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3.3.3 Ensure broadcast & multicast icmp requests are ignored

Information

Broadcast ICMP request is an ICMP packet sent to all hosts in a subnet, used for network diagnostics like discovering hosts or testing connectivity, but often disabled for security reasons. Multicast ICMP request targets a specific group of subscribed hosts, allowing efficient data distribution for applications like streaming media, reducing bandwidth usage by sending data to multiple recipients simultaneously.

This 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 :

 - net.inet.icmp.bmcastecho=0
 - net.inet.icmp.tstamprepl=0

Example:

# printf "
net.inet.icmp.bmcastecho=0
net.inet.icmp.tstamprepl=0
" >> /etc/sysctl.conf

Run the following command to set the active kernel parameters:

# sysctl net.inet.icmp.bmcastecho=0
# sysctl net.inet.icmp.tstamprepl=0

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/19044

Item Details

Category: CONFIGURATION MANAGEMENT, IDENTIFICATION AND AUTHENTICATION

References: 800-53|CM-1, 800-53|CM-2, 800-53|CM-6, 800-53|CM-7, 800-53|IA-5, CSCv7|9.2

Plugin: Unix

Control ID: 70b63aa3d91b9653f10fb787ba41e0dad31be877d9041d346a9140a636bace6c