[ntp:questions] NTP to multiple networks via one interface.

detha detha at foad.co.za
Sat Dec 14 15:58:34 UTC 2013

On Thu, 12 Dec 2013 20:08:43 -0800, sc0tt.v3rn0n wrote:

> Hi,
> We have a xenserver host.  We manage it via an interface on x.x.75.x which
> is the only network that is actually plumbed in on the host.  It has other
> interfaces but they are virtualised by xen and don't have IP's.
> We have several VM's on this host on the same x.x.75.x network so we have
> set the xenserver to broadcast ntp (it syncs from network switches) and
> the VM's to be broadcast clients .  NTP is working ok in this scenario.
> However we've added a new network to xenserver on x.x.69.x but VM's on
> this network don't receive the NTP broadcast packets as they don't have an
> interface on the x.x.75 network.
> Is there any way to get the NTP packets to all VM's without dual homing
> either the xenserver or the VM's.
> Thanks in advance,
> Scott

Broadcast works in an L2 broadcast domain that the NTP server is in. If
ntpd currently only knows about the x.x.75.x network, that's all it
will broadcast on.

I'm assuming the x.x.69.x network has a virtual interface/bridge on the
Dom0 (otherwise, how do you get to the DomUs), so specifying multiple
broadcast networks (including x.x.69.x) in ntp.conf on the Dom0 should
make it also bind to that interface.

Alternatively you might want to look at a multicast setup.

But in both cases, the machine running ntpd should have an interface,
virtual or physical, in the relevant subnet.


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