Using the router to find TCP/UDP ports

• John Zobeck
• Published: 10 May 2001

To find out what TCP/UDP ports are being used by an application, simply make up a temporary access-list (ACL) and debug it on your router. For example, to find out what ports PCANYWHERE uses, make a temporary ACL and debug it.
Example:

access-list 199 permit ip host 123.123.123.123 any
and add another line that says "access-list 199 permit ip any host 123.123.123.123."
Then run the following command: "debug ip packet detail 199." Make sure you clear IP routes and cache and do a term mon before debugging and you will see the ports 5631 and 5632 for PCANYWHERE. Just make sure to do a u all to disable all debugging when finished.