Flexible Packet Filtering: Providing a Rich Toolbox

The BSD/OS IPFW packet filtering system is a well engineered, flexible kernel framework for filtering (accepting, rejecting, logging, or modifying) IP packets. IPFW uses the well understood, widely available Berkeley Packet Filter (BPF) system as the basis of its packet matching abilities, and extends BPF in several straightforward areas. Since the first implementation of IPFW, the system has been enhanced several times to support additional functions, such as rate filtering, network address translation (NAT), and traffic flow monitoring. This paper examines the motivation behind IPFW and the design of the system. Comparisons with some contemporary packet filtering systems are provided. Potential future enhancements for the IPFW system are discussed.