Permalink
Fetching contributors…
Cannot retrieve contributors at this time
1457 lines (1096 sloc) 65.3 KB
Network Working Group J. Hall
Internet-Draft CDT
Intended status: Informational M. Aaron
Expires: September 19, 2016 CU Boulder
B. Jones
GA Tech
March 18, 2016
A Survey of Worldwide Censorship Techniques
draft-hall-censorship-tech-02
Abstract
This document describes the technical mechanisms used by censorship
regimes around the world to block or impair 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 19, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Hall, et al. Expires September 19, 2016 [Page 1]
Internet-Draft draft-hall-censorship-tech March 2016
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Technical Prescription . . . . . . . . . . . . . . . . . . . 3
3. Technical Identification . . . . . . . . . . . . . . . . . . 3
3.1. Points of Control . . . . . . . . . . . . . . . . . . . . 3
3.2. Application Layer . . . . . . . . . . . . . . . . . . . . 5
3.2.1. HTTP Request Header Identification . . . . . . . . . 5
3.2.2. HTTP Response Header Identification . . . . . . . . . 6
3.2.3. Instrumenting Content Providers . . . . . . . . . . . 6
3.2.4. Deep Packet Inspection (DPI) Identification . . . . . 7
3.2.5. Server Name Indication . . . . . . . . . . . . . . . 8
3.3. Transport Layer . . . . . . . . . . . . . . . . . . . . . 9
3.3.1. TCP/IP Header Identification . . . . . . . . . . . . 9
3.3.2. Protocol Identification . . . . . . . . . . . . . . . 10
4. Technical Interference . . . . . . . . . . . . . . . . . . . 11
4.1. Performance Degradation . . . . . . . . . . . . . . . . . 11
4.2. Packet Dropping . . . . . . . . . . . . . . . . . . . . . 11
4.3. RST Packet Injection . . . . . . . . . . . . . . . . . . 12
4.4. DNS Interference . . . . . . . . . . . . . . . . . . . . 13
4.5. Distributed Denial of Service (DDoS) . . . . . . . . . . 14
4.6. Network Disconnection or Adversarial Route Announcement . 15
5. Non-Technical Prescription . . . . . . . . . . . . . . . . . 16
6. Non-Technical Interference . . . . . . . . . . . . . . . . . 16
6.1. Self Censorship . . . . . . . . . . . . . . . . . . . . . 16
6.2. Domain Name Reallocation . . . . . . . . . . . . . . . . 17
6.3. Server Takedown . . . . . . . . . . . . . . . . . . . . . 17
6.4. Notice and Takedown . . . . . . . . . . . . . . . . . . . 17
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17
8. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Censorship is where an entity in a position of power - such as a
government, organization, or individual - suppresses communication
that it considers objectionable, harmful, sensitive, politically
incorrect or inconvenient. (While censors that engage in censorship
or establish censorship regimes must do so through legal, military,
or other sources of power, this document focuses largely on technical
mechanisms used to achieve network censorship.)
Hall, et al. Expires September 19, 2016 [Page 2]
Internet-Draft draft-hall-censorship-tech March 2016
This document describes the technical mechanisms used by censorship
regimes around the world to block or impair Internet traffic (see
[RFC7754] for a discussion of Internet blocking and filtering in
terms of Internet architecture). To that end, we describe three
elements of Internet censorship: prescription, identification, and
interference. Prescription is the process by which censors determine
what types of material they should block, i.e. they decide to block a
list of pornographic websites. Identification is the process by
which censors classify specific traffic to be blocked or impaired,
i.e. the censor blocks or impairs all webpages containing "sex" in
the title or traffic to sex.com. Interference is the process by
which the censor intercedes in communication and prevents access to
censored materials by blocking access or impairing the connection.
2. Technical Prescription
Prescription is the process of figuring out what censors would like
to block [Glanville-2008]. Generally, censors aggregate "to block"
information in blacklists or using real-time heuristic assessment of
content [Ding-1999]. There are typically three types 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,
which allows filtering from broad categories, such as gambling or
pornography, that they would like to block. In these cases, the
private services embark on an attempt to label every semi-
questionable website as to allow for metatag blocking (similarly,
they tune real-time content heuristic systems to map their
assessments onto categories of objectionable content). 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
Internet 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
Hall, et al. Expires September 19, 2016 [Page 3]
Internet-Draft draft-hall-censorship-tech March 2016
interception mechanism located somewhere, logically or physically,
implicates various 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
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.
o Services: Application service providers can be pressured, coerced,
or legally required to censor specific content or flows of data.
Service providers naturally face incentives to maximize their
potential customer base and potential service shutdowns or legal
liability due to censorship efforts may seem much less attractive
than potentially excluding content, users, or uses of their
service.
o Certificate Authorities: Authorities that issue cryptographically
secured resources can be a significant point of control.
Certificate Authorities that issue certificates to domain holders
for TLS/HTTPS or Regional/Local Internet Registries that issue
Route Origination Authorizations to BGP operators can be forced to
issue rogue certificates that may allow compromises in
confidentiatlity guarantees - allowing censorship software to
engage in identification and interference where not possible
before - or integrity degrees - allowing, for example, adversarial
routing of traffic.
