NETWORK WORKING GROUP                                            J. Hall
Internet-Draft                                                  M. Aaron
Intended status: Informational       Center for Democracy and Technology
Expires: April 30, 2015                                 October 27, 2014

              A Survey of Worldwide Censorship Techniques


   This document describes the technical mechanisms used by censorship
   regimes around the world to block or degrade internet traffic.  It
   aims to make designers, implementers, and users of Internet protocols
   aware of the properties being exploited and mechanisms used to censor
   end-user access to information.  This document makes no suggestions
   on individual protocol considerations, and is purely informational,
   intended to be a reference.

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   This Internet-Draft will expire on April 30, 2015.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

2.  Technical Aggregation

   Aggregation is the process of figuring out what censors would like to
   block.  Generally, censors aggregate "to block" information in three
   possible sorts of blacklists: Keyword, Domain Name, or IP.  Keyword
   and Domain Name blocking take place at the application level (e.g.
   HTTP), whereas IP blocking tends to take place in the TCP/IP header.
   The mechanisms for building up these blacklists are varied.  Many
   times private industries that sell "content control" software, such
   as SmartFilter, provide their services to nations which can then pick
   from broad categories, such as gambling or pornography, that they
   would like to block [ref-1].  In these cases, the private services
   embark on an attempt to label every semi-questionable website as to
   allow for this metatag blocking.  Countries that are more interested
   in retaining specific political control, a desire which requires
   swift and decisive action, often have ministries or organizations,
   such as the Ministry of Industry and Information Technology in China
   or the Ministry of Culture and Islamic Guidance in Iran, which
   maintain their own blacklists.

3.  Technical Identification

3.1.  Points of Control

   Digital censorship, necessarily, takes place over a network.  Network
   design gives censors a number of different points-of-control where
   they can identify the content they are interested in filtering.  An
   important aspect of pervasive technical interception is the necessity
   to rely on software or hardware to intercept the content the censor
   is interested in.  This requirement, the need to have the
   interception mechanism located somewhere, logically or physically,
   implicates four general points-of-control:

   o  Internet Backbone: If a censor controls the gateways into a
      region, they can filter undesirable traffic that is traveling into
      and out of the region by sniffing and mirroring at the relevant
      exchange points.  Censorship at this point-of-control is most
      effective at controlling the flow of information between a region
      and the rest of the internet, but is ineffective at identifying
      content traveling between the users within a region.

   o  Internet Service Providers: Internet Service Providers are perhaps
      the most natural point-of-control.  They have a benefit of being

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      easily enumerable by a censor paired with the ability to identify
      the regional and international traffic of all their users.  The
      censor's filtration mechanisms can be placed on an ISP via
      governmental mandates, ownership, or voluntary/coercive influence.

   o  Institutions: Private institutions such as corporations, schools,
      and cyber cafes can put filtration mechanisms in place.  These
      mechanisms are occasionally at the request of a censor, but are
      more often implemented to help achieve institutional goals, such
      as to prevent the viewing of pornography on school computers.

   o  Personal Devices: Censors can mandate censorship software be
      installed on the device level.  This has many disadvantages in
      terms of scalability, ease-of-circumvention, and operating system
      requirements.  The emergence of mobile devices exacerbate these
      feasibility problems.

   At all levels of the network hierarchy, the filtration mechanisms
   used to detect undesirable traffic are essentially the same: a censor
   sniffs transmitting packets and identifies undesirable content, and
   then uses a blocking or shaping mechanism to prevent or degrade
   access.  Identification of undesirable traffic can occur at the
   application, transport, or network layer of the IP stack.  Censors
   are almost always concerned with web traffic, so the relevant
   protocols tend to be filtered in predictable ways.  For example, a
   subversive image would always make it past a keyword filter, but the
   IP address of the site serving the image may be blacklisted when
   identified as a provider of undesirable content.

