HTTP 421 Misdirected Request vs 501 Not Implemented
HTTP 421 (Misdirected Request) is a 4xx Client Error response, while 501 (Not Implemented) 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, 501 means that the server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability 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
The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
When You See It
When using an HTTP method (like PATCH) that the server doesn't support.
How to Fix
Use a different HTTP method that the server supports. Check server documentation.
Key Differences
421 is a 4xx Client Error response, while 501 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 501: The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability 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 501 when when using an HTTP method (like PATCH) that the server doesn't support.
When to Use Which
For 421 (Misdirected Request): Retry the request on a new connection. For 501 (Not Implemented): Use a different HTTP method that the server supports. Check server documentation.