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Re: [Openvpn-users] openvpn and time changes


  • Subject: Re: [Openvpn-users] openvpn and time changes
  • From: Joachim Banzhaf <joachimbanzhaf@xxxxxxxxxxxxx>
  • Date: Thu, 10 Nov 2005 08:08:04 +0100

Hi Jason,

Am Mittwoch, 9. November 2005 16:39 schrieb Jason Keltz:
> Hi Jason,
>
> In my case, ntp is running, but it doesn't help the situation.  Let's
> assume a machine is off when the time changes.  The machine comes back
> on, and the hardware clock is now, say, 2:00 AM when the real time is
> say, 1:00 AM.  Let's say OpenVPN starts at 2:00 AM in the boot sequence,
> but then ntpdate runs (in our ntpd start script) and sets the clock back
> to 1:00 AM.   ntp (at least with our configuration) wouldn't have
> touched a time that was out by an hour, and even if it would, I was
> wondering whether the slew would cause any difficulty for OpenVPN.  A
> slew backwards slowly or making the time jump back during boot would
> basically be the same thing, the key component being that the time would
> be going backwards.  If OpenVPN wasn't happy with time going back, it
> wouldn't matter whether ntpdate was doing it or ntpd...  It seems like

it is not the same thing!
ntpd is _very_ careful to change the time in so small amounts at a time, that 
no other process is able to notice (the reason why it takes sooo long until 
it reaches synced state when the difference is more than a few seconds). 
ntpdate however can cause arbitrary sized time jumps back and forth.
So my advice is: _dont disable security features_ but fix your time settings. 
I think, it may be enough, to start ntpd init script with ntpdate before 
openvpn or other processes that could be confused by time jumps. At least 
that is the way I do it.
If clients time jumps backward: their problem. They should fix that (in the 
same way as you should: jump in time _before_ it matters to someone)

regards,

Joachim Banzhaf

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