Proxy servers are used to communicate with external servers on behalf of internal clients. A proxy service is set up and torn down in response to a client request, rather than existing on a static basis. The term proxy server typically refers to an application-level gateway, although a circuit-level gateway is also a form of proxy server. The gateway can be configured to support an application-level proxy on inbound connections and a circuit-level proxy on outbound connections. Application proxies forward packets only when a connection has been established using some known protocol. When the connection closes, a firewall using application proxies rejects individual packets, even if they contain port numbers allowed by a rule set. In contrast, circuit proxies always forward packets containing a given port number if that port number is permitted by the rule set. Thus, the key difference between application and circuit proxies is that the latter are static and will always set up a connection if the DUT/SUT's rule set allows it. Each proxy is configured to allow access only to specific host systems.
The audit log is an essential tool for detecting and terminating intruder attacks. Therefore, each proxy maintains detailed audit information by logging all traffic, each connection, and the duration of each connection.
Since a proxy module is a relatively small software package specifically designed for network security, it is easier to check such modules for security flaws.
Each proxy is independent of other proxies on the bastion host. If there is a problem with the operation of any proxy, or if future vulnerability is discovered, it is easy to replace the proxy without affecting the operation of the proxy's applications. If the support of a new service is required, the network administrator can easily install the required proxy on the bastion host.
A proxy generally performs no disk access other than to read its initial configuration file. This makes it difficult for an intruder to install Trojan horse sniffers or other dangerous files on the bastion host.