net/portmapper: add pcp portmapping

This adds PCP portmapping, hooking into the existing PMP portmapping.

Signed-off-by: julianknodt <julianknodt@gmail.com>

net/portmapper/pcp_test.go

@@ -0,0 +1,27 @@
+// 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 (
+	"testing"
+
+	"inet.af/netaddr"
+)
+
+var examplePCPMapResponse = []byte{2, 129, 0, 0, 0, 0, 28, 32, 0, 2, 155, 237, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 129, 112, 9, 24, 241, 208, 251, 45, 157, 76, 10, 188, 17, 0, 0, 0, 4, 210, 4, 210, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 255, 135, 180, 175, 246}
+
+func TestParsePCPMapResponse(t *testing.T) {
+	mapping, err := parsePCPMapResponse(examplePCPMapResponse)
+	if err != nil {
+		t.Fatalf("failed to parse PCP Map Response: %v", err)
+	}
+	if mapping == nil {
+		t.Fatalf("got nil mapping when expected non-nil")
+	}
+	expectedAddr := netaddr.MustParseIPPort("135.180.175.246:1234")
+	if mapping.external != expectedAddr {
+		t.Errorf("mismatched external address, got: %v, want: %v", mapping.external, expectedAddr)
+	}
+}