Proxy Service
A firewall is at the center of a whole collection of technologies designed to protect and simplify the internal network and confine the unpredictable and potentially insecure Internet activity to the perimeter. Another related technology is known as proxy service. A proxy server intercepts requests for Internet resources and forwards the requests on behalf of the client, acting as an intermediary between the client and the server that is the target of the request (See Figure 10.6). Although a proxy server is not necessarily sufficient to protect the network by itself, it is often used in conjunction with a firewall (particularly in the context of a Network Address Translation environment, which you learn about in Hour 12, “Automatic Configuration”).
Figure 10.6. A proxy server requests services on behalf of the client.
By placing and receiving Internet requests on behalf of the client, the proxy server protects the client from direct contact with possibly malicious web resources. Some proxies perform a kind of content filtering to watch for blacklisted servers or potentially dangerous content. Proxy servers are also used to limit the range of browsing options for clients on the internal network. For instance, a school network might use a proxy server to prevent students from surfing to exhilarating sites that are intended for the category of adult education.
In many situations, the primary purpose of a proxy server is performance rather than security. Proxy servers often perform a service known as content caching. A content-caching proxy server stores a copy of the web pages it accesses. Future requests for the page can thus be served locally with a much faster response than if the request were served from the Internet. This might seem like a lot of trouble just to help a user visit the same site twice, but if you consider the browsing habits of a typical user, it is quite common to click around several times at a website and visit a page more than once—or to leave the page and come back after only a short interval. The proxy server is usually configured to hold the page only for a specific time interval before releasing the cache and requesting a new version of the page.