Application-level firewall - an explanation

Firewall system providing service by processes that maintain complete TCP connection state and sequencing. Often re-addresses traffic so outgoing traffic appears to have originated from firewall, rather than internal host.An Application Level Firewall is a firewall where one application-level (i.e., not kernel) process is used to forward each session that an internal user makes to a network resource on the public network. Firewall is a system which limits network access between two or more networks. Normally, a Firewall is deployed between a trusted, protected private network and an untrusted public network. For example, the trusted network might be a corporate network, and the public network might be the Internet. A Firewall might grant or revoke access based on user Authentication, source and destination network addresses, network protocol, time of day, network service or any combination of these. It might be implemented as an Application Level Firewall or a Packet Level Firewall. It might also implement a Content Filter, such as a Virus Wall.

Application Level Firewall's are considered to be the most secure type of Firewall's, but they incur a significant performance penalty. The penalty arises because a new process must be started each time a user starts a new session -- for instance by following a URL to a new World-Wide Web site.

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