The HTTP 506 status code stands for "Variant Also Negotiates". It is a server error response that indicates a conflict in content negotiation. This error occurs when the server is unable to fulfill the request due to a misconfiguration in its content negotiation mechanism.

When is HTTP 506 Used?

  • The 506 Variant Also Negotiates error is returned when the server's content negotiation configuration itself leads to a conflict. Specifically, it occurs when the server cannot determine the appropriate variant of the requested resource because the content negotiation process is itself ambiguous or recursive.
  • Content negotiation is a mechanism used by HTTP servers to select the appropriate version of a resource based on parameters like language, encoding, or content type.

Common Causes of HTTP 506 Errors

  • Configuration issues: The server's content negotiation configuration might be misconfigured, causing it to create an infinite loop or ambiguous scenario where it cannot decide which variant of the resource to serve.
  • Conflicting content negotiation headers: If the server's configuration allows conflicting or contradictory negotiation criteria (for example, conflicting Accept headers from the client and Content-Language from the server), the server may return a 506 error.
  • Recursive negotiation: This error can arise when content negotiation itself requires further negotiation, creating a loop that the server cannot resolve.

Example Scenario

A server might be configured to negotiate content based on both language and encoding headers. However, due to a misconfiguration, the server cannot resolve which content variant to serve and ends up in a recursive loop or conflict, triggering a 506 error.

Example

Client Request (Standard Request)

    
        GET /home HTTP/1.1
        Host: example.com
        Accept-Language: en
        Accept-Encoding: gzip, deflate

(The client sends a request with Accept-Language and Accept-Encoding headers, but the server encounters a conflict during content negotiation.)

Server Response (506 Variant Also Negotiates)

    
        HTTP/1.1 506 Variant Also Negotiates
        Content-Type: text/html

        <html>
          <body>
            <h1>506 Variant Also Negotiates</h1>
            <p>The server encountered an internal error and was unable to complete the request due to a configuration issue with content negotiation.</p>
          </body>
        </html>

The server responds with a 506 error, indicating that it cannot resolve the content negotiation conflict.

Best Practices for Handling HTTP 506

  • Fix content negotiation configurations: Ensure that the server’s content negotiation settings are properly configured to avoid conflicting or recursive negotiation rules. This may involve checking how the server handles headers like Accept, Accept-Language, and Content-Type.
  • Test content negotiation logic: Server administrators should thoroughly test content negotiation logic for different headers and scenarios to prevent conflicts.
  • Provide fallback options: If content negotiation fails, it’s helpful to provide a fallback response or a default version of the resource to avoid server errors.

Summary

HTTP 506 indicates a content negotiation conflict where the server cannot determine which variant of a resource to serve due to misconfiguration. This can result from recursive or ambiguous content negotiation rules. To resolve a 506 error, the server’s content negotiation configuration needs to be reviewed and corrected to avoid conflicts or loops.