Configure systemd-timesyncd with approved time servers and keep it running

Description

systemd-timesyncd is a simple SNTP client. Its servers are set with NTP= in /etc/systemd/timesyncd.conf or in a drop-in under /etc/systemd/timesyncd.conf.d/. Without NTP=, it only uses the FallbackNTP= list compiled into the package. The service must be enabled and running.

The check reads the merged configuration with systemd-analyze cat-config and keeps the last NTP= line, which is the one systemd applies. Ubuntu ships no NTP= line (only the commented FallbackNTP=ntp.ubuntu.com), so the control fails on a fresh install of 20.04 to 24.04. On 26.04, chrony replaces systemd-timesyncd and the control is not applicable. The fix writes /etc/systemd/timesyncd.conf.d/60-cirsec.conf.

Check and swap the default servers. time_servers defaults to ntp.ubuntu.com, the server Ubuntu already falls back to, so that the fix works everywhere. It is not an approved source: replace it in the tailoring file with your organization's own time servers (internal NTP, domain controllers, or a provider you trust) before applying the fix in production.

Rationale

Explicit servers make the time source a documented choice: internal servers for isolated networks, or a public pool the organization trusts. A stopped client lets the clock drift, which breaks log correlation and time-based authentication.

Impact

The system contacts the configured servers on UDP port 123; the firewall must allow it. When the default list is kept, the server is ntp.ubuntu.com, run by Canonical.

Applies when

Detected: when: systemd-timesyncd is installed A scanner reports the control not applicable when one is false.

Setting

Parameters

The value used is your override (tailoring file), else the profile's, else the one of the profile it extends, else the default. Bounds limit what an override may set.

Audit

1. Run systemd-analyze cat-config systemd/timesyncd.conf. The last NTP= line must list at least one approved server.
2. systemctl is-enabled systemd-timesyncd must print enabled and systemctl is-active systemd-timesyncd must print active.

Automated: every check must pass. Scanners report each check by its id.

servers

Time servers set with NTP= in the merged configuration.

script: |
  o=$(systemd-analyze cat-config systemd/timesyncd.conf 2>/dev/null | grep -E '^[[:space:]]*NTP=' | tail -n 1)
  v=$(printf '%s' "$o" | sed 's/^[[:space:]]*NTP=[[:space:]]*//')
  echo "NTP=${v:-<not set>}"
  [ -n "$v" ]

enabled

systemd-timesyncd is enabled.

unit: systemd-timesyncd.service
state: enabled

active

systemd-timesyncd is running.

script: systemctl is-active systemd-timesyncd.service || exit 1

Remediation

Create /etc/systemd/timesyncd.conf.d/60-cirsec.conf containing:

[Time]
NTP={{ time_servers }}

then run systemctl enable systemd-timesyncd and systemctl restart systemd-timesyncd. Check with timedatectl timesync-status.

Rollback

Delete /etc/systemd/timesyncd.conf.d/60-cirsec.conf and restart systemd-timesyncd.

enabled

Generated from check enabled: the engine writes the fix for every output, it sets the expected value and the check runs again after.

servers

f=/etc/systemd/timesyncd.conf.d/60-cirsec.conf
want=$(printf '# Written by CIRSEC Baselines CB-UBU-SVC-015\n[Time]\nNTP=%s' "$CB_TIME_SERVERS")
if [ "$(cat "$f" 2>/dev/null)" != "$want" ]; then
  cb_backup "$f"
  mkdir -p /etc/systemd/timesyncd.conf.d
  printf '%s\n' "$want" >"$f"
fi
systemctl is-active --quiet systemd-timesyncd.service || systemctl start systemd-timesyncd.service

Handwritten code reads parameters from CB_<NAME> environment variables. Each release turns these fixes into remediate.sh and an Ansible role per profile: dry run by default, a fix runs only when its check fails.

References

• timesyncd.conf(5), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• systemd-timesyncd.service(8), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• Ubuntu Server documentation - Synchronize time using timedatectl and timesyncd Canonical, vendor-doc, accessed 2026-10-19

Mappings

How this control relates to the other framework's: equivalent, stricter, looser, partial or related.

Threats mitigated

Related controls

• related CB-UBU-SVC-014 Run exactly one time synchronization daemon

People and lifecycle