Change back dns_lookup_kdc to the default
    
    The specifications recommend against using TXT records to mapping
    hostnames to realms. However they do not recommend against using
    SRV records to lookup the KDC.
    
    Change back to the MIT default of enabling DNS for KDC lookup.
    This allows automatic configuration and failover.
    
    A theoretical attack involving SRV records could be similarly
    accomplished by a similar attack involving the A records for
    the KDC hosts.