Probing Chinese search engine filtering



ONI conducted a research probe into the practice of search engine filtering in China, with a focus on Baidu.com and Yisou.com, two popular Chinese search engines. Our probe concludes that the Yisou and Baidu search engines are indeed actively filtering keyword search requests and that searches for certain keywords are sometimes obstructed by China’s gateway filtering. The full bulleting is available here.

The part I find the most interesting is the use of the TCP reset functionality in conjuction with keyword filtering. When making an HTTP request with a banned keyword in the GET an RST packet is sent and a ZeroWindow size is advertized. Another request transmission cannot be made until the host advertises a non-zero window size. In effect, you cannot connect to the same host from the same IP address for a period of time.

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