5xx Server errors · Cloudflare (not part of the HTTP standard)
521 Web Server Is Down
Cloudflare couldn't connect to the origin server: the connection was refused.
Common causes
- The web server isn't running.
- The origin's firewall blocks Cloudflare.
How to fix it
- Start the web server and check it listens on port 80/443.
- Allow Cloudflare's IP ranges in the origin's firewall.
Find out the moment your site returns 521
Spot Downtime monitors count this as down and alert you, since visitors see an error. (The public “Is it down?” checker treats 4xx answers from big sites as online, because many refuse automated requests.)