Network Terminal Protocol (Telnet)

The Network Terminal Protocol (Telnet) is a bi-directional character oriented communications application primarily designed to provide a Virtual Terminal process across a network. The original application was intended to allow two computers on a network to communicate with one another so that a user on a terminal connected to the first computer, the client, could communicate with the second computer, the server, as if the users terminal was directly connected to the second computer. It was implemented so that users could communicate with computers (client-server), users could communicate with other users (client-client), and computers could communicate directly with other computers (server-server), all using the same basic protocol and conventions. The Telnet protocol is documented in RFC 854. It is a TCP/IP applications layer protocol that uses Transmission Control Protocol (TCP) to establish a bi-directional ASCII text conversation. TCP uses a pair of ports numbers, one for source and one for destination, to identify each communications link. By default a Telnet client application will contact the remote server using TCP/IP application port number 23 as the destination port, and will select at random a port from the dynamic or private range for the source port number. The remote server will normally respond by contacting the client system using the private port number as the destination and port number 23 as the source. Most server systems on the Internet that allow incoming Telnet connections on port 23 run a program on that port that requests a user name and ID, and authenticates this against a list of valid users. On most UNIX systems a valid user is logged onto the system as a standard terminal session. Telnet servers are more rare on Microsoft Windows systems because of the GUI based orientation of the system. Most Windows systems that support Telnet connections across the Internet provide a validated user with a limited DOS command prompt style session not unlike the UNIX terminal session. The Telnet protocol has been extended and expanded to allow many additional features. It can negotiate terminal and communication line features such as flow control, screen size, and terminal type. By identifying the terminal type to be used in a conversation, such as DEC VT-100 or ANSI, the server can send special codes to manipulate the users screen for applications such as full screen editors and interactive data entry screens. Telnet has as option that gives it a very useful ability. On most Telnet client applications, the user can select the Port number on the destination to be used for the communication link. This allows a user to contact network application programs other than the Telnet server. This can be used as a valuable troubleshooting aid, and as a substitute for a client application program in some cases. For example, if a user does not have an E-Mail program, or suspects that there are problems with the one being used, Telnet can be used instead to send E-Mail messages. The user Telnets to the recipients mail host on port 25. Then, using text commands from the Telnet client, the user can identify themself as a sender, identify the recipient, enter a simple text message, and have it delivered. If there is a problem with the receiving system, the user should see a text error message at the appropriate point in this conversation, helping them to identify the problem. By allowing a user to select the port to contact, and by the virtue that most common applications layer protocols on the Internet are text oriented, Telnet becomes a very powerful tool for testing and troubleshooting a TCP/IP network. In addition to contacting remote servers for terminal sessions, and troubleshooting applications layer services, Telnet has other uses. It is used to contact network equipment for configuration and troubleshooting. For example, most routing equipment supports Telnet for remote management. This allows an administrator or technician to contact almost any router, anywhere on a network, to verify its operation and make changes to the configuration. Normally it is only the least expensive or limited equipment that does not have this ability.