89. Telnet

Telnet, which stands for Telecommunications Network, is a protocol that provides a way for users (or clients) to connect to multi-user computers (or servers) on the Internet, whether in the next building or across the other side of the world. In most cases, users use Telnet to communicate with a remote login service. Telnet (Telecommunication network) is a network protocol used on the Internet or local area network (LAN) connections. 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 OSes with a TCP/IP stack support some kind of Telnet service server for their remote configuration (including ones based on Windows NT). Because of security issues with Telnet, its use has waned in favor of SSH for remote access.

"To telnet" is also used as a verb, meaning to establish an interactive connection with the Telnet protocol. 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. The Telnet program runs on your computer and connects your PC to a server on the network. You can then enter commands through the Telnet program and they will be executed as if you were entering them directly on the server console. This enables you to control the server and communicate with other servers on the network. To start a Telnet session, you must log in to a server by entering a valid username and password. Telnet is a common way to remotely control Web servers.

The term telnet also refers to software which implements the client part of the protocol. TELNET clients have been available on most UNIX systems for many years and are available for virtually all platforms. Most network equipment and OSs with a TCP/IP stack support some kind of TELNET service server for their remote configuration (including ones based on Windows NT). Recently, Secure Shell has begun to dominate remote access for Unix-based machines. 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.

On the Internet, the ability to connect with another machine is made possible by the Transmission Control Protocol (TCP), which enables two machines to transmit data back and forth in a manner coherent to the operating systems of each device, and the Internet Protocol (IP), which provides a unique, 32-bit address for each machine connected to the network. The telecommunications application built over these capabilities provides the local terminal with the means to emulate a terminal compatible with the remote computer. The telnet protocol gives you the ability to connect to a machine, by giving commands and instructions interactively to that machine, thus creating an interactive connection. In such a case, the local system becomes transparent to the user, who gets the feeling that he is connected directly to the remote computer. The commands typed by the user are transmitted directly to the remote machine and the response from the remote machine is displayed on the user’s monitor screen. An interactive connection is also known as remote login.