Towards a Comprehensive Picture of the Great Firewall's DNS Censorship

China’s Great Firewall passively inspects network traffic and disrupts unwanted communication by injecting forged DNS replies or TCP resets. We attempted to comprehensively examine the structure of the DNS injector, using queries from both within and outside China. Using these probes, we were able to localize the DNS monitors’ locations, extract the firewall’s DNS blacklist of approximately 15,000 keywords, and estimate the cluster structure and active response rate by utilizing an information leakage in the Great Firewall’s design.