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Circuit Level Gateways
These operate at the level of the TCP connection
(or circuit). In effect, the application gateway
transfers a copy of application-level data between the internal
network and the outside Internet instead of the IP packets
themselves.
Such a structure might be used where an internal host wishes to,
for example, fetch a WWW page from a server in the external
Internet. The client web browser establishes a connection to a
server which is running on the gateway host. The gateway server in
turn establishes a second connection to the actual
WWW server on the external Internet, and passes the incoming HTML
document back to the internal browser process. Such a server
(running on the gateway, or bastion, host) is called a
proxy.
One of the advantages of a proxy server is the
caching of external web pages, thereby potentially
reducing traffic on the connection to the outside Internet.
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