Are new cracks appearing in China's great firewall?
来源：未知 作者：廉廛 时间：2017-06-23 05:01:02
By Jim Giles Some internet pages previously blocked by the Chinese government may now be available in China following changes in the way officials censor what users can access. News stories and other pages that deal with sensitive subjects are among parts of the web that may now be accessible – as long as they do not include banned keywords in their page titles. This is the conclusion of two US-based researchers who have discovered previously unknown changes to China’s suite of internet censorship tools, although precisely how much previously censored material now gets though is unclear. The Golden Shield project, often known abroad as the great firewall of China, allows the government to control what its citizens can access on the web. At its heart is a system that checks the names and internet addresses of web pages that users in China attempt to open. Wikipedia pages on sensitive topics such as Tibet or the 1989 Tiananmen Square protests are automatically blocked. The same often happens to social networking sites like Facebook and Twitter. Another element originally included in the censorship software is an HTML filtering system that examines incoming internet content for keywords such as “Tiananmen incident”. If a banned word is identified, the filter inserts a message into the flow of data to terminate the connection between the computer requesting the page and the foreign server delivering it. When the system works, any page containing banned words is impossible to reach. But it now appears that HTML filtering has been largely abandoned, according to Jong Chun Park and Jedidiah Crandall at the University of New Mexico in Albuquerque. Park and Crandall remotely accessed a number of computers in China and attempted to deliver a web page containing part of the term “Falun Gong”, the name of a religious sect that is banned there. They then monitored activity to see whether attempts were made to terminate the connection. In August 2008, filtering was in place on six of 10 machines, but when they checked last year they found only one case of filtering in each of two sets of 15 and 20 computers. Others forms of censorship were still in place. When a browser talks to a web server, each packet of information is labelled according to its position in the stream of data that flows back and forth. To terminate the connection, the filtering device has to insert a packet with the correct label. Because large numbers of packets of information are exchanged, the filter often mislabels the termination message, so the browser ignores it. Park and Crandall estimate HTML filtering only blocked around half of the pages the Chinese authorities wanted it to capture, and that this lies behind the decision to give up on it. While some previously unobtainable material may now be available to users in China, local-level filtering by internet service providers may be blocking some of the holes that a lack of HTML filtering has left, Crandall says. Censors may also have banned more sites outright, adds Steven Murdoch at the University of Cambridge. “If there is any practical impact, the Chinese censorship authorities might be a bit more eager to block a site, because they can no longer rely on HTML filtering to deal with banned content,” he says. The decision to abandon HTML filtering may be linked to the introduction the of Green Dam web-filtering software that the Chinese government originally declared would be required on all new computers sold in the country. Several western manufacturers refused to install Green Dam, but computer makers in Asia said they would. Following protests from within China as well as by foreign governments and companies, the authorities backed down in the middle of last year, stating the Green Dam software would only be required on public computers such as those in schools and internet cafes. It could be used to block the same content as HTML filtering, but at the browser level. Park and Crandall’s will be presenting a paper on their work in June at the 30th International Conference on Distributed Computing Systems in Genoa,