net: Windows build with netgo doesn't repsect hosts file, can not resolve localhost

[wxiaoguang]

What version of Go are you using (go version)?

$ go version
go version go1.19.5 windows/amd64
go version go1.20rc2 windows/amd64

What did you do?

Set Windows system DNS to a standard server which doesn't resolve localhost, then use Go net package to resolve a domain defined in local hosts file (C:\Windows\System32\Drivers\etc\hosts).

package main

import (
	"fmt"
	"net"
)

func main() {
	addr, err := net.ResolveTCPAddr("tcp", "localhost:12345")
	fmt.Printf("addr=%v, err=%v\n", addr, err)
}

What did you expect to see?

Go should respect C:\Windows\System32\Drivers\etc\hosts file and resolve localhost correctly with -tags=netgo.

What did you see instead?

Go can not resolve localhost with -tags=netgo

C:\work> go run resolvtest.go
addr=127.0.0.1:12345, err=<nil>

C:\work> go run -tags=netgo resolvtest.go
addr=<nil>, err=lookup localhost on 8.8.8.8:53: no such host

[bcmills]

That seems like this TODO in the code, added in CL 409234 for #33097 (attn @bradfitz @ianlancetaylor)
https://cs.opensource.google/go/go/+/master:src/net/conf.go;l=137-141;drc=d52883f443e1d564b0300acdd382af1769bf0477

(CC @neild @golang/windows)

[qmuntal]

I naively changed etc/hosts to C:/Windows/System32/Drivers/etc/hosts in https://cs.opensource.google/go/go/+/master:src/net/hook.go;l=16 and exercised the ResolveTCPAddr function with some addresses only known to my local hosts file, and all of them could be successfully resolved.

Everything seems to be wired up on the Windows side, only the hosts file path needs to be changed, and probably add some tests.

[zeripath]

The underlying problem is likely in:

go/src/net/conf.go

Lines 134 to 145 in d52883f

	if c.netGo || r.preferGo() {
	switch c.goos {
	case "windows":
	// TODO(bradfitz): implement files-based
	// lookup on Windows too? I guess /etc/hosts
	// kinda exists on Windows. But for now, only
	// do DNS.
	fallbackOrder = hostLookupDNS
	default:
	fallbackOrder = hostLookupFilesDNS
	}
	}

The result of L141 is that DNS lookups cannot check the /etc/hosts file (or equivalent) and thus I bet the lookups for localhost are going out to an external server and we're getting the external name for the host.