[ntp:questions] multicast - wrong interface

Danny Mayer mayer at ntp.isc.org
Mon Feb 19 14:20:10 UTC 2007

Frank Kardel wrote:
> mayer at ntp.isc.org (Danny Mayer) writes:
>> Martin wrote:
>>> Hello !
>>> My NTP server sends all multicast traffic from eth0 and I don't
>>> know how to configure it to send it to the LAN connected via eth2.
>>> The details are below. Could you please give me some
>>> hints or help ?
>> Right now you can't do it. I am aware of this issue. A related issue is
>> in Bug #725 though that's for IPv6. Part of the question is whether or
>> not there should be additional arguments to specify which interface to
>> broadcast on or whether to broadcast on all interfaces which is much
>> harder to do as there would need to be a lot of changes.
> I looked into that issue today on NetBSD. As it turns out the IPv4 mcast
> implementation will copy the multicast pakets to any interface where
> the respective multicast groups have been joined. For that to happen
> on a multihomed host (one with several multicast capable interfaces) you
> need to run mrouted or some other multicast routing daemon.
> It is relatively easy to modify ntpd to find out the preferred/default
> multicast interface, but it should not be neccessary when a
> multicast routing daemon installs the appropriate Multicast Forwarding
> Cache entries in the kernel.

Yes, this is an unfinished piece of work. Figuring out which interface
to use is rather difficult, particularly if there is more than one to
choose from. Even worse, every O/S usually has sightly different and
incompatible requirements which makes getting it right particularly hard.


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