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. A government can try to prevent its citizens from viewing these even if it has no control over the websites themselves. Filtering can be based on a blacklist or be dynamic. In the case of a blacklist, that list is usually not published. The list may be produced manually or automatically.
Unless the government has total control over all Internet-connected computers, such as in North Korea, total censorship of information on the Internet is very difficult (or impossible) to achieve due to the underlying distributed technology of the Internet. Pseudonymity and data havens (such as Freenet) allow unconditional free speech, as the technology guarantees that material cannot be removed and the author of any information is impossible to link to a physical identity or organization.
In some cases, Internet censorship may involve deceit. In such cases the censoring authority may block content while leading the public to believe that censorship has not been applied. This may be done by having the ISP provide a fake "Not Found" error message upon the request of an Internet page that is actually found but blocked (see 404 error for details).
In November 2007, "Father of the Internet" Vint Cerf stated that he sees control of the Internet failing due to private ownership. Many Internet experts use the term "splinternet" to describe some of the effects of national firewalls. The verb "rivercrab" colloquially refers to censorship of the Internet, particularly in Asia.
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| Internet Censorship Ratings by Countries |
No censorship
Some censorship
Country under surveillance from Reporters Without Borders
Most heavily censored nations
The level of censorship in Tunisia has changed since Zine El Abidine Ben Ali's government was overthrown on 14 January 2011.
Technical censorship
Some commonly used methods for (partial) censoring content are:
- IP 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 like Google have allocated additional IP addresses to circumvent the block, but later the block was extended to cover the new IPs.
- DNS filtering and redirection. Don't resolve domain names, or return incorrect IP addresses. This affects all IP-based protocols such as HTTP, FTP and POP. A typical circumvention method is to find a Alternative DNS root that resolves domain names correctly, but domain name servers are subject to blockage as well, especially IP blocking. Another workaround is to bypass DNS if the IP address is obtainable from other sources and is not blocked. Examples are modifying the Hosts file or typing the IP address instead of the domain name in a Web browser.
- Uniform Resource Locator (URL) filtering. Scan the requested URL string 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.[5]
- 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 will also be blocked for up to 30 minutes. Depending on the location of the block, other users or websites may also be blocked if the communication is routed to the location of the block. A circumvention method is to ignore the reset packet sent by the firewall.
- Reverse surveillance. Computers accessing certain websites including Google are automatically exposed to reverse scanning from the ISP in an apparent attempt to extract further information from the "offending" system.
One of the most popular filtering software programmes is SmartFilter, owned by Secure Computing in California, which has recently been bought by McAfee. SmartFilter has been used by Tunisia, Saudi Arabia and Sudan, as well as in the US and the UK.
Full block
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). This appears to have been the case on 27/28 January 2011 during the 2011 Egyptian protests, in what has been widely described as an "unprecedented" internet block. About 3500 Border Gateway Protocol (BGP) routes to Egyptian networks were shut down from about 22:10 to 22:35 UTC 27 January. This full block was implemented without cutting off major intercontinental fibre-optic links, with Renesys stating on 27 January, "Critical European-Asian fiber-optic routes through Egypt appear to be unaffected for now."
Commonly targeted websites
- Pornographic and fetishism websites.
- Pedophile-related and other websites considered highly objectionable (see also CIRCAMP)
- Media sharing website (e.g. Flickr and YouTube)
- Social networks (e.g. Facebook, MySpace and Twitter)
- Wikipedia
- Wikileaks
- Political blogs
- Nazi and similar websites — particularly in France and Germany
- Religious websites
- Erowid
- Filesharing and P2P-related websites
- Google - particularly in mainland China and Cuba
- Censorship-circumvention websites
- Foreign websites
- 4chan
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