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.

There are notable differences between Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6). There is only a directive to disable sending of IPv4 redirected packets. Refer to RFC4294 for an explanation of "IPv6 Node Requirements", which resulted in this difference between IPv4 and IPv6.

Check

Verify OL 8 does not allow interfaces to perform Internet Protocol version 4 (IPv4) ICMP redirects by default.

Note: If IPv4 is disabled on the system, this requirement does not apply.

Check the value of the "default send_redirects" variables with the following command:

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

net.ipv4.conf.default.send_redirects=0

If the returned line does not have a value of "0" or a line is not returned, this is a finding.

Fix

Configure OL 8 to not allow interfaces to perform Internet Protocol version 4 (IPv4) ICMP redirects by default with the following command:

$ sudo sysctl -w net.ipv4.conf.default.send_redirects=0

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

net.ipv4.conf.default.send_redirects=0