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.

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Set the net.ipv4.icmp_echo_ignore_broadcasts parameter to 1 in /etc/sysctl.conf: net.ipv4.icmp_echo_ignore_broadcasts=1 Modify active kernel parameters to match: # /sbin/sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1 # /sbin/sysctl -w net.ipv4.route.flush=1