net: LookupIP("127.0.0.1") does not work when CGO_ENABLED=1

[neild]

package main
import (
        "fmt"
        "net"
)
func main() {
        fmt.Println(net.LookupIP("127.0.0.1"))
}

$ go version
go version go1.4.2 linux/amd64
$ go run main.go
[127.0.0.1] <nil>

$ /tmp/g/go version
go version devel +e38bcb5 Wed Jun 17 18:17:50 2015 +0000 linux/amd64
$ /tmp/g/go run main.go
[] lookup 127.0.0.1: invalid domain name

$ uname -a
Linux kinoko.mtv.corp.google.com 3.13.0-52-generic #86-Ubuntu SMP Mon May 4 04:32:59 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

The net.LookupIP docs are unclear on what it does with IPs as opposed to hostnames, but changing the behavior seems wrong.

[mikioh]

I think this is expected behavior from go1.5, and a consequence of the new feature described in 4a0ba7a.

[bradfitz]

@ianlancetaylor, @rsc, any opinions on this? Apparently glibc lets you do a DNS lookup on an IP address? Is that part of the net.LookupIP contract? Do we care if this behavior changes?

[rsc]

It wasn't intended but I think it's probably worth keeping.

[rsc]

That is, we should keep the 1.4 behavior.

[bradfitz]

Easy enough. I assume we can just recognize IP literals (IPv4 or IPv6) and return them immediately, without regard to the system's nsswitch.conf or resolv.conf, etc. I hope that glibc isn't doing anything fancier? @mikioh, do you know?

[ianlancetaylor]

That is what glibc's getaddrinfo function does: it calls inet_aton on the name, and, if that succeeds, returns the result.

[gopherbot]

CL https://golang.org/cl/11420 mentions this issue.