One major point of differentiation between firewalls is their ability to perform application-layer inspection. Many firewalls simply don't have application-layer inspection, while others implement basic functionality (such as URL filtering). Some products, like Secure Computing Corp.'s Sidewinder G2 firewall and F5 Networks' BIG-IP Application Security Manager, have deep application inspection capabilities. These types of firewalls allow for complex application rule bases that limit the types of actions carried out over a connection. For example, you might limit inbound HTTP requests from the Internet to GET commands, while internal users might be able to issue POST commands. This functionality allows you to protect the enterprise against application-based attacks as well as network-based attacks.
Finally, consider the vendor itself. When investing in a firewall product, you're making a long-term decision. The financial commitment is only the tip of the iceberg; your firewall administrators will invest significant time and energy building and customizing a rule base for that particular product. In general, rule bases are not portable between platforms, so any future platform change will require a substantial commitment of human resources, so it's wise to make sure the vendors on your short list are all stable companies with solid financials. You certainly don't want to get on board a sinking ship.
This was first published in July 2007