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Print Formattednetworking > protocols > telnet

Telnet

Operating systems have command functions that can control its operation. These functions can be programmed in a general way that allows the abstraction of the user interface to a series of bytes sent into the operating system and as a result received from the operating system. This abstraction means connections can be made with local keyboards and video displays can be made just as easily with a TCP connection from a remote host's keyboard/display combination.

With multi-tasking and multiple user logins it is possible to have many users with multiple logins via Telnet to a single shared host or server. The operating systems based on the UNIX model, such as Linux, are capable of reliably providing such connections. The power of the UNIX style command line makes this sharing very accessible and flexible. These commands are easily invoked using a Telnet connection and thus completely controlling the Telnet server.

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The complimentary host/server arrangement is also possible where a single user can connect to many other hosts or servers. For example using a Graphic User Telnet Interface it will allow multiple windows each with its own Telnet session on the GUI to be connected to remote servers.

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Using a command prompt in Windows, type C:\telnet and ?/help to see the available commands.

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Notice that open hostname can have a port number appended to the connection (the default port number is usually 23), and means that a mail or news server can be interrogated by using the Telnet connection. It is possible, though somewhat awkward compared to the speed a properly designed mail client can read email from a server, to "manually" interact with the server to download the email. Obviously the email protocol must be followed to the letter to keep the connection alive and responding. It is certainly a grass roots way of investigating some protocols.

Practical Example:

An application called PuTTY (Download from www.tucows.com) is a much more flexible and sophisticated application for Telnet connections than the simple DOS prompt style window. To use this to investigate the NNTP protocol, change the Protocol radio button to Raw and open a news server, e.g. news.BizzEB.com.au and use Port 119. When logged in, type in LIST and press enter. You should see a long list of all the news groups the server keeps up to date. You must be logged onto the Internet and have permissions to access the news server to be able to do this.

For example, if BizzEB is the ISP in the above example that you are logged into, then it will most likely provide a news server originating with the BizzEB.com.au domain name you can access ie news.BizzEB.com.au. The list of commands to interact at this level and the resulting behaviour of the server is what makes up a protocol. It is sort of like a machine based dialogue exchanging information about the user’s data and also information about the machines themselves.

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This is a freeware program that can be downloaded from various sources and provides good Telnet service on a Microsoft Windows based computer.

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