question about the personal-firewall

I have a connection to the Internet via fiber optics and eth0. I have configured the firewall, but, as when the computer is switched on there is no connection, etho is not active and the Firewall is initialized, but not activated. What do I have to do to activate the Firewall after the login procedure, i.e. when I connect to the internet and issue the rcnetwork restart command to restart eth0 ? Thanx, Riccardo,Italy.