1. Several concepts
Firewall Policy
It can be customized based on the source / destination address, port number, protocol, application and other information of the traffic
Firewall function
Although firewalls are divided into hardware and software, their main function is to filter the traffic passing through the firewall itself according to the policy. If the traffic matches a policy rule, the corresponding processing is performed, otherwise, it is discarded.
iptables & firewalld
Both iptables and firewalld are just firewall management tools used to define firewall policies. They are just services.
The iptables service passes the configured firewall policy to the netfilter network filter at the kernel level for processing
The firewalld service passes the configured firewall policy to the nftables packet filtering framework at the kernel level for processing