Enable Kernel Paremeter to Log Martian Packets on all IPv4 Interfaces by Default
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Description
To set the runtime status of the net.ipv4.conf.default.log_martians
kernel parameter, run the following command:
$ sudo sysctl -w net.ipv4.conf.default.log_martians=1
To make sure that the setting is persistent, add the following line to a file in the directory /etc/sysctl.d
:
net.ipv4.conf.default.log_martians = 1
Rationale
The presence of “martian” packets (which have impossible addresses)
as well as spoofed packets, source-routed packets, and redirects could be a
sign of nefarious network activity. Logging these packets enables this activity
to be detected.
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 [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then
# Comment out any occurrences of net.ipv4.conf.default.log_martians 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=$(grep -P '^(?!#).*[\s]*net.ipv4.conf.default.log_martians.*$' $f | uniq )
if ! test -z "$matching_list"; then
while IFS= read -r entry; do
escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
# comment out "net.ipv4.conf.default.log_martians" 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_conf_default_log_martians_value='1'
#
# Set runtime for net.ipv4.conf.default.log_martians
#
/sbin/sysctl -q -n -w net.ipv4.conf.default.log_martians="$sysctl_net_ipv4_conf_default_log_martians_value"
#
# If net.ipv4.conf.default.log_martians present in /etc/sysctl.conf, change value to appropriate value
# else, add "net.ipv4.conf.default.log_martians = value" to /etc/sysctl.conf
#
# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped_key=$(sed 's/[\^=\$,;+]*//g' <<< "^net.ipv4.conf.default.log_martians")
# shellcheck disable=SC2059
printf -v formatted_output "%s = %s" "$stripped_key" "$sysctl_net_ipv4_conf_default_log_martians_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.conf.default.log_martians\\>" "${SYSCONFIG_FILE}"; then
escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
LC_ALL=C sed -i --follow-symlinks "s/^net.ipv4.conf.default.log_martians\\>.*/$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: List /etc/sysctl.d/*.conf files
find:
paths:
- /etc/sysctl.d/
- /run/sysctl.d/
- /usr/local/lib/sysctl.d/
contains: ^[\s]*net.ipv4.conf.default.log_martians.*$
patterns: '*.conf'
file_type: any
register: find_sysctl_d
when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
tags:
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5(3)(a)
- disable_strategy
- low_complexity
- medium_disruption
- reboot_required
- sysctl_net_ipv4_conf_default_log_martians
- unknown_severity
- name: Comment out any occurrences of net.ipv4.conf.default.log_martians from config
files
replace:
path: '{{ item.path }}'
regexp: ^[\s]*net.ipv4.conf.default.log_martians
replace: '#net.ipv4.conf.default.log_martians'
loop: '{{ find_sysctl_d.files }}'
when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
tags:
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5(3)(a)
- disable_strategy
- low_complexity
- medium_disruption
- reboot_required
- sysctl_net_ipv4_conf_default_log_martians
- unknown_severity
- name: XCCDF Value sysctl_net_ipv4_conf_default_log_martians_value # promote to variable
set_fact:
sysctl_net_ipv4_conf_default_log_martians_value: !!str 1
tags:
- always
- name: Ensure sysctl net.ipv4.conf.default.log_martians is set
sysctl:
name: net.ipv4.conf.default.log_martians
value: '{{ sysctl_net_ipv4_conf_default_log_martians_value }}'
sysctl_file: /etc/sysctl.conf
state: present
reload: true
when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
tags:
- NIST-800-171-3.1.20
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-5(3)(a)
- disable_strategy
- low_complexity
- medium_disruption
- reboot_required
- sysctl_net_ipv4_conf_default_log_martians
- unknown_severity