http://ronnybull.com/2015/11/20/how-to-centos-7-router/
Enable IP forwarding
Add ZONE= to ifcfg files
sudo firewall-cmd --permanent --direct --passthrough ipv4 -t nat -I POSTROUTING -o em1 -j MASQUERADE -s x.x.x.0/24
sudo firewall-cmd --set-default-zone=internal