Hall, et al. Expires September 19, 2016 [Page 4]
Internet-Draft draft-hall-censorship-tech March 2016
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 impair
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
An HTTP header contains a lot of useful information for traffic
identification; although host is the only required field in an HTTP
request header (for HTTP/1.1 and later), an 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. A
censor 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 (see
Section 3.3.1) 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 (see Section 3.3.1) is
necessary for filtering of HTTPS. However, some countermeasures such
as URL obfuscation [RSF-2005] can trivially defeat simple forms of
HTTP Request Header Identification.
Empirical Examples: Studies exploring censorship mechanisms have
found evidence of HTTP header/ URL filtering in many countries,
including Bangladesh, Bahrain, China, India, Iran, Malaysia,
Pakistan, Russia, Saudi Arabia, South Korea, Thailand, and Turkey
[Verkamp-2012] [Nabi-2013] [Aryan-2012]. Commercial technologies
such as the McAfee SmartFilter and NetSweeper are often purchased by
censors [Dalek-2013]. These commercial technologies use a
combination of HTTP Request Identification and TCP/IP Header
Identification to filter specific URLs. Dalek et al. and Jones et
al. identified the use of these products in the wild [Dalek-2013]
[Jones-2014].
Hall, et al. Expires September 19, 2016 [Page 5]
Internet-Draft draft-hall-censorship-tech March 2016
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.
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 may allow
some content through and the user may be able to detect that the
censor is actively interfering with undesirable content.
Empirical Examples: In 2009, Jong Park et al. at the University of
New Mexico demonstrated that the Great Firewall of China (GFW) used
this technique [Crandall-2010]. However, Jong Park et al. found that
the GFW discontinued this practice during the course of the study.
Due to the overlap in HTTP response filtering and keyword filtering
(see Section 3.2.3), it is likely that most censors rely on keyword
filtering over TCP streams instead of HTTP response filtering.
3.2.3. Instrumenting Content Providers
In addition to censorship by the state, many governments pressure
content providers to censor themselves. Due to the extensive reach
of government censorship, we need to define content provider as any
service that provides utility to users, including everything from web
sites to locally installed programs. The defining factor of keyword
identification by content providers is the choice of content
providers to detect restricted terms on their platform. The terms to
look for may be provided by the government or the content provider
may be expected to come up with their own list.
Tradeoffs: By instrumenting content providers to identify restricted
content, the censor can gain new information at the cost of political
capital with the companies it forces or encourages to participate in
censorship. For example, the censor can gain insight about the
content of encrypted traffic by coercing web sites to identify
restricted content, but this may drive away potential investment.
Coercing content providers may encourage self censorship, an
additional advantage for censors. The tradeoffs for instrumenting
Hall, et al. Expires September 19, 2016 [Page 6]
Internet-Draft draft-hall-censorship-tech March 2016
content providers are highly dependent on the content provider and
the requested assistance.
Empirical Examples: Researchers have discovered keyword
identification by content providers on platforms ranging from instant
messaging applications [Senft-2013] to search engines [Rushe-2015]
[Cheng-2010] [Whittaker-2013] [BBC-2013] [Condliffe-2013]. To
demonstrate the prevalence of this type of keyword identification, we
look to search engine censorship.
Search engine censorship demonstrates keyword identification by
content providers and 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 most likely maintained by the search engine
provider. China requires search engine providers to "voluntarily"
maintain search term blacklists to acquire/keep an Internet content
provider (ICP) license [Cheng-2010]. It is clear these blacklists
are maintained by each search engine provider based on the slight
variations in the intercepted searches [Zhu-2011] [Whittaker-2013].
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 [BBC-2013] [Condliffe-2013].
Depending on the output, search engine keyword identification may be
difficult or easy to detect. In some cases specialized or blank
results provide a trivial enumeration mechanism, but more subtle
censorship can be difficult to detect. In February 2015, Microsoft's
search engine, Bing, was accused of censoring Chinese content outside
of China [Rushe-2015] because Bing returned different results for
censored terms in Chinese and English. However, it is possible that
censorship of the largest base of Chinese search users, China, biased
Bing's results so that the more popular results in China (the
uncensored results) were also more popular for Chinese speakers
outside of China.
3.2.4. Deep Packet Inspection (DPI) Identification
Deep Packet Inspection has become computationally feasible as a
censorship mechanism in recent years [Wagner-2009]. Unlike other
techniques, DPI reassembles network flows to examine the application
"data" section, as opposed to only the header, and is therefore often
used for keyword identification. DPI also differs from other
identification technologies because it can leverage additional packet
and flow characteristics, i.e. packet sizes and timings, to identify
content. To prevent substantial quality of service (QoS) impacts,
DPI normally analyzes a copy of data while the original packets
Hall, et al. Expires September 19, 2016 [Page 7]
Internet-Draft draft-hall-censorship-tech March 2016
continue to be routed. Typically, the traffic is split using either
a mirror switch or fiber splitter, and analyzed on a cluster of
machines running Intrusion Detection Systems (IDS) configured for
censorship.
Tradeoffs: DPI is one of the most expensive identification mechanisms
and can have a large QoS impact [Porter-2010]. When used as a
keyword filter for TCP flows, DPI systems can cause also major
overblocking problems. Like other techniques, DPI is less useful
against encrypted data, though DPI can leverage unencrypted elements
of an encrypted data flow (e.g., the Server Name Indicator (SNI) sent
in the clear for TLS) or statistical information about an encrypted
flow (e.g., video takes more bandwidth than audio or textual forms of
communication) to identify traffic.
