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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteFor a Linux server that only needs accurate time, install a client daemon and point it at the NTP Pool. With chrony, the essential configuration is pool pool.ntp.org iburst, normally accompanied by a drift file, an early correction rule, and hardware-clock synchronization. Restart the daemon, then verify it with chronyc or ntpq. This configures your server as an NTP client; serving time to other machines is a separate role with additional access controls.
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What you are configuring
Network Time Protocol (NTP) keeps the Linux system clock aligned with reference clocks reached over the network. A normal application server needs an NTP client: it periodically asks remote time servers for measurements and disciplines its own clock. Installing a daemon does not automatically make the host a public time server.
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The NTP Pool Project is a distributed collection of public NTP servers. Its DNS names resolve to changing addresses rather than one permanent machine. The pool names 0.pool.ntp.org through 3.pool.ntp.org point to a random set that changes every hour, according to NTP Pool Project documentation. A client therefore has to handle changing associations instead of treating one IP address as permanent.
This guide covers the two daemon families commonly found on Linux:
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- chrony, the usual choice on many current distributions and the recommended path for a new installation.
- ntpd, the traditional implementation, including its current
pooldirective and the numbered-server syntax retained for older versions.
Choose one daemon for the host. Running competing time daemons at the same time can produce conflicts and makes diagnosis harder.
Before editing a configuration
Confirm the host can reach the pool
- DNS resolution must work, because the pool name is resolved dynamically.
- Outbound UDP port 123 must be permitted by the host firewall and any upstream firewall.
- The machine should have a roughly correct date and a functioning network connection. A very large initial error can require a step correction, which chrony can be configured to allow.
Do not open inbound UDP 123 merely to synchronize this host. Inbound access is needed only when you intentionally configure the machine to serve time to other clients.
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Package names, configuration paths, and service-unit names vary. Check which implementation is already installed before adding another:
command -v chronyc
command -v ntpq
systemctl list-unit-files | grep -E 'chrony|chronyd|ntp'If another time-synchronization service is active, stop or disable it according to your distribution’s documentation before enabling the daemon you selected.
Configure chrony with the NTP Pool
1. Install and enable chrony
Install the distribution’s chrony package using its normal package manager. On Debian- or Ubuntu-based systems, the package is commonly named chrony; on RPM-based systems it is also commonly chrony, although the service unit may be called chronyd. Use your distribution’s supported command and do not assume the unit name from the package name.
# Debian or Ubuntu example
sudo apt update
sudo apt install chrony
# Fedora, RHEL-compatible, or similar example
sudo dnf install chronyEnable the service so it starts at boot. Depending on the distribution, the unit is chrony or chronyd:
sudo systemctl enable --now chrony
# or
sudo systemctl enable --now chronydIf the command reports that the unit does not exist, list the available units and use the name supplied by your package.
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2. Back up and edit the configuration
The usual file is /etc/chrony/chrony.conf; some distributions use /etc/chrony.conf. Make a dated backup before changing it:
sudo cp -a /etc/chrony/chrony.conf /etc/chrony/chrony.conf.backup
sudo editor /etc/chrony/chrony.confWhen the file contains vendor-supplied public server or pool lines, remove or comment out conflicting public sources if your intention is to use the NTP Pool. Keep local policy directives that your distribution requires, such as logging or an existing trusted internal source.
Add the following client configuration, adjusting only paths required by your distribution:
pool pool.ntp.org iburst
driftfile /var/lib/chrony/drift
makestep 1 3
rtcsync| Directive | Purpose | Operational implication |
|---|---|---|
pool pool.ntp.org iburst |
Uses one DNS name that represents multiple changing NTP servers. | Chrony can create and refresh multiple associations as DNS results change. The iburst option sends a burst of requests at startup to speed initial synchronization. |
driftfile /var/lib/chrony/drift |
Persists chrony’s estimate of the system clock’s rate error. | The estimate survives restarts, reducing the work needed to settle after a reboot. The directory and path must be writable by the chrony service account and may differ by distribution. |
makestep 1 3 |
Allows a large correction during the first three clock updates when the measured error exceeds one second. | A step changes the clock immediately. Restricting it to startup updates avoids unexpected jumps during normal operation. |
rtcsync |
Copies system time to the hardware real-time clock periodically on supported Linux systems. | The hardware clock is closer to the correct time after shutdown or reboot. Support depends on the operating system and hardware. |
Chrony’s manual specifically recommends the pool directive when one name represents several servers that can change over time. Its FAQ also advises using at least three servers for reliable operation; a functioning pool name normally supplies multiple candidates, but you should confirm the number of usable sources in the status output.
