Is 421 a client or server issue?

It is typically a server/infrastructure issue — the server received a request it cannot handle on that connection.

How does this relate to CDNs?

CDNs with shared IPs may trigger 421 if a connection meant for one domain gets reused for another domain on the same edge server.

HTTP 421 Misdirected Request — TLS/HTTP2 Routing Error

Warning4xx client error

Overview

HTTP 421 Misdirected Request means the request was directed at a server that cannot produce a response, often due to HTTP/2 connection coalescing issues.

Key Details

Introduced in HTTP/2 specification (RFC 7540) for connection multiplexing issues
HTTP/2 connection coalescing allows reusing connections for different hostnames on the same IP
421 occurs when the server cannot serve the requested hostname on the reused connection
The TLS certificate may not cover the requested hostname
Client should retry on a new, dedicated connection to the correct server

Common Causes

HTTP/2 connection coalescing sending request to wrong virtual host
TLS certificate does not cover the requested domain name
CDN routing request to a server that does not host the requested domain
Load balancer misconfiguration with shared IP addresses

Steps

1Retry the request — client should open a new connection and try again
2Verify TLS certificate covers the domain using openssl s_client -connect host:443
3Check CDN configuration to ensure correct origin mapping for each domain
4If using shared hosting, verify SNI is properly configured
5Disable HTTP/2 connection coalescing as a temporary workaround