# Copyright (c) 2013-2021 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
# distribution and is available at:
#
# http://www.eclipse.org/legal/epl-2.0/
#
# This program may also be made available under the following secondary
# licenses when the conditions for such availability set forth in the
# Eclipse Public License v2.0 are satisfied:
#
# GNU General Public License, Version 2.0, or any later versions of
# that license
#
# SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-or-later
#
# Contributors:
# Ron Frederick - initial implementation, API, and documentation
"""SSH server protocol handler"""
from typing import TYPE_CHECKING, Optional, Tuple, Union
from .auth import KbdIntChallenge, KbdIntResponse
from .listener import SSHListener
from .misc import MaybeAwait
from .public_key import SSHKey
from .stream import SSHSocketSessionFactory, SSHServerSessionFactory
if TYPE_CHECKING:
# pylint: disable=cyclic-import
from .connection import SSHServerConnection
from .channel import SSHServerChannel, SSHTCPChannel, SSHUNIXChannel
from .session import SSHServerSession, SSHTCPSession, SSHUNIXSession
_NewSession = Union[bool, 'SSHServerSession', SSHServerSessionFactory,
Tuple['SSHServerChannel', 'SSHServerSession'],
Tuple['SSHServerChannel', SSHServerSessionFactory]]
_NewTCPSession = Union[bool, 'SSHTCPSession', SSHSocketSessionFactory,
Tuple['SSHTCPChannel', 'SSHTCPSession'],
Tuple['SSHTCPChannel', SSHSocketSessionFactory]]
_NewUNIXSession = Union[bool, 'SSHUNIXSession', SSHSocketSessionFactory,
Tuple['SSHUNIXChannel', 'SSHUNIXSession'],
Tuple['SSHUNIXChannel', SSHSocketSessionFactory]]
_NewListener = Union[bool, SSHListener]
[docs]class SSHServer:
"""SSH server protocol handler
Applications may subclass this when implementing an SSH server to
provide custom authentication and request handlers.
The method :meth:`begin_auth` can be overridden decide whether
or not authentication is required, and additional callbacks are
provided for each form of authentication in cases where authentication
information is not provided in the call to :func:`create_server`.
In addition, the methods :meth:`session_requested`,
:meth:`connection_requested`, :meth:`server_requested`,
:meth:`unix_connection_requested`, or :meth:`unix_server_requested`
can be overridden to handle requests to open sessions or direct
connections or set up listeners for forwarded connections.
.. note:: The authentication callbacks described here can be
defined as coroutines. However, they may be cancelled if
they are running when the SSH connection is closed by
the client. If they attempt to catch the CancelledError
exception to perform cleanup, they should make sure to
re-raise it to allow AsyncSSH to finish its own cleanup.
"""
# pylint: disable=no-self-use,unused-argument
[docs] def connection_made(self, conn: 'SSHServerConnection') -> None:
"""Called when a connection is made
This method is called when a new TCP connection is accepted. The
`conn` parameter should be stored if needed for later use.
:param conn:
The connection which was successfully opened
:type conn: :class:`SSHServerConnection`
"""
[docs] def connection_lost(self, exc: Optional[Exception]) -> None:
"""Called when a connection is lost or closed
This method is called when a connection is closed. If the
connection is shut down cleanly, *exc* will be `None`.
Otherwise, it will be an exception explaining the reason for
the disconnect.
"""
[docs] def debug_msg_received(self, msg: str, lang: str,
always_display: bool) -> None:
"""A debug message was received on this connection
This method is called when the other end of the connection sends
a debug message. Applications should implement this method if
they wish to process these debug messages.
:param msg:
The debug message sent
:param lang:
The language the message is in
:param always_display:
Whether or not to display the message
:type msg: `str`
:type lang: `str`
:type always_display: `bool`
"""
[docs] def begin_auth(self, username: str) -> MaybeAwait[bool]:
"""Authentication has been requested by the client
This method will be called when authentication is attempted for
the specified user. Applications should use this method to
prepare whatever state they need to complete the authentication,
such as loading in the set of authorized keys for that user. If
no authentication is required for this user, this method should
return `False` to cause the authentication to immediately
succeed. Otherwise, it should return `True` to indicate that
authentication should proceed.
If blocking operations need to be performed to prepare the
state needed to complete the authentication, this method may
be defined as a coroutine.
