Packet Filter: The Engine of pfSense

Packet FilterpfSense is, as most users know, a specialized version of FreeBSD. It can be configured an upgraded through a web-based interface and often runs on embedded systems; it requires no knowledge of the underlying FreeBSD system. Those curious enough to find out, however, will be interested to know that pfSense is based on OpenBSD’s powerful pf (Packet Filter) software, which was released in late 2001 and has since been ported to many other operating systems including FreeBSD, NetBSD, DragonFly BSD, Debian GNU/Free BSD, and Mac OS X 10.7 “Lion” and later.

OpenBSD and the other BSD variants are direct descendants of BSD Unix. BSD, sometimes called Berkeley Unix, is a Unix operating system derivative developed and distributed by the Computer Systems Research Group (CSRG) of the University of California, Berkeley, from 1977 to 1995. The final release from Berkely was 1995’s 4.4BSD-Lite Release 2, after which the CSRG was dissolved and development of BSD at Berkeley ceased. In the meantime, as CSRG was winding down, small groups of enthusiasts around the world began working on further development of the code. Several different variants of BSD Unix came into existence. The OpenBSD group became known as the most security-oriented of the BSDs. For its packet filtering needs, it used a program called IPFilter, written by Darren Reed.


The packet filter’s main function is to filter network packets by matching the properties of individual packets and the network connections build from those packets agains the filtering criteria defined in its configuration files. The packet filter is responsible for deciding what to do with those packets, which could mean passing them through or rejecting them, or triggering events that other parts of the operating system or external applications are set up to handle.

The IPFilter subsystem performed this function within OpenBSD, but in May 2001, IPFilter was removed from the OpenBSD source tree due to licensing issues. The OpenBSD version of IPFilter contained several changes and customizations that, as it turned out, were not allowed under the licensing agreement. For a few weeks, the development version of OpenBSD did not include any firewalling software.


Enter Packet Filter

However, Daniel Hartmeier had already begun work on his own packet filtering software, called PF, and after a few months of development, it was ready to be added to OpenBSD. PF was added as a default part of the OpenBSD 3.0 base system in December 2001. Migrating from IPFilter to PF sense did not pose major problems, as their configuration languages were similar. Moreover, PF has done well in performance tests, performing equally well or better under stress on OpenBSD 3.1 than either IPFilter on OpenBSD 3.1 (or iptables on Linux). PF’s overhead is relatively low, and is capable of running effectively on rather modest hardware.

Future installments of this series of articles will go into greater depth on how to configure and run PF. For now, it should be mentioned that in order to run PF, you need to install and run a BSD system such as OpenBSD, FreeBSD, NetBSD, or DragonFly BSD. OpenBSD is the BSD variant of choice for many, as this is where PF was first introduced and where essentially all PF development happens. Moreover, the newest and most-up-to-date PF code is always to be found on OpenBSD. If you are planning to run PF on FreeBSD, NetBSD, DragonFly BSD, or another system, you should check your system’s release notes and other documentation about which version of PF is included.

External Links:

PF documentation in the OpenBSD FAQ

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