4×× CLIENT ERROR
421 MISDIRECTED REQUEST
The request was directed at a server that is not able to produce a response. This can be sent by a server that is not configured to produce responses for the combination of scheme and authority that are included in the request URI.
Clients receiving a 421 Misdirected Request response from a server MAY retry the request — whether the request method is idempotent or not — over a different connection. This is possible if a connection is reused or if an alternative service is selected ALT-SVC.
This status code MUST NOT be generated by proxies.
A 421 response is cacheable by default, i.e., unless otherwise indicated by the method definition or explicit cache controls.
Connection Reuse RFC7540 Section 9.1.1
Calculating Heuristic Freshness RFC7234 Section 4.2.2
421 MISDIRECTED REQUEST Source: RFC7540 Section 9.1.2
421 CODE REFERENCES
Rails HTTP Status Symbol