PF (firewall)
PF (Packet Filter, also written pf) is a BSD licensed stateful packet filter, a central piece of software for firewalling. It is comparable to iptables, ipfw and ipfilter. PF is developed on OpenBSD, but has been ported to many other operating systems including FreeBSD, NetBSD, DragonFly BSD, Debian GNU/kFreeBSD, and Mac OS 10.7 ‘Lion’.
Read more about PF (firewall): History, Features, Ports, Annotated Example Pf.conf File