Unix and Linux Security:  An Introduction — Packet Filters, 
    Firewalls and ACLs
Back up to Unix and Linux Sec: An Intro
In this section we consider:
- host-based packet-filters and (stateful) firewalls, such as 
    IPTables (Linux) and IPFilter (Solaris);
 - router-based access-control lists (ACLs);
 
and we look at some particular examples:
  
- dealing with RPC-based services — which do not operate with fixed 
    ports;
 - firewalls which are default-deny outward and inward too —
    pinhole/pinprick firewalls;
 - and practical steps to building a firewall on a production system, i.e.,
    a system that is already in service.