don't trip over truncated udp packets

and log such events, plus any other errors during decoding, nicely,
including the connection on which this occured.

Closes #123.

router/crypto.go

@@ -224,13 +224,20 @@ func (nd *NonDecryptor) IterateFrames(fun FrameConsumer, packet *UDPPacket) erro
 		dstNameByte := buf[:NameSize]
 		buf = buf[NameSize:]
 		length := binary.BigEndian.Uint16(buf[:2])
-		frame := buf[2 : 2+length]
-		buf = buf[2+length:]
+		buf = buf[2:]
+		if len(buf) < int(length) {
+			return PacketDecodingError{Desc: fmt.Sprintf("too short; expected frame of length %d, got %d", length, len(buf))}
+		}
+		frame := buf[:length]
+		buf = buf[length:]
 		err := fun(nd.conn, packet.Sender, srcNameByte, dstNameByte, length, frame)
 		if err != nil {
 			return err
 		}
 	}
+	if len(buf) > 0 {
+		return PacketDecodingError{Desc: fmt.Sprintf("%d octets of trailing garbage", len(buf))}
+	}
 	return nil
 }
 
@@ -279,7 +286,7 @@ func (nd *NaClDecryptor) ReceiveNonce(msg []byte) {
 func (nd *NaClDecryptor) IterateFrames(fun FrameConsumer, packet *UDPPacket) error {
 	buf, err := nd.decrypt(packet.Packet)
 	if err != nil {
-		return err
+		return PacketDecodingError{Desc: fmt.Sprint("decryption failed; ", err)}
 	}
 	packet.Packet = buf
 	return nd.NonDecryptor.IterateFrames(fun, packet)

router/router.go

@@ -204,7 +204,12 @@ func (router *Router) udpReader(conn *net.UDPConn, po PacketSink) {
 		if !ok {
 			continue
 		}
-		checkWarn(relayConn.Decryptor.IterateFrames(handleUDPPacket, udpPacket))
+		err = relayConn.Decryptor.IterateFrames(handleUDPPacket, udpPacket)
+		if pde, ok := err.(PacketDecodingError); ok {
+			relayConn.log(pde.Error())
+		} else {
+			checkWarn(err)
+		}
 	}
 }
 

router/types.go

@@ -129,6 +129,10 @@ type NameCollisionError struct {
 	Name PeerName
 }
 
+type PacketDecodingError struct {
+	Desc string
+}
+
 type LocalAddress struct {
 	ip      net.IP
 	network *net.IPNet

router/utils.go

@@ -46,6 +46,10 @@ func (nce NameCollisionError) Error() string {
 	return fmt.Sprint("Multiple peers found with same name: ", nce.Name)
 }
 
+func (pde PacketDecodingError) Error() string {
+	return fmt.Sprint("Failed to decode packet: ", pde.Desc)
+}
+
 func (packet UDPPacket) String() string {
 	return fmt.Sprintf("UDP Packet\n name: %s\n sender: %v\n payload: % X", packet.Name, packet.Sender, packet.Packet)
 }