THE ART OF COMPUTER VIRUS RESEARCH AND DEFENSE [Electronic resources]

14.3. Firewall Protection

There are three basic kinds of firewalls: stateful, nonstateful, and proxy⁴. Stateful firewall solutions, as the name implies, track the state of network traffic (such as connections) and compare it against a policy. Some stateful firewalls, such as Cisco PIX, also can inspect some application-level protocols to see whether only regular commands are used on some known protocols, such as SMTP. If your SMTP server receives non-SMTP commands, the firewall will pretend to the sender that the bogus commands were accepted.