:param username:
The name of the user being authenticated
:type username: `str`
:returns: A `bool` indicating whether authentication is required
"""
return True # pragma: no cover
[docs] def auth_completed(self) -> None:
"""Authentication was completed successfully
This method is called when authentication has completed
succesfully. Applications may use this method to perform
processing based on the authenticated username or options in
the authorized keys list or certificate associated with the
user before any sessions are opened or forwarding requests
are handled.
"""
[docs] def validate_gss_principal(self, username: str, user_principal: str,
host_principal: str) -> MaybeAwait[bool]:
"""Return whether a GSS principal is valid for this user
This method should return `True` if the specified user
principal is valid for the user being authenticated. It can
be overridden by applications wishing to perform their own
authentication.
If blocking operations need to be performed to determine the
validity of the principal, this method may be defined as a
coroutine.
By default, this method will return `True` only when the
name in the user principal exactly matches the username and
the domain of the user principal matches the domain of the
host principal.
:param username:
The user being authenticated
:param user_principal:
The user principal sent by the client
:param host_principal:
The host principal sent by the server
:type username: `str`
:type user_principal: `str`
:type host_principal: `str`
:returns: A `bool` indicating if the specified user principal
is valid for the user being authenticated
"""
host_domain = host_principal.rsplit('@')[-1]
return user_principal == username + '@' + host_domain
[docs] def host_based_auth_supported(self) -> bool:
"""Return whether or not host-based authentication is supported
This method should return `True` if client host-based
authentication is supported. Applications wishing to support
it must have this method return `True` and implement
:meth:`validate_host_public_key` and/or :meth:`validate_host_ca_key`
to return whether or not the key provided by the client is valid
for the client host being authenticated.
By default, it returns `False` indicating the client host
based authentication is not supported.
:returns: A `bool` indicating if host-based authentication is
supported or not
"""
return False # pragma: no cover
[docs] def validate_host_public_key(self, client_host: str, client_addr: str,
client_port: int, key: SSHKey) -> bool:
"""Return whether key is an authorized host key for this client host
Host key based client authentication can be supported by
passing authorized host keys in the `known_client_hosts`
argument of :func:`create_server`. However, for more flexibility
in matching on the allowed set of keys, this method can be
implemented by the application to do the matching itself. It
should return `True` if the specified key is a valid host key
for the client host being authenticated.
This method may be called multiple times with different keys
provided by the client. Applications should precompute as
much as possible in the :meth:`begin_auth` method so that
this function can quickly return whether the key provided is
in the list.
By default, this method returns `False` for all client host keys.
.. note:: This function only needs to report whether the
public key provided is a valid key for this client
host. If it is, AsyncSSH will verify that the
client possesses the corresponding private key
before allowing the authentication to succeed.
:param client_host:
The hostname of the client host
:param client_addr:
The IP address of the client host
:param client_port:
The port number on the client host
:param key:
The host public key sent by the client
:type client_host: `str`
:type client_addr: `str`
:type client_port: `int`
:type key: :class:`SSHKey` *public key*
:returns: A `bool` indicating if the specified key is a valid
key for the client host being authenticated
"""
return False # pragma: no cover
[docs] def validate_host_ca_key(self, client_host: str, client_addr: str,
client_port: int, key: SSHKey) -> bool:
"""Return whether key is an authorized CA key for this client host
Certificate based client host authentication can be
supported by passing authorized host CA keys in the
`known_client_hosts` argument of :func:`create_server`.
However, for more flexibility in matching on the allowed
set of keys, this method can be implemented by the application
to do the matching itself. It should return `True` if the
specified key is a valid certificate authority key for the
client host being authenticated.
This method may be called multiple times with different keys
provided by the client. Applications should precompute as
much as possible in the :meth:`begin_auth` method so that
this function can quickly return whether the key provided is
in the list.
By default, this method returns `False` for all CA keys.
.. note:: This function only needs to report whether the
public key provided is a valid CA key for this
client host. If it is, AsyncSSH will verify that
the certificate is valid, that the client host is
one of the valid principals for the certificate,
and that the client possesses the private key
corresponding to the public key in the certificate
before allowing the authentication to succeed.
