/lib/paramiko/transport.py

# Copyright (C) 2003-2007  Robey Pointer <robey@lag.net>
#
# This file is part of paramiko.
#
# Paramiko is free software; you can redistribute it and/or modify it under the
# terms of the GNU Lesser General Public License as published by the Free
# Software Foundation; either version 2.1 of the License, or (at your option)
# any later version.
#
# Paramiko is distrubuted in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
# details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with Paramiko; if not, write to the Free Software Foundation, Inc.,
# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA.

"""
L{Transport} handles the core SSH2 protocol.
"""

import os
import socket
import string
import struct
import sys
import threading
import time
import weakref

from paramiko import util
from paramiko.auth_handler import AuthHandler
from paramiko.channel import Channel
from paramiko.common import *
from paramiko.compress import ZlibCompressor, ZlibDecompressor
from paramiko.dsskey import DSSKey
from paramiko.kex_gex import KexGex
from paramiko.kex_group1 import KexGroup1
from paramiko.message import Message
from paramiko.packet import Packetizer, NeedRekeyException
from paramiko.primes import ModulusPack
from paramiko.rsakey import RSAKey
from paramiko.server import ServerInterface
from paramiko.sftp_client import SFTPClient
from paramiko.ssh_exception import SSHException, BadAuthenticationType, ChannelException

# these come from PyCrypt
#     http://www.amk.ca/python/writing/pycrypt/
# i believe this on the standards track.
# PyCrypt compiled for Win32 can be downloaded from the HashTar homepage:
#     http://nitace.bsd.uchicago.edu:8080/hashtar
from Crypto.Cipher import Blowfish, AES, DES3
from Crypto.Hash import SHA, MD5


# for thread cleanup
_active_threads = []
def _join_lingering_threads():
    for thr in _active_threads:
        thr.stop_thread()
import atexit
atexit.register(_join_lingering_threads)


class SecurityOptions (object):
    """
    Simple object containing the security preferences of an ssh transport.
    These are tuples of acceptable ciphers, digests, key types, and key
    exchange algorithms, listed in order of preference.

    Changing the contents and/or order of these fields affects the underlying
    L{Transport} (but only if you change them before starting the session).
    If you try to add an algorithm that paramiko doesn't recognize,
    C{ValueError} will be raised.  If you try to assign something besides a
    tuple to one of the fields, C{TypeError} will be raised.
    """
    __slots__ = [ 'ciphers', 'digests', 'key_types', 'kex', 'compression', '_transport' ]

    def __init__(self, transport):
        self._transport = transport

    def __repr__(self):
        """
        Returns a string representation of this object, for debugging.

        @rtype: str
        """
        return '<paramiko.SecurityOptions for %s>' % repr(self._transport)

    def _get_ciphers(self):
        return self._transport._preferred_ciphers

    def _get_digests(self):
        return self._transport._preferred_macs

    def _get_key_types(self):
        return self._transport._preferred_keys

    def _get_kex(self):
        return self._transport._preferred_kex
        
    def _get_compression(self):
        return self._transport._preferred_compression

    def _set(self, name, orig, x):
        if type(x) is list:
            x = tuple(x)
        if type(x) is not tuple:
            raise TypeError('expected tuple or list')
        possible = getattr(self._transport, orig).keys()
        forbidden = filter(lambda n: n not in possible, x)
        if len(forbidden) > 0:
            raise ValueError('unknown cipher')
        setattr(self._transport, name, x)

    def _set_ciphers(self, x):
        self._set('_preferred_ciphers', '_cipher_info', x)

    def _set_digests(self, x):
        self._set('_preferred_macs', '_mac_info', x)

    def _set_key_types(self, x):
        self._set('_preferred_keys', '_key_info', x)

    def _set_kex(self, x):
        self._set('_preferred_kex', '_kex_info', x)
    
    def _set_compression(self, x):
        self._set('_preferred_compression', '_compression_info', x)

    ciphers = property(_get_ciphers, _set_ciphers, None,
                       "Symmetric encryption ciphers")
    digests = property(_get_digests, _set_digests, None,
                       "Digest (one-way hash) algorithms")
    key_types = property(_get_key_types, _set_key_types, None,
                         "Public-key algorithms")
    kex = property(_get_kex, _set_kex, None, "Key exchange algorithms")
    compression = property(_get_compression, _set_compression, None,
                           "Compression algorithms")


class Transport (threading.Thread):
    """
    An SSH Transport attaches to a stream (usually a socket), negotiates an
    encrypted session, authenticates, and then creates stream tunnels, called
    L{Channel}s, across the session.  Multiple channels can be multiplexed
    across a single session (and often are, in the case of port forwardings).
    """

    _PROTO_ID = '2.0'
    _CLIENT_ID = 'paramiko_1.7'

