Cross-domain linking works by the referring site adding parameters to the URL which the receiving site parses. (It's a little trickier with form submissions, but also doable.)

The main limitation is that first party cookies cannot be shared across different top level domains, i.e., x.com and y.com.

If we implement this feature, I propose:

• site would use setDomains('y.com')
• copy the idsite, uuid (from the visitor cookie), referring URL and timestamp (from the referral conversion cookie), and (maybe) custom variables
• setAllowLinker(true) (or perhaps a more obvious name to enable this feature, so as to not confuse with enableLinkTracking)

Maybe: tracker.enableCrossDomainLinker( boolCopyCookies );

tracker.enableCrossDomainsLinker (with a "s") maybe?

was your idea to pass the parameters, automatically on all clicks to domains that were specified in setDomains('y.com') ?

I think it's a must have indeed (especially with Ecommerce sites).

I envision it being automatic.

Notes
In GA it used to be done with:

_gaq.push(['_setDomainName', 'none']);
_gaq.push(['_setAllowLinker', true]);

Update 2014: In Universal analytics it is done this way:

ga('require', 'linker'); // Load the plugin.

// Define which domains to autoLink.
ga(linker:autoLink, [3-party.com, 3-party-domain.com]); //add as many as you need third party sites

on third party domain add

ga(create, 'UA-XXXXXX-X', { allowLinker: true });

See also this forum post

+1

I think we can partially accomplish it by exposing a setVisitorId method on the JS tracker, it will still require manual work for reading, and transferring the visitor id between domains, but it's still better then the current situation.

Yes, I believe that's already done and the docs are here: http://piwik.org/docs/user-id/
What I'm hoping for is a native baked-in solution that generates a unique ID and appends it to all the links that go to alias domains on the page, so that we can consistently track the same user across multiple domains (not just subdomains).

+100

PR: #11243

Cross domain tracking is available in Piwik 3.0.2-b1 release.

To use it configure it as follows in the Piwik Tracker code:

_paq.push(['setDomains', ['domain-1.net', 'domain-2.net']]);
_paq.push(['enableCrossDomainLinking']);
_paq.push(['setSiteId', X]);

(when adding both these lines in domain-1.net and domain-2.net -> users clicking from either website to the other website, will be automatically recognised as the same visitor and tracked in the same visit in website id X)

Before closing the issue:

• Document cross domain feature in the JavaScript tracking guide
• Add new FAQ
• Create a separate issue for backend improvement: ensure that Visitor ID is the same across all visits of a given visitor and re-uses the last visitor id value set or similar

when adding both these lines in domain-1.net and domain-2.net

We need to clearly mention it needs to be enabled on all domains / websites

backend improvement: ensure that Visitor ID is the same across all visits of a given visitor and re-uses the last visitor id value set

We should do it actually as part of this issue, it is in most cases not working without it.

Note: Cross Domain feature is also useful when the data is tracked into different websites in Piwik. In that case, there won't be just "one visit" created, but "one visit per website". The advantage of using cross domains is that those two visits across website IDs will have the same Visitor ID. Could be super useful and we need to mention it in docs 👍

Important:

• Need to add a new checkbox in JavaScript Tracker Code screen, to let users enable cross domain tracking which will 1) enable the setDomains existing checkbox automatically and 2) output in the code the _paq.push(['enableCrossDomainLinking']);

• Created new FAQ: How do I accurately measure a same visitor across multiple domain names (cross domain linking)?
• Added link into related faqs https://piwik.org/faq/new-to-piwik/faq_104/ and into manage websites user guide
• and developer guide https://github.com/piwik/developer-documentation/pull/175/files for JS Tracker code guide