support "ip route get" as fallback on linux systems

this fixes gateway lookups on android, where neither `netstat -rn`
nor `ip route show` display the gateway.

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.0.1.0        0.0.0.0         255.255.255.0   U     0      0        0 eth0

$ ip route show
10.0.1.0/24 dev eth0  proto kernel  scope link  src 10.0.1.36

$ ip route get 8.8.8.8
8.8.8.8 via 10.0.1.1 dev eth0  src 10.0.1.36  uid 2000
    cache

gateway_common.go

@@ -39,7 +39,7 @@ func parseWindowsRoutePrint(output []byte) (net.IP, error) {
 	return nil, errNoGateway
 }
 
-func parseLinuxIPRoute(output []byte) (net.IP, error) {
+func parseLinuxIPRouteShow(output []byte) (net.IP, error) {
 	// Linux '/usr/bin/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
@@ -57,6 +57,23 @@ func parseLinuxIPRoute(output []byte) (net.IP, error) {
 	return nil, errNoGateway
 }
 
+func parseLinuxIPRouteGet(output []byte) (net.IP, error) {
+	// Linux '/usr/bin/ip route get 8.8.8.8' format looks like this:
+	// 8.8.8.8 via 10.0.1.1 dev eth0  src 10.0.1.36  uid 2000
+	lines := strings.Split(string(output), "\n")
+	for _, line := range lines {
+		fields := strings.Fields(line)
+		if len(fields) >= 2 && fields[1] == "via" {
+			ip := net.ParseIP(fields[2])
+			if ip != nil {
+				return ip, nil
+			}
+		}
+	}
+
+	return nil, errNoGateway
+}
+
 func parseLinuxRoute(output []byte) (net.IP, error) {
 	// Linux route out format is always like this:
 	// Kernel IP routing table

gateway_linux.go

@@ -8,19 +8,32 @@ import (
 func DiscoverGateway() (ip net.IP, err error) {
 	ip, err = discoverGatewayUsingRoute()
 	if err != nil {
-		ip, err = discoverGatewayUsingIp()
+		ip, err = discoverGatewayUsingIpRouteShow()
+	}
+	if err != nil {
+		ip, err = discoverGatewayUsingIpRouteGet()
 	}
 	return
 }
 
-func discoverGatewayUsingIp() (net.IP, error) {
+func discoverGatewayUsingIpRouteShow() (net.IP, error) {
 	routeCmd := exec.Command("ip", "route", "show")
 	output, err := routeCmd.CombinedOutput()
 	if err != nil {
 		return nil, err
 	}
 
-	return parseLinuxIPRoute(output)
+	return parseLinuxIPRouteShow(output)
+}
+
+func discoverGatewayUsingIpRouteGet() (net.IP, error) {
+	routeCmd := exec.Command("ip", "route", "get", "8.8.8.8")
+	output, err := routeCmd.CombinedOutput()
+	if err != nil {
+		return nil, err
+	}
+
+	return parseLinuxIPRouteGet(output)
 }
 
 func discoverGatewayUsingRoute() (net.IP, error) {

gateway_test.go

@@ -59,7 +59,7 @@ Persistent Routes:
 	test(t, testcases, parseWindowsRoutePrint)
 }
 
-func TestParseLinuxIPRoutePrint(t *testing.T) {
+func TestParseLinuxIPRouteShow(t *testing.T) {
 	correctData := []byte(`
 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
@@ -85,7 +85,40 @@ default via foo dev wlp3s0  metric 303
 		{badRoute, false, ""},
 	}
 
-	test(t, testcases, parseLinuxIPRoute)
+	test(t, testcases, parseLinuxIPRouteShow)
+}
+
+func TestParseLinuxIPRouteGet(t *testing.T) {
+	correctData := []byte(`
+8.8.8.8 via 10.0.1.1 dev eth0  src 10.0.1.36  uid 2000
+    cache`)
+	randomData := []byte(`
+test
+Lorem ipsum dolor sit amet, consectetur adipiscing elit,
+sed do eiusmod tempor incididunt ut labore et dolore magna
+aliqua. Ut enim ad minim veniam, quis nostrud exercitation
+`)
+	noRoute := []byte(`
+broadcast 255.255.255.255 dev eth0  src 10.0.1.36  uid 2000
+	cache <local,brd>
+`)
+	badRoute := []byte(`
+local 0.0.0.0 dev lo  src 127.0.0.1  uid 2000
+    cache <local>
+`)
+	errorRoute := []byte(`
+RTNETLINK answers: Invalid argument
+`)
+
+	testcases := []testcase{
+		{correctData, true, "10.0.1.1"},
+		{randomData, false, ""},
+		{noRoute, false, ""},
+		{badRoute, false, ""},
+		{errorRoute, false, ""},
+	}
+
+	test(t, testcases, parseLinuxIPRouteGet)
 }
 
 func TestParseLinuxRoutePrint(t *testing.T) {
@@ -108,13 +141,19 @@ Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
 Kernel IP routing table
 Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
 0.0.0.0         foo     0.0.0.0         UG    0      0        0 eth0
+`)
+	missingRoute := []byte(`
+Kernel IP routing table
+Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
+10.0.1.0        0.0.0.0         255.255.255.0   U     0      0        0 eth0
 `)
 
 	testcases := []testcase{
 		{correctData, true, "192.168.1.1"},
 		{randomData, false, ""},
 		{noRoute, false, ""},
 		{badRoute, false, ""},
+		{missingRoute, false, ""},
 	}
 
 	test(t, testcases, parseLinuxRoute)