gate# cat /etc/krb5.conf
[libdefaults]
        default_realm = CORPX.UN

gate# kinit user1

[gate:~] # cat /etc/pam.d/system 
...
auth   sufficient      pam_krb5.so     no_warn try_first_pass
auth   required        pam_unix.so     no_warn try_first_pass
...

root@gate:~# apt-get install libpam-heimdal

root@server:~# apt-get install libpam-krb5

root@gate:~# cat /etc/pam.d/login
...
auth     sufficient      pam_krb5.so
# Standard Un*x authentication.
...