HTTP Error 400, also known as "Bad Request," occurs when the client sends a request to the server with invalid syntax. This can happen for various reasons, but one common cause is when the HTTP request uses an unsupported or incorrect HTTP verb (request method) for the specified URL.

In IIS (Internet Information Services) 8.5, the default HTTP verbs that are supported are GET, POST, HEAD, PUT, DELETE, OPTIONS, and TRACE.

Here are some steps you can take to troubleshoot and resolve the "HTTP Error 400: The request verb is invalid" issue in IIS 8.5:

  1. Check the HTTP Verb (Request Method): Review the client's HTTP request and make sure it is using a valid HTTP verb. Verify that the HTTP verb being used is one of the supported verbs mentioned above.

  2. Check for Typos or Incorrect URL: Ensure that the URL in the client's HTTP request is correct and free of typos. Verify that the URL is pointing to a valid endpoint on the server.

  3. Check for URL Rewriting or Proxy Configuration: If you are using URL rewriting or a reverse proxy in your IIS configuration, make sure that it is not causing conflicts with the HTTP request's verb. Incorrectly configured rewriting or proxy rules can lead to a "Bad Request" error.

  4. Inspect the Request Headers and Body: Use tools like Fiddler or browser developer tools to inspect the request headers and body. Verify that the request is well-formed and contains the correct headers for the chosen HTTP verb.

  5. Check for Firewall or Security Software Interference: Ensure that there are no firewall rules or security software that may be interfering with the request and causing it to be blocked or modified.

  6. Check for Server-Side Code Issues: If the request is reaching the server, verify that there are no issues with the server-side code that could be causing the error. For example, check for any custom HTTP modules or handlers that may be affecting the request processing.

  7. Restart IIS: Sometimes, restarting IIS can help resolve temporary issues that may be causing the "Bad Request" error.

  8. Check the Server Logs: Review the server logs for any additional details or error messages that could provide more insights into the cause of the error.

If you are still unable to identify the root cause of the issue, consider providing more details about the specific request and configuration, so that further assistance can be provided.

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