:param client_host:
The hostname of the client host
:param client_addr:
The IP address of the client host
:param client_port:
The port number on the client host
:param key:
The public key which signed the certificate sent by the client
:type client_host: `str`
:type client_addr: `str`
:type client_port: `int`
:type key: :class:`SSHKey` *public key*
:returns: A `bool` indicating if the specified key is a valid
CA key for the client host being authenticated
"""
return False # pragma: no cover
[docs] def validate_host_based_user(self, username: str, client_host: str,
client_username: str) -> MaybeAwait[bool]:
"""Return whether remote host and user is authorized for this user
This method should return `True` if the specified client host
and user is valid for the user being authenticated. It can be
overridden by applications wishing to enforce restrictions on
which remote users are allowed to authenticate as particular
local users.
If blocking operations need to be performed to determine the
validity of the client host and user, this method may be defined
as a coroutine.
By default, this method will return `True` when the client
username matches the name of the user being authenticated.
:param username:
The user being authenticated
:param client_host:
The hostname of the client host making the request
:param client_username:
The username of the user on the client host
:type username: `str`
:type client_host: `str`
:type client_username: `str`
:returns: A `bool` indicating if the specified client host
and user is valid for the user being authenticated
"""
return username == client_username
[docs] def public_key_auth_supported(self) -> bool:
"""Return whether or not public key authentication is supported
This method should return `True` if client public key
authentication is supported. Applications wishing to support
it must have this method return `True` and implement
:meth:`validate_public_key` and/or :meth:`validate_ca_key`
to return whether or not the key provided by the client is
valid for the user being authenticated.
By default, it returns `False` indicating the client public
key authentication is not supported.
:returns: A `bool` indicating if public key authentication is
supported or not
"""
return False # pragma: no cover
[docs] def validate_public_key(self, username: str, key: SSHKey) -> \
MaybeAwait[bool]:
"""Return whether key is an authorized client key for this user
Key based client authentication can be supported by
passing authorized keys in the `authorized_client_keys`
argument of :func:`create_server`, or by calling
:meth:`set_authorized_keys
<SSHServerConnection.set_authorized_keys>` on the server
connection from the :meth:`begin_auth` method. However, for
more flexibility in matching on the allowed set of keys, this
method can be implemented by the application to do the
matching itself. It should return `True` if the specified
key is a valid client key for the user being authenticated.
This method may be called multiple times with different keys
provided by the client. Applications should precompute as
much as possible in the :meth:`begin_auth` method so that
this function can quickly return whether the key provided is
in the list.
If blocking operations need to be performed to determine the
validity of the key, this method may be defined as a coroutine.
By default, this method returns `False` for all client keys.
.. note:: This function only needs to report whether the
public key provided is a valid client key for this
user. If it is, AsyncSSH will verify that the
client possesses the corresponding private key
before allowing the authentication to succeed.
:param username:
The user being authenticated
:param key:
The public key sent by the client
:type username: `str`
:type key: :class:`SSHKey` *public key*
:returns: A `bool` indicating if the specified key is a valid
client key for the user being authenticated
"""
return False # pragma: no cover
[docs] def validate_ca_key(self, username: str, key: SSHKey) -> MaybeAwait[bool]:
"""Return whether key is an authorized CA key for this user
Certificate based client authentication can be supported by
passing authorized CA keys in the `authorized_client_keys`
argument of :func:`create_server`, or by calling
:meth:`set_authorized_keys
<SSHServerConnection.set_authorized_keys>` on the server
connection from the :meth:`begin_auth` method. However, for
more flexibility in matching on the allowed set of keys, this
method can be implemented by the application to do the
matching itself. It should return `True` if the specified
key is a valid certificate authority key for the user being
authenticated.
This method may be called multiple times with different keys
provided by the client. Applications should precompute as
much as possible in the :meth:`begin_auth` method so that
this function can quickly return whether the key provided is
in the list.
If blocking operations need to be performed to determine the
validity of the key, this method may be defined as a coroutine.
By default, this method returns `False` for all CA keys.
.. note:: This function only needs to report whether the
public key provided is a valid CA key for this
user. If it is, AsyncSSH will verify that the
certificate is valid, that the user is one of
the valid principals for the certificate, and
that the client possesses the private key
corresponding to the public key in the certificate
before allowing the authentication to succeed.
:param username:
The user being authenticated
:param key:
The public key which signed the certificate sent by the client
:type username: `str`
:type key: :class:`SSHKey` *public key*
:returns: A `bool` indicating if the specified key is a valid
CA key for the user being authenticated
"""
return False # pragma: no cover
[docs] def password_auth_supported(self) -> bool:
"""Return whether or not password authentication is supported
This method should return `True` if password authentication
is supported. Applications wishing to support it must have
this method return `True` and implement :meth:`validate_password`
to return whether or not the password provided by the client
is valid for the user being authenticated.