    _preferred_ciphers = ( 'aes128-cbc', 'blowfish-cbc', 'aes256-cbc', '3des-cbc' )
    _preferred_macs = ( 'hmac-sha1', 'hmac-md5', 'hmac-sha1-96', 'hmac-md5-96' )
    _preferred_keys = ( 'ssh-rsa', 'ssh-dss' )
    _preferred_kex = ( 'diffie-hellman-group1-sha1', 'diffie-hellman-group-exchange-sha1' )
    _preferred_compression = ( 'none', )
    
    _cipher_info = {
        'blowfish-cbc': { 'class': Blowfish, 'mode': Blowfish.MODE_CBC, 'block-size': 8, 'key-size': 16 },
        'aes128-cbc': { 'class': AES, 'mode': AES.MODE_CBC, 'block-size': 16, 'key-size': 16 },
        'aes256-cbc': { 'class': AES, 'mode': AES.MODE_CBC, 'block-size': 16, 'key-size': 32 },
        '3des-cbc': { 'class': DES3, 'mode': DES3.MODE_CBC, 'block-size': 8, 'key-size': 24 },
        }

    _mac_info = {
        'hmac-sha1': { 'class': SHA, 'size': 20 },
        'hmac-sha1-96': { 'class': SHA, 'size': 12 },
        'hmac-md5': { 'class': MD5, 'size': 16 },
        'hmac-md5-96': { 'class': MD5, 'size': 12 },
        }

    _key_info = {
        'ssh-rsa': RSAKey,
        'ssh-dss': DSSKey,
        }

    _kex_info = {
        'diffie-hellman-group1-sha1': KexGroup1,
        'diffie-hellman-group-exchange-sha1': KexGex,
        }
    
    _compression_info = {
        # zlib@openssh.com is just zlib, but only turned on after a successful
        # authentication.  openssh servers may only offer this type because
        # they've had troubles with security holes in zlib in the past.
        'zlib@openssh.com': ( ZlibCompressor, ZlibDecompressor ),
        'zlib': ( ZlibCompressor, ZlibDecompressor ),
        'none': ( None, None ),
    }


    _modulus_pack = None

    def __init__(self, sock):
        """
        Create a new SSH session over an existing socket, or socket-like
        object.  This only creates the Transport object; it doesn't begin the
        SSH session yet.  Use L{connect} or L{start_client} to begin a client
        session, or L{start_server} to begin a server session.

        If the object is not actually a socket, it must have the following
        methods:
            - C{send(str)}: Writes from 1 to C{len(str)} bytes, and
              returns an int representing the number of bytes written.  Returns
              0 or raises C{EOFError} if the stream has been closed.
            - C{recv(int)}: Reads from 1 to C{int} bytes and returns them as a
              string.  Returns 0 or raises C{EOFError} if the stream has been
              closed.
            - C{close()}: Closes the socket.
            - C{settimeout(n)}: Sets a (float) timeout on I/O operations.

        For ease of use, you may also pass in an address (as a tuple) or a host
        string as the C{sock} argument.  (A host string is a hostname with an
        optional port (separated by C{":"}) which will be converted into a
        tuple of C{(hostname, port)}.)  A socket will be connected to this
        address and used for communication.  Exceptions from the C{socket} call
        may be thrown in this case.

        @param sock: a socket or socket-like object to create the session over.
        @type sock: socket
        """
        if type(sock) is str:
            # convert "host:port" into (host, port)
            hl = sock.split(':', 1)
            if len(hl) == 1:
                sock = (hl[0], 22)
            else:
                sock = (hl[0], int(hl[1]))
        if type(sock) is tuple:
            # connect to the given (host, port)
            hostname, port = sock
            sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            sock.connect((hostname, port))
        # okay, normal socket-ish flow here...
        threading.Thread.__init__(self)
        self.randpool = randpool
        self.sock = sock
        # Python < 2.3 doesn't have the settimeout method - RogerB
        try:
            # we set the timeout so we can check self.active periodically to
            # see if we should bail.  socket.timeout exception is never
            # propagated.
            self.sock.settimeout(0.1)
        except AttributeError:
            pass

        # negotiated crypto parameters
        self.packetizer = Packetizer(sock)
        self.local_version = 'SSH-' + self._PROTO_ID + '-' + self._CLIENT_ID
        self.remote_version = ''
        self.local_cipher = self.remote_cipher = ''
        self.local_kex_init = self.remote_kex_init = None
        self.local_mac = self.remote_mac = None
        self.local_compression = self.remote_compression = None
        self.session_id = None
        self.host_key_type = None
        self.host_key = None
        
        # state used during negotiation
        self.kex_engine = None
        self.H = None
        self.K = None

        self.active = False
        self.initial_kex_done = False
        self.in_kex = False
        self.authenticated = False
        self._expected_packet = tuple()
        self.lock = threading.Lock()    # synchronization (always higher level than write_lock)

