When encountering an IIS 500 error, 1. First check whether the Web.config file has syntax errors or configuration conflicts, such as the tag is not closed or repeated configuration; 2. Confirm whether the application pool status and settings are correct, including the running status, .NET CLR version and access permissions; 3. Turn on detailed error information to obtain specific error clues, which can be implemented through IIS manager or web.config configuration; 4. Check for code exceptions and dependency problems, such as database connection failure, DLL missing or unhandled backend exceptions. The above steps help accurately locate and resolve the specific causes of 500 errors.

Encountering an IIS 500 error usually means that the server is having problems processing the request, but the specific reasons may be varied. This type of error will not directly tell you where the error occurred, and you need to troubleshoot according to the log and configuration.

Check whether the Web.config file has syntax errors

IIS reads the web.config file when loading the website configuration. If there is a typo in this file, the label is not closed, or the non-existent module is referenced, it will result in a 500 error.

• Use the XML Verification Tool to check if the format is correct
• If you have recently modified web.config, you can try to restore to the previous version to see if the problem disappears
• Pay attention to configuration items added by third-party modules, such as URL rewrite rules or custom error settings

A common example is that if a duplicate <modules></modules> configuration is accidentally added to the <system.webserver></system.webserver> node, an error will be reported if IIS cannot handle it.

View application pool status and settings

If the application pool does not run correctly, it will also throw a 500 error. for example:

• The application pool is in a "stop" state
• The .NET CLR version settings do not match (for example, the code using .NET 4.0 is configured as 2.0)
• Program pool ID does not have access rights (especially when using a custom account)

You can view the status of the application pool through IIS Manager or find relevant error information in the Event Viewer.

If you are not sure about the settings of the current program pool, you can try creating a new program pool and binding the site to past tests.

Turn on detailed error information and locate specific problems

By default, IIS may only return a common 500 error page, which is not very helpful for troubleshooting. You can temporarily enable detailed error information to get more clues.

The operation steps are as follows:

• Open IIS Manager and find your site
• Double-click the "Error Page" function
• Click "Edit Function Settings" on the right
• Change "Error Response" to "Detailed Error"

In addition, you can also add the following configuration in web.config :

 <configuration>
  <system.webServer>
    <httpErrors errorMode="Detailed" />
  </system.webServer>
  <system.web>
    <customErrors mode="Off" />
  </system.web>
</configuration>

This way the browser will display a specific error stack, which will help determine whether it is a code or configuration problem.

Check for code exceptions and dependencies

Sometimes the 500 error is actually caused by an unhandled exception thrown by the backend code, especially in ASP.NET applications.

• View Applications logs in Windows Event Viewer
• Check whether there are any problems such as database connection failure, insufficient permissions, missing DLLs, etc.
• If you are deploying a .NET Core application, confirm that the corresponding runtime is installed

For example, if your application attempts to connect to an unreachable database without exception capture, it may cause the entire request to fail and return 500.

Basically these common situations. Although the error of IIS 500 seems scary, it can be solved most of the time just by stepping down. The key is to open the logs, look at the details, and don't be scared by the surface error codes.

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