If you are not familar with firewalld and the firewall-cmd, check out our Getting Started article.

The --add-port=port_number/protocol option can be used to allow connections to a port number. This is typically used when a protocol is not using the default port associated with the protocol, such as when HTTP is using 8080. --add-service is used when the protocol is using it's common port, such as when HTTP is using port 80.

firewall-cmd --add-port=12345/tcp --permanent
firewall-cmd --reload

Or --remove-port=port_number/protocol to remove a port.

firewall-cmd --remove-port=8080/tcp --permanent
firewall-cmd --reload

The --check-config command can be used to ensure there are no configuration errors.

~]$ firewall-cmd --check-config
success

firewall-cmd --list-all will show the allowed ports.

~]# firewall-cmd --list-all
public
  target: default
  icmp-block-inversion: no
  interfaces: eth0
  sources:
  services:
  ports: 12345/tcp
  protocols:
  masquerade: no
  forward-ports:
  sourceports:
  icmp-blocks:
  rich rules: