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From Wikipedia, the free encyclopedia

Telnet (Telecommunication network) is a network protocol used on the Internet or local area networks. It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet standards. Typically, Telnet provides access to a command-line interface on a remote machine.

The term telnet also refers to software which implements the client part of the protocol. Telnet clients 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 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).

The Internet Protocol Suite
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Contents 1 Protocol details 2 Security 3 Telnet 5250 4 Current status 4.1 Related RFCs 4.2 Telnet clients 5 See also 6 References 7 External links

[edit] 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 5th, 1973, Telnet was an ad-hoc protocol with no official definition ^[1]. 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 5th, 1973, a Telnet protocol standard was defined at UCLA^[2] 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.

[edit] 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 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 that would ensure 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 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.

[edit] 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.

[edit] 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:

• 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.
• Internet game clubs, like the Internet Chess Club, the Free Internet Chess Server and the Internet Go server.
• embedded systems
• Oxford University students and faculty members; it is still used as an interface for library catalogue searches, and many prefer it over more modern interfaces.

[edit] Related RFCs

[edit] Telnet clients

• PuTTY is a free, open source SSH, Telnet, rlogin, and raw TCP client for Windows, Linux, and Unix.
• NCSA Telnet

[edit] See also

• Virtual terminal
• Reverse telnet
• Telnet 3270 (TN3270)
• HyTelnet
• Kermit
• NCSA Telnet
• SSH

[edit] References

[edit] External links

• Telnet Options - The official list of assigned option numbers at iana.org
• Telnet Interactions Described as a Sequence Diagram
• Telnet protocol description, with NVT reference