By default, this method returns `False` indicating that
password authentication is not supported.
:returns: A `bool` indicating if password authentication is
supported or not
"""
return False # pragma: no cover
[docs] def validate_password(self, username: str, password: str) -> \
MaybeAwait[bool]:
"""Return whether password is valid for this user
This method should return `True` if the specified password
is a valid password for the user being authenticated. It must
be overridden by applications wishing to support password
authentication.
If the password provided is valid but expired, this method
may raise :exc:`PasswordChangeRequired` to request that the
client provide a new password before authentication is
allowed to complete. In this case, the application must
override :meth:`change_password` to handle the password
change request.
This method may be called multiple times with different
passwords provided by the client. Applications may wish
to limit the number of attempts which are allowed. This
can be done by having :meth:`password_auth_supported` begin
returning `False` after the maximum number of attempts is
exceeded.
If blocking operations need to be performed to determine the
validity of the password, this method may be defined as a
coroutine.
By default, this method returns `False` for all passwords.
:param username:
The user being authenticated
:param password:
The password sent by the client
:type username: `str`
:type password: `str`
:returns: A `bool` indicating if the specified password is
valid for the user being authenticated
:raises: :exc:`PasswordChangeRequired` if the password
provided is expired and needs to be changed
"""
return False # pragma: no cover
[docs] def change_password(self, username: str, old_password: str,
new_password: str) -> MaybeAwait[bool]:
"""Handle a request to change a user's password
This method is called when a user makes a request to
change their password. It should first validate that
the old password provided is correct and then attempt
to change the user's password to the new value.
If the old password provided is valid and the change to
the new password is successful, this method should
return `True`. If the old password is not valid or
password changes are not supported, it should return
`False`. It may also raise :exc:`PasswordChangeRequired`
to request that the client try again if the new password
is not acceptable for some reason.
If blocking operations need to be performed to determine the
validity of the old password or to change to the new password,
this method may be defined as a coroutine.
By default, this method returns `False`, rejecting all
password changes.
:param username:
The user whose password should be changed
:param old_password:
The user's current password
:param new_password:
The new password being requested
:type username: `str`
:type old_password: `str`
:type new_password: `str`
:returns: A `bool` indicating if the password change
is successful or not
:raises: :exc:`PasswordChangeRequired` if the new password
is not acceptable and the client should be asked
to provide another
"""
return False # pragma: no cover
[docs] def kbdint_auth_supported(self) -> bool:
"""Return whether or not keyboard-interactive authentication
is supported
This method should return `True` if keyboard-interactive
authentication is supported. Applications wishing to support
it must have this method return `True` and implement
:meth:`get_kbdint_challenge` and :meth:`validate_kbdint_response`
to generate the apporiate challenges and validate the responses
for the user being authenticated.
By default, this method returns `NotImplemented` tying
this authentication to password authentication. If the
application implements password authentication and this
method is not overridden, keyboard-interactive authentication
will be supported by prompting for a password and passing
that to the password authentication callbacks.
:returns: A `bool` indicating if keyboard-interactive
authentication is supported or not
"""
return NotImplemented # pragma: no cover
[docs] def get_kbdint_challenge(self, username: str, lang: str,
submethods: str) -> MaybeAwait[KbdIntChallenge]:
"""Return a keyboard-interactive auth challenge
This method should return `True` if authentication should
succeed without any challenge, `False` if authentication
should fail without any challenge, or an auth challenge
consisting of a challenge name, instructions, a language tag,
and a list of tuples containing prompt strings and booleans
indicating whether input should be echoed when a value is
entered for that prompt.
If blocking operations need to be performed to determine the
challenge to issue, this method may be defined as a coroutine.
:param username:
The user being authenticated
:param lang:
The language requested by the client for the challenge
:param submethods:
A comma-separated list of the types of challenges the client
can support, or the empty string if the server should choose
:type username: `str`
:type lang: `str`
:type submethods: `str`
:returns: An authentication challenge as described above
"""
return False # pragma: no cover
[docs] def validate_kbdint_response(
self, username: str, responses: KbdIntResponse) -> \
MaybeAwait[KbdIntChallenge]:
"""Return whether the keyboard-interactive response is valid
for this user
This method should validate the keyboard-interactive responses
provided and return `True` if authentication should succeed
with no further challenge, `False` if authentication should
fail, or an additional auth challenge in the same format returned
by :meth:`get_kbdint_challenge`. Any series of challenges can be
returned this way. To print a message in the middle of a sequence
of challenges without prompting for additional data, a challenge
can be returned with an empty list of prompts. After the client
acknowledges this message, this function will be called again
with an empty list of responses to continue the authentication.
