Circuit-level gateway
Firewalls. The pocketknife for enforcing computer security

What circuit-level gateways (TCP) have in common with application-level gateways is that they require two connections to be established: one between the client and the gateway, and another between the gateway and the server. The similarities end there, as circuit-level gateways filter at the transport layer and, unlike application-level gateways, can benefit any TCP/IP service.

The filtering performed by circuit-level gateways covers the same fields as packet filters, but control is more flexible in that the relay gateway can momentarily block traffic and thus perform additional processing. Here are the different filtering rules that can be implemented:

• authorize a connection on a port for a fixed maximum duration ;

• allow reuse of the same port only after a certain period of time;...