The Resolver panel gives access to determining how the server resolves recursive queries. A resolver can be set to either resolve for itself, or to use another server to do its resolving. The latter is called "forwarding". To use a forwarder click the appropriate radio button and enter the appropriate IP address in the area below.

If you enter the IP addresses to multiple forwarders, all the forwarders will be queried simultaneously, and the first response will be accepted.

If a server using a forwarder does not receive a response after a few seconds it will attempt to resolve the query itself. This behavior can be changed by clicking the "Work as slave server" checkbox. When acting as a slave server, a server will never attempt to resolve a query itself.

It should be noted that this "slave server" status regarding forwarding behavior is entirely unrelated to acting as a slave server for a particular zone.