An OL 8 firewall must employ a deny-all, allow-by-exception policy for allowing connections to other systems.

Failure to restrict network connectivity only to authorized systems permits inbound connections from malicious systems. It also permits outbound connections that may facilitate exfiltration of DoW data.

OL 8 incorporates the "firewalld" daemon, which allows for many different configurations. One of these configurations is zones. Zones can be used in a deny-all, allow-by-exception approach. The default "drop" zone will drop all incoming network packets unless it is explicitly allowed by the configuration file or is related to an outgoing network connection.

Verify "firewalld" is configured to employ a deny-all, allow-by-exception policy for allowing connections to other systems with the following commands:

     $ sudo firewall-cmd --state 
     running 

     $ sudo firewall-cmd --get-active-zones 
     [custom] 
     interfaces: ens33 

     $ sudo firewall-cmd --info-zone=[custom] | grep target 
     target: DROP 

If no zones are active on the OL 8 interfaces or if the target is set to an option other than "DROP", this is a finding.

If the "firewalld" package is not installed, ask the System Administrator if an alternate firewall (such as iptables) is installed and in use, and how is it configured to employ a deny-all, allow-by-exception policy.

If the alternate firewall is not configured to employ a deny-all, allow-by-exception policy, this is a finding.

If no firewall is installed, this is a finding.

Configure the "firewalld" daemon to employ a deny-all, allow-by-exception policy with the following commands:

     $ sudo firewall-cmd --permanent --new-zone=[custom]

     $ sudo cp /usr/lib/firewalld/zones/drop.xml /etc/firewalld/zones/[custom].xml

This will provide a clean configuration file to work with that employs a deny-all approach.

Note: Add the exceptions that are required for mission functionality and update the short title in the xml file to match the [custom] zone name.

Reload the firewall rules to make the new [custom] zone available to load: $ sudo firewall-cmd --reload

Set the default zone to the new [custom] zone: $ sudo firewall-cmd --set-default-zone=[custom]

Note: This is a runtime and permanent change.

Add any interfaces to the new [custom] zone: $ sudo firewall-cmd --permanent --zone=[custom] --change-interface=ens33

Reload the firewall rules for changes to take effect: $ sudo firewall-cmd --reload