Vulnerability Discussion
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.
Check
Verify the system does not send IPv4 ICMP redirect messages.
# grep -r net.ipv4.conf.all.send_redirects /run/sysctl.d/* /etc/sysctl.d/* /usr/local/lib/sysctl.d/* /usr/lib/sysctl.d/* /lib/sysctl.d/* /etc/sysctl.conf 2> /dev/null
If "net.ipv4.conf.all.send_redirects" is not configured in the /etc/sysctl.conf file or in any of the other sysctl.d directories, is commented out or does not have a value of "0", this is a finding.
Check that the operating system implements the "all send_redirects" variables with the following command:
# /sbin/sysctl -a | grep net.ipv4.conf.all.send_redirects
net.ipv4.conf.all.send_redirects = 0
If the returned line does not have a value of "0", this is a finding.
If conflicting results are returned, this is a finding.
Fix
Configure the system to not allow interfaces to perform IPv4 ICMP redirects.
Set the system to the required kernel parameter 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.send_redirects = 0
Issue the following command to make the changes take effect:
# sysctl --system