Google Analytics 4 Cross-Domain Tracking

Overview

Cross-domain tracking in Google Analytics 4 ensures user sessions persist when visitors navigate between domains you own. GA4 uses the _gl linker parameter to pass client identifiers across domain boundaries, preventing session fragmentation and preserving accurate attribution.

When Cross-Domain Tracking Is Required

• Separate checkout domains: Main site on www.example.com, checkout on checkout.example.com
• Third-party payment processors: Redirects to payment gateways (Stripe, PayPal) that return users to confirmation pages
• Regional or brand domains: example.com, example.co.uk, brand.example.com
• Microsites and landing pages: Campaign-specific domains that funnel to conversion points
• App-to-web experiences: Mobile app links opening web views on different domains

Configuration Methods

Google Tag Manager Setup

1. Navigate to your GA4 Configuration tag
2. Expand Configure tag settings > More Settings > Cross-domain measurement
3. Add all domains that should share sessions:
   • example.com
   • checkout.example.com
   • payments.thirdparty.com
4. Enable Enable cross-domain measurement

Alternatively, configure in the data stream settings:

1. Go to Admin > Data Streams > [Your Stream] > Configure tag settings
2. Click Configure your domains
3. Add match conditions for each domain

gtag.js Direct Implementation

gtag('config', 'G-XXXXXXXXXX', {
  linker: {
    domains: ['example.com', 'checkout.example.com', 'partner.com'],
    accept_incoming: true,
    decorate_forms: true
  }
});

Domain Configuration Options

Referral Exclusion Configuration

Prevent internal domains from appearing as referral traffic sources:

1. Go to Admin > Data Streams > [Your Stream] > Configure tag settings
2. Click List unwanted referrals
3. Add match conditions:
   • Match type: Contains
   • Domain: checkout.example.com

Without referral exclusions, users returning from checkout or payment domains will start new sessions, breaking attribution.

How the Linker Works

When a user clicks a cross-domain link:

1. GA4 detects the link destination matches a configured domain
2. The _gl parameter is appended with encoded client/session data
3. The destination domain reads the parameter and restores the session

Example decorated URL:

https://checkout.example.com/cart?_gl=1*abc123*_ga*MTIzNDU2Nzg5MA..*_ga_XXXXXXXX*MTY4...

The _gl parameter contains:

• Client ID (_ga)
• Session ID and timestamp
• Cross-domain linking metadata

Third-Party Checkout Integration

When using external payment providers:

Supported Flow

example.com → stripe.com → example.com/confirmation

1. Pre-redirect: Store the GA client_id in a server session or URL parameter
2. Payment callback: Include the _gl parameter in the return URL
3. Confirmation page: GA4 reads the linker and restores the session

Configuration for Payment Providers

Most payment providers support custom return URLs:

// Stripe checkout session
const session = await stripe.checkout.sessions.create({
  success_url: 'https://example.com/success?session_id={CHECKOUT_SESSION_ID}&_gl=' + getGALinker(),
  cancel_url: 'https://example.com/cart'
});

Retrieve the linker parameter:

function getGALinker() {
  return new URLSearchParams(window.location.search).get('_gl') || '';
}

Edge Cases and Considerations

iFrame Embeds

When content is embedded via iframes across domains:

• Parent and child frames need separate GA4 instances
• Use postMessage to pass client_id between frames
• Consider whether iframe tracking genuinely adds value

Single Page Applications (SPAs)

Ensure the linker decorator attaches to dynamically rendered links:

// After SPA navigation or dynamic content load
gtag('event', 'page_view', {
  page_location: window.location.href,
  page_title: document.title
});

Link Decoration Blockers

Safari ITP and privacy extensions may strip URL parameters:

• Monitor cross-domain conversion rates for drops indicating blocking
• Implement server-side session stitching as a fallback
• Use first-party subdomains where possible

Consent Mode Integration

Cross-domain linking respects Consent Mode settings:

• With analytics_storage: denied, the linker still works but uses cookieless pings
• Client identity may be less persistent without cookies
• Ensure consent state is consistent across all domains

Validation and Testing

GTM Preview Mode

1. Enable GTM Preview on the source domain
2. Click a cross-domain link
3. Verify the _gl parameter appears in the URL
4. Open GTM Preview on the destination domain
5. Confirm the same client_id appears in both contexts

GA4 DebugView

1. Enable DebugView in GA4 (Admin > DebugView)
2. Install the GA Debugger browser extension
3. Navigate through the cross-domain flow
4. Verify session_start only fires once at the journey start
5. Confirm client_id remains consistent across domains

Network Tab Inspection

Check that the linker parameters persist:

1. Open browser DevTools > Network
2. Click a cross-domain link
3. Inspect the request URL for _gl parameter
4. On the destination, verify the collect request includes the restored client_id

Real-Time Reports

After implementing:

1. Open GA4 Real-Time reports
2. Navigate through the cross-domain journey
3. Confirm a single user appears moving through pages on different domains
4. Check that traffic source attribution is preserved

Troubleshooting

BigQuery Validation

For sites with BigQuery export enabled, validate cross-domain tracking with SQL:

SELECT
  user_pseudo_id,
  event_name,
  (SELECT value.string_value FROM UNNEST(event_params) WHERE key = 'page_location') AS page_location,
  event_timestamp
FROM `project.analytics_XXXXXX.events_*`
WHERE _TABLE_SUFFIX BETWEEN '20240101' AND '20240131'
  AND user_pseudo_id = 'your_test_client_id'
ORDER BY event_timestamp;

This query reveals the full journey with consistent user_pseudo_id across domains.