3.2.  Application Layer

3.2.1.  HTTP Request Header Identification

   A HTTP header contains a lot of useful information for traffic
   identification; although host is the only required field in a HTTP
   request header, a HTTP method field is necessary to do anything
   useful.  As such, the method and host fields are the two fields used
   most often for ubiquitous censorship.  As a censor, I can sniff
   traffic and identify a specific domain name (host) and usually a page
   name (GET /page) as well.  This identification technique is usually
   paired with TCP/IP header identification for a more robust method.
   Tradeoffs: Request Identification is a technically straight-forward
   identification method that can be easily implemented at the Backbone
   or ISP level.  The hardware needed for this sort of identification is
   cheap and easy-to-acquire, making it desirable when budget and scope
   are a concern.  HTTPS will encrypt the relevant request and response
   fields, so pairing with TCP/IP identification is necessary for
   filtering of HTTPS.  Empirical Examples: Empirical examples of pure

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   HTTP Request Identification are unusually hard to identify due to the
   lack of distinguishing charistics.  Commercial technologies such as
   the McAfee SmartFilter and NetSweeper are often purchased by censors
   [ref-2].  These commercial technologies use a combination of HTTP
   Request Identification and TCP/IP Header Identification to filter
   specific URLs.  There has not been research conducted to try and
   identify if only one of these two techniques is being used.

3.2.2.  HTTP Response Header Identification

   While HTTP Request Header Identification relies on the information
   contained in the HTTP request from client to server, response
   identification uses information sent in response by the server to
   client to identify undesirable content.  Usually implemented at the
   Backbone or ISP level, the technique normally relies on mirroring, or
   duplicating the packets such that one can provide uninterrupted
   service while inspecting the duplicates for undesirable content, to
   prevent QoS degradation [ref-3] - the mirrored traffic is identified
   by relevant response fields (such as Server or Via).  Tradeoffs: As
   with HTTP Request Header Identification, the techniques used to
   identify HTTP traffic are well-known, cheap, and relatively easy to
   implement, but is made useless by HTTPS, because the response in
   HTTPS is encrypted, including headers.  The response fields are also
   less helpful for identifying content than request fields, as Server
   could easily be identified using HTTP Request Header identification,
   and Via is rarely relevant.  HTTP Response censorship mechanisms
   normally let the first n packets through while the mirrored traffic
   is being processed; this can let a page momentarily load before
   blocking mechanisms kick in; giving the user a very clear indication
   that the censor is actively interfering with undesirable content.
   Empirical Examples: pointing to the "smoking-gun" examples in
   response header identification is difficult for the same reasons
   identifying requests is difficult.  The best targeted evidence comes
   from a 2010 study conducted by Jong Park at the University of New
   Mexico.  The study strongly indicates HTTP Response Header
   Identification was being used as a censorship identification
   technique in China from August 2008-January 2009 [ref-4].

3.2.3.  Search Engine Keyword Identification

   While technically similar to a HTTP request filter, the pervasiveness
   of search engines blacklisting search terms warrants its own
   attention.  Search Engine Keyword Identification differentiates
   itself from other keyword identification techniques by being
   controlled by the company managing the search engine.  Identification
   can be regional or worldwide.  Implementation is occasionally
   voluntary, but normally is based on laws and regulations of the
   country a search engine is operating in.  The keyword blacklists are

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   most likely maintained by the search engine provider.  Tradeoffs:
   Search Engine Keyword Identification is an inconvenience as opposed
   to a hard block.  As around half of all web traffic comes from search
   [ref-5], disrupting the flow of users to undesirable content is an
   effective method to redirect non-dedicated, curious users to less
   subversive content.  It is also likely an effective method at
   encouraging self-censorship (see below) around the blocked content.
   Empirical Examples: Search Engine Keyword Identification is one of
   the easiest mechanisms to detect given the clear indicators, such as
   a specialized or blank results, paired with a trivial enumeration
   mechanism.  China requires search engine providers to "voluntarily"
   maintain search term blacklists to acquire/keep an ICP license
   [ref-6].  It is clear these blacklists are maintained by each search
   engine provider based on the slight variations in the intercepted
   searches [ref-7][ref-8].  The United Kingdom has been pushing search
   engines to self censor with the threat of litigation if they don't do
   it themselves: Google and Microsoft have agreed to block more than
   100,00 queries in U.K. to help combat abuse [ref-9][ref-10].

