HTTP 421 Misdirected Request
HTTP 421 Misdirected Request (RFC 9110) is returned when a server receives a request directed at an authority (host) it cannot or is unwilling to produce a response for over the current connection. It frequently arises with HTTP/2 connection coalescing, where a client reuses one TLS connection for multiple hostnames that share a certificate but are not all served by that backend.
What this means
Under HTTP/2, a client may coalesce requests for multiple hostnames onto a single connection when those hosts are covered by the same TLS certificate (for example a wildcard or multi-SAN cert) and resolve to the same IP. This saves connections but assumes the backend serves all of those hosts.
If the backend on that connection is not configured to serve the requested authority, it returns 421 Misdirected Request. RFC 9110 specifies that a client receiving 421 may retry the request on a fresh connection.
Common causes and fixes
421s typically come from shared certificates spanning hostnames that live on different origins, or from CDN/load-balancer configurations where coalescing routes a request to a node that does not own the host. The page is fine; the connection routing is not.
Fixes include ensuring every host covered by a shared certificate is actually served by the backends that terminate that certificate, separating certificates where origins differ, or configuring the edge to discourage coalescing across hosts that do not share a backend. Clients generally recover by retrying on a new connection, but persistent 421s indicate a real configuration gap.
- Triggered by HTTP/2 connection coalescing across hostnames
- Server is not configured to serve the requested authority on that connection
- Fix the certificate/virtual-host mapping or discourage coalescing
How it appears in analytics and logs
A 421 means the request reached a server that is not configured for the requested host on that connection. It signals a mismatch between connection coalescing and virtual-host/certificate configuration, not a problem with the page itself.
Diagnostic use case
Diagnose intermittent failures where a crawler or browser reuses an HTTP/2 connection for a hostname the target server does not actually serve, returning 421.
What WebmasterID can help detect
WebmasterID records the status returned to crawler fetches, so recurring 421s flag an HTTP/2 coalescing or virtual-host misconfiguration affecting how crawlers reach specific hostnames.
Common mistakes
- Sharing one certificate across hosts that are not all served by the same backend.
- Assuming a 421 is a page error rather than a connection-routing mismatch.
- Ignoring intermittent 421s because a retry on a new connection often succeeds.
Privacy and accuracy notes
A 421 is a routing/protocol response with no visitor identity attached. WebmasterID records crawler fetch statuses without linking them to a person.
Related pages
- HTTP vs HTTPS canonicalization
https://example.com and http://example.com are different URLs, so serving content on both creates duplication and mixed signals. The standard fix is to force HTTPS: 301-redirect HTTP to HTTPS, reference only HTTPS in links, sitemaps, and canonicals, and use HSTS so clients default to the secure scheme.
- www vs non-www canonicalization
To a crawler, https://www.example.com and https://example.com are distinct URLs that can serve the same content, creating duplication. The fix is to choose one canonical host, redirect the other to it with a 301, and keep internal links, sitemaps, and canonical tags consistent with the chosen version.
- How CDNs interact with crawlers
A CDN sits between crawlers and your origin, so it shapes what crawlers see: cached responses, edge-served status codes, bot-management challenges, and region-specific edges. Configured well, a CDN speeds crawling and absorbs load; configured poorly, it can block legitimate crawlers, serve stale or wrong content, or return CDN-specific errors that look like origin problems. Understanding the interaction prevents silent crawl failures.
- Website observability
Surface HTTP/2 routing status codes like 421 returned to crawlers, server-side.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.