Description
The dhcpd service should be disabled on
any system that does not need to act as a DHCP server.
The isc-dhcp-server service can be disabled with the following command:
$ sudo systemctl mask --now isc-dhcp-server.service
Rationale
Unmanaged or unintentionally activated DHCP servers may provide faulty information
to clients, interfering with the operation of a legitimate site
DHCP server if there is one.
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='${db:Status-Status}' 'linux-base' 2>/dev/null | grep -q '^installed$'; then
SYSTEMCTL_EXEC='/usr/bin/systemctl'
if [[ $("$SYSTEMCTL_EXEC" is-system-running) != "offline" ]]; then
"$SYSTEMCTL_EXEC" stop 'isc-dhcp-server.service'
fi
"$SYSTEMCTL_EXEC" disable 'isc-dhcp-server.service'
"$SYSTEMCTL_EXEC" mask 'isc-dhcp-server.service'
# Disable socket activation if we have a unit file for it
if "$SYSTEMCTL_EXEC" -q list-unit-files isc-dhcp-server.socket; then
if [[ $("$SYSTEMCTL_EXEC" is-system-running) != "offline" ]]; then
"$SYSTEMCTL_EXEC" stop 'isc-dhcp-server.socket'
fi
"$SYSTEMCTL_EXEC" mask 'isc-dhcp-server.socket'
fi
# The service may not be running because it has been started and failed,
# so let's reset the state so OVAL checks pass.
# Service should be 'inactive', not 'failed' after reboot though.
"$SYSTEMCTL_EXEC" reset-failed 'isc-dhcp-server.service' || true
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:
- NIST-800-53-CM-6(a)
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- disable_strategy
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- service_dhcpd_disabled
- name: Disable DHCP Service - Collect systemd Services Present in the System
ansible.builtin.command: systemctl -q list-unit-files --type service
register: service_exists
changed_when: false
failed_when: service_exists.rc not in [0, 1]
check_mode: false
when: '"linux-base" in ansible_facts.packages'
tags:
- NIST-800-53-CM-6(a)
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- disable_strategy
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- service_dhcpd_disabled
- name: Disable DHCP Service - Ensure isc-dhcp-server.service is Masked
ansible.builtin.systemd:
name: isc-dhcp-server.service
state: stopped
enabled: false
masked: true
when:
- '"linux-base" in ansible_facts.packages'
- service_exists.stdout_lines is search("isc-dhcp-server.service", multiline=True)
tags:
- NIST-800-53-CM-6(a)
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- disable_strategy
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- service_dhcpd_disabled
- name: Unit Socket Exists - isc-dhcp-server.socket
ansible.builtin.command: systemctl -q list-unit-files isc-dhcp-server.socket
register: socket_file_exists
changed_when: false
failed_when: socket_file_exists.rc not in [0, 1]
check_mode: false
when: '"linux-base" in ansible_facts.packages'
tags:
- NIST-800-53-CM-6(a)
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- disable_strategy
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- service_dhcpd_disabled
- name: Disable DHCP Service - Disable Socket isc-dhcp-server
ansible.builtin.systemd:
name: isc-dhcp-server.socket
enabled: false
state: stopped
masked: true
when:
- '"linux-base" in ansible_facts.packages'
- socket_file_exists.stdout_lines is search("isc-dhcp-server.socket", multiline=True)
tags:
- NIST-800-53-CM-6(a)
- NIST-800-53-CM-7(a)
- NIST-800-53-CM-7(b)
- disable_strategy
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- service_dhcpd_disabled
