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521 Web Server Is Down
The origin server refused Cloudflare's connection.
Unofficial code. 521 is used only by Cloudflare. It is not in the IANA registry or any HTTP standard, and other software may give the number a different meaning or none.
- Code
521- Reason phrase
- Web Server Is Down
- Class
- 5xx Server error
- Used by
- Cloudflare
- Cacheable by default
- No
What it means
Server error: the request may be fine, but the server failed to carry it out.
Usual causes. The web server is stopped, or a firewall on the origin blocks Cloudflare's IP ranges.
What to do. Start the web server and allow Cloudflare's published IP ranges through the firewall.
How clients and crawlers treat it
Google Search. Googlebot sees the 5xx code Cloudflare sends and treats it like any server error: it slows down crawling, and URLs that keep failing are eventually dropped from the index. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.
Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.
Example response
HTTP/1.1 521 Web Server Is Down Content-Type: text/plain; charset=utf-8 Web Server Is Down
Related codes
- 511Network Authentication RequiredYou need to sign in to the network before you can reach the internet.
- 510Not ExtendedThe request needs an HTTP extension the server does not have. The code is now obsolete.
- 508Loop DetectedThe server found an infinite loop while processing a WebDAV request.
- 507Insufficient StorageThe server has no space left to store what the request needs.
All status codes · All 5xx codes
Names and numbers from the IANA HTTP Status Code Registry (this code is not in it). Google Search behaviour as documented by Google Search Central.