Vulnerability Discussion

Responding to broadcast ICMP echoes facilitates network mapping and provides a vector for amplification attacks.

There are notable differences between Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6). IPv6 does not implement the same method of broadcast as IPv4. Instead, IPv6 uses multicast addressing to the all-hosts multicast group. Refer to RFC4294 for an explanation of "IPv6 Node Requirements", which resulted in this difference between IPv4 and IPv6.

Check

Verify RHEL 8 does not respond to ICMP echoes sent to a broadcast address.

Note: If either IPv4 or IPv6 is disabled on the system, this requirement only applies to the active internet protocol version.

Check the value of the "icmp_echo_ignore_broadcasts" variable with the following command:

$ sudo sysctl net.ipv4.icmp_echo_ignore_broadcasts

net.ipv4.icmp_echo_ignore_broadcasts = 1

If the returned line does not have a value of "1", a line is not returned, or the retuned line is commented out, this is a finding.

Fix

Configure RHEL 8 to not respond to IPv4 ICMP echoes sent to a broadcast address with the following command:

$ sudo sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1

If "1" is not the system's default value then add or update the following line in the appropriate file under "/etc/sysctl.d":

net.ipv4.icmp_echo_ignore_broadcasts=1