Enable Kernel Parameter to Use Reverse Path Filtering on all IPv4 Interfaces by Default
Description
To set the runtime status of the net.ipv4.conf.default.rp_filter
kernel parameter, run the following command:
$ sudo sysctl -w net.ipv4.conf.default.rp_filter=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.rp_filter = 1
Rationale
Enabling reverse path filtering drops packets with source addresses
that should not have been able to be received on the interface they were
received on. It should not be used on systems which are routers for
complicated networks, but is helpful for end hosts and routers serving small
networks.
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.conf.default.rp\_filter 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=$(grep -P '^(?!#).\*[\s]\*net.ipv4.conf.default.rp\_filter.\*$' $f | uniq )
if ! test -z "$matching\_list"; then
while IFS= read -r entry; do
escaped\_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
# comment out "net.ipv4.conf.default.rp\_filter" 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\_default\_rp\_filter\_value=''
#
# Set runtime for net.ipv4.conf.default.rp\_filter
#
/sbin/sysctl -q -n -w net.ipv4.conf.default.rp\_filter="$sysctl\_net\_ipv4\_conf\_default\_rp\_filter\_value"
#
# If net.ipv4.conf.default.rp\_filter present in /etc/sysctl.conf, change value to appropriate value
# else, add "net.ipv4.conf.default.rp\_filter = value" to /etc/sysctl.conf
#
# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped\_key=$(sed 's/[\^=\$,;+]\*//g' <<< "^net.ipv4.conf.default.rp\_filter")
# shellcheck disable=SC2059
printf -v formatted\_output "%s = %s" "$stripped\_key" "$sysctl\_net\_ipv4\_conf\_default\_rp\_filter\_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.rp\_filter\\>" "${SYSCONFIG\_FILE}"; then
escaped\_formatted\_output=$(sed -e 's|/|\\/|g' <<< "$formatted\_output")
LC\_ALL=C sed -i --follow-symlinks "s/^net.ipv4.conf.default.rp\_filter\\>.\*/$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.conf.default.rp\_filter.\*$
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-7(a)
- disable\_strategy
- low\_complexity
- medium\_disruption
- medium\_severity
- reboot\_required
- sysctl\_net\_ipv4\_conf\_default\_rp\_filter
- name: Comment out any occurrences of net.ipv4.conf.default.rp\_filter from config
files
replace:
path: '{{ item.path }}'
regexp: ^[\s]\*net.ipv4.conf.default.rp\_filter
replace: '#net.ipv4.conf.default.rp\_filter'
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-7(a)
- disable\_strategy
- low\_complexity
- medium\_disruption
- medium\_severity
- reboot\_required
- sysctl\_net\_ipv4\_conf\_default\_rp\_filter
- name: XCCDF Value sysctl\_net\_ipv4\_conf\_default\_rp\_filter\_value # promote to variable
set\_fact:
sysctl\_net\_ipv4\_conf\_default\_rp\_filter\_value: !!str
tags:
- always
- name: Ensure sysctl net.ipv4.conf.default.rp\_filter is set
sysctl:
name: net.ipv4.conf.default.rp\_filter
value: '{{ sysctl\_net\_ipv4\_conf\_default\_rp\_filter\_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-6(a)
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- NIST-800-53-SC-7(a)
- disable\_strategy
- low\_complexity
- medium\_disruption
- medium\_severity
- reboot\_required
- sysctl\_net\_ipv4\_conf\_default\_rp\_filter