Firewall configuration Vs. SW platform equipment

[Enrico Venturi]

Hello colleagues,
I'm not an expert of firewalls, but I guess they act through the filtering of the IP packets, source/destination and port....
So, If my system hosts several applications, the firewall should be configured to let them to send and receive any needed messages ....
I guess again that all the applications communications occur through the port, or sockets, where the service is provided.
If I consider the machine as a black box, how can I determine the needs, maybe the used ports?, that the application have to configure the firewall?
May I simply run a UNIX command to list the used ports?

thanks a lot
Enrico

[Arunvijai_4]

Check /etc/services for known ports, /etc/inetd.conf for known services. You can use lsof to get the list all used ports.
#netstat -na will also help.

-Arun

[Enrico Venturi]

How can I exactly determine which are the used ports?
lsof needs options??