Google HTTP rel=”canonical” Header support

Google’s search now supports the rel=”canonical” HTTP header allowing for canonical page and file linking (HTML and other formats)

The feature, though just introduced, only supports link headers for the Google web search but Google is hoping to increase support over time depending on how webmasters choose to use said elements. Google also introduced potential examples for how to implement canonical linking.

The example details a website which offers a white paper both as an HTML page and downloadable PDF. The PDF can be used as a substitute for the .HTML equivalent with the use of canonical:

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: <http://www.example.com/white-paper.html>; rel="canonical"
Content-Length: 785710
(... rest of HTTP response headers...)

Source – Google Webmaster Central Blog