Read Your Firewall Logs!
Laura Taylor, of
ZDNet Business and Technology, reminds that installing a firewall and setting its rules is not enough. A conscientious systems administrator will check the firewall's logs daily, so that "you can determine whether new IP addresses are trying to probe your network, and whether you want to write new and stronger firewall rules to block them, or trace the probes and take some sort of management action."
Read Your Firewall Logs! (ZDNet IT Resource Centers, July 5, 2001)