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How to Open Blocked Sites: Keyword Blocking Filters

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Part III of How To Open Blocked Sites: Bypass Proxy Sites, VPNs and Other Tricks to Get Around Censorship


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Slightly less advanced than stateful analysis filtering, but also expensive, is keyword block filtering – a means of blocking access to any page containing a prohibited word. This type of filtering may also conduct deep packet inspection to analyze content passing through the censoring servers or router but will not record it for pattern analysis. The URLs (as well as content if DPI is used) are scanned and searched for the banned keywords or phrases. They may be politically sensitive words, ones relating to banned services (such as Facebook or Youtube), or words that suggest the site offers a tool for internet censorship circumvention (eg. bypass proxy sites). Generically prohibited words like porn and games are often added to the banned keyword lists as well.

Again, the sure-fire solution is to use a VPN, as when all packets of data, including URLs, are encrypted, the keyword blocking scans have nothing to work with. It is true that SSL proxies with the encrypt or encode URL feature may also help, as long as you always access the proxy with an HTTPS connection from the outset and the domain name of the site does not betray any sign of its proxy function. This is important, because even encrypted URLs do not encrypt the URL stem, so any site showing proxy or unblock in the URL stem will be shut out. Providers of the best school proxy sites know this, however, and generally use domain names for their bypass proxy sites which do not give away the proxy function (eg. www.webinfo.net).

Bear in mind, there is a good chance that DNS and IP blocklists are used by the filters in addition to keyword blocking, so you may need a fresh proxy also, as discussed earlier.

Part IV: How to Open Blocked Sites: IP Block Filters

Written by: 
Robin Welles; expats team, internet security team