[ntp:questions] multicast - wrong interface

Frank Kardel kardel at ntp.org
Sun Feb 18 21:34:13 UTC 2007

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.


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