3.2.4.  Deep Packet Inspection (DPI) Identification

   Deep Packet Inspection has become computationally feasible as a
   censorship mechanism in the past 5 years [ref-11].  DPI differs from
   other filtration techniques in that it examine the application "data"
   section of traversing packets as opposed to only the header.  To
   prevent substantial QoS impacts, DPI normally works by splitting the
   traffic, using either a mirror switch or fiber splitter, and
   analyzing a copy of the traffic.  Keyword identification is often
   times used to flag undesirable content.  Tradeoffs: While DPI can be
   employed across entire networks, it is one of the most expensive
   technical filtration mechanisms to implement and avoiding a large
   impact to QoS is difficult [ref-12].  Often times a targeted approach
   proves itself more feasible.  Any encryption on the application
   level, such as HTTPS, also makes DPI useless as a censorship
   technique as the content typically analyzed is encrypted in this
   case.  DPI, when paired with a keyword filter, can cause major
   overblocking problems if used indiscriminately.  Empirical Evidence:
   Identifying deep packet inspection censorship is non-trivial; one
   must be sure that the undesirable content being filtered isn't being
   caught by simpler mechanisms before claiming more advanced DPI
   techniques are being used.  The Tor project claims that China, Iran,
   Ethiopia, and others must being using DPI to block the obsf2 protocol
   [ref-13].  Malaysia has been accused of using target DPI, paired with
   DDoS, to identify and subsequently knockout pro-opposition material
   [ref-14].  It also seems likely that organizations not so worried
   about blocking content in real-time could use DPI to sort and
   categorically search gathered traffic using technologies such as
   NarusInsight [ref-15].

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3.3.  Transport Layer

3.3.1.  TCP/IP Header Identification

   TCP/IP Header Identification is the most pervasive, reliable, and
   predictable type of identification.  TCP/IP headers contain a few
   invaluable pieces of information that must be transparent for traffic
   to be successfully routed: destination and source IP address and
   port.  Destination and Source IP are doubly useful, as not only does
   it allow a cto block undesirable content via IP blacklisting, but
   also allows a censor to identify the IP of the user making the
   request.  Port is useful for whitelisting certain applications or
   forcing an HTTP proxy for non-technical users.  Trade-offs: This
   method of filtration is popular due to its simplicity, relative
   cheapness, and wide availability.  It is trivial to implement a
   filtration mechanism at the Backbone, ISP, or Institutional level
   that compares the IP address of a packet with a blacklist of IP
   addresses.  IP blocking is relatively crude, often leading to
   overblocking, and one of the simplest to circumvent via VPN or proxy
   as those either mask transport protocol within a tunnel or reroute
   data that might have been blocked otherwise.  Port blocking is semi-
   effective at best.  A censor can block communication on the default
   port of an undesirable application (for example uTorrent defaults to
   32459), but almost all applications allow the user to change ports.
   Port whitelisting, where a censor only allow communication on
   approved ports, such as 80 for HTTP traffic, is more often used.
   This identification mechanism is often used in conjunction with HTTP
   Identification.  Empirical Examples: TCP/IP Header Identification is
   pervasive.  Some form of TCP/IP Header Identification is used by
   most, if not all, ISP and backbone censors.  Any time an IP blacklist
   is being used, TCP/IP Header Identification is probably the technique
   being used to match the request against the blacklist.  The examples
   of TCP/IP Header Identification are too numerous to enumerate in any
   meaningful way.

3.3.2.  Protocol Identification

   Protocol identification is a network analysis technique where one
   attempts to identify the protocols being used based on a variety of
   traffic characteristics.  There have been a number well documented
   cases where traffic identification has been used to filter
   undesirable protocols.  A very simple example of traffic
   identification would be to recognize all TCP traffic over port 80 as
   HTTP, but much more sophisticated methods, such as analyzing
   statistical properties of payload data and flow behavior, have been
   used [ref-16][ref-17].  Trade-offs: Protocol Identification
   necessarily only provides insight into the way information is
   traveling, and not the information itself.  This can lead to massive