Despite these problems, DPI is the most powerful identification
method and is widely used in practice. The Great Firewall of China
(GFW), the largest censorship system in the world, uses DPI to
identify restricted content over HTTP and DNS and inject TCP RSTs and
bad DNS responses, respectively, into connections [Crandall-2010]
[Clayton-2006] [Anonymous-2014].
Empirical Evidence: Several studies have found evidence of DPI being
used to censor content and tools. Clayton et al. Crandal et al.,
Anonymous, and Khattak et al., all explored the GFW and Khattak et
al. even probed the firewall to discover implementation details like
how much state it stores [Crandall-2010] [Clayton-2006]
[Anonymous-2014] [Khattak-2013]. The Tor project claims that China,
Iran, Ethiopia, and others must have used DPI to block the obsf2
protocol [Wilde-2012]. Malaysia has been accused of using targeted
DPI, paired with DDoS, to identify and subsequently knockout pro-
opposition material [Wagstaff-2013]. 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 [Hepting-2011].
3.2.5. Server Name Indication
In encrypted connections using Transport Layer Security (TLS), there
may be servers that host multiple "virtual servers" at a give network
address, and the client will need to specify in the (unencrypted)
Client Hello message which domain name it seeks to connect to (so
that the server can respond with the appropriate TLS certificate)
using the Server Name Indication (SNI) TLS extension [RFC6066].
Since SNI is sent in the clear, censors and filtering software can
use it as a basis for blocking, filtering, or impairment by dropping
connections to domains that match prohibited content (e.g.,
bad.foo.com may be censored while good.foo.com is not) [Shbair-2015].
Hall, et al. Expires September 19, 2016 [Page 8]
Internet-Draft draft-hall-censorship-tech March 2016
Tradeoffs: Some clients do not send the SNI extension (e.g., clients
that only support versions of SSL and not TLS) or will fall back to
SSL if a TLS connection fails, rendering this method ineffective. In
addition, this technique requires deep packet inspection techniques
that can be computationally and infrastructurally expensive and
improper configuration of an SNI-based block can result in
significant overblocking, e.g., when a second-level domain like
google.com is inadvertently blocked.
Empirical Evidence: While there are many examples of security firms
that offer SNI-based filtering [Trustwave-2015] [Sophos-2015]
[Shbair-2015], the authors currently know of no specific examples or
reports of SNI-based filtering observed in the field used for
censorship purposes.
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 censor to 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.
Trade-offs: TCP/IP identification is popular due to its simplicity,
availability, and robustness.
TCP/IP identification is trivial to implement, but is difficult to
implement in backbone or ISP routers at scale, and is therefore
typically implemented with DPI. Blacklisting an IP is equivalent to
installing a /32 route on a router and due to limited flow table
space, this cannot scale beyond a few thousand IPs at most. IP
blocking is also relatively crude, leading to overblocking, and
cannot deal with some services like Content Distribution Networks
(CDN), that host content at hundreds or thousands of IP addresses.
Despite these limitations, IP blocking is extremely effective because
the user needs to proxy their traffic through another destination to
circumvent this type of identification.
Port-blocking is generally not useful because many types of content
share the same port and it is possible for censored applications to
change their port. For example, most HTTP traffic goes over port 80,
so the censor cannot differentiate between restricted and allowed
content solely on the basis of port. Port whitelisting is
Hall, et al. Expires September 19, 2016 [Page 9]
Internet-Draft draft-hall-censorship-tech March 2016
occasionally used, where a censor limits communication to approved
ports, such as 80 for HTTP traffic and is most effective when used in
conjunction with other identification mechanisms. For example, a
censor could block the default HTTPS port, port 443, thereby forcing
most users to fall back to HTTP.
3.3.2. Protocol Identification
Censors sometimes identify entire protocols to be blocked using a
variety of traffic characteristics. For example, Iran impairs the
performance of HTTPS traffic, a protocol that prevents further
analysis, to encourage users to switch to HTTP, a protocol that they
can analyze [Aryan-2012]. A simple protocol identification would be
to recognize all TCP traffic over port 443 as HTTPS, but more
sophisticated analysis of the statistical properties of payload data
and flow behavior, would be more effective, even when port 443 is not
used [Hjelmvik-2010] [Sandvine-2014].
If censors can detect circumvention tools, they can block them, so
censors like China are extremely interested in identifying the
protocols for censorship circumvention tools. In recent years, this
has devolved into an arms race between censors and circumvention tool
developers. As part of this arms race, China developed an extremely
effective protocol identification technique that researchers call
active probing or active scanning.
In active probing, the censor determines whether hosts are running a
circumvention protocol by trying to initiate communication using the
circumvention protocol. If the host and the censor successfully
negotiate a connection, then the censor conclusively knows that host
is running a circumvention tool. China has used active scanning to
great effect to block Tor [Winter-2012].
Trade-offs: Protocol Identification necessarily only provides insight
into the way information is traveling, and not the information
itself.
Protocol identification is useful for detecting and blocking
circumvention tools, like Tor, or traffic that is difficult to
analyze, like VoIP or SSL, because the censor can assume that this
traffic should be blocked. However, this can lead to overblocking
problems when used with popular protocols. These methods are
expensive, both computationally and financially, due to the use of
statistical analysis, and can be ineffective due to its imprecise
nature.
