A mirror site is a replica of a website hosted on a different server under a different URL. In SEO, the concern is identical content being available at more than one address.
What are mirror sites used for?
The term covers two different situations.
- Legitimate mirrors. According to Wikipedia, operators run mirrors to reduce traffic on a main server, speed up access, keep content available and provide backups. They are common for software archives and open-source downloads, for example Linux distributions. Modern mirrors support HTTPS and IPv6.
- Copied content. Third parties copy a site onto their own domain. Google’s spam policies call taking content from other sites to manipulate rankings scraping and list republishing it with only minor changes as an example.
Mirror sites and SEO
Several URLs with the same content give search engines a duplicate problem. Google recommends telling it which address is preferred. The documentation names redirects and rel="canonical" as strong signals and listing the URL in a sitemap as a weak one. Without a signal, Google picks the version it considers best for users. It advises against using robots.txt for canonicalization, because a blocked URL can still be indexed without its content.
For a mirror you run yourself, each copy can carry a canonical tag to the original or redirect to it with a 301, so that the original is the version shown in results.
Example
<!-- On the copy https://mirror.example.com/guide/ -->
<link rel="canonical" href="https://www.example.com/guide/">
Common mistakes
- A mirror without a canonical tag or redirect, so two domains compete for the same queries.
- Blocking the mirror in robots.txt as a canonicalization method.
- A forgotten test server or old domain copy that stays indexable.
Sources
- Mirror site – Wikipedia (secondary)en.wikipedia.org
- How to specify a canonical URL – Google Search Centraldevelopers.google.com
- Spam policies for Google web search (scraping) – Google Search Centraldevelopers.google.com
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