Chapter 9: Securing Network Connectivity with firewalld

A great mentor and technologist working with military restricted environments once told me that "The only secure system is the one that is switched off, disconnected from any network, and buried in the middle of the desert." He is right, of course, but we must provide a service to make the system useful. This means having it running and connected to a network.

One of the techniques that's used in security to reduce incidents, such as avoiding unexpected exposure to a vulnerability and enabling unauthorized remote access, is reducing the attack surface and applying defense in depth principles. When you do that in a network, step one is filtering connections using a firewall. The firewall management tool that's included in Red Hat Enterprise Linux (RHEL) is firewalld and it helps us manage zones, profiles, services, and ports. It also includes a command-line tool called firewall-cmd and a systemd service...