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Telnet is a protocol used on the or local area networks to provide a bidirectional interactive text-oriented communication facility using a virtual connection. User data is interspersed in-band with Telnet control information in an 8-bit data connection over the Transmission Control Protocol (TCP).

Telnet was developed in 1969 beginning with RFC 15, extended in RFC 854, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards. The name stands for " teletype network".Andrew Wheen. Dot-dash to Dot.Com: How Modern Telecommunications Evolved from the Telegraph to the Internet. Springer, 2011. p. 132. .Christoph Meinel; Harald Sack. Internetworking: Technological Foundations and Applications. X.media.publishing, 2013. p. 57. .

Historically, Telnet provided access to a command-line interface (usually, of an ) on a remote host, including most network equipment and with a configuration utility (including systems based on ). However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of .

The term telnet is also used to refer to the software that implements the client part of the protocol. Telnet client applications are available for virtually all computer platforms. Telnet is also used as a . To telnet means to establish a connection using the Telnet protocol, either with command line client or with a programmatic interface. For example, a common directive might be: " To change your password, telnet into the server, log in and run the passwd command." Most often, a user will be telnetting to a server system or a network device (such as a router) and obtaining a login prompt to a command line text interface or a character-based full-screen manager.


History and standards
Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically, this protocol is used to establish a connection to Transmission Control Protocol (TCP) 23, where a Telnet server application (telnetd) is listening. Telnet, however, predates TCP/IP and was originally run over Network Control Program (NCP) protocols.

Even though Telnet was an ad hoc protocol with no official definition until March 5, 1973,RFC 318 — documentation of old ad hoc telnet protocol the name actually referred to Teletype Over Network Protocol as the RFC 206 (NIC 7176) on Telnet makes the connection clear:Garth O. Bruen. WHOIS Running the Internet: Protocol, Policy, and Privacy. Wiley, 1st edition (November 2, 2015). p. 25. .

Essentially, it used an 8-bit channel to exchange 7-bit ASCII data. Any byte with the high bit set was a special Telnet character. On March 5, 1973, a Telnet protocol standard was defined at UCLARFC 495 — announcement of Telnet protocol with the publication of two NIC documents: Telnet Protocol Specification, NIC 15372, and Telnet Option Specifications, NIC 15373.
     

Many extensions were made for Telnet because of its negotiable options protocol architecture. Some of these extensions have been adopted as Internet standards, IETF documents STD 27 through STD 32. Some extensions have been widely implemented and others are proposed standards on the IETF standards track (see below) Telnet is best understood in the context of a user with a simple terminal using the local Telnet program (known as the client program) to run a logon session on a remote computer where the user's communications needs are handled by a Telnet server program.


Security
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 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 hack other people's servers, made encrypted alternatives necessary.

Experts in computer security, such as , 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 any data sent over the connection (including passwords), and so it is often feasible to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, , or gateway located on the network between the two hosts where Telnet is being used can intercept the packets passing by and obtain login, password and whatever else is typed with a .
  • Most implementations of Telnet have no authentication that would ensure communication is carried out between the two desired hosts and not intercepted in the middle.
  • Several vulnerabilities have been discovered over the years in commonly used Telnet daemons.

These security-related shortcomings have seen the usage of the Telnet protocol drop rapidly, especially on the public , in favor of the (SSH) protocol, first released in 1995. SSH has practically replaced Telnet, and the older protocol is used these days only in rare cases to access decades old legacy equipment that does not support more modern protocols. 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 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 Transport Layer Security (TLS) security and Simple Authentication and Security Layer (SASL) authentication that address the above concerns. 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.

It is of note that there are a large number of industrial and scientific devices which have only Telnet available as a communication option. Some are built with only a standard RS-232 port and use a serial server hardware appliance to provide the translation between the TCP/Telnet data and the RS-232 serial data. In such cases, SSH is not an option unless the interface appliance can be configured for SSH.


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 (also known as ), port 992 is the default port for secured telnet.


