421

Misdirected Request

4xx Client Error

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ELI5

You sent your request to the wrong server. It's like calling the wrong phone number - the person who answered can't help you.

Server perspective

Use 421 when the request reached a server that cannot produce a response for that scheme-and-authority combination.

When to use

  • •Return 421 for connection coalescing, SNI, or virtual-host mismatches
  • •Use it when the failure is about the connection being routed to the wrong server rather than the resource being missing
  • •HTTP/2 connection coalescing issues
  • •Server cannot handle specific Host header
  • •SNI (Server Name Indication) mismatches
  • •Load balancer routing errors
  • •Virtual host configuration issues

How to respond

  • •Keep 421 tied to connection-target mismatch, not generic routing errors
  • •Do not let caches treat it like a resource-level property

Headers to consider

  • •No status-specific header is required; connection authority, TLS/SNI, and routing context are the important signals.

Response body

  • •A body is optional; include a short explanation only if it will not expose routing details.

Server-side pitfalls

  • •Do not misuse 421 as a substitute for 404 or 400
  • •Do not cache a 421 as though it describes the resource itself
  • •Resource doesn't exist (use 404 Not Found)
  • •Bad request format (use 400 Bad Request)
  • •Authentication required (use 401 Unauthorized)

Examples

HTTP/2 connection coalescing mismatch

Request:GET https://api.example.test/data # Headers Host: cdn.example.com
Response:421 Misdirected Request # Headers Content-Type: application/json # Body { "error": "misdirected_request" }

Server handling cdn.example.com cannot serve api.example.com requests

Virtual host mismatch

Request:GET https://api.example.test/api/users # Headers Host: wrong.example.com
Response:421 Misdirected Request # Headers Content-Type: application/json # Body { "error": "misdirected_request" }

Server received request for domain it doesn't serve

References

Related Status Codes

400

Bad Request

Your request doesn't make sense or has mistakes in it. Check what you're sending and try again.

404

Not Found

The thing you're looking for doesn't exist here. It's like knocking on a door and nobody's home.