NTPと時刻同期サービス

Accurate clocks are essential for networked systems. Logs, TLS certificates, authentication tokens, and distributed databases all break when clocks drift apart. The Network Time Protocol (NTP) keeps them aligned.

NTP clients query time servers, measure the network round-trip, and gradually adjust the local clock. It corrects for delay so even over the internet machines stay within milliseconds.

NTP servers are ranked by stratum. Stratum 0 is a reference clock (like GPS), stratum 1 servers connect directly to it, and each layer below is one step further. Lower stratum means closer to the source.

The simplest check is timedatectl, which shows the current time, timezone, and whether NTP synchronization is active.

Turn on automatic synchronization with one command. The system then keeps the clock in sync in the background.

chrony is the modern NTP implementation used by most Linux distributions. It handles intermittent connections and large time jumps better than the older ntpd.

List the time servers chrony is using and how it rates them with chronyc sources. The asterisk marks the currently selected source.

chronyc tracking shows how far off the clock is and how it is being corrected, including the estimated offset from true time.

Edit /etc/chrony/chrony.conf to point at preferred pools or internal time servers, then restart the service. Many setups use the public NTP pool.

Time synchronization sets the absolute moment (UTC), while the timezone affects how it is displayed. Set the zone separately and keep the hardware clock in UTC.

After a long downtime the clock may be far off. chronyc makestep forces an immediate correction instead of slowly slewing, useful right after boot.

You learned to manage time synchronization.

• NTP keeps clocks aligned, which is critical for logs, TLS, and auth.
• Use timedatectl to check and enable sync; chrony is the modern daemon.
• chronyc sources and tracking show servers and accuracy.