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OSI Model : Telnet Services and Protocol- VI

February 06, 2009 , ,

OSI Model : Telnet Services and Protocol- VI 



People utilized text-based systems, which were frequently merely display terminals physically connected to a central computer, long before desktop computers with sophisticated graphical interfaces emerged. When networks became accessible, individuals need a method to remotely access computer systems in the same way that they did with physically connected terminals.

Telnet was created to address this requirement. Telnet is one of the earliest Application layer protocols and services in the TCP/IP family, dating back to the early 1970s. Telnet is a protocol for mimicking text-based terminal devices via a data network. Telnet refers to both the protocol itself and the client software that implements the protocol.

A Telnet connection is appropriately referred to as a Virtual Terminal (VTY) session or connection. Rather than connecting to the server with a physical device, Telnet uses software to establish a virtual device that gives the same functions as a terminal session as well as access to the server command line interface (CLI).

The server runs a service called the Telnet daemon to facilitate Telnet client connections. A Telnet client application is used to establish a virtual terminal connection from an end device. An Application layer Telnet client is included in the majority of operating systems. Telnet may be launched from the command prompt on a Microsoft Windows PC. HyperTerminal and Minicom are some typical terminal apps that function as Telnet clients.

Once a Telnet connection is established, users can perform any authorized function on the server, just as if they were using a command line session on the server itself. If authorized, they can start and stop processes, configure the device, and even shut down the system.

Telnet is a client/server protocol and it specifies how a VTY session is established and terminated. It also provides the syntax and order of the commands used to initiate the Telnet session, as well as control commands that can be issued during a session. Each Telnet command consists of at least two bytes. The first byte is a special character called the Interpret as Command (IAC) character. As its name implies, the IAC defines the next byte as a command rather than text. 

Some sample Telnet protocol commands include:

  • Are You There (AYT) - Lets the user request that something appear on the terminal screen to indicate that the VTY session is active.
  • Erase Line (EL) - Deletes all text from the current line. 
  • Interrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program. 

While the Telnet protocol supports user authentication, it does not support the transport of encrypted data. All data exchanged during a Telnet sessions is transported as plain text across the network. This means that the data can be intercepted and easily understood.

If security is a concern, the Secure Shell (SSH) protocol offers an alternate and secure method for server access. SSH provides the structure for secure remote login and other secure network services. It also provides stronger authentication than Telnet and supports the transport of session data using encryption. As a best practice, network professionals should always use SSH in place of Telnet, whenever possible.

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