An application-level proxy server provides all the basic proxy features and also provides extensive packet analysis. When packets from the outside arrive at the gateway, they are examined and evaluated to determine if the security policy allows the packet to enter into the internal network. Not only does the server evaluate IP addresses, it also looks at the data in the packets to stop hackers from hiding information in the packets.
A typical application-level gateway can provide proxy services for applications and protocols like Telnet, FTP (file transfers), HTTP (Web services), and SMTP (e-mail). Note that a separate proxy must be installed for each application-level service (some vendors achieve security by simply not providing proxies for some services, so be careful in your evaluation). With proxies, security policies can be much more powerful and flexible because all of the information in packets can be used by administrators to write the rules that determine how packets are handled by the gateway. It is easy to audit just about everything that happens on the gateway. You can also strip computer names to hide internal systems, and you can evaluate the contents of packets for appropriateness and security.
NOTE: Appropriateness is an interesting option. You might set up a filter that discards any e-mail messages that contain "dirty" words.
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