TOSS 5 must not send Internet Control Message Protocol (ICMP) redirects.

ICMP redirect messages are used by routers to inform hosts that a more direct route exists for a particular destination. These messages contain information from the system's route table possibly revealing portions of the network topology.

The ability to send ICMP redirects is only appropriate for systems acting as routers.

Verify TOSS 5 does not IPv4 ICMP redirect messages using the following command:

$ sudo sysctl net.ipv4.conf.all.send_redirects

net.ipv4.conf.all.send_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 not allow interfaces to perform IPv4 ICMP redirects.

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

net.ipv4.conf.all.send_redirects = 0

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

$ sudo sysctl --system