Cloudflare error 520: web server returned an unknown error
Cloudflare connected to your origin server, but the answer was empty, malformed or not something Cloudflare understands.
Where it failed: the origin behind Cloudflare
What visitors see
Cloudflare's "Web server is returning an unknown error" page with a Ray ID.
Usual causes
- The origin crashed or reset the connection mid-response.
- Response headers over Cloudflare's size limit (very large or many cookies).
- An empty reply from the application, or a reply that is not valid HTTP.
- A firewall on the origin that drops some of Cloudflare's addresses.
How to tell which one it is
- Request the page from the origin directly (curl --resolve example.com:443:<origin IP> https://example.com/) to see what it really sends.
- The origin's logs at the time of the Ray ID show whether the request arrived.
What to do
- Check the origin's error logs for crashes or resets.
- Reduce response header size (too many or too large Set-Cookie headers).
- Allow Cloudflare's IP ranges (cloudflare.com/ips) in the origin's firewall.
Check the site now
The Down Checker names the layer that failed (DNS, connection, TLS or the server) and decodes the error page it gets.
Free, no sign-up. Scan only sites you own or may test. 60 scans an hour per address.