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Every published application answers on a *.vertraweb.app subdomain. A custom domain lets the same application answer on an address you own, like api.yourcompany.com, with an HTTPS certificate the platform issues and renews for you.

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Before you start

The application is published to the web (Pro plan or higher, at least 512 MB of RAM).
Your account is on the Intermediary plan or higher.
You can edit the DNS records of the domain at your registrar or DNS provider.
Each application accepts one custom domain. Use a subdomain (app.example.com, www.example.com): DNS doesn’t allow a CNAME on the bare root (example.com) at most providers.

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Step by step

1

Add the domain in the dashboard

Open the application, go to Network and type the full hostname, for example app.example.com. The dashboard shows the target address for the CNAME record.
2

Create the CNAME record

At your DNS provider, create one record:Remove any existing A, AAAA or CNAME record with the same name first: two records for one name is the most common reason the domain never validates.
3

Wait for DNS and the certificate

Most providers propagate in minutes; the worst case is 48 hours. Once the record resolves, the platform validates it and provisions the SSL certificate automatically. HTTP is redirected to HTTPS.
4

Check it

The first command must print the target address; the second must answer with your application’s status code.

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Using Cloudflare

If the domain’s DNS lives on Cloudflare, create the CNAME with the proxy off (grey cloud, “DNS only”) until the certificate is issued. With the proxy on, validation sees Cloudflare’s addresses instead of your record. After HTTPS works you may turn the proxy on; in that case set the SSL mode to Full (strict), since the application already serves a valid certificate. The Flexible mode causes a redirect loop, because the platform redirects HTTP to HTTPS.

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Troubleshooting

Run dig +short app.example.com. No answer means the record doesn’t exist yet or has a typo; an IP address instead of the target means there’s still an old A record, or the Cloudflare proxy is on.
The certificate is only issued after DNS validation. Wait a few minutes after the record resolves, then reload. If it persists, remove and add the domain again in Network.
You’re behind a proxy in Flexible SSL mode. Switch it to Full (strict).
The domain is fine; the application isn’t listening. It must listen on 0.0.0.0 and the port in PORT (80). See Configuration.
Unpublishing the application removes its custom domain along with the subdomain. Publishing again means adding the domain again.