syncthing/vendor/github.com/jackpal/gateway/gateway_linux.go

package gateway

import (
	"bytes"
	"io/ioutil"
	"net"
	"os/exec"
)

func discoverGatewayUsingIp() (ip net.IP, err error) {
	routeCmd := exec.Command("ip", "route", "show")
	stdOut, err := routeCmd.StdoutPipe()
	if err != nil {
		return
	}
	if err = routeCmd.Start(); err != nil {
		return
	}
	output, err := ioutil.ReadAll(stdOut)
	if err != nil {
		return
	}

	// Linux 'ip route show' format looks like this:
	// default via 192.168.178.1 dev wlp3s0  metric 303
	// 192.168.178.0/24 dev wlp3s0  proto kernel  scope link  src 192.168.178.76  metric 303
	outputLines := bytes.Split(output, []byte("\n"))
	for _, line := range outputLines {
		if bytes.Contains(line, []byte("default")) {
			ipFields := bytes.Fields(line)
			ip = net.ParseIP(string(ipFields[2]))
			break
		}
	}
	err = routeCmd.Wait()
	return
}

func discoverGatewayUsingRoute() (ip net.IP, err error) {
	routeCmd := exec.Command("route", "-n")
	stdOut, err := routeCmd.StdoutPipe()
	if err != nil {
		return
	}
	if err = routeCmd.Start(); err != nil {
		return
	}
	output, err := ioutil.ReadAll(stdOut)
	if err != nil {
		return
	}

	// Linux route out format is always like this:
	// Kernel IP routing table
	// Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
	// 0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth0
	outputLines := bytes.Split(output, []byte("\n"))
	for _, line := range outputLines {
		if bytes.Contains(line, []byte("0.0.0.0")) {
			ipFields := bytes.Fields(line)
			ip = net.ParseIP(string(ipFields[1]))
			break
		}
	}
	err = routeCmd.Wait()
	return
}

func DiscoverGateway() (ip net.IP, err error) {
	ip, err = discoverGatewayUsingRoute()
	if err != nil {
		ip, err = discoverGatewayUsingIp()
	}
	return
}