Empirical Examples: Protocol identification can be easy to detect if
it is conducted in real time and only a particular protocol is
Hall, et al. Expires September 19, 2016 [Page 10]
Internet-Draft draft-hall-censorship-tech March 2016
blocked, but some types of protocol identification, like active
scanning, are much more difficult to detect. Protocol identification
has been used by Iran to identify and throttle SSH traffic to make it
unusable [Anonymous-2007] and by China to identify and block Tor
relays [Winter-2012]. Protocol Identification has also been used for
traffic management, such as the 2007 case where Comcast in the United
States used RST injection to interrupt BitTorrent Traffic
[Winter-2012].
4. Technical Interference
4.1. Performance Degradation
While other interference techniques outlined in this section mostly
focus on blocking or preventing access to content, it can be an
effective censorship strategy in some cases to not entirely block
access to a given destination, or service but instead degrade the
performance of the relevant network connection. The resulting user
experience for a site or service under performance degradation can be
so bad that users opt to use a different site, service, or method of
communication, or may not engage in communication at all if there are
no alternatives. Traffic shaping techniques that rate-limit the
bandwidth available to certain types of traffic is one example of a
performance degradation.
Trade offs: While implementing a performance degradation will not
always eliminate the ability of people to access a desire resource,
it may force them to use other means of communication where
censorship (or surveillance) is more easily accomplished.
Empirical examples: Iran is known to shape the bandwidth available to
HTTPS traffic to encourage unencrypted HTTP traffic [Aryan-2012].
4.2. 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
IPs based on a single subversive sub-domain; blogging services and
Hall, et al. Expires September 19, 2016 [Page 11]
Internet-Draft draft-hall-censorship-tech March 2016
github repositories are good examples. China famously dropped all
github packets for three days based on a single repository hosting
undesirable content [Anonymous-2013]. 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 interference 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 [Ensafi-2013]. Iran also
uses Packet Dropping as the mechanisms for throttling SSH
[Aryan-2012]. These are but two examples of a ubiquitous censorship
practice.
4.3. RST Packet Injection
Packet injection, generally, refers to a man-in-the-middle (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 as a censorship technique. RST Packet Injection is
an out-of-band interference 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 [Weaver-2009] (this asynchronous version of a MITM is
often called a Man-on-the-Side (MOTS)). 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. 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 (TCP)
sequence number. Sequence number is the hardest to get correct, as
[RFC0793] specifies an RST Packet should be in-sequence to be
accepted, although the RFC also recommends allowing in-window packets
as "good enough". This in-window recommendation is important, as if
it is implemented it allows for successful Blind RST Injection
attacks [Netsec-2011]. When in-window sequencing is allowed, It is
trivial to conduct a Blind RST Injection, a blind injection implies
Hall, et al. Expires September 19, 2016 [Page 12]
Internet-Draft draft-hall-censorship-tech March 2016
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 [Schoen-2007], this later led to a US Federal
Communications Commission ruling against Comcast [VonLohmann-2008].
China has also been known to use RST Packet Injection for censorship
purposes. This interference is especially evident in the
interruption of encrypted/obfuscated protocols, such as those used by
Tor [Winter-2012].
4.4. DNS Interference
There are a variety of mechanisms that censors can use to block or
filter access to content by altering responses from the DNS
[AFNIC-2013] [ICANN-SSAC-2012], including blocking the response,
replying with an error message, or responding with an incorrect
address (potentially to a server that can communicate to the end-user
a reason for blocking access to that resource, for example using HTTP
Status Code 451 [RFC7725]).
DNS poisoning refers to a mechanism where a censor interferes with
the response sent by a DNS server to the requesting device by
injecting an alternative IP address into the response message on the
return path [ViewDNS-2011]. 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 server that initially
forwarded the request. During this caching process if an undesirable
keyword is recognized, the resolved IP is "poisoned" and an
alternative IP (or NXDOMAIN error) is returned. The alternative IPs
usually direct to a nonsense domain or a warning page.
Alternatively, Iranian censorship appears to prevent the
communication en-route, preventing a response from ever being sent
[Aryan-2012].
Trade-offs: DNS interference requires the censor to force a user to
traverse a controlled DNS hierarchy (or intervening network on which
the censor serves as a Active Pervasive Attacker [RFC7624] to rewrite
Hall, et al. Expires September 19, 2016 [Page 13]
Internet-Draft draft-hall-censorship-tech March 2016
DNS responses) for the mechanism to be effective. It can be
circumvented by a technical savvy user that opts to use alternative
DNS resolvers (such as the public DNS resolvers provided by Google or
OpenDNS) or Virtual Private Network technology. DNS poisoning also
implies returning an incorrect IP to those attempting to resolve a
domain name, but in some cases the destination may be technically
accessible; over HTTP, for example, the user may have another method
of obtaining the IP address of the desired site and may be able to
access it if the site is configured to be the default server
listening at this IP address. Blocking overflow has also been a
problem, as occasionally users outside of the censors region will be
directed through a DNS servers or DNS-rewriting network equipment
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 interference a partial, difficult, and less than
ideal censorship mechanism.
