5xx Server Error
Not part of the HTTP standard
520 Web Server Returned an Unknown Error
Cloudflare-specific: the origin returned something Cloudflare could not interpret.
What causes it
- The origin returned an empty response, or a response with too many headers.
- The origin crashed mid-response, or reset the TCP connection.
How to fix it
- Bypass Cloudflare (hit the origin IP directly with a `Host` header) to see the real response.
- Check origin logs for a crash at the same timestamp; 520 is almost always an origin bug.
At a glance
- Class
- 5xx Server Error
- Reference
- Cloudflare (non-standard)
- Standardised
- No — vendor extension
- Retry the request?
- Only if idempotent
Other 5xx server error codes
- 500 Internal Server Error
- 501 Not Implemented
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Gateway Timeout
- 505 HTTP Version Not Supported
- 506 Variant Also Negotiates
- 507 Insufficient Storage
- 508 Loop Detected
- 510 Not Extended
- 511 Network Authentication Required
- 521 Web Server Is Down
- 522 Connection Timed Out
- 523 Origin Is Unreachable
- 524 A Timeout Occurred
- 525 SSL Handshake Failed
- 526 Invalid SSL Certificate