Troubleshoot ASP.NET Core on Azure App Service and IIS

App startup errors

In Visual Studio, an ASP.NET Core project defaults to IIS Express hosting during debugging. A 502.5 - Process Failure or a 500.30 - Start Failure that occurs when debugging locally can be diagnosed using the advice in this topic.

403.14 Forbidden

The app fails to start. The following error is logged:

The Web server is configured to not list the contents of this directory.

The error is usually caused by a broken deployment on the hosting system, which includes any of the following scenarios:

The app is deployed to the wrong folder on the hosting system.
The deployment process failed to move all of the app's files and folders to the deployment folder on the hosting system.
The web.config file is missing from the deployment, or the web.config file contents are malformed.

Perform the following steps:

Delete all of the files and folders from the deployment folder on the hosting system.

Redeploy the contents of the app's publish folder to the hosting system using your normal method of deployment, such as Visual Studio, PowerShell, or manual deployment:

Confirm that the web.config file is present in the deployment and that its contents are correct.
When hosting on Azure App Service, confirm that the app is deployed to the D:\home\site\wwwroot folder.
When the app is hosted by IIS, confirm that the app is deployed to the IIS Physical path shown in IIS Manager's Basic Settings.

Confirm that all of the app's files and folders are deployed by comparing the deployment on the hosting system to the contents of the project's publish folder.

500 Internal Server Error

The app starts, but an error prevents the server from fulfilling the request.

This error occurs within the app's code during startup or while creating a response. The response may contain no content, or the response may appear as a 500 Internal Server Error in the browser. The Application Event Log usually states that the app started normally. From the server's perspective, that's correct. The app did start, but it can't generate a valid response. Run the app at a command prompt on the server or enable the ASP.NET Core Module stdout log to troubleshoot the problem.

This error also may occur when the .NET Core Hosting Bundle isn't installed or is corrupted. Installing or repairing the installation of the .NET Core Hosting Bundle (for IIS) or Visual Studio (for IIS Express) may fix the problem.

500.0 In-Process Handler Load Failure

The worker process fails. The app doesn't start.

An unknown error occurred loading ASP.NET Core Module components. Take one of the following actions:

Contact Microsoft Support (select Developer Tools then ASP.NET Core).
Ask a question on Stack Overflow.
File an issue on our GitHub repository.

500.30 In-Process Startup Failure

The worker process fails. The app doesn't start.

The ASP.NET Core Module attempts to start the .NET Core CLR in-process, but it fails to start. The cause of a process startup failure can usually be determined from entries in the Application Event Log and the ASP.NET Core Module stdout log.

Common failure conditions:

The app is misconfigured due to targeting a version of the ASP.NET Core shared framework that isn't present. Check which versions of the ASP.NET Core shared framework are installed on the target machine.

Using Azure Key Vault, lack of permissions to the Key Vault. Check the access policies in the targeted Key Vault to ensure that the correct permissions are granted.

500.31 ANCM Failed to Find Native Dependencies

The worker process fails. The app doesn't start.

The ASP.NET Core Module attempts to start the .NET Core runtime in-process, but it fails to start. The most common cause of this startup failure is when the Microsoft.NETCore.App or Microsoft.AspNetCore.App runtime isn't installed. If the app is deployed to target ASP.NET Core 3.0 and that version doesn't exist on the machine, this error occurs. An example error message follows:

The specified framework 'Microsoft.NETCore.App', version '3.0.0' was not found.
- The following frameworks were found:
2.2.1 at [C:\Program Files\dotnet\x64\shared\Microsoft.NETCore.App]
3.0.0-preview5-27626-15 at [C:\Program Files\dotnet\x64\shared\Microsoft.NETCore.App]
3.0.0-preview6-27713-13 at [C:\Program Files\dotnet\x64\shared\Microsoft.NETCore.App]
3.0.0-preview6-27714-15 at [C:\Program Files\dotnet\x64\shared\Microsoft.NETCore.App]
3.0.0-preview6-27723-08 at [C:\Program Files\dotnet\x64\shared\Microsoft.NETCore.App]

The error message lists all the installed .NET Core versions and the version requested by the app. To fix this error, either:

Install the appropriate version of .NET Core on the machine.
Change the app to target a version of .NET Core that's present on the machine.
Publish the app as a self-contained deployment.

