Quick DDoS Protection

One of my client's website got under a DDoS attack and I needed to act quickly. But first I needed to confirm it was a DDoS attack and not something else:

At first, I noticed there was a spike in CPU usage on the hosting Dashboard which could indicate many things not necessarily a DDoS attack but maybe a stuck request, an infinite loop or a bug in one of WordPress plugins or even an issue with the MySQL engine.
I logged into the server an ran the command ps -aux to check all currently running processes and see which one consumes the highest CPU usage and it turns out it was the PHP Engine php-fpm
I checked the logs and found out there was a massive number of requests which just confirmed it's a DDoS attack.

Logs

That's where it was confirmed it was a DDoS attack and needed to stop it in few easy steps:

First, I updated Nginx configs with a directive to limit the number of open connections per ip to only 10 connections

limit_conn_zone $binary_remote_addr zone=addr:10m;

server {
    limit_conn addr 10;
}

I then added a deny directive to the problematic IP address

http {
    deny 94.158.245.100
}

Voilà! the CPU usage just dropped to almost normal usage thanks to Nginx's high throughput to just deny a high amount of requests easily. Of course a better option is to use CloudFlare but when under attack, one must act quickly to keep the website afloat compared to just shutting down the server!

CPU Usage down