June 14, 2021

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How Great is the Great Firewall? Measuring China’s DNS Censorship. (arXiv:2106.02167v1 [cs.CR])

The DNS filtering apparatus of China’s Great Firewall (GFW) has evolved
considerably over the past two decades. However, most prior studies of China’s
DNS filtering were performed over short time periods, leading to unnoticed
changes in the GFW’s behavior. In this study, we introduce GFWatch, a
large-scale, longitudinal measurement platform capable of testing hundreds of
millions of domains daily, enabling continuous monitoring of the GFW’s DNS
filtering behavior.

We present the results of running GFWatch over a nine-month period, during
which we tested an average of 411M domains per day and detected a total of 311K
domains censored by GFW’s DNS filter. To the best of our knowledge, this is the
largest number of domains tested and censored domains discovered in the
literature. We further reverse engineer regular expressions used by the GFW and
find 41K innocuous domains that match these filters, resulting in overblocking
of their content. We also observe bogus IPv6 and globally routable IPv4
addresses injected by the GFW, including addresses owned by US companies, such
as Facebook, Dropbox, and Twitter.

Using data from GFWatch, we studied the impact of GFW blocking on the global
DNS system. We found 77K censored domains with DNS resource records polluted in
popular public DNS resolvers, such as Google and Cloudflare. Finally, we
propose strategies to detect poisoned responses that can (1) sanitize poisoned
DNS records from the cache of public DNS resolvers, and (2) assist in the
development of circumvention tools to bypass the GFW’s DNS censorship.