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   overblocking problems if used with popular protocols.  Most often
   undesirable protocols are those which can be used to transmit
   information that is otherwise hard to analyze or considered to likely
   cary undesirable information; VoIP, P2P, SSL, and Tor have all been
   targets of protocol identification in the past.  As statistical
   analysis is used, the methods tend to be expensive, both
   computationally and financially, and are occasionally imprecise and
   under-filter obfuscated protocols.  Empirical Examples: Protocol
   Identification is easy to prove given the ubiquitous nature of the
   throttling/interruption; If only a specific protocol(s) are being
   prevented, then Protocol Identification is the most likely culprit.
   Iran censors have used Protocol Identification to identify and
   throttle SSH traffic by such a large amount as to make it unusable
   [ref-18].  The method used by censors in China to identify Tor
   connections could also be viewed as a type of Protocol
   Identification[ref-19].  Protocol Identification has also been used
   by industry from traffic management, such as the 2007 case where
   Comcast in the United States was using RST injection to interrupt
   BitTorrent Traffic [ref-20].

4.  Technical Prevention

4.1.  Packet Dropping

   Packet dropping is a simple mechanism to prevent undesirable traffic.
   The censor identifies undesirable traffic and chooses to not properly
   forward any packets it sees associated with the traversing
   undesirable traffic instead of following a normal routing protocol.
   This can be paired with any of the previously described mechanisms so
   long as the censor knows the user must route traffic through a
   controlled router.  Trade offs: Packet Dropping is most successful
   when every traversing packet has transparent information linked to
   undesirable content, such as a Destination IP.  One downside Packet
   Dropping suffers from is the necessity of overblocking all content
   from otherwise allowable IP's based on a single subversive sub-
   domain; blogging services and github repositories are good examples.
   China famously dropped all github packets for three days based on a
   single repository hosting undesirable content [ref-21].  The need to
   inspect every traversing packet in close to real time also makes
   Packet Dropping somewhat challenging from a QoS perspective.
   Empirical Examples: Packet Dropping is a very common form of
   technical prevention and lends itself to accurate detection given the
   unique nature of the time-out requests it leaves in its wake.  The
   Great Firewall of China uses packet dropping as one of its primary
   mechanisms of technical censorship [ref-22].  Iran also uses Packet
   Dropping as the mechanisms for throttling SSH [ref-23].  These are
   but two examples of a ubiquitous censorship practice.

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4.2.  RST Packet Injection

   Packet injection, generally, refers to a MITM network interference
   technique that spoofs packets in an established traffic stream.  RST
   packets are normally used to let one side of TCP connection know the
   other side has stopped sending information, and thus the receiver
   should close the connection.  RST Packet Injection is a specific type
   of packet injection attack that is used to interrupt an established
   stream by sending RST packets to both sides of a TCP connection; as
   each receiver thinks the other has dropped the connection, the
   session is terminated.  Trade-offs: RST Packet Injection has a few
   advantages that make it extremely popular is a censorship technique.
   RST Packet Injection is an out-of-band prevention mechanism, allowing
   the avoidance of the the QoS bottleneck one can encounter with inline
   techniques such as Packet Dropping.  This out-of-band property allows
   a censor to inspect a copy of the information, usually mirrored by an
   optical splitter, making it an ideal pairing for DPI and Protocol
   Identification[ref-24].  RST Packet Injection also has the advantage
   of only requiring one of the two endpoints to accept the spoofed
   packet for the connection to be interrupted[ref-25].  The difficult
   part of RST Packet Injection is spoofing "enough" correct information
   to ensure one end-point accepts a RST packet as legitimate; this
   generally implies a correct IP, port, and sequence number.  Sequence
   number is the hardest to get correct, as RFC 793 specifies an RST
   Packet should be in-sequence to be accepted, although the RFC also
   recommends allowing in-window packets as "good enough"[ref-26].  This
   in-window recommendation is important, as if it is implement it
   allows for successful Blind RST Injection attacks[ref-27].  When in-
   window sequencing is allowed, It is trivial to conduct a Blind RST
   Injection, a blind injection implies the censor doesn't know any
   sensitive (encrypted) sequencing information about the TCP stream
   they are injecting into, they can simply enumerate the ~70000
   possible windows; this is particularly useful for interrupting
   encrypted/obfuscated protocols such as SSH or Tor. RST Packet
   Injection relies on a stateful network, making it useless against UDP
   connections.  RST Packet Injection is among the most popular
   censorship techniques used today given its versatile nature and
   effectiveness against all types of TCP traffic.  Empirical Examples:
   RST Packet Injection, as mentioned above, is most often paired with
   identification techniques that require splitting, such as DPI or
   Protocol Identification.  In 2007 Comcast was accused of using RST
   Packet Injection to interrupt traffic it identified as BitTorrent
   [ref-28], this later led to a FCC ruling against Comcast [ref-29].
   China has also been known to use RST Packet Injection for censorship
   purposes.  This prevention is especially evident in the interruption
   of encrypted/obfuscated protocols, such as those used by Tor

