Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces

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Description

To set the runtime status of the net.ipv4.conf.all.forwarding kernel parameter, run the following command:

$ sudo sysctl -w net.ipv4.conf.all.forwarding=0

To make sure that the setting is persistent, add the following line to a file in the directory /etc/sysctl.d:

net.ipv4.conf.all.forwarding = 0

Rationale

IP forwarding permits the kernel to forward packets from one network interface to another. The ability to forward packets between two networks is only appropriate for systems acting as routers.

Remediation

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 rpm --quiet -q kernel-core; then

# Comment out any occurrences of net.ipv4.conf.all.forwarding 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=$(grep -P '^(?!#).*[\s]*net.ipv4.conf.all.forwarding.*$' $f | uniq )
  if ! test -z "$matching_list"; then
    while IFS= read -r entry; do
      escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
      # comment out "net.ipv4.conf.all.forwarding" 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_all_forwarding_value='0'


#
# Set runtime for net.ipv4.conf.all.forwarding
#
if ! { rpm --quiet -q kernel rpm-ostree bootc && ! rpm --quiet -q openshift-kubelet && { [ -f "/run/.containerenv" ] || [ -f "/.containerenv" ]; }; } ; then
    /sbin/sysctl -q -n -w net.ipv4.conf.all.forwarding="$sysctl_net_ipv4_conf_all_forwarding_value"
fi

#
# If net.ipv4.conf.all.forwarding present in /etc/sysctl.conf, change value to appropriate value
#	else, add "net.ipv4.conf.all.forwarding = 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.all.forwarding")

# shellcheck disable=SC2059
printf -v formatted_output "%s = %s" "$stripped_key" "$sysctl_net_ipv4_conf_all_forwarding_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.all.forwarding\\>" "${SYSCONFIG_FILE}"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^net.ipv4.conf.all.forwarding\\>.*/$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
    cce="CCE-87420-6"
    printf '# Per %s: Set %s in %s\n' "${cce}" "${formatted_output}" "${SYSCONFIG_FILE}" >> "${SYSCONFIG_FILE}"
    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:
  - CCE-87420-6
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding
- name: XCCDF Value sysctl_net_ipv4_conf_all_forwarding_value # promote to variable
  set_fact:
    sysctl_net_ipv4_conf_all_forwarding_value: !!str 0
  tags:
    - always

- name: Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces - Set
    fact for sysctl paths
  ansible.builtin.set_fact:
    sysctl_paths:
    - /etc/sysctl.d/
    - /run/sysctl.d/
    - /usr/local/lib/sysctl.d/
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-87420-6
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

- name: Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces - Find
    all files that contain net.ipv4.conf.all.forwarding
  ansible.builtin.shell:
    cmd: find -L {{ sysctl_paths | join(" ") }} -type f -name '*.conf' | xargs grep
      -HP '^\s*net.ipv4.conf.all.forwarding\s*=\s*.*$'
  register: find_all_values
  check_mode: false
  changed_when: false
  failed_when: false
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-87420-6
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

- name: Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces - Find
    all files that set net.ipv4.conf.all.forwarding to correct value
  ansible.builtin.shell:
    cmd: find -L {{ sysctl_paths | join(" ") }} -type f -name '*.conf' | xargs grep
      -HP '^\s*net.ipv4.conf.all.forwarding\s*=\s*{{ sysctl_net_ipv4_conf_all_forwarding_value
      }}$'
  register: find_correct_value
  check_mode: false
  changed_when: false
  failed_when: false
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-87420-6
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

- name: Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces - Comment
    out any occurrences of net.ipv4.conf.all.forwarding from config files
  ansible.builtin.replace:
    path: '{{ item | split(":") | first }}'
    regexp: ^[\s]*net.ipv4.conf.all.forwarding
    replace: '#net.ipv4.conf.all.forwarding'
  loop: '{{ find_all_values.stdout_lines }}'
  when:
  - '"kernel-core" in ansible_facts.packages'
  - find_correct_value.stdout_lines | length == 0 or find_all_values.stdout_lines
    | length > find_correct_value.stdout_lines | length
  tags:
  - CCE-87420-6
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

- name: Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces - Ensure
    sysctl net.ipv4.conf.all.forwarding is set
  ansible.posix.sysctl:
    name: net.ipv4.conf.all.forwarding
    value: '{{ sysctl_net_ipv4_conf_all_forwarding_value }}'
    sysctl_file: /etc/sysctl.conf
    state: present
    reload: true
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-87420-6
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

Warning

There might be cases when certain applications can systematically override this option. One such case is Libvirt; a toolkit for managing of virtualization platforms. By default, Libvirt requires IP forwarding to be enabled to facilitate network communication between the virtualization host and guest machines. It enables IP forwarding after every reboot.