Updated 25 May 2021: Added information about using this with GA4. As Google Analytics 4 does not have a mechanism to disable cookie storage, only the second solution (send dataLayer events from iframe to the parent) described in this article will work for GA4. Here I am, back with <iframe> and cross-domain tracking. I’ve published a couple of articles before on the topic, with my upgraded solution being the most recent one.
**Last updated 18 September 2020: Due to how most browsers now have third-party cookie protections in place, this solution will be very ineffective going forwards. You should instead take a look at a cookieless solution. Some years ago, I wrote a post on how to track cross-domain iframes when using Google Tag Manager and Google Analytics. That solution relied on hitCallback to decorate the iframe, and now that I look back on it, it has its shortcomings.
NOTE! This solution has been upgraded, and the new approach can be found here. If you’re unfamiliar with the lingo, cross-domain tracking is a hack used by Google Analytics to circumvent the web browser’s same-origin policy. Essentially, the policy dictates that browser cookies can only be shared with a parent domain and all its sub-domains. In other words, domainA.com and domainB.com do not share cookies. Since Google Analytics calculates sessions and users by using a cookie, this is problematic.