From e54707cfe9ce5ca6216367f3f6454fd86900c7fa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E4=B8=96=E7=95=8C?= <i@sekai.icu>
Date: Tue, 24 Mar 2026 14:01:28 +0800
Subject: [PATCH] Return v3 registration protocol errors

---
 protocol/cloudflare/datagram_v3.go      |  4 ++--
 protocol/cloudflare/datagram_v3_test.go | 26 +++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/protocol/cloudflare/datagram_v3.go b/protocol/cloudflare/datagram_v3.go
index c40459117..ea23ed21c 100644
--- a/protocol/cloudflare/datagram_v3.go
+++ b/protocol/cloudflare/datagram_v3.go
@@ -120,7 +120,7 @@ func (m *DatagramV3Muxer) handleRegistration(ctx context.Context, data []byte) {
 
 	if flags&v3FlagIPv6 != 0 {
 		if len(data) < offset+v3IPv6AddrLen {
-			m.logger.Debug("V3 registration too short for IPv6")
+			m.sendRegistrationResponse(requestID, v3ResponseErrorWithMsg, "registration too short for IPv6")
 			return
 		}
 		var addr [16]byte
@@ -129,7 +129,7 @@ func (m *DatagramV3Muxer) handleRegistration(ctx context.Context, data []byte) {
 		offset += v3IPv6AddrLen
 	} else {
 		if len(data) < offset+v3IPv4AddrLen {
-			m.logger.Debug("V3 registration too short for IPv4")
+			m.sendRegistrationResponse(requestID, v3ResponseErrorWithMsg, "registration too short for IPv4")
 			return
 		}
 		var addr [4]byte
diff --git a/protocol/cloudflare/datagram_v3_test.go b/protocol/cloudflare/datagram_v3_test.go
index ae41d8ca9..5703310c0 100644
--- a/protocol/cloudflare/datagram_v3_test.go
+++ b/protocol/cloudflare/datagram_v3_test.go
@@ -36,3 +36,29 @@ func TestDatagramV3RegistrationDestinationUnreachable(t *testing.T) {
 		t.Fatalf("unexpected datagram response: %v", sender.sent[0])
 	}
 }
+
+func TestDatagramV3RegistrationErrorWithMessage(t *testing.T) {
+	sender := &captureDatagramSender{}
+	inboundInstance := &Inbound{
+		Adapter:     inbound.NewAdapter(C.TypeCloudflareTunnel, "test"),
+		flowLimiter: &FlowLimiter{},
+	}
+	muxer := NewDatagramV3Muxer(inboundInstance, sender, nil)
+
+	requestID := RequestID{}
+	requestID[15] = 2
+	payload := make([]byte, 1+2+2+16+1)
+	payload[0] = 1
+	binary.BigEndian.PutUint16(payload[1:3], 53)
+	binary.BigEndian.PutUint16(payload[3:5], 30)
+	copy(payload[5:21], requestID[:])
+	payload[21] = 0xaa
+
+	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] != v3ResponseErrorWithMsg {
+		t.Fatalf("unexpected datagram response: %v", sender.sent[0])
+	}
+}