Mirror sites or mirrors are replicas of other websites or any network node. The concept of mirroring applies to network services accessible through any protocol, such as HTTP or FTP. Such sites have different URLs than the original site, but host identical or near-identical content. Mirror sites are often located in a different geographic region than the original, or upstream site. The purpose of mirrors is to reduce network traffic, improve access speed, ensure availability of the original site for technical or political reasons, or provide a real-time backup of the original site. Mirror sites are particularly important in developing countries, where internet access may be slower or less reliable. The maintainers of some mirrors choose not to replicate the entire contents of the upstream server they are mirroring because of technical constraints, or selecting only a subset relevant to their purpose, such as software written in a particular programming language, runnable on a single computer platform, or written by one author. These sites are called partial mirrors or secondary mirrors.
Mirror sites were heavily used on the early internet, when most users accessed through dialup and the Internet backbone had much lower bandwidth than today, making a geographically-localized mirror network a worthwhile benefit. Download archives such as Info-Mac, Tucows and CPAN maintained worldwide networks mirroring their content accessible over HTTP or anonymous FTP. Some of these networks, such as Info-Mac or Tucows are no longer active or have removed their mirrored download sections, but some like CPAN or the Debian package mirrors are still active in 2019. Debian removed FTP access to its mirrors in 2017 because of declining use and the relative stagnation of the FTP protocol, mentioning FTP servers' lack of support for techniques such as caching and load balancing that are available to HTTP. New Mirrors use HTTPS and support IPv6 along with IPv4. The primary mirror sites distribute to secondary mirrors by using rsync to ensure integrity of files and file consistency with the master mirror.
Notable websites with mirrors include Project Gutenberg, KickassTorrents, The Pirate Bay, WikiLeaks, the website of the Environmental Protection Agency, and Wikipedia. Some notable partial mirrors include free and open-source software projects such as GNU, in particular Linux distributions CentOS  Debian and Fedora;, Ubuntu   such projects provide mirrors of the download sites (since those are expected to have high load). Many open Source application providers such as VideoLAN to distribute VLC Media Player. and The Document Foundation to distribute LibreOffice.
It was once common for tech companies such as Microsoft, Hewlet-Packard or Apple Computer to maintain a network of mirrors accessible over HTTP or anonymous FTP, hosting software updates, sample code and various freely-downloadable utilities. Much of these sites were shut down in the first decades of the 21st century, with Apple shutting down its FTP services in 2012 and Microsoft stopping updates in 2010. Today, the contents of a number of these mirror sites are archived at https://archive.org/details/ftpsites&tab=collection
- Glushko, Robert J. (2014-08-25). The Discipline of Organizing: Core Concepts Edition. "O'Reilly Media, Inc.". ISBN 9781491912812. Retrieved 29 April 2017.
- "Debian worldwide mirror sites".
Using a nearby server will probably speed up your download, and also reduce the load on our central servers and on the Internet as a whole.
- "Impending Trump has Internet Archive mirror to Canada". 2016-11-29.
The Internet Archive has several mirrors up right now, and Canada is set to be its next. This move is taking place specifically because of the new presidential elect Trump here in the United States.
- "What is Mirror Site? Webopedia Definition". www.webopedia.com. Retrieved 29 April 2017.
- "What is Mirror Site? - Definition from Techopedia". Techopedia.com. Retrieved 29 April 2017.
- Wisshak, Max; Tapanila, Leif (2008-06-02). Current Developments in Bioerosion. Springer Science & Business Media. ISBN 9783540775973. Retrieved 29 April 2017.
- Sekikawa, A.; Sa, E. R.; Acosta, B.; Aaron, D. J.; Laporte, R. E. (2000). "Internet mirror sites - The Lancet". Lancet. 355 (9219): 2000. doi:10.1016/s0140-6736(05)72944-5. PMID 10859070. S2CID 32218172.
We all become frustrated when web pages take minutes to unfold. This can increase the gap between infrastructure haves and have-nots. Downloading time is important for other reasons; users connecting to the internet via telephone line in many countries are charged per minute and slow downloading itself may make users lose interest.
- "Debian worldwide mirror sites".
