SLEM 5 must not allow interfaces to accept Internet Protocol version 4 (IPv4) Internet Control Message Protocol (ICMP) redirect messages by default.

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.

Verify SLEM 5 does not accept IPv4 ICMP redirect messages by default with the following command:

     > sudo sysctl net.ipv4.conf.default.accept_redirects
     net.ipv4.conf.default.accept_redirects = 0

If the network parameter "ipv4.conf.default.accept_redirects" is not equal to "0", or nothing is returned, this is a finding.

Configure SLEM 5 to not accept IPv4 ICMP redirect messages by default by running the following command as an administrator:

     > sudo sysctl -w net.ipv4.conf.default.accept_redirects=0

If "0" is not the system's default value, add or update the following line in "/etc/sysctl.d/99-stig.conf":

     > sudo sh -c 'echo "net.ipv4.conf.default.accept_redirects=0" >> /etc/sysctl.d/99-stig.conf'

     > sudo sysctl --system