We are building a true white-label SaaS platform and are trying to use Cloudflare for SaaS Custom Hostnames with Railway.

Architecture:

Customer Domain

→ Cloudflare for SaaS Custom Hostname

→ tenants-frontend.websarathi.com (Railway service)

→ Next.js application

Examples:

• xyz.com
• admin.xyz.com
• api.xyz.com

Each customer should be able to use their own domain without us manually adding domains inside Railway.

What we observed:

1. Railway service is available at:

   tenants-frontend.websarathi.com

2. Accessing:

   https://tenants-frontend.websarathi.com/api/host

returns:

{

"host": "tenants-frontend.websarathi.com",

"xForwardedHost": "tenants-frontend.websarathi.com"

}

1. We created a Cloudflare for SaaS custom hostname:

   test-customer.atmrtx.in

2. DNS setup:

   test-customer.atmrtx.in CNAME tenants-frontend.websarathi.com

3. When the hostname is NOT added as a custom domain in Railway:

   https://test-customer.atmrtx.in

returns:

HTTP 404

with header:

x-railway-fallback: true

and the Railway "The train has not arrived at the station" page.

1. If we add test-customer.atmrtx.in as a custom domain in Railway, the request immediately starts working.

Question:

Does Railway Edge validate incoming Host headers before routing requests to services?

If so, is there any supported way to allow arbitrary customer domains (Cloudflare for SaaS style) without manually registering every customer hostname inside Railway?

Are Cloudflare for SaaS Custom Hostnames officially supported with Railway as the origin?

Any guidance or recommended architecture would be appreciated.

Additional context:

We are using Cloudflare for SaaS Custom Hostnames.

Customer domains are configured as:

customer-domain.com

→ CNAME

→ tenants-frontend.websarathi.com

Cloudflare then forwards traffic to our Railway service.

Our goal is a true white-label SaaS where customer domains are onboarded dynamically and we do NOT manually register each customer hostname in Railway.

Current observation:

• If the customer hostname is NOT registered in Railway, we receive:

  x-railway-fallback: true

• If the same hostname IS registered in Railway, traffic reaches our Next.js application immediately.

Can Railway be used as the origin behind Cloudflare for SaaS custom hostnames without manually registering every customer domain?

Or does Railway Edge require explicit hostname registration for all incoming Host headers?

Additional note:

We already use Railway wildcard domains successfully for tenant websites. For example:

• silvara.websarathi.in
• kiaautocar.websarathi.in
• other dynamic subdomains

All are served by a single Railway service registered as:

*.websarathi.in

The application resolves tenants dynamically from the Host header and serves content correctly.

The issue only occurs when using Cloudflare for SaaS Custom Hostnames where customer-owned domains (e.g. customer-domain.com) are CNAME'd to our platform. In that scenario, Railway appears to return x-railway-fallback: true unless the hostname is explicitly registered.