        # tracking open channels
        self.channels = weakref.WeakValueDictionary()   # (id -> Channel)
        self.channel_events = { }       # (id -> Event)
        self.channels_seen = { }        # (id -> True)
        self._channel_counter = 1
        self.window_size = 65536
        self.max_packet_size = 34816
        self._x11_handler = None
        self._tcp_handler = None

        self.saved_exception = None
        self.clear_to_send = threading.Event()
        self.clear_to_send_lock = threading.Lock()
        self.log_name = 'paramiko.transport'
        self.logger = util.get_logger(self.log_name)
        self.packetizer.set_log(self.logger)
        self.auth_handler = None
        self.global_response = None     # response Message from an arbitrary global request
        self.completion_event = None    # user-defined event callbacks
        self.banner_timeout = 15        # how long (seconds) to wait for the SSH banner

        # server mode:
        self.server_mode = False
        self.server_object = None
        self.server_key_dict = { }
        self.server_accepts = [ ]
        self.server_accept_cv = threading.Condition(self.lock)
        self.subsystem_table = { }

    def __repr__(self):
        """
        Returns a string representation of this object, for debugging.

        @rtype: str
        """
        out = '<paramiko.Transport at %s' % hex(long(id(self)) & 0xffffffffL)
        if not self.active:
            out += ' (unconnected)'
        else:
            if self.local_cipher != '':
                out += ' (cipher %s, %d bits)' % (self.local_cipher,
                                                  self._cipher_info[self.local_cipher]['key-size'] * 8)
            if self.is_authenticated():
                if len(self.channels) == 1:
                    out += ' (active; 1 open channel)'
                else:
                    out += ' (active; %d open channels)' % len(self.channels)
            elif self.initial_kex_done:
                out += ' (connected; awaiting auth)'
            else:
                out += ' (connecting)'
        out += '>'
        return out
    
    def atfork(self):
        """
        Terminate this Transport without closing the session.  On posix
        systems, if a Transport is open during process forking, both parent
        and child will share the underlying socket, but only one process can
        use the connection (without corrupting the session).  Use this method
        to clean up a Transport object without disrupting the other process.
        
        @since: 1.5.3
        """
        self.sock.close()
        self.close()

    def get_security_options(self):
        """
        Return a L{SecurityOptions} object which can be used to tweak the
        encryption algorithms this transport will permit, and the order of
        preference for them.

        @return: an object that can be used to change the preferred algorithms
            for encryption, digest (hash), public key, and key exchange.
        @rtype: L{SecurityOptions}
        """
        return SecurityOptions(self)

    def start_client(self, event=None):
        """
        Negotiate a new SSH2 session as a client.  This is the first step after
        creating a new L{Transport}.  A separate thread is created for protocol
        negotiation.
        
        If an event is passed in, this method returns immediately.  When
        negotiation is done (successful or not), the given C{Event} will
        be triggered.  On failure, L{is_active} will return C{False}.
        
        (Since 1.4) If C{event} is C{None}, this method will not return until
        negotation is done.  On success, the method returns normally.
        Otherwise an SSHException is raised.

        After a successful negotiation, you will usually want to authenticate,
        calling L{auth_password <Transport.auth_password>} or
        L{auth_publickey <Transport.auth_publickey>}.

        @note: L{connect} is a simpler method for connecting as a client.
        
        @note: After calling this method (or L{start_server} or L{connect}),
            you should no longer directly read from or write to the original
            socket object.

        @param event: an event to trigger when negotiation is complete
            (optional)
        @type event: threading.Event

        @raise SSHException: if negotiation fails (and no C{event} was passed
            in)
        """
        self.active = True
        if event is not None:
            # async, return immediately and let the app poll for completion
            self.completion_event = event
            self.start()
            return

        # synchronous, wait for a result
        self.completion_event = event = threading.Event()
        self.start()
        while True:
            event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is not None:
                    raise e
                raise SSHException('Negotiation failed.')
            if event.isSet():
                break

    def start_server(self, event=None, server=None):
        """
        Negotiate a new SSH2 session as a server.  This is the first step after
        creating a new L{Transport} and setting up your server host key(s).  A
        separate thread is created for protocol negotiation.
        
        If an event is passed in, this method returns immediately.  When
        negotiation is done (successful or not), the given C{Event} will
        be triggered.  On failure, L{is_active} will return C{False}.
        
        (Since 1.4) If C{event} is C{None}, this method will not return until
        negotation is done.  On success, the method returns normally.
        Otherwise an SSHException is raised.

        After a successful negotiation, the client will need to authenticate.
        Override the methods
        L{get_allowed_auths <ServerInterface.get_allowed_auths>},
        L{check_auth_none <ServerInterface.check_auth_none>},
        L{check_auth_password <ServerInterface.check_auth_password>}, and
        L{check_auth_publickey <ServerInterface.check_auth_publickey>} in the
        given C{server} object to control the authentication process.

        After a successful authentication, the client should request to open
        a channel.  Override
        L{check_channel_request <ServerInterface.check_channel_request>} in the
        given C{server} object to allow channels to be opened.

        @note: After calling this method (or L{start_client} or L{connect}),
            you should no longer directly read from or write to the original
            socket object.

