MCCP2/3 --compression or --no-compression

Author: jquast

README.rst

@@ -127,6 +127,32 @@ If GA causes unwanted output for your use case, disable it::
 For PTY shells, GA is sent after 500ms of output idle time to avoid
 injecting GA in the middle of streaming output.
 
+Compression (MCCP)
+~~~~~~~~~~~~~~~~~~
+
+MCCP2 (server-to-client) and MCCP3 (client-to-server) zlib compression are
+supported, widely used by MUD servers to reduce bandwidth::
+
+    # connect to a MUD that offers MCCP compression
+    telnetlib3-client dunemud.net 6789
+
+    # or with TLS (compression auto-disabled over TLS, CRIME/BREACH mitigation)
+    telnetlib3-client --ssl dunemud.net 6788
+
+    # actively request compression from a server
+    telnetlib3-client --compression dunemud.net 6789
+
+    # reject compression even if the server offers it
+    telnetlib3-client --no-compression dunemud.net 6789
+
+    # host a MUD server that advertises MCCP2/MCCP3
+    telnetlib3-server --compression --shell=my_mud.shell
+
+By default (without ``--compression`` or ``--no-compression``), the client
+passively accepts compression when offered by the server, and the server does
+not advertise compression. Compression is automatically disabled over TLS
+connections to avoid CRIME/BREACH attacks.
+
 
 Asyncio Protocol
 ----------------

docs/rfcs.rst

@@ -81,16 +81,23 @@ Dungeon) servers and clients.
 * `MSSP`_ (MUD Server Status Protocol, option 70). Server metadata protocol
   for MUD crawlers and directories, providing server name, player count,
   codebase, and other listing information.
+* `MCCP2`_ (MUD Client Compression Protocol v2, option 86). Server-to-client
+  zlib compression, reducing bandwidth for output-heavy sessions. Activated
+  via ``IAC SB MCCP2 IAC SE``; all subsequent server output is compressed.
+* `MCCP3`_ (MUD Client Compression Protocol v3, option 87). Client-to-server
+  zlib compression, the reverse direction of MCCP2.
 
 .. _GMCP: https://www.gammon.com.au/gmcp
 .. _MSDP: https://tintin.mudhalla.net/protocols/msdp/
 .. _MSSP: https://tintin.mudhalla.net/protocols/mssp/
+.. _MCCP2: https://tintin.mudhalla.net/protocols/mccp/
+.. _MCCP3: https://tintin.mudhalla.net/protocols/mccp/
 
 MUDs Not Implemented
 --------------------
 
 Constants are also defined for the following MUD options, though their handlers
-are not implemented: MCCP/MCCP2 (85/86, compression), MXP (91, markup), ZMP
+are not implemented: MCCP (85, legacy compression), MXP (91, markup), ZMP
 (93, messaging), MSP (90, sound), and ATCP (200, Achaea-specific).
 
 Additional Resources

telnetlib3/_base.py

@@ -45,6 +45,9 @@ def _process_data_chunk(
         or ``None`` when only IAC (255) is special.
     :param log_fn: Callable for logging exceptions (e.g. ``logger.warning``).
     :returns: ``True`` if any IAC/SB command was observed.
+
+    When MCCP2 is activated mid-chunk, the remaining compressed bytes are
+    stored in ``writer._compressed_remainder`` for the caller to consume.
     """
     cmd_received = False
     n = len(data)
@@ -84,6 +87,12 @@ def _process_data_chunk(
         out_start = i
         feeding_oob = bool(writer.is_oob)
 
+        if writer._mccp2_activated:
+            writer._mccp2_activated = False
+            writer.mccp2_active = True
+            writer._compressed_remainder = data[i:] if i < n else b""
+            return True
+
     return cmd_received
 
 

telnetlib3/client.py

@@ -65,13 +65,15 @@ def __init__(
         force_binary: bool = False,
         connect_minwait: float = 0,
         connect_maxwait: float = 4.0,
+        compression: Optional[bool] = None,
         limit: Optional[int] = None,
         waiter_closed: Optional[asyncio.Future[None]] = None,
         _waiter_connected: Optional[asyncio.Future[None]] = None,
         gmcp_modules: Optional[List[str]] = None,
         gmcp_log: bool = False,
     ) -> None:
         """Initialize TelnetClient with terminal parameters."""
+        self._compression = compression
         super().__init__(
             shell=shell,
             encoding=encoding,
@@ -118,6 +120,9 @@ def connection_made(self, transport: asyncio.BaseTransport) -> None:
 
         super().connection_made(transport)
 
