We have a strange problem regarding Kerberos authentication with Apache mod_auth_kerb. We use a very simple krb5.conf, where only a single (main) AD server is configured. There are many domains in the forest, and it seems that SSO is working for most of them, except one.

I don't know what is special about that domain, the error message that I see in the Apache logs is "Server not found in Kerberos database":

[Wed Aug 31 14:56:02 2011] [debug] src/mod_auth_kerb.c(1025): [client xx.xxx.xxx.xxx] Using HTTP/[email protected] as server principal for password verification
[Wed Aug 31 14:56:02 2011] [debug] src/mod_auth_kerb.c(714): [client xx.xxx.xxx.xxx] Trying to get TGT for user [email protected]
[Wed Aug 31 14:56:02 2011] [debug] src/mod_auth_kerb.c(625): [client xx.xxx.xxx.xxx] Trying to verify authenticity of KDC using principal HTTP/[email protected]
[Wed Aug 31 14:56:02 2011] [debug] src/mod_auth_kerb.c(640): [client xx.xxx.xxx.xxx] krb5_get_credentials() failed when verifying KDC
[Wed Aug 31 14:56:02 2011] [error] [client xx.xxx.xxx.xxx] failed to verify krb5 credentials: Server not found in Kerberos database
[Wed Aug 31 14:56:02 2011] [debug] src/mod_auth_kerb.c(1110): [client xx.xxx.xxx.xxx] kerb_authenticate_user_krb5pwd ret=401 user=(NULL) authtype=(NULL)

When I try to kinit that user on the machine on which Apache is running, it works. I also checked that DNS lookups work, including reverse lookup.

Who can tell me whats going?