🕐 NTP & Time Synchronisation

Client verification (ntpd / chrony)

1ntpstat    # see which NTP server we synchronise with, and whether the sync is good.

1synchronised to NTP server (192.168.1.12) at stratum 4
2   time correct to within 68 ms
3   polling server every 1024 s

1ntpq -p     # see the state of the peers.
2ntpq -c peers

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+192.168.1.11     192.168.2.4    4 u  259 1024  373    0.731   -0.980   0.557
*192.168.1.12     192.168.3.21   3 u  385 1024  377    0.773    0.146   0.365
 192.168.4.255    .BCST.         16 u    -   64    0    0.000    0.000   0.000

• The server preceded by an asterisk (*) is the one being used.
• Those preceded by a - are currently discarded by the server-selection algorithm.
• Those whose name is preceded by a + are possible synchronisation candidates.
• A server preceded by a space is either unreachable or too distant.

Column meaning

• remote — the server name.
• refid — the parent server’s identifier.
• st — the server’s stratum.
• t — the server type.
• when — seconds elapsed since the last contact.
• poll — seconds between each contact.
• reach — bitmask of successful contacts (octal): the server considers itself synchronised when reach reaches 177; a quality, stable connection shows 377.
• delay — estimated round-trip time (ms) of the UDP packet.
• offset — estimated difference between the peer’s clock and the internal clock.
• jitter — dispersion of the reference values obtained from this peer.

Restart the NTP daemon

1service ntpd restart      # or: systemctl restart ntpd

Configuration & logs

1cat /etc/ntp.conf    # "server example.com" + restart ntpd + enable
2/var/log/ntpstats

ntpdate (legacy)

Old service that synchronises NTP at boot (install the package first).

1ntpdate 192.168.1.12          # + enable the ntpdate service
2ntpdate -ubd 192.168.1.12     # -b sync immediately, -u unprivileged port, -d debug mode to see all the steps
3/etc/ntp/step-tickers

NTP server side

1ntpdc -c monlist    # normally used on an NTP server to list its clients.

Know the stratum level of an NTP server

1ntptrace
2ntp-server: stratum 3, offset 0.000791, synch distance 0.069005
3192.168.5.33: stratum 2, offset -0.000125, synch distance 0.016285

timedatectl / systemd-timesyncd (RHEL 7+, Ubuntu 15+)

1timedatectl                       # = ntpdate + ntp combined.
2timedatectl status
3timedatectl set-ntp true
4
5timedatectl list-timezones | grep Paris
6sudo timedatectl set-timezone Europe/Paris
7
8/etc/systemd/timesyncd.conf       # timesyncd = ntpd

chrony

1chronyc sources -v

[root@server ~]# chronyc tracking
Reference ID    : 00000000 ()
Stratum         : 0
Ref time (UTC)  : Thu Jan 01 00:00:00 1970
System time     : 0.000000000 seconds fast of NTP time
Last offset     : +0.000000000 seconds
...
Leap status     : Not synchronised

1chronyc sourcestats
2210 Number of sources = 2
3Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
4==============================================================================
5ntp1.example.com            0   0     0     +0.000   2000.000     +0ns  4000ms
6ntp2.example.com            0   0     0     +0.000   2000.000     +0ns  4000ms

1systemctl restart chronyd
2watch chronyc tracking