A friend of mine noticed something odd while visiting Hong Kong last weekend. Despite Hong Kong not being subject to the heavy internet blocks and censorship on the mainland he still couldn’t get Facebook or Twitter to work on his iPhone (which had international roaming turned on). Hong Kong residents using one of the local providers don’t face any such restrictions and after testing out a few different apps as well as making sure the sites worked fine on a normal laptop we began to become suspicious.
He was roaming using a China Mobile SIM card but surely the blocks couldn’t extend outside the geographic boundaries of their own network? Further investigation was required…
Using the same SIM card we tried switching between many of the local HK networks including: CSL, Vodaphone and the local subsidiary of China Mobile. All refused to let us access sites blocked on the mainland indicating that somehow our traffic was being re-routed by the local Hong Kong carrier back to the motherland. When switching on a VPN connection everything worked fine again.
To prove our theory further we installed a traceroute application on the iPhone and ran traces against the same blocked websites. As can be seen from the screenshots above traffic was indeed being routed back to China hence why the sites were blocked even though we were physically in Hong Kong.
Is it normal practice for cell phone carriers to re-route data traffic back to the original provider for those roaming on their network?
The implications here are massive; in essence it allows the hand of the Chinese government and potentially other regimes wanting to limit the free-flow of information to reach globally. Chinese citizens travelling abroad are being kept in a prison via their phone and it would seem that the cell carriers are complicit in allowing it. I’d love to hear from someone who understands a bit more about how these things work.
Update: Accoring to @WildPixels this is normal routing for data roaming. Apparently using a non-China SIM in China also bypasses the great firewall as well (at high cost).