telnetsrv 1.0.1

Telnet server handler library

telnetsrvlib

Telnet server using gevent or threading.

Copied from http://pytelnetsrvlib.sourceforge.net/ and modified to support gevent and eventlet, better input handling, clean asynchronous messages and much more. Licensed under the LGPL, as per the SourceForge notes.

Requires Python 3.10 or later.

This library allows you to easily create a Telnet or SSH server powered by your Python code. The library negotiates with a Telnet client, parses commands, provides an automated help command, optionally provides login queries, then allows you to define your own commands. An optional SSH handler is provided to wrap the defined Telnet handler into an SSH handler.

You use the library to create your own handler, then pass that handler to a StreamServer or TCPServer to perform the actual connection tasks.

This library includes two flavors of the server handler, one uses separate threads, the other uses greenlets (green pseudo-threads) via gevent or eventlet.

The threaded version uses a separate thread to process the input buffer and semaphores reading and writing. The provided test server only handles a single connection at a time.

The green version moves the input buffer processing into a greenlet to allow cooperative multi-processing. This results in significantly less memory usage and nearly no idle processing. The provided test server handles a large number of connections.

Install

telnetsrv is available on PyPI and can be installed with pip:

pip install telnetsrv

Note that there are no required dependencies beyond the Python standard library, but if you want to use the green version you must also install gevent or eventlet, and if you wish to use the SSH server you must also install paramiko:

pip install telnetsrv[green]   # includes gevent
pip install telnetsrv[ssh]     # includes paramiko

To Use

Import TelnetHandler, the cmd decorator, and the Commands base class from either the threaded, green, or evtlet module. Define your commands by subclassing Commands, then point your TelnetHandler subclass at that commands class.

Threaded

from telnetsrv.threaded import TelnetHandler, cmd, Commands

class MyCommands(Commands):
   ...

class MyHandler(TelnetHandler):
   commands_class = MyCommands

Green

from telnetsrv.green import TelnetHandler, cmd, Commands

class MyCommands(Commands):
   ...

class MyHandler(TelnetHandler):
   commands_class = MyCommands

Eventlet

from telnetsrv.evtlet import TelnetHandler, cmd, Commands

class MyCommands(Commands):
   ...

class MyHandler(TelnetHandler):
   commands_class = MyCommands

Async (asyncio)

from telnetsrv.aio import TelnetHandler, cmd, Commands

class MyCommands(Commands):
   ...

class MyHandler(TelnetHandler):
   commands_class = MyCommands

Adding Commands

Commands are defined as methods of a Commands subclass, decorated with @cmd.

class MyCommands(Commands):
    @cmd('echo')
    def echo(self, params):
        self.handler.writeresponse(' '.join(params))

class MyHandler(TelnetHandler):
    commands_class = MyCommands

Within a command method, self is the Commands instance. Use self.handler to reach the telnet handler and all of its I/O methods and session attributes.

Old Style

Commands can also be defined by prefixing any method name with cmd. For example, this also creates an echo command:

class MyCommands(Commands):
    def cmdECHO(self, params):
        self.handler.writeresponse(' '.join(params))

This method is less flexible and may not be supported in future versions.

Command Parameters

Any command parameters will be passed to this function automatically. The parameters are contained in a list. The user input is parsed similar to the way Bash parses text: space delimited, quoted parameters are kept together and default behavior can be modified with the \ character. If you need to access the raw text input, inspect self.handler.input.raw.

Telnet Server> echo 1  "2    3"

params == ['1', '2    3']
self.handler.input.raw == 'echo 1 "2    3"\n'

Telnet Server> echo 1 \
... 2 "3
... 4"  "5\
... 6"

params == ['1', '2', '3\n4', '56']

Telnet Server> echo 1\ 2

params == ['1 2']

Command Help Text

The command's docstring is used for generating the console help information, and must be formatted with at least 3 lines:

• Line 0: Command parameter(s) if any. (Can be blank line)
• Line 1: Short descriptive text. (Mandatory)
• Line 2+: Long descriptive text. (Can be blank line)

If there is no line 2, line 1 will be used for the long description as well.

class MyCommands(Commands):
    @cmd('echo')
    def echo(self, params):
        '''<text to echo>
        Echo text back to the console.
        This command simply echos the provided text
        back to the console.
        '''
        self.handler.writeresponse(' '.join(params))

Telnet Server> help
? [<command>] - Display help
BYE - Exit the command shell
ECHO <text to echo> - Echo text back to the console.
...


