libads: disable dns_lookup_realm in auto-generated krb5.conf files

libads: disable dns_lookup_realm in auto-generated krb5.conf files

This patch sets dns_lookup_realm=false in samba-generated krb5.conf.

Disabling dns_lookup_realm in krb5.conf is the recommended practice for
Kerberos usage in Active Directory environment. dns_lookup_realm is enabled
by default, at least in Heimdal.

When used by samba, Kerberos libraries operate based on either the system
krb5.conf, or a private krb5.conf generated specifically for the domain by
samba code. In the former case, it's the responsibility of the administrator
to set dns_lookup_realm=false. In the latter case, it's the responsibility
of samba - which is what this patch does.

In many usage scenarios the value of this variable is of no consequence
since samba knows the realm in which it is operating, and knows how to
generate service principal names. However, there are some scenarios
in which samba calls kerberos_get_principal_from_service_hostname(),
and here samba consults the Kerberos libraries and this parameter comes
into play. One primary example is cli_full_connection() function.

Not setting dns_lookup_realm leads to a series of DNS TXT record lookups.
This can be observed by running "net ads join -k -U <user>".

In AD environments, the TXT queries  typically fail quickly, but test setups
or misconfigured DNS may lead to large timeouts (for example, if the domain
is dept.example.com but there's no parent example.com domain and no DNS
zones for example.com). At the very least we want to avoid those lookups
because they are hardly documented and lead to confusion.

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>