A circuit-level gateway is a type of firewall.
Circuit level gateways work at the session layer of the OSI model, or as a "shim-layer" between the application layer and the transport layer of the TCP/IP stack. They monitor TCP handshaking between packets to determine whether a requested session is legitimate. Information passed to a remote computer through a circuit level gateway appears to have originated from the gateway. Circuit-level firewall applications represent the technology of next to first generation. Firewall technology supervises TCP handshaking among packets to confirm a session is genuine. Firewall traffic is clean based on particular session rules and may be controlled to acknowledged computers only. Circuit-level firewalls conceal the network itself from the external, which is helpful for interdicting access to impostors. But Circuit-level firewalls do not clean entity packets. This is useful for hiding information about protected networks. Circuit level gateways are relatively inexpensive and have the advantage of hiding information about the private network they protect. On the other hand, they do not filter individual packets.
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