Telnet Server> help echo
ECHO <text to echo>

This command simply echos the provided text
back to the console.
Telnet Server>

Command Aliases

To create aliases for a command, pass a list of names to the decorator:

@cmd(['echo', 'copy'])
def echo(self, params):
   ...

The decorator may be stacked, which adds each list to the aliases:

@cmd('echo')
@cmd(['copy', 'repeat'])
@cmd('ditto')
def echo(self, params):
   ...

Hidden Commands

To hide the command (and any alias for that command) from the help text output, pass in hidden=True to the decorator:

@cmd('echo', hidden=True)
def echo(self, params):
   ...

The command will not show when the user invokes help by itself, but the detailed help text will show if the user invokes help echo.

When stacking decorators, any one of the stack may define the hidden parameter to hide the command.

Handling Unknown Commands

If the user enters a command that is not defined, _command_not_found is called on the Commands instance. By default it writes a reasonable error message back to the client. Override this method in your Commands subclass to provide custom handling — for example, to implement a dynamic command dispatcher or to log unrecognised input.

class MyCommands(Commands):
    def _command_not_found(self, command, params):
        '''
        Called when no registered command matches the user's input.

        ``command`` is the uppercased command name the user typed.
        ``params``  is the list of parsed arguments (may be empty).
        '''
        self.handler.writeerror(f"Unknown command '{command}'")

A common use is to act as a catch-all that forwards commands to another system:

class MyCommands(Commands):
    def _command_not_found(self, command, params):
        result = my_backend.run(command, params)
        if result is None:
            self.handler.writeerror(f"Unknown command '{command}'")
        else:
            self.handler.writeresponse(result)

When using the async handler, _command_not_found may also be a coroutine:

class MyCommands(Commands):
    async def _command_not_found(self, command, params):
        result = await my_backend.run(command, params)
        if result is None:
            self.handler.writeerror(f"Unknown command '{command}'")
        else:
            self.handler.writeresponse(result)

Console Information

These handler attributes are available for inspection. Within a command method, access them via self.handler; within handler lifecycle methods (session_start, session_end, etc.), access them directly as self.

• TERM — String ID describing the currently connected terminal
• WIDTH — Integer describing the width of the terminal at connection time.
• HEIGHT — Integer describing the height of the terminal at connection time.
• username — Set after authentication succeeds, name of the logged in user. If no authentication was requested, will be None.
• history — List containing the command history. This can be manipulated directly.

class MyCommands(Commands):
    @cmd('info')
    def info(self, params):
        '''
        Provides some information about the current terminal.
        '''
        self.handler.writeresponse(
            "Username: %s, terminal type: %s" % (self.handler.username, self.handler.TERM)
        )
        self.handler.writeresponse(
            "Width: %s, height: %s" % (self.handler.WIDTH, self.handler.HEIGHT)
        )
        self.handler.writeresponse("Command history:")
        for c in self.handler.history:
            self.handler.writeresponse("  %r" % c)

Console Communication

Send Text to the Client

Within a command method, reach the handler via self.handler:

Lower level functions:

• self.handler.writeline( TEXT )
• self.handler.write( TEXT )

Higher level functions:

• self.handler.writemessage( TEXT ) — for clean, asynchronous writing. Any interrupted input is rebuilt.
• self.handler.writeresponse( TEXT ) — to emit a line of expected output
• self.handler.writeerror( TEXT ) — to emit error messages

The writemessage method is intended to send messages to the console without interrupting any current input. If the user has entered text at the prompt, the prompt and text will be seamlessly regenerated following the message. It is ideal for asynchronous messages that aren't generated from the direct user input.

