Oracle Database 12c on Oracle Linux: Firewall configuration to access Enterprise Manager on http://host:5500/em

If you have installed your database on Oracle Linux, first step is to access the enterprise manager via port 5500 (https://localhost:5500/em). If you want to access this URL from another host, you have to check and change the firewall settings:

[root@localhost system]# service firewalld status
Redirecting to /bin/systemctl status firewalld.service
● firewalld.service - firewalld - dynamic firewall daemon
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
   Active: active (running) since So 2017-10-01 18:22:30 CEST; 21h ago
     Docs: man:firewalld(1)
 Main PID: 684 (firewalld)
   CGroup: /system.slice/firewalld.service
           └─684 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid

For a quick check disabling the firewall with

service firewalld stop

might be ok, but the better way is to allow port 5500.
Therefor check the active zone and the services:

[root@localhost system]# firewall-cmd --get-active-zone
public
  interfaces: enp0s3
[root@localhost system]# firewall-cmd --zone=public --list-services
ssh dhcpv6-client

 With this knowlegde you can add port 5500 to your firewall:

[root@localhost system]# firewall-cmd --zone=public --add-port=5500/tcp
success
[root@localhost system]# firewall-cmd --permanent --zone=public --add-port=5500/tcp
success

Now you should get the following:

[root@localhost system]# firewall-cmd --zone=public --list-ports
5500/tcp

and you will get in your Browser by accessing the URL https://hostname:5500/em:

 (still Flash...)