CensMon: A Web Censorship Monitor

Over a 14-day evaluation in April 2011, CensMon tested 4,950 unique URLs from 2,500 domains across 174 agents in 33 countries, detecting 951 unique URLs from 193 domains as filtered. Manual verification of all 193 flagged domains found only 3 false positives, demonstrating high precision for an automated distributed monitor.

§4.2 evaluation measurement-platformip-blockingdns-poisoningkeyword-filtering cntr

The single Chinese PlanetLab node reported 176 censored domains — more than all other 173 agents combined. Turkey (6 domains), Jordan (5), and Hungary (1) were the only other countries with any detected filtering. 86% of agent nodes across 33 countries reported zero filtering events.

§4.2, Table 2 evaluation measurement-platform cntr

Among all filtered URLs detected, HTTP filtering accounted for 48.5%, IP address blocking for 33.3%, and DNS manipulation for 18.2%. Of the domains blocked at the HTTP layer in China, 71% were blocked due to URL keyword filtering rather than HTML response content filtering.

§4.2, Figure 2 evaluation keyword-filteringip-blockingdns-poisoning cn

CensMon detected zero instances of partial web-page content filtering across 4,950 tested URLs during April 2011, indicating that censors at that time uniformly applied coarse-grained techniques — full URL block, IP blacklist, or DNS hijack — rather than inline content modification at the sub-page level.

§4.2 evaluation keyword-filteringip-blockingdns-poisoningmeasurement-platform cn

21% of all URLs that CensMon began tracking were found accessible on the very first re-probe, indicating initial inaccessibility was a transient network failure rather than censorship. The false-network-failure rate fell to near zero after 3 consecutive tracking attempts, providing a practical threshold for classifying persistent inaccessibility as filtering.

§4.2, Figure 4 evaluation measurement-platform generic