lib/pmp: Add NAT-PMP support (ref #698)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2968

vendor/github.com/jackpal/gateway/gateway_windows.go

@@ -0,0 +1,43 @@
+package gateway
+
+import (
+	"bytes"
+	"io/ioutil"
+	"net"
+	"os/exec"
+)
+
+func DiscoverGateway() (ip net.IP, err error) {
+	routeCmd := exec.Command("route", "print", "0.0.0.0")
+	stdOut, err := routeCmd.StdoutPipe()
+	if err != nil {
+		return
+	}
+	if err = routeCmd.Start(); err != nil {
+		return
+	}
+	output, err := ioutil.ReadAll(stdOut)
+	if err != nil {
+		return
+	}
+
+	// Windows route output format is always like this:
+	// ===========================================================================
+	// Active Routes:
+	// Network Destination        Netmask          Gateway       Interface  Metric
+	//           0.0.0.0          0.0.0.0      192.168.1.1    192.168.1.100     20
+	// ===========================================================================
+	// I'm trying to pick the active route,
+	// then jump 2 lines and pick the third IP
+	// Not using regex because output is quite standard from Windows XP to 8 (NEEDS TESTING)
+	outputLines := bytes.Split(output, []byte("\n"))
+	for idx, line := range outputLines {
+		if bytes.Contains(line, []byte("Active Routes:")) {
+			ipFields := bytes.Fields(outputLines[idx+2])
+			ip = net.ParseIP(string(ipFields[2]))
+			break
+		}
+	}
+	err = routeCmd.Wait()
+	return
+}