Report unreachable cloudflare v3 registrations

protocol/cloudflare/datagram_v3.go

@@ -142,6 +142,10 @@ func (m *DatagramV3Muxer) handleRegistration(ctx context.Context, data []byte) {
 	if closeAfterIdle == 0 {
 		closeAfterIdle = 210 * time.Second
 	}
+	if !destination.Addr().IsValid() || destination.Addr().IsUnspecified() || destination.Port() == 0 {
+		m.sendRegistrationResponse(requestID, v3ResponseDestinationUnreachable, "")
+		return
+	}
 
 	m.sessionAccess.Lock()
 	if existing, exists := m.sessions[requestID]; exists {

protocol/cloudflare/datagram_v3_test.go

@@ -0,0 +1,38 @@
+//go:build with_cloudflare_tunnel
+
+package cloudflare
+
+import (
+	"context"
+	"encoding/binary"
+	"testing"
+
+	"github.com/sagernet/sing-box/adapter/inbound"
+	C "github.com/sagernet/sing-box/constant"
+)
+
+func TestDatagramV3RegistrationDestinationUnreachable(t *testing.T) {
+	sender := &captureDatagramSender{}
+	inboundInstance := &Inbound{
+		Adapter:     inbound.NewAdapter(C.TypeCloudflareTunnel, "test"),
+		flowLimiter: &FlowLimiter{},
+	}
+	muxer := NewDatagramV3Muxer(inboundInstance, sender, nil)
+
+	requestID := RequestID{}
+	requestID[15] = 1
+	payload := make([]byte, 1+2+2+16+4)
+	payload[0] = 0
+	binary.BigEndian.PutUint16(payload[1:3], 0)
+	binary.BigEndian.PutUint16(payload[3:5], 30)
+	copy(payload[5:21], requestID[:])
+	copy(payload[21:25], []byte{0, 0, 0, 0})
+
+	muxer.handleRegistration(context.Background(), payload)
+	if len(sender.sent) != 1 {
+		t.Fatalf("expected one registration response, got %d", len(sender.sent))
+	}
+	if sender.sent[0][0] != byte(DatagramV3TypeRegistrationResponse) || sender.sent[0][1] != v3ResponseDestinationUnreachable {
+		t.Fatalf("unexpected datagram response: %v", sender.sent[0])
+	}
+}