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4.3.  DNS Cache Poisoning

   DNS Cache Poisoning refers to a mechanism where a censor interferes
   with the response sent by a DNS resolver to the requesting device by
   injecting an alternative IP address on the return path.  Cache
   poisoning occurs after the requested site's name servers resolve the
   request and attempt to forward the IP back to the requesting device;
   on the return route the resolved IP is recursively cached by each DNS
   servers that initially forwarded the request.  During this caching
   process if an undesirable keyword is recognized, the resolved IP is
   poisoned and an alternative IP is returned.  These alternative IP's
   usually direct to a nonsense domain or a warning page[ref-31].
   Alternatively, Iranian censorship appears to prevent the
   communication en-route, preventing a response from ever being
   sent[ref-32].  Trade-offs: DNS Cache Poisoning is one of the rarer
   forms of prevention due to a number of shortcomings.  DNS Cache
   Poisoning requires the censor to force a user to traverse a
   controlled DNS resolver for the mechanism to be effective, it is
   easily circumvented by a technical savvy user that opts to use
   alternative DNS resolvers, such as the public DNS
   resolvers provided by Google.  DNS Cache Poisoning also implies
   returning an incorrect IP to those attempting to resolve a domain
   name, but the site is still technically unblocked if the user has
   another method to acquire the IP address of the desired site.
   Blocking overflow has also been a problem, as occasionally users
   outside of the censors region will be directed through a DNS server
   controlled by a censor, causing the request to fail.  The ease of
   circumvention paired with the large risk of overblocking and blocking
   overflow make DNS Cache Poisoning a partial, difficult, and less than
   ideal censorship mechanism.  Empirical Evidence: DNS Cache Poisoning,
   when properly implemented, is easy to identify based on the
   shortcomings identified above.  Turkey relied on DNS Cache Poisoning
   for its country-wide block of websites such Twitter and Youtube for
   almost week in March of 2014 but the ease of circumvention resulted
   in an increase in the popularity of Twitter until Turkish ISP's
   implementing an IP blacklist to achieve the governmental
   mandate[ref-33].  To drive proverbial "nail in the coffin" Turkish
   ISPs started hijacking all requests to Google and Level 3's
   international DNS resolvers [ref-34].  DNS Cache Poisoning, when
   incorrectly implemented, has as has resulted in some of the largest
   "censorship disasters".  In January 2014 China started directing all
   requests passing through the Great Fire Wall to a single domain,, due to an improperly configured DNS Cache Poisoning
   attempt; this incident is thought to be the largest internet-service
   outage in history [ref-35][ref-36].  Countries such as China, Iran,
   Turkey, and the United States have discussed blocking entire TLDs as
   well, but only Iran has acted by blocking all Israeli (.il) domains

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4.4.  Distributed Denial of Service (DDoS)

