Analyzing Syria Censorship and Web Filtering

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fic in the logs. We find a total of 338,168 BitTorrent announce
ship circumvention tools, with a relatively high effectiveness.
requests from 38,575 users (identified by peerID) for 35,331
While some tools and websites are monitored and blocked
unique contents in the Dfull dataset.14 Most of these requests
(e.g., Hotspot Shield), many others are successful in bypass-
(99.97%) are allowed. Censored requests can be explained,
ing censorship. Our study also shows that some tools that
once again, with the occurrence of blacklisted keywords, e.g.,
were not necessarily designed as circumvention tools, such as
proxy, in the request URL. For instance, all announce requests
BitTorrent and Google cache, could provide additional ways
sent to the tracker on tracker-proxy.furk.net are censored.
to access censored content if proper precautions are taken,
Using the hashes of torrent files provided in the announce
especially considering that Syrian ISPs started to block Tor
messages, we crawl torrentz.eu and torrentproject.com to ex-
relays and bridges in December 2012.
tract the titles of these torrent files, achieving a success rate
of 77.4%. The five blacklisted keywords, reported in Table
8. DISCUSSION
11, are actually present in the titles of some of the BitTor-
rent files, yet the associated announce requests are allowed.
Economics of Censorship. Our analysis shows that Syrian
While we do not find any content that can be directly associ-
authorities deploy several techniques to filter Internet traffic,
ated with sensitive topics like Syrian revolution or Arab
ranging from blocking entire subnets to filtering based on
spring (these files may still be shared via BitTorrent without
specific keywords. This range of techniques can be explained
publicly announcing the content), we do identify content titles
by the cost/benefit tradeoff of censorship, as described by
which relate to anti-censorship software, such as UltraSurf
Danezis and Anderson [9]. In particular, while censoring
(2,703 requests for all versions), HideMyAss (176 requests),
the vast majority of the Israeli network regardless of the
Auto Hide IP (532 requests) and anonymous browsers (393
actual content can be explained on geo-political grounds,
requests). Our findings suggest that peer-to-peer networks are
completely denying the access to social networks, such as
indeed used by users inside Syria to circumvent censorship to
Facebook, could generate unrest. For instance, facing the
a certain extent. Also note that BitTorrent is used to download
Arab spring uprisings, the Syrian authorities decided to al-
Instant Messaging software, such as Skype, MSN messenger,
low access to Facebook, Twitter, and Youtube in February
and Yahoo! Messenger, which cannot be downloaded directly
2011. Nonetheless, these websites are monitored and selec-
from the official download pages due to censorship.
tively censored. Our analysis shows that censorship aims at a
more subtle control of the Internet, by only denying access to
7.4 Google Cache
a predefined set of websites, as well as a set of keywords. This
When searching for terms in Google s search engine, the re-
shift is achievable as the proxy appliances seamlessly support
sult pages allow access to cached versions of suggested pages.
Deep Packet Inspection (DPI), thus allowing fine-grained
While Google Cache is not intended as an anti-censorship
censorship in real-time.
tool, a simple analysis of the logs shows that it provides a
Censorship s target. Censored traffic encompasses a large
way to access content that is otherwise censored.
variety of content, mostly aiming to prevent users from using
We identify a total of 4,860 requests accessing Google s
cache on webcache.googleusercontent.com in the Dfull Instant Messaging software (e.g., Skype), video sharing web-
sites (e.g., metacafe.com, upload.youtube.com), Wikipedia,
dataset. Only 12 of them are censored due to an occur-
as well as sites related to news and opposition parties
rence of a blacklisted keyword in the URL, and a single
(e.g., islammemo.cc, alquds.co.uk). Censors also deliber-
request to retrieve a cached version of http://ar-ar.facebook.
ately block any requests related to a set of predefined anti-
com/SYRIANREVOLUTION.K.N.N has policy denied, al-
censorship tools (e.g., proxy ). This mechanism, however,
though it is not categorized as a Blocked Site. However,
has several side effects as it denies the access to any page
the rest of the requests are allowed. Interestingly, some of
containing these keywords, including those that have nothing
the allowed requests, although small in number, relate to
to do with censorship circumvention.
cached versions of webpages that are otherwise censored,
such as www.panet.co.il, aawsat.com, www.facebook.com/
Censorship Circumvention. Our study has also shown that
Syrian.Revolution, and www.free-syria.com.
users do take actions to circumvent censorship. One inter-
While the use of Google cache to access censored content
esting way is using BitTorrent to download anti-censorship
14 tools such as UltraSurf as well as Instant Messaging software.
BitTorrent clients send announce requests to BitTorrent servers [ Pobierz całość w formacie PDF ]

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