6. The 421 (Misdirected Request) HTTP Status Code

The 421 (Misdirected Request) status code is defined in Section 9.1.2 of [RFC7540] to indicate that the current server instance is not authoritative for the requested resource. This can be used to indicate that an alternative service is not authoritative; see Section 2).

Clients receiving 421 (Misdirected Request) from an alternative service MUST remove the corresponding entry from its alternative service cache (see Section 2.2) for that origin. Regardless of the idempotency of the request method, they MAY retry the request, either at another alternative server, or at the origin.

An Alt-Svc header field in a 421 (Misdirected Request) response MUST be ignored.