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

Change-Id: Icd318f75b3ed8c73ccd588be8122379b263651fb
Reviewed-on: https://go-review.googlesource.com/41514
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>