        @param event: an event to trigger when negotiation is complete.
        @type event: threading.Event
        @param server: an object used to perform authentication and create
            L{Channel}s.
        @type server: L{server.ServerInterface}

        @raise SSHException: if negotiation fails (and no C{event} was passed
            in)
        """
        if server is None:
            server = ServerInterface()
        self.server_mode = True
        self.server_object = server
        self.active = True
        if event is not None:
            # async, return immediately and let the app poll for completion
            self.completion_event = event
            self.start()
            return

        # synchronous, wait for a result
        self.completion_event = event = threading.Event()
        self.start()
        while True:
            event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is not None:
                    raise e
                raise SSHException('Negotiation failed.')
            if event.isSet():
                break

    def add_server_key(self, key):
        """
        Add a host key to the list of keys used for server mode.  When behaving
        as a server, the host key is used to sign certain packets during the
        SSH2 negotiation, so that the client can trust that we are who we say
        we are.  Because this is used for signing, the key must contain private
        key info, not just the public half.  Only one key of each type (RSA or
        DSS) is kept.
        
        @param key: the host key to add, usually an L{RSAKey <rsakey.RSAKey>} or
            L{DSSKey <dsskey.DSSKey>}.
        @type key: L{PKey <pkey.PKey>}
        """
        self.server_key_dict[key.get_name()] = key

    def get_server_key(self):
        """
        Return the active host key, in server mode.  After negotiating with the
        client, this method will return the negotiated host key.  If only one
        type of host key was set with L{add_server_key}, that's the only key
        that will ever be returned.  But in cases where you have set more than
        one type of host key (for example, an RSA key and a DSS key), the key
        type will be negotiated by the client, and this method will return the
        key of the type agreed on.  If the host key has not been negotiated
        yet, C{None} is returned.  In client mode, the behavior is undefined.

        @return: host key of the type negotiated by the client, or C{None}.
        @rtype: L{PKey <pkey.PKey>}
        """
        try:
            return self.server_key_dict[self.host_key_type]
        except KeyError:
            pass
        return None

    def load_server_moduli(filename=None):
        """
        I{(optional)}
        Load a file of prime moduli for use in doing group-exchange key
        negotiation in server mode.  It's a rather obscure option and can be
        safely ignored.

        In server mode, the remote client may request "group-exchange" key
        negotiation, which asks the server to send a random prime number that
        fits certain criteria.  These primes are pretty difficult to compute,
        so they can't be generated on demand.  But many systems contain a file
        of suitable primes (usually named something like C{/etc/ssh/moduli}).
        If you call C{load_server_moduli} and it returns C{True}, then this
        file of primes has been loaded and we will support "group-exchange" in
        server mode.  Otherwise server mode will just claim that it doesn't
        support that method of key negotiation.

        @param filename: optional path to the moduli file, if you happen to
            know that it's not in a standard location.
        @type filename: str
        @return: True if a moduli file was successfully loaded; False
            otherwise.
        @rtype: bool
        
        @note: This has no effect when used in client mode.
        """
        Transport._modulus_pack = ModulusPack(randpool)
        # places to look for the openssh "moduli" file
        file_list = [ '/etc/ssh/moduli', '/usr/local/etc/moduli' ]
        if filename is not None:
            file_list.insert(0, filename)
        for fn in file_list:
            try:
                Transport._modulus_pack.read_file(fn)
                return True
            except IOError:
                pass
        # none succeeded
        Transport._modulus_pack = None
        return False
    load_server_moduli = staticmethod(load_server_moduli)

    def close(self):
        """
        Close this session, and any open channels that are tied to it.
        """
        if not self.active:
            return
        self.active = False
        self.packetizer.close()
        self.join()
        for chan in self.channels.values():
            chan._unlink()

    def get_remote_server_key(self):
        """
        Return the host key of the server (in client mode).

        @note: Previously this call returned a tuple of (key type, key string).
            You can get the same effect by calling
            L{PKey.get_name <pkey.PKey.get_name>} for the key type, and
            C{str(key)} for the key string.

        @raise SSHException: if no session is currently active.
        
        @return: public key of the remote server
        @rtype: L{PKey <pkey.PKey>}
        """
        if (not self.active) or (not self.initial_kex_done):
            raise SSHException('No existing session')
        return self.host_key

    def is_active(self):
        """
        Return true if this session is active (open).

        @return: True if the session is still active (open); False if the
            session is closed
        @rtype: bool
        """
        return self.active

    def open_session(self):
        """
        Request a new channel to the server, of type C{"session"}.  This
        is just an alias for C{open_channel('session')}.

        @return: a new L{Channel}
        @rtype: L{Channel}
        
        @raise SSHException: if the request is rejected or the session ends
            prematurely
        """
        return self.open_channel('session')

    def open_x11_channel(self, src_addr=None):
        """
        Request a new channel to the client, of type C{"x11"}.  This
        is just an alias for C{open_channel('x11', src_addr=src_addr)}.

