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3.8.7. Understanding The Direct Interface

firewalld has a so called direct interface, which enables directly passing rules to iptables, ip6tables and ebtables. It is intended for use by applications and not users. It is dangerous to use the direct interface if you are not very familiar with iptables as you could inadvertently cause a breach in the firewall. firewalld still tracks what has been added, so it is still possible to query firewalld and see the changes made by an application using the direct interface mode. The direct interface is used by adding the --direct option to firewall-cmd.
The direct interface mode is intended for services or applications to add specific firewall rules during run time. The rules are not permanent and need to be applied every time after receiving the start, restart or reload message from firewalld using D-BUS.