What Is Cross-Domain Tracking in GA4?
Cross-domain tracking connects a visitor's activity across related domains, such as a storefront and a separate checkout.
In GA4, the domain linker transfers measurement identifiers through the _gl URL parameter so the destination can continue the same user and session when consent permits. Both domains need the same tag ID from the same web data stream.
- URL parameter
- _gl
- Both domains need
- the same tag IDthe same web data stream
Daniil Maximkin Published Reviewed
- In practice
What is passed between domains?
First-party measurement cookies belong to their own domain. The linker adds their values to the destination URL as
_gl; the destination tag reads a valid linker and stores the measurement identifiers there.Google recommends configuring GA4 domains in Admin, in the web stream’s tag settings under Configure your domains. I first check that the domains belong to one measured journey and that I can configure the tag on both sides.
- In practice
When does the handoff break?
A redirect can remove
_glbefore the destination tag reads it. An incorrect domain rule can leave links undecorated. JavaScript navigation or another script stopping click propagation can also prevent decoration.I check the destination stream and consent state as well as the URL. Seeing
_glin the address bar proves a parameter arrived; it does not prove the destination used the expected identity. - Not to be confused with
Not to be confused with Is not Is unwanted-referral rule the linker's transfer of measurement identifiers An unwanted-referral rule tells GA4 to ignore a matching referrer as a traffic source. It does not perform the linker’s transfer of measurement identifiers. I test identity continuity separately from referral handling.
A payment provider I cannot tag is a different situation from two domains I can configure. Adding it to a domain list does not install a tag there or create events it never sent.
- Where to check
How do I check cross-domain tracking?
- In a test journey, confirm both domains use the same GA4 tag ID and web stream. Review the configured domain rules against the actual link destination.
- Navigate through the real link with the relevant consent state. Check that
_glis added and the destination loads. - Preserve the browser network log and follow redirects. Find any step that removes the linker before the destination tag reads it.
- Compare the client and session identifiers in the measurement requests before and after the transition. Use the same browser and a continuous test journey.
- Check the resulting journey and source in a permitted test property. If the IDs change, trace the handoff before changing attribution settings.
- Sources
