Users report certificate warnings or cannot reach the App Service API through the custom domain. How would you debug it?

Debug a TLS certificate or domain binding incident in App Service

Investigate production failures caused by custom domain, DNS, TLS certificate, or HTTPS binding problems.

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I would check whether the problem is DNS resolution, App Service custom domain binding, TLS certificate validity, certificate binding, HTTPS-only behavior, or a proxy in front of App Service. I would test from outside the corporate network and inspect the certificate presented to the client, including hostname and expiration. If only some users are affected, I would consider DNS propagation, caching, regional routing, or a CDN/front-door layer. I would communicate clearly because certificate and domain incidents look severe to users.

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DNS, custom domain bindings, TLS certificates, HTTPS enforcement, proxy boundaries, and communication during edge incidents.

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Edge incidents are visible and urgent because users cannot even reach the application path. This question tests basic but important production boundary thinking.
I would check whether the problem is DNS resolution, App Service custom domain binding, TLS certificate validity, certificate binding, HTTPS-only behavior, or a proxy in front of App Service. I would test from outside the corporate network and inspect the certificate presented to the client, including hostname and expiration. If only some users are affected, I would consider DNS propagation, caching, regional routing, or a CDN/front-door layer. I would communicate clearly because certificate and domain incidents look severe to users.
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I would compare the public path with the App Service configuration. The custom domain needs to point to the intended app, the app needs a matching binding, and the certificate needs to cover the hostname being used. If a gateway, Front Door, or CDN terminates TLS before App Service, I would identify where the bad certificate is presented and where redirects are generated. I would also verify that app-generated links, auth callbacks, and CORS settings use the public domain after the edge fix.

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Details

Checking only the Azure default hostname.
Ignoring where TLS terminates when a proxy is involved.
Forgetting auth callbacks and generated links after domain changes.
Not monitoring certificate expiration.

Users report certificate warnings or cannot reach the App Service API through the custom domain. How would you debug it?

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