An application proxy (sometimes called an application-level gateway) often acts as a middle layer. These proxy applications are run on top of the firewall. The users connect the gateway using applications like Telnet, FTP, or web connect. Users are prompted to enter resources they want to access and are also asked to validate their credentials. After successful authentication, the gateway will establish a connection to a remote application and relay data between the application and end user. Since application gateways are application aware, it becomes very easy from a gateway management point of view. In a typical firewall, either ports will be closed or open, and policies are defined by the administrator.
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