They recognize the contents of each packet and can provide fine-grained control and logging of each application. For example, each request to download a file from an FTP server or a Web page can be logged, and requests to send files with FTP can be blocked. The main disadvantage of application gateways is the lack of flexibility. There must be an application gateway for every Internet service your require. These include ANS, Blackhole, Checkpoint, Digital, IBM and Sunscreen.
Some people consider the lack of flexibility in a positive light.
You can't accidentally enable dangerous services if no application gateway exists to support those services. Also, the failure mode of the application gateway is desirable. If the application gateway shuts down, no traffic passes through. Amateurs Beware Firewall products vary in the amount of installation assistance included with them. Training also is optional with most products, although those requiring on-site installation always include some training in the package.