only change remote address after authenticating a packet

fixes #395

session.go

@@ -272,10 +272,14 @@ func (s *Session) handlePacketImpl(p *receivedPacket) error {
 		utils.Debugf("<- Reading packet 0x%x (%d bytes) for connection %x @ %s", hdr.PacketNumber, len(data)+len(hdr.Raw), hdr.ConnectionID, time.Now().Format("15:04:05.000"))
 	}
 
-	// TODO: Only do this after authenticating
-	s.conn.setCurrentRemoteAddr(p.remoteAddr)
-
 	packet, err := s.unpacker.Unpack(hdr.Raw, hdr, data)
+	// if the decryption failed, this might be a packet sent by an attacker
+	// don't update the remote address
+	if quicErr, ok := err.(*qerr.QuicError); ok && quicErr.ErrorCode == qerr.DecryptionFailure {
+		return err
+	}
+	// update the remote address, even if unpacking failed for any other reason than a decryption error
+	s.conn.setCurrentRemoteAddr(p.remoteAddr)
 	if err != nil {
 		return err
 	}