IPFilter is an open source packet filter software package written by Darren Reed. It was originally written for SunOS and Solaris, but now runs on a number of Unix platforms. It is now the standard packet filter package for Solaris 10, and one of the three shipped with FreeBSD and NetBSD. It shipped with OpenBSD until 2001, when, after failed license negotiations between Theo De Raadt and its author, it was replaced by pf.
IPFilter is a stateful packet filter that also ships with a small suite of proxies to handle difficult protocols like FTP, H.323 and TFTP. It supports state tracking for TCP and OSPF and state approximation for all other IP protocols. It supports both IPv4 and IPv6.
IPFilter is supported under many Unices, including AIX, IRIX, HP-UX and Tru64 Unix. Though it works under Linux, Linux includes a packet filter called Netfilter which is equally featureful and better integrated.