Google Now Supporting rel="canonical" Across Domains

Google announced that it now offering cross-domain support of the rel="canonical" link element. If you are unfamiliar with this link element, Google's Matt Cutts discussed it. Basically, it's a way to avoid duplicate content issues, but until now, you couldn't use it across domains.

"For some sites, there are legitimate reasons to [have] duplicate content across different websites — for instance, to migrate to a new domain name using a web server that cannot create server-side redirects," says John Mueller, Webmaster Trends Analyst with Google Zürich.
"There are situations where it's not easily possible to set up redirects," he says. "This could be the case when you need to move your website from a server that does not feature server-side redirects. In a situation like this, you can use the rel='canonical' link element across domains to specify the exact URL of whichever domain is preferred for indexing. While the rel='canonical' link element is seen as a hint and not an absolute directive, we do try to follow it where possible."

Mueller gives the following ways of handling cross-domain content duplication:

- Choose your preferred domain
- Reduce in-site duplication
- Enable crawling and use 301 (permanent) redirects where possible
- Use the cross-domain rel="canonical" link element

Barry Schwartz at Search Engine Roundtable gives three reasons why the addition of cross-domain support for the rel="canonical" link element is really important:

1. Some hosts don't allow webmasters to deploy 301 redirects
2. Some site owners aren't technical enough to implement a 301 redirect
3. In some cases, webmasters do not want to redirect users but rather only search engines (i.e. pagination, weird filtering, tracking parameters added to URLs, etc).

To use the link element, pages don't have to be identical, but they should be similar. According to Google, slight differences are fine. You should not point point rel="canonical" to the home page of the preferred site. Google says this can result in problems, and that a mapping from an old URL to a new URL for each URL on the old site is the best way to go.

You should not use a nonindex robots meta tag on pages with a rel="canonical" link element because those pages would not be equivalent with regards to indexing, Google says. One would be allowed while the other would be blocked. Google also says it's important that these pages aren't disallowed from crawling through a robots.txt file, because search engine crawlers won't be able to discover the rel="canonical" link element.