Firewall design: consistency, completeness, and compactness

Abstract

A firewall is often placed at the entrance of each private network in the Internet. The function of a firewall is to examine each packet that passes through the entrance and decide whether to accept the packet and allow it to proceed or to discard the packet. A firewall is usually designed as a sequence of rules. To make a decision concerning some packets, the firewall rules are compared, one by one, with the packet until one rule is found to be satisfied by the packet: this rule determines the fate of the packet. We present the first ever method for designing the sequence of rules in a firewall to be consistent, complete, and compact. Consistency means that the rules are ordered correctly, completeness means that every packet satisfies at least one rule in the firewall, and compactness means that the firewall has no redundant rules. Our method starts by designing a firewall decision diagram (FDD, for short) whose consistency and completeness can be checked systematically (by an algorithm). We then apply a sequence of five algorithms to this FDD to generate, reduce and simplify the target firewall rules while maintaining the consistency and completeness of the original FDD.