421 Misdirected Request
A server should emit 421 Misdirected Request when it receives a HTTP
request that was not intended for that server.
This status was introduced with HTTP/2, but it may also be applicable to HTTP/1 servers. For example, a server might receive the following HTTP request:
GET /contact.html HTTP/1.1
Host: foo.example.org
Since HTTP/1.1 a client is required to include a Host header. A server might
already know that the foo.example.org domain is not configured on that server,
cand could respond with a 421 Misdirected Request response to tell the client
that it connected to the wrong server:
HTTP/1.1 421 Misdirected Request
Content-Type: text/html
<h1>Switchboard operator error</h1>
So when can that actually happen? One example of this is that the DNS for a server was set to the wrong IP or CNAME, or simply a server configuration error.
HTTP/2
There’s other examples where this might occur. In HTTP/2 it will be possible for a single HTTP/2 connection to be used for multiple domainnames via connection coalescing.
If a client is trying to use this feature but a server doesn’t support it,
it must return 421 Misdirected Request This is actually the real reason
this status got introduced.
References
- RFC7540, Section 9.1.2 - 412 Misdirected Request