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Firewall builder alternative12/25/2023 ![]() ![]() Second, more seriously, most tools do not understand the firewall’s match conditions. First, calling and returning from custom chains, due to the possibility of complex nested chain calls. We identified two concrete issues which prevent tools from “understanding” real-world firewalls. Removing the first two rules (in particular the call in the DOS_PROTECT chain) lets ITVal compute the expected result. However, ITVal erroneously only reports one IP range: the universe. The expected result is a set of two IP ranges: the local network 192.168.0.0/16 and the “rest”. 1 into equivalence classes (i.e., ranges with the same access rights). We used ITVal to partition the IP space of Fig. Firewall models used in related work are surveyed in Sect. However, ITVal’s firewall model is representative of the model used by the majority of tools therefore, the problems described here also apply to a large class of other tools. To illustrate the problem, we decided to use ITVal because it natively supports iptables, is open source, and supports calls to user-defined chains. The output of the (unverified) verification tools itself cannot be trusted. Finally, the firewall allows all packets from the local network 192.168.0.0/16 and discards all other packets. There, some services, identified by their ports, are blocked (and any packets with those destination ports will never create an established connection). In the example, the subsequent rules are the interesting ones which shape the firewall’s connectivity policy. Once a packet is accepted, further packets for this connection are treated as ESTABLISHED. The interesting aspect is when a firewall accepts a packet which does not yet belong to an established connection. However, it is barely interesting for the actual policy (“ who may connect to whom”) enforced by the firewall. Often, the ESTABLISHED rule accepts most packets and is placed at the beginning of a ruleset for performance reasons. ![]() This is generally considered good practice. In this second rule, the firewall allows all packets which belong to already ESTABLISHED (or RELATED) connections. 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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