Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces

An XCCDF Rule

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

warning alert: 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.

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.

ID: xccdf_org.ssgproject.content_rule_sysctl_net_ipv4_conf_all_forwarding
Severity: Medium
References: CCI-000366

CM-6(b)

SRG-OS-000480-GPOS-00227
Updated: about 1 year ago

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; 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
  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 "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='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sysctl_net_ipv4_conf_all_forwarding_value" use="legacy"/>'


#
# Set runtime for net.ipv4.conf.all.forwarding
#
/sbin/sysctl -q -n -w net.ipv4.conf.all.forwarding="$sysctl_net_ipv4_conf_all_forwarding_value"

#
# 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=""
    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

- 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.all.forwarding.*$
    patterns: '*.conf'
    file_type: any
  register: find_sysctl_d
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

- name: Comment out any occurrences of net.ipv4.conf.all.forwarding from config files
  replace:
    path: '{{ item.path }}'
    regexp: ^[\s]*net.ipv4.conf.all.forwarding
    replace: '#net.ipv4.conf.all.forwarding'
  loop: '{{ find_sysctl_d.files }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - 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 <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sysctl_net_ipv4_conf_all_forwarding_value" use="legacy"/>
  tags:
    - always

- name: Ensure sysctl net.ipv4.conf.all.forwarding is set
  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: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-53-CM-6(b)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_conf_all_forwarding

Disable Kernel Parameter for IPv4 Forwarding on all IPv4 Interfaces

Description

Rationale

Remediation - Shell Script

Remediation - Ansible