Urchin Q and A: Utmlinker Function and UTM Cookies Across Domains

Question: Can you describe in detail how the utmlinker function and utm cookies work across domains?

Answer: Urchin cookies are 1st party. Thus, if you intend to track visitors across a primary domain and/or sub-domains of that primary domain, you have to set cookies to the primary domain. We recommend you set _udn equal to whatever the primary domain is the page resides on. The reason for __utmLinker() is to copy values from one set of cookies to another set, usually between different primary domains all together. The uLink option enables Urchin to execute utmLinker() and also to look for utmLinker values in the URI.
If your site spans multiple primary domains, you will also want to set _uLink=1.