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Error 521: Web server is down in plesk with cloudflare

Symptoms

When visiting the website, it fails to load with error 521. The error occurs because the origin web server refused the connection from CloudFlare
CloudFlare is used
Website uses Nginx as a reverse-proxy;

Cause

CloudFlare IP addresses were blocked by nginx because of outdated contents of /etc/nginx/conf.d/cloudflare.conf.
/etc/nginx/conf.d/cloudflare.conf contains old CloudFlare IP addresses list.

Solution

Make sure that you're not blocking CloudFlare IPs in .htaccess, iptables, or your firewall.
Make sure that you're operating off of the most recent versions of Bad Behavior or mod_security. You want to ensure that mod_security's core rules aren't blocking CloudFlare requests;
If you are running custom Apache modules, such as mod_antiloris and mod_reqtimeout, disable and unload the modules. These modules will block any time an IP that connects more than 22 times. Since all connections are now coming from a CloudFlare IP, you will definitely hit the limit causing the error page. As soon as you unload the module, the issue will disappear.
Make sure that /etc/nginx/conf.d/cloudflare.conf is up to date and contains all IP addresses found here: https://www.cloudflare.com/ips;

Make sure your provider doesn't rate limiting or blocking IP requests from the CloudFlare IPs and ask them to whitelist the IP addresses found here: https://www.cloudflare.com/ips;
If the issue persists, please visit CloudFlare article regarding this issue.

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  1. Vaishal Patel1 October 2018 at 11:46

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