Telnet is a protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual text terminal connection. User data is interspersed in-band with Telnet control information in an 8-bit byte oriented 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 operating system) on a remote host, including most network equipment and with a configuration utility (including systems based on Windows NT). 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 Secure Shell.
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 verb. 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 Unix-like 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.
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.
Experts in computer security, such as SANS Institute, recommend that the use of Telnet for remote logins should be discontinued under all normal circumstances, for the following reasons:
These security-related shortcomings have seen the usage of the Telnet protocol drop rapidly, especially on the public Internet, in favor of the Secure Shell (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 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 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.
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.
|go-telnet go-telnet repository go-telnet documentation||*BSD, GNU, macOS, Windows||Go|
|libtelnet libtelnet repository||*BSD, GNU, macOS, Windows||C|
|telnetlib telnetlib documentation||*BSD, GNU, macOS, Windows||Python|