        @param src_addr: the source address of the x11 server (port is the
            x11 port, ie. 6010)
        @type src_addr: (str, int)
        @return: a new L{Channel}
        @rtype: L{Channel}
        
        @raise SSHException: if the request is rejected or the session ends
            prematurely
        """
        return self.open_channel('x11', src_addr=src_addr)
    
    def open_forwarded_tcpip_channel(self, (src_addr, src_port), (dest_addr, dest_port)):
        """
        Request a new channel back to the client, of type C{"forwarded-tcpip"}.
        This is used after a client has requested port forwarding, for sending
        incoming connections back to the client.
        
        @param src_addr: originator's address
        @param src_port: originator's port
        @param dest_addr: local (server) connected address
        @param dest_port: local (server) connected port
        """
        return self.open_channel('forwarded-tcpip', (dest_addr, dest_port), (src_addr, src_port))
    
    def open_channel(self, kind, dest_addr=None, src_addr=None):
        """
        Request a new channel to the server.  L{Channel}s are socket-like
        objects used for the actual transfer of data across the session.
        You may only request a channel after negotiating encryption (using
        L{connect} or L{start_client}) and authenticating.

        @param kind: the kind of channel requested (usually C{"session"},
            C{"forwarded-tcpip"}, C{"direct-tcpip"}, or C{"x11"})
        @type kind: str
        @param dest_addr: the destination address of this port forwarding,
            if C{kind} is C{"forwarded-tcpip"} or C{"direct-tcpip"} (ignored
            for other channel types)
        @type dest_addr: (str, int)
        @param src_addr: the source address of this port forwarding, if
            C{kind} is C{"forwarded-tcpip"}, C{"direct-tcpip"}, or C{"x11"}
        @type src_addr: (str, int)
        @return: a new L{Channel} on success
        @rtype: L{Channel}

        @raise SSHException: if the request is rejected or the session ends
            prematurely
        """
        chan = None
        if not self.active:
            # don't bother trying to allocate a channel
            return None
        self.lock.acquire()
        try:
            chanid = self._next_channel()
            m = Message()
            m.add_byte(chr(MSG_CHANNEL_OPEN))
            m.add_string(kind)
            m.add_int(chanid)
            m.add_int(self.window_size)
            m.add_int(self.max_packet_size)
            if (kind == 'forwarded-tcpip') or (kind == 'direct-tcpip'):
                m.add_string(dest_addr[0])
                m.add_int(dest_addr[1])
                m.add_string(src_addr[0])
                m.add_int(src_addr[1])
            elif kind == 'x11':
                m.add_string(src_addr[0])
                m.add_int(src_addr[1])
            self.channels[chanid] = chan = Channel(chanid)
            self.channel_events[chanid] = event = threading.Event()
            self.channels_seen[chanid] = True
            chan._set_transport(self)
            chan._set_window(self.window_size, self.max_packet_size)
        finally:
            self.lock.release()
        self._send_user_message(m)
        while True:
            event.wait(0.1);
            if not self.active:
                e = self.get_exception()
                if e is None:
                    e = SSHException('Unable to open channel.')
                raise e
            if event.isSet():
                break
        self.lock.acquire()
        try:
            if chanid in self.channels:
                return chan
        finally:
            self.lock.release()
        e = self.get_exception()
        if e is None:
            e = SSHException('Unable to open channel.')
        raise e

    def request_port_forward(self, address, port, handler=None):
        """
        Ask the server to forward TCP connections from a listening port on
        the server, across this SSH session.
        
        If a handler is given, that handler is called from a different thread
        whenever a forwarded connection arrives.  The handler parameters are::
        
            handler(channel, (origin_addr, origin_port), (server_addr, server_port))
            
        where C{server_addr} and C{server_port} are the address and port that
        the server was listening on.
        
        If no handler is set, the default behavior is to send new incoming
        forwarded connections into the accept queue, to be picked up via
        L{accept}.
        
        @param address: the address to bind when forwarding
        @type address: str
        @param port: the port to forward, or 0 to ask the server to allocate
            any port
        @type port: int
        @param handler: optional handler for incoming forwarded connections
        @type handler: function(Channel, (str, int), (str, int))
        @return: the port # allocated by the server
        @rtype: int
        
        @raise SSHException: if the server refused the TCP forward request
        """
        if not self.active:
            raise SSHException('SSH session not active')
        address = str(address)
        port = int(port)
        response = self.global_request('tcpip-forward', (address, port), wait=True)
        if response is None:
            raise SSHException('TCP forwarding request denied')
        if port == 0:
            port = response.get_int()
        if handler is None:
            def default_handler(channel, (src_addr, src_port), (dest_addr, dest_port)):
                self._queue_incoming_channel(channel)
            handler = default_handler
        self._tcp_handler = handler
        return port

    def cancel_port_forward(self, address, port):
        """
        Ask the server to cancel a previous port-forwarding request.  No more
        connections to the given address & port will be forwarded across this
        ssh connection.
        