Empirical Evidence: DNS interference, when properly implemented, is
easy to identify based on the shortcomings identified above. Turkey
relied on DNS interference 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 ISPs implementing an IP blacklist to achieve
the governmental mandate [Zmijewki-2014]. Ultimately, Turkish ISPs
started hijacking all requests to Google and Level 3's international
DNS resolvers [Zmijewki-2014]. DNS interference, when incorrectly
implemented, 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,
dongtaiwang.com, due to an improperly configured DNS poisoning
attempt; this incident is thought to be the largest Internet-service
outage in history [AFP-2014] [Anon-SIGCOMM12]. 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 [Albert-2011].
4.5. 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 [Wikip-DoS], 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.
Hall, et al. Expires September 19, 2016 [Page 14]
Internet-Draft draft-hall-censorship-tech March 2016
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 [Schone-2014] [CERT-2000].
Dissenting opinion websites are frequently victims of DDoS around
politically sensitive events in Burma [Villeneuve-2011]. Controlling
parties in Russia [Kravtsova-2012], Zimbabwe [Orion-2013], and
Malaysia [Muncaster-2013] have been accused of using DDoS to
interrupt opposition support and access during elections. In 2015,
China launched a DDoS attack using a true MITM system colocated with
the Great Firewall, dubbed "Great Cannon", that was able to inject
JavaScript code into web visits to a Chinese search engine that
comandeered those user agents to send DDoS traffic to various sites
[Marczak-2015].
4.6. Network Disconnection or Adversarial Route Announcement
While it is perhaps 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 logically cut off in a region when a censoring
body withdraws all of the Boarder Gateway Protocol (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
[Dobie-2007]. China disconnected the network in the Xinjiang region
Hall, et al. Expires September 19, 2016 [Page 15]
Internet-Draft draft-hall-censorship-tech March 2016
during unrest in 2009 in an effort to prevent the protests from
spreading to other regions [Heacock-2009]. The Arab Spring saw the
the most frequent usage of Network Disconnection, with events in
Egypt and Libya in 2011 [Cowie-2011] [Cowie-2011b], and Syria in 2012
[Thomson-2012].
5. Non-Technical Prescription
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 it 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 [BBC-2013b] - but more commonly manual
filtering occurs on an institutional level. Internet Content
Providers 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 ICPs
being held liable for the offending content by the Chinese government
[BBC-2013b].
6. Non-Technical Interference
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
[Calamur-2013] [AP-2012] [Hopkins-2011] [Guardian-2014]
[Johnson-2010]. A good example of swaying public thought is China's
"50-Cent Party", composed of somewhere between 20,000 [Bristow-2013]
Hall, et al. Expires September 19, 2016 [Page 16]
Internet-Draft draft-hall-censorship-tech March 2016
and 300,000 [Fareed-2008] contributors who are paid to "guide public
thought" on local and regional issues as 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 [Gao-2014].
6.2. Domain Name Reallocation
As Domain Names are resolved recursively, if a TLD deregisters a
domain all other DNS servers 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 .cn or .ru [Anderson-2011] or where legal processes
in countries like the United States result in domain name seizures
and/or DNS redirection by the government [Kopel-2013].
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 [Anderson-2011].
6.4. Notice and Takedown
In some countries, legal mechanisms exist where an individual can
issue a legal request to a content host that requires the host to
take down content. Examples include the voluntary systems employed
by companies like Google to comply with "Right to be Forgotten"
policies in the European Union [Google-RTBF] and the copyright-
oriented notice and takedown regime of the United States Digital
Millennium Copyright Act (DMCA) Section 512 [DMLP-512].
7. Contributors
This document benefited from discussions with Stephane Bortzmeyer,
Nick Feamster, and Martin Nilsson.
8. Informative References
[AFNIC-2013]
AFNIC, "Report of the AFNIC Scientific Council:
Consequences of DNS-based Internet filtering", 2013,
<http://www.afnic.fr/medias/documents/conseilscientifique/
SC-consequences-of-DNS-based-Internet-filtering.pdf>.
Hall, et al. Expires September 19, 2016 [Page 17]
Internet-Draft draft-hall-censorship-tech March 2016
[AFP-2014]
AFP, "China Has Massive Internet Breakdown Reportedly
Caused By Their Own Censoring Tools", 2014,
<http://www.businessinsider.com/chinas-internet-breakdown-
reportedly-caused-by-censoring-tools-2014-1>.
[Albert-2011]
Albert, K., "DNS Tampering and the new ICANN gTLD Rules",
2011, <https://opennet.net/blog/2011/06/dns-tampering-and-
new-icann-gtld-rules>.
[Anderson-2011]
Anderson, R. and S. Murdoch, "Access Denied: Tools and
Technology of Internet Filtering", 2011,
<http://access.opennet.net/wp-content/uploads/2011/12/
accessdenied-chapter-3.pdf>.
[Anon-SIGCOMM12]
Anonymous, "The Collateral Damage of Internet Censorship
by DNS Injection", 2012,
<http://www.sigcomm.org/sites/default/files/ccr/
papers/2012/July/2317307-2317311.pdf>.
[Anonymous-2007]
Anonymous, "How to Bypass Comcast's Bittorrent
Throttling", 2012, <https://torrentfreak.com/how-to-
bypass-comcast-bittorrent-throttling-071021>.
[Anonymous-2013]
Anonymous, "GitHub blocked in China - how it happened, how
to get around it, and where it will take us", 2013,
<https://en.greatfire.org/blog/2013/jan/github-blocked-
china-how-it-happened-how-get-around-it-and-where-it-will-
take-us>.
