Support "forward only" and "forward first" options


#1

In order to prevent MAAS DNS servers from querying root DNS servers in some cases “forward only” option needs to be used. This is quite relevant for offline or restricted environments.

https://kb.isc.org/docs/aa-00538

There are global and per-zone “forwarders” and “forward” directives:

options {
# ...
 forward ( first | only );
 forwarders [ port integer ] [ dscp integer ] { ( ipv4_address
     | ipv6_address ) [ port integer ] [ dscp integer ]; ... };

https://ftp.isc.org/isc/bind9/9.13.3/doc/arm/Bv9ARM.ch05.html#zone_statement_grammar

zone string [ class ] {
# ...
 forward ( first | only );
# ...
 forwarders [ port integer ] [ dscp integer ] { ( ipv4_address | ipv6_address ) [ port integer ] [ dscp integer ]; ... };