HTTP 421 Misdirected Request vs 510 Not Extended
HTTP 421 (Misdirected Request) is a 4xx Client Error response, while 510 (Not Extended) is a 5xx Server Error response. 421 indicates that the request was directed at a server that is unable to produce a response for the combination of scheme and authority in the URI. In contrast, 510 means that further extensions to the request are required for the server to fulfill it.
Description
The request was directed at a server that is unable to produce a response for the combination of scheme and authority in the URI.
When You See It
With HTTP/2 when a connection is reused for a domain the certificate doesn't cover.
How to Fix
Retry the request on a new connection.
Description
Further extensions to the request are required for the server to fulfill it.
When You See It
When an HTTP extension required by the server is not present in the request.
How to Fix
Add the required extension headers to your request.
Key Differences
421 is a 4xx Client Error response, while 510 is a 5xx Server Error response.
HTTP 421: The request was directed at a server that is unable to produce a response for the combination of scheme and authority in the URI.
HTTP 510: Further extensions to the request are required for the server to fulfill it.
You encounter 421 when with HTTP/2 when a connection is reused for a domain the certificate doesn't cover.
You encounter 510 when when an HTTP extension required by the server is not present in the request.
When to Use Which
For 421 (Misdirected Request): Retry the request on a new connection. For 510 (Not Extended): Add the required extension headers to your request.