+        # Set compression policy on writer
+        self.writer.compression = self._compression
+
         # Wire extended rfc callbacks for requests of
         # terminal attributes, environment values, etc.
         for opt, func in (
@@ -472,6 +477,7 @@ async def open_connection(
     connect_minwait: float = 0,
     connect_maxwait: float = 3.0,
     connect_timeout: Optional[float] = None,
+    compression: Optional[bool] = None,
     waiter_closed: Optional[asyncio.Future[None]] = None,
     _waiter_connected: Optional[asyncio.Future[None]] = None,
     limit: Optional[int] = None,
@@ -531,6 +537,9 @@ async def open_connection(
         connection attempt may block indefinitely.  When specified, a
         :exc:`ConnectionError` is raised if the connection is not established
         within the given time.
+    :param compression: MCCP compression policy.  ``None`` (default) passively
+        accepts compression when offered by the server.  ``True`` actively
+        requests MCCP2/MCCP3.  ``False`` rejects all compression offers.
 
     :param force_binary: When ``True``, the encoding is used regardless
         of BINARY mode negotiation.
@@ -565,6 +574,7 @@ def connection_factory() -> client_base.BaseClient:
             shell=shell,
             connect_minwait=connect_minwait,
             connect_maxwait=connect_maxwait,
+            compression=compression,
             waiter_closed=waiter_closed,
             _waiter_connected=_waiter_connected,
             limit=limit,
@@ -897,6 +907,13 @@ def _get_argument_parser() -> argparse.ArgumentParser:
         "keys instead of encoding-specific control codes.  Use for "
         "BBSes that expect ANSI cursor sequences.",
     )
+    parser.add_argument(
+        "--compression",
+        action=argparse.BooleanOptionalAction,
+        default=None,
+        help="MCCP compression: --compression to request, --no-compression to reject, "
+        "omit to passively accept (default)",
+    )
     parser.add_argument(
         "--ssl", action="store_true", default=False, help="connect using TLS (TELNETS)"
     )
@@ -1029,6 +1046,7 @@ def _transform_args(args: argparse.Namespace) -> Dict[str, Any]:
             if args.gmcp_modules
             else None
         ),
+        "compression": args.compression,
         "gmcp_log": args.gmcp_log,
         "typescript": args.typescript,
     }

telnetlib3/client_base.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 # std imports
+import zlib
 import asyncio
 import logging
 import weakref
@@ -66,6 +67,12 @@ def __init__(
         self.writer: Optional[Union[TelnetWriter, TelnetWriterUnicode]] = None
         self._limit = limit
 
+        # MCCP2: server→client decompression
+        self._mccp2_decompressor: Optional[zlib.Decompress] = None
+        # MCCP3: client→server compression
+        self._mccp3_compressor: Optional[zlib.Compress] = None
+        self._mccp3_orig_write: Any = None
+
         # High-throughput receive pipeline
         self._rx_queue: collections.deque[bytes] = collections.deque()
         self._rx_bytes = 0
@@ -93,6 +100,11 @@ def connection_lost(self, exc: Optional[Exception]) -> None:
             return
         self._closing = True
 
+        # Clean up MCCP compressors/decompressors
+        self._mccp2_decompressor = None
+        self._mccp3_compressor = None
+        self._mccp3_orig_write = None
+
         # Drain any pending rx data before signalling EOF to prevent
         # _process_rx from calling feed_data() after feed_eof().
         self._rx_queue.clear()
@@ -343,6 +355,26 @@ def _process_chunk(self, data: bytes) -> bool:
         """Process a chunk of received bytes; return True if any IAC/SB cmd observed."""
         self._last_received = datetime.datetime.now()
 
+        # MCCP2: decompress server→client data when active
+        if self._mccp2_decompressor is not None:
+            try:
+                data = self._mccp2_decompressor.decompress(data)
+            except zlib.error:
+                self.log.warning("MCCP2 decompression error, disabling")
+                self._mccp2_end()
+                return False
+            if self._mccp2_decompressor.eof:
+                unused = self._mccp2_decompressor.unused_data
+                self._mccp2_end()
+                cmd = self._process_chunk_inner(data)
+                if unused:
+                    cmd = self._process_chunk(unused) or cmd
+                return cmd
+
+        return self._process_chunk_inner(data)
+
+    def _process_chunk_inner(self, data: bytes) -> bool:
+        """Inner chunk processing with IAC interpretation and mid-chunk MCCP2 detection."""
         try:
             mode = self.writer.mode
         except Exception:
@@ -355,7 +387,22 @@ def _process_chunk(self, data: bytes) -> bool:
         else:
             slc_special = None
 
