Telnet

Telnet (Telecommunication network) is a network protocol used on the Internet or local area network (LAN) connections. It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet standards.

The term "telnet" also refers to software which implements the client part of the protocol. Telnet clients are available for virtually all platforms. Most network equipment and OSes with a TCP/IP stack support some kind of Telnet service server for their remote configuration (including ones based on Windows NT). Because of security issues with Telnet, its use has waned as it is replaced by the use of SSH for remote access.

"To telnet" is also used as a verb meaning to establish or use a Telnet or other interactive TCP connection, as in, "To change your password, telnet to the server and run the passwd command".

Most often, a user will be "telnetting" to a Unix-like server system or a simple network device such as a router. For example, a user might "telnet in from home to check his mail at school". In doing so, he would be using a telnet client to connect from his computer to one of his servers. Once the connection is established, he would then log in with his account information and execute operating system commands remotely on that computer, such as ls or cd.

On many systems, the client may also be used to make interactive raw-TCP sessions. It is commonly believed that a telnet session which does not include an IAC (character 255) is functionally identical. This is not the case however due to special NVT (Network Virtual Terminal) rules such as the requirement for a bare CR (ASCII 13) to be followed by a NULL (ASCII 0).

Protocol details

Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically this protocol used to establish a connection to TCP port 23, where a getty-equivalent program (telnetd) is listening, although Telnet predates TCP/IP and was originally run on NCP.

Initially, On March 5th, 1973, a meeting was held at UCLA [ [ftp://ftp.rfc-editor.org/in-notes/rfc495.txt RFC 495 - Announcement of Telnet protocol] ] where "New Telnet" was defined in two NIC The protocol has many extensions, some of which have been adopted as Internet standards. IETF standards STD 27 through STD 32 define various extensions, most of which are extremely common. Other extensions are on the IETF standards track as proposed standards.

ecurity

When Telnet was initially developed in 1969, most users of networked computers were in the computer departments of academic institutions, or at large private and government research facilities. In this environment, security was not nearly as much of a concern as it became after the bandwidth explosion of the 1990s. The rise in the number of people with access to the Internet, and by extension, the number of people attempting to crack other people's servers made encrypted alternatives much more of a necessity.

Experts in computer security, such as SANS Institute, and the members of the comp.os.linux.security newsgroup recommend that the use of Telnet for remote logins should be discontinued under all normal circumstances, for the following reasons:

* Telnet, by default, does not encrypt any data sent over the connection (including passwords), and so it is often practical to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, switch, hub or gateway located on the network between the two hosts where Telnet is being used can intercept the packets passing by and obtain login and password information (and whatever else is typed) with any of several common utilities like tcpdump and Wireshark.
* Most implementations of Telnet have no authentication to ensure that communication is carried out between the two desired hosts and not intercepted in the middle.
* Commonly used Telnet daemons have several vulnerabilities discovered over the years.

These security-related shortcomings have seen the usage of the Telnet protocol drop rapidly, especially on the public Internet, in favor of the ssh protocol, first released in 1995. SSH provides much of the functionality of telnet, with the addition of strong encryption to prevent sensitive data such as passwords from being intercepted, and public key authentication, to ensure that the remote computer is actually who it claims to be.

As has happened with other early Internet protocols, extensions to the Telnet protocol provide TLS security and SASL authentication that address the above issues. However, most Telnet implementations do not support these extensions; and there has been relatively little interest in implementing these as SSH is adequate for most purposes. The main advantage of TLS-Telnet would be the ability to use certificate-authority signed server certificates to authenticate a server host to a client that does not yet have the server key stored. In SSH, there is a weakness in that the user must trust the first session to a host when it has not yet acquired the server key.

Telnet 5250

IBM 5250 or 3270 workstation emulation is supported via custom telnet clients, TN5250/TN3270, and IBM servers. Clients and servers designed to pass IBM 5250 data streams over Telnet generally do support SSL encryption, as SSH does not include 5250 emulation. Under OS/400, port 992 is the default port for secured telnet.

Current status

As of the mid-2000s, while the Telnet protocol itself has been mostly superseded for remote login, Telnet clients are still used, often when diagnosing problems, to manually "talk" to other services without specialized client software. For example, it is sometimes used in debugging network services such as an SMTP, IRC, HTTP, FTP or POP3 server, by serving as a simple way to send commands to the server and examine the responses.