        @param address: the address to stop forwarding
        @type address: str
        @param port: the port to stop forwarding
        @type port: int
        """
        if not self.active:
            return
        self._tcp_handler = None
        self.global_request('cancel-tcpip-forward', (address, port), wait=True)
        
    def open_sftp_client(self):
        """
        Create an SFTP client channel from an open transport.  On success,
        an SFTP session will be opened with the remote host, and a new
        SFTPClient object will be returned.

        @return: a new L{SFTPClient} object, referring to an sftp session
            (channel) across this transport
        @rtype: L{SFTPClient}
        """
        return SFTPClient.from_transport(self)

    def send_ignore(self, bytes=None):
        """
        Send a junk packet across the encrypted link.  This is sometimes used
        to add "noise" to a connection to confuse would-be attackers.  It can
        also be used as a keep-alive for long lived connections traversing
        firewalls.

        @param bytes: the number of random bytes to send in the payload of the
            ignored packet -- defaults to a random number from 10 to 41.
        @type bytes: int
        """
        m = Message()
        m.add_byte(chr(MSG_IGNORE))
        randpool.stir()
        if bytes is None:
            bytes = (ord(randpool.get_bytes(1)) % 32) + 10
        m.add_bytes(randpool.get_bytes(bytes))
        self._send_user_message(m)

    def renegotiate_keys(self):
        """
        Force this session to switch to new keys.  Normally this is done
        automatically after the session hits a certain number of packets or
        bytes sent or received, but this method gives you the option of forcing
        new keys whenever you want.  Negotiating new keys causes a pause in
        traffic both ways as the two sides swap keys and do computations.  This
        method returns when the session has switched to new keys.

        @raise SSHException: if the key renegotiation failed (which causes the
            session to end)
        """
        self.completion_event = threading.Event()
        self._send_kex_init()
        while True:
            self.completion_event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is not None:
                    raise e
                raise SSHException('Negotiation failed.')
            if self.completion_event.isSet():
                break
        return

    def set_keepalive(self, interval):
        """
        Turn on/off keepalive packets (default is off).  If this is set, after
        C{interval} seconds without sending any data over the connection, a
        "keepalive" packet will be sent (and ignored by the remote host).  This
        can be useful to keep connections alive over a NAT, for example.
        
        @param interval: seconds to wait before sending a keepalive packet (or
            0 to disable keepalives).
        @type interval: int
        """
        self.packetizer.set_keepalive(interval,
            lambda x=weakref.proxy(self): x.global_request('keepalive@lag.net', wait=False))

    def global_request(self, kind, data=None, wait=True):
        """
        Make a global request to the remote host.  These are normally
        extensions to the SSH2 protocol.

        @param kind: name of the request.
        @type kind: str
        @param data: an optional tuple containing additional data to attach
            to the request.
        @type data: tuple
        @param wait: C{True} if this method should not return until a response
            is received; C{False} otherwise.
        @type wait: bool
        @return: a L{Message} containing possible additional data if the
            request was successful (or an empty L{Message} if C{wait} was
            C{False}); C{None} if the request was denied.
        @rtype: L{Message}
        """
        if wait:
            self.completion_event = threading.Event()
        m = Message()
        m.add_byte(chr(MSG_GLOBAL_REQUEST))
        m.add_string(kind)
        m.add_boolean(wait)
        if data is not None:
            m.add(*data)
        self._log(DEBUG, 'Sending global request "%s"' % kind)
        self._send_user_message(m)
        if not wait:
            return None
        while True:
            self.completion_event.wait(0.1)
            if not self.active:
                return None
            if self.completion_event.isSet():
                break
        return self.global_response

    def accept(self, timeout=None):
        """
        Return the next channel opened by the client over this transport, in
        server mode.  If no channel is opened before the given timeout, C{None}
        is returned.
        
        @param timeout: seconds to wait for a channel, or C{None} to wait
            forever
        @type timeout: int
        @return: a new Channel opened by the client
        @rtype: L{Channel}
        """
        self.lock.acquire()
        try:
            if len(self.server_accepts) > 0:
                chan = self.server_accepts.pop(0)
            else:
                self.server_accept_cv.wait(timeout)
                if len(self.server_accepts) > 0:
                    chan = self.server_accepts.pop(0)
                else:
                    # timeout
                    chan = None
        finally:
            self.lock.release()
        return chan

    def connect(self, hostkey=None, username='', password=None, pkey=None):
        """
        Negotiate an SSH2 session, and optionally verify the server's host key
        and authenticate using a password or private key.  This is a shortcut
        for L{start_client}, L{get_remote_server_key}, and
        L{Transport.auth_password} or L{Transport.auth_publickey}.  Use those
        methods if you want more control.

        You can use this method immediately after creating a Transport to
        negotiate encryption with a server.  If it fails, an exception will be
        thrown.  On success, the method will return cleanly, and an encrypted
        session exists.  You may immediately call L{open_channel} or
        L{open_session} to get a L{Channel} object, which is used for data
        transfer.

        @note: If you fail to supply a password or private key, this method may
        succeed, but a subsequent L{open_channel} or L{open_session} call may
        fail because you haven't authenticated yet.

