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 'net.ipv4.conf.all.send_redirects' /etc/sysctl.conf /etc/sysctl.d/*

If "net.ipv4.conf.all.send_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 "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.

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