A customer tries an Apple Inc. iPhone 6 at an Apple store in the China Central Mall in Beijing, China, on Tuesday, Nov. 11, 2014. Apple, which ended the fiscal fourth quarter with $155.2 billion in cash, is forecasting a record holiday sales quarter after introducing new bigger-screen iPhones and slimmer iPads. Photographer: Tomohiro Ohsumi/Bloombergout

Internet users in mainland China are increasingly finding that the technology they use to circumvent the country’s “Great Firewall” has been blocked or is only intermittently available.

Virtual private networks, which encrypt and redirect web traffic to get around Beijing’s blocking technology, are either unavailable or appear to suffer more frequent terminations.

Amid an increase in online censorship by Beijing, VPNs, which typically charge a monthly fee of around $5, have become essential to access blocked foreign sites blocked including Facebook, Twitter, and Google.

A representative of Astrill, the most popular VPN service in China, confirmed that its VPN protocols used in Apple’s iOS operating system for mobile devices such as the iPhone have been blocked. “It’s been blocked since new year but worked with few servers; now it is completely blocked,” said a member of Astrill’s support staff.

While Astrill’s iPhone app is apparently blocked, it does remain accessible via a complicated process. The desktop version, which uses a different internet protocol, appears to be working but suffers frequent terminations. Users of other VPN services report similar disruptions.

Runox.us, a Chinese VPN provider, began experiencing problems on December 31 and has suggested a sophisticated work around to its clients, saying that it expects the present disruptions to be “a long-term measure”.

The blockages follow the start of intermittent blocking of Google’s Gmail on third-party services in December. Last June, Google’s main websites were blocked in China. VPNs are the last route for many users to their Gmail accounts.

“We have seen increased web censorship over the past year and I think drawing a correlation to the disruption of consumer-facing VPN services is not a stretch,” said Charlie Smith of Greatfire.org, which monitors censorship in China.

What exactly is happening is hard for experts to discern because no monitoring of VPN connections in China is done for public consumption, and the VPN services themselves generally do not provide detail on traffic interruptions. However, anecdotal evidence suggests a sharp increase in the instability of VPN connections since the end of last year.

“VPNs use a fundamentally fragile architecture that makes them easy for censors to block in a number of ways,” said Adam Fisk of Getlantern.org, a censorship circumvention tool. “VPNs use centralised servers with fixed IP addresses that can be blocked even if those IP addresses change quickly.”

He and other experts say they have detected greater sophistication in China’s blocking efforts recently. While previously China’s Great Firewall had to know the precise IP address of each VPN provider it is now able to identify VPN traffic during transit and shut it down. “VPNs frequently use unique protocols that can be blocked with little to no collateral damage to other services,” said Mr Fisk.

Pao-pao.net, a Chinese language website devoted to news about the internet, in an online chat conversation said the main impediment to cracking down harder on VPNs is not technical, but that authorities instead would likely assess the amount of damage they could do to business interests.

“In reality, the [Great Firewall] is very cautious when it comes to cutting off connections like this, mainly because a lot of commercial interests are at stake with respect to VPNs.”

“While they won’t go so far as to block all VPNs, they can increase the cost of using a VPN by intermittently disrupting services.“

Additional reporting by Ma Fangjing

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