3. Select an appropriate pool zone
pool.ntp.org is the general pool name. The project also publishes regional, continental, and country zones. A country zone may be unavailable or have too few participating servers. Use a country-specific name only when the NTP Pool zone page shows that it is available and adequately populated; otherwise choose an available continental zone or the global pool name. Do not invent a regional hostname.
Keep the zone change in the same pool directive, for example:
pool <zone-name-listed-by-ntp-pool> iburstReplace the angle-bracketed text with the exact zone hostname published by the project. The pool’s DNS answers are dynamic, so a successful lookup today does not identify a permanent set of servers.
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4. Restart chrony after the edit
Chrony reads its configuration at startup. Restart the service after saving the file:
sudo systemctl restart chrony
# or, on systems using the chronyd unit
sudo systemctl restart chronydCheck for syntax or startup failures immediately:
sudo systemctl status chrony --no-pager
# or
sudo systemctl status chronyd --no-pager
sudo journalctl -u chrony -b --no-pager
# substitute chronyd when that is the unit nameIf the service fails, the journal normally identifies an invalid directive, an unreadable drift-file path, a permissions problem, or a name-resolution failure. Restore the backup only after recording the error; otherwise you may hide the configuration mistake.
5. Verify synchronization with chronyc
Use chrony’s query tool rather than judging synchronization by the displayed wall clock alone:
chronyc tracking
chronyc sources -vchronyc tracking reports the state of the clock discipline, including the reference identifier, stratum, measured offset, frequency estimate, and whether the clock is currently synchronized. chronyc sources -v lists candidate sources and marks the source selected for synchronization. Look for a selected source and an offset that is small and trending toward stability. The actual convergence period depends on the initial error, network conditions, source quality, and whether a step was permitted; do not assume a fixed number of seconds.
Run the commands again after a short interval if the first result shows no selected source. A source can be temporarily unreachable while DNS results change or while chrony collects enough measurements.
Configure classic ntpd with the NTP Pool
Current ntpd: prefer the pool directive
In NTP 4.2.7 and later, the improved pool implementation is the preferred way to describe a dynamic pool. The ntp.conf manual defines it as creating persistent client-mode associations to multiple remote servers. A minimal current configuration is:
pool pool.ntp.org iburstRetain or add a drift file so ntpd can preserve its estimate of the local clock rate:
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driftfile /var/lib/ntp/ntp.drift
pool pool.ntp.org iburstThe exact drift-file directory can differ between packages. Keep the path supplied by your distribution if its default configuration uses another location.
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Older ntpd: numbered server lines
The NTP Pool Project publishes this compatibility form for implementations that do not support the improved pool behavior:
driftfile /var/lib/ntp/ntp.drift
server 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org
server 3.pool.ntp.orgThe four names resolve to random servers and the set changes every hour. This syntax remains useful for older ntpd releases, but do not use it merely out of habit on a current 4.2.7-or-newer implementation when the pool directive is available.
Install, edit, and restart ntpd
Install the ntpd package supplied by your distribution if it is not already present. The configuration is commonly /etc/ntp.conf, although a package may use an equivalent path. Back up the file, remove or comment out unrelated public sources, and add either the current pool form or the compatibility form appropriate to your version.
sudo cp -a /etc/ntp.conf /etc/ntp.conf.backup
sudo editor /etc/ntp.conf
sudo systemctl restart ntp
# some distributions use ntpd as the unit name
sudo systemctl status ntp --no-pagerUse systemctl list-unit-files | grep ntp if the service name is uncertain. Check the journal for parse errors or blocked network access before moving to query commands.
Verify ntpd with ntpq
Run:
ntpq -pnThe peer table shows the numeric addresses, stratum, reachability, delay, offset, and jitter for each association. A leading selection character identifies the peer ntpd currently prefers; a reachability value that remains zero indicates that replies are not arriving. An empty or all-unreachable table points to DNS, firewall, routing, a stopped daemon, or an invalid configuration. Repeat the query rather than promising a universal convergence time: initial synchronization varies with the machine’s starting clock and network path.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Chrony and ntpd compared
| Decision point | chrony | ntpd |
|---|---|---|
| Typical availability | Common default or supported package on current Linux distributions. | Traditional implementation; availability and default status depend on the distribution. |
| Dynamic pool syntax | pool name iburst; the manual recommends it for names that resolve to changing groups. |
Use pool name iburst on NTP 4.2.7 and later; older implementations use numbered server lines. |
| Common configuration file | /etc/chrony/chrony.conf or /etc/chrony.conf. |
/etc/ntp.conf or the distribution’s equivalent. |
| Restart unit | chrony or chronyd, depending on packaging. |
ntp or ntpd, depending on packaging. |
| Verification tool | chronyc tracking and chronyc sources -v. |
ntpq -pn. |
| Client versus server role | These examples are client-only. Serving other hosts requires explicit server and access policy. | These examples are client-only. Server-mode examples must include restrictions before exposing UDP 123. |
Choosing between global and regional pool names
Use the global name when availability is uncertain
The unqualified pool.ntp.org name is the safest starting point when you have not confirmed a local zone. It lets the project return participating servers without assuming that a particular country has enough capacity.