Receive Text from the Client

self.handler.readline( prompt=TEXT )

Setting the prompt is important to recreate the user input following a writemessage interruption.

When requesting sensitive information from the user (such as requesting a new password) the input should not be shown nor should the input line be written to the command history. readline accepts two optional parameters to control this, echo and use_history.

self.handler.readline( prompt=TEXT, echo=False, use_history=False )

When echo is set to False, the input will not echo back to the user. When use_history is set to False, the user will not have access to the command history (up arrow) nor will the entered data be stored in the command history.

Handler Options

Override these class members on TelnetHandler to change the handler's behavior. Within these methods, self is the handler, so I/O methods are called directly (self.writeline, etc.).

• PROMPT — Default: "Telnet Server> "
• CONTINUE_PROMPT — Default: "... "
• WELCOME — Displayed after a successful connection, after the username/password is accepted, if configured. Default: "You have connected to the telnet server."
• session_start(self) — Called after the WELCOME text is displayed. Default: pass
• session_end(self) — Called after the console is disconnected. Default: pass
• authCallback(self, username, password) — Reference to authentication function. If this is not defined, no username or password is requested. Should raise an exception if authentication fails. Default: None
• authNeedUser — Should a username be requested? Default: False
• authNeedPass — Should a password be requested? Default: False

Handler Display Modification

If you want to change how the output is displayed, override one or all of the write methods on TelnetHandler. Make sure you call back to the base class when doing so. This is a good way to provide color to your console by using ANSI color commands. See the ANSI Wikipedia article

• writemessage( TEXT )
• writeresponse( TEXT )
• writeerror( TEXT )

class MyHandler(TelnetHandler):
    def writeerror(self, text):
        '''Write errors in red'''
        TelnetHandler.writeerror(self, "\x1b[91m%s\x1b[0m" % text )

Serving the Handler

Now you have a shiny new handler class, but it doesn't serve itself - it must be called from an appropriate server. The server will create an instance of the TelnetHandler class for each new connection. The handler class will work with either a gevent StreamServer instance (for the green version) or with a socketserver.TCPServer instance (for the threaded version).

Threaded

import socketserver
class TelnetServer(socketserver.TCPServer):
    allow_reuse_address = True

server = TelnetServer(("0.0.0.0", 8023), MyHandler)
server.serve_forever()

Green

The TelnetHandler class includes a streamserver_handle class method to translate the required fields from a StreamServer, allowing use with the gevent StreamServer (and possibly others).

import gevent.server
server = gevent.server.StreamServer(("", 8023), MyHandler.streamserver_handle)
server.serve_forever()

Async

Pass the asyncio_handle classmethod to asyncio.start_server. No extra dependencies are required — asyncio is part of the Python standard library.

import asyncio

async def main():
    server = await asyncio.start_server(
        MyHandler.asyncio_handle, host="", port=8023
    )
    async with server:
        await server.serve_forever()

asyncio.run(main())

Command methods may be regular functions or coroutines; the handler detects and awaits coroutines automatically:

class MyCommands(Commands):
    @cmd('timer')
    async def timer(self, params):
        '''<time> <message>
        In <time> seconds, display <message>.
        '''
        try:
            delay, message = int(params[0]), params[1]
        except (ValueError, IndexError):
            self.handler.writeerror("Need both a time and a message")
            return
        self.handler.writeresponse(f"Waiting {delay} seconds...")
        await asyncio.sleep(delay)
        self.handler.writemessage(message)

The write methods (writeresponse, writeerror, writemessage, writeline, write) are not coroutines and should not be awaited. They buffer output synchronously into the asyncio stream; the data is flushed to the network automatically between command invocations.

session_start and session_end may also be defined as coroutines in the async handler.

Short Example

import gevent, gevent.server
from telnetsrv.green import TelnetHandler, cmd, Commands

class MyCommands(Commands):
    @cmd(['echo', 'copy', 'repeat'])
    def echo(self, params):
        '''<text to echo>
        Echo text back to the console.

