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HTTP 521 Web Server Is Down — Cloudflare Origin Server Unreachable

Critical5xx server error

Overview

Fix Cloudflare HTTP 521 error when the origin web server refuses or is unable to accept connections from Cloudflare's proxy.

Key Details

  • HTTP 521 is a Cloudflare-specific error indicating the origin server actively refused the connection
  • Cloudflare acts as a reverse proxy and must connect to your origin server to serve content
  • This error means Cloudflare cannot reach your web server at all — not even a timeout, but a direct refusal
  • The origin server may be down, firewalled, or not listening on the expected port
  • Unlike 522 (connection timed out), 521 means the connection was immediately rejected

Common Causes

  • Origin web server (Nginx, Apache) is stopped or crashed
  • Firewall on the origin server blocking Cloudflare IP ranges
  • Origin server listening on wrong port (e.g., only 8080 instead of 443)
  • Hosting provider suspended the account or server

Steps

  1. 1Verify the origin web server is running: systemctl status nginx or systemctl status apache2
  2. 2Whitelist all Cloudflare IP ranges in your server firewall (cloudflare.com/ips)
  3. 3Confirm the server is listening on port 443 (HTTPS) or 80 (HTTP): ss -tlnp | grep ':443'
  4. 4Check hosting provider dashboard for any account suspensions or resource limits
  5. 5Temporarily pause Cloudflare proxy (grey cloud) to test direct connection to origin

Tags

cloudflare521origin-serverweb-server-downproxy

More in 5xx Server Error

http-500-internal-server-errorHTTP 500 Internal Server Error — What It Means & How to Fix It
Critical
http-501-not-implementedHTTP 501 Not Implemented — What It Means & How to Fix It
Critical
http-502-bad-gatewayHTTP 502 Bad Gateway — What It Means & How to Fix It
Critical
http-503-service-unavailableHTTP 503 Service Unavailable — What It Means & How to Fix It
Critical
http-504-gateway-timeoutHTTP 504 Gateway Timeout — What It Means & How to Fix It
Critical
http-505-http-version-not-supportedHTTP 505 HTTP Version Not Supported — What It Means & How to Fix It
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Frequently Asked Questions

HTTP 521 is not an official HTTP status code. It is specific to Cloudflare's proxy infrastructure, indicating their edge servers cannot connect to your origin server.