[Anonymous-2014]
Anonymous, "Towards a Comprehensive Picture of the Great
Firewall's DNS Censorship", 2014,
<https://www.usenix.org/system/files/conference/foci14/
foci14-anonymous.pdf>.
[AP-2012] Associated Press, "Sattar Beheshit, Iranian Blogger, Was
Beaten In Prison According To Prosecutor", 2012,
<http://www.huffingtonpost.com/2012/12/03/
sattar-beheshit-iran_n_2233125.html>.
Hall, et al. Expires September 19, 2016 [Page 18]
Internet-Draft draft-hall-censorship-tech March 2016
[Aryan-2012]
Aryan, S., Aryan, H., and J. Halderman, "Internet
Censorship in Iran: A First Look", 2012,
<https://jhalderm.com/pub/papers/iran-foci13.pdf>.
[BBC-2013]
BBC News, "Google and Microsoft agree steps to block abuse
images", 2013, <http://www.bbc.com/news/uk-24980765>.
[BBC-2013b]
BBC, "China employs two million microblog monitors state
media say", 2013,
<http://www.bbc.com/news/world-asia-china-2439695>.
[Bristow-2013]
Bristow, M., "China's internet 'spin doctors'", 2013,
<http://news.bbc.co.uk/2/hi/asia-pacific/7783640.stm>.
[Calamur-2013]
Calamur, K., "Prominent Egyptian Blogger Arrested", 2013,
<http://www.npr.org/blogs/thetwo-way/2013/11/29/247820503/
prominent-egyptian-blogger-arrested>.
[CERT-2000]
CERT, "TCP SYN Flooding and IP Spoofing Attacks", 2000,
<http://www.cert.org/historical/advisories/
CA-1996-21.cfm>.
[Cheng-2010]
Cheng, J., "Google stops Hong Kong auto-redirect as China
plays hardball", 2010, <http://arstechnica.com/tech-
policy/2010/06/
google-tweaks-china-to-hong-kong-redirect-same-results/>.
[Clayton-2006]
Clayton, R., "Ignoring the Great Firewall of China", 2006,
<http://link.springer.com/chapter/10.1007/11957454_2>.
[Condliffe-2013]
Condliffe, J., "Google Announces Massive New Restrictions
on Child Abuse Search Terms", 2013, <http://gizmodo.com/
google-announces-massive-new-restrictions-on-child-abus-
1466539163>.
[Cowie-2011]
Cowie, J., "Egypt Leaves the Internet", 2011,
<http://www.renesys.com/2011/01/
egypt-leaves-the-internet/>.
Hall, et al. Expires September 19, 2016 [Page 19]
Internet-Draft draft-hall-censorship-tech March 2016
[Cowie-2011b]
Cowie, J., "Libyan Disconnect", 2011,
<http://www.renesys.com/2011/02/libyan-disconnect-1/>.
[Crandall-2010]
Crandall, J., "Empirical Study of a National-Scale
Distributed Intrusion Detection System: Backbone-Level
Filtering of HTML Responses in China", 2010,
<http://www.cs.unm.edu/~crandall/icdcs2010.pdf>.
[Dalek-2013]
Dalek, J., "A Method for Identifying and Confirming the
Use of URL Filtering Products for Censorship", 2013,
<http://www.cs.stonybrook.edu/~phillipa/papers/
imc112s-dalek.pdf>.
[Ding-1999]
Ding, C., Chi, C., Deng, J., and C. Dong, "Centralized
Content-Based Web Filtering and Blocking: How Far Can It
Go?", 1999, <http://citeseerx.ist.psu.edu/viewdoc/
download?doi=10.1.1.132.3302&rep=rep1&type=pdf>.
[DMLP-512]
Digital Media Law Project, "Protecting Yourself Against
Copyright Claims Based on User Content", 2012,
<http://www.dmlp.org/legal-guide/protecting-yourself-
against-copyright-claims-based-user-content>.
[Dobie-2007]
Dobie, M., "Junta tightens media screw", 2007,
<http://news.bbc.co.uk/2/hi/asia-pacific/7016238.stm>.
[Ensafi-2013]
Ensafi, R., "Detecting Intentional Packet Drops on the
Internet via TCP/IP Side Channels", 2013,
<http://arxiv.org/pdf/1312.5739v1.pdf>.
[Fareed-2008]
Fareed, M., "China joins a turf war", 2008,
<http://www.theguardian.com/media/2008/sep/22/
chinathemedia.marketingandpr>.
[Gao-2014]
Gao, H., "Tiananmen, Forgotten", 2014,
<http://www.nytimes.com/2014/06/04/opinion/
tiananmen-forgotten.html>.
Hall, et al. Expires September 19, 2016 [Page 20]
Internet-Draft draft-hall-censorship-tech March 2016
[Glanville-2008]
Glanville, J., "The Big Business of Net Censorship", 2008,
<http://www.theguardian.com/commentisfree/2008/nov/17/
censorship-internet>.
[Google-RTBF]
Google, Inc., "Search removal request under data
protection law in Europe", 2015,
<https://support.google.com/legal/contact/
lr_eudpa?product=websearch>.
[Guardian-2014]
The Gaurdian, "Chinese blogger jailed under crackdown on
'internet rumours'", 2014,
<http://www.theguardian.com/world/2014/apr/17/chinese-
blogger-jailed-crackdown-internet-rumours-qin-zhihui>.
[Heacock-2009]
Heacock, R., "China Shuts Down Internet in Xinjiang Region
After Riots", 2009, <https://opennet.net/blog/2009/07/
china-shuts-down-internet-xinjiang-region-after-riots>.
[Hepting-2011]
Electronic Frontier Foundation, "Hepting vs. AT&T", 2011,
<https://www.eff.org/cases/hepting>.
[Hjelmvik-2010]
Hjelmvik, E., "Breaking and Improving Protocol
Obfuscation", 2010, <https://www.iis.se/docs/
hjelmvik_breaking.pdf>.
[Hopkins-2011]
Hopkins, C., "Communications Blocked in Libya, Qatari
Blogger Arrested: This Week in Online Tyranny", 2011,
<http://readwrite.com/2011/03/03/
communications_blocked_in_libya_this_week_in_onlin>.
[ICANN-SSAC-2012]
ICANN Security and Stability Advisory Committee (SSAC),
"SAC 056: SSAC Advisory on Impacts of Content Blocking via
the Domain Name System", 2012,
<https://www.icann.org/en/system/files/files/sac-
056-en.pdf>.
[Johnson-2010]
Johnson, L., "Torture feared in arrest of Iraqi blogger",
2011, <http://seattlepostglobe.org/2010/02/05/
torture-feared-in-arrest-of-iraqi-blogger/>.
Hall, et al. Expires September 19, 2016 [Page 21]
Internet-Draft draft-hall-censorship-tech March 2016
[Jones-2014]
Jones, B., "Automated Detection and Fingerprinting of
Censorship Block Pages", 2014,
<http://conferences2.sigcomm.org/imc/2014/papers/
p299.pdf>.
[Khattak-2013]
Khattak, S., "Towards Illuminating a Censorship Monitor's
Model to Facilitate Evasion", 2013, <http://0b4af6cdc2f0c5
998459-c0245c5c937c5dedcca3f1764ecc9b2f.r43.cf2.rackcdn.co
m/12389-foci13-khattak.pdf>.
[Kopel-2013]
Kopel, K., "Operation Seizing Our Sites: How the Federal
Government is Taking Domain Names Without Prior Notice",
2013, <http://dx.doi.org/doi:10.15779/Z384Q3M>.
[Kravtsova-2012]
Kravtsova, Y., "Cyberattacks Disrupt Opposition's
Election", 2012,
<http://www.themoscowtimes.com/news/article/
cyberattacks-disrupt-oppositions-election/470119.html>.
[Marczak-2015]
Marczak, B., Weaver, N., Dalek, J., Ensafi, R., Fifield,
D., McKune, S., Rey, A., Scott-Railton, J., Deibert, R.,
and V. Paxson, "An Analysis of China's "Great Cannon"",
2015,
<https://www.usenix.org/system/files/conference/foci15/
foci15-paper-marczak.pdf>.
[Muncaster-2013]
Muncaster, P., "Malaysian election sparks web blocking/
DDoS claims", 2013,
<http://www.theregister.co.uk/2013/05/09/
malaysia_fraud_elections_ddos_web_blocking/>.
[Nabi-2013]
Nabi, Z., "The Anatomy of Web Censorship in Pakistan",
2013, <http://0b4af6cdc2f0c5998459-c0245c5c937c5dedcca3f17
64ecc9b2f.r43.cf2.rackcdn.com/12387-foci13-nabi.pdf>.
[Netsec-2011]
n3t2.3c, "TCP-RST Injection", 2011, <https://nets.ec/TCP-
RST_Injection>.
Hall, et al. Expires September 19, 2016 [Page 22]
Internet-Draft draft-hall-censorship-tech March 2016
[Orion-2013]
Orion, E., "Zimbabwe election hit by hacking and DDoS
attacks", 2013,
<http://www.theinquirer.net/inquirer/news/2287433/
zimbabwe-election-hit-by-hacking-and-ddos-attacks>.
[Porter-2010]
Porter, T., "The Perils of Deep Packet Inspection", 2010,
<http://www.symantec.com/connect/articles/
perils-deep-packet-inspection>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<http://www.rfc-editor.org/info/rfc7624>.
[RFC7725] Bray, T., "An HTTP Status Code to Report Legal Obstacles",
RFC 7725, DOI 10.17487/RFC7725, February 2016,
<http://www.rfc-editor.org/info/rfc7725>.
[RFC7754] Barnes, R., Cooper, A., Kolkman, O., Thaler, D., and E.
Nordmark, "Technical Considerations for Internet Service
Blocking and Filtering", RFC 7754, DOI 10.17487/RFC7754,
March 2016, <http://www.rfc-editor.org/info/rfc7754>.
[RSF-2005]
Reporters Sans Frontieres, "Technical ways to get around
censorship", 2005, <http://archives.rsf.org/
print-blogs.php3?id_article=15013>.
[Rushe-2015]
Rushe, D., "Bing censoring Chinese language search results
for users in the US", 2013,
<http://www.theguardian.com/technology/2014/feb/11/
bing-censors-chinese-language-search-results>.
Hall, et al. Expires September 19, 2016 [Page 23]
Internet-Draft draft-hall-censorship-tech March 2016
[Sandvine-2014]
Sandvine, "Technology Showcase on Traffic Classification:
Why Measurements and Freeform Policy Matter", 2014,
<https://www.sandvine.com/downloads/general/technology/
sandvine-technology-showcases/sandvine-technology-
showcase-traffic-classification.pdf>.
