CentOS 7 Router/DHCP Server: Second NIC adapter plugged in but not sending internet through the cable

Solution 1:

You need to enable IP Forwarding in the kernel and net the firewall do NAT in the router. Just because you have two NICs, you machine does not "automagically" become a router

Let the kernel handle IP traffic:

vi /etc/sysctl_d/ip_forward.conf

and add the following line:

net.ipv4.ip_forward = 1

activate the change:

sysctl -p /etc/sysctl.d/ip_forward.conf

Next step is most likely to configure the firewall to allow IP masquerading between the public and private interfaces. Here ens224 is the public interface and 10.73.0.0/24 is the private network.

firewall-cmd --permanent --direct --passthrough ipv4 -t nat -I POSTROUTING -o ens224 -j MASQUERADE -s 10.73.0.0/24

Don't forget to assign the interface to the external firewall zone:

firewall-cmd --change-interface=ens224 --zone=external --permanent

The Default zone should (most likely) be internal:

firewall-cmd --set-default-zone=internal

Reload firewall's config

firewall-cmd --complete-reload

Restart all networking and firewall services:

systemctl restart network && systemctl restart firewalld

Verify the firewall settings:

firewall-cmd --list-all
firewall-cmd --list-all --zone=external

Set the internal's interface IP as the default gateway on your clients.