TOSS 5 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 TOSS 5 will not accept IPv4 ICMP redirect messages using the following command:

$ sudo 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", or a line is not returned, this is a finding.

Configure TOSS 5 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 using the following command:

$ sudo sysctl --system