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.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)"
l_ufw_file="$([ -f /etc/default/ufw ] && \
awk -F= '/^\s*IPT_SYSCTL=/ {print $2}' /etc/default/ufw)"
[ -f "$(readlink -e "$l_ufw_file")" ] && \
a_files+=("$l_ufw_file"); 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 < <("$l_systemdsysctl" --cat-config | tac | \
grep -Pio -- '^\h*#\h*\/[^#\n\r\h]+\.conf\b')
for l_file in "${a_files[@]}"; do
grep -Poi -- '\h*'"$l_grep"'\h*=\h*\H+\b' "$l_file" \
| grep -Pivq -- '^\h*'"$l_grep"'\h*=\h*'"$l_value"'\b' && \
sed -ri '/^\s*'"$l_grep"'\s*=\s*(0|[2-9]|1[0-9]+)/s/^/# /' "$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
Note: If the UFW file was the first file listed in the audit, the entry will be commented out as part of the first step, however updating Uncomplicated Firewall (UFW) may update this change. In this case the updated entry will supersede the entry being created as part of this step.
- Run the following command to load all sysctl configuration filles:
# sysctl --system
See Also
Item Details
Audit Name: CIS Rocky Linux 8 v3.0.0 L1 Server
Category: CONFIGURATION MANAGEMENT
References: 800-53|CM-6b., CSCv7|9.2
Plugin: Unix
Control ID: 4995a05d79dd1d7422f7f7f24ae60aca3df8c74df95fc3a2535cee4bee72647f