3.3.4 Ensure broadcast icmp requests are ignored

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.

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 parameter in /etc/sysctl.conf or a file in /etc/sysctl.d/ ending inconf :

- net.ipv4.icmp_echo_ignore_broadcasts = 1

Example:

# printf '%s
' "net.ipv4.icmp_echo_ignore_broadcasts = 1" >> /etc/sysctl.d/60-netipv4_sysctl.conf

Run the following script to set the active kernel parameters:

#!/usr/bin/env bash

{
sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
sysctl -w net.ipv4.route.flush=1
}

Note: If these settings appear in a canonically later file, or later in the same file, these settings will be overwritten

See Also

https://workbench.cisecurity.org/benchmarks/17045

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6, 800-53|CM-7, CSCv7|9.2

Plugin: Unix

Control ID: 5239b00b2976a803ad925d5ca757039652a6301d3768cd4d96d5de182396069c