net: Listen("tcp", ":port") fails in an AF_INET6-only environment

[pmarks-net]

Given a hacked-up IPv6-only Linux kernel:

• socket(AF_INET6) ok
• setsockopt(IPV6_V6ONLY, 0) fails
• socket(AF_INET) fails

The following program:

package main

import (
        "net"
        "fmt"
        "flag"
)

var port = flag.Int("port", 8080, "XXX")

func main() {
        flag.Parse()
        ln, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
        if err != nil {
                fmt.Printf("FAIL: %v\n", err)
                return
        }
        fmt.Printf("Listening on %v; sleeping.\n", ln.Addr())
        select{}
}

Does not work:

$ ./listen 
FAIL: listen tcp :8080: socket: address family not supported by protocol

The problem is, favoriteAddrFamily sees that it can't use dual-stack sockets, and falls back to AF_INET without first checking supportsIPv4.

You can use "[::]:port" as a workaround, but ":port" ought to work. I'm preparing a patch for this.

[gopherbot]

CL https://golang.org/cl/14334 mentions this issue.