A firewall is a way to protect machines from any unwanted traffic from outside. Administrators use a firewall to control incoming network traffic on host machines by defining a set of firewall rules. These rules are used to sort the incoming traffic and either block it or allow it through.

firewalld is a firewall service daemon that provides a dynamic, customizable firewall with a D-Bus interface. This dynamic architecture allows administrators to create, change, and delete rules without restarting the service after every modification.

You can use firewalld to configure packet filtering required by the majority of typical cases. If firewalld does not cover your scenario, or you want to have complete control of rules, use the nftables framework.

firewalld uses the concepts of zones, policies, and services to simplify traffic management. Zones logically separate a network. Network interfaces and sources can be assigned to a zone. Policies are used to deny or allow traffic flowing between zones. Firewall services are predefined rules that cover all necessary settings to allow incoming traffic for a specific service, and they apply within a zone.

Services use one or more ports or addresses for network communication. Firewalls filter communication based on ports. To allow network traffic for a service, its ports must be open. firewalld blocks all traffic on ports that are not explicitly set as open. Some zones, such as trusted, allow all traffic by default.

firewalld maintains separate runtime and permanent configurations. This allows runtime-only changes. The runtime configuration does not persist after firewalld reload or restart. At startup, it is populated from the permanent configuration.