Kerberos Login Failed

Kerberos Login Failed:
The specified realm is not in your configuration file or does not exist

Why do I get this error message, and how can I fix the problem?

The realm is intentionally not in my configuration file. I've selected "Configure additional realms automatically using DNS" in the Settings pane of the Edit Realms dialog summoned from the Kerberos Edit menu.

Of course the realm exists. This should work - Kerberos just cant find the realm as the DNS is broken. Autoconfiguration should be achieved using using DNS TXT records [RFC 1035]. Some, not RFC compliant, DNS servers (in particular the one shipped with my SpeedTouch Alcatel 510 modem - I have an old 510v3; the problem occurs with both LEFTAA3.721 andLEFTAA3.716 firmware) can't resolve the TXT record query.

A solution is to provide explicitly the IP address of a competent DNS server (such as the one your ISP provides) in the TCP/IP pane of Network preferences. Once this is done, autoconfiguration works fine.