A type of firewall that provides session-level control over network traffic. Similar in operation to packet filtering routers, circuit-level gateways operate at a higher layer of the Open Systems Interconnection (OSI) reference model protocol stack.
Circuit-level gateways are host-based and reside on individual clients and servers inside the network, rather than on a dedicated machine as they do with other types of firewalls. Circuit-level gateways examine incoming Internet Protocol (IP) packets at the session level—Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)—and act as relays by handing off incoming packets to other hosts.
Circuit-level gateways are rarely used as a stand-alone firewall solution; instead, they are typically used in combination with application layer proxy services and packet filtering features in dedicated firewall applications.
Microsoft Proxy Server combines the features of packet filtering, circuit-level gateways, and application layer proxy to provide a full firewall solution for protecting your corporate network. Proxy Server supports both the SOCKS protocol, which provides nontransparent circuit-level gateway security, and the Winsock Proxy, which provides transparent circuit-level gateway security.