Issue
How to open port 1521 in firewall under Oracle Linux 7.5?
Solution
Use this command to find your active zone(s):
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:
Then remember to reload the firewall for changes to take effect.
Otherwise, substitute 'public' for your zone, for example, if your zone is dmz:
Environment
+ Oracle Linux 7.5 - x86-64