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Telnet Protocol server and shell using tulip / PEP3156.

Project description

About is an ISC-licensed Telnet Client and Telnet Server library.

It is hosted on github. Currently in development stage, feedback is encouraged. Feel free to make use of github’s fork and “Issues” service to report any bugs or grievances.

This project uses the asyncio module of python 3.4, also available as a package on pypi for python 3.3, for which this is currently targeted. asyncio aims to be compatible with existing frameworks such as Twisted and gevent.


Telnet Server and Client example programs are provided in the github_examples sub-folder of the source project. demonstrates a multi-user server (chat program). provides a debugging shell for testing compatible clients, using the default telsh shell. provides a simple client for connecting to telnet servers. provides a sort of honepot for gathering client details. and Favorite boards

Most notably, examples/ provides a –cp437 command-line option that can be used to connect to DOS Bulletin-board systems that would otherwise require a DOS-emulating client. This client demonstrates python’s powerful encoding capabilities of translating the DOS codepage cp437 to utf-8, allowing fancyful ansi artwork to be displayed directly in xterm, rxvt, iTerm, or any other client supporting utf-8. In this way, this simple client program example replaces the need for SyncTerm, mtelnet, netrunner, or other special-purpose DOS emulating clients.

Some of my favorite telnet destinations,

  • (cp437 only)
  • (cp437 only)
  • (cp437 only)
  • (utf8 or cp437 only)
  • (cp437 only)
  • (ascii)
  • (ascii or latin1)


In adition 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 utf-8 input/output, etc.


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-adapation of the World Wide Web in the mid 90’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 Transports, 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).


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, UTF-8, 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 (‘utf-8’ by default). The example shell ‘telsh’ allows changing encoding on the fly by setting the ‘CHARSET’ session environment value at the command prompt by issuing command ‘set CHARSET=utf-8’.

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 UTF-8 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 utf-8).

Remote Linemode

This project is the only known Server-side implementation of Special Linemode Character (SLC) negotiation and Remote line editing (rfc1184), 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 editing facilities (such as backspace, kill line, etc.), remotely.

The Server and Client agree on a series of Special Line Mode (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 “magic” 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 provies 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 Supress 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 wether 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 GA signal. For this reason, this server takes the modern recommendation of supressing the GA signal (WILL-SGA) by default; those clients wishing to make use of the GA signal must explicitly request IAC-DONT-SGA to enable the GA signal.

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

Local Linemode

Unless otherwise negotiatied, 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 implmentation 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’ (GA) command.
  • Client signals end of input (send) by CR, LF (Carriage Return, Linefeed).

“Synch” Mechanism


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” (AO), “Interrupt Process” (IP), or others, and then presumably return to normal processing.

Consider the description of a PDP-10 session in rfc139 (May 1971), presented here as a faux naif 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 GA), is by a special line signal wired seperately from the teletype keyboard. This is the ‘BREAK’ or ‘ATTN’ key.

The terminal driver may then signal the ‘supervisor’, which then sends ‘INS’ (rfc139). 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 (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). Though, 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 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 isgreived 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 wether the last received byte is still marked urgent, by continuing to test errorfds (third argument to, 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() alogorithm, which also sets the neturgent pointer.

Carriage Return

There are five supported signalling 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: The Telnet protocol defines the sequence CR LF to mean “end-of-line”. The default implementation strips CL LF, and fires line_received on CR.

CR NUL: An interpretation of rfc854 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’ vs ‘enter’ key). The default implementation strips CL NUL, and fires line_received on CR.

CR: CR alone may be received, though a client is not RFC-complaint to do so. The default implementation strips CL, and fires line_received.

LF: LF aline may be received, though a client is not RFC-complaint to do so. The default implementation strips LF, and fires line_received.

IAC EOR: 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 newline 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”. Its meaning as received by server is not known ..

Not Implemented

RFC-1416 “Telnet Authentication Option”, RFC-1411 “Telnet Authentication: Kerberos Version 4”, and RFC-1412 “Telnet Authentication: SPX” are supported by the BSD telnetd.c, but there are no plans to implement any of them.

RFC-861 “Telnet Extended Options List”, May 1983. describes a method of negotiating options after all possible 255 option bytes are exausted by future implementations. This never happened (about 100 remain), it was perhaps, ambitious in thinking more protocols would incorperate 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 rfc1492. Essentially, the user’s credentials are forwarded to a TACAS 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 sesssion the banner is relevant. This was implemented by Mitre for DOD installations that much, for instance, need to 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 Carnnige Mellon University in the mid-80’s, this was a mechanism to identify a Terminal by ID, which would then be read and forwarded by gatewaying hosts. So that user travelling 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 send a terminal ID inband as ASCII.

