Prefetch keeps popular items in the cache from expiring.

• 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.