[Schoen-2007]
Schoen, S., "EFF tests agree with AP: Comcast is forging
packets to interfere with user traffic", 2007,
<https://www.eff.org/deeplinks/2007/10/eff-tests-agree-ap-
comcast-forging-packets-to-interfere>.
[Schone-2014]
Schone, M., Esposito, R., Cole, M., and G. Greenwald,
"Snowden Docs Show UK Spies Attacked Anonymous, Hackers",
2014, <http://www.nbcnews.com/feature/edward-snowden-
interview/exclusive-snowden-docs-show-uk-spies-attacked-
anonymous-hackers-n21361>.
[Senft-2013]
Senft, A., "Asia Chats: Analyzing Information Controls and
Privacy in Asian Messaging Applications", 2013,
<https://citizenlab.org/2013/11/asia-chats-analyzing-
information-controls-privacy-asian-messaging-
applications/>.
[Shbair-2015]
Shbair, W., Cholez, T., Goichot, A., and I. Chrisment,
"Efficiently Bypassing SNI-based HTTPS Filtering", 2015,
<https://hal.inria.fr/hal-01202712/document>.
[Sophos-2015]
Sophos, "Understanding Sophos Web Filtering", 2015,
<https://www.sophos.com/en-us/support/
knowledgebase/115865.aspx>.
[Thomson-2012]
Thomson, I., "Syria Cuts off Internet and Mobile
Communication", 2012,
<http://www.theregister.co.uk/2012/11/29/
syria_internet_blackout/>.
[Trustwave-2015]
Trustwave, "Filter: SNI extension feature and HTTPS
blocking", 2015,
<https://www3.trustwave.com/software/8e6/hlp/r3000/
files/1system_filter.html>.
Hall, et al. Expires September 19, 2016 [Page 24]
Internet-Draft draft-hall-censorship-tech March 2016
[Verkamp-2012]
Verkamp, J. and M. Gupta, "Inferring Mechanics of Web
Censorship Around the World", 2012,
<https://www.usenix.org/system/files/conference/foci12/
foci12-final1.pdf>.
[ViewDNS-2011]
ViewDNS.info, "DNS Cache Poisoning in the People's
Republic of China", 2011, <http://viewdns.info/research/
dns-cache-poisoning-in-the-peoples-republic-of-china/>.
[Villeneuve-2011]
Villeneuve, N., "Open Access: Chapter 8, Control and
Resistance, Attacks on Burmese Opposition Media", 2011,
<http://access.opennet.net/wp-content/uploads/2011/12/
accesscontested-chapter-08.pdf>.
[VonLohmann-2008]
VonLohmann, F., "FCC Rules Against Comcast for BitTorrent
Blocking", 2008, <https://www.eff.org/deeplinks/2008/08/
fcc-rules-against-comcast-bit-torrent-blocking>.
[Wagner-2009]
Wagner, B., "Deep Packet Inspection and Internet
Censorship: International Convergence on an 'Integrated
Technology of Control'", 2009,
<http://advocacy.globalvoicesonline.org/wp-
content/uploads/2009/06/
deeppacketinspectionandinternet-censorship2.pdf>.
[Wagstaff-2013]
Wagstaff, J., "In Malaysia, online election battles take a
nasty turn", 2013,
<http://www.reuters.com/article/2013/05/04/
uk-malaysia-election-online-idUKBRE94309G20130504>.
[Weaver-2009]
Weaver, N., Sommer, R., and V. Paxson, "Detecting Forged
TCP Packets", 2009, <http://www.icir.org/vern/papers/
reset-injection.ndss09.pdf>.
[Whittaker-2013]
Whittaker, Z., "1,168 keywords Skype uses to censor,
monitor its Chinese users", 2013,
<http://www.zdnet.com/1168-keywords-skype-uses-to-censor-
monitor-its-chinese-users-7000012328/>.
Hall, et al. Expires September 19, 2016 [Page 25]
Internet-Draft draft-hall-censorship-tech March 2016
[Wikip-DoS]
Wikipedia, "Denial of Service Attacks", 2016,
<https://en.wikipedia.org/w/index.php?title=Denial-of-
service_attack&oldid=710558258>.
[Wilde-2012]
Wilde, T., "Knock Knock Knockin' on Bridges Doors", 2012,
<https://blog.torproject.org/blog/knock-knock-knockin-
bridges-doors>.
[Winter-2012]
Winter, P., "How China is Blocking Tor", 2012,
<http://arxiv.org/pdf/1204.0447v1.pdf>.
[Zhu-2011]
Zhu, T., "An Analysis of Chinese Search Engine Filtering",
2011,
<http://arxiv.org/ftp/arxiv/papers/1107/1107.3794.pdf>.
[Zmijewki-2014]
Zmijewki, E., "Turkish Internet Censorship Takes a New
Turn", 2014, <http://www.renesys.com/2014/03/
turkish-internet-censorship/>.
Authors' Addresses
Joseph Lorenzo Hall
CDT
Email: joe@cdt.org
Michael D. Aaron
CU Boulder
Email: michael.aaron@colorado.edu
Ben Jones
GA Tech
Email: bjones99@gatech.edu
Hall, et al. Expires September 19, 2016 [Page 26]