Linux ntp

Declaimer

Most these commands have been used on Ubuntu systems.

Time manipulation and troubleshooting

timedatectl - In recent Ubuntu releases timedatectl replaces ntpdate. By default timedatectl syncs the time once on boot and later on uses socket activation to recheck once network connections become active.

timesyncd - In recent Ubuntu releases timesyncd replaces the client portion of ntpd. By default timesyncd regularly checks and keeps the time in sync. It also stores time updates locally, so that after reboots monotonically advances if applicable.

New NTP packages

sudo update-rc.d -f ntpdate remove     -remove deprecated ntpdate service
sudo apt-get install ntp

Show ntp logs

cat /var/log/messages | grep ntp

Show RTC (Real Time Clock) hardware clock

sudo hwclock --show  
hwclock -r
hwclock --show --utc

Sync system clock -> hardware clock

sudo hwclock --systohc

Sync hardware clock -> system clock

hwclock --hctosys

Display time details (Ubuntu 16.04)

$ timedatectl
      Local time: Tue 2017-08-08 09:31:14 UTC
  Universal time: Tue 2017-08-08 09:31:14 UTC
        RTC time: Tue 2017-08-08 09:31:14
       Time zone: Etc/UTC (UTC, +0000)
 Network time on: yes
NTP synchronized: no
 RTC in local TZ: no

Set timezone (Ubuntu 16.04)

sudo timedatectl set-timezone Europe/London

Force ntpd to sync time

sudo systemctl stop ntp.service
sudo ntpd -gq #-gq tells the ntp daemon to correct the time regardless of the offset (g) and exit immediately (q) after setting the time; it can take 5-10 sec
sudo systemctl start ntp.service   #you should notice in timedatectl output NTP synchronized: yes

Troubleshooting NTP server

sudo tcpdump port 123    #enable ntp packet logging
timedatectl status    
ntpdc -p     #offset in seconds
ntpq -p      #offset in milliseconds

ntpq --numeric --peers   #show peers (time providers)
ntpq -c lpeer            #show peers (time providers)
ntpq -d <time_server>       #debugging mode (not sure if will update local clock)
ntpdate -d <time_server>    #debugging mode, in which ntpdate will go through all the steps, but doesn't adjust the local clock
ntpstats
ntpdate -s a_stratum_1_server_address

ntpdc -c loopinfo   #will display the combined offset in seconds, as seen at the last poll
ntpdc -c kerninfo   #will display the current remaining correction, just as ntptime does

Show difference/offset between local and remote time server

ntpdate -q <time_server>    #just query NTP server and it will display time difference/offset

References

Ntp Ubuntu wiki