Listening on port

Question

Hello,
I have to listen on a port and catch any transportation going in/out to/from it, from any software using it to connect to another computer. An example: If I want a spam filter for my arriving e-mail messages, I want to listen to port 110 (the default for POP3), scan every message arriving, and let only messages that don't meet the filter criteria pass on to the mail agent. How can I do it?
Thanks.

Answer 1

Are you trying to write an application that acts as a proxy, i.e. connects to a Mailserver, retrieves mail, and allows another application (e.g. Mail Client application) to connect on a different port and collect the mail from your new application? This is how a lot of e-mail Anti-virus software works....
....or, are you trying to write a "packet sniffer" type application that intercepts all the traffic on a specific port, without the application that already connects on that port being aware, or needing to be reconfigured? This would be more like the way something like a software firewall works.
If it's the former, it's relatively straightforward, if it's the latter, I would say it's almost impossible in Java. You would need access to the underlying operating system TCP/IP implementation, which you would need to handle through JNI to be able to use in Java, but this would certainly not be a trivial task.
Hope this helps you, if you need more help with the first possibility, post again....
D