Detections
Analytics
3.3.1.7 Ensure net.ipv4.icmp_echo_ignore_broadcasts is configured
Information
net.ipv4.icmp_echo_ignore_broadcasts controls if all ICMP echo and timestamp requests to broadcast and multicast addresses are ignored.More information about the kernel parameter configuration files, their location, and load preference is available in the "Configure Network Kernel Parameters" section overview.
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
- Review all files being used by systemd sysctl and comment out or remove all net.ipv4.icmp_echo_ignore_broadcasts lines that are not net.ipv4.icmp_echo_ignore_broadcasts=1.Example script:
#!/usr/bin/env bash
{
l_option="net.ipv4.icmp_echo_ignore_broadcasts" l_value="1"
l_grep="${l_option//./(\\.|\\/)}" a_files=()
l_systemdsysctl="$(readlink -e /lib/systemd/systemd-sysctl \
|| readlink -e /usr/lib/systemd/systemd-sysctl)"
a_files+=("/etc/sysctl.conf")
while IFS= read -r l_fname; do
l_file="$(readlink -e "${l_fname//# /}")"
[ -n "$l_file" ] && ! grep -Psiq -- '(^|\h+)'"$l_file"'\b' \
<<< "${a_files[*]}" && a_files+=("$l_file")
done < <("$l_systemdsysctl" --cat-config | tac | \
grep -Psio -- '^\h*#\h*\/[^#\n\r\h]+\.conf\b')
for l_file in "${a_files[@]}"; do
grep -Psio -- '\h*'"$l_grep"'\h*=\h*\H+\b' "$l_file" \
| grep -Psivq -- '^\h*'"$l_grep"'\h*=\h*'"$l_value"'\b' && \
sed -ri '/^\s*'"$l_grep"'\s*=\s*(1[0-9]*)/s/^/# /' "$l_file"
done
}
- Create or edit a file in the /etc/sysctl.d/ directory ending in .conf and edit or add the following line:
net.ipv4.icmp_echo_ignore_broadcasts = 1
Example:
# [ ! -d "/etc/sysctl.d/" ] && mkdir -p /etc/sysctl.d/
# printf '%s\n' "" "net.ipv4.icmp_echo_ignore_broadcasts = 1" \
>> /etc/sysctl.d/60-ipv4_sysctl.conf
- Run the following command to load all sysctl configuration filles:
# sysctl --system
Impact:
The system will no longer respond to ICMP echo requests sent to broadcast or multicast addresses. Broadcast-based network diagnostics, such as discovering live hosts with a broadcast ping, will not receive a response from this system.
See Also
Item Details
Category: CONFIGURATION MANAGEMENT
References: 800-53|CM-6, 800-53|CM-6b., 800-53|CM-7, CSCv7|9.2
Plugin: Unix
Control ID: 343fa90529d5ae0f500e86dd8e85418969683e262f9b243535be739b053c8874