Using firewall-cmd and ufw

Using built-in firewalls has been a de facto standard in Linux for more than two decades now. Ever since the invention of ipfwadm (kernel v2.0), Linux kernel developers have been piling up functionality and a firewall has been one of those things. ipfwadm was followed by ipchains (kernel v2.2), iptables (kernel v2.4), and today it's all about firewalld (CentOS) and ufw (Ubuntu). Let's go through both of these concepts so that we can use them when we need them regardless of the Linux distribution we're working on.

Getting ready

As a part of this recipe, we are going to go through a list of dozens of different scenarios covering firewalld and ufw. In other words, we are going to introduce the necessary commands to do configuration changes for some of the most commonly used scenarios. First, let's install the necessary packages for CentOS (on our client2 machine) and Ubuntu (client1 machine). So, for CentOS, we need to type the following...