A glitch in the "Great Firewall" of China likely caused many of that country's half-billion Internet users to be cut off from the World Wide Web for more than two hours Thursday.
The firewall allows Chinese censors to block access to foreign websites on a government blacklist.
On Thursday, Internet users in China were unable to access some major Chinese Web portals and many popular foreign websites not normally blocked by censors, according to multiple postings by users on social networking sites.
"Can't get to any overseas websites!" wrote Li Kaifu, an executive who once ran Microsoft's and Google's China operations, to his 33.5 million followers on the Chinese microblogging site Sina Weibo.
At the same time, users in Hong Kong, North America and elsewhere began reporting trouble accessing key Chinese sites, such as the search engine Baidu and the website of the People's Bank of China.
Although the outages appeared to have been fixed within two hours, their scale and sporadic quality – some connections seemed to work fine, according to other users – set off a storm of speculation about the cause.
The Chinese telecommunications ministry and several major Internet companies said they were looking into the outage reports, according to the Guardian.
The Chinese Embassy in Washington did not respond to a request for comment. China routinely denies that it censors the Internet.
"Most likely it was some kind of change, update or patch" to the Great Firewall, said Adam Rice, a consultant who was for several years head of security for Tata Communications, the world's largest wholesaler of Internet service.
Mr. Rice's observation jibes with a posting from a Beijing-based Internet market analysis firm often quoted by China's official media.
"A large number of overseas [Internet] addresses have been blocked. It's possible that the Great Firewall is undergoing adjustments and additions to the blocked list have caused a number of IP addresses to be accidentally included. Details are unknown," read the posting from Data Center of China Internet.
The Weibo posting was removed by censors shortly after it went up.
Another posting, from Guan Peng, deputy general manager of China's Yanhuang Network, said "a number of domain name servers have been blocked," according to China Tech News.
Domain Name System (DNS) servers are the traffic cops of the Web, routing the electronic packets of Internet communications to the correct destination addresses.
Controlling access to DNS servers is an important element of any system of Internet control and censorship, said Mr. Rice.
"In China they pay very close attention to to [Internet] routing," said Mr. Rice, "You can do some very clever and naughty things" by interfering with it.
He said some botched or premature effort by Chinese authorities to re-route Web traffic might be another explanation for Thursday's outage.
On April 9, 2010, nearly 15 percent of the world's Internet traffic, including that of many U.S. government and military sites, was briefly redirected through computer servers in China because of DNS routing instructions issued by a Chinese telecommunications company, according to a congressional commission that looked into the incident.
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