Error 520: Web server is returning an unknown error
Cloudflare got an empty or invalid response
Seen in Cloudflare
Cloudflare connected to your origin, but the response was empty or not valid HTTP.
Common causes
- The origin crashed or reset the connection.
- Response headers too large (often huge cookies).
- A firewall on the origin blocking Cloudflare's IPs.
How to fix it
- Check origin server logs at the time of the error.
- Trim oversized headers and cookies.
- Allow Cloudflare's IP ranges on the origin firewall.
Got a different message? Paste it into the error explainer.
Related errors
- 502 Bad Gateway · The proxy got a bad response from the server behind it
- 504 Gateway Timeout · The server behind the proxy took too long
- 429 Too Many Requests · You've hit a rate limit
- 400 Bad Request · The server could not understand the request
- 401 Unauthorized · The request needs valid credentials
- 403 Forbidden · The server refuses to allow this request
- 404 Not Found · Nothing exists at this URL
- 405 Method Not Allowed · This URL does not accept that HTTP method
- 410 Gone · The resource was removed on purpose
- 414 Request-URI Too Large · The URL is longer than the server accepts
- 500 Internal Server Error · The application hit an error it did not handle
- 400 Bad Request: The plain HTTP request was sent to HTTPS port · Plain HTTP was sent to a port that expects TLS