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pf is OpenBSD's stateful packet filter, written by Daniel Hartmeier. It replaced Darren Reed's IPFilter in OpenBSD, due to problems with its license. pf has evolved quickly, and has now several advantages over others. The filtering syntax is similar to ipf, though it has been modified to make it more clean. Network Address Translation (NAT) and Quality of Service (QoS) have been tightly integrated into pf, to allow greater flexibility. QoS is achieved by merging ALTQ into pf itself.
pf has also been ported to NetBSD-current by itojun and there is a kernel module for FreeBSD.