Enable Kernel Parameter to Ignore ICMP Broadcast Echo Requests on IPv4 Interfaces
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Description
To set the runtime status of the net.ipv4.icmp_echo_ignore_broadcasts
kernel parameter, run the following command:
$ sudo sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
To make sure that the setting is persistent, add the following line to a file in the directory /etc/sysctl.d
:
net.ipv4.icmp_echo_ignore_broadcasts = 1
Rationale
Responding to broadcast (ICMP) echoes facilitates network mapping
and provides a vector for amplification attacks.
Ignoring ICMP echo requests (pings) sent to broadcast or multicast
addresses makes the system slightly more difficult to enumerate on the network.
Shell script
The following script can be run on the host to remediate the issue.
#!/bin/bash
# Remediation is applicable only in certain platforms
if dpkg-query --show --showformat='${db:Status-Status}
' 'kernel' 2>/dev/null | grep -q installed; then
# Comment out any occurrences of net.ipv4.icmp_echo_ignore_broadcasts from /etc/sysctl.d/*.conf files
for f in /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf; do
# skip systemd-sysctl symlink (/etc/sysctl.d/99-sysctl.conf -> /etc/sysctl.conf)
if [[ "$(readlink -f "$f")" == "/etc/sysctl.conf" ]]; then continue; fi
matching_list=$(grep -P '^(?!#).*[\s]*net.ipv4.icmp_echo_ignore_broadcasts.*$' $f | uniq )
if ! test -z "$matching_list"; then
while IFS= read -r entry; do
escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
# comment out "net.ipv4.icmp_echo_ignore_broadcasts" matches to preserve user data
sed -i --follow-symlinks "s/^${escaped_entry}$/# &/g" $f
done <<< "$matching_list"
fi
done
#
# Set sysctl config file which to save the desired value
#
SYSCONFIG_FILE="/etc/sysctl.conf"
sysctl_net_ipv4_icmp_echo_ignore_broadcasts_value='1'
#
# Set runtime for net.ipv4.icmp_echo_ignore_broadcasts
#
/sbin/sysctl -q -n -w net.ipv4.icmp_echo_ignore_broadcasts="$sysctl_net_ipv4_icmp_echo_ignore_broadcasts_value"
#
# If net.ipv4.icmp_echo_ignore_broadcasts present in /etc/sysctl.conf, change value to appropriate value
# else, add "net.ipv4.icmp_echo_ignore_broadcasts = value" to /etc/sysctl.conf
#
# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped_key=$(sed 's/[\^=\$,;+]*//g' <<< "^net.ipv4.icmp_echo_ignore_broadcasts")
# shellcheck disable=SC2059
printf -v formatted_output "%s = %s" "$stripped_key" "$sysctl_net_ipv4_icmp_echo_ignore_broadcasts_value"
# If the key exists, change it. Otherwise, add it to the config_file.
# We search for the key string followed by a word boundary (matched by \>),
# so if we search for 'setting', 'setting2' won't match.
if LC_ALL=C grep -q -m 1 -i -e "^net.ipv4.icmp_echo_ignore_broadcasts\\>" "${SYSCONFIG_FILE}"; then
escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
LC_ALL=C sed -i --follow-symlinks "s/^net.ipv4.icmp_echo_ignore_broadcasts\\>.*/$escaped_formatted_output/gi" "${SYSCONFIG_FILE}"
else
if [[ -s "${SYSCONFIG_FILE}" ]] && [[ -n "$(tail -c 1 -- "${SYSCONFIG_FILE}" || true)" ]]; then
LC_ALL=C sed -i --follow-symlinks '$a'\\ "${SYSCONFIG_FILE}"
fi
printf '%s\n' "$formatted_output" >> "${SYSCONFIG_FILE}"
fi
else
>&2 echo 'Remediation is not applicable, nothing was done'
fi
Ansible playbook
The following playbook can be run with Ansible to remediate the issue.
- name: Gather the package facts
package_facts:
manager: auto
tags:
- CJIS-5.10.1.1
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5
- PCI-DSS-Req-1.4.3
- PCI-DSSv4-1.4
- PCI-DSSv4-1.4.2
- disable_strategy
- low_complexity
- medium_disruption
- medium_severity
- reboot_required
- sysctl_net_ipv4_icmp_echo_ignore_broadcasts
- name: List /etc/sysctl.d/*.conf files
find:
paths:
- /etc/sysctl.d/
- /run/sysctl.d/
- /usr/local/lib/sysctl.d/
contains: ^[\s]*net.ipv4.icmp_echo_ignore_broadcasts.*$
patterns: '*.conf'
file_type: any
register: find_sysctl_d
when: '"kernel" in ansible_facts.packages'
tags:
- CJIS-5.10.1.1
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5
- PCI-DSS-Req-1.4.3
- PCI-DSSv4-1.4
- PCI-DSSv4-1.4.2
- disable_strategy
- low_complexity
- medium_disruption
- medium_severity
- reboot_required
- sysctl_net_ipv4_icmp_echo_ignore_broadcasts
- name: Comment out any occurrences of net.ipv4.icmp_echo_ignore_broadcasts from config
files
replace:
path: '{{ item.path }}'
regexp: ^[\s]*net.ipv4.icmp_echo_ignore_broadcasts
replace: '#net.ipv4.icmp_echo_ignore_broadcasts'
loop: '{{ find_sysctl_d.files }}'
when: '"kernel" in ansible_facts.packages'
tags:
- CJIS-5.10.1.1
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5
- PCI-DSS-Req-1.4.3
- PCI-DSSv4-1.4
- PCI-DSSv4-1.4.2
- disable_strategy
- low_complexity
- medium_disruption
- medium_severity
- reboot_required
- sysctl_net_ipv4_icmp_echo_ignore_broadcasts
- name: XCCDF Value sysctl_net_ipv4_icmp_echo_ignore_broadcasts_value # promote to variable
set_fact:
sysctl_net_ipv4_icmp_echo_ignore_broadcasts_value: !!str 1
tags:
- always
- name: Ensure sysctl net.ipv4.icmp_echo_ignore_broadcasts is set
sysctl:
name: net.ipv4.icmp_echo_ignore_broadcasts
value: '{{ sysctl_net_ipv4_icmp_echo_ignore_broadcasts_value }}'
sysctl_file: /etc/sysctl.conf
state: present
reload: true
when: '"kernel" in ansible_facts.packages'
tags:
- CJIS-5.10.1.1
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5
- PCI-DSS-Req-1.4.3
- PCI-DSSv4-1.4
- PCI-DSSv4-1.4.2
- disable_strategy
- low_complexity
- medium_disruption
- medium_severity
- reboot_required
- sysctl_net_ipv4_icmp_echo_ignore_broadcasts