The command that should be run on the DNS forwarder server to accomplish the task is firewall- cmd --zone=public --add-port=53/udp --permanent. The firewall-cmd command is a tool for managing firewalld, which is a firewall service that provides dynamic and persistent network security on Linux systems. The firewalld uses zones and services to define the rules and policies for the network traffic. The zones are logical groups of network interfaces and sources that have the same level of trust and security. The services are predefined sets of ports and protocols that are associated with certain applications or functions. The --zone=public option specifies the zone name that the rule applies to. The public zone is the default zone that represents the untrusted network, such as the internet. The - -add-port=53/udp option adds a port and protocol to the zone. The 53 is the port number that is used by the DNS service. The udp is the protocol that is used by the DNS service. The -- permanent option makes the change persistent across reboots.
The command firewall-cmd -- zone=public --add-port=53/udp --permanent will modify the firewall on the server for the DNS forwarder to allow the internal servers to communicate to it and make the changes persistent between server reboots.