   Distributed Denial of Service attacks are a common attack mechanism
   used by "hacktivists" and black-hat hackers, but censors have used
   DDoS in the past for a variety of reasons.  There is a huge variety
   of DDoS attacks[ref-38], but on a high level two possible impacts
   tend to occur; a flood attack results in the service being unusable
   while resources are being spent to flood the service, a crash attack
   aims to crash the service so resources can be reallocated elsewhere
   without "releasing" the service.  Trade-offs: DDoS is an appealing
   mechanism when a censor would like to prevent all access to
   undesirable content, instead of only access in their region for a
   limited period of time, but this is really the only uniquely
   beneficial feature for DDoS as a censorship technique.  The resources
   required to carry out a successful DDoS against major targets are
   computationally expensive, usually requiring renting or owning a
   malicious distributed platform such as a botnet, and imprecise.  DDoS
   is an incredibly crude censorship technique, and appears to largely
   be used as a timely, easy-to-access mechanism for blocking
   undesirable content for a limited period of time.  Empirical
   Examples: In 2012 the U.K.'s GCHQ used DDoS to temporarily shutdown
   IRC chat rooms frequented by members of Anonymous using the Syn Flood
   DDoS method; Syn Flood exploits the handshake used by TCP to overload
   the victim server with so many requests that legitimate traffic
   becomes slow or impossible [ref-39][ref-40].  Dissenting opinion
   websites are frequently victims of DDoS around politically sensitive
   events in Burma [ref-41].  Controlling parties in Russia[ref-42],
   Zimbabwe[ref-43], and Malaysia[ref-44] have been accused of using
   DDoS to interrupt opposition support and access during elections.

4.5.  Network Disconnection or Adversarial Route Announcement

   Network Disconnection or Adversarial Route Announcement The crudest
   of all censorship techniques, there is no more effective way of
   making sure undesirable information isn't allowed to propagate on the
   web than by shutting off the network.  The network can be cut off in
   a region when a censoring body withdraws all of the BGP prefixes
   routing through the censor's country.  Trade-offs: The impact to a
   network disconnection in a region is huge and absolute; the censor
   pays for absolute control over digital information with all the
   benefits the internet brings; this is never a long-term solution for
   any rational censor and is normally only used as a last resort in
   times of substantial unrest.  Empirical Examples: Network
   Disconnections tend to only happen in times of substantial unrest,
   largely due to the huge social, political, and economic impact such a
   move has.  One of the first, highly covered occurrences was with the
   Junta in Myanmar employing Network Disconnection to help Junta forces
   quash a rebellion in 2007 [ref-45].  China disconnected the network

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   in the Xinjiang region during unrest in 2009 in an effort to prevent
   the protests from spreading to other regions [ref-46].  The Arab
   Spring saw the the most frequent usage of Network Disconnection, with
   events in Egypt and Libya in 2011 [ref-47][ref-48], and Syria in 2012

5.  Non-Technical Aggregation

   As the name implies, sometimes manpower is the easiest way to figure
   out which content to block.  Manual Filtering differs from the common
   tactic of building up blacklists in that is doesn't necessarily
   target a specific IP or DNS, but instead removes or flags content.
   Given the imprecise nature of automatic filtering, manually sorting
   through content and flagging dissenting websites, blogs, articles and
   other media for filtration can be an effective technique.  This
   filtration can occur on the Backbone/ISP level, China's army of
   monitors is a good example [ref-50]; more commonly manual filtering
   occurs on an institutional level.  ICP's, such as Google or Weibo,
   require a business license to operate in China.  One of the
   prerequisites for a business license is an agreement to sign a
   "voluntary pledge" known as the "Public Pledge on Self-discipline for
   the Chinese Internet Industry".  The failure to " energetically
   uphold" the pledged values can lead to the ICP's being held liable
   for the offending content by the Chinese government [ref-51].

6.  Non-Technical Prevention

6.1.  Self Censorship

   Self censorship is one of the most interesting and effective types of
   censorship; a mix of Bentham's Panopticon, cultural manipulation,
   intelligence gathering, and meatspace enforcement.  Simply put, self
   censorship is when a censor creates an atmosphere where users censor
   themselves.  This can be achieved through controlling information,
   intimidating would-be dissidents, swaying public thought, and
   creating apathy.  Self censorship is difficult to document, as when
   it is implemented effectively the only noticeable tracing is a lack
   of undesirable content; instead one must look at the tools and
   techniques used by censors to encourage self-censorship.  Controlling
   Information relies on traditional censorship techniques, or by
   forcing all users to connect through an intranet, such as in North
   Korea.  Intimidation is often achieved through allowing internet
   users to post "whatever they want", but arresting those who post
   about dissenting views, this technique is incredibly
   common[ref-52][ref-53][ref-54][ref-55][ref-56].  A good example of
   swaying public thought is China's "50-Cent Party", composed of
   somewhere between 20,000[ref-57] and 300,000[ref-58] contributors who
   are paid to "guide public thought" on local and regional issues as

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   directed by the Ministry of Culture.  Creating apathy can be a side-
   effect of successfully controlling information over time and is ideal
   for a censorship regime [ref-59].

