This is not perfect but is enough for during the dual IP stack era.

Fixes golang/go#19298.

Change-Id: I5f16b10fda5ddfe470edb021d9a6d7e2a642d267
Reviewed-on: https://go-review.googlesource.com/41515
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>