Independent Error Code Reference
Find Your Error Code
Clear explanations for HTTP status codes and web errors. Fast to scan, free to use, and built for quick understanding.
37+ Error Codes
Independent Reference
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HTTP status codes and web errors explained for developers.
Most Common Error Codes
High severity first — the most frequently searched error codes
520
High
Web Server Returned an Unknown Error — the origin sent unexpected responses to the proxy
4 common causes
521
High
Web Server Is Down — Cloudflare cannot establish a TCP connection to the origin server
4 common causes
522
High
Connection Timed Out — the origin server took too long to complete the TCP handshake
4 common causes
523
High
Origin Is Unreachable — Cloudflare cannot locate a network path to the origin IP
4 common causes
524
High
A Timeout Occurred — the origin server connected but failed to reply in time
4 common causes
525
High
SSL Handshake Failed — Cloudflare could not negotiate a secure connection to the origin
4 common causes