Global Voices Summit: Detecting Internet Filtering

The leading expert on Internet filtering, Nart Villeneuve, gave the second presentation. He provided an overview on strategies to deal with filtering.

  • Distinguish between errors and actual censorship;
  • https is encrypted, not subject to key word filtering;
  • Test root domain name before testing entire path;
  • Some websites can be blocked because they include an image or other file from a blocked website;
  • in addition to checking website, check IP address; websites often have multiple IP addresses, not all IP addresses may be blocked; sub-domain names can also have different IP addresses;
  • Some blocked websites are not blocked when you remove the www prefix;
  • For DNS tampering, you can use a remote DNS server such as;
  • If an IP address is blocked, which is easily done, checking for this can be done using;
  • If filtering proxies is taking place, compare http headers, sometimes you get a 200, 403 and 302. Filtering software often have unique identifying headers;
  • Looking at source code of the page can give you information on the category that a particular website has been blocked under. several reverse IP tools can also be used;

Patrick Philippe Meier

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