AlmaLinux OS 9 must ignore Internet Control Message Protocol (ICMP) redirect messages.

ICMP redirect messages are used by routers to inform hosts that a more direct route exists for a particular destination. These messages modify the host's route table and are unauthenticated. An illicit ICMP redirect message could result in a man-in-the-middle attack.

This feature of the IP protocol has few legitimate uses. It should be disabled unless absolutely required.

Verify AlmaLinux OS 9 will not accept ICMP redirect messages.

Check the value of the "accept_redirects" variables with the following command:

$ sysctl -a | grep accept_redirects

net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.default.accept_redirects = 0 net.ipv4.conf.enp1s0.accept_redirects = 0 net.ipv4.conf.lo.accept_redirects = 0 net.ipv6.conf.all.accept_redirects = 0 net.ipv6.conf.default.accept_redirects = 0 net.ipv6.conf.enp1s0.accept_redirects = 0 net.ipv6.conf.lo.accept_redirects = 0

If the returned lines do not all have a value of "0", this is a finding.

Configure AlmaLinux OS 9 to ignore ICMP redirect messages.

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

net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.default.accept_redirects = 0 net.ipv4.conf.lo.accept_redirects = 0 net.ipv6.conf.all.accept_redirects = 0 net.ipv6.conf.default.accept_redirects = 0 net.ipv6.conf.lo.accept_redirects = 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