The Great Firewall of China: How It Blocks Tor and Why It Is Hard to Pinpoint

Internet censorship is no longer a phenomenon limited to countries with a weak human rights record—the Western world is beginning to embrace the idea. This development leads to a fundamental research question: how can we know where the roadblocks are on the Internet and the details of how they work? In this article, we use the “Great Firewall of China” (GFC) to illustrate how complex of a problem it can be to find the network filtering devices, and how sophisticated the filtering itself can be when directed at an advanced target such as the Tor anonymity network.