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Telnet server handler library

Project description

Telnet server using gevent or threading.

Copied from and modified to support gevent, better input handling, clean asynchronous messages and much more. Licensed under the LGPL, as per the SourceForge notes.

This library allows you to easily create a Telnet 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.

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.

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.

To Use

Import the TelnetHandler base class and command function decorator from either the green class or threaded class, then subclass TelnetHandler to add your own commands as specially named methods.


> from telnetsrv.threaded import TelnetHandler, command
> class MyHandler(TelnetHandler):
>    ...


> from import TelnetHandler, command
> class MyHandler(TelnetHandler):
>    ...

Add Commands

Commands can be defined by using the command function decorator.

>   @command('echo')
>   def command_echo(self, params):
>      ...

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 the self.input.raw variable.

Telnet Server> echo 1  "2    3"
>   params == ['1', '2    3']
>   self.raw_input == 'echo 1 "2    3"'
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.

>    @command('echo')
>    def command_echo(self, params):
>        '''<text to echo>
>        Echo text back to the console.
>        This command simply echos the provided text
>        back to the console.
>        '''
>        pass
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 an alias for the new command, set the method’s name to a list:

>   @command(['echo', 'copy'])
>   def comand_echo(self, params):
>      ...

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

>   @command('echo')
>   @command(['copy', 'repeat'])
>   @command('ditto')
>   def comand_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:

>   @command('echo', hidden=True)
>   def comand_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.

Console Communication

To communicate with the connected Telnet client, use:

  • self.writeline( TEXT )
  • self.write( TEXT )
  • self.readline( prompt=TEXT )
  • self.writemessage( TEXT ) - for clean, asynchronous writing. Any interrupted input is rebuilt.
  • self.writeresult( TEXT ) - to emit a line of expected output
  • self.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.

Handler Options

Override these class members to change the handler’s behavior.

Default: pass
Default: "Telnet Server> "
Default: "... "

Displayed after a successful connection, after the username/password is accepted, if configured.

Default: "You have connected to the telnet server."


Called after the WELCOME text is displayed.

Default: pass


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


Should a username be requested?

Default: False


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 classes. 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

  • writemessage( TEXT )
  • writeresult( TEXT )
  • writeerror( TEXT )
>    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).


> import SocketServer
> class TelnetServer(SocketServer.TCPServer):
>     allow_reuse_address = True
> server = TelnetServer(("", 8023), MyHandler)
> server.serve_forever()


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.server_forever()

Short Example

> import gevent, gevent.server
> from import TelnetHandler, command
> class MyTelnetHandler(TelnetHandler):
>     WELCOME = "Welcome to my server."
>     @command(['echo', 'copy', 'repeat'])
>     def command_echo(self, params):
>         '''<text to echo>
>         Echo text back to the console.
>         '''
>         self.writeresult( ' '.join(params) )
>     @command('timer')
>     def command_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]
>             time = int(timestr)
>         except ValueError:
>             self.writeerror( "Need both a time and a message" )
>             return
>         self.writeresult("Waiting %d seconds...", time)
>         gevent.spawn_later(time, self.writemessage, message)
> server = gevent.server.StreamServer(("", 8023), MyTelnetHandler.streamserver_handle)
> server.server_forever()

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