I just hit a problem in grpc-go (grpc/grpc-go#1058) where their tests were assuming they could write code like this:

        lis, err := net.Listen("tcp", ":0")
        if err != nil {
                t.Fatalf("Failed to listen: %v", err)
        }
        conn, err := net.Dial("tcp", lis.Addr().String())

The code looks reasonable, and often works (on both Go 1.7 and Go 1.8, on Linux and Darwin).

But not always.

lis.Addr().String() returns [::]:NNNNN, which looks like an IPv6 address literal for all zeros (like 0.0.0.0), regardless of whether the machine has IPv6.

And if a machine doesn't have an IPv6 address on the loopback interface, then net.Dial("tcp", "[::]:12345") fails.

But it passes otherwise.

The net.Dial documentation (https://beta.golang.org/pkg/net/#Dial) is kinda fuzzy on whether dialing the all zero address works. It does document and implement this behavior this, though:

If the host is empty, as in ":80", the local system is assumed.

Thus, the solution for gRPC's tests and anybody else is apparently adding something hacky like:

// dialAddr returns a valid net.Dial address to dial using network "tcp".
func dialAddr(ln net.Listener) string {
	addr := ln.Addr().String()
	addr = strings.TrimPrefix(addr, "[::]")
	addr = strings.TrimPrefix(addr, "0.0.0.0")
	return addr
}

But that's kinda lame and tedious.

Maybe we should just treat a net.Dial of [::] or 0.0.0.0 the same as an empty string for the host part?

/cc @mikioh @rsc @ianlancetaylor @MakMukhi