[ntp:questions] serving multicast ntp in multihomed server

Diego Torres diego.torres+ntp at gmail.com
Wed Oct 22 20:36:24 UTC 2008

hi there!

After reading this list archives, and after playing around with the
broadcast option in ntp.conf to enable multicast in a ntp server,I'm
unable to do a couple of things that should work without problems.

I'm using servers that come with three network cards. Our network
topology uses two of them to send data, and the third one is used for
server management.

The problem is that it doesn't matter what I do, multicast group is
always sended through the last configured nic (eth2).

My boxes run linux, and multicast routes are configured:

ifconfig eth0
ifconfig eth1
ifconfig eth2
route add -net mask dev eth0
route add -net mask dev eth1
route add default gw


Running tcpdump shows that multicat packets output through eth2... I'm
running no mcast daemon. Since ntpd asks the kernel the multicast
route, something is going wrong since it is outputing data to the
wrong interface, with the wrong source address.

If ntpd doesn't rely on the routing tables to output its data, it
should let the user specify the source ip of the outgoing multicast
packet, so it will leave the box using the correct interface. I was
thinking about the possibility of modifying ntpd, to add a "source ip"
and a "destination interface"  to the broadcast directive.

Maybe a better approach is to use a multicast routing daemon? There
are not plenty of them... I've only found one, smcroute, which insert
static multicast routes in kernel. mrouted no longer exists, zebra is

Could someone bring some light to this, thanks in advance!

-- Use of a keyboard or mouse may be linked to serious injuries or disorders.
diego dot torres at gmail dot com - Madrid / Spain

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