On Encountering an HTTP 421 Error

One-line summary

Let’s stop using HTTP/1.1.

Detailed notes

• Symptom:
  • A 421 (Misredirected Request) error occurred on the CDN server
  • The end user’s web browser received a 200 OK response, but the file actually returned was abnormal
• Cause: When there are two CDNs using the same host name, SNI behaves unstably under HTTP/1.1 (it sends the request to the wrong host)
• Fix: Configure it to use only the HTTP/2 protocol, which strictly manages TCP connection reuse
  • What does “strictly manages TCP connections” mean?
    • HTTP/1.1 performs SNI based only on the Host in the request header, so when a TCP connection is reused, it may send the request to the wrong host that merely shares the same Host.
    • HTTP/2 reuses a TCP connection only after verifying the Host, port, and TLS certificate all together.

Of course, the 421 error can still occur under HTTP/2. But it’s still better than HTTP/1.1.

EOD

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