Is my NTP daemon working?

Published on March 29, 2019 Fedora 3 minutes

If the time on your workstation or server is not stable, strange errors might appear, such as:

$ tar zxvf /tmp/archive.tgz
tar: my-file: time stamp 2019-03-28 14:04:45 is 0.042713488 s in the future

This can happen when your NTP daemon is not synchronized. This means it cannot reliably determine the current time.

First, make sure your NTP daemon is started:

$ sudo systemctl status ntpd
● ntpd.service - Network Time Service
   Loaded: loaded (/usr/lib/systemd/system/ntpd.service; enabled; vendor preset: disabled)
   Active: active (running) since Thu 2019-03-28 17:13:10 CET; 16h ago
  Process: 33844 ExecStart=/usr/sbin/ntpd -u ntp:ntp $OPTIONS (code=exited, status=0/SUCCESS)
 Main PID: 33845 (ntpd)
    Tasks: 1
   Memory: 924.0K
   CGroup: /system.slice/ntpd.service
           └─33845 /usr/sbin/ntpd -u ntp:ntp -g

For your NTP daemon to be started, the state must be active (running).

Then, you can query its status with the ntpdc command:

$ sudo ntpdc -c sysinfo
system peer:          www.almaprovence.fr
system peer mode:     client
leap indicator:       00
stratum:              3
precision:            -25
root distance:        0.00130 s
root dispersion:      0.02950 s
reference ID:         [212.83.145.32]
reference time:       e0485cce.58fb4baf  Fri, Mar 29 2019  9:58:54.347
system flags:         auth ntp kernel stats 
jitter:               0.000259 s
stability:            0.000 ppm
broadcastdelay:       0.000000 s
authdelay:            0.000000 s

In this output, you need to check the stratum.

The stratum tells you how far you are from the reference clock (the world atomic clock). The closer you are, the lower is the stratum and the more stable your clock will be.

Usually, your stratum will be between 3 and 5. By convention, 16 means “unsynchronized”.

You can query the list of peers your NTP daemon is synchronized to with the ntpq command:

$ sudo ntpq -c peers
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
-stardust.ploup. 195.13.23.5      3 u  782 1024  377    4.272   -1.984   2.479
+server.bertold. 193.190.230.66   2 u  679 1024  377    4.235   -0.152   0.629
*www.almaprovenc 145.238.203.14   2 u  854 1024  377    0.321   -0.225   0.502
+ns.rail.eu.org  138.96.64.10     2 u  563 1024  377    0.455    0.413   0.630

Again, check the st column (abbreviation for stratum) and make sure your peers have a correct stratum. As above, 16 means “unsynchronized”.

The offset and jitter column are also useful:

the offset tells you how far away in time your peers are.
the jitter tells you how stable your peers are.

Last but not least, you can troubleshoot network issues with the ntpdate command.

For the test to be meaningful, you need to shut down temporarily the NTP daemon:

sudo systemctl stop ntpd

Then, run the ntpdate command:

$ sudo ntpdate -q 0.rhel.pool.ntp.org
server 91.121.88.161, stratum 2, offset -0.000393, delay 0.02974
server 129.250.35.251, stratum 2, offset 0.004071, delay 0.02733
server 5.196.192.58, stratum 2, offset -0.005378, delay 0.03003
server 51.15.182.163, stratum 2, offset 0.000207, delay 0.02658
29 Mar 10:45:09 ntpdate[121271]: adjust time server 51.15.182.163 offset 0.000207 sec

It is important to shut down the NTP daemon and run the ntpdate command with sudo in order for ntpdate to successfully bind to the NTP port, 123.

If the ntpdate command fails, it is a strong indication that there is a network issue between your host and the NTP peers.

After this step, do not forget to restart your NTP daemon:

sudo systemctl start ntpd