Issue How to open port 1521 in firewall under Oracle Linux 7.5? Solution Use this command to find your active zone(s): firewall-cmd --get-active-zones It will say either 'public', 'dmz' or something else. You should only apply to the zones required. In case of 'public' use the following command: firewall-cmd --zone=public --add-port=1521/tcp --perm...