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HTTP Error 400 – the size of the request headers is too long

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Error Description

The “HTTP Error 400. The size of the request headers is too long” is a server-side error indicating that the total size of the headers sent with the request exceeds the server’s limit. This error typically occurs in web browsers when a user tries to access a website, but it can also be encountered by web applications interacting with APIs.

Reason: Root Cause

The root cause of the HTTP Error 400 related to request header size is often due to one of the following scenarios:

  • An unusually large number of cookies sent in the request.
  • A single cookie that has grown too large over time (perhaps due to excessive data being stored).
  • Redundant or accumulated request headers due to misconfigured redirects or authentication loops.

Resolving the Error

Once the cause of the error is identified, you can proceed with the following solutions:

Clearing Browser Cookies

  1. Open the browser settings.
  2. Navigate to the Privacy or Security section.
  3. Locate the option to clear browsing data, ensuring cookies and cache are selected.
  4. Clear the data for the affected site or for all sites if necessary.

Increasing Server Limits

If you manage the server and find that legitimate requests are being rejected due to the header size:

  1. Locate the server’s configuration file.
  2. Depending on the server, find the directive responsible for limiting request header size, such as LimitRequestFieldSize in Apache.
  3. Increase the value judiciously, ensuring it is within reasonable limits to prevent abuse.
  4. Restart the server to apply changes.

Reviewing Application Code

If the error originates from a misconfigured application:

  1. Inspect and debug the application code to identify where excessive cookies or headers are being set.
  2. Refactor the code to reduce the size and number of headers being sent.

Configuring Reverse Proxies

If using a reverse proxy like Nginx:

  1. Edit the proxy configuration file.
  2. Set the large_client_header_buffers directive with appropriate buffer size and number.
  3. Restart the proxy to apply the changes.

Testing and Verification

After applying the fixes, it is crucial to test and verify that the HTTP Error 400 has been resolved. Here’s how you can do that:

  1. Attempt to reaccess the website or resource that was previously causing the error.
  2. Ensure that the web browser or application can successfully complete the request without errors.
  3. Conduct thorough testing across different browsers or environments to confirm the issue is resolved.
  4. Monitor server logs to make sure that the change does not inadvertently affect other parts of the system.
  5. If the error persists or reoccurs, revisit the debugging steps and confirm that all configurations have been applied correctly.

By following these guidelines, you should be able to effectively identify and rectify the “HTTP Error 400. The size of the request headers is too long” error without introducing new issues into the system.

ByRavishanker Kusuma
Updated on