Disable Kernel Parameter for IP Forwarding on IPv4 Interfaces

Classification:

compliance

Framework:

Control:

Description

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

$ sudo sysctl -w net.ipv4.ip_forward=0

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

net.ipv4.ip_forward = 0

Rationale

Routing protocol daemons are typically used on routers to exchange network topology information with other routers. If this capability is used when not required, system network information may be unnecessarily transmitted across the network.

Remediation

Shell script

The following script can be run on the host to remediate the issue.

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

# Comment out any occurrences of net.ipv4.ip\_forward from /etc/sysctl.d/\*.conf files

for f in /etc/sysctl.d/\*.conf /run/sysctl.d/\*.conf /usr/local/lib/sysctl.d/\*.conf /usr/lib/sysctl.d/\*.conf; do

 matching\_list=$(grep -P '^(?!#).\*[\s]\*net.ipv4.ip\_forward.\*$' $f | uniq )
 if ! test -z "$matching\_list"; then
 while IFS= read -r entry; do
 escaped\_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
 # comment out "net.ipv4.ip\_forward" 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"


#
# Set runtime for net.ipv4.ip\_forward
#
/sbin/sysctl -q -n -w net.ipv4.ip\_forward="0"

#
# If net.ipv4.ip\_forward present in /etc/sysctl.conf, change value to "0"
# else, add "net.ipv4.ip\_forward = 0" 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.ip\_forward")

# shellcheck disable=SC2059
printf -v formatted\_output "%s = %s" "$stripped\_key" "0"

# 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.ip\_forward\\>" "${SYSCONFIG\_FILE}"; then
 escaped\_formatted\_output=$(sed -e 's|/|\\/|g' <<< "$formatted\_output")
 LC\_ALL=C sed -i --follow-symlinks "s/^net.ipv4.ip\_forward\\>.\*/$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/
 - /usr/lib/sysctl.d/
 contains: ^[\s]\*net.ipv4.ip\_forward.\*$
 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-6(a)
 - NIST-800-53-CM-7(a)
 - NIST-800-53-CM-7(b)
 - NIST-800-53-SC-5
 - NIST-800-53-SC-7(a)
 - PCI-DSS-Req-1.3.1
 - PCI-DSS-Req-1.3.2
 - PCI-DSSv4-1.4.2
 - disable\_strategy
 - low\_complexity
 - medium\_disruption
 - medium\_severity
 - reboot\_required
 - sysctl\_net\_ipv4\_ip\_forward

- name: Comment out any occurrences of net.ipv4.ip\_forward from config files
 replace:
 path: '{{ item.path }}'
 regexp: ^[\s]\*net.ipv4.ip\_forward
 replace: '#net.ipv4.ip\_forward'
 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-6(a)
 - NIST-800-53-CM-7(a)
 - NIST-800-53-CM-7(b)
 - NIST-800-53-SC-5
 - NIST-800-53-SC-7(a)
 - PCI-DSS-Req-1.3.1
 - PCI-DSS-Req-1.3.2
 - PCI-DSSv4-1.4.2
 - disable\_strategy
 - low\_complexity
 - medium\_disruption
 - medium\_severity
 - reboot\_required
 - sysctl\_net\_ipv4\_ip\_forward

- name: Ensure sysctl net.ipv4.ip\_forward is set to 0
 sysctl:
 name: net.ipv4.ip\_forward
 value: '0'
 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-6(a)
 - NIST-800-53-CM-7(a)
 - NIST-800-53-CM-7(b)
 - NIST-800-53-SC-5
 - NIST-800-53-SC-7(a)
 - PCI-DSS-Req-1.3.1
 - PCI-DSS-Req-1.3.2
 - PCI-DSSv4-1.4.2
 - disable\_strategy
 - low\_complexity
 - medium\_disruption
 - medium\_severity
 - reboot\_required
 - sysctl\_net\_ipv4\_ip\_forward

Warning

Certain technologies such as virtual machines, containers, etc. rely on IPv4 forwarding to enable and use networking. Disabling IPv4 forwarding would cause those technologies to stop working. Therefore, this rule should not be used in profiles or benchmarks that target usage of IPv4 forwarding.