        '''
        self.handler.writeresponse(' '.join(params))

    @cmd('timer')
    def timer(self, params):
        '''<time> <message>
        In <time> seconds, display <message>.
        Send a message after a delay.
        <time> is in seconds.
        If <message> is more than one word, quotes are required.
        example:
        > TIMER 5 "hello world!"
        '''
        try:
            timestr, message = params[:2]
            delay = int(timestr)
        except ValueError:
            self.handler.writeerror("Need both a time and a message")
            return
        self.handler.writeresponse("Waiting %d seconds..." % delay)
        gevent.spawn_later(delay, self.handler.writemessage, message)


class MyTelnetHandler(TelnetHandler):
    WELCOME = "Welcome to my server."
    commands_class = MyCommands


server = gevent.server.StreamServer(("", 8023), MyTelnetHandler.streamserver_handle)
server.serve_forever()

SSH

If the paramiko library is installed, the TelnetHandler can be used via an SSH server for significantly improved security. paramiko_ssh contains SSHHandler and getRsaKeyFile to make setting up the server trivial. Since the authentication is done prior to invoking the TelnetHandler, any authCallback defined in the TelnetHandler is ignored.

Green

If using the green version of the TelnetHandler, you must use Gevent's monkey patch_all prior to importing from paramiko_ssh.

from gevent import monkey; monkey.patch_all()
from telnetsrv.paramiko_ssh import SSHHandler, getRsaKeyFile

Eventlet

If using the eventlet version of the TelnetHandler, you must use Eventlet's monkey patch_all prior to importing from paramiko_ssh.

import eventlet; eventlet.monkey_patch(all=True)
from telnetsrv.paramiko_ssh import SSHHandler, getRsaKeyFile

Async

The aio_ssh module provides AsyncSSHHandler, which pairs with an aio.TelnetHandler subclass. Paramiko's SSH transport is still blocking and thread-based; once a PTY channel is established, asyncio.run() creates a fresh event loop in that thread and runs the async handler inside it. No monkey-patching is needed.

from telnetsrv.aio_ssh import AsyncSSHHandler, getRsaKeyFile
from telnetsrv.aio import TelnetHandler, cmd, Commands

Operation Overview

The socketserver.TCPServer or gevent/eventlet StreamServer sets up the socket then passes that to an SSHHandler class which authenticates then starts the SSH transport. Within the SSH transport, the client requests a PTY channel (and possibly other channel types, which are denied) and the SSHHandler sets up a TelnetHandler class as the PTY for the channel. If the client never requests a PTY channel, the transport will disconnect after a timeout.

SSH Host Key

To thwart man-in-the-middle attacks, every SSH server provides a key as a unique fingerprint. This unique key should never change, and should be stored in a local file or a database. getKeyFile makes this easy by reading the given key file if it exists, or generating a new Ed25519 key if it does not. The result should be read once and set in the class definition.

Easy way:

host_key = getKeyFile( FILENAME ) — If FILENAME can be read, the key is read in and returned. Reads any key type supported by paramiko (Ed25519, RSA, ECDSA). If the file can't be read, generates a new Ed25519 key and stores it in that file.

Long way:

from paramiko import Ed25519Key, PKey

# Generate a new key - should only be done once per server during setup
new_key = Ed25519Key.generate()
new_key.write_private_key_file('server.key')

...

host_key = PKey.from_path('server.key')

Upgrading from paramiko < 5: Existing RSA host key files are read without modification — getKeyFile auto-detects the key type. Clients connecting via the legacy ssh-rsa algorithm (SHA-1) will no longer be able to connect; modern SSH clients use rsa-sha2-256, rsa-sha2-512, or Ed25519 and are unaffected.

getRsaKeyFile is deprecated but still works as an alias for getKeyFile.

SSH Authentication

Users can authenticate with just a username, a username/publickey or a username/password. Up to three callbacks can be defined, and if all three are defined, all three will be tried before denying the authentication attempt. An SSH client will always provide a username. If no authCallbackXX is defined, the SSH authentication will be set to "none" and any username will be able to log in.

