Telnet

Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual 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 , extended in , and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards. The name stands for " teletype network".

Historically, Telnet provided access to a command-line interface on a remote host. 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 SSH.

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 a command line client or with a graphical interface. For example, a common directive might be: "''To change your password, telnet into the server, log in and run the passwd command.''" In most cases, a user would be ''telnetting'' into a Unix-like server system or a network device (such as a router).

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

Even though Telnet was an ad hoc protocol with no official definition until March 5, 1973, the name actually referred to ''Teletype Over Network Protocol'' as the RFC 206 (NIC 7176) on Telnet makes the connection clear:
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.

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 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 feasible 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, password and whatever else is typed with a packet analyzer.
* 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 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 (or is replaced with one supporting SSH).

Telnet is still used by hobbyists, especially among amateur radio operators. The Winlink protocol supports packet radio via a Telnet connection.

Telnet 5250

IBM 5250 or 3270 workstation emulation is supported via custom telnet clients, TN5250/TN3270, and IBM i systems. 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 IBM i (also known as OS/400), 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 must be escaped by doubling it when sending data over the telnet protocol.)

Telnet client applications can establish an interactive TCP session to a port other than the Telnet server port. Connections to such ports do not use IAC and all octets are sent to the server without interpretation. For example, a command line telnet client could make an HTTP request to a web server on TCP port 80 as follows:

$ telnet www.example.com 80
GET /path/to/file.html HTTP/1.1
Host: www.example.com
Connection: close

There are other TCP terminal clients, such as netcat or socat on UNIX and PuTTY on Windows, which handle such requirements. Nevertheless, Telnet may still be used in debugging network services such as SMTP, IRC, HTTP, FTP or POP3, to issue commands to a server and examine the responses.

Another difference between Telnet and other TCP terminal clients is that Telnet is not 8-bit clean by default. 8-bit mode may be negotiated, but octets with the high bit set may be garbled until this mode is requested, as 7-bit is the default mode. The 8-bit mode (so named ''binary option'') is intended to transmit binary data, not ASCII characters. 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, but nothing is known about its actual software support.

Related RFCs

Internet Standards

* , Telnet Protocol Specification
* , Telnet Option Specifications
* , Telnet Binary Transmission
* , Telnet Echo Option
* , Telnet Suppress Go Ahead Option
* , Telnet Status Option
* , Telnet Timing Mark Option
* , Telnet Extended Options: List Option

Proposed Standards

* , Telnet End of Record Option
* , Telnet Window Size Option
* , Telnet Terminal Speed Option
* , Telnet Terminal-Type Option
* , Telnet X Display Location Option
* , Requirements for Internet Hosts - Application and Support
* , Telnet Linemode Option
* , Telnet Remote Flow Control Option
* , Telnet Environment Option
* , Telnet Authentication Option
* , Telnet Authentication: Kerberos Version 5
* , TELNET Authentication Using DSA
* , Telnet Authentication: SRP
* , Telnet Data Encryption Option
* , The telnet URI Scheme

Informational/experimental

* , The Q Method of Implementing TELNET Option Negotiation
* , Telnet Environment Option Interoperability Issues

Other RFCs

* , Telnet 3270 Regime Option
* , 5250 Telnet Interface
* , Telnet Com Port Control Option
* , IBM's iSeries Telnet Enhancements

Telnet clients

* PuTTY and plink command line are a free, open-source SSH, Telnet, rlogin, and raw TCP client for Windows, Linux, and Unix.
* AbsoluteTelnet is a telnet client for Windows. It also supports SSH and SFTP,
* RUMBA (Terminal Emulator)
* Line Mode Browser, a command line web browser
* NCSA Telnet
* TeraTerm
* SecureCRT from Van Dyke Software
* ZOC Terminal
* SyncTERM BBS terminal program supporting Telnet, SSHv2, RLogin, Serial, Windows, *nix, and Mac OS X platforms, X/Y/ZMODEM and various BBS terminal emulations
*Rtelnet is a SOCKS client version of Telnet, providing similar functionality of telnet to those hosts which are behind firewall and NAT.
* Inetutils includes a telnet client and server and is installed by default on many Linux distributions.
* telnet.exe command line utility included in default installation of many versions of Microsoft Windows.

In popular culture

Star Wars: Episode IV – A New Hope from 1977 has been recreated as a text art movie served through Telnet.

See also

* List of terminal emulators
* Banner grabbing
* Virtual terminal
* Reverse telnet
* HyTelnet
* Kermit
* SSH

References

External links

Telnet Options
— the official list of assigned option numbers at iana.org

Telnet Interactions Described as a Sequence Diagram

Telnet configuration



Microsoft TechNet:Telnet commands

TELNET: The Mother of All (Application) Protocols

Troubleshoot Telnet Errors in Windows Operating System
* Contains a list of telnet addresses and list of telnet clients

{{URI scheme

Application layer protocols
History of the Internet
Internet Protocol based network software
Internet protocols
Internet Standards
Remote administration software
Unix network-related software
URI schemes