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A proxy server sits between a user on your network and a server out on the Internet. Instead of communicating with each other directly, each talks to the proxy (in other words, to a “stand-in”). From the user’s point of view, the proxy server presents the illusion that the user is dealing with a genuine Internet server. To the real server on the Internet, the proxy server gives the illusion that the real server is dealing directly with the user on the internal network. So a proxy server can be both a client and a server; it depends on which way you are facing. The point to remember here is that the user is never in direct contact with the Internet server, as Figure 3.6 illustrates.

FIGURE 3.6 How a proxy server works

Actual Connection ■ Apparent Connection

The proxy server does more than just forward requests from your users to the Internet and back. Because it examines and makes decisions about the requests that it processes, it can control what your users can do. Depending on the details of your security policy, client requests can be approved and forwarded, or they can be denied. And rather than requiring that the same restrictions be enforced for all users, many advanced proxy server packages can offer different capabilities to different users.

A proxy server can be effective only if it is the only type of connection between an internal network and the Internet. As soon as you allow a connection that does not go through a proxy server, your network is at risk.

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