Linux ntp

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.

Update NTP managment system

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

hwclock --show  
hwclock -r
hwclock --show --utc
hwclock --systohc #sync system clock -> hardware clock
hwclock --hctosys #sync hardware clock -> system clock

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

Set time zone on Amazon Linux, to UTC

cat /etc/*rel*
NAME="Amazon Linux AMI"
VERSION="2014.09"

$ date
Fri  4 Sep 15:32:12 BST 2020

sudo vi /etc/sysconfig/clock # edit if needed
ZONE="UTC"
UTC=true

# List available time zones
ls -l /etc/localtime

# Symlink (cp) 'localtime' file with content of desired TZ
sudo mv /etc/localtime /etc/localtime.original
sudo ln -s /usr/share/zoneinfo/UTC /etc/localtime

date # no reboot required
Fri  4 Sep 14:50:26 UTC 2020

Force ntpd to sync time

Worked for Ubuntu 18.4

sudo systemctl stop ntp.service
sudo ntpd -gq 
# -g tells the ntp daemon to correct the time regardless of the offset 
# -q exit immediately 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    #capture ntp packets
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

chrony

Every pre-RHEL8 uses ntpd but recently chrony becomes a standard time client.

# RHEL8
sudo yum -y install chrony
sudo systemctl start chronyd && sudo systemctl enable chronyd
sudo vi /etc/chrony.conf # add NTP server, service restart is required to apply changes

References

Ntp Ubuntu wiki