RFC 1041, “Telnet 3270 Regime Option”, Jan 1988 RFC 1043, “TELNET Data Entry Terminal Option”, Feb 1988 RFC 1143, “The Q Method of Implementing .. Option Negotiation”, Feb 1990 RFC 1097, “Telnet Subliminal-Message Option”, Apr 1989 RFC 1205, “5250 Telnet Interface”, Feb 1991 RFC 1411, “Telnet Authentication: Kerberos Version 4”, Jan 1993 RFC 2217, “Telnet Com Port Control Option”, Oct 1997


  • Proper XON/XOFF/AO, this was previously implemented with the proposed pause/resume_writing() methods for tulip, but these have been complicated a great deal upstream in asyncio/py 3.4, looks like a custom StreamWriter will need to be implemented ?
  • Proper abort_output()/IAC rewind, have yet
  • handling duplication of echo and flow control by LINEMODE opts need to cross-wire these all correctly
  • TelnetClient (also using TelnetStreamReader) this is rudimentarily implemented, but we need to implement a tty-driven interface of some sort to rapidly test it
  • fingerprinting Client & Server somewhere there with examples/
  • tests :-)
  • example MUD server, or nibbles, or pong, something worth playing
  • example client


  1. = Not implemented in bsd telnet (rare!)
  2. = Required by specification (complies!)
  • RFC 727 TELNET Logout Option. (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 Supress 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 1096, “Telnet X Display Location Option”, Mar 1989
  • RFC 1184, “Telnet Linemode Option (extended options)”, Oct 1990
  • RFC 1123, “Requirements for Internet Hosts”, Oct 1989 (2)
  • RFC 2066, “Telnet Charset Option”, Jan 1997 (1)
  • 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


These RFCs predate RFC 854, but are often relevant (or not)

RFC 97 A FIRST CUT AT A PROPOSED TELNET PROTOCOL RFC 137 TELNET Protocol. RFC 139 Discussion of TELNET Protocol. RFC 318 Telnet Protocol. RFC 328 Suggested Telnet Protocol Changes. RFC 340 PROPOSED TELNET CHANGES. RFC 393 Comments on TELNET Protocol Changes. RFC 435 TELNET Issues. RFC 513 COMMENTS ON THE NEW TELNET SPECIFICATIONS. RFC 529 A Note on Protocol Synch Sequences. RFC 559 Comments on the new TELNET Protocol and its Implementation. RFC 563 Comments on the RCTE TELNET Option. RFC 593 Telnet and FTP Implementation Schedule Change. RFC 595 Some Thoughts in Defense of the TELNET Go-Ahead. RFC 596 Second Thoughts on Telnet Go-Ahead. RFC 652 Telnet Output Carriage-Return Disposition Option. RFC 653 TELNET OUTPUT HORIZONTAL TABSTOPS OPTION. RFC 654 TELNET OUTPUT HORIZONTAL TAB DISPOSITION OPTION. RFC 655 TELNET OUTPUT FORMFEED DISPOSITION OPTION. RFC 656 TELNET OUTPUT VERTICAL TABSTOPS OPTION. RFC 657 TELNET OUTPUT VERTICAL TAB DISPOSITION OPTION. RFC 658 TELNET OUTPUT LINEFEED DISPOSITION. RFC 659 Announcing Addtional Telnet Options. RFC 698 TELNET EXTENDED ASCII OPTION. RFC 701 AUGUST, 1974, SURVEY OF NEW-PROTOCOL TELNET SERVERS. RFC 702 SEPTEMBER, 1974, SURVEY OF NEW-PROTOCOL TELNET SERVERS. RFC 703 July, 1975, Survey of New-Protocol TELNET Servers. RFC 718 Comments on RCTE from the TENEX Implementation Experience. RFC 719 Discussion on RCTE. RFC 726 Remote Controlled Transmssion and Echoing Telnet Option. RFC 728 A Minor Pitfall in the Telnet Protocol. RFC 732 Telnet Data Entry Terminal Option (Obsoletes: RFC 731) RFC 734 SUPDUP Protocol. RFC 735 Revised TELNET Byte Macro Option (Obsoletes: RFC 729, RFC 736) RFC 749 Telnet SUPDUP-OUTPUT Option. RFC 818 The Remote User Telnet Service.

“Telnet Protocol,” MIL-STD-1782, U.S. Department of Defense, May 1984. “Mud Terminal Type Standard,” “Telnet Protocol in C-Kermit 8.0 and Kermit 95 2.0,” “Telnet Negotiation Concepts,” “Telnet RFCs,”” “Telnet Options”,


It should be said as historical source code, BSD 2.11’s telnet source of UCLA and NSCA 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.

RFC 495

RFC 495, NIC #15371 “TELNET Protocol Specification.” 1 May 1973, A. McKenzie, lists the following “attatched documents”, which are not available to the public or in digitized form.

[…] 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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