HTTP status code

The HTTP 431 status code stands for "Request Header Fields Too Large". It is a client error response indicating that the server is refusing to process the request because the headers sent by the client are too large or contain too many fields.

When is HTTP 431 Used?

• The 431 Request Header Fields Too Large status code is used when the client’s request headers exceed the server’s configured limit. HTTP headers are used to convey metadata about the request, such as authentication tokens, cookies, and content types.
• This status code helps prevent a DoS (Denial of Service) attack or resource exhaustion due to excessive header sizes, which could overwhelm the server’s capacity to handle incoming requests.

Common Causes of HTTP 431 Errors

• Too many cookies: If a client sends a request with a large number of cookies, the total size of the cookies might exceed the server's configured limit.
• Large request headers: If the request headers contain very large amounts of data (e.g., large authentication tokens, cookies, or custom headers), the server might respond with a 431 error.
• Overly long URLs: Sometimes, excessively long URLs (which may include query parameters or other data in the URL) can contribute to large headers, potentially triggering this error.
• Excessive custom headers: Applications that add a large number of custom headers or non-standard headers to requests may cause the overall header size to grow beyond the server's acceptable limits.

Example Scenarios

• A client sends a login request with a very large number of cookies, causing the request header size to exceed the server’s limit, triggering a 431 error.
• An API request with large custom headers (e.g., multiple authorization tokens or lengthy user-agent strings) may lead to a 431 error if the total header size is too large.

Example

Client Request (Excessive Header Size)

    
        GET /api/v1/data HTTP/1.1
        Host: example.com
        Cookie: sessionId=abc123xyz... (very large cookie data)
        Authorization: Bearer <long-token>
        X-Custom-Header: <large-custom-data>
    

(The client sends a request with large headers, such as excessive cookies or long custom headers.)

Server Response (431 Request Header Fields Too Large)

    
        HTTP/1.1 431 Request Header Fields Too Large
        Content-Type: application/json

        {
          "error": "The request headers are too large. Please reduce the size of the headers and try again."
        }
    

Best Practices for Handling HTTP 431

• Reduce header size: Clients should minimize the size of their request headers. This includes reducing the number of cookies, limiting the size of individual header fields, and shortening URLs or custom headers if possible.
• Use more efficient authentication methods: If using large authentication tokens or session identifiers, consider alternatives that use smaller tokens, or use session management that avoids sending large amounts of data in the headers.
• Consider compression: If the headers must contain large amounts of data, compression techniques may be used to reduce their size.
• Monitor server limits: Server-side applications should configure appropriate header size limits and handle situations where those limits are exceeded. This may include informing clients about acceptable header size limits in API documentation.
• Handle retries gracefully: When a 431 error occurs, clients should respond by reducing the size of their request headers and retrying the request. This can include clearing cookies, reducing the number of headers, or splitting large data into smaller requests.

Summary

HTTP 431 indicates that the client's request headers are too large for the server to process. This can happen when headers contain too many fields, large cookies, lengthy URLs, or large custom headers. To resolve the issue, clients should reduce the size of their headers and retry the request. Servers may configure appropriate limits and provide feedback to clients on how to comply with these limits.