Forwarding Visitor’s Real-IP in Nginx Proxy/Fastcgi backend correctly

Here, we are dealing with 2 nginx servers. A front-end nginx, proxying request to another nginx-server running behind firewall. This backend-nginx is a WordPress setup, using PHP-FPM (fastcgi) on our case. Article is valid for any code/application running behind fastcgi upstream. To manage security properly, we need to pass-on visitors real-IP to backend-nginx. In backend-nginx, WordPress do some