ICF is a stateful packet filter. The tracking of state allows ICF to make better decisions and support a more comprehensive ruleset than traditional packet filters. By default, the ICF ruleset is very secure and denies all traffic from the Internet, including ICMP echo requests (ping packets). This makes your computer virtually invisible to attackers attempting to probe your machine.

The ICF ruleset can be modified manually by configuring “services” or programmatically by using the ICF API. Out of the box, ICF contains standard services such as FTP and HTTP. In addition, ICF allows you add custom services for services that use ports that are not provided in the custom list. However, what is lacking from the specification of the ruleset is the ability to restrict access to a particular IP address. If you allow port 80 for HTTP services, you are allowing the whole world to connect to port 80. You can’t restrict it a specific IP address or a range of IP addresses. This is a huge shortcoming of ICF.

[ Read more ]