Cross-Domain Tracking Setup

Cross-domain tracking is needed when users travel between two separate domains — say yourstore.com and checkout.payments-provider.com — and you want GA4 to treat the entire journey as a single session. Without it, the payment domain will show yourstore.com as a referrer, breaking attribution.

Tip

Cross-domain tracking is NOT needed for subdomains of the same root domain (e.g., shop.example.com and blog.example.com). GA4 handles subdomains automatically with the default cookie configuration. See Subdomain Tracking for that scenario.

How GA4 cross-domain tracking works

GA4 appends a _gl parameter to links that cross domain boundaries. This parameter encodes the current session information. When the user lands on the second domain, GA4 reads the _gl parameter and restores the session — keeping the user in the same session rather than starting a new one.

Example URL with _gl parameter:

https://checkout.payments-provider.com/pay?cart=abc&_gl=1*abc123*_ga*...

The second domain’s GA4 measurement reads the _gl parameter before setting its own cookie, carrying the session forward.

Step 1 — Configure cross-domain in GA4

1. Go to GA4 → Admin → Data Streams → [your stream] → Configure tag settings

2. Expand Configure your domains

3. Add the conditions for ALL domains in your user journey:

   • yourstore.com
   • checkout.payments-provider.com
   • Any other domains users might travel through

4. Match type options:

   • Contains: most flexible, use for payments-provider.com (covers all subdomains)
   • Equals: exact domain match
   • Begins with: for paths that differ
   • Matches regex: for complex patterns

5. Save changes

Step 2 — Configure cross-domain in GTM (if using Google Tag)

If you deploy GA4 via GTM using the Google Tag, configure cross-domain in the tag:

1. Open your Google Tag configuration in GTM (not the GA4 Event tag — the Google Tag itself)

2. Expand Configuration settings → Fields to set

3. Add a field:

   • Field name: linker
   • Value: a JavaScript object (use a Custom JavaScript Variable)

   Create a Custom JavaScript Variable named Linker Configuration:

   function() {
     return {
       domains: ['yourstore.com', 'checkout.payments-provider.com']
     };
   }

   Set linker → {{Linker Configuration}}

   Or simply use the Cross-domain settings section in newer Google Tag configurations and list the domains directly.

4. Publish the container changes

Step 3 — Add referral exclusions in GA4

When a user returns from checkout.payments-provider.com to your confirmation page on yourstore.com, GA4 may count this as a new session with payments-provider.com as the source. Prevent this by adding referral exclusions.

1. Go to GA4 → Admin → Data Streams → [your stream] → Configure tag settings
2. Expand List unwanted referrals
3. Add payments-provider.com (or all domains in your cross-domain setup)
4. Save changes

This prevents the return journey from being counted as a referral.

Verifying cross-domain tracking works

1. Open your main site in a browser
2. Navigate to a page and open the browser console
3. Run: document.cookie — look for the _ga cookie value, e.g., GA1.1.123456789.1616000000
4. Click a link that goes to your second domain
5. In the URL bar, check that _gl= is appended to the URL before the page loads on the second domain
6. On the second domain, run document.cookie again — the _ga value should match the first domain
7. In GA4 DebugView, verify both domains show as a single session in the stream view

If the _gl parameter is not being appended, cross-domain linking is not configured correctly. Double-check the domain list in GA4 or the Google Tag linker configuration.

Common multi-domain scenarios

Main site + third-party checkout:

• Add the checkout domain to your GA4 cross-domain configuration
• Add it to referral exclusions
• Verify the checkout platform allows the _ga cookie (some platforms strip unknown cookies)

Multi-brand / multi-country domains:

• brand-uk.com and brand-us.com running the same GA4 property
• List both in the cross-domain configuration
• Consider whether you actually want the same property — sometimes two properties is the right answer

Landing page platform (Unbounce, Leadpages) → main site:

• If the landing page uses your GA4 measurement ID, add it to cross-domain
• If the landing page uses a different GA4 property, cross-domain will not help — use UTM parameters instead

Test it

1. Navigate from your main domain to the second domain by clicking an internal link
2. Inspect the URL in the address bar — _gl= should appear in the query string
3. In GA4 DebugView: the session should appear in both domains without a new session starting
4. Check GA4 → Reports → Acquisition → Traffic Acquisition — payments-provider.com should not appear as a referral source

Common gotchas

Links open in new tabs. When target="_blank" is used, the _gl parameter is still appended. This is correct behavior.

JavaScript-based navigation. If your site uses window.location.href = 'https://other-domain.com' programmatically, the linker may not append _gl. Use window.open() or anchor tags instead, or call the get() method from the gtag linker plugin to get the URL with _gl appended.

Consent Mode blocks cookies. If the user has not consented and analytics_storage is denied, GA4 will not set or read the _ga cookie. Cross-domain tracking will not work for these users — this is expected and correct behavior from a privacy perspective.

Subdomain Tracking Track across subdomains — a much simpler scenario.

iFrame Tracking When the other domain is embedded as an iframe.