Allow not handled packets for UDP Forwarder

pkg/tcpip/adapters/gonet/gonet_test.go

@@ -480,7 +480,7 @@ func TestUDPForwarder(t *testing.T) {
 	}
 
 	done := make(chan struct{})
-	fwd := udp.NewForwarder(s, func(r *udp.ForwarderRequest) {
+	fwd := udp.NewForwarder(s, func(r *udp.ForwarderRequest) bool {
 		defer close(done)
 
 		var wq waiter.Queue
@@ -496,11 +496,15 @@ func TestUDPForwarder(t *testing.T) {
 		n, e := c.Read(buf)
 		if e != nil {
 			t.Errorf("c.Read() = %v", e)
+			return true
 		}
 
 		if _, e := c.Write(buf[:n]); e != nil {
 			t.Errorf("c.Write() = %v", e)
+			return true
 		}
+
+		return true
 	})
 	s.SetTransportProtocolHandler(udp.ProtocolNumber, fwd.HandlePacket)
 

pkg/tcpip/transport/udp/forwarder.go

@@ -20,19 +20,24 @@ import (
 	"gvisor.dev/gvisor/pkg/waiter"
 )
 
+// ForwarderHandler handles incoming requests. Returning true marks the
+// request as handled, returning false marks the request as unhandled.
+// Stack may send an ICMP port unreachable message for unhandled requests.
+type ForwarderHandler func(*ForwarderRequest) (handled bool)
+
 // Forwarder is a session request forwarder, which allows clients to decide
 // what to do with a session request, for example: ignore it, or process it.
 //
 // The canonical way of using it is to pass the Forwarder.HandlePacket function
 // to stack.SetTransportProtocolHandler.
 type Forwarder struct {
-	handler func(*ForwarderRequest)
+	handler ForwarderHandler
 
 	stack *stack.Stack
 }
 
 // NewForwarder allocates and initializes a new forwarder.
-func NewForwarder(s *stack.Stack, handler func(*ForwarderRequest)) *Forwarder {
+func NewForwarder(s *stack.Stack, handler ForwarderHandler) *Forwarder {
 	return &Forwarder{
 		stack:   s,
 		handler: handler,
@@ -44,13 +49,11 @@ func NewForwarder(s *stack.Stack, handler func(*ForwarderRequest)) *Forwarder {
 // This function is expected to be passed as an argument to the
 // stack.SetTransportProtocolHandler function.
 func (f *Forwarder) HandlePacket(id stack.TransportEndpointID, pkt *stack.PacketBuffer) bool {
-	f.handler(&ForwarderRequest{
+	return f.handler(&ForwarderRequest{
 		stack: f.stack,
 		id:    id,
 		pkt:   pkt.Clone(),
 	})
-
-	return true
 }
 
 // ForwarderRequest represents a session request received by the forwarder and