-        return _process_data_chunk(data, self.writer, self.reader, slc_special, self.log.warning)
+        cmd_received = _process_data_chunk(
+            data, self.writer, self.reader, slc_special, self.log.warning
+        )
+
+        if self.writer._compressed_remainder is not None:
+            remainder = self.writer._compressed_remainder
+            self.writer._compressed_remainder = None
+            self._mccp2_start()
+            if remainder:
+                cmd_received = self._process_chunk(remainder) or cmd_received
+
+        # MCCP3: start compressor when writer signals activation
+        if self.writer.mccp3_active and self._mccp3_compressor is None:
+            self._mccp3_start()
+
+        return cmd_received
 
     async def _process_rx(self) -> None:
         """Async processor for receive queue that yields control and applies backpressure."""
@@ -395,6 +442,51 @@ async def _process_rx(self) -> None:
             if any_cmd and not self._waiter_connected.done():
                 self._check_negotiation_timer()
 
+    def _mccp2_start(self) -> None:
+        """Start MCCP2 decompression of server→client data."""
+        self._mccp2_decompressor = zlib.decompressobj()
+        self.log.debug("MCCP2 decompression started (server→client)")
+
+    def _mccp2_end(self) -> None:
+        """Stop MCCP2 decompression."""
+        self._mccp2_decompressor = None
+        self.writer.mccp2_active = False
+        self.log.debug("MCCP2 decompression ended (server→client)")
+
+    def _mccp3_start(self) -> None:
+        """Start MCCP3 compression of client→server data."""
+        self._mccp3_compressor = zlib.compressobj(
+            zlib.Z_BEST_COMPRESSION, zlib.DEFLATED, 12, 5, zlib.Z_DEFAULT_STRATEGY
+        )
+        # Wrap transport.write so all outbound bytes are compressed
+        transport = self.writer._transport
+        orig_write = transport.write
+
+        def compressed_write(data: bytes) -> None:
+            if self._mccp3_compressor is not None:
+                compressed = self._mccp3_compressor.compress(data)
+                compressed += self._mccp3_compressor.flush(zlib.Z_SYNC_FLUSH)
+                orig_write(compressed)
+            else:
+                orig_write(data)
+
+        transport.write = compressed_write  # type: ignore[assignment]
+        self._mccp3_orig_write = orig_write
+        self.log.debug("MCCP3 compression started (client→server)")
+
+    def _mccp3_end(self) -> None:
+        """Stop MCCP3 compression, flush Z_FINISH."""
+        if self._mccp3_compressor is not None:
+            if not self.writer.is_closing():
+                self._mccp3_orig_write(
+                    self._mccp3_compressor.flush(zlib.Z_FINISH)
+                )
+            self._mccp3_compressor = None
+            # Restore original transport.write
+            self.writer._transport.write = self._mccp3_orig_write  # type: ignore[method-assign]
+        self.writer.mccp3_active = False
+        self.log.debug("MCCP3 compression ended (client→server)")
+
     def _check_negotiation_timer(self) -> None:
         self._check_later.cancel()
         self._tasks.remove(self._check_later)

telnetlib3/fingerprinting.py

@@ -83,6 +83,8 @@
     SUPDUPOUTPUT,
     VT3270REGIME,
     AUTHENTICATION,
+    MCCP2_COMPRESS,
+    MCCP3_COMPRESS,
     COM_PORT_OPTION,
     PRAGMA_HEARTBEAT,
     SUPPRESS_LOCAL_ECHO,
@@ -260,6 +262,8 @@ class FingerprintingServer(FingerprintingTelnetServer, TelnetServer):
 # returning a hard error for anything else. GMCP-capable MUD clients
 # typically self-announce via IAC WILL GMCP, so probing is unnecessary.
 EXTENDED_OPTIONS = [
+    (MCCP2_COMPRESS, "MCCP2", "MUD Client Compression Protocol v2"),
+    (MCCP3_COMPRESS, "MCCP3", "MUD Client Compression Protocol v3"),
     (GMCP, "GMCP", "Generic MUD Communication Protocol"),
     (MSDP, "MSDP", "MUD Server Data Protocol"),
     (MSSP, "MSSP", "MUD Server Status Protocol"),