This approach has limitations as Telnet clients speak is close to, but not equivalent to, raw mode (due to terminal control handshaking and the special rules regarding 377 and 15). Thus, other software such as "nc" (netcat) or "socat" on Unix (or PuTTY on Windows) are finding greater favor with some system administrators for testing purposes, as they can be called with arguments not to send any terminal control handshaking data. Also netcat does not distort the 377 octet, which allows raw access to TCP socket, unlike any standard-compliant Telnet software.

Telnet is popular with:
* enterprise networks to access host applications, e.g. on IBM Mainframes.
* administration of network elements, e.g., in commissioning, integration and maintenance of core network elements in mobile communication networks.
* MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the resurgent BBS community.
* embedded systems

ee also

* Reverse telnet
* Telnet 3270 (TN3270)
* HyTelnet
* Kermit
* NCSA Telnet
* SSH

References

External links

* [http://www.iana.org/assignments/telnet-options Telnet Options] - The official list of assigned option numbers at iana.org
* [http://www.eventhelix.com/RealtimeMantra/Networking/Telnet.pdf Telnet Interactions Described as a Sequence Diagram]
* [http://www.pcmicro.com/netfoss/telnet.html Telnet protocol description, with NVT reference]

Related RFCs

* [http://tools.ietf.org/html/rfc854 RFC 854] TELNET protocol specification
* [http://tools.ietf.org/html/rfc855 RFC 855] TELNET option specifications
* [http://tools.ietf.org/html/rfc856 RFC 856] TELNET binary transmission
* [http://tools.ietf.org/html/rfc857 RFC 857] TELNET echo option
* [http://tools.ietf.org/html/rfc858 RFC 858] TELNET suppress Go Ahead option
* [http://tools.ietf.org/html/rfc859 RFC 859] TELNET status option
* [http://tools.ietf.org/html/rfc860 RFC 860] TELNET timing mark option
* [http://tools.ietf.org/html/rfc861 RFC 861] TELNET extended options - list option
* [http://tools.ietf.org/html/rfc885 RFC 885] Telnet end of record option
* [http://tools.ietf.org/html/rfc1041 RFC 1041] Telnet 3270 regime option
* [http://tools.ietf.org/html/rfc1073 RFC 1073] Telnet Window Size Option
* [http://tools.ietf.org/html/rfc1079 RFC 1079] Telnet terminal speed option
* [http://tools.ietf.org/html/rfc1091 RFC 1091] Telnet terminal-type option
* [http://tools.ietf.org/html/rfc1096 RFC 1096] Telnet X display location option
* [http://tools.ietf.org/html/rfc1184 RFC 1184] Telnet linemode option
* [http://tools.ietf.org/html/rfc1205 RFC 1205] 5250 Telnet interface
* [http://tools.ietf.org/html/rfc1372 RFC 1372] Telnet remote flow control option
* [http://tools.ietf.org/html/rfc2217 RFC 2217] Telnet Com Port Control Option
* [http://tools.ietf.org/html/rfc2941 RFC 2941] Telnet Authentication Option
* [http://tools.ietf.org/html/rfc2942 RFC 2942] Telnet Authentication: Kerberos Version 5
* [http://tools.ietf.org/html/rfc2943 RFC 2943] TELNET Authentication Using DSA
* [http://tools.ietf.org/html/rfc2944 RFC 2944] Telnet Authentication: SRP
* [http://tools.ietf.org/html/rfc2946 RFC 2946] Telnet Data Encryption Option
* [http://tools.ietf.org/html/rfc4248 RFC 4248] The telnet URI Scheme
* [http://tools.ietf.org/html/rfc4777 RFC 4777] IBM's iSeries Telnet Enhancements

Telnet clients

* PuTTY is a free, open source SSH, Telnet, rlogin, and raw TCP client for Windows, Linux, and Unix.
* [http://syncterm.bbsdev.net/ SyncTERM] is a free open-source Telnet/RLogin/SSH client/terminal for Windows and *nix platforms (including Mac OS X) with windowed/full-screen and Zmodem support.
* [http://www.celestialsoftware.net AbsoluteTelnet] is a terminal application (telnet, ssh, serial, dialup) for Windows desktop platforms. It includes file transfer capabilities such a zmodem and sftp.
* [http://mobilevt.altervista.org MobileVT] is a telnet, ssh, vt100, dialup terminal client for Windows mobile and Pocket PC. It includes file transfer and script capabilities.