Advanced Policy Firewall

Advanced Policy Firewall (APF) is an iptables(netfilter) based firewall system designed around the essential needs of today’s Linux servers. The configuration is designed to be very informative and easy to follow. The management on a day-to-day basis is conducted from the command line with the ‘apf’ command, which includes detailed usage information on all the features. The technical side of APF is such that it utilizes the latest stable features from the iptables (netfilter) project to provide a very robust and powerful firewall. The filtering performed by APF is three fold:

1. Static rule based policies (not to be confused with a “static firewall”)
2. Connection based stateful policies
3. Sanity based policies The first, static rule based policies, is the most traditional method of firewalling. This is when the firewall has an unchanging set of instructions (rules) on how traffic should be handled in certain conditions. An example of a static rule based policy would be when you allow/deny an address access to the server with the trust system or open a new port with conf.apf. So the short of it is rules that infrequently or never change while the firewall is running. The second, connection based stateful policies, is a means to distinguish legitimate packets for different types of connections. Only packets matching a known connection will be allowed by the firewall; others will be rejected. An example of this would be FTP data transfers, in an older era of firewalling you would have to define a complex set of static policies to allow FTA data transfers to flow without a problem. That is not so with stateful policies, the firewall can see that an address has established a connection to port 21 then “relate” that address to the data transfer portion of the connection and dynamically alter the firewall to allow the traffic. ... and much much more. See site for further details.