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>

wgengine/netstack/netstack.go

@@ -1326,6 +1326,50 @@ func (ns *Impl) forwardTCP(getClient func(...tcpip.SettableSocketOption) *gonet.
 	return
 }
 
+// ListenPacket listens for incoming packets for the given network and address.
+// Address must be of the form "ip:port" or "[ip]:port".
+//
+// As of 2024-05-18, only udp4 and udp6 are supported.
+func (ns *Impl) ListenPacket(network, address string) (net.PacketConn, error) {
+	ap, err := netip.ParseAddrPort(address)
+	if err != nil {
+		return nil, fmt.Errorf("netstack: ParseAddrPort(%q): %v", address, err)
+	}
+
+	var networkProto tcpip.NetworkProtocolNumber
+	switch network {
+	case "udp":
+		return nil, fmt.Errorf("netstack: udp not supported; use udp4 or udp6")
+	case "udp4":
+		networkProto = ipv4.ProtocolNumber
+		if !ap.Addr().Is4() {
+			return nil, fmt.Errorf("netstack: udp4 requires an IPv4 address")
+		}
+	case "udp6":
+		networkProto = ipv6.ProtocolNumber
+		if !ap.Addr().Is6() {
+			return nil, fmt.Errorf("netstack: udp6 requires an IPv6 address")
+		}
+	default:
+		return nil, fmt.Errorf("netstack: unsupported network %q", network)
+	}
+	var wq waiter.Queue
+	ep, nserr := ns.ipstack.NewEndpoint(udp.ProtocolNumber, networkProto, &wq)
+	if nserr != nil {
+		return nil, fmt.Errorf("netstack: NewEndpoint: %v", nserr)
+	}
+	localAddress := tcpip.FullAddress{
+		NIC:  nicID,
+		Addr: tcpip.AddrFromSlice(ap.Addr().AsSlice()),
+		Port: ap.Port(),
+	}
+	if err := ep.Bind(localAddress); err != nil {
+		ep.Close()
+		return nil, fmt.Errorf("netstack: Bind(%v): %v", localAddress, err)
+	}
+	return gonet.NewUDPConn(&wq, ep), nil
+}
+
 func (ns *Impl) acceptUDP(r *udp.ForwarderRequest) {
 	sess := r.ID()
 	if debugNetstack() {