OL 9 must prevent IPv4 Internet Control Message Protocol (ICMP) redirect messages from being accepted.

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 IPv4 protocol has few legitimate uses. It must be disabled unless absolutely required.

Verify that OL 9 will not accept IPv4 ICMP redirect messages. Check the value of the default "accept_redirects" variables with the following command: $ sysctl net.ipv4.conf.default.accept_redirects net.ipv4.conf.default.accept_redirects = 0 If the returned line does not have a value of "0", a line is not returned, or the line is commented out, this is a finding.

Configure OL 9 to prevent IPv4 ICMP redirect messages from being accepted. Add or edit the following line in a single system configuration file in the "/etc/sysctl.d/" directory: net.ipv4.conf.default.accept_redirects = 0 Load settings from all system configuration files with the following command: $ sudo sysctl --system