A secondary mirror site may have restrictions on what they mirror
- "Debian Project to Shut Down Its Public FTP Services, Developers Are Not Affected".
The decision to close the Debian FTP services for users was made because the FTP servers in their current state lack support for acceleration or caching, and they aren't quite used lately due to the fact that the Debian Installer no longer provides an FTP option for accessing mirrors since more than ten years ago... FTP as a protocol appears to no longer be efficient, requiring adding strange workarounds to firewalls and load-balancing daemons.
- "CSpace Hostings Public Mirror".
This page and mirror are available over IPv4 and IPv6 and accessible over HTTP, HTTPS and Rsync
- "Create Public CentOS Mirror".
If your mirror has an IPv6 address, you should check IPv4 and IPv6 separately.We are currently concentrating on HTTP and RSYNC mirrors. HTTPS is currently not used for mirrors, but we have started collecting HTTPS URLs for possible future inclusion to the mirrorlist. If you set up HTTPS, please do not redirect HTTP requests to HTTPS. There may be organizations that allow outbound HTTP connections but not HTTPS connections, and any redirects may cause problems for them.
- "Project Gutenberg, nonprofit organization".
In addition, dozens of “mirror” Web sites were created around the world, where the e-books were also stored and available for downloading.
- Russon, Mary-Ann (22 July 2016). "Kickass Torrents is back: New domains, mirrors and proxies show business is as usual". International Business Times UK. Retrieved 29 April 2017.
- Clark, Bryan (21 July 2016). "IsoHunt just launched a working KickassTorrent mirror". The Next Web. Retrieved 29 April 2017.
- "Mexican Police Target Popular KickassTorrents 'Clone,' Seize Domain - TorrentFreak". TorrentFreak. 23 September 2016. Retrieved 29 April 2017.
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- "The Piratebay Blocked By Chrome, Mirror Sites Accessible". iTech Post. 8 October 2016. Retrieved 29 April 2017.
- "The Pirate Bay is blocked Australia wide... except it really isn't". CNET. Retrieved 29 April 2017.
- "Pirate Bay Mirror Shut Down: Alternative Clone Had Kickass Torrents Skin, Vows To Continue". Tech Times. 24 September 2016. Retrieved 29 April 2017.
- "Pirate Bay Blocked By Google Chrome And Firefox: Kickass Torrents Mirror, Extratorrent, Torrentz And Other Clones Accessible". Tech Times. 8 October 2016. Retrieved 29 April 2017.
- Greenemeier, Larry. "How Has WikiLeaks Managed to Keep Its Web Site Up and Running?". Scientific American. Retrieved 29 April 2017.
- Schroeder, Stan. "WikiLeaks Now Has Hundreds of Mirrors". Mashable. Retrieved 29 April 2017.
- "The EPA Posted a Mirror of Its Website Before Trump Can Gut the Real One". Vice. 2017-02-16. Retrieved 29 April 2017.
- Hiltzik, Michael (24 April 2017). "Did 'people power' save a trove of EPA data from a shutdown by Trump?". Los Angeles Times. Retrieved 29 April 2017.
- "How to set up your own copy of Wikipedia - ExtremeTech". ExtremeTech. 18 January 2012. Retrieved 29 April 2017.
- Broughton, John (2008). Wikipedia: The Missing Manual. "O'Reilly Media, Inc.". ISBN 9780596515164. Retrieved 29 April 2017.
- Ayers, Phoebe; Matthews, Charles; Yates, Ben (2008). How Wikipedia Works: And how You Can be a Part of it. No Starch Press. ISBN 9781593271763. Retrieved 29 April 2017.
- "gnu.org". www.gnu.org. Retrieved 2017-08-27.
- "CentOS Mirror". CentOS Mirror.
- "Debian worldwide mirror sites". www.debian.org. Retrieved 2017-08-27.
- "Home - MirrorManager". admin.fedoraproject.org. Retrieved 2017-08-27.
- "Mirrors : Ubuntu". Ubuntu Archive Mirrors.
- "Mirrors : Ubuntu". Ubuntu CD Mirrors.
- "Mirrors - VideoLAN". www.videolan.org.
- "The Document Foundation Mirrors". download.documentfoundation.org.
- "How do I access the MICROSOFT FTP server?".