Disable Kernel Parameter for Sending ICMP Redirects on all IPv4 Interfaces

Classification:

compliance

Framework:

Control:

Description

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

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

Rationale

ICMP redirect messages are used by routers to inform hosts that a more direct route exists for a particular destination. These messages contain information from the system’s route table possibly revealing portions of the network topology.

The ability to send ICMP redirects is only appropriate for systems acting as routers.

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.conf.all.send\_redirects 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=$(grep -P '^(?!#).\*[\s]\*net.ipv4.conf.all.send\_redirects.\*$' $f | uniq )
 if ! test -z "$matching\_list"; then
 while IFS= read -r entry; do
 escaped\_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
 # comment out "net.ipv4.conf.all.send\_redirects" 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.conf.all.send\_redirects
#
/sbin/sysctl -q -n -w net.ipv4.conf.all.send\_redirects="0"

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

# 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.conf.all.send\_redirects\\>" "${SYSCONFIG\_FILE}"; then
 escaped\_formatted\_output=$(sed -e 's|/|\\/|g' <<< "$formatted\_output")
 LC\_ALL=C sed -i --follow-symlinks "s/^net.ipv4.conf.all.send\_redirects\\>.\*/$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.all.send\_redirects.\*$
 patterns: '\*.conf'
 file\_type: any
 register: find\_sysctl\_d
 when: ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
 tags:
 - CJIS-5.10.1.1
 - 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-DSSv4-1.4.2
 - disable\_strategy
 - low\_complexity
 - medium\_disruption
 - medium\_severity
 - reboot\_required
 - sysctl\_net\_ipv4\_conf\_all\_send\_redirects

- name: Comment out any occurrences of net.ipv4.conf.all.send\_redirects from config
 files
 replace:
 path: '{{ item.path }}'
 regexp: ^[\s]\*net.ipv4.conf.all.send\_redirects
 replace: '#net.ipv4.conf.all.send\_redirects'
 loop: '{{ find\_sysctl\_d.files }}'
 when: ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
 tags:
 - CJIS-5.10.1.1
 - 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-DSSv4-1.4.2
 - disable\_strategy
 - low\_complexity
 - medium\_disruption
 - medium\_severity
 - reboot\_required
 - sysctl\_net\_ipv4\_conf\_all\_send\_redirects

- name: Ensure sysctl net.ipv4.conf.all.send\_redirects is set to 0
 sysctl:
 name: net.ipv4.conf.all.send\_redirects
 value: '0'
 sysctl\_file: /etc/sysctl.conf
 state: present
 reload: true
 when: ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
 tags:
 - CJIS-5.10.1.1
 - 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-DSSv4-1.4.2
 - disable\_strategy
 - low\_complexity
 - medium\_disruption
 - medium\_severity
 - reboot\_required
 - sysctl\_net\_ipv4\_conf\_all\_send\_redirects