The Great Firewall of China and how it blocks Tor traffic

Diagram showing how GFW filters/censors tor traffic
China’s firewall is now able to dynamically recognise Tor usage and block the respective relays and bridges. The diagram above illustrates how this works: 1) the firewall searches for a bunch of bytes which identify a network connection as Tor. If these bytes are found, 2) the firewall initiates a scan of the host which is believed to be a bridge. In particular, 3) the scan is run by seemingly arbitrary Chinese computers which connect to the bridge and try to “speak Tor” to it. If this succeeds, the bridge is blocked.

(via phw’s blog on Tor Project)

Over the last years, we learned a lot about how the Great Firewall of China is blocking Tor. Some questions remained unanswered, however. Roya, Mueen, Jed, and I just published a project which seeks to answer some of these open questions. Being curious as we are, we tried to find answers to the following questions:

  • Is the filtering decentralised (i.e., happening in provinces) or centralised (i.e., happening in Internet exchange points (IXP))?
  • Are there any temporal patterns in the filtering? Or in other words, are there certain times when people are more likely to be able to connect to Tor?
  • Similarly, are there any spatial patterns? Are folks in some special regions of China able to connect to Tor while others cannot?
  • When a computer in China tries to connect to a Tor relay, what part of the TCP handshake is blocked?

It turns out that some of these questions are quite tricky to answer. For example, to find spatial patterns, we need to be able to measure the connectivity between many Tor relays and many clients in China. However, we are not able to control even a single one of these machines. So how do we proceed from here? As so often, side channels come to the rescue! In particular, we made use of two neat network measurement side channels which are the hybrid idle scan and the SYN backlog scan. The backlog scan is a new side channel we discovered and discuss in our paper. Equipped with these two powerful techniques, we were able to infer if there is packet loss between relay A and client B even though we cannot control A and B.

You might notice that our measurement techniques are quite different from most other Internet censorship studies which rely on machines inside the censoring country. While our techniques give us a lot more geographical coverage, they come at a price which is flexibility; we are limited to measuring Internet filtering on the IP layer. More sophisticated filtering techniques such as deep packet inspection remain outside our scope.

Now what we did was to measure the connectivity between several dozen Tor relays and computers in China over four weeks which means that we collected plenty of data points, each of which telling us “was A able to talk to B at time T?”. These data points reveal a number of interesting things:

  • It appears that many IP addresses inside the China Education and Research Network (CERNET) are able to connect to at least our Tor relay.
  • Apart from the CERNET netblock, the filtering seems to be quite effective despite occasional country-wide downtimes.
  • It seems like the filtering is centralised at the IXP level instead of being decentralised at the provincial level. That makes sense from the censor’s point of view because it is cheap, effective, and easy to control.

Now what does all of this mean for Tor users? Our results show that China still has a tight grip on its communication infrastructure, especially on the IP and TCP layer. That is why our circumvention efforts mostly focus on the application layer (with meek being an exception) and pluggable transport protocols such as ScrambleSuit (which is now part of the experimental version of TorBrowser) and obfs4 are specifically designed to thwart the firewall’s active probing attacks.

Check out the comments section of the original blog post at Tor Project for interesting discussion … Also, see “How The Great Firewall of China Is Blocking Tor” (PDF)

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