[Netty 5] DnsResolver

[normanmaurer]

Netty’s asynchronous DNS resolver APIs have organically grown over the lifetime of Netty 4.1 and continue to be unstable. The scope has increased from the original design to including but not limited to EDNS, redirects, IPv4 and IPv6 preferences, caching name servers, retry, parsing /etc/hosts and system configuration, observing query lifecycle events for visibility / load balancing. We should revisit the different APIs now that we have a better handle on scope and refine/consolidate where possible.

Caches don’t account for inflight queries. If they did we could prevent duplicate queries for the same address in flight at any given time and reduce bursts of duplicate queries. The duplicate query could instead listen to the result of the in-flight query, and use this result. This is interesting when we have to resolve the name server’s address, or when there are CNAME/redirects involved.