howto: keep serving from cache after TTL if servers down ?

[Sean Darcy]

I'm running a voip application, asterisk, that dies if it can't
resolve the addresses for its SIP registrations. Importantly there are
other connections, PSTN, that work even if the internet connection is
down. Also, I want the app to stay alive so that it can reconnect to
the SIP servers once the internet comes back up. For whatever reason,
asterisk itself will not have a fix for this.

I have unbound working on the machine. I'd like to have unbound serve
an address from its cache even if it's beyond the ttl IF it can't
refresh the address. In other words, just serve the stale address
until it can be refreshed.

Any way to do this ?

sean