Network Firewalls   «Prev 

Circuit-level Gateways

There is also a fourth type of firewall. A dynamic packet filter is a combination of a packet filter and a circuit-level gateway, and it often has application layer semantics as well.
1) The transmission process begins when the internal system sends out a series of packets destined for the internet.

2) These packets, then go the circuit-level gateway, which checks them against its predetermined set of rules. If the packets do not violate any rules, the circuit-level gateway sends out the same packets on behalf of the internal system.

3) The packets that appear on the internet originate from the circuit-level gateway's external port's IP address, which is also the address that resolves any replies.