page, it will be returned directly from the cache, rather than connecting to the actual Web
server a second time. From the user’s perspective this is better since the Web page will appear faster, as they never have to connect outside their own ISP. It is also better for the ISP, as
connecting to the Web server will consume (expensive) bandwidth, and rather than having to
transfer pages from a popular site hundreds of times, they need only do this once. Figure 3.4
shows how the use of a proxy differs from the normal case.
However, as well as improving performance, an HTTP proxy can also block Web sites. The
proxy decides whether requests for Web pages should be permitted, and if so, sends the request to the Web server hosting the requested content. Since the full content of the request is
available, individual Web pages can be filtered, not just entire Web servers or domains.
An HTTP proxy may be nontransparent, requiring that users configure their Web browsers
to send requests via it, but its use can be forced by deploying TCP/IP header filtering to block
normal Web traffic. Alternatively, a transparent HTTP proxy may intercept outgoing Web
Figure 3.4
Normal Web browsing with a proxy.
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Steven J. Murdoch and Ross Anderson
server a second time. From the user’s perspective this is better since the Web page will appear faster, as they never have to connect outside their own ISP. It is also better for the ISP, as
connecting to the Web server will consume (expensive) bandwidth, and rather than having to
transfer pages from a popular site hundreds of times, they need only do this once. Figure 3.4
shows how the use of a proxy differs from the normal case.
However, as well as improving performance, an HTTP proxy can also block Web sites. The
proxy decides whether requests for Web pages should be permitted, and if so, sends the request to the Web server hosting the requested content. Since the full content of the request is
available, individual Web pages can be filtered, not just entire Web servers or domains.
An HTTP proxy may be nontransparent, requiring that users configure their Web browsers
to send requests via it, but its use can be forced by deploying TCP/IP header filtering to block
normal Web traffic. Alternatively, a transparent HTTP proxy may intercept outgoing Web
Figure 3.4
Normal Web browsing with a proxy.
62
Steven J. Murdoch and Ross Anderson