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Publication: Examining How the Great Firewall Discovers Hidden Circumvention Servers

Published 2015 in Internet Measurement Conference

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Publication Details

SummaryComprehensive analysis of how the Great Firewall of China blocks circumvention proxies, mostly focused on the Tor network.
Start Time2010-01-01 05:00 UTC (+0000)
End Time2015-08-01 04:00 UTC (+0000)
Duration2037 days 23:00:00 (176079600.0 s)
Data formatspcap
Primary contactRoya Ensafi
AuthorsRoya Ensafi, David Fifield, Philipp Winter, Nick Feamster, Nicholas Weaver, Vern Paxson
KeywordsActive Probing, Censorship Circumvention, Deep Packet Inspection, Great Firewall of China, Tor
Publication abstract
Recently, the operators of the national censorship infrastructure of China began to employ "active probing" to detect and block the use of privacy tools. This probing works by passively monitoring the network for suspicious traffic, then actively probing the corresponding servers, and blocking any that are determined to run circumvention servers such as Tor.

We draw upon multiple forms of measurements, some spanning years, to illuminate the nature of this probing. We identify the different types of probing, develop fingerprinting techniques to infer the physical structure of the system, localize the sensors that trigger probing---showing that they differ from the "Great Firewall" infrastructure---and assess probing's efficacy in blocking different versions of Tor. We conclude with a discussion of the implications for designing circumvention servers that resist such probing mechanisms.
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Record Details

ContributorPhilipp Winter
Contributed2015-09-03 16:17:36.751 UTC (+0000)
Last Modified2015-09-06 17:03:04.380 UTC (+0000)