[ntp:questions] Multicast question

Danny Mayer mayer at ntp.isc.org
Sun Mar 2 01:50:05 UTC 2008

Frank Kardel wrote:
> Danny Mayer wrote:
>> John Vossler wrote:
>>> Greetings,
>>> I have a new system running Solaris 10 set up as an NTP server.  IT 
>>> is synchronizing correctly but I cannot get it to multicast on any 
>>> interface except the systems primary Ethernet interface, bge0.  I 
>>> need it to multicast on interfaces bge1-bge3 and ce0 - ce7.
>>> Client systems reside on the network segments on these other interfaces.
>>> Anyone have any suggestions on getting the server to multicast on 
>>> these other interfaces?
>>> John
>> Are you talking about outgoing packets to a multicast address? I think 
>> that there may be some bugs in that area since the addresses are 
>> limited and choosing an outgoing address becomes an issue and it will 
>> only send it out on one address. I'm not sure if it's possible to set 
>> up a server (at least not easily) to send via different interfaces.
> It is. But on a multihomed server the server must have multicast routing
> functionality. Either this is already provided in the kernel or you
> need to run an mcast-routing daemon like mrouted, zebra or others.
> The ntpd setup is not different on multihomed hosts. Just the multicast
> routing must be working.

Even if that's true, we don't know apriori whether or not that is the 
*intent* of the sysop. Most times you are likely to have a server with 
two NICs but one is used as an outbound interface to the external 
servers and the other NIC is used to multicast packets to the LAN. NTPD 
has no way of knowing that. I assume that something like mrouted can be 
configured in some way to know how to send those packets. Note that such 
a configuration is likely to introduce additional delays and jitter to 
the outgoing multicast packet.

>> To do this correctly you need to be able to set up a different 
>> multicast address to use and associate it with a particular server 
>> address for each interface.
> NO - see above.

Yes, it turns out you can do this from within ntpd but you have to 
program this.

>> The only way that I can think of to accomplish this is to be able to 
>> specify a specific binding address in the config file. We would have 
>> to add that as an optional parameter to the broadcast directive.
> not needed.
>> Danny
> Frank

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