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Telnet server and client Protocol library using asyncio

Project Description


telnetlib3 is a Telnet Client and Server Protocol library for python.

It is hosted on github. Currently in development stage, feedback is encouraged. Feel free to make use of fork, pull and Issues services to report any bugs, grievances, or enhancements.

This project uses the asyncio module of python 3.4, available on pypi for python 3.3, for which this is currently targeted.

Example scripts are provided that make use of the protocol.


  1. Install python 3

  2. Install pip

  3. Ensure pip is up-to-date:

    pip install --upgrade pip
  4. Install telnetlib3:

    pip install telnetlib3


  • telnet-client: This provides a simple interactive shell for terminals for communicating with any telnet server and the keyboard & screen. Most notably, it provides a --cp437 argument that allows connecting to telnet BBS systems from any posix shell, that otherwise would require a DOS Emulating program SyncTerm, mtelnet, netrunner. Instead, these systems may be used with a standard terminal emulator, such as xterm, rxvt, or iTerm2. Some example telnet destinations: * Supports UTF8 or CP437 encodings (enthral). * Supports UTF8 or CP437 encodings (x/84). * Supports latin1, CP437, or UTF8 encoding (dgamelaunch). * DOS-based CP437 bbs, requires a 80x24 window (mystic). * DOS-based CP437 bbs, requires a 80x24 window (synchronet).
  • telnet-server: This provides a simple cmd-line interface (telsh) for interactively toggling and displaying telnet session parameters. This serves as an example of a basic prompting server with which commands may be issued.
  • telnet-talker: An example multi-user telnet server shell that is basically a simple chat system, sometimes called a talker.


In addition to remote line editing as described below, a pure-python shell, telsh is provided to allow toggling of server options and session parameters. In this way, it provides a suitable interface for testing telnet client capabilities.

It is only in the interest of this project to provide enough shell-like capabilities to demonstrate remote line editing and an extensible environment for session introspection. An example of this is assigning a new value to CHARSET, toggling in and outbinary, thereby enabling UTF8 input/output, etc.


CHARSET (RFC 2066) specifies a codepage, not an encoding. At the time, this was more or less limited to specifying the codepage used to display bytes of the range 127 through 255. Unimplemented in BSD client, and generally found implemented only in recent MUD client (Atlantis) and servers. Most common values are: ASCII, UTF8, BIG5, and LATIN1.

The default preferred encoding for clients that negotiate BINARY but not CHARSET, such as the BSD client, is defined by the TelnetServer keyword argument default_encoding (‘UTF8’ by default).

The example shell telsh allows changing encoding on the fly by setting the ‘CHARSET’ session environment value at the telsh command prompt by issuing command:


Setting binary for only a single direction (‘outbinary’ or ‘inbinary’) is supported. Client support of one does not immediately toggle the other, it must be negotiated both ways for full UTF8 input and output.

Some clients (TinTin++) incorrectly negotiation either directions (WILL, DO/WONT, DONT) as a single option, causing only one reply for a request of either ‘outbinary’ or ‘inbinary’ for which it always declines, only once, for either request (Even when configured for UTF8).


Additionally, a contrib.cp437 module is included (authored by tehmaze) which translates output meant to be translated by DOS Emulating programs to their comparable UTF-8 font. This is used by argument –cp437 of the telnet-client program.

Some bulletin-board systems will send extended ascii characters (such as those used by


The Telnet protocol is over 40 years old and still in use today. Telnet predates TCP, and was used over a wide array of transports, especially on academic and military systems. Nearly all computer networking that interacted with human interfaces was done using the Telnet protocol prior to the mass-adoption of the World Wide Web in the mid 1990’s, when SSH became more commonplace.

Naturally, Telnet as a code project inevitably must handle a wide variety of connecting clients and hosts, due to limitations of their networking Transport , Terminals, their drivers, and host operating systems.

This implementation aims to implement only those capabilities “found in the wild”, and includes, or does not include, mechanisms that are suitable only for legacy or vendor-implemented options. It even makes one of its own: the encoding’ used in binary mode is the value replied by the CHARSET negotation (RFC 2066).

Remote LineMode

This project is the only known Server-side implementation of Special Linemode Character (SLC) negotiation and Remote line editing (RFC 1184), other than BSD telnet, which was used as a guide for the bulk of this python implementation.

Remote line editing is a comprehensive approach to providing responsive, low-latency output of characters received over slow network links, allowing incomplete lines to be buffered, while still providing remote editing facilities, such as backspace, kill line, etc.

