3.2.5 Ensure broadcast ICMP requests are ignored - 'net.ipv4.icmp_echo_ignore_broadcasts = 1'

Information

Setting net.ipv4.icmp_echo_ignore_broadcasts to 1 will cause the system to ignore all
ICMP echo and timestamp requests to broadcast and multicast addresses.

Rationale:

Accepting ICMP echo and timestamp requests with broadcast or multicast destinations for
your network could be used to trick your host into starting (or participating) in a Smurf
attack. A Smurf attack relies on an attacker sending large amounts of ICMP broadcast
messages with a spoofed source address. All hosts receiving this message and responding
would send echo-reply messages back to the spoofed address, which is probably not
routable. If many hosts respond to the packets, the amount of traffic on the network could
be significantly multiplied.

Solution

Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file:

net.ipv4.icmp_echo_ignore_broadcasts = 1

Run the following commands to set the active kernel parameters:

# sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
# sysctl -w net.ipv4.route.flush=1

See Also

https://workbench.cisecurity.org/files/2420