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TELNET (TELetype NETwork) is a network protocol used on the Internet or local area network (LAN) connections. It was developed in 1969 and standardized as IETF STD 8, one of the first Internet standards. It has limitations that are considered to be security risks.
"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 telneting to a unix-like server system or a simple network device such as a switch. 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.
Protocol detailsTELNET is a client-server protocol, based on a reliable connection-oriented transport. Typically this is TCP port 23, although TELNET predates TCP/IP and was originally run on NCP. 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. SecurityWhen 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 into other people's servers made encrypted alternatives much more necessary. 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:
These security-related shortcomings have seen the usage of the TELNET protocol drop rapidly, especially on the public Internet, in favor of a more secure and functional protocol called SSH, first released in 1995. SSH provides all 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. Current statusAs of the mid-2000s, while the TELNET protocol itself has been mostly superseded, TELNET clients are still used, usually 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 or HTTP server, by serving as a simple way to send commands to the server and examine the responses. On UNIX, though, other software such as nc (netcat) or socat are finding greater favor with some system administrators for testing purposes, as they can be called with arguments to not send any terminal control handshaking data. TELNET is still very popular in enterprise networks to access host applications, i.e. on IBM Mainframes. TELNET is also heavily used for MUD games played over the Internet, as well as talkers, MUSH es, MUCKs and MOOes. By using image-to-ASCII algorithms, it can also be used for primitive "video" streaming. Recently, ASCII-WM offered live broadcasts of the 2006 World Cup. TELNET can also be used as a rudimentary IRC client if one knows the protocol well enough. In the 2007 Microsoft Windows release, Windows Vista, Telnet.exe is no longer installed by default, but is still included as an installable feature.[1] TELNET clientsWindows
Macintosh
Multiplatform
Free with Source Code
Non-free Source Code
TELNET serversFree with Source Code
Related RFCs
See also
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