  Researchers have studied Internet censorship for nearly as long as attempts
to censor contents have taken place. Most studies have however been limited to
a short period of time and/or a few countries; the few exceptions have traded
off detail for breadth of coverage. Collecting enough data for a comprehensive,
global, longitudinal perspective remains challenging. In this work, we present
ICLab, an Internet measurement platform specialized for censorship research. It
achieves a new balance between breadth of coverage and detail of measurements,
by using commercial VPNs as vantage points distributed around the world. ICLab
has been operated continuously since late 2016. It can currently detect DNS
manipulation and TCP packet injection, and overt "block pages" however they are
delivered. ICLab records and archives raw observations in detail, making
retrospective analysis with new techniques possible. At every stage of
processing, ICLab seeks to minimize false positives and manual validation.
  Within 53,906,532 measurements of individual web pages, collected by ICLab in
2017 and 2018, we observe blocking of 3,602 unique URLs in 60 countries. Using
this data, we compare how different blocking techniques are deployed in
different regions and/or against different types of content. Our longitudinal
monitoring pinpoints changes in censorship in India and Turkey concurrent with
political shifts, and our clustering techniques discover 48 previously unknown
block pages. ICLab's broad and detailed measurements also expose other forms of
network interference, such as surveillance and malware injection.
