Configure Firewalld to Restrict Loopback Traffic

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Description

Configure firewalld to restrict loopback traffic to the lo interface.

The loopback traffic must be trusted by assigning the lo interface to the firewalld trusted zone. However, the loopback traffic must be restricted to the loopback interface as an anti-spoofing measure.

To configure firewalld to restrict loopback traffic to the lo interface, run the following commands:


sudo firewall-cmd --permanent --zone=trusted --add-rich-rule='rule family=ipv4 source address="127.0.0.1" destination not address="127.0.0.1" drop'
sudo firewall-cmd --permanent --zone=trusted --add-rich-rule='rule family=ipv6 source address="::1" destination not address="::1" drop'

To ensure firewalld settings are applied in runtime, run the following command:

firewall-cmd --reload

Rationale

Loopback traffic is generated between processes on machine and is typically critical to operation of the system. The loopback interface is the only place that loopback network traffic should be seen, all other interfaces should ignore traffic on this network as an anti-spoofing measure.

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 [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

if ! rpm -q --quiet "firewalld" ; then
    yum install -y "firewalld"
fi

ipv4_rule='rule family=ipv4 source address="127.0.0.1" destination not address="127.0.0.1" drop'
ipv6_rule='rule family=ipv6 source address="::1" destination not address="::1" drop'

if test "$(stat -c %d:%i /)" != "$(stat -c %d:%i /proc/1/root/.)"; then
    firewall-offline-cmd --zone=trusted --add-rich-rule="${ipv4_rule}"
    firewall-offline-cmd --zone=trusted --add-rich-rule="${ipv6_rule}"
elif systemctl is-active firewalld; then
    firewall-cmd --permanent --zone=trusted --add-rich-rule="${ipv4_rule}"
    firewall-cmd --permanent --zone=trusted --add-rich-rule="${ipv6_rule}"
    firewall-cmd --reload
else
    echo "
    firewalld service is not active. Remediation aborted!
    This remediation could not be applied because it depends on firewalld service running.
    The service is not started by this remediation in order to prevent connection issues."
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: Configure Firewalld to Restrict Loopback Traffic - Ensure firewalld Package
    is Installed
  ansible.builtin.package:
    name: '{{ item }}'
    state: present
  with_items:
  - firewalld
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-87272-1
  - PCI-DSSv4-1.4.1
  - configure_strategy
  - firewalld_loopback_traffic_restricted
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed

- name: Configure Firewalld to Restrict Loopback Traffic - Collect Facts About System
    Services
  ansible.builtin.service_facts: null
  register: result_services_states
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-87272-1
  - PCI-DSSv4-1.4.1
  - configure_strategy
  - firewalld_loopback_traffic_restricted
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed

- name: Configure Firewalld to Restrict Loopback Traffic - Remediation is Applicable
    if firewalld Service is Running
  block:

  - name: Configure Firewalld to Restrict Loopback Traffic - Ensure firewalld trusted
      Zone Restricts IPv4 Loopback Traffic
    ansible.builtin.command:
      cmd: firewall-cmd --permanent --zone=trusted --add-rich-rule='rule family=ipv4
        source address="127.0.0.1" destination not address="127.0.0.1" drop'
    register: result_trusted_ipv4_restriction
    changed_when:
    - '''ALREADY_ENABLED'' not in result_trusted_ipv4_restriction.stderr'

  - name: Configure Firewalld to Restrict Loopback Traffic - Ensure firewalld trusted
      Zone Restricts IPv6 Loopback Traffic
    ansible.builtin.command:
      cmd: firewall-cmd --permanent --zone=trusted --add-rich-rule='rule family=ipv6
        source address="::1" destination not address="::1" drop'
    register: result_trusted_ipv6_restriction
    changed_when:
    - '''ALREADY_ENABLED'' not in result_trusted_ipv6_restriction.stderr'

  - name: Configure Firewalld to Restrict Loopback Traffic - Ensure firewalld Changes
      are Applied
    ansible.builtin.service:
      name: firewalld
      state: reloaded
    when:
    - result_trusted_ipv4_restriction is changed or result_trusted_ipv6_restriction
      is changed
  when:
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  - ansible_facts.services['firewalld.service'].state == 'running'
  tags:
  - CCE-87272-1
  - PCI-DSSv4-1.4.1
  - configure_strategy
  - firewalld_loopback_traffic_restricted
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed

- name: Configure Firewalld to Restrict Loopback Traffic - Informative Message Based
    on Service State
  ansible.builtin.assert:
    that:
    - ansible_facts.services['firewalld.service'].state == 'running'
    fail_msg:
    - firewalld service is not active. Remediation aborted!
    - This remediation could not be applied because it depends on firewalld service
      running.
    - The service is not started by this remediation in order to prevent connection
      issues.
    success_msg:
    - Configure Firewalld to Restrict Loopback Traffic remediation successfully executed
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-87272-1
  - PCI-DSSv4-1.4.1
  - configure_strategy
  - firewalld_loopback_traffic_restricted
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed