files/en-us/web/http/reference/status/510/index.md
The HTTP 510 Not Extended server error response status code is sent when the client request declares an HTTP Extension ({{RFC("2774")}}) that should be used to process the request, but the extension is not supported.
510 Not Extended
In the following example, a client sends a request with a mandatory extension specified in the C-MAN header.
The {{HTTPHeader("Connection")}} header specifies that these extensions are to be handled on a hop-by-hop basis.
A {{Glossary("Proxy_server", "proxy")}} forwards the extended request, but the {{HTTPHeader("Connection")}} header is stripped out in transit.
Because the origin server doesn't receive any information about the M-GET method, it sends a 510 in response:
M-GET /document HTTP/1.1
Host: example.com
C-Man: "http://www.example.org/"
Connection: C-Man
HTTP/1.1 510 Not Extended
{{Specifications}}