IP Filter : Portable UNIX Packet Filter
IP Filter is a software package that can be used to provide network address translation (NAT) or firewall services. It can either be used as a loadable kernel module or incorporated into your UNIX kernel; use as a loadable kernel module where possible is highly recommended. Scripts are provided to install and patch system files, as required. IP Filter is distributed with FreeBSD, NetBSD, and Solaris. OpenBSD users should see Openbsd PF and Linux users Netfilter.