cf-custom-codes

Fri, 24 Jul 2026 14:57:36 -0400

Cloudflare Custom Codes

The full 520–527 range, each with what actually happened, whose knob it is, and the honest blame assignment the error page won’t give you:

520: “Web server returns an unknown error.”

The garbage drawer. CF got something it couldn’t parse as HTTP: connection reset mid-response, empty reply, malformed headers, headers over CF’s ~32KB limit (classic: apps echoing huge cookies), or origin closing the socket. Blame: genuinely origin-side usually, but it’s a catch-all by construction, so it also swallows MTU/middlebox weirdness on the path. The worst-designed of the eight — it’s “something broke, we refuse to say what,” and a decade of forum archaeology exists because the diagnostic detail lives only in CF’s internal logs (Enterprise customers can see it; everyone else guesses). An RFC 9457 problem-details body would have deleted thousands of support threads.

rtfrfc

cf-custom-codes

Cloudflare Custom Codes

520: “Web server returns an unknown error.”