Discussion of Internet censorship usually focuses on China and its “Great Firewall.” But the Chinese Communist Party isn’t the only regime that censors its Internet. Iran does too.
Little is known about Iran’s censorship system because Iranian citizens who probe the network from inside the country risk reprisals from the government. But earlier this year, two anonymous Iranians teamed up with Alex Halderman, a computer science professor at the University of Michigan, to conduct one of the first systematic studies of Iranian Internet censorship to be published outside Iran. Halderman presented his findings at a Tuesday talk at the Usenix Security conference in Washington, D.C.
Iran has an extensive list of blacklisted sites. Users who attempt to visit a banned site see a notice that looks something like this:
What gets censored? To find out, the researchers attempted to visit the sites on Alexa’s top 500 lists in various categories. Almost half of the 500 most popular sites on the Internet are censored. Unsurprisingly, the theocratic Iranian regime censors pornographic Web sites most heavily. But a high percentage of sites in the “art,” “society,” and “news” categories are also blocked:
In addition to banning particular sites, the Iranian network also filters traffic based on its content. In one test, the researchers created a file called sex.htm that was hosted outside the United States. Access to this page from inside Iran was blocked.
The Iranian Internet is also configured to discourage the use of certain encrypted protocols. Web traffic is allowed through at full speed. Traffic that uses the encrypted SSH protocol, which can be used to “tunnel” other types of traffic out of the country, run at less than 20 percent of the network’s full speed. Traffic the Iranian firewall doesn’t recognize is throttled even more dramatically, and gets cut off altogether after about 60 seconds.
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http://www.washingtonpost.com/blogs/the-switch/wp/2013/08/15/heres-how-iran-censors-the-internet/