Circuit-level gateway

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. 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.

Circuit level gateways work at the TCP level.
TCP connections are relayed through a computer that runs a program to copy bytes between 2 connections while logging or caching the contents.
IP packets do not flow from end to end as the relay host works above that level. All the IP problems like fragments etc are terminated at the relay host which is better equipped to handle them
Circuit level gateways can bridge 2 networks that do not share any IP connectivity or DNS processing and circuit relays are generally used to create specific connections between isolated networks.
Circuit-Gsteway is a variation of the Application level gateway which does some extra functions as compared to AWG.

CGW creates a new connection between itself and the remote host. The user is not aware of this and thinks that there is a direct connection.

Also Circuit-Gsteway changes source IP address in the packets from end user’s IP address to its own. Therefore the IP address of the computer of the internal user are hidden from the outside world.

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