If blocking operations need to be performed to determine the
validity of the response or the next challenge to issue, this
method may be defined as a coroutine.
:param username:
The user being authenticated
:param responses:
A list of responses to the last challenge
:type username: `str`
:type responses: `list` of `str`
:returns: `True`, `False`, or the next challenge
"""
return False # pragma: no cover
[docs] def session_requested(self) -> MaybeAwait[_NewSession]:
"""Handle an incoming session request
This method is called when a session open request is received
from the client, indicating it wishes to open a channel to be
used for running a shell, executing a command, or connecting
to a subsystem. If the application wishes to accept the session,
it must override this method to return either an
:class:`SSHServerSession` object to use to process
the data received on the channel or a tuple consisting of an
:class:`SSHServerChannel` object created with
:meth:`create_server_channel
<SSHServerConnection.create_server_channel>` and an
:class:`SSHServerSession`, if the application
wishes to pass non-default arguments when creating the channel.
If blocking operations need to be performed before the session
can be created, a coroutine which returns an
:class:`SSHServerSession` object can be returned instead of
the session iself. This can be either returned directly or as
a part of a tuple with an :class:`SSHServerChannel` object.
To reject this request, this method should return `False`
to send back a "Session refused" response or raise a
:exc:`ChannelOpenError` exception with the reason for
the failure.
The details of what type of session the client wants to start
will be delivered to methods on the :class:`SSHServerSession`
object which is returned, along with other information such
as environment variables, terminal type, size, and modes.
By default, all session requests are rejected.
:returns: One of the following:
* An :class:`SSHServerSession` object or a coroutine
which returns an :class:`SSHServerSession`
* A tuple consisting of an :class:`SSHServerChannel`
and the above
* A `callable` or coroutine handler function which
takes AsyncSSH stream objects for stdin, stdout,
and stderr as arguments
* A tuple consisting of an :class:`SSHServerChannel`
and the above
* `False` to refuse the request
:raises: :exc:`ChannelOpenError` if the session shouldn't
be accepted
"""
return False # pragma: no cover
[docs] def connection_requested(self, dest_host: str, dest_port: int,
orig_host: str, orig_port: int) -> _NewTCPSession:
"""Handle a direct TCP/IP connection request
This method is called when a direct TCP/IP connection
request is received by the server. Applications wishing
to accept such connections must override this method.
To allow standard port forwarding of data on the connection
to the requested destination host and port, this method
should return `True`.
To reject this request, this method should return `False`
to send back a "Connection refused" response or raise an
:exc:`ChannelOpenError` exception with the reason for
the failure.
If the application wishes to process the data on the
connection itself, this method should return either an
:class:`SSHTCPSession` object which can be used to process the
data received on the channel or a tuple consisting of of an
:class:`SSHTCPChannel` object created with
:meth:`create_tcp_channel()
<SSHServerConnection.create_tcp_channel>` and an
:class:`SSHTCPSession`, if the application wishes
to pass non-default arguments when creating the channel.
If blocking operations need to be performed before the session
can be created, a coroutine which returns an
:class:`SSHTCPSession` object can be returned instead of
the session iself. This can be either returned directly or as
a part of a tuple with an :class:`SSHTCPChannel` object.
By default, all connection requests are rejected.
:param dest_host:
The address the client wishes to connect to
:param dest_port:
The port the client wishes to connect to
:param orig_host:
The address the connection was originated from
:param orig_port:
The port the connection was originated from
:type dest_host: `str`
:type dest_port: `int`
:type orig_host: `str`
:type orig_port: `int`
:returns: One of the following:
* An :class:`SSHTCPSession` object or a coroutine
which returns an :class:`SSHTCPSession`
* A tuple consisting of an :class:`SSHTCPChannel`
and the above
* A `callable` or coroutine handler function which
takes AsyncSSH stream objects for reading from
and writing to the connection
* A tuple consisting of an :class:`SSHTCPChannel`
and the above
* `True` to request standard port forwarding
* `False` to refuse the connection
:raises: :exc:`ChannelOpenError` if the connection shouldn't
be accepted
"""
return False # pragma: no cover
[docs] def server_requested(self, listen_host: str,
listen_port: int) -> MaybeAwait[_NewListener]:
"""Handle a request to listen on a TCP/IP address and port
This method is called when a client makes a request to
listen on an address and port for incoming TCP connections.
