PFSense - DNS - DNS Resolver - Prefetch Support

Prefetch keeps popular items in the cache from expiring.

Navigate to Services → DNS Resolver → Advanced Settings.

In Advanced Resolver Options:

NOTE: It will only look up or prefetch a record if a client requests that record and the ttl is within 10% of expiration.

Lets say www.domain.com had a ttl of 600 seconds.
Client asks for www.domain.com while the ttl is at 300 seconds.
The record is returned to the client. No prefetch done as the ttl is only at 50%.

Now another client asks for www.domain.com while there is say 59 seconds left on the ttl.
Client will get their answer, and then unbound will go resolve www.domain.com again to refresh the cache.