On the Forwarders Tab of the server Properties, there is a checkbox next to an option Use root hints if no forwarders are available.
This option much more clearly describes the Forward First behaviour, which is the default (box checked). This option is entitled Use root hints if no forwarders are available. This is not technically accurate or at least it is incomplete. The servers that are being forwarded to may certainly be available and responding. It’s just that in addition to a Timeout (server not being available) the response may be an NXDOMAIN or SERVFAIL.
Broadly then we can say:
- Forward First is implemented by checking the checkbox
- Forward Only is implemented by NOT checking the checkbox