Enable Kernel Parameter to Use TCP RFC 1337 on IPv4 Interfaces

An XCCDF Rule

Description

To set the runtime status of the net.ipv4.tcp_rfc1337 kernel parameter, run the following command:

$ sudo sysctl -w net.ipv4.tcp_rfc1337=1

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

net.ipv4.tcp_rfc1337 = 1

Rationale

Enable TCP behavior conformant with RFC 1337. When disabled, if a RST is received in TIME_WAIT state, we close the socket immediately without waiting for the end of the TIME_WAIT period.

ID: xccdf_org.ssgproject.content_rule_sysctl_net_ipv4_tcp_rfc1337
Severity: Medium
References: BP28(R22)
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.tcp_rfc1337 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=$(grep -P '^(?!#).*[\s]*net.ipv4.tcp_rfc1337.*$' $f | uniq )
  if ! test -z "$matching_list"; then
    while IFS= read -r entry; do
      escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
      # comment out "net.ipv4.tcp_rfc1337" 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_tcp_rfc1337_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_tcp_rfc1337_value" use="legacy"/>'


#
# Set runtime for net.ipv4.tcp_rfc1337
#
/sbin/sysctl -q -n -w net.ipv4.tcp_rfc1337="$sysctl_net_ipv4_tcp_rfc1337_value"

#
# If net.ipv4.tcp_rfc1337 present in /etc/sysctl.conf, change value to appropriate value
#	else, add "net.ipv4.tcp_rfc1337 = 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.tcp_rfc1337")

# shellcheck disable=SC2059
printf -v formatted_output "%s = %s" "$stripped_key" "$sysctl_net_ipv4_tcp_rfc1337_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.tcp_rfc1337\\>" "${SYSCONFIG_FILE}"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^net.ipv4.tcp_rfc1337\\>.*/$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.tcp_rfc1337.*$
    patterns: '*.conf'
    file_type: any
  register: find_sysctl_d
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_tcp_rfc1337

- name: Comment out any occurrences of net.ipv4.tcp_rfc1337 from config files
  replace:
    path: '{{ item.path }}'
    regexp: ^[\s]*net.ipv4.tcp_rfc1337
    replace: '#net.ipv4.tcp_rfc1337'
  loop: '{{ find_sysctl_d.files }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_tcp_rfc1337
- name: XCCDF Value sysctl_net_ipv4_tcp_rfc1337_value # promote to variable
  set_fact:
    sysctl_net_ipv4_tcp_rfc1337_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_tcp_rfc1337_value" use="legacy"/>
  tags:
    - always

- name: Ensure sysctl net.ipv4.tcp_rfc1337 is set
  sysctl:
    name: net.ipv4.tcp_rfc1337
    value: '{{ sysctl_net_ipv4_tcp_rfc1337_value }}'
    sysctl_file: /etc/sysctl.conf
    state: present
    reload: true
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_net_ipv4_tcp_rfc1337

Enable Kernel Parameter to Use TCP RFC 1337 on IPv4 Interfaces

Description

Rationale

Remediation - Shell Script

Remediation - Ansible