telnetlib3 0.2.3

About

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.

Installing

1. Install python 3

2. Install pip

3. Ensure pip is up-to-date:

   pip install --upgrade pip

4. Install telnetlib3:

   pip install telnetlib3

Scripts

• 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,

  • htc.zapto.org: Supports UTF8 or CP437 encodings (enthral).

  • 1984.ws: Supports UTF8 or CP437 encodings (x/84).

  • nethack.alt.org: Supports latin1, CP437, or UTF8 encoding (dgamelaunch).

  • blackflag.acid.org: DOS-based CP437 bbs, requires a 80x24 window (mystic).

  • bbs.pharcyde.org: 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.

telsh

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.

UTF8

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:

set CHARSET=UTF8

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).

CP437

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

Telnet

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).

socket.send(IAC, socket.MSG_OOB).

/*
 * 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
 (...)

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

telnet> send synch

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

#define         TIOCPKT_FLUSHWRITE      0x02    /* flush packet */

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.

• 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 Additional 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 Transmission 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,” http://tintin.sourceforge.net/mtts/

• “Mud Client Protocol, Version 2.1,” http://www.moo.mud.org/mcp/mcp2.html

• “Telnet Protocol in C-Kermit 8.0 and Kermit 95 2.0,” http://www.columbia.edu/kermit/telnet80.html

• “Telnet Negotiation Concepts,” http://lpc.psyc.eu/doc/concepts/negotiation

• “Telnet RFCs,” http://www.omnifarious.org/~hopper/telnet-rfc.html”

• “Telnet Options”, http://www.iana.org/assignments/telnet-options/telnet-options.xml

[...] 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).