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3.3 What are proxy servers and how do they work?

  A proxy server (sometimes referred to as an application gateway or forwarder) is an application that mediates traffic between a protected network and the Internet. Proxies are often used instead of router-based traffic controls, to prevent traffic from passing directly between networks. Many proxies contain extra logging or support for user authentication. Since proxies must ``understand'' the application protocol being used, they can also implement protocol specific security (e.g., an FTP proxy might be configurable to permit incoming FTP and block outgoing FTP).

Proxy servers are application specific. In order to support a new protocol via a proxy, a proxy must be developed for it. One popular set of proxy servers is the TIS Internet Firewall Toolkit (``FWTK'') which includes proxies for Telnet, rlogin, FTP, X-Window, HTTP/Web, and NNTP/Usenet news. SOCKS is a generic proxy system that can be compiled into a client-side application to make it work through a firewall. Its advantage is that it's easy to use, but it doesn't support the addition of authentication hooks or protocol specific logging. For more information on SOCKS, see http://www.socks.nec.com/ .

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