A comprehensive review of tools and technology for filtering Internet and is described by Murdoch Anderson (08) and ranges from technical filtering domain-registration and denial of service attacks. In addition, they also discussed briefly surveillance and censorship non-technical methods. Murdoch and Anderson (08) articulate the following filtering mechanisms: - Filtering header TCP / IP : With this method, the router of the censorship can inspect the Internet Protocol [IP] address and port number of the destination. If the destination is being blacklisted, the connection is interrupted or redirected to a page indicating that access is denied to the destination
- TCP / IP content filtering . This is a similar method to filter header, except that the router censorship inspects packet contents for patterns or keywords that can be blacklisted. The focus is not on the content but rather on where the packets going to or from
- Domain Name Server (DNS) Falsification :. Normally, domain name servers are accessed by users' computers to retrieve the corresponding IP address of a given domain. With a falsification server domain name, the domain name resolution may fail as the router could send a wrong answer that does not contain the IP address on the right, where the connection fails.
- Hypertext Transfer Protocol (HTTP) Proxy filtering : In some cases, users are forced to use an HTTP proxy that are assigned to access the Internet. These proxies may be the only way to access the Internet, so they can monitor all traffic that passes through them. Such a method is more powerful than TCP / IP header and DNS filtering
- Hybrid TCP / IP and HTTP Proxy Filtering :. Because the use of HTTP proxy filtering is often demanding, a solution has been designed to use only the HTTP Proxy filtering for a list of IP addresses known to have prohibited content. If any of these IP addresses is available, traffic is redirected to a transparent HTTP proxy, which inspects the transferred flow and filter any content prohibited.
- Denial-of-Service (DoS) : Denial of service may be launched on the host server. Such attacks are usually made by having a large number of computers requesting services from a particular server and therefore overwhelming with too much traffic which causes the server and connecting it to stall.
- Server takedown : Through legal methods, extra-legal or pressure, a hosting company for a specific server can take down and disconnect from the Internet . The owner of the server may be able to transfer the content from the server, however - provided that a backup copy exists - to another hosting company within hours
- Monitoring :. professionally monitored through logging transfers between the host and the user of the Internet. If the content is forbidden in the transferred flux shares - legal or extra-legal - could be taken against the user, host, or both. Such acts could trigger a sense of fear, causing the host to not publish such content and causing the user to hesitate to access
- social engineering :. This includes the requirement to show photo identification (ID) before using public computers in libraries or Internet cafes; social or religious norms that force Internet users to avoid opening a particular content are another form of social censure. Families who place the computer in the living room to allow monitoring of Internet use of their children is another example of a social technique of censorship.
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