Scalable Remote Measurement of Application-Layer Censorship
Quack: Scalable Remote Measurement of Application-Layer Censorship Benjamin VanderSloot, Allison McDonald, Will Scott, J. Alex Halderman, and Roya Ensafi University of Michigan {benvds, amcdon, willscott, jhalderm, ensafi}@umich.edu Abstract under repressive or secretive government controls, coop- erating with security researchers has substantial risks. Remote censorship measurement tools can now detect An emerging body of work addresses these problems DNS- and IP-based blocking at global scale. However, by using existing protocols and infrastructure to remotely a major unmonitored form of interference is blocking measure network interference. Such approaches have triggered by deep packet inspection of application-layer been effective in measuring DNS poisoning [35,41] and data. We close this gap by introducing Quack, a scalable, for detecting interference in TCP/IP-connectivity between remote measurement system that can efficiently detect remote machines [17,34]. There has not yet been a global, application-layer interference. remote method for detecting another broadly deployed We show that Quack can effectively detect application- censorship technique: application-layer censorship. layer blocking triggered on HTTP and TLS headers, and Application-layer censorship has become increasingly it is flexible enough to support many other diverse pro- important with the rise of content delivery networks tocols. In experiments, we test for blocking across 4458 (CDNs). CDNs use a small number of network entry- autonomous systems, an order of magnitude larger than points for a large number of customers, resulting in siz- provided by country probes used by OONI over a one able collateral damage to IP-based blocking techniques. week span. We also test a corpus of 100,000 keywords When an adversary wishes to block some, but not all, of from vantage points in 40 countries to produce detailed these sites, they must look into the content of requests national blocklists.
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