- https://www.mjhall.org/centos7-firewalld-nat-router/
 - https://www.lisenet.com/2016/firewalld-rich-and-direct-rules-setup-rhel-7-server-as-a-router/
 - http://linuxtechlab.com/turning-centosrhel-6-7-machine-router/
 - https://linuxhint.com/centos7_router/
 
- https://www.server-world.info/en/note?os=CentOS_7&p=dhcp
 - http://www.itzgeek.com/how-tos/linux/ubuntu-how-tos/install-and-configure-dhcp-server-on-centos-7-ubuntu-14-04.html