Tracking anonymous visitors across multiple domains is difficult because cookies can’t be shared across domains. One way around this is to use an iframe to set the cookie, which works well in Chrome, Firefox, and Edge. However, Safari prevents a challenge because it prevents third-party cookies from being set.

I haven’t found a solution for the Safari issue yet, but thought I’d share the method I’ve worked out for cross domain tracking with Mixpanel for the remaining browsers.

For the example, I’ll use domains a A.com and B.com. A.com is the primary website and B.com is a marketing website that refers traffic to A.com.

On domain A.com you’ll need to set up an URL that loads the tracking code when it’s called inside of an iframe. We can pass tracking data from B.com to the iframe using a query string, and an efficient method for passing that data is by base64 encoding an JSON object. Continue reading →