Telnet data
All data octets except 0xff are transmitted over Telnet as is. (0xff, or 255 in decimal, is the IAC byte (Interpret As Command) which signals that the next byte is a telnet command. The command to insert 0xff into the stream is 0xff, so 0xff need to be escaped by doubling it when sending data over the telnet protocol.) Therefore, a Telnet client application may also be used to establish an interactive raw TCP session, and it is commonly believed that such session which does not use the IAC is functionally identical. This is not the case, however, because there are other network virtual terminal (NVT) rules, such as the requirement for a bare carriage return character (CR, 13) to be followed by a NUL (ASCII 0) character, that distinguish the telnet protocol from raw TCP sessions. On the other hand, many systems now possess true raw TCP clients, such as or on UNIX and on Windows, which also can be used to manually "talk" to other services without specialized client software. Nevertheless, Telnet is still sometimes used in network services such as , IRC, , or POP3 servers, to issue commands to a server and examine the responses, but of all these protocols only FTP really uses Telnet data format.

Another difference of Telnet from a raw TCP session is that Telnet is not 8-bit clean by default. 8-bit mode may be negotiated, but high-bit-set octets may be garbled until this mode is requested, and it will not be requested in non-Telnet connection. The 8-bit mode (so named binary option) is intended to transmit binary data, not characters though. The standard suggests the interpretation of codes 0000–0176 as ASCII, but does not offer any meaning for high-bit-set data octets. There was an attempt to introduce a switchable character encoding support like HTTP has,RFC 2066 — TELNET CHARSET Option but nothing is known about its actual software support.


Related RFCs

Internet Standards
  • RFC 854, Telnet Protocol Specification
  • RFC 855, Telnet Option Specifications
  • RFC 856, Telnet Binary Transmission
  • RFC 857, Telnet Echo Option
  • RFC 858, Telnet Suppress Go Ahead Option
  • RFC 859, Telnet Status Option
  • RFC 860, Telnet Timing Mark Option
  • RFC 861, Telnet Extended Options: List Option


Proposed Standards
  • RFC 885, Telnet end of record option
  • RFC 1073, Telnet Window Size Option
  • RFC 1079, Telnet terminal speed option
  • RFC 1091, Telnet terminal-type option
  • RFC 1096, Telnet X display location option
  • RFC 1123, Requirements for Internet Hosts - Application and Support
  • RFC 1184, Telnet Linemode Option
  • RFC 1372, Telnet Remote Flow Control Option
  • RFC 1572, Telnet Environment Option
  • RFC 2941, Telnet Authentication Option
  • RFC 2942, Telnet Authentication: Kerberos Version 5
  • RFC 2943, TELNET Authentication Using DSA
  • RFC 2944, Telnet Authentication: SRP
  • RFC 2946, Telnet Data Encryption Option
  • RFC 4248, The telnet URI Scheme


Informational/experimental
  • RFC 1143, The Q Method of Implementing TELNET Option Negotiation
  • RFC 1571, Telnet Environment Option Interoperability Issues


Other RFCs
  • RFC 1041, Telnet 3270 regime option
  • RFC 1205, 5250 Telnet interface
  • RFC 2217, Telnet Com Port Control Option
  • RFC 4777, IBM's iSeries Telnet Enhancements


Telnet clients
  • and plink command line are a free, open-source , Telnet, , and raw TCP client for Windows, , and .
  • is a telnet client for Windows. It also supports and SFTP,
  • RUMBA (Terminal Emulator)
  • Line Mode Browser, a command line
  • from Van Dyke Software
  • ZOC Terminal
  • BBS terminal program supporting Telnet, SSHv2, RLogin, Serial, Windows, *nix, and Mac OS X platforms, X/Y/ZMODEM and various BBS terminal emulations
  • NetRunner BBS telnet client for ANSI-BBS. Uses a modern SDL2-based emulation engine to provide very accurate and clear emulation of DOS and Amiga systems. Available for Linux and Windows.
  • PowerTerm InterConnect from available for , Mac OS X, , and supports 35 terminal emulation types including TN3270, TN5250, VT420, and others with SSH and SSL.
  • is a client version of Telnet, providing similar functionality of telnet to those hosts which are behind firewall and NAT.
  • includes a telnet client and server and is installed by default on many GNU/Linux distributions.
  • telnet.exe command line utility included in default installation of many version of Microsoft Windows OS.


See also


External links

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