Assisted Firewall Policy Repair Using Examples and History

Firewall policies can be extremely complex and difficult to maintain, especially on networks with more than a few hundred machines. The difficulty of configuring a firewall properly often leads to serious errors in the firewall configuration or discourage system administrators from implementing restrictive policies. In previous research, we developed a technique for modeling firewall policies using Multiway Decision Diagrams and performing logical queries against a decision diagram model. Using the query logic, the system administrator can detect errors in the policy and gain a deeper understanding of the behavior of the firewall. The technique is extremely efficient and can process policies with thousands of rules in just a few seconds. While queries are a significant improvement over manual inspection of the policy for detecting that errors exist, they provide only limited assistance in repairing a broken policy. In this paper we present two extensions to our work, examples and history, which enable the administrator to more easily repair a policy which contains errors. An example is a representative packet which illustrates that the firewall complies with or (more importantly) deviates from its expected behavior. History records the specific rules involved in the deviation. Examples and history provide guidance in finding and fixing faults in a firewall rule set. These contributions can be also be used with the equivalence class analysis to reduce the burden of designing a complicated set of assertions.