When I use cloudflare, sometimes 525 error and 520 error

When I use Cloudflare DNS, there will 525 error and 520 error sometimes. There’s no error when I use other DNS. I don’t know why. I use nginx in my origin server.

Look to see if there is a firewall at the origin. If so, make sure Cloudflare IPs are whitelisted. When 52X errors are found, it’s typically a firewall blocking Cloudflare.

yes, there is a firewall at the origin. But when I stop the firewall, the error sometime show again. Not always show, in some pages some img show 525 error

Whitelisting the Cloudflare IPs is a good idea and might be the answer.

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I have the same problem. Nothing in nginx logs. How to solve?