A post on Google’s Webmaster Central Blog offers information on preventing cross-domain URL issues and announces a new notifications feature for webmasters. If their algorithms choose to index a URL outside of your site, they will attempt to notify you within the Webmaster Tools system.
There are a number of reasons similar/same content may appear on more than one URL: similar content targeting different geographical areas or multilingual sites, plagiarized content, server misconfiguration, hacking via 301 redirects, or hijacking the HTML or HTTP header, among others. Google chooses one of the URLs to index, whether it’s within the same domain or not. When similar/duplicate content spans across domains and Google has to choose one to index, they’re making a cross-domain URL decision.
Depending on whether you own all of the properties involved, the wrong decision can either be a bit of a nuisance or a huge problem. It may not be a big deal if a URL on another of your sites gets indexed over the one you preferred (though it could be). A worse scenario might be a hacker sticking a rel="canonical" link in your header that points to a spam site. Until now, it’s been difficult for webmasters to get answers as to why a cross-domain URL decision went the way it did.
Webmaster Tools Cross-Domain URL Notifications - What to Expect
In their blog post announcing the change, Google offers a number of actions webmasters can take to prevent/correct duplicate content, canonicalization, server configuration, and hacking issues. This information is reflected in an updated Webmaster Tools Help article.
The notifications feature is certainly a welcome addition to the toolkit, but there are a few things you need to know. If you’re moving your entire site, for example, the volume of cross-domain URL selections may trigger Google to stop the notifications; you could receive some and not others. They also won’t send notifications multiple times for the same URL.
Set up email delivery for WMT messages to get these notifications right away. Remember that notifications are only available for your top pages, the ones your WMT account is already monitoring.
The actual language in Google’s WMT Help article says, “...we may send a message via Webmaster Tools to explain that a cross-domain URL selection has happened.” It’s not clear whether they intend to say whyit has happened (I’m guessing not).
The links and resources offered in the blog post seem to indicate that the notification will alert the webmaster to a situation they probably would have discovered in short order anyway, when traffic to the affected page took a nose-dive. It would then be up to the user to determine which, among the reasons Google offers as possible causes, is the culprit. It will be interesting to see how much information the notifications provide.
This is the latest in a series of updates to Webmaster Tools; early last month, Google integrated WMT with Analytics. In September, they added the Website Health feature and changed how external and internal links are categorized by making subdomain links internal.