APITester| Status code | 521 |
|---|---|
| Status text | Web Server Is Down |
| Category | 5xx Server Error |
| Response has body | Usually |
| Defined by | Cloudflare (not an IANA-registered code) |
What does 521 Web Server Is Down mean?
Cloudflare tried to connect to the origin server and the connection was refused. It is numbered in the 5xx Server Error range but is not a registered HTTP status code, so the general meaning of that class is only a rough guide to it.
When do APIs return 521?
On sites behind Cloudflare, when nothing is accepting connections on the origin's web port. Either the web server process is stopped, or a firewall on the origin is rejecting Cloudflare's IP addresses.
How to fix a 521 Web Server Is Down
Confirm the web server is running and listening on the port Cloudflare connects to. Then check the origin's firewall and any security software for rules that block Cloudflare's published IP ranges; rate-limiting tools often ban them by mistake, because every visitor appears to come from the same few addresses.
Reproduce a 521 response
The fastest way to understand a status code is to trigger one and read the full response. Open the
API tester and send a request to https://httpbin.org/status/521 — you will
see the 521 status along with every response header the server sent. Comparing those headers against
a working request is often what reveals the cause.
That response only imitates the number, which is useful for checking how your own client, alerting or retry logic treats a 521. A real one comes from the software named in the table above, in the circumstances this page describes.
When you are debugging a real endpoint, check the response headers as carefully as the body, and confirm you are using the right HTTP method for the operation.
Related 5xx status codes
500 Internal Server Error
The server hit an unexpected condition and could not complete the request.
501 Not Implemented
The server does not support the functionality required to fulfil the request.
502 Bad Gateway
A server acting as a gateway or proxy received an invalid response from the upstream server.
503 Service Unavailable
The server is temporarily unable to handle the request.
504 Gateway Timeout
A gateway or proxy did not receive a timely response from the upstream server.
505 HTTP Version Not Supported
The server does not support the major HTTP version named in the request.
See also
- 520 Web Server Returned an Unknown Error — Cloudflare reached the origin server and got back something it could not interpret as a valid HTTP response.
- 522 Connection Timed Out — Cloudflare tried to open a TCP connection to the origin server and got no answer in time.
- 523 Origin Is Unreachable — Cloudflare could not find a network route to the origin server.
References
- Cloudflare Docs: Error 521 — Cloudflare's own description of the cause and the checks to run.
- IANA: HTTP Status Code Registry — The official list of registered codes. This one is not on it.