Feature Request: async dns

[binarycraft007]

Hi Mitchell,

Nice library, can we add async dns to the roadmap, because it's really helpful for network programming.

Thanks

[mitchellh]

I'd be happy to add this if there is a reasonable way to do this by operating system. I'm unfamiliar with the low level APIs available since I always used a higher level library for async DNS. It would help if you could suggest the low-level APIs on Linux, Windows, and Mac that could enable this.

[rsepassi]

libuv seems to just run getaddrinfo (Unix-like, GetAddrInfoW for Windows) in a thread pool.

https://github.com/c-ares/c-ares is what libcurl uses and seems to be manually constructing the DNS requests and directly communicating with the nameserver over TCP. Didn't see how it's getting the nameservers.