Cloudflare Error Reference

Cloudflare is a global edge network that operates as a reverse proxy between visitors and origin servers. Every request passes through Cloudflare's proxy layer before reaching the origin, which means Cloudflare observes both the visitor-to-edge path and the edge-to-origin path. When the origin is unreachable, returns malformed data, or fails security checks, Cloudflare generates its own error codes that do not exist in the HTTP specification.

Cloudflare errors fall into two families that serve different diagnostic purposes. The 5xx family (520–527) signals failures on the edge-to-origin path: the origin responded with something unexpected, refused the TCP connection, timed out, or had SSL handshake problems. The 1xxx family (1000–1020) signals failures at the edge itself: DNS resolution, WAF blocks, rate limiting, and Worker runtime errors. Neither family is defined by RFC 9110, and both require knowledge of the Cloudflare proxy architecture to interpret correctly.

FixerCode documents 15 Cloudflare error codes across 1 model. The reference covers the full edge-to-origin failure surface, from connection establishment problems (521–524) to SSL handshake failures (525–527) to edge-level access controls (1015, 1020). Each entry explains what Cloudflare measured, what the origin or client did, and how the code differs from its closest standard HTTP equivalent.