To assign the eth0 network interface temporarily to the internal zone, type:

$ firewall-cmd --zone=internal --change-interface=eth0
success

To assign the eth0 network interface permanently to the internal zone (a file called internal.xml is created in the /etc/firewalld/zones directory), type:

$ firewall-cmd --permanent --zone=internal --change-interface=eth0
success

Source: RHEL7: How to get started with Firewalld. – CertDepot

Firewalld is a complete firewall solution available by default on CentOS and Fedora servers. In this guide, we will cover how to set up a basic firewall for your server and show you the basics of managing the firewall with firewall-cmd, its command-li

Source: How To Set Up a Firewall Using FirewallD on CentOS 7 | DigitalOcean