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Internet censorship is the control or suppression of what can be accessed, published, or viewed on the Internet enacted by regulators, or on their own initiative. Individuals and organizations may engage in self-censorship for moral, religious, or business reasons, to conform to societal norms, due to intimidation, or out of fear of legal or other consequences.

The extent of Internet censorship varies on a country-to-country basis. While most democratic countries have moderate Internet censorship, other countries go as far as to limit the access of information such as news and suppress discussion among citizens. Internet censorship also occurs in response to or in anticipation of events such as elections, protests, and riots. An example is the increased censorship due to the events of the Arab Spring. Other types of censorship include the use of copyrights, defamation, harassment, and obscene material claims as a way to suppress content.

Support for and opposition to Internet censorship also varies. In a 2012 Internet Society survey 71% of respondents agreed that "censorship should exist in some form on the Internet". In the same survey 83% agreed that "access to the Internet should be considered a basic human right" and 86% agreed that "freedom of expression should be guaranteed on the Internet". Perception of internet censorship in the US is largely based on the First Amendment and the right for expansive free speech and access to content without regard to the consequences. According to GlobalWebIndex, over 400 million people use virtual private networks to circumvent censorship or for increased user privacy.

Overview

Many of the challenges associated with Internet censorship are similar to those for offline censorship of more traditional media such as newspapers, magazines, books, music, radio, television, and film. One difference is that national borders are more permeable online: residents of a country that bans certain information can find it on websites hosted outside the country. Thus censors must work to prevent access to information even though they lack physical or legal control over the websites themselves. This in turn requires the use of technical censorship methods that are unique to the Internet, such as site blocking and content filtering.

Views about the feasibility and effectiveness of Internet censorship have evolved in parallel with the development of the Internet and censorship technologies:
  • A 1993 Time Magazine article quotes computer scientist John Gilmore, one of the founders of the Electronic Frontier Foundation, as saying "The Net interprets censorship as damage and routes around it."
  • In November 2007, "Father of the Internet" Vint Cerf stated that he sees government control of the Internet failing because the Web is almost entirely privately owned.
  • A report of research conducted in 2007 and published in 2009 by the Berkman Center for Internet & Society at Harvard University stated that: "We are confident that the [ censorship circumvention ] tool developers will for the most part keep ahead of the governments' blocking efforts", but also that "...we believe that less than two percent of all filtered Internet users use circumvention tools".
  • In contrast, a 2011 report by researchers at the Oxford Internet Institute published by UNESCO concludes "... the control of information on the Internet and Web is certainly feasible, and technological advances do not therefore guarantee greater freedom of speech."
Blocking and filtering can be based on relatively static blacklists or be determined more dynamically based on a real-time examination of the information being exchanged. Blacklists may be produced manually or automatically and are often not available to non-customers of the blocking software. Blocking or filtering can be done at a centralized national level, at a decentralized sub-national level, or at an institutional level, for example in libraries, universities or Internet cafes. Blocking and filtering may also vary within a country across different ISPs. Countries may filter sensitive content on an ongoing basis and/or introduce temporary filtering during key time periods such as elections. In some cases the censoring authorities may surreptitiously block content to mislead the public into believing that censorship has not been applied. This is achieved by returning a fake "Not Found" error message when an attempt is made to access a blocked website.

Unless the censor has total control over all Internet-connected computers, such as in North Korea (who employ an intranet that only privileged citizens can access), or Cuba, total censorship of information is very difficult or impossible to achieve due to the underlying distributed technology of the Internet. Pseudonymity and data havens (such as Freenet) protect free speech using technologies that guarantee material cannot be removed and prevents the identification of authors. Technologically savvy users can often find ways to access blocked content. Nevertheless, blocking remains an effective means of limiting access to sensitive information for most users when censors, such as those in China, are able to devote significant resources to building and maintaining a comprehensive censorship system.

The term "splinternet" is sometimes used to describe the effects of national firewalls. The verb "rivercrab" colloquially refers to censorship of the Internet, particularly in Asia.

Content suppression methods

Technical censorship

Various parties are using different technical methods of preventing public access to undesirable resources, with varying levels of effectiveness, costs and side effects.

Blacklists

Entities mandating and implementing the censorship usually identify them by one of the following items: keywords, domain names and IP addresses. Lists are populated from different sources, ranging from private suppliers through courts to specialized government agencies (Ministry of Industry and Information Technology of China, Islamic Guidance in Iran).

As per Hoffmann, different methods are used to block certain websites or pages including DNS poisoning, blocking access to IPs, analyzing and filtering URLs, inspecting filter packets and resetting connections.

