Error 521
Error 521 occurs when the origin web server refuses connections from Cloudflare. Security solutions at your origin may block legitimate connections from certain Cloudflare IP addresses ↗.
The two most common causes of 521 errors are:
- Offlined origin web server application.
- Blocked Cloudflare requests.
Contact your hosting provider or site administrator and share the necessary error details to assist in troubleshooting these common causes:
- Ensure your origin web server is responsive.
- Review origin web server error logs to identify web server application crashes or outages.
- Confirm Cloudflare IP addresses ↗ are not blocked or rate limited.
- Allow all Cloudflare IP ranges ↗ in your origin web server's firewall or other security software.
- Confirm that — if you have your SSL/TLS mode set to Full or Full (Strict) — your origin supports HTTPS and/or you have installed a Cloudflare Origin Certificate or a certificate matching the requirements for these modes.
- Ensure that your origin web server application is actively bound and listening on the port required by your SSL/TLS mode: Port 80 for Flexible, or Port 443 for Full and Full (Strict).
- Find additional troubleshooting information on the Cloudflare Community ↗.