Note that if Always Online is enabled, this error will trigger an Always Online page to be served. While the 520 error can be triggered by unique and seemingly strange edge-case scenarios, they are generally caused by: Connection resets (following a successful TCP handshake), Headers exceed the Cloudflare header size limit (over 8kb), An empty response from origin, An invalid HTTP response, Missing response headers from an HTTP response, or the Presence of multiple Content-Length headers.
While you’re having a conversation with your hosting provider, make sure they include Cloudflare IPs on their allowed list. The Code Igniter PHP framework has some known bugs around this, too. This is often caused by runaway scripts that return too many cookies, for example. Often this can happen if your web server is returning too many/too large headers. Your web server returned an invalid response that exceeded our limits. Check your web server error logs with your hosting provider for the time that the error occurred to see any error messages. Sometimes when a web server crashes it will reset the connection. Your web server or networking equipment, any http aware device like Firewall, Load Balancer, et al, reset the TCP connection after it was established. When this happens, you’ll see “Error 520: Web server is returning an unknown error”.
The most common reason why this would occur is that either a program, cron job, or resource is taking up more resources than it should causing the server not to be able to respond to all requests properly. A 520 error occurs when the connection started on the origin web server, but that the request was not completed. Try the suggestions in this Community Tip to help you fix “Error 520: Web server is returning an unknown error”.Įrror 520 is essentially a catch-all response when something unexpected happens or when the origin server incorrectly interprets or does not tolerate a request due to a protocol violation or an empty response.
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