Howie Jacobson, author of AdWords for Dummies, asked me about the SEO implications of aliasing multiple domain names to one IP address as a workaround for Google’s new AdWords rule change. As of April 1st, Google no longer allows a display URL in your Google ad if it redirects to a different domain name. This rule change is unfortunate, as it restricts search marketers’ ability to A/B test different domain names in their Google ads. So if you wanted to test www.widgetshelp.com versus your main URL of www.widgetsllc.com, you’d need to remove the redirect from www.widgetshelp.com. Thus you’d have a CNAME or A record in your DNS server for widgetshelp.com, and you’d have a “ServerAlias www.widgetshelp.com widgetshelp.com” line in your Apache config. But without a redirect, search engine spiders won’t know which URL is the canonical one. Widgetshelp.com and widgetsllc.com would look like duplicates of each other, and the link juice to each URL would be split up rather than aggregated together. This becomes an issue when there are links to the alternate domain, as the spiders will find that duplicate site. By the way, don’t worry about spiders following the link in the Google ad, as that is not a spiderable link. Unfortunately the “Preferred Domain” tool in Google Webmaster Central is not going to be a help here, because it only lets you select between www vs. non-www. You can’t specify multiple domain names — at least not yet. Back in 2006 I recommended to Google that they Extend their canonicalization feature in Webmaster Central. Even though they haven’t heeded my request, I’m still hopeful that they someday will. In the meantime, there’s a gray hat solution you could implement — a conditional redirect that only spiders see. It’s not without risks, however. You have been warned. Alternatively, you could contact the site owners who have linked to your alternate domains and ask them to update the link to your main URL. That can only really work if you only have just a few links pointing to your alternate domain. And obviously, don’t link to your alternate domain yourself!