only send Version Negotiation packets for packets larger than 1200 bytes

internal/protocol/protocol.go

@@ -64,6 +64,10 @@ const MaxReceivePacketSize ByteCount = 1452
 // MinInitialPacketSize is the minimum size an Initial packet is required to have.
 const MinInitialPacketSize = 1200
 
+// MinUnknownVersionPacketSize is the minimum size a packet with an unknown version
+// needs to have in order to trigger a Version Negotiation packet.
+const MinUnknownVersionPacketSize = MinInitialPacketSize
+
 // MinStatelessResetSize is the minimum size of a stateless reset packet that we send
 const MinStatelessResetSize = 1 /* first byte */ + 20 /* max. conn ID length */ + 4 /* max. packet number length */ + 1 /* min. payload length */ + 16 /* token */
 

server.go

@@ -351,6 +351,13 @@ func (s *baseServer) handlePacketImpl(p *receivedPacket) bool /* is the buffer s
 	}
 	// send a Version Negotiation Packet if the client is speaking a different protocol version
 	if !protocol.IsSupportedVersion(s.config.Versions, hdr.Version) {
+		if p.Size() < protocol.MinUnknownVersionPacketSize {
+			s.logger.Debugf("Dropping a packet with an unknown version that is too small (%d bytes)", p.Size())
+			if s.config.Tracer != nil {
+				s.config.Tracer.DroppedPacket(p.remoteAddr, logging.PacketTypeNotDetermined, p.Size(), logging.PacketDropUnexpectedPacket)
+			}
+			return false
+		}
 		go s.sendVersionNegotiationPacket(p, hdr)
 		return false
 	}

server_test.go

@@ -385,7 +385,7 @@ var _ = Describe("Server", func() {
 					SrcConnectionID:  srcConnID,
 					DestConnectionID: destConnID,
 					Version:          0x42,
-				}, make([]byte, protocol.MinInitialPacketSize))
+				}, make([]byte, protocol.MinUnknownVersionPacketSize))
 				raddr := &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 1337}
 				packet.remoteAddr = raddr
 				tracer.EXPECT().SentPacket(packet.remoteAddr, gomock.Any(), gomock.Any(), nil).Do(func(_ net.Addr, replyHdr *logging.Header, _ logging.ByteCount, _ []logging.Frame) {
@@ -431,6 +431,29 @@ var _ = Describe("Server", func() {
 				time.Sleep(scaleDuration(20 * time.Millisecond))
 			})
 
+			It("doesn't send a Version Negotiation Packet for unsupported versions, if the packet is too small", func() {
+				srcConnID := protocol.ConnectionID{1, 2, 3, 4, 5}
+				destConnID := protocol.ConnectionID{1, 2, 3, 4, 5, 6}
+				p := getPacket(&wire.Header{
+					IsLongHeader:     true,
+					Type:             protocol.PacketTypeHandshake,
+					SrcConnectionID:  srcConnID,
+					DestConnectionID: destConnID,
+					Version:          0x42,
+				}, make([]byte, protocol.MinUnknownVersionPacketSize-50))
+				Expect(p.Size()).To(BeNumerically("<", protocol.MinUnknownVersionPacketSize))
+				raddr := &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 1337}
+				p.remoteAddr = raddr
+				done := make(chan struct{})
+				tracer.EXPECT().DroppedPacket(raddr, logging.PacketTypeNotDetermined, p.Size(), logging.PacketDropUnexpectedPacket).Do(func(net.Addr, logging.PacketType, protocol.ByteCount, logging.PacketDropReason) {
+					close(done)
+				})
+				serv.handlePacket(p)
+				Eventually(done).Should(BeClosed())
+				// make sure no other packet is sent
+				time.Sleep(scaleDuration(20 * time.Millisecond))
+			})
+
 			It("replies with a Retry packet, if a Token is required", func() {
 				serv.config.AcceptToken = func(_ net.Addr, _ *Token) bool { return false }
 				hdr := &wire.Header{