What is Telnet?
Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers. Through Telnet, an administrator or another user can access someone else's computer remotely. On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a user of that computer. With Telnet, you log on as a regular user with whatever privileges you may have been granted to the specific application and data on that computer.
A Telnet command request looks like this (the computer name is made-up):
telnet the.libraryat.whatis.edu
The result of this request would be an invitation to log on with a userid and a prompt for a password. If accepted, you would be logged on like any user who used this computer every day.
Telnet is most likely to be used by program developers and anyone who has a need to use specific applications or data located at a particular host computer.
Telnet is a network protocol used on the Internet or local area networks 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 RFC 15, extended in RFC 854, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards.
Historically, Telnet provided access to a command-line interface (usually, of an operating system) on a remote host. Most network equipment and operating systems 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 for this purpose has waned in favor of SSH.
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. Telnet is also used as averb. 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, log in 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.
Telnet and Other Technologies
Telnet is closely related to two other technologies that provide an alternative for remote command console sessions, depending on your needs for distributed computing, security, configuration requirements, and so on.
Windows Server 2003 Terminal Services
Telnet and Terminal Services are similar in that they are both used for remote sessions. However, Terminal Services extends the model of distributed computing by allowing client computers to operate in a server-based computing environment. Whereas Telnet only allows terminal emulation between a Telnet server and client, Terminal Services running on Windows Server 2003-based computers allows clients to run applications, while data processing, and data storage occur on the server. Applications and user sessions are transmitted over the network and displayed via terminal emulation software. Similarly, print streams, keyboard input, and mouse clicks are also transmitted over the network between the server and the terminal emulation software. Each user logs on and sees only their individual session, which is managed transparently by the server operating system and is independent of any other client session. You might want to consider using Windows Server 2003 Terminal Services if you require more extensive distributed computing.
Windows Services for UNIX 3.5
Windows Services for UNIX 3.5 includes a Telnet character-mode client that provides functionality not included in Windows Server 2003. The Windows Services for UNIX character-mode client supports both stream mode and console mode. It also provides for logging and additional configuration settings.
Windows Services for UNIX 3.5 includes two Telnet servers:
- The default, Windows-based Telnet server, which is functionally similar to the one included with all versions of Microsoft Windows since Windows 2000
- The Interix telnetd
The Windows Services for UNIX 3.5 Telnet server accepts logons from a variety of clients, including the Telnet clients shipped with Windows 2000, Windows NT, Windows 95, and Windows 98, as well as a variety of character-mode terminal clients from virtually any operating system. Additionally, it can be configured to meet specific site requirements to improve security, simplify logons, support stream or console mode, and so forth.
The Windows Services for UNIX 3.5 Telnet server should be familiar to users of Windows 2000, Windows XP, and Windows Server 2003. It is essentially the same as the server included in Windows XP Professional and Windows Server 2003, and it is very similar to the one included in Windows 2000 editions. It uses the Windows command shell (Cmd.exe) as the default shell. You can start and stop this server from either the Services MMC (Services.msc) or from the Windows Services for UNIX Administration MMC (Sfumgmt.msc). If you are using Windows and UNIX in a mixed networking environment, you might want to consider installing Windows Services for UNIX 3.5 to extend the capabilities of Telnet.
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