better BSD detection without calling external commands

[MrDefacto]

My full example. Feel free to implement in your package.

package main

import (
    "net"
    "fmt"
    "errors"
    "syscall"
    "golang.org/x/net/route"
)

var (
    errNoGateway = errors.New("No gateway")
)

func GetGateway() (ip net.IP, err error) {
    rib, err := route.FetchRIB(syscall.AF_INET, syscall.NET_RT_DUMP, 0)
    if err != nil {
	return nil, err
    }

    msgs, err := route.ParseRIB(syscall.NET_RT_DUMP, rib)
    if err != nil {
	return nil, err
    }

    for _, m := range msgs {
	switch m := m.(type) {
	case *route.RouteMessage:
		var ip net.IP
		switch sa := m.Addrs[syscall.RTAX_GATEWAY].(type) {
		case *route.Inet4Addr:
		    ip = net.IPv4(sa.IP[0], sa.IP[1], sa.IP[2], sa.IP[3])
		    return ip, nil
		case *route.Inet6Addr:
		    ip = make(net.IP, net.IPv6len)
		    copy(ip, sa.IP[:])
		    return ip, nil
		}
	}
    }
    return nil, errNoGateway
}

func main() {
    gateway, err := GetGateway()
    if err != nil {
	fmt.Println(err)
    } else {
	fmt.Println("Gateway:", gateway.String())
    }
}