521 — Origin web server is down
What does this error mean?
Cloudflare cannot establish a connection to the origin web server.
Where can it appear?
This error may appear in application logs, client messages, monitoring systems or protocol traces. Keep the complete timestamp, request context and surrounding events before troubleshooting.
Likely causes
- Origin web server availability or performance
- Firewall or routing between Cloudflare and the origin
- Invalid or delayed HTTP response from the origin
Checks and resolution
- Test the origin directly when possible
- Review web server, PHP/application and firewall logs
- Confirm DNS records and Cloudflare-to-origin connectivity
Caution : Avoid disabling security or proxying permanently before the origin cause is understood.