Simplest possible setup (NetBSD)

Introduced to NetBSD 2.0 as the package security/pflkm.

From NetBSD 3.0 on, PF is in the base system

Your kernel configuration file needs these non-default options:

pseudo-device  pf		# PF packet filter
pseudo-device  pflog		# PF log interface

Add to /etc/rc.conf:

pf=YES 
pflogd=YES
NetBSD$ sudo modload /usr/lkm/pf.o; 

or

NetBSD$ sudo pfctl -e

NetBSD$ sudo /etc/rc.d/pf start ; sudo /etc/rc.d/pflogd start

in /etc/lkm.conf:

/usr/lkm/pf.o - - - - BEFORENET

Also see NetBSD PF pages.