AlmaLinux OS 9 must not accept router advertisements on all IPv6 interfaces.

An illicit router advertisement message could result in a man-in-the-middle attack.

Note: If IPv6 is disabled on the system, this requirement is Not Applicable.

Verify AlmaLinux OS 9 does not accept router advertisements on any IPv6 interfaces, unless the system is a router.

Determine if router advertisements are not accepted by using the following command:

$ sysctl -a | grep 'accept_ra '

net.ipv6.conf.all.accept_ra = 1 net.ipv6.conf.default.accept_ra = 1 net.ipv6.conf.enp1s0.accept_ra = 0 net.ipv6.conf.lo.accept_ra = 1

If any of the returned lines are not set to "0" and it is not documented with the information system security officer (ISSO) as an operational requirement, this is a finding.

Configure AlmaLinux OS 9 to not accept router advertisements on all IPv6 interfaces unless the system is a router.

Create a numbered *.conf file in /etc/sysctl.d/ with the following content:

net.ipv6.conf.all.accept_ra = 0 net.ipv6.conf.default.accept_ra = 0 net.ipv6.conf.lo.accept_ra = 0

The system configuration files need to be reloaded for the changes to take effect. To reload the contents of the files, run the following command:

$ sysctl –system