6.2.  Domain Name Reallocation

   As Domain Names are resolved recursively, if a TLD deregisters a
   domain all other DNS resolvers will be unable to properly forward and
   cache the site.  Domain name registration is only really a risk where
   undesirable content is hosted on TLD controlled by the censoring
   country, such as .ch or .ru [ref-60].

6.3.  Server Takedown

   Servers must have a physical location somewhere in the world.  If
   undesirable content is hosted in the censoring country the servers
   can be physically seized or the hosting provider can be required to
   prevent access [ref-61].

7.  References

   [ref-1]    Glanville, J., ""The Big Business of Net Censorship"",
              November 2008 ,

   [ref-2]    Dalek, J., ""A Method for Identifying and Confirming the
              Use of URL Filtering Products for Censorship"", October
              2013 , <

   [ref-3]    EF, A., ""EFA Filtering Overview"", May 2009 ,

   [ref-4]    Crandall, J., ""Empirical Study of a National-Scale
              Distributed Intrusion Detection System: Backbone-Level
              Filtering of HTML Responses in China"", June 2010 ,
              < >.

   [ref-5]    Dobie, M., ""Junta Tightens Military Screws"", September
              2007 ,

   [ref-6]    Cheng, J., ""Google stops Hong Kong auto-redirect as China
              plays hardball"", June 2010, <

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   [ref-7]    Zhu, T., ""An Analysis of Chinese Search Engine
              Filtering"", July 2011 , <

   [ref-8]    Whittaker, Z., ""1,168 keywords Skype uses to censor,
              monitor its Chinese users"", March 2013 ,

   [ref-9]    News, B., ""Google and Microsoft agree steps to block
              abuse images"", November 2013 ,

   [ref-10]   Condliffe, J., ""Google Announces Massive New Restrictions
              on Child Abuse Search Terms"", November 2013 ,

   [ref-11]   Wagner, B., ""Deep Packet Inspection and Internet
              Censorship: International Convergence on an 'Integrated
              Technology of Control'"", June 2009 ,

   [ref-12]   Porter, T., ""The Perils of Deep Packet Inspection"", Oct
              2010, <

   [ref-13]   Wilde, T., ""Knock Knock Knockin' on Bridges Doors"",
              January 2012, <

   [ref-14]   Wagstaff, J., ""In Malaysia, online election battles take
              a nasty turn"", May 2013,

   [ref-15]   EFF, T., ""Hepting vs. ATand T"", Updated December,

   [ref-16]   Hjelmvik, E., "July 2010 7", "Breaking and,

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   [ref-17]   Vine, S., ""Technology Showcase on Traffic Classification:
              Why Measurements and Freeform Policy Matter"", May 2014,
              showcase-traffic-classification.pdf#page=3 >.

   [ref-18]   Anonymous, A., ""How to Bypass Comcast's Bittorrent
              Throttling"", October 2007, <

   [ref-19]   Lee, T., ""Here's how Iran censors the Internet"", August
              2013, <

   [ref-20]   Winter, P., ""How China is Blocking Tor"", April 2012,

   [ref-21]   Anonymous, A., ""GitHub blocked in China - how it
              happened, how to get around it, and where it will take
              us"", January 2013,

   [ref-22]   Ensafi, R., ""Detecting Intentional Packet Drops on the
              Internet via TCP/IP Side Channels"", December 2013,

   [ref-23]   Aryan*, A., ""Internet Censorship in Iran: A First Look"",
              August 2013 , <

   [ref-24]   Weaver, S., ""Detecting Forged TCP Packets"", June 2009 ,

   [ref-25]   Weaver, S., ""Detecting Forged TCP Packets"", June 2009 ,

   [ref-26]   Weaver, S., ""Detecting Forged TCP Packets"", June 2009 ,

   [ref-27]   Anonymous, A., ""TCP-RST Injection"", June 210 ,
              < >.