        @param hostkey: the host key expected from the server, or C{None} if
            you don't want to do host key verification.
        @type hostkey: L{PKey<pkey.PKey>}
        @param username: the username to authenticate as.
        @type username: str
        @param password: a password to use for authentication, if you want to
            use password authentication; otherwise C{None}.
        @type password: str
        @param pkey: a private key to use for authentication, if you want to
            use private key authentication; otherwise C{None}.
        @type pkey: L{PKey<pkey.PKey>}
        
        @raise SSHException: if the SSH2 negotiation fails, the host key
            supplied by the server is incorrect, or authentication fails.
        """
        if hostkey is not None:
            self._preferred_keys = [ hostkey.get_name() ]

        self.start_client()

        # check host key if we were given one
        if (hostkey is not None):
            key = self.get_remote_server_key()
            if (key.get_name() != hostkey.get_name()) or (str(key) != str(hostkey)):
                self._log(DEBUG, 'Bad host key from server')
                self._log(DEBUG, 'Expected: %s: %s' % (hostkey.get_name(), repr(str(hostkey))))
                self._log(DEBUG, 'Got     : %s: %s' % (key.get_name(), repr(str(key))))
                raise SSHException('Bad host key from server')
            self._log(DEBUG, 'Host key verified (%s)' % hostkey.get_name())

        if (pkey is not None) or (password is not None):
            if password is not None:
                self._log(DEBUG, 'Attempting password auth...')
                self.auth_password(username, password)
            else:
                self._log(DEBUG, 'Attempting public-key auth...')
                self.auth_publickey(username, pkey)

        return
        
    def get_exception(self):
        """
        Return any exception that happened during the last server request.
        This can be used to fetch more specific error information after using
        calls like L{start_client}.  The exception (if any) is cleared after
        this call.
        
        @return: an exception, or C{None} if there is no stored exception.
        @rtype: Exception
        
        @since: 1.1
        """
        self.lock.acquire()
        try:
            e = self.saved_exception
            self.saved_exception = None
            return e
        finally:
            self.lock.release()

    def set_subsystem_handler(self, name, handler, *larg, **kwarg):
        """
        Set the handler class for a subsystem in server mode.  If a request
        for this subsystem is made on an open ssh channel later, this handler
        will be constructed and called -- see L{SubsystemHandler} for more
        detailed documentation.

        Any extra parameters (including keyword arguments) are saved and
        passed to the L{SubsystemHandler} constructor later.

        @param name: name of the subsystem.
        @type name: str
        @param handler: subclass of L{SubsystemHandler} that handles this
            subsystem.
        @type handler: class
        """
        try:
            self.lock.acquire()
            self.subsystem_table[name] = (handler, larg, kwarg)
        finally:
            self.lock.release()
    
    def is_authenticated(self):
        """
        Return true if this session is active and authenticated.

        @return: True if the session is still open and has been authenticated
            successfully; False if authentication failed and/or the session is
            closed.
        @rtype: bool
        """
        return self.active and (self.auth_handler is not None) and self.auth_handler.is_authenticated()
    
    def get_username(self):
        """
        Return the username this connection is authenticated for.  If the
        session is not authenticated (or authentication failed), this method
        returns C{None}.

        @return: username that was authenticated, or C{None}.
        @rtype: string
        """
        if not self.active or (self.auth_handler is None):
            return None
        return self.auth_handler.get_username()

    def auth_none(self, username):
        """
        Try to authenticate to the server using no authentication at all.
        This will almost always fail.  It may be useful for determining the
        list of authentication types supported by the server, by catching the
        L{BadAuthenticationType} exception raised.
        
        @param username: the username to authenticate as
        @type username: string
        @return: list of auth types permissible for the next stage of
            authentication (normally empty)
        @rtype: list

        @raise BadAuthenticationType: if "none" authentication isn't allowed
            by the server for this user
        @raise SSHException: if the authentication failed due to a network
            error
            
        @since: 1.5
        """
        if (not self.active) or (not self.initial_kex_done):
            raise SSHException('No existing session')
        my_event = threading.Event()
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_none(username, my_event)
        return self.auth_handler.wait_for_response(my_event)

    def auth_password(self, username, password, event=None, fallback=True):
        """
        Authenticate to the server using a password.  The username and password
        are sent over an encrypted link.
        
        If an C{event} is passed in, this method will return immediately, and
        the event will be triggered once authentication succeeds or fails.  On
        success, L{is_authenticated} will return C{True}.  On failure, you may
        use L{get_exception} to get more detailed error information.

        Since 1.1, if no event is passed, this method will block until the
        authentication succeeds or fails.  On failure, an exception is raised.
        Otherwise, the method simply returns.
        
        Since 1.5, if no event is passed and C{fallback} is C{True} (the
        default), if the server doesn't support plain password authentication
        but does support so-called "keyboard-interactive" mode, an attempt
        will be made to authenticate using this interactive mode.  If it fails,
        the normal exception will be thrown as if the attempt had never been
        made.  This is useful for some recent Gentoo and Debian distributions,
        which turn off plain password authentication in a misguided belief
        that interactive authentication is "more secure".  (It's not.)
        
