busybox ntpd

busybox ntpd -w -d -p SERVER sends NTP requests to one server and prints each reply: the offset (how far this clock is from the server's, in seconds), the round-trip delay, the server's stratum and reference. It is an SNTP-style probe: one server, no voting. Without -w it would set the clock, which no lab device does: the containers share one kernel clock with the host.

busybox ntpd command usage

busybox ntpd -w -d [-q] -p SERVER

Options

-w

only query the servers, never set the clock

-d

print each exchange (on stderr)

-p SERVER

the server to ask; repeat for more

-q

quit after the first good answer

busybox ntpd examples

timeout 3 busybox ntpd -w -d -p timeServer.lab 2>&1 | grep reply

Asks timeServer for the time for three seconds and prints its answers.

tcpdump -nn -v -i eth0 udp port 123

Shows NTP requests and answers with their four timestamps.

Pitfalls

• It runs until stopped: wrap it in timeout.
• A negative offset means this clock is ahead of the server's.