Make DNS lookups async in Go's net package

[bradfitz]

Go's net package on Windows currently consumes a thread while doing DNS lookups, which means Tailscale's ExitDNS service (the DNS server that runs when you're running an exit node) consumes one thread per outstanding DNS lookup.

@dblohm7 wrote a proof-of-concept of using DnsQueryEx to do them async. It requires Windows 8, but we should be able to move that code to Go's net package so everybody benefits, without changing the Go net Lookup APIs.

[DentonGentry]

It requires Windows 8

The day is coming where Windows 7 support will not be possible. golang/go#52188

[bradfitz]

@dblohm7 still got that code around somewhere? I forgot its branch name.

[cristaloleg]

Hi @dblohm7 is this change available somewhere?

[DentonGentry]

@dblohm7 wrote a proof-of-concept of using DnsQueryEx to do them async. It requires Windows 8, but we should be able to move that code to Go's net package so everybody benefits, without changing the Go net Lookup APIs.

Tailscale 1.42 is the last release which will support Windows 7 & 8. https://github.com/tailscale/tailscale/releases/tag/v1.42.0

Future releases will target Windows 10 and later.