Get default gateway by parsing RIB information using the net/route package. BSD Only.

default_gateway.go

package main

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

var defaultRoute = [4]byte{0, 0, 0, 0}

func main() {
	rib, _ := route.FetchRIB(0, route.RIBTypeRoute, 0)
	messages, err := route.ParseRIB(route.RIBTypeRoute, rib)

	if err != nil {
		return
	}

	for _, message := range messages {
		route_message := message.(*route.RouteMessage)
		addresses := route_message.Addrs

		var destination, gateway *route.Inet4Addr
		ok := false

		if destination, ok = addresses[0].(*route.Inet4Addr); !ok {
			continue
		}

		if gateway, ok = addresses[1].(*route.Inet4Addr); !ok {
			continue
		}

		if destination == nil || gateway == nil {
			continue
		}

		if destination.IP == defaultRoute {
			fmt.Println(gateway.IP)
		}
	}
}