Friday, June 17, 2011
Based on your feedback, we’re happy to announce that Google web search now supports
specified in HTTP headers as per the syntax described in
section 5 of IETF RFC 5988.
Webmasters can use
rel="canonical" HTTP headers to signal the canonical URL for both
HTML documents and other types of content such as PDF files.
To see the
rel="canonical" HTTP header in action, let’s look at the scenario of a
website offering a white paper both as an HTML page and as a downloadable PDF alternative, under
these two URLs:
In this case, the webmaster can signal to Google that the canonical URL for the PDF download is
the HTML document by using a
rel="canonical" 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: <http://www.example.com/white-paper.html>; rel="canonical" Content-Length: 785710 (... rest of HTTP response headers...)
Another common situation in which
rel="canonical" HTTP headers may help is when a
website serves the same file from multiple URLs (for example when using a content distribution
network) and the webmaster wishes to signal to Google the preferred URL.
We currently support these link header elements for web search only. As we see how webmasters are
using these elements, we're hoping to add support for them in our other properties. For more
information, please see our Help Center articles about
canonicalization and the
If you have any questions, please ask in our
Webmaster Help Forum.