realm = false
    dns_lookup_kdc = false
    ticket_lifetime = 24h
    forwardable = yes

[realms]
    YOURDOMAIN.ORG = {
        kdc = server_address_of_this_machine:88
        admin_server = server_address_of_this_machine:749
        default_domain = yourdomain.org
    }

[domain_realm]
    .yourdomain.org = YOURDOMAIN.ORG
    yourdomain.org = YOURDOMAIN.ORG

Replace YOURDOMAIN.ORG / yourdomain.org with the domain name of your domain the server should act for (names must be specified in UPPER- / lowercase as shown above). Replace server_address_of_this_machine with the host name/IP adress of this server you're setting up.

Starting the server

Start the server by issuing

/etc/init.d/krb5kdc start

This should create the /etc/krb5kdc/ directory with the following files

-rw-------    1 root     root         8192 Feb 13 11:17 principal
-rw-------    1 root     root         8192 Feb 13 09:12 principal.kadm5
-rw-------    1 root     root            0 Feb 13 09:12 principal.kadm5.lock
-rw-------    1 root     root            0 Feb 13 11:17 principal.ok

In case you don't get any error messages check your server by logging on with kadmin.local In case everything works well you will see the following message

root@bridge:~# kadmin.local
Authenticating as principal xxxxxxx/[email protected] with password.
kadmin.local:

Start on boot

To enable/disable automatic start on boot:

/etc/init.d/krb5kdc enable

this simply creates a symlink: /etc/rc.d/S60krb5kdc ? /etc/init.d/krb5kdc

/etc/init.d/krb5kdc disable

this removes the symlink again

References

See Also: