Use iptables for CentOS 7 instead of firewalld

CentOS 7 is now using firewalld instead of iptables. To use iptables, here are the steps:

1. Disable firewalld
systemctrl disable firewalld

2. Stop firewalld
systemctrl stop firewalld

3. Install iptables-services
yum install iptables-services

4. Enable iptables
systemctl enable iptables

5. Start iptables
systemctl start iptables

 6. Configure iptables as usual (/etc/sysconfig/iptables)