firewalld can not be used with iptables.
By default, firewalld is enabled, so disabled firewalld if using iptables, you must install iptables-service and enable iptables.
If you want to configure easily to work with external services with api, firewalld.
If you want to configure easy + to work with external service with api, firewalld.
If you want to do complex things, it is better to use iptables.
[vagrant@localhost ~]$ cat /etc/redhat-release
CentOS release 6.10 (Final)
[vagrant@localhost ~]$ sudo /etc/rc.d/init.d/iptables status
iptables: ファイアウォールが稼働していません。
