521
Web Server Is Down
5xx Server error
Class
Cloudflare (non-standard)
Defined in
What does HTTP 521 mean?
Cloudflare could not connect to the origin server — the connection was refused. The origin is offline or actively refusing Cloudflare’s connections.
5xx Server error: The request looked valid, but the server failed to fulfil it. The fix is on the server side.
Common causes of a 521 error
- Web server not running.
- Origin firewall or security plugin blocking Cloudflare IPs.
How to fix 521 Web Server Is Down
- Start the web server and confirm it listens on ports 80/443.
- Allow Cloudflare IP ranges.
521 and SEO
Treated like a 5xx error.
Example response
HTTP/1.1 521 Web Server Is DownRelated status codes
- 520 Web Server Returned an Unknown Error — Cloudflare received an empty, unknown or unexpected response from the origin server.
- 522 Connection Timed Out — Cloudflare timed out while connecting to the origin server.
- 502 Bad Gateway — A gateway or proxy got an invalid response from the upstream server.
Check which status code a URL returns with the HTTP header checker.
All HTTP status codesSearch the full list of status codes from 100 to 599 with their meanings and SEO impact.
Open the full list Frequently asked questions
What does status code 521 mean?
521 Web Server Is Down: Cloudflare could not connect to the origin server — the connection was refused.
How do I fix a 521 error?
Start the web server and confirm it listens on ports 80/443. Allow Cloudflare IP ranges.
Does 521 affect SEO?
Treated like a 5xx error.