AlmaLinux OS 9 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.

AlmaLinux OS 9 incorporates the "firewalld" daemon, which allows for many different configurations. One of these configurations is zones. Zones can be used to 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 the AlmaLinux OS 9 "firewalld" is configured to employ a deny-all, allow-by-exception policy for allowing connections to other systems.

First ensure firewalld is running:

$  firewall-cmd --state

running

Next, get the active zones:

$ firewall-cmd --get-active-zones

public interfaces: enp1s0

Check the target of the zones returned from the previous command:

$ firewall-cmd --info-zone=public | grep target

target: DROP

Check the runtime and permanent rules match:

$ firewall-cmd --permanent --info-zone=public | grep target

target: DROP

If no zones are active on the AlmaLinux OS 9 interfaces or if runtime and permanent targets are set to a different option other than "DROP", this is a finding.

Configure the "firewalld" daemon to employ a deny-all, allow-by-exception.

Start by adding the exceptions that are required for mission functionality to the "drop" zone. If SSH access on port 22 is needed for example, run the following command:

$ firewall-cmd --permanent --add-service=ssh --zone=drop

Set the default zone to the "drop" zone:

$ firewall-cmd --set-default-zone=drop

Note: This is a runtime and a permanent change.

Add any interfaces to the newly modified "drop" zone:

$ firewall-cmd --permanent --zone=drop --change-interface=enp1s0

Reload the firewall rules for changes to take effect:

$ firewall-cmd --reload

Check zones and interfaces:

$ firewall-cmd --get-active-zones

drop interfaces: enp1s0

Check new default zone's target is set to "DROP":

$ firewall-cmd --permanent --info-zone=drop | grep target

target: DROP

The same outcome is achieved by creating a new zone, for example:

$ firewall-cmd --permanent --new-zone=stig
$ firewall-cmd --reload
$ firewall-cmd --permanent --change-interface=enp1s0 --zone=stig 
$ firewall-cmd --permanent --add-service=ssh --zone=stig
$ firewall-cmd --permanent --set-target=DROP --zone=stig
$ firewall-cmd --set-default-zone=stig