Hmm.. i thought ipfilter would be like iptables for linux: you setup the kernel to accept it, and use an userspace program to add/delete rules dinamically. But I discovered it isn't how it works.. I found this text here,
http://www.defcon1.org/~ghostrdr/Fre..._IPFILTER.html
Which you might like... Oh, something else: http://www.freebsddiary.org/ipfilter.php