Points of control

Enforcement of the censor-nominated technologies can be applied at various levels of countries and Internet infrastructure:
  • Internet backbone, including Internet exchange points (IXP) with international networks (Autonomous Systems), operators of submarine communications cables, satellite Internet access points, international optical fibre links etc. In addition to facing huge performance challenges due to large bandwidths involved, these do not give censors access to information exchanged within the country.
  • Internet Service Providers, which involves installation of voluntary (as in UK) or mandatory (as in Russia) Internet surveillance and blocking equipment.
  • Individual institutions, which in most cases implement some form of Internet access controls to enforce their own policies, but, especially in case of public or educational institutions, may be requested or coerced to do this on the request from the government.
  • Personal devices, whose manufacturers or vendors may be required by law to install censorship software.
  • Application service providers (e.g. social media companies), who may be legally required to remove particular content. Foreign providers with business presence in given country may be also coerced into restricting access to specific contents for visitors from the requesting country.
  • Certificate authorities may be required to issue counterfeit X.509 certificates controlled by the government, allowing man-in-the-middle surveillance of TLS encrypted connections.
  • Content Delivery Network providers who tend to aggregate large amounts of content (e.g. images) may be also attractive target for censorship authorities.

Approaches

Internet content is subject to technical censorship methods, including:
  • Internet Protocol (IP) address blocking: Access to a certain IP address is denied. If the target Web site is hosted in a shared hosting server, all websites on the same server will be blocked. This affects IP-based protocols such as HTTP, FTP and POP. A typical circumvention method is to find proxies that have access to the target websites, but proxies may be jammed or blocked, and some Web sites, such as Wikipedia (when editing), also block proxies. Some large websites such as Google have allocated additional IP addresses to circumvent the block, but later the block was extended to cover the new addresses. Due to challenges with geolocation, geo-blocking is normally implemented via IP address blocking.
  • Domain name system (DNS) filtering and redirection: Blocked domain names are not resolved, or an incorrect IP address is returned via DNS hijacking or other means. This affects all IP-based protocols such as HTTP, FTP and POP. A typical circumvention method is to find an alternative DNS resolver that resolves domain names correctly, but domain name servers are subject to blockage as well, especially IP address blocking. Another workaround is to bypass DNS if the IP address is obtainable from other sources and is not itself blocked. Examples are modifying the Hosts file or typing the IP address instead of the domain name as part of a URL given to a Web browser.
  • Uniform Resource Locator (URL) filtering: URL strings are scanned for target keywords regardless of the domain name specified in the URL. This affects the HTTP protocol. Typical circumvention methods are to use escaped characters in the URL, or to use encrypted protocols such as VPN and TLS/SSL.
  • Packet filtering: Terminate TCP packet transmissions when a certain number of controversial keywords are detected. This affects all TCP-based protocols such as HTTP, FTP and POP, but Search engine results pages are more likely to be censored. Typical circumvention methods are to use encrypted connections – such as VPN and TLS/SSL – to escape the HTML content, or by reducing the TCP/IP stack's MTU/MSS to reduce the amount of text contained in a given packet.
  • Connection reset: If a previous TCP connection is blocked by the filter, future connection attempts from both sides can also be blocked for some variable amount of time. Depending on the location of the block, other users or websites may also be blocked, if the communication is routed through the blocking location. A circumvention method is to ignore the reset packet sent by the firewall.
  • Network disconnection: A technically simpler method of Internet censorship is to completely cut off all routers, either by software or by hardware (turning off machines, pulling out cables). A circumvention method could be to use a satellite ISP to access Internet.
  • Portal censorship and search result removal: Major portals, including search engines, may exclude web sites that they would ordinarily include. This renders a site invisible to people who do not know where to find it. When a major portal does this, it has a similar effect as censorship. Sometimes this exclusion is done to satisfy a legal or other requirement, other times it is purely at the discretion of the portal. For example, Google.de and Google.fr remove Neo-Nazi and other listings in compliance with German and French law.
  • Computer network attacks: Denial-of-service attacks and attacks that deface opposition websites can produce the same result as other blocking techniques, preventing or limiting access to certain websites or other online services, although only for a limited period of time. This technique might be used during the lead up to an election or some other sensitive period. It is more frequently used by non-state actors seeking to disrupt services.

Over and under blocking

Technical censorship techniques are subject to both over- and under-blocking since it is often impossible to always block exactly the targeted content without blocking other permissible material or allowing some access to targeted material and so providing more or less protection than desired. An example is blocking an IP-address of a server that hosts multiple websites, which prevents access to all of the websites rather than just those that contain content deemed offensive.

Use of commercial filtering software