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   [ref-28]   Schoen, S., ""EFF tests agree with AP: Comcast is forging
              packets to interfere with user traffic"", October 19th,,

   [ref-29]   VonLohmann, F., ""FCC Rules Against Comcast for BitTorrent
              Blocking"", August 3rd,,

   [ref-30]   Phillip Winter, S., ""How China Is Blocking Tor"", April
              2nd,, <>.

   [ref-31]   DNS, V., ""DNS Cache Poisoning in the People's Republic of
              China"", September 6th, <

   [ref-32]   Aryan*, A., ""Internet Censorship in Iran: A First Look"",
              August 2013 , <

   [ref-33]   Zmijewki, E., ""Turkish Internet Censorship Takes a New
              Turn"", March 2014, <

   [ref-34]   Zmijewki, E., ""Turkish Internet Censorship Takes a New
              Turn"", March 2014, <

   [ref-35]   AFP, .A., ""China Has Massive Internet Breakdown
              Reportedly Caused By Their Own Censoring Tools"", January
              2014, <

   [ref-36]   Anonymous, A., ""The Collateral Damage of Internet
              Censorship by DNS Injection"", July 2012 ,

   [ref-37]   Albert, K., ""DNS Tampering and the new ICANN gTLD
              Rules"", June 2011, <

   [ref-38]   Anonymous, A., ""Denial of Service Attacks (Wikipedia)"",
              N/A N/A, <

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   [ref-39]   Esposito, S., ""Snowden Docs Show UK Spies Attacked
              Anonymous, Hackers"", February 2014,

   [ref-40]   CMU, .C., ""TCP SYN Flooding and IP Spoofing Attacks"",
              November 2000, <

   [ref-41]   Villeneuve, N., ""Open Access: Chapter 8, Control and
              Resistance, Attacks on Burmese Opposition Media"",
              December 2011 , <

   [ref-42]   Kravtsova, Y., ""Cyberattacks Disrupt Opposition's
              Election"", October 2012,

   [ref-43]   Orion, E., ""Zimbabwe election hit by hacking and DDoS
              attacks"", August 2013,

   [ref-44]   Muncaster, P., ""Malaysian election sparks web blocking/
              DDoS claims"", May 2013,

   [ref-45]   Dobie, M., ""Junta tightens media screw"", September 2007,

   [ref-46]   Heacock, R., ""China Shuts Down Internet in Xinjiang
              Region After Riots"", July 2009,

   [ref-47]   Cowie, J., ""Egypt Leaves the Internet"", January 2011,

   [ref-48]   Cowie, J., ""Libyan Disconnect"", February 2011,

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   [ref-49]   Thomson, I., ""Syria Cuts off Internet and Mobile
              Communication"", November 2012,

   [ref-50]   News, B., ""China employs two million microblog monitors
              state media say"", October 2013,

   [ref-51]   MacKinnon, R., ""'Race to the Bottom' Corporate Complicity
              in Chinese Internet Censorship"", August 2006 ,

   [ref-52]   Calamur, K., ""Prominent Egyptian Blogger Arrested"",
              November 2013, <

   [ref-53]   Press, A., ""Sattar Beheshit, Iranian Blogger, Was Beaten
              In Prison According To Prosecutor"", December 2012,

   [ref-54]   Hopkins, C., ""Communications Blocked in Libya, Qatari
              Blogger Arrested: This Week in Online Tyranny"", March
              2011, <

   [ref-55]   Gaurdian, T., ""Chinese blogger jailed under crackdown on
              'internet rumours'"", April 2014,

   [ref-56]   Johnson, L., ""Torture feared in arrest of Iraqi
              blogger"", Febuary 2010,

   [ref-57]   Bristow, M., ""China's internet 'spin doctors'"", November

   [ref-58]   Fareed, M., ""China joins a turf war"", September 2008,

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   [ref-59]   Gao, H., ""Tiananmen, Forgotten"", June 2014,

   [ref-60]   Anderson, R., ""Access Denied: Tools and Technology of
              Internet Filtering"", December 2011 ,

   [ref-61]   Murdoch, S., ""Access Denied: Tools and Technology of
              Internet Filtering"", December 2011 ,

Authors' Addresses

   Joeseph L. Hall
   Center for Democracy and Technology


   Michael D. Aaron
   Center for Democracy and Technology


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