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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[2]. 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 ironbark 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!
[2] 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 #04.
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Copyright © 2001 by
Philip Scott, La Trobe
University.
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