Google Now Supports Rel="canonical" HTTP Headers

Google now supports the use of rel="canonical" at the server level.

Google has recently announced that its web search now extends its support for link rel="canonical" in HTTP headers, which is part of a link tag, found in the section of your HTML page.

This recent development is Google’s countermeasure to prevent the hacking of canonical tags in which Google has warned webmasters for the recent trend wherein websites are hacked in order to insert canonical tags which will point to the hacker’s site.

rel="canonical" HTTP headers can be used by webmasters or search engine optimization experts to identify the canonical (preferred) URL for both HTML documents and other types of content such as PDF files.

Used by professional SEOs to help Google recognize pages with similar content, the rel="canonical" tag can help save them from getting penalized by Google for duplicate content.

According to Google, the rel="canonical" tag could be used to indicate to Google your preferred URL for instance, for both HTML documents and PDF file such as seen in this example given by Google:

Google says:

In a website offering, a white paper both as an HTML page and as a downloadable PDF alternative is under these two URLs:

http://www.example.com/white-paper.html

http://www.example.com/white-paper.pdf

The webmaster can indicate to Google that the preferred (canonical) URL for the PDF download is the HTML document by using a rel="canonical" tag in the HTTP header when the PDF file is requested; for example:

GET /white-paper.pdf HTTP/1.1

Host: www.example.com

(...rest of HTTP request headers...)

HTTP/1.1 200 OK

Content-Type: application/pdf

Link: ; rel="canonical"
Content-Length: 785710
(... rest of HTTP response headers...)


Google also says that these link header elements can only be used for web search only for now as Google is hoping to add support for them in our other properties.

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