Lecture 3: Applications #1: Intro and Telnet

Telnet as a "Debugging Weapon"

A telnet program can be used to connect to other services than the standard telnet (ie, login) server at port 23. Most telnet implementations allow the user to specify a port number on the command line, and will open a TCP connection to that port. This can be very useful in debugging communications protocols.

The reason this works is that virtually all "traditional" Internet application protocols are based on the telnet idea of exchanging lines of text^[3]. In fact, as we shall see, they usually use the telnet NVT specification.

In this subject, we will use telnet to demonstrate the operation of various Internet application protocols. For example, to investigate the Internet "email delivery" protocol SMTP (see next lecture) we could do:

telnet redgum 25

A final comment: telnet is a valuable tool to learn about network applications. In fact, it's so powerful that in some educational institutions, possession of a copy of telnet is regarded as prima facie evidence of intending to "hack into" computer systems... Be careful!

^[3] In some cases this is only true for "commands" and "responses" -- so-called "8-bit data" can subsequently be transferred.

The tutorial for this lecture is Tutorial #03.
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