421 Misdirected Request (HTTP Status Code 421)

The HTTP status code 421 indicates a “Misdirected Request.” This is a relatively new status code introduced in the HTTP/2 specification.

Definition

The 421 status code signifies that the server believes the request was misdirected to a server that cannot produce a response.

Reason for Introduction

This status code was primarily introduced to address specific issues in HTTP/2, particularly those related to connection multiplexing.

Use Cases

When a client sends a request over an HTTP/2 connection, but the server cannot provide service for that specific request.
In cases of misconfiguration with load balancers or proxy servers.
When a request reaches the wrong server, which knows it should not handle the request.

Specific Examples

In a web application using multiple virtual hosts, if a request reaches the wrong virtual host, the server might return a 421.
In environments using TLS SNI (Server Name Indication), if the client connects to the incorrect server.

Server Behavior

When returning a 421 status code, the server should close the connection, or if using HTTP/2, it may only close the corresponding stream.

Client Behavior

Upon receiving a 421 response, the client should typically:
- Attempt to re-establish the connection and resend the request.
- Check whether it is using the correct server address and port.

Distinction from Other Status Codes

Unlike 404 (Not Found), 421 indicates that the entire server is incorrect, rather than just that the requested resource does not exist.
It is also different from 503 (Service Unavailable), as 421 explicitly states that the request was misdirected, rather than the server being temporarily unable to handle the request.

Importance in HTTP/2

Since HTTP/2 allows multiple requests to be multiplexed over a single TCP connection, 421 can help clients identify when a new connection is needed for a specific request.

Implementation Considerations

Server developers should use this status code judiciously, ensuring it is returned only in genuinely suitable cases.
Client developers should be prepared to handle this status code, especially when implementing HTTP/2 clients.

Potential Issues

Overuse of 421 may lead to unnecessary connection re-establishment, increasing network load and latency.

Although this status code is not as widely used as some more common codes, it can be a valuable tool in modern web architectures, particularly in complex server setups, load balancing, and HTTP/2 implementations. Understanding and correctly using 421 can help developers build more robust and efficient web applications and services.