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 --permanent 

Then remember to reload the firewall for changes to take effect.

firewall-cmd --reload 

Otherwise, substitute 'public' for your zone, for example, if your zone is dmz: 

firewall-cmd --zone=dmz --add-port=1521/tcp --permanent 

Environment
+ Oracle Linux 7.5 - x86-64