Use a regional or continental zone when the project lists it
A closer zone can reduce network distance, but proximity is not a guarantee of availability or better accuracy. Check the NTP Pool zone page first. If a country zone is absent or sparsely populated, select an available continental zone instead. Keep at least three usable sources, as advised in the Chrony FAQ, and verify the result with the daemon’s query command.
Expect DNS answers to change
Do not hard-code the IP addresses returned by a pool lookup. The project intentionally rotates the set; the numbered names are documented as changing every hour. A static firewall allow-list or permanent server list can therefore become stale. Permit DNS resolution and outbound NTP traffic to the pool’s changing destinations.
Client-only configuration versus serving time
A client configuration sends requests to public sources and disciplines the local clock. It does not require your host to accept NTP requests from the Internet. Avoid copying examples that contain broad server-mode permissions into a client-only setup.
If other machines on a private network must use this host as their time source, treat that as a separate design:
- Choose the internal networks that are allowed to query the daemon.
- Add restrictive access rules appropriate to chrony or ntpd rather than allowing every address.
- Open inbound UDP 123 only on the interfaces and firewall zones that need it.
- Continue monitoring the host’s own upstream synchronization; an unsynchronized relay can spread bad time to every client.
The NTP Pool compatibility guidance warns that access restrictions are required for a public-facing daemon. A server intended only as a client should not be made Internet-facing by adding those server-mode examples.
Troubleshooting by symptom
The service will not start after editing
- Check the unit journal with
journalctl -u chrony -b,journalctl -u chronyd -b, or the corresponding ntpd unit. - Look for a misspelled directive, an invalid pool name, or a duplicate/conflicting setting.
- Confirm that the drift-file directory exists and is writable by the daemon.
- Restore the backed-up file temporarily if you need to return the host to a known-good state, then reapply one change at a time.
No sources appear in the status command
- Test DNS resolution for the configured pool name using the resolver tools installed on the host.
- Check routing and outbound UDP 123 rules. A TCP-only firewall rule does not permit NTP.
- Confirm the daemon is running under the expected service name and that another time daemon is not competing for the clock.
- For ntpd, run
ntpq -pnrepeatedly and inspect reachability. For chrony, runchronyc sources -vand look for unreachable or unresolved entries.
The clock is far from correct
Chrony’s makestep 1 3 allows a correction greater than one second during its first three updates. That can produce an intentional jump, which may affect applications that assume time never moves backward or forward abruptly. Schedule the initial synchronization before starting workloads that depend on monotonic wall-clock timestamps, and use a monotonic clock in application code where elapsed-time measurement matters.
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Do not assume that one source provides the redundancy expected from a pool. Check the selected zone’s population, DNS results, firewall policy, and the daemon output. Switch to the global pool or an available continental zone when the project indicates that a country zone has too few servers.
Synchronization stops after a network change
Because pool addresses are dynamic, a network change can invalidate cached routes or firewall assumptions. Recheck DNS resolution, outbound UDP 123, and the daemon’s source list. Restarting the daemon is appropriate after a configuration change; it is not a substitute for fixing blocked traffic.
Operational checks after deployment
- Confirm exactly one time daemon is enabled and active.
- Verify that the configuration contains the intended pool directive and no accidental duplicate public sources.
- Restart the daemon after every configuration edit.
- Run
chronyc trackingpluschronyc sources -v, or runntpq -pnfor ntpd. - Record the actual selected source, offset, stratum, and reachability rather than treating a successful service status as proof of synchronization.
- Check again after a reboot to confirm that the drift file is retained and the hardware-clock synchronization policy behaves as expected.
- Review logs when DNS, firewall, routing, or provider changes occur; the pool’s returned addresses are expected to change.
With those checks complete, the Linux host is configured as an NTP Pool client and has a repeatable way to prove that synchronization is working.
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