Pick fastest resolver

[fantom-x]

The only relevant parameter I found is round_robin_upstreams, which either uses the list of resolvers as ordered or all of them. Some DNS providers have two resolvers with very different response times (15ms vs 180ms at this moment and could flip from time to time). Any plan to add a mode to pick the fastest resolver on startup or periodically?

[saradickinson]

@fantom-x We don't have a selection based on RTT at the moment, no immediate plan to add but I like this as a feature request. Right now we will use an upstream as long as it 'works' which is currently defined by 3 tests:

1. it has had less than tls_connection_retries (default=2) failures at making a TLS connection
2. it has not had more than 5 * tls_connection_retries occurrences of the upstream shutting a connection unexpectedly
3. on successful connections it is sending responses and has not had more than 5 timeouts If it fails the test a 'backoff' is enacted with increasing intervals before trying that upstream again (you can see this in the full logs)

@wtoorop is RTT based selection something planned in the getdns connection re-factoring?