A packet which does not exceed certain limits can make it through this chain without getting DROPped by RETURNing back to the second rule of the INPUT chain. In the first rule, all incoming packets are sent directly to the user-defined DOS_PROTECT chain, where some rate limiting is applied. The ruleset reads as follows: processing starts at the INPUT chain. 1, taken from an NAS (network-attached storage) device. Further possible actions include jumping to other chains and continue processing from there.Īs an example, we use the firewall rules in Fig. Ultimately, the Linux kernel needs to determine whether to ACCEPT or DROP the packet, hence, those are the common actions. In general, an iptables ruleset is processed by the Linux kernel for each packet comparably to a batch program: rules are evaluated sequentially, but the action (sometimes called target) is only applied if the packet matches the criteria of the rule. The predominant firewall of Linux is iptables. Consequently, studies regularly report insufficient quality of firewall rulesets. However, the prevalent methodology for configuring firewalls has not changed. While vulnerabilities in firewall software itself are comparatively rare, it has been known for over a decade that many firewalls enforce poorly written rulesets. Operating and managing firewalls is challenging as rulesets are usually written manually. Several firewall solutions, ranging from open source to commercial, exist. An evaluation of our work on a large set of real-world firewall rulesets shows that our framework provides interesting results in many situations, and can both help and out-compete other static analysis frameworks found in related work.įirewalls are a fundamental security mechanism for computer networks. Based on that, we implement the verified generation of IP space partitions and minimal service matrices. We provide and verify procedures that translate from the complex iptables language into this simple model. For analysis purposes, we describe a simplified model of firewalls that only supports a single list of rules with limited expressiveness. Around that, we organize a set of simplification procedures and their correctness proofs: we include procedures that can unfold calls to user-defined chains, simplify match expressions, and construct approximations removing unknown or unwanted match expressions. This semantics is tailored to the specifics of the filter table and supports arbitrary match expressions, even new ones that may be added in the future. We build our work around a formal semantics of the behavior of iptables firewalls. This article summarizes our efforts around the formally verified static analysis of iptables rulesets using Isabelle/HOL.
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