PF is OpenBSD's stateful packet filter firewall. PF is developed on OpenBSD, but has been ported to many other operating systems. It is the only firewall that supports both IPv4 and IPv6 traffic filtering:
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A recent linux.com story highlighted the wonderfulness of OpenBSD's spamd with an empasis on greylisting. This tutorial, field tested at user groups and conferences, gives a slightly broader view of the things you can do with PF on OpenBSD and the other BSDs. Last seen at AsiaBSDCon 2007, but on the menu for BSDCan 2007 and EuroBSDCon 2007 too....
...This lecture will be about firewalls and related functions, starting from a little theory along with a number of examples of filtering and other network traffic directing. As in any number of other endeavors, the things I discuss can be done in more than one way. Under any circumstances I will urge you to interrupt me when you need to. That is, if you will permit me to use what I learn from your comments later, either in revised versions of this lecture or in practice at a later time.