From https://github.com/golang/go/issues/17659#issuecomment-423113606 ...

> In kubernetes , isDomainName reject Pods "A Record" "pod-ip-address",
> for example: "172-17-0-16", as RFC 3696 section 2 requires
> "top-level domain names not be all-numeric", but this example has
> three hyphen, so I think it should not be reject.

Updates #17659

Change-Id: Ibd8ffb9473d69c45c91525953c09c6749233ca20
Reviewed-on: https://go-review.googlesource.com/136900
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>