        If the server requires multi-step authentication (which is very rare),
        this method will return a list of auth types permissible for the next
        step.  Otherwise, in the normal case, an empty list is returned.
        
        @param username: the username to authenticate as
        @type username: string
        @param password: the password to authenticate with
        @type password: string
        @param event: an event to trigger when the authentication attempt is
            complete (whether it was successful or not)
        @type event: threading.Event
        @param fallback: C{True} if an attempt at an automated "interactive"
            password auth should be made if the server doesn't support normal
            password auth
        @type fallback: bool
        @return: list of auth types permissible for the next stage of
            authentication (normally empty)
        @rtype: list
        
        @raise BadAuthenticationType: if password authentication isn't
            allowed by the server for this user (and no event was passed in)
        @raise AuthenticationException: if the authentication failed (and no
            event was passed in)
        @raise SSHException: if there was a network error
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to send the password unless we're on a secure link
            raise SSHException('No existing session')
        if event is None:
            my_event = threading.Event()
        else:
            my_event = event
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_password(username, password, my_event)
        if event is not None:
            # caller wants to wait for event themselves
            return []
        try:
            return self.auth_handler.wait_for_response(my_event)
        except BadAuthenticationType, x:
            # if password auth isn't allowed, but keyboard-interactive *is*, try to fudge it
            if not fallback or ('keyboard-interactive' not in x.allowed_types):
                raise
            try:
                def handler(title, instructions, fields):
                    if len(fields) > 1:
                        raise SSHException('Fallback authentication failed.')
                    if len(fields) == 0:
                        # for some reason, at least on os x, a 2nd request will
                        # be made with zero fields requested.  maybe it's just
                        # to try to fake out automated scripting of the exact
                        # type we're doing here.  *shrug* :)
                        return []
                    return [ password ]
                return self.auth_interactive(username, handler)
            except SSHException, ignored:
                # attempt failed; just raise the original exception
                raise x
        return None

    def auth_publickey(self, username, key, event=None):
        """
        Authenticate to the server using a private key.  The key is used to
        sign data from the server, so it must include the private part.
        
        If an C{event} is passed in, this method will return immediately, and
        the event will be triggered once authentication succeeds or fails.  On
        success, L{is_authenticated} will return C{True}.  On failure, you may
        use L{get_exception} to get more detailed error information.
        
        Since 1.1, if no event is passed, this method will block until the
        authentication succeeds or fails.  On failure, an exception is raised.
        Otherwise, the method simply returns.

        If the server requires multi-step authentication (which is very rare),
        this method will return a list of auth types permissible for the next
        step.  Otherwise, in the normal case, an empty list is returned.

        @param username: the username to authenticate as
        @type username: string
        @param key: the private key to authenticate with
        @type key: L{PKey <pkey.PKey>}
        @param event: an event to trigger when the authentication attempt is
            complete (whether it was successful or not)
        @type event: threading.Event
        @return: list of auth types permissible for the next stage of
            authentication (normally empty)
        @rtype: list
        
        @raise BadAuthenticationType: if public-key authentication isn't
            allowed by the server for this user (and no event was passed in)
        @raise AuthenticationException: if the authentication failed (and no
            event was passed in)
        @raise SSHException: if there was a network error
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to authenticate unless we're on a secure link
            raise SSHException('No existing session')
        if event is None:
            my_event = threading.Event()
        else:
            my_event = event
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_publickey(username, key, my_event)
        if event is not None:
            # caller wants to wait for event themselves
            return []
        return self.auth_handler.wait_for_response(my_event)
    
    def auth_interactive(self, username, handler, submethods=''):
        """
        Authenticate to the server interactively.  A handler is used to answer
        arbitrary questions from the server.  On many servers, this is just a
        dumb wrapper around PAM.
        
        This method will block until the authentication succeeds or fails,
        peroidically calling the handler asynchronously to get answers to
        authentication questions.  The handler may be called more than once
        if the server continues to ask questions.
        
        The handler is expected to be a callable that will handle calls of the
        form: C{handler(title, instructions, prompt_list)}.  The C{title} is
        meant to be a dialog-window title, and the C{instructions} are user
        instructions (both are strings).  C{prompt_list} will be a list of
        prompts, each prompt being a tuple of C{(str, bool)}.  The string is
        the prompt and the boolean indicates whether the user text should be
        echoed.
        
        A sample call would thus be:
        C{handler('title', 'instructions', [('Password:', False)])}.
        
        The handler should return a list or tuple of answers to the server's
        questions.
        
        If the server requires multi-step authentication (which is very rare),
        this method will return a list of auth types permissible for the next
        step.  Otherwise, in the normal case, an empty list is returned.

        @param username: the username to authenticate as
        @type username: string
        @param handler: a handler for responding to server questions
        @type handler: callable
        @param submethods: a string list of desired submethods (optional)
        @type submethods: str
        @return: list of auth types permissible for the next stag