Proxy Applications

The application level gateway provides another service beyond that of re-addressing. It has a separate proxy application for each Internet service it supports. For example, in order for authorized e-mail packets to get through the gateway, there must be an e-mail proxy running on the firewall.

The same is true for every Internet application (service). A World Wide Web proxy is required for WWW packets to be processed and etc.An Internet application for which there is no proxy or for which the proxy has been disabled, cannot go through the firewall.

The proxy applications are responsible for examining in detail the contents of every packet. In particular, they scan the session and application data inside each packet. Each proxy has intimate knowledge of the Internet application which it serves.