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An anonymous proxy is a website which allows the user to browse through it to other sites without disclosing their URLs to any filtering software. All that the filtering software will see is the URL or IP address of the proxy. As illustrated below, the proxy is typically presented to the user as a form.
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Often, the URL of the proxy is misleading - education administrators recently finding (the now defunct) mathshelp.info and examstudies.com littering their logs. Although they look like legitimate sites, they are actually anonymous proxies!
How do your users find an anonymous proxy? Easy - try entering "unblock myspace" into Google!
Hundreds of thousands of sites offering the same thing - anonymous proxying. To make matters totally impossible for a URL filter, the addresses of these sites change frequently, rendering a URL blocklist impossible to maintain.
Dynamic Content Analysis is the answer. Guardian examines all received web pages, not just for objectionable content and malware but also for the tell-tale signatures of a proxy. Despite the efforts of many proxy sites to hide their function, Guardian will detect them based on the proxy signatures which are present in one of the dozens of categories included in Guardian's daily blocklist update.
Some proxy sites and applications (such as Ultrasurf) also use HTTPS/SSL Certificates to evade detection. For information on how to block secure proxies like these, see Blocking UltraSurf and HTTPS Proxies.
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