For those who enjoy, are used to, or are nostalgic for the OS that powered Cisco's former line of PIX firewalls, we offer OpenPIX: an open souce PIX commad-line interface/shell that aims to offer as similar as possible a look, feel, command structure and usage as Cisco's PIX OS.
The idea behind OpenPIX is to provide 3 layers of functionality:
1) A command-line interface/shell,
2) A pluggable systems abstraction layer, and
3) Pluggable backends with support for open source firewall/packet filter solutions such as pf+pfctl, netfilter+iptables, ipf, ipfw, etc.