tsnet,wgengine/netstack: add ListenPacket and tests

This adds a new ListenPacket function on tsnet.Server
which acts mostly like `net.ListenPacket`.

Unlike `Server.Listen`, this requires listening on a
specific IP and does not automatically listen on both
V4 and V6 addresses of the Server when the IP is unspecified.

To test this, it also adds UDP support to tsdial.Dialer.UserDial
and plumbs it through the localapi. Then an associated test
to make sure the UDP functionality works from both sides.

Updates #12182

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tsnet/tsnet_test.go

@@ -745,3 +745,73 @@ func TestCapturePcap(t *testing.T) {
 		t.Errorf("s2 pcap file size = %d, want > pcapHeaderSize(%d)", got, pcapHeaderSize)
 	}
 }
+
+func TestUDPConn(t *testing.T) {
+	tstest.ResourceCheck(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	controlURL, _ := startControl(t)
+	s1, s1ip, _ := startServer(t, ctx, controlURL, "s1")
+	s2, s2ip, _ := startServer(t, ctx, controlURL, "s2")
+
+	lc2, err := s2.LocalClient()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// ping to make sure the connection is up.
+	res, err := lc2.Ping(ctx, s1ip, tailcfg.PingICMP)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("ping success: %#+v", res)
+
+	pc := must.Get(s1.ListenPacket("udp", fmt.Sprintf("%s:8081", s1ip)))
+	defer pc.Close()
+
+	// Dial to s1 from s2
+	w, err := s2.Dial(ctx, "udp", fmt.Sprintf("%s:8081", s1ip))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer w.Close()
+
+	// Send a packet from s2 to s1
+	want := "hello"
+	if _, err := io.WriteString(w, want); err != nil {
+		t.Fatal(err)
+	}
+
+	// Receive the packet on s1
+	got := make([]byte, 1024)
+	n, from, err := pc.ReadFrom(got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got = got[:n]
+	t.Logf("got: %q", got)
+	if string(got) != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+	if from.(*net.UDPAddr).AddrPort().Addr() != s2ip {
+		t.Errorf("got from %v, want %v", from, s2ip)
+	}
+
+	// Write a response back to s2
+	if _, err := pc.WriteTo([]byte("world"), from); err != nil {
+		t.Fatal(err)
+	}
+
+	// Receive the response on s2
+	got = make([]byte, 1024)
+	n, err = w.Read(got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got = got[:n]
+	t.Logf("got: %q", got)
+	if string(got) != "world" {
+		t.Errorf("got %q, want world", got)
+	}
+}