A host that runs no time daemon has come up several minutes fast. Before starting the applications you want one command that contacts an NTP server, sets the system clock from the reply and then exits. Which command is the traditional tool for that?
LPIC-1 Exam 102-500, objective 108. Essential system services medium
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The options
Correct ntpdate pool.ntp.org
Correct. ntpdate queries the servers named on its command line, adjusts the system clock once and terminates. It is the classic boot-time or cron one-shot from the reference NTP distribution.
Not correct ntpq -p pool.ntp.org
Wrong. ntpq is the query program for a running ntpd; -p prints that daemon's peer list. It reports state and never sets the clock.
Not correct chronyc sources
Wrong. chronyc talks to a running chronyd over its control socket, and `sources` only lists the sources that daemon knows about. With no daemon running it has nothing to talk to.
Not correct hwclock --systohc
Wrong. That copies the current, still incorrect, system time into the hardware clock. No network server is involved.
Why
ntpdate steps the clock in one jump rather than slewing it, which is acceptable at boot and dangerous once services are running and relying on monotonic timestamps. It is deprecated in the reference implementation, whose replacements are `ntpd -gq` and `sntp`; on a chrony host the equivalent is `chronyd -q` or `chronyc makestep`. Note that ntpdate cannot run while a daemon already holds UDP port 123.
Where this comes from
- Cited
- manual page ntpdate(8)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 108 and space the ones you get wrong.
More questions on this objective
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- In /etc/ntp.conf you find the line `server 0.pool.ntp.org iburst`. What does the `iburst` keyword do? machine-checked
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- On a systemd host, type the single command that turns on automatic network time synchronisation (do not include a path). machine-checked