Circuit-level Firewalls

Circuit-level firewallsare similar in operation to packet-filtering firewalls, but they operate at the transport and session layers of the OSI model. The biggest difference between a packet-filtering firewall and a circuit-level firewall is that a circuit-level firewall validates TCP and UDP sessions before opening a connection, or circuit, through the firewall. When the session is established, the firewall maintains a table of valid connections and lets data pass through when session information matches an entry in the table. The table entry is removed, and the circuit is closed when the session is terminated.

Application Gateway Firewalls

The application gateway firewall is the most functional of all the firewall types. As its name suggests, the application gateway firewall functionality is implemented through an application. Application gateway firewall systems can implement sophisticated rules and closely control traffic that passes through. Features of these firewalls can include user authentication systems and the capability to control which systems an outside user can access on the internal network. Some also provide bandwidth control mechanisms. Because application gateway firewalls operate above the Session layer of the OSI model, they can provide protection against any software-based network traffic that attempts to pass through them.

Firewalls are now a common sight in businesses and homes alike. As the Internet becomes an ever more hostile place, firewalls and the individuals who understand them are likely to become an essential part of the IT landscape.