# Copyright (c) 2013-2017 by Ron Frederick <ronf@timeheart.net>.
# All rights reserved.
#
# This program and the accompanying materials are made available under
# the terms of the Eclipse Public License v1.0 which accompanies this
# distribution and is available at:
#
#     http://www.eclipse.org/legal/epl-v10.html
#
# Contributors:
#     Ron Frederick - initial implementation, API, and documentation

"""SSH server protocol handler"""


class SSHServer:
    """SSH server protocol handler

       Applications should subclass this when implementing an SSH server.
       At a minimum, one or more of the authentication handlers will need
       to be overridden to perform authentication, or :meth:`begin_auth`
       should be overridden to return ``False`` to indicate that no
       authentication is required.

       In addition, one or more of the :meth:`session_requested`,
       :meth:`connection_requested`, :meth:`server_requested`,
       :meth:`unix_connection_requested`, or :meth:`unix_server_requested`
       methods will need to 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

    def connection_made(self, connection):
        """Called when a connection is made

           This method is called when a new TCP connection is accepted. The
           connection parameter should be stored if needed for later use.

        """

        pass # pragma: no cover

    def connection_lost(self, exc):
        """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.

        """

        pass # pragma: no cover

    def debug_msg_received(self, msg, lang, always_display):
        """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 str msg:
               The debug message sent
           :param str lang:
               The language the message is in
           :param bool always_display:
               Whether or not to display the message

        """

        pass # pragma: no cover

    def begin_auth(self, username):
        """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.

           :param str username:
               The name of the user being authenticated

           :returns: A bool indicating whether authentication is required

        """

        return True # pragma: no cover

    def validate_gss_principal(self, username, user_principal,
                               host_principal):
        """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
           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 str username:
               The user being authenticated
           :param str user_principal:
               The user principal sent by the client
           :param str host_principal:
               The host principal sent by the server

           :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

    def public_key_auth_supported(self):
        """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` 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

    def validate_public_key(self, username, key):
        """Return whether key is an authorized client key for this user

           Basic 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 str username:
               The user being authenticated
           :param key:
               The public key sent by the client
           :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

    def validate_ca_key(self, username, key):
        """Return whether key is an authorized CA key for this user

           Basic 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 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 str username:
               The user being authenticated
           :param key:
               The public key which signed the certificate sent by the client
           :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

    def password_auth_supported(self):
        """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

    def validate_password(self, username, password):
        """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 str username:
               The user being authenticated
           :param str password:
               The password sent by the client

           :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

    def change_password(self, username, old_password, new_password):
        """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 str username:
               The user whose password should be changed
           :param str old_password:
               The user's current password
           :param str new_password:
               The new password being requested

           :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

    def kbdint_auth_supported(self):
        """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

    def get_kbdint_challenge(self, username, lang, submethods):
        """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 str username:
               The user being authenticated
           :param str lang:
               The language requested by the client for the challenge
           :param str submethods:
               A comma-separated list of the types of challenges the client
               can support, or the empty string if the server should choose

           :returns: An authentication challenge as described above

        """

        return False # pragma: no cover

    def validate_kbdint_response(self, username, responses):
        """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 str username:
               The user being authenticated
           :param responses:
               A list of responses to the last challenge
           :type responses: list of str

           :returns: ``True``, ``False``, or the next challenge

        """

        return False # pragma: no cover

    def session_requested(self):
        """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

    def connection_requested(self, dest_host, dest_port, orig_host, orig_port):
        """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 str dest_host:
               The address the client wishes to connect to
           :param int dest_port:
               The port the client wishes to connect to
           :param str orig_host:
               The address the connection was originated from
           :param int orig_port:
               The port the connection was originated from

           :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

    def server_requested(self, listen_host, listen_port):
        """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 str listen_host:
               The address the server should listen on
           :param int listen_port:
               The port the server should listen on, or the value ``0``
               to request that the server dynamically allocate a port

           :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 port forwarding
                     * ``False`` to reject the request

        """

        return False # pragma: no cover

    def unix_connection_requested(self, dest_path):
        """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 str dest_path:
               The path the client wishes to connect to

           :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

    def unix_server_requested(self, listen_path):
        """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 str listen_path:
               The path the server should listen on

           :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
