net/portmapper: add sketch of a TestIGD for portmapper testing

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/portmapper/igd_test.go

@@ -0,0 +1,78 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package portmapper
+
+import (
+	"net"
+	"net/http"
+	"net/http/httptest"
+)
+
+// TestIGD is an IGD (Intenet Gateway Device) for testing. It supports fake
+// implementations of NAT-PMP, PCP, and/or UPnP to test clients against.
+type TestIGD struct {
+	upnpConn net.PacketConn // for UPnP discovery
+	pxpConn  net.PacketConn // for NAT-PMP and/or PCP
+	ts       *httptest.Server
+
+	doPMP  bool
+	doPCP  bool
+	doUPnP bool // TODO: more options for 3 flavors of UPnP services
+}
+
+func NewTestIGD() (*TestIGD, error) {
+	d := &TestIGD{
+		doPMP:  true,
+		doPCP:  true,
+		doUPnP: true,
+	}
+	var err error
+	if d.upnpConn, err = net.ListenPacket("udp", "127.0.0.1:1900"); err != nil {
+		return nil, err
+	}
+	if d.pxpConn, err = net.ListenPacket("udp", "127.0.0.1:5351"); err != nil {
+		return nil, err
+	}
+	d.ts = httptest.NewServer(http.HandlerFunc(d.serveUPnPHTTP))
+	go d.serveUPnPDiscovery()
+	go d.servePxP()
+	return d, nil
+}
+
+func (d *TestIGD) Close() error {
+	d.ts.Close()
+	d.upnpConn.Close()
+	d.pxpConn.Close()
+	return nil
+}
+
+func (d *TestIGD) serveUPnPHTTP(w http.ResponseWriter, r *http.Request) {
+	http.NotFound(w, r) // TODO
+}
+
+func (d *TestIGD) serveUPnPDiscovery() {
+	buf := make([]byte, 1500)
+	for {
+		n, addr, err := d.upnpConn.ReadFrom(buf)
+		if err != nil {
+			return
+		}
+		pkt := buf[:n]
+		_, _ = pkt, addr // TODO
+	}
+}
+
+// servePxP serves NAT-PMP and PCP, which share a port number.
+func (d *TestIGD) servePxP() {
+	buf := make([]byte, 1500)
+	for {
+		n, addr, err := d.pxpConn.ReadFrom(buf)
+		if err != nil {
+			return
+		}
+		pkt := buf[:n]
+		_, _ = pkt, addr // TODO
+	}
+}