A mirror in computing is a direct copy of a data set. The mirror sites are copies of web sites. A mirror site is a very common occurrence when downloading something off the Internet.

Contents

1 Rationale 2 Examples 3 History 4 Programs

Rationale

Mirroring of sites occurs for a variety of reasons:

• To protect data from a failure, usually in hardware. See disk mirror.
• To combat censorship, and deception; while promoting freedom of information. For example: an activist might post pictures of a company conducting illegal activities or information on government activity and be litigated for such. Other internet users will post the content in question on other servers when the legal action results in the cancellation of ISP or DNS services for the original activist. (more examples decss)
• To allow faster downloads for users in one area: for example, a US server could be mirrored in Japan, allowing Japanese Internet users to download content faster from the Japanese server than the American
• To balance load - if one server is extremely popular a mirror may help relieve this load: for example if a Linux distribution is released as an ISO image onto the distribution producer's own server, this server may be overloaded with demand and so mirrors may help reduce the load off this main server.
• As a temporary measure to counterbalance a sudden, temporary increase in traffic. For example, slashdotted websites will often be mirrored by a few slashdot posters until the article is pushed off the front page.
• To keep historic content: economic reasons may prevent the maintainers of a server from keeping older and unsupported content for users who still may desire them - a mirror may be made to keep this content from disappearing
• To increase a site's ranking in a search engine by placing hyperlinks to each mirror within each other mirror. This is viewed as unethical by most search engine administrators and websurfers.
• Rarely, as a form of plagiarism. Usually pointless since a website popular enough to be worth plagiarizing will find out as soon as one of their many readers stumbles onto the plagiarized site. Generally regarded with the same amount of respect as other forms of plagiarism, possibly less.

Examples

A perfect example of this is the well-known Source Forge.net site. The basis of the Sourceforge concept is, primarily, open source, but secondarily, the use of many different locations to achieve one goal: to get the product to the user, you. Many innovative and amazing computer projects host their sites and files on Source Forge, which provides mirrors in several states and countries; from Dublin, Ireland to Tokyo, Japan.

Other, even larger mirror networks include those of Debian, FreeBSD and similar free software projects.

History

Many sites have been mirrored in the past due to historical or social events.

• IsoNews - a warez-listing site that was raided and taken over by the FBI in 2003. It was subsequently mirrored by its advocates in protest of this action.
• Google - When Google's website was banned in 2002 by China, the mirror Elgoog was set up to circumvent the ban. As it turns out, the regulators were trying to stop the use of Google's cache to visit site s that were already previously banned.

Programs

There are numerous computer programs that provide automated mirroring of entire sites. Some are oriented towards personal use, which increases load times. Others are intended to be used by public mirror maintainers.

Examples include:

• HTTrack [1]
• wget
• rsync