OL 9 must use a reverse-path filter for IPv4 network traffic, when possible, by default.

Enabling reverse path filtering drops packets with source addresses that should not have been able to be received on the interface on which they were received. It must not be used on systems that are routers for complicated networks but is helpful for end hosts and routers serving small networks.

Verify OL 9 uses reverse path filtering on IPv4 interfaces with the following commands:

$ sysctl net.ipv4.conf.default.rp_filter net.ipv4.conf.default.rp_filter = 1

If the returned line does not have a value of "1", or a line is not returned, this is a finding.

Configure OL 9 to use reverse path filtering on IPv4 interfaces by default.

Add or edit the following line in a single system configuration file in the "/etc/sysctl.d/" directory:

net.ipv4.conf.default.rp_filter = 1

Load settings from all system configuration files with the following command:

$ sudo sysctl --system