Configuring the Directory Server firewall
On the LDAP host, make sure that the firewall allows access to the Directory Server’s
secure (636) port, so that LDAP clients can access the Directory
Server.
Leave the default unsecure port (389) closed.
# firewall-cmd --zone=public --add-port=636/tcp
# firewall-cmd --zone=public --add-port=636/tcp --permanent