ntpdate-like functionality in ntpd

[Denys Vlasenko]

On Wed, Feb 18, 2015 at 10:10 AM, Guillermo Rodriguez Garcia
<guille.rodriguez at gmail.com> wrote:
> 2015-02-18 9:45 GMT+01:00 Denys Vlasenko <vda.linux at googlemail.com>:
>> On Wed, Feb 18, 2015 at 8:38 AM, Guillermo Rodriguez Garcia
>> <guille.rodriguez at gmail.com> wrote:
>>> I ended up using rdate on this particular case but I think it would be
>>> nice if ntpd could be used as described.
>>>
>>> I don't have enough knowledge about the protocol to know what are the
>>> implications of not waiting for the burst mode to end for option -q
>>> (as per Miroslav's patch). Can anyone shed some light?
>>
>> Time will be set after 2 reply packets from one peer.
>> Which normally would take 2-3 seconds.
>>
>> If network is down or NTP server is not replying at all,
>> "ntpd -q" can still wait indefinitely. I guess the same is true
>> for ntpdate.
>
> In my tests, if the network is down, or the server is unreachable or
> does not reply, ntpdate returns almost immediately with an error
> message.
>
>>
>> If you plan for your machines to not hang at boot time
>> in such a case, you need to think (and test) booting
>> with network down.
>
> Can you provide any recommendations about this? What would be the best
> way to run ntpd to make sure it does not hang at boot if the network
> is down and/or the NTP server does not reply, but still sync the time
> as soon as the network is back up and/or the NTP server becomes
> available?

I do not try to set time super-duper-early. I don't see a particular reason
why that is important.

I start and run ntpd just the same as I run any other daemon.

I use this method:

http://git.busybox.net/busybox/tree/examples/var_service/README

http://git.busybox.net/busybox/tree/examples/var_service/ntpd/README