From Red Hat Enterprise Linux 7 Security Technical Implementation Guide
Part of SRG-OS-000480-GPOS-00227
Associated with: CCI-000366
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 the system ignores IPv4 ICMP redirect messages. # grep 'net.ipv4.conf.all.accept_redirects' /etc/sysctl.conf /etc/sysctl.d/* If " net.ipv4.conf.all.accept_redirects " is not configured in the /etc/sysctl.conf file or in the /etc/sysctl.d/ directory, is commented out, or does not have a value of "0", this is a finding. Check that the operating system implements the "accept_redirects" variables with the following command: # /sbin/sysctl -a | grep 'net.ipv4.conf.all.accept_redirects' net.ipv4.conf.all.accept_redirects = 0 If the returned line does not have a value of "0", this is a finding.
Set the system to ignore IPv4 ICMP redirect messages by adding the following line to "/etc/sysctl.conf" or a configuration file in the /etc/sysctl.d/ directory (or modify the line to have the required value): net.ipv4.conf.all.accept_redirects = 0 Issue the following command to make the changes take effect: # sysctl --system
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