HTTP Status Codes

510 Not Extended

The server needs a further protocol extension, declared by the client, to fulfil the request under the (rarely used) HTTP extension framework.

What it means

510 Not Extended (RFC 2774) belongs to an experimental HTTP extension framework: the server requires the request to declare additional extensions before it can be fulfilled. The framework saw little adoption, so this status is very rare.

Why it matters

  • It is a historical artefact of an extension-negotiation mechanism that never gained traction.

Common mistakes

  • Expecting broad client support for the HTTP extension framework behind 510.

How WebInspect checks this

  • WebInspect reports whatever status the server returns, including obscure codes like 510.