Ensure a Table Exists for Nftables

Description

Tables in nftables hold chains. Each table only has one address family and only applies to packets of this family. Tables can have one of six families.

Rationale

Nftables doesn’t have any default tables. Without a table being built, nftables will not filter network traffic.

Remediation

Shell script

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

#!/bin/bash

# Remediation is applicable only in certain platforms
if dpkg-query --show --showformat='${db:Status-Status}' 'nftables' 2>/dev/null | grep -q '^installed$'; then

var_nftables_family='inet'

var_nftables_table='filter'


if ! nft list table $var_nftables_family $var_nftables_table; then
  nft create table "$var_nftables_family" "$var_nftables_table"
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: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - set_nftables_table
- name: XCCDF Value var_nftables_family # promote to variable
  set_fact:
    var_nftables_family: !!str inet
  tags:
    - always
- name: XCCDF Value var_nftables_table # promote to variable
  set_fact:
    var_nftables_table: !!str filter
  tags:
    - always

- name: Collect Existing Nftables
  ansible.builtin.command: nft list table {{ var_nftables_family }} {{ var_nftables_table
    }}
  register: result_nftables_table_family
  changed_when: false
  failed_when: result_nftables_table_family.rc not in [0, 1]
  when: '"nftables" in ansible_facts.packages'
  tags:
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - set_nftables_table

- name: Set Nftable Table
  ansible.builtin.command: nft create table {{ var_nftables_family }} {{ var_nftables_table
    }}
  when:
  - '"nftables" in ansible_facts.packages'
  - result_nftables_table_family is not skipped
  - result_nftables_table_family.rc != 0
  tags:
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - set_nftables_table

Warning

Adding or editing rules in a running nftables can cause loss of connectivity to the system.