When "none" authentication is active, a warning is logged each time a connection is established as a reminder that the server is unauthenticated. If this is intentional (e.g. a local development server), suppress the warning by setting warn_of_insecure_auth = False in your SSHHandler subclass:

class MySSHHandler(SSHHandler):
    host_key = getKeyFile('server_fingerprint.key')
    telnet_handler = MyTelnetHandler
    warn_of_insecure_auth = False  # suppress warning: no auth is intentional

• authCallbackUsername(self, username) — Reference to username-only authentication function. Define this function to permit specific usernames to log in without any further authentication. Raise any exception to deny this authentication attempt. If defined, this is always tried first. Default: None
• authCallbackKey(self, username, key) — Reference to username/key authentication function. If this is defined, users can log in the SSH client automatically with a key. Raise any exception to deny this authentication attempt. Default: None
• authCallback(self, username, password) — Reference to username/password authentication function. If this is defined, a password is requested. Raise any exception to deny this authentication attempt. If defined, this is always tried last. Default: None

SSHHandler uses Paramiko's ServerInterface as one of its base classes. If you are familiar with Paramiko, feel free to instead override the authentication callbacks as needed.

Short SSH Example

from gevent import monkey; monkey.patch_all()
import gevent.server
from telnetsrv.paramiko_ssh import SSHHandler, getKeyFile
from telnetsrv.green import TelnetHandler, cmd, Commands

class MyCommands(Commands):
    @cmd(['echo', 'copy', 'repeat'])
    def echo(self, params):
        '''<text to echo>
        Echo text back to the console.

        '''
        self.handler.writeresponse(' '.join(params))

class MyTelnetHandler(TelnetHandler):
    WELCOME = "Welcome to my server."
    commands_class = MyCommands

class MySSHHandler(SSHHandler):
    # Set the unique host key
    host_key = getKeyFile('server_fingerprint.key')

    # Instruct this SSH handler to use MyTelnetHandler for any PTY connections
    telnet_handler = MyTelnetHandler

    def authCallbackUsername(self, username):
        # These users do not require a password
        if username not in ['john', 'eric', 'terry', 'graham']:
           raise RuntimeError('Not a Python!')

    def authCallback(self, username, password):
        # Super secret password:
        if password != 'concord':
           raise RuntimeError('Wrong password!')

# Start a telnet server for just the localhost on port 8023.  (Will not request any authentication.)
telnetserver = gevent.server.StreamServer(('127.0.0.1', 8023), MyTelnetHandler.streamserver_handle)
telnetserver.start()

# Start an SSH server for any local or remote host on port 8022
sshserver = gevent.server.StreamServer(("", 8022), MySSHHandler.streamserver_handle)
sshserver.serve_forever()

Short Async SSH Example

import socketserver
from telnetsrv.aio_ssh import AsyncSSHHandler, getKeyFile
from telnetsrv.aio import TelnetHandler, cmd, Commands

class MyCommands(Commands):
    @cmd(['echo', 'copy', 'repeat'])
    async def echo(self, params):
        '''<text to echo>
        Echo text back to the console.

        '''
        self.handler.writeresponse(' '.join(params))

class MyTelnetHandler(TelnetHandler):
    WELCOME = "Welcome to my server."
    commands_class = MyCommands

class MySSHHandler(AsyncSSHHandler):
    host_key = getKeyFile('server_fingerprint.key')
    telnet_handler = MyTelnetHandler

    def authCallbackUsername(self, username):
        if username not in ['john', 'eric', 'terry', 'graham']:
            raise RuntimeError('Not a Python!')

    def authCallback(self, username, password):
        if password != 'concord':
            raise RuntimeError('Wrong password!')

class TelnetServer(socketserver.TCPServer):
    allow_reuse_address = True

server = TelnetServer(('', 8022), MySSHHandler)
server.serve_forever()

Longer Example

See https://github.com/ianepperson/telnetsrvlib/blob/master/example.py