HTTP 521 (Server Is Down): meaning and handling

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: HTTP 521 is a Cloudflare response indicating that the origin server refused the connection.

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HTTP_STATUS_UNKNOWN_ERROR	HTTP_STATUS_CONNECTION_TIMED_OUT

HTTP_STATUS_SERVER_IS_DOWN

Cloudflare documents two common causes: the origin web service is offline, or a firewall/security product blocks Cloudflare IP ranges. It differs from an application-generated 5xx response because the connection did not reach the origin application.

Verify that the origin is listening on the expected port, accepts direct health checks, and permits Cloudflare network ranges. Preserve the Ray ID for correlation with support and server logs.

For a 521, Cloudflare can resolve and reach the origin network but the origin refuses the connection or otherwise does not accept it. Check whether the web server is running and listening on the expected address and port, and whether firewall or rate-limit rules reject Cloudflare source networks.

Cloudflare: Error 521

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