# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-SVC-001
title: Disable DHCP and DNS server services the system does not provide
finding:
  title: DHCP or DNS server services can run on a system that does not provide them
  description: An unneeded DHCP or DNS server answers on the network and can hand out wrong addresses or names.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [services, dhcp, dns, attack-surface]

description: |
  Ubuntu packages several network infrastructure servers:

  - DHCP: `isc-dhcp-server` (`isc-dhcp-server`, `isc-dhcp-server6`) and Kea (`kea-dhcp4-server`,
    `kea-dhcp6-server`, `kea-dhcp-ddns-server`);
  - DNS: BIND (`named`);
  - dnsmasq, a combined DNS forwarder and DHCP server (`dnsmasq`).

  The control passes when each unit is masked or not installed at all.
rationale: |
  These daemons answer requests from any host on the network. A rogue or forgotten DHCP or DNS
  server can redirect clients to an attacker, and every running daemon is code that can be
  exploited remotely.
impact: |
  Masking a unit stops it and prevents it from starting again, even as a dependency. Systems
  whose role is to provide DHCP or DNS must exclude this control in their tailoring file.
  The dnsmasq binary used by libvirt or NetworkManager (`dnsmasq-base`) is not affected: only the
  stand-alone `dnsmasq` service is masked.
notes: |
  The check reads `systemctl is-enabled`: a unit that does not exist counts as compliant, so
  removing the package (`apt purge`) is an equally valid fix. The packages are not installed on
  a default Ubuntu install (server, desktop or cloud image).

audit:
  manual: |
    For each unit, run `systemctl is-enabled <unit>`. It must print `masked`, or fail because the
    unit does not exist: `isc-dhcp-server.service`, `isc-dhcp-server6.service`,
    `kea-dhcp4-server.service`, `kea-dhcp6-server.service`, `kea-dhcp-ddns-server.service`,
    `named.service`, `dnsmasq.service`.
  logic: all
  checks:
    - id: isc-dhcp-server
      engine: systemd-unit
      description: ISC DHCPv4 server.
      spec: { unit: isc-dhcp-server.service, state: masked }
    - id: isc-dhcp-server6
      engine: systemd-unit
      description: ISC DHCPv6 server.
      spec: { unit: isc-dhcp-server6.service, state: masked }
    - id: kea-dhcp4
      engine: systemd-unit
      description: Kea DHCPv4 server.
      spec: { unit: kea-dhcp4-server.service, state: masked }
    - id: kea-dhcp6
      engine: systemd-unit
      description: Kea DHCPv6 server.
      spec: { unit: kea-dhcp6-server.service, state: masked }
    - id: kea-dhcp-ddns
      engine: systemd-unit
      description: Kea dynamic DNS update server.
      spec: { unit: kea-dhcp-ddns-server.service, state: masked }
    - id: named
      engine: systemd-unit
      description: BIND DNS server.
      spec: { unit: named.service, state: masked }
    - id: dnsmasq
      engine: systemd-unit
      description: dnsmasq DNS and DHCP server.
      spec: { unit: dnsmasq.service, state: masked }

remediation:
  manual: |
    Stop and mask each unit that is installed: `systemctl mask --now <unit>`. If the package is not
    needed at all, remove it instead: `apt purge isc-dhcp-server kea-dhcp4-server kea-dhcp6-server
    kea-dhcp-ddns-server bind9 dnsmasq`.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Run `systemctl unmask <unit>` then `systemctl enable --now <unit>`.
  scripts:
    - id: isc-dhcp-server
      from_check: isc-dhcp-server
    - id: isc-dhcp-server6
      from_check: isc-dhcp-server6
    - id: kea-dhcp4
      from_check: kea-dhcp4
    - id: kea-dhcp6
      from_check: kea-dhcp6
    - id: kea-dhcp-ddns
      from_check: kea-dhcp-ddns
    - id: named
      from_check: named
    - id: dnsmasq
      from_check: dnsmasq

references:
  - title: systemctl(1), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man1/systemctl.1.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: dhcpd(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/dhcpd.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: named(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/named.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: dnsmasq(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/dnsmasq.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R62)
    url: https://messervices.cyber.gouv.fr/documents-guides/fr_np_linux_configuration-v2.0.pdf
    publisher: ANSSI
    kind: guide
    accessed: 2026-10-19
mappings:
  - framework: anssi-linux
    framework_version: "2.0"
    id: R62
    relation: partial
    note: R62 asks to disable every unneeded service; this control covers DHCP and DNS servers.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7
    relation: partial
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "2.1.3"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "2.1.4"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "2.1.5"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "2.1.3"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "2.1.4"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "2.1.5"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "2.1.5"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "2.1.7"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "2.1.9"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "2.1.3"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "2.1.4"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "2.1.5"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1210
  - framework: mitre-attack
    id: T1557

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["20.04"]
    environment: Proxmox VM, Ubuntu 20.04 cloud image, kernel 5.4.0-216-generic
    notes: "Stock image passes (units absent); with isc-dhcp-server, kea-dhcp4-server, bind9 and dnsmasq installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["22.04"]
    environment: Proxmox VM, Ubuntu 22.04 cloud image, kernel 5.15.0-198-generic
    notes: "Stock image passes (units absent); with isc-dhcp-server, kea-dhcp4-server, bind9 and dnsmasq installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["24.04"]
    environment: Proxmox VM, Ubuntu 24.04 cloud image, kernel 6.8.0-147-generic
    notes: "Stock image passes (units absent); with isc-dhcp-server, kea-dhcp4-server, bind9 and dnsmasq installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["26.04"]
    environment: Proxmox VM, Ubuntu 26.04 cloud image, kernel 7.0.0-38-generic
    notes: "Stock image passes (units absent); with isc-dhcp-server, kea-dhcp4-server, bind9 and dnsmasq installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19