Basic NTP Client Test: ntpdate & sntp

During my work with a couple of NTP servers, I had many situations in which I just wanted to know whether an NTP server is up and running or not. For this purpose, I used two small Linux tools that fulfil almost the same: single CLI command while not actually updating any clock but only displaying the result. That is: ntpdate & sntp. Of course, the usage of IPv6 is mandatory as well as the possibility to test NTP authentication.

This article is one of many blogposts within this NTP series. Please have a look!

Refer to my “Packet Capture: Network Time Protocol (NTP)” blog post in order to download a pcap with different NTP packets, or at least heaving a view of them at the screenshots.


You can use ntpdate with the “-q” switch to “Query only – don’t set the clock”. This is a very basic run:

When using an FQDN with a couple of A/AAAA records, ntpdate queries all of them:

Furthermore, you can query several names at once:

Debugging & NTP Authentication

Even more relevant for my lab tests is the ability to test NTP authentication. Therefore I am using the debugging mode “-d” which will print out all steps (while still not updating the local clock) in conjunction with -a <key-id> and -k <keyfile>. For this example I used two SHA1 keys from one of my NTP servers (

While key number 11 is indeed correct (receive: authentication passed):

I falsified key number 12 on purpose to test the output (receive: authentication failed):

Mission accomplished.


Just as an alternative you can use sntp as well. Sntp by default writes “the estimated correct local date and time (i.e. not UTC) to the standard output”:

The “kod_init_kod_db” warning is normal and can be ignored.

Debugging & NTP Authentication

Fortunately, the options are quite the same when it comes to debugging and NTP authentication. That is: -d -a <key-id> -k <keyfile>. Having the same two keys (11 ok, 12 not ok) in place, gives the following output for “authenticated using key id 11“:

as well as for “Crypto NAK“:

Please note that to my mind there was a bug at least in sntp version 4.2.8p10, since I got this “Segmentation fault” output from another machine (while the above-listed outputs were from version 4.2.8p12):

That’s it. ;) Happy troubleshooting.

Featured image “Reading glasses over a book” by Marco Verch is licensed under CC BY 2.0.

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