The Server and Client agree on a series of Special Linemode Character (SLC) function values, to agree on the keyboard characters used for Backspace, Interrupt Process (^C), Repaint (^R), Erase Word (^W), etc.

Kludge Mode

In kludge mode, SLC characters are simulated for remote editing, provide an almost readline-like experience for all telnet clients, except those that perform only local editing, which are unaffected.

The sequence sent by server, WILL-SGA, WILL-ECHO enables “kludge mode”, a form of line mode editing that is compatible with all minimally implemented telnet clients. This is the most frequent implementation used by Windows 98 telnet, SyncTerm, netrunner, or TinTin++ to provide character-at-a-time editing.

Consider that kludge mode provides no way to determine which bytes, received at any indeterminate time, of any indeterminate length, or none at all, are received as the result of which input characters sent.

Accordingly, with Suppress Go-Ahead (SGA) enabled, there can be any indeterminable state: (1) the remote program is hung, (2) receiving and/or processing, (3) has responded with output but not yet received by transport, and (4) has received some, but not yet all output by transport.

This is detrimental to a user experience with character-at-a-time processing, as a user cannot know whether the input was legal, ignored, or not yet replied to, causing some frustration over high latency links.


The IAC-GA signal would seemingly be of little use over today’s bi-directional TCP protocol and virtual terminal emulators – its original purpose was to coordinate transmission on half-duplex protocols and terminals.

Only a few 1970-era hosts (AMES-67, UCLA-CON) require the IAC-GA signal. For this reason, this server takes the modern recommendation of suppressing the IAC-GA signal (IAC-WILL-SGA) by default; those clients wishing to make use of the IAC-GA signal must explicitly request IAC-DONT-SGA to enable the IAC-GA signal.

The IAC-GA signal has been recently restored for character-at-a-time servers, such as the competition nethack server, targeted at client scripts that play using AI decision-making routines.

Local Line Mode

Unless otherwise negotiated, the specification describes Telnet’s default mode as half-duplex, local line editing. This most basic “dummy” mode is modeled after a Teletype 33, which runs in “half-duplex” mode.

A Telnet implementation attached to 7-bit ASCII teletype may implement the Telnet protocol by hardware circuit, or by minimal changes to their terminal line drivers: when the connecting CPU is without MMU or process control, an IAC interpreter or hardware device could be “interrupted” when the 8th bit is set high, “Out of band” in regards to 7-bit terminals, the receipt of value 255 indicates that the byte following it Is-A-Command (IAC).

Default Telnet Mode

  • Each end transmits only 7-bit ASCII, (except as used in the interpreter).
  • A server’s prompt must be followed by the ‘Go-Ahead’ (IAC-GA) command.
  • Client signals end of input (send) by CR, LF (Carriage Return, Linefeed).

“Synch” Mechanism, not supported

A supervisor connecting a (7-bit) teletype to a telnet (8-bit) data line would simply pipe the streams together by the 7 bits; The teletypist may press ‘BREAK’ at any time to signal a control line: the supervisor then enters Telnet Synch” mode by sending an “Urgent” mechanism, and ceases printing data received on the transport.

A user could then instruct “Abort Output” (IAC-AO), “Interrupt Process” (IAC-IP), or others, and then presumably return to normal processing.

Consider the description of a PDP-10 session in RFC 139 (May 1971), presented here as a simple unix session:

  1. Teletype sends command input:

    find /usr -name 'telop.c'<CR>
  2. Server begins output – perhaps, after some minutes of pause, many rows of ‘Permission Denied’. Meanwhile, the user has already filled his teletype’s input buffer, and later deciding to abort the previous program:

    ed /usr/local/s^t/tel^t^c

At this point, however, the half-dupex Teletype cannot transmit any input.

The only way to signal the attention of the supervisor, which is currently blocking the half-duplex transmission with output (having not yet received IAC-GA), is by a special line signal wired separately from the teletype keyboard. This is the BREAK or ATTN key.

The terminal driver may then signal the ‘supervisor’, which then sends INS (RFC 139). Although the teletype is capable of “flushing” its input buffer, it does not flush control codes. Remaining control codes from the teletype (^t^t^c) continues to the remote end, but is discarded by that end, until the Data-Mark (IAC-DM) is sent by the supervisor.

This ensures the ^t and ^c characters are not received by the remote program.

