Why Does Azure App Service Return HTTP 404?

When an application deployed to Azure App Service suddenly starts returning HTTP 404 – Not Found, it can be confusing —especially when: 

• The deployment completed successfully 
• The App Service shows as Running 
• No obvious errors appear in the portal 

This behaviour is more common than it appears and is often linked to routing, configuration, or platform :

In this article, I’ll walk through real-world reasons why Azure App Service can return HTTP 404 errors, based on issues . The goal is to help you systematically isolate the root cause—whether it’s application-level, configuration-related, or platform-specific. 

 

What Does HTTP 404 Mean in Azure App Service? 

An HTTP 404 response from Azure App Service means: 

The incoming request successfully reached Azure App Service, but neither the platform nor the application could locate the requested resource. 

 

This distinction is important. Unlike connectivity or DNS issues, a 404 confirms that: 

• DNS resolution worked 
• The request hit the App Service front end 
• The failure happened after request routing 

 

1. Incorrect Application URL or Route 

This is the most common cause of 404 errors. 

Typical scenarios 

• Accessing the root URL (https://<app>.azurewebsites.net) for a Web API that exposes only API routes 
• Missing route prefixes such as /api , /v1controller/action name segments 
• Case sensitivity mismatches on Linux App Service 

Example 

• https://myapp.azurewebsites.net   

Returns 404, but: 

• https://myapp.azurewebsites.net/weatherforecast 

Works as expected. 

✅ Tip: Always validate your routing locally and confirm the exact same path is being accessed in Azure. 

2. Application Appears Running, but Startup Failed Partially 

It is possible for an App Service to show Running even when the application failed to initialize fully. 

Common causes 

• Missing or incorrect environment variables 
• Invalid connection strings 
• Exceptions thrown during Program.cs / Startup.cs 
• Dependency initialization failures at startup 

In such scenarios, the app may start the host process but fail to register routes—resulting in 404 responses instead of 500 errors. 

✅ Where to check 

• Application logs 
• Deployment logs 
• Kudu → LogFiles 

3. Static Files Not Found or Not Being Served 

For applications hosting static content (HTML, JavaScript, images, JSON files), a 404 can occur even when files exist. 

Common reasons 

• Files not deployed to the expected directory (wwor root, /home/site/wwwroot) 
• Missing or unsupported MIME type configuration (commonly seen with .json) 
• Static file middleware not enabled in ASP.NET Core applications 

✅ Quick validation: Deploy a simple test.html to wwwroot and try accessing it directly. 

 

4. Windows vs Linux App Service Differences 

Behaviour can differ significantly between Windows App Service and Linux App Service. 

Common pitfalls on Linux 

• Case-sensitive file paths (Index.html ≠ index.html) 
• Missing or incorrect startup command 
• Differences in request routing handled by Nginx 

✅ Tip: If the app works on Windows App Service but fails on Linux, always recheck file casing and startup configuration first. 

 

5. Custom Domain and Networking Configuration Issues 

In some cases, requests reach the App Service but fail due to domain or network constraints. 

Possible causes 

• Incorrect custom domain binding 

✅ Isolation step: Always test using the default *.azurewebsites.net specific issues the issue is domain-specific. 

 

    6. Health Checks or Monitoring Probes Targeting Invalid Paths 

Seeing periodic 404 entries in logs—every few minutes—is often a sign of misconfigured probes. 

Typical scenarios 

• App Service Health Check configured with a non-existent endpoint 
• External monitoring tools probing /health or paths that do no exist

✅ Fix: Ensure the health check path maps to a valid endpoint implemented by the application.

 

   7.Missing or Corrupted Deployment Artifacts 

Even when deployments report success, application files may not be where the runtime expects them. 

Commonly observed with 

• Zip deployments 
• WEBSITE_RUN_FROM_PACKAGE misconfigurations 
• Partial or interrupted deployments 

✅ Verify using Kudu: Browse /home/site/wwwroot and check files are present. 

 

Quick Troubleshooting Checklist 

If your Azure App Service is returning HTTP 404: 

1. Verify the exact URL and route 
2. Test hostingstart.html or a static file (for example, /hostingstart.html) 
3. Review startup and application logs 
4. Inspect deployed artifacts via Kudu 
5. Validate Windows vs Linux behaviour differences 
6. Review networking, authentication, and health check settings 

 

 8. Application Gateway infront of App Service

If you have Application gateway infront of app service , please check the re-write rules so that the request is being sent to correct path.

 

Final Thoughts 

HTTP 404 errors on Azure App Service are rarely random. In most cases, they point to: 

• Routing mismatches 
• Startup or configuration failures 
• Platform-specific behavior differences 

 

By breaking the investigation into platform → configuration → application, you can systematically narrow down the root cause and resolve the issue. 

Happy debugging 🚀