Java Network Programming, Second Edition

Investigating Protocols with Telnet

In this chapter, you’ll see clients that use sockets to communicate with a number of well-known Internet services such as HTTP, echo, and more. The sockets themselves are simple enough; however, the protocols to communicate with different servers make life complex.

To get a feel for how a protocol operates, you can use Telnet to connect to a server, type different commands to it, and watch its responses. By default, Telnet attempts to connect to port 23. To connect to servers on different ports, specify the port you want to connect to like this:^[16]

% telnet localhost 25

This example requests a connection to port 25, the SMTP port, on the local machine; SMTP is the protocol used to transfer email between servers or between a mail client and a server. If you know the commands to interact with an SMTP server, you can send email without going through a mail program. This trick can be used to forge email. For example, a few years ago, the summer students at the National Solar Observatory in Sunspot, New Mexico, made it appear that the party that one of the scientists was throwing after the annual volleyball match between the staff and the students was in fact a victory party for the students. (Of course, the author of this book had absolutely nothing to do with such despicable behavior. ;-) ) The interaction with the SMTP server went something like this; input that you type is shown in bold (the names have been changed to protect the gullible):

flare% ...