TCP Implementation

In the TCP implementation of telnet, where presumably a half-duplex terminal may still interconnect, the INS marker referenced in pre-TCP documents is, instead, marked by sending the TCP Urgent option:

socket.send(IAC, socket.MSG_OOB).

The value of the byte does not seem to matter, can be of any length, and can continue sending socket.MSG_OOB (presumably, along with the remaining ^t^t^c described previously). The BSD server sends only a single byte:

 * In 4.2 (and 4.3) systems, there is some question about
 * what byte in a sendOOB operation is the "OOB" data.
 * To make ourselves compatible, we only send ONE byte
 * out of band, the one WE THINK should be OOB

All input is discarded by the IAC interpreter until IAC-DM is received; including IAC or 8-bit commands. This was used to some abuse to “piggyback” telnet by breaking out of IAC and into another “protocol” all together, and is grieved about in RFC 529:

The Telnet SYNCH mechanism is being misused by attempting to give
it meaning at two different levels of protocol.

The BSD client may be instructed to send this legacy mechanism by escaping and using the command send synch:

telnet> send synch

This sends IAC marked MSG_OOB, followed by DM, not marked MSG_OOB. The BSD server at this point would continue testing whether the last received byte is still marked urgent, by continuing to test errorfds (third argument to select select, a modern implementation might rather use sockatmark(3)).

Abort Output

BSD Telnet Server sets “Packet mode” with the pty driver:

(void) ioctl(p, TIOCPKT, (char *)&on);

And when TIOCPKT_FLUSHWRITE is signaled by the pty driver:

#define         TIOCPKT_FLUSHWRITE      0x02    /* flush packet */

Awaiting data buffered on the write transport is cleared; taking care to ensure all IAC commands were sent in the netclear() algorithm, which also sets the neturgent pointer.

Carriage Return

There are five supported signaling mechanisms for “send” or “end of line” received by clients. The default implementation supplies remote line editing and callback of line_received with all client-supported carriage returns, but may cause loss of data for implementors wishing to distinguish among them.

Namely, the difference between ‘return’ and ‘enter’ or raw file transfers. Those implementors should directly override data_received, or carefully deriving their own implementations of editing_received and character_received.

An overview of the primary callbacks and their interaction with carriage returns are described below for those wishing to extend the basic remote line editing or ‘character-at-a-time’ capabilities.

  • CR LF (Carriage Return, Linefeed): The Telnet protocol defines the sequence CR LF to mean “end-of-line”. The default implementation strips CL LF, and fires line_received on receipt of CR byte.
  • CR NUL (Carriage Return, Null): An interpretation of RFC 854 may be that CR NUL should be sent when only a single CR is intended on a client and server host capable of distinguishing between CR and CR LF (return key vs enter key). The default implementation strips CL NUL, and fires line_received on receipt of CR byte.
  • CR (Carriage Return): CR alone may be received, though a client is not RFC-complaint to do so. The default implementation strips CR, and fires line_received.
  • LF (Linefeed): LF alone may be received, though a client is not RFC-complaint to do so. The default implementation strips LF, and fires line_received.
  • IAC EOR (Is-A-Command, End-Of-Record): In addition to line-oriented or character-oriented terminals, IAC EOR is used to delimit logical records (e.g., “screens”) on Data Entry Terminals (DETs), or end of multi-line input on vendor-implemented and some MUD clients, or, together with BINARY, a mechanism to signal vendor-implemented newline outside of CR LF during file transfers. MUD clients may read IAC EOR as meaning ‘Go Ahead’, marking the current line to be displayed as a “prompt”, optionally not included in the client “history buffer”. To register receipt of IAC EOR, a client must call set_iac_callback(telopt.EOR, func).

