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Telnet (teletype network) is a network protocol used on the Internet or local area networks to provide a bidirectional interactive communications facility. Typically, telnet provides access to a command-line interface on a remote host via a virtual terminal connection which consists of an 8-bit byte oriented data connection over the Transmission Control Protocol (TCP). User data is interspersed in-band with TELNET control information.

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 term telnet may also refer to the software that implements the client part of the protocol. Telnet client applications are available for virtually all computer platforms. Most network equipment and operating system with a TCP/IP stack support a Telnet service for remote configuration (including systems based on Windows NT). Because of security issues with Telnet, its use has waned in favor of SSH for remote access.

Telnet is also used as a verb. To telnet means to establish a connection with 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 to the server, login 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 obtain a login prompt to a command line text interface or a character-based full-screen manager.

On many systems, a Telnet client application may also be used to establish interactive raw-TCP sessions. It is commonly believed that a Telnet session which does not use the IAC (character 255) 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, ASCII 13) to be followed by a NULL (ASCII 0) character, that distinguish the telnet protocol from raw-TCP sessions.

Protocol details

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) port number 23, where a Telnet server application (telnetd) is listening. Telnet, however, predates TCP/IP and was originally run over Network Control Program (NCP) protocols.

Before March 5, 1973, Telnet was an ad-hoc protocol with no official definition. 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 UCLA with the publication of two NIC documents: Telnet Protocol Specification, NIC #15372, and Telnet Option Specifications, NIC #15373.

The protocol has many extensions, some of which 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.


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 server made encrypted alternatives much more of a necessity.

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:

  • 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 that would ensure communication is carried out between the two desired host and not intercepted in the middle.
  • Commonly used Telnet daemon 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 Secure Shell (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 Transport Layer Security (TLS) security and Simple Authentication and Security Layer (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 SMTP, IRC, HTTP, FTP or POP3 servers, to issue commands to a server and examine the responses.

This approach has limitations, however, because Telnet clients do not use a raw character 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 popular among system administrators, as they can be called with arguments not to send any terminal control handshaking data.

Telnet is popular in various application areas:

Related RFCs

  • 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
  • RFC 885, Telnet end of record option
  • RFC 1041, Telnet 3270 regime 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 1184, Telnet linemode option
  • RFC 1205, 5250 Telnet interface
  • RFC 1372, Telnet remote flow control option
  • RFC 1572, Telnet Environment Option
  • RFC 2217, Telnet Com Port Control 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
  • RFC 4777, IBM's iSeries Telnet Enhancements

Telnet clients

See also


  1. RFC 318 - documentation of old ad-hoc telnet protocol
  2. RFC 495 - Announcement of Telnet protocol

External links

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