Google's Gmail appears to have been blocked by China at IP level

Internet performance monitoring company, Dyn Research, said Sunday that China was blocking Gmail at the IP level when served from Hong Kong.

All Gmail traffic to China is through Hong Kong and affected by the block, unless users deploy evasion techniques, wrote Earl Zmijewski, Dyn's vice president of analytics, in an email.

Google's real-time measure of traffic patterns for its services also indicated a sharp drop in Gmail traffic in China after Christmas. "We've checked and there's nothing wrong on our end," Google said via email.

DNS, or the Domain Name System, is a naming system for resources on the Internet that translates a domain name into numerical IP (Internet Protocol) addresses that computers can understand to access the Internet. IP address blocking is often used by regimes to censor content to a particular region.

"DNS returns only Hong Kong IPs from all of our locations throughout China," Zmijewski wrote. "These IP addresses are then blocked on backbone routers, so the Hong Kong servers cannot be reached from the mainland." Google returns Hong Kong IPs to their Chinese users as these are the closest servers, he added.

Zmijewski pointed out that Google operates Gmail servers in 20 different countries and he can access them from China when doing a spot check. "Thus, using a non-Hong Kong server (if you happen to be able to discover the appropriate IP address) or a VPN based in another country, should enable Chinese users to get around this block," he added.

China has a long history of censoring and disrupting sites that could be used for distributing anti-government content and has also blocked some international services like Facebook and Twitter. Google pulled out partially from China in 2010 rather than self-censor its search engine, after it said it had found a cyberattack that targeted the accounts of human rights activists. The company started redirecting users visiting its Chinese search site to its Hong Kong website.

A number of Google services were disrupted this year, ahead of the 25th anniversary on June 4 of China's crackdown on pro-democracy activists in Tiananmen Square, according to GreatFire.org, a website that tracks Internet access in the country.

Google's transparency report shows consistent access to Gmail from Chinese users over the years until the current block, Zmijewski said.

On Twitter and other forums, users complained about the lack of access to Gmail. Liu Yuan, a user in Shanghai, wrote: "mail from or to gmail from china may be returned. btw, which is the best vpn"

John Ribeiro covers outsourcing and general technology breaking news from India for The IDG News Service. Follow John on Twitter at @Johnribeiro. John's e-mail address is john_ribeiro@idg.com