RFCs Implemented

  • RFC 727 “Telnet Logout Option,” Apr 1977. (1)
  • RFC 779 “Telnet Send-Location Option”, Apr 1981. (1)
  • RFC 854 “Telnet Protocol Specification”, May 1983. (2)
  • RFC 855 “Telnet Option Specifications”, May 1983. (2)
  • RFC 856 “Telnet Binary Transmission”, May 1983.
  • RFC 857 “Telnet Echo Option”, May 1983. (2)
  • RFC 858 “Telnet Suppress Go Ahead Option”, May 1983. (2)
  • RFC 859 “Telnet Status Option”, May 1983.
  • RFC 860 “Telnet Timing mark Option”, May 1983. (2)
  • RFC 885 “Telnet End of Record Option”, Dec 1983. (1)
  • RFC 1073, “Telnet Window Size Option”, Oct 1988.
  • RFC 1079, “Telnet Terminal Speed Option”, Dec 1988.
  • RFC 1091, “Telnet Terminal-Type Option”, Feb 1989. (2)
  • RFC 1123, “Requirements for Internet Hosts”, Oct 1989. (2)
  • RFC 1184, “Telnet Linemode Option (extended options)”, Oct 1990.
  • RFC 1096, “Telnet X Display Location Option”, Mar 1989.
  • RFC 1372, “Telnet Remote Flow Control Option”, Oct 1992.
  • RFC 1408, “Telnet Environment Option”, Jan 1993.
  • RFC 1571, “Telnet Environment Option Interoperability Issues”, Jan 1994.
  • RFC 1572, “Telnet Environment Option”, Jan 1994.
  • RFC 2066, “Telnet Charset Option”, Jan 1997. (1)

(1): Not implemented in BSD telnet (rare!)

(2): Required by specification (complies!)

RFCs Not Implemented

  • RFC 861, “Telnet Extended Options List”, May 1983. describes a method of negotiating options after all possible 255 option bytes are exhausted by future implementations. This never happened (about 100 remain), it was perhaps, ambitious in thinking more protocols would incorporate Telnet (such as FTP did).
  • RFC 927, “TACACS User Identification Telnet Option”, describes a method of identifying terminal clients by a 32-bit UUID, providing a form of ‘rlogin’. This system, published in 1984, was designed for MILNET by BBN, and the actual TACACS implementation is undocumented, though partially re-imagined by Cisco in RFC 1492. Essentially, the user’s credentials are forwarded to a TACACS daemon to verify that the client does in fact have access. The UUID is a form of an early Kerberos token.
  • RFC 933, “Output Marking Telnet Option”, describes a method of sending banners”, such as displayed on login, with an associated ID to be stored by the client. The server may then indicate at which time during the session the banner is relevant. This was implemented by Mitre for DOD installations that might, for example, display various levels of “TOP SECRET” messages each time a record is opened – preferably on the top, bottom, left or right of the screen.
  • RFC 946, “Telnet Terminal Location Number Option”, only known to be implemented at Carnegie Mellon University in the mid-1980’s, this was a mechanism to identify a Terminal by ID, which would then be read and forwarded by gatewaying hosts. So that user traveling from host A -> B -> C appears as though his “from” address is host A in the system “who” and “finger” services. There exists more appropriate solutions, such as the “Report Terminal ID” sequences CSI + c and CSI + 0c for vt102, and ESC + z (vt52), which sends a terminal ID in-band as ASCII.
  • RFC 1041, “Telnet 3270 Regime Option”, Jan 1988
  • RFC 1043, “Telnet Data Entry Terminal Option”, Feb 1988
  • RFC 1097, “Telnet Subliminal-Message Option”, Apr 1989
  • RFC 1143, “The Q Method of Implementing .. Option Negotiation”, Feb 1990
  • RFC 1205, “5250 Telnet Interface”, Feb 1991
  • RFC 1411, “Telnet Authentication: Kerberos Version 4”, Jan 1993
  • RFC 1412, “Telnet Authentication: SPX”
  • RFC 1416, “Telnet Authentication Option”
  • RFC 2217, “Telnet Com Port Control Option”, Oct 1997

Additional Resources

These RFCs predate, or are superseded by, RFC 854, but may be relevant.


It should be said as historical source code, BSD 2.11’s telnet source of UCLA and NCSA_Telnet client of Univ. of IL for MacOS is most notable. There are also a few modern Telnet servers. Some modern Telnet clients support only kludge mode, with the exception of MUD clients, which are often Linemode only. TinTin++ is the only known client to support both modes.

Finding RFC 495

RFC 495, NIC #15371 “TELNET Protocol Specification.” 1 May 1973, A. McKenzie, lists the following attached documents, which are not available:

[...] specifications for TELNET options which allow negotiation of:

        o binary transmission
        o echoing
        o reconnection
        o suppression of "Go Ahead"
        o approximate message size
        o use of a "timing mark"
        o discussion of status
        o extension of option code set

These specifications have been prepared by Dave Walden (BBN-NET) with
the help of Bernie Cosell, Ray Tomlinson (BBN-TENEX) and Bob Thomas;
by Jerry Burchfiel (BBN-TENEX); and by David Crocker (ULCA-NMC).

If anybody can locate these documents, please forward them along.

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