When running in development (the ASPNETCORE_ENVIRONMENT environment variable is set to Development), the specific error is written to the HTTP response. The cause of a process startup failure is also found in the Application Event Log.

500.32 ANCM Failed to Load dll

The worker process fails. The app doesn't start.

The most common cause for this error is that the app is published for an incompatible processor architecture. If the worker process is running as a 32-bit app and the app was published to target 64-bit, this error occurs.

To fix this error, either:

Republish the app for the same processor architecture as the worker process.
Publish the app as a framework-dependent deployment.

500.33 ANCM Request Handler Load Failure

The worker process fails. The app doesn't start.

The app didn't reference the Microsoft.AspNetCore.App framework. Only apps targeting the Microsoft.AspNetCore.App framework can be hosted by the ASP.NET Core Module.

To fix this error, confirm that the app is targeting the Microsoft.AspNetCore.App framework. Check the .runtimeconfig.json to verify the framework targeted by the app.

500.34 ANCM Mixed Hosting Models Not Supported

The worker process can't run both an in-process app and an out-of-process app in the same process.
To fix this error, run apps in separate IIS application pools.

500.35 ANCM Multiple In-Process Applications in same Process

The worker process can't run multiple in-process apps in the same process.
To fix this error, run apps in separate IIS application pools.

500.36 ANCM Out-Of-Process Handler Load Failure

The out-of-process request handler, aspnetcorev2_outofprocess.dll, isn't next to the aspnetcorev2.dll file. This indicates a corrupted installation of the ASP.NET Core Module.

To fix this error, repair the installation of the .NET Core Hosting Bundle (for IIS) or Visual Studio (for IIS Express).

500.37 ANCM Failed to Start Within Startup Time Limit

ANCM failed to start within the provided startup time limit. By default, the timeout is 120 seconds.

This error can occur when starting a large number of apps on the same machine. Check for CPU/Memory usage spikes on the server during startup. You may need to stagger the startup process of multiple apps.

500.38 ANCM Application DLL Not Found

ANCM failed to locate the application DLL, which should be next to the executable.

This error occurs when hosting an app packaged as a single-file executable using the in-process hosting model. The in-process model requires that the ANCM load the .NET Core app into the existing IIS process. This scenario isn't supported by the single-file deployment model. Use one of the following approaches in the app's project file to fix this error:

Disable single-file publishing by setting the PublishSingleFile MSBuild property to false.

Switch to the out-of-process hosting model by setting the AspNetCoreHostingModel MSBuild property to OutOfProcess.

502.5 Process Failure

The worker process fails. The app doesn't start.

The ASP.NET Core Module attempts to start the worker process but it fails to start. The cause of a process startup failure can usually be determined from entries in the Application Event Log and the ASP.NET Core Module stdout log.

A common failure condition is the app is misconfigured due to targeting a version of the ASP.NET Core shared framework that isn't present. Check which versions of the ASP.NET Core shared framework are installed on the target machine. The shared framework is the set of assemblies (.dll files) that are installed on the machine and referenced by a metapackage such as Microsoft.AspNetCore.App. The metapackage reference can specify a minimum required version. For more information, see The shared framework.

The 502.5 Process Failure error page is returned when a hosting or app misconfiguration causes the worker process to fail:

Failed to start application (ErrorCode '0x800700c1')

EventID: 1010
Source: IIS AspNetCore Module V2
Failed to start application '/LM/W3SVC/6/ROOT/', ErrorCode '0x800700c1'.

The app failed to start because the app's assembly (.dll) couldn't be loaded.

This error occurs when there's a bitness mismatch between the published app and the w3wp/iisexpress process.

Confirm that the app pool's 32-bit setting is correct: