Change compression to be disabled by default

This commit changes compression to default to off when AsyncSSH is
acting as a client, only enabling it if the server requires it. This
matches the OpenSSH default and avoids a potential bottleneck where
running with compression enabled can actually slow down performance
signficantly on high-bandwidth connections.

This also cleans up some unintended cosmetic differences between the
documentation for client and server connection options.

asyncssh/compression.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2013-2021 by Ron Frederick <ronf@timeheart.net> and others.
+# Copyright (c) 2013-2024 by Ron Frederick <ronf@timeheart.net> and others.
 #
 # This program and the accompanying materials are made available under
 # the terms of the Eclipse Public License v2.0 which accompanies this
@@ -149,9 +149,9 @@ def get_decompressor(alg: bytes) -> Optional[Decompressor]:
 
     return _cmp_decompressors[alg]()
 
+register_compression_alg(b'none',
+                         _none,         _none,           False, True)
 register_compression_alg(b'zlib@openssh.com',
                          _ZLibCompress, _ZLibDecompress, True,  True)
 register_compression_alg(b'zlib',
                          _ZLibCompress, _ZLibDecompress, False, False)
-register_compression_alg(b'none',
-                         _none,         _none,           False, True)

asyncssh/connection.py

@@ -7544,16 +7544,17 @@ class SSHClientConnectionOptions(SSHConnectionOptions):
        :param compression_algs: (optional)
            A list of compression algorithms to use during the SSH handshake,
            taken from :ref:`compression algorithms <CompressionAlgs>`, or
-           `None` to disable compression.
+           `None` to disable compression. The client prefers to disable
+           compression, but will enable it if the server requires it.
        :param signature_algs: (optional)
            A list of public key signature algorithms to use during the SSH
            handshake, taken from :ref:`signature algorithms <SignatureAlgs>`.
        :param rekey_bytes: (optional)
            The number of bytes which can be sent before the SSH session
-           key is renegotiated. This defaults to 1 GB.
+           key is renegotiated, defaulting to 1 GB.
        :param rekey_seconds: (optional)
            The maximum time in seconds before the SSH session key is
-           renegotiated. This defaults to 1 hour.
+           renegotiated, defaulting to 1 hour.
        :param connect_timeout: (optional)
            The maximum time in seconds allowed to complete an outbound
            SSH connection. This includes the time to establish the TCP
@@ -8289,26 +8290,28 @@ class SSHServerConnectionOptions(SSHConnectionOptions):
            this server, defaulting to `'AsyncSSH'` and its version number.
        :param kex_algs: (optional)
            A list of allowed key exchange algorithms in the SSH handshake,
-           taken from :ref:`key exchange algorithms <KexAlgs>`
+           taken from :ref:`key exchange algorithms <KexAlgs>`,
        :param encryption_algs: (optional)
            A list of encryption algorithms to use during the SSH handshake,
-           taken from :ref:`encryption algorithms <EncryptionAlgs>`
+           taken from :ref:`encryption algorithms <EncryptionAlgs>`.
        :param mac_algs: (optional)
            A list of MAC algorithms to use during the SSH handshake, taken
-           from :ref:`MAC algorithms <MACAlgs>`
+           from :ref:`MAC algorithms <MACAlgs>`.
        :param compression_algs: (optional)
            A list of compression algorithms to use during the SSH handshake,
            taken from :ref:`compression algorithms <CompressionAlgs>`, or
-           `None` to disable compression
+           `None` to disable compression. The server defaults to allowing
+           either no compression or compression after auth, depending on
+           what the client requests.
        :param signature_algs: (optional)
            A list of public key signature algorithms to use during the SSH
-           handshake, taken from :ref:`signature algorithms <SignatureAlgs>`
+           handshake, taken from :ref:`signature algorithms <SignatureAlgs>`.
        :param rekey_bytes: (optional)
            The number of bytes which can be sent before the SSH session
-           key is renegotiated, defaulting to 1 GB
+           key is renegotiated, defaulting to 1 GB.
        :param rekey_seconds: (optional)
            The maximum time in seconds before the SSH session key is
-           renegotiated, defaulting to 1 hour
+           renegotiated, defaulting to 1 hour.
        :param connect_timeout: (optional)
            The maximum time in seconds allowed to complete an outbound
            SSH connection. This includes the time to establish the TCP
@@ -8318,8 +8321,8 @@ class SSHServerConnectionOptions(SSHConnectionOptions):
            and AsyncSSH's login timeout.
        :param login_timeout: (optional)
            The maximum time in seconds allowed for authentication to
-           complete, defaulting to 2 minutes. Setting this to 0
-           will disable the login timeout.
+           complete, defaulting to 2 minutes. Setting this to 0 will
+           disable the login timeout.
 
                .. note:: This timeout only applies after the SSH TCP
                          connection is established. To set a timeout

tests/test_channel.py

@@ -1848,8 +1848,9 @@ async def test_dropbear_client(self):
         """Test reduced dropbear send packet size"""
 
         with patch('asyncssh.connection.SSHServerChannel', _ServerChannel):
-            async with self.connect(client_version='dropbear',
-                                    max_pktsize=32759) as conn:
+            async with self.connect(
+                    client_version='dropbear', max_pktsize=32759,
+                    compression_algs=['zlib@openssh.com']) as conn:
                 _, stdout, _ = await conn.open_session('send_pktsize')
                 self.assertEqual((await stdout.read()), '32758')
 
@@ -1875,7 +1876,8 @@ async def test_dropbear_server(self):
         """Test reduced dropbear send packet size"""
 
         with patch('asyncssh.connection.SSHClientChannel', _ClientChannel):
-            async with self.connect() as conn:
+            async with self.connect(
+                    compression_algs='zlib@openssh.com') as conn:
                 stdin, _, _ = await conn.open_session()
                 self.assertEqual(stdin.channel.get_send_pktsize(), 32758)
 

tests/test_connection.py

@@ -1260,6 +1260,24 @@ def send_newkeys(self, k, h):
             with self.assertRaises(asyncssh.ProtocolError):
                 await self.connect()
 
+    @asynctest
+    async def test_client_decompression_failure(self):
+        """Test client decompression failure"""
+
+        def send_packet(self, pkttype, *args, **kwargs):
+            """Send an SSH packet"""
+
+            asyncssh.connection.SSHConnection.send_packet(
+                self, pkttype, *args, **kwargs)
+
+            if pkttype == MSG_USERAUTH_SUCCESS:
+                self._compressor = None
+                self.send_debug('Test')
+
+        with patch('asyncssh.connection.SSHServerConnection.send_packet',
+                   send_packet):
+            await self.connect(compression_algs=['zlib@openssh.com'])
+
     @asynctest
     async def test_packet_decode_error(self):
         """Test SSH packet decode error"""