The port to listen on may be `0` to request a dynamically
allocated port. Applications wishing to allow TCP/IP connection
forwarding must override this method.
To set up standard port forwarding of connections received
on this address and port, this method should return `True`.
If the application wishes to manage listening for incoming
connections itself, this method should return an
:class:`SSHListener` object that listens for new connections
and calls :meth:`create_connection
<SSHServerConnection.create_connection>` on each of them to
forward them back to the client or return `None` if the
listener can't be set up.
If blocking operations need to be performed to set up the
listener, a coroutine which returns an :class:`SSHListener`
can be returned instead of the listener itself.
To reject this request, this method should return `False`.
By default, this method rejects all server requests.
:param listen_host:
The address the server should listen on
:param listen_port:
The port the server should listen on, or the value `0`
to request that the server dynamically allocate a port
:type listen_host: `str`
:type listen_port: `int`
:returns: One of the following:
* An :class:`SSHListener` object
* `True` to set up standard port forwarding
* `False` to reject the request
* A coroutine object which returns one of the above
"""
return False # pragma: no cover
[docs] def unix_connection_requested(self, dest_path: str) -> _NewUNIXSession:
"""Handle a direct UNIX domain socket connection request
This method is called when a direct UNIX domain socket connection
request is received by the server. Applications wishing to accept
such connections must override this method.
To allow standard path forwarding of data on the connection to the
requested destination path, this method should return `True`.
To reject this request, this method should return `False`
to send back a "Connection refused" response or raise an
:exc:`ChannelOpenError` exception with the reason for
the failure.
If the application wishes to process the data on the
connection itself, this method should return either an
:class:`SSHUNIXSession` object which can be used to process the
data received on the channel or a tuple consisting of of an
:class:`SSHUNIXChannel` object created with
:meth:`create_unix_channel()
<SSHServerConnection.create_unix_channel>` and an
:class:`SSHUNIXSession`, if the application wishes
to pass non-default arguments when creating the channel.
If blocking operations need to be performed before the session
can be created, a coroutine which returns an
:class:`SSHUNIXSession` object can be returned instead of
the session iself. This can be either returned directly or as
a part of a tuple with an :class:`SSHUNIXChannel` object.
By default, all connection requests are rejected.
:param dest_path:
The path the client wishes to connect to
:type dest_path: `str`
:returns: One of the following:
* An :class:`SSHUNIXSession` object or a coroutine
which returns an :class:`SSHUNIXSession`
* A tuple consisting of an :class:`SSHUNIXChannel`
and the above
* A `callable` or coroutine handler function which
takes AsyncSSH stream objects for reading from
and writing to the connection
* A tuple consisting of an :class:`SSHUNIXChannel`
and the above
* `True` to request standard path forwarding
* `False` to refuse the connection
:raises: :exc:`ChannelOpenError` if the connection shouldn't
be accepted
"""
return False # pragma: no cover
[docs] def unix_server_requested(self, listen_path: str) -> \
MaybeAwait[_NewListener]:
"""Handle a request to listen on a UNIX domain socket
This method is called when a client makes a request to
listen on a path for incoming UNIX domain socket connections.
Applications wishing to allow UNIX domain socket forwarding
must override this method.
To set up standard path forwarding of connections received
on this path, this method should return `True`.
If the application wishes to manage listening for incoming
connections itself, this method should return an
:class:`SSHListener` object that listens for new connections
and calls :meth:`create_unix_connection
<SSHServerConnection.create_unix_connection>` on each of them to
forward them back to the client or return `None` if the
listener can't be set up.
If blocking operations need to be performed to set up the
listener, a coroutine which returns an :class:`SSHListener`
can be returned instead of the listener itself.
To reject this request, this method should return `False`.
By default, this method rejects all server requests.
:param listen_path:
The path the server should listen on
:type listen_path: `str`
:returns: One of the following:
* An :class:`SSHListener` object or a coroutine
which returns an :class:`SSHListener` or `False`
if the listener can't be opened
* `True` to set up standard path forwarding
* `False` to reject the request
"""
return False # pragma: no cover