Firewall modules and modular firewalls

A firewall is a packet filter placed at an entry point of a network in the Internet. Each packet that goes through this entry point is checked by the firewall to determine whether to accept or discard the packet. The firewall makes this determination based on a specified sequence of overlapping rules. The firewall uses the first-match criterion to determine which rule in the sequence should be applied to which packet. Thus, to compute the set of packets to which a rule is applied, the firewall designer needs to consider all the rules that precede this rule in the sequence. This “rule dependency” complicates the task of designing firewalls (especially those with thousands of rules), and makes firewalls hard to understand. In this paper, we present a metric, called the dependency metric, for measuring the complexity of firewalls. This metric, though accurate, does not seem to suggest ways to design firewalls whose dependency metrics are small. Thus, we present another metric, called the inversion metric, and develop methods for designing firewalls with small inversion metrics. We show that the dependency metric and the inversion metric are correlated for some classes of firewalls. So by aiming to design firewalls with small inversion metrics, the designer may end up with firewalls whose dependency metrics are small as well. We present a method for designing modular firewalls whose inversion metrics are very small. Each modular firewall consists of several components, called firewall modules. The inversion metric of each firewall module is very small - in fact, 1 or 2. Thus, we conclude that modular firewalls are easy to design and easy to understand.