Proxy servers were originally developed to cache frequently accessed web pages for computers behind a common Internet connection. In the early days of the Internet, wide area links were very slow, the Web was relatively small, and web pages were static. The entire Web consisted of only a few thousand websites shared by scientists and academicians. Whenever an important news element hit a website, many scientists in the same organization would visit that page (how many times have you forwarded a link inside your company?). By caching that page on a local server, proxies could eliminate redundant Internet access to retrieve the same page over and over. So, proxies were originally very effective at web caching.
When the Web went supernova, proxies became markedly less effective at caching, the Web was now vast, web pages were frequently dynamic, and the interests of users within a single organization might range across a million web pages before the same site was hit three times. These factors presented a difficult caching problem indeed and proxies became largely ineffective, except in extremely large organizations or in ISPs. Although support for proxy servers was built into all the standard browsers, by 1996 it was seldom used. But the new Web also has its seedier element, and proxy servers showed a remarkably serendipitous side effect.
They can hide all the real users of a network behind a single machine, they can filter URLs, and they can drop suspicious or illegal content. So although originally created as non−security caches, the primary purpose of the majority of proxy servers has now become firewalling. Proxy servers regenerate high−level service requests on an external network on behalf of their clients on a private network. This effectively hides the identity and number of clients on the internal network from examination by the external network. Because of their position between a number of internal clients and public servers, proxies can also cache frequently accessed content from the public network to reduce access to the public network through high−cost wide−area links. For the sake of understanding, this chapter discusses only “pure” proxies—those that operate on the principle of service protocol forwarding.
Most actual implementations of security proxies include the services of packet filtering and Network Address Translation to form a complete firewall. Those technologies can be combined with proxies to eliminate some of the attacks to which pure proxies are vulnerable. Many proxy service alternatives exist, ranging from the Application layer filter functionality of true firewalls like Checkpoint’s Firewall−1, to general−purpose pure “proxy only” applications like WinGate, to simple single−service proxies like Jigsaw for HTTP. Pure proxies are subject to a number of problems, most based on the fact that the base operating system is not protected by the proxy software against denial−of−service attacks and the exploitation of other services that may be running on the server.
Proxy servers are most often associated with the HTTP World Wide Web service because proxies were first developed for this service. Since that time, proxy functionality has been applied to most other common Internet services. Examples in this chapter will use the HTTP service, but the functionality remains largely the same for other services.
How Proxy Work
Proxies work by listening for service requests from internal clients and then sending those requests on the external network as if the proxy server itself were the originating client. When the proxy server receives a response from the public server, it returns that response to the original internal client as if it were the originating public server. image shows this process in detail. The next sections discuss the advantages and disadvantages of proxy servers.
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