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