[ntp:questions] Multicast question

Frank Kardel kardel at ntp.org
Sat Mar 1 10:28:59 UTC 2008

Martin Burnicki wrote:
> Danny Mayer wrote:
>> Martin Burnicki wrote:
>>> Danny Mayer wrote:
>>>> John Vossler wrote:
>>> [...]
>>>>> My initial reaction to this issue was to determine if there was an OS
>>>>> (Solaris) directive to include the multicast for other interfaces in
>>>>> the
>>>>> group as reported by "netstat -g".  Or a method to specify the
>>>>> multicast interfaces in the ntp.conf file (apparently not, or not
>>>>> easily).
>>>> No, and certainly it has not been implemented in NTP.
>>> If ntpd has been configured to send multicasts, shouldn't it by default
>>> send multicasts on every interface it actually uses?
>>> I had expected ntpd would already do so.
>> How could you do that? You have one packet that you are sending out. It
>> can only go out on exactly one interface. You can only choose which
>> interface to use, you cannot select more than one.
> I'd expect that either the kernel routed multicast packets to all interfaces
> (isn't that what routers do with multicasts, contrarily to broadcasts?),
Correct in both accounts. The kernel will do the replication if (!) 
instructed to do so. Usually these instructions come from the multicast 
routing software. That's why routers will do that and that's why your
multihomed system needs to become a multicast router.

> the application would send an individual packet on each interface.
> Anyway, if I have a server with several interfaces I'd expect those
> multicasts to appear on every single subnet connected to one of those
> interfaces.
That expectation is right, but the packets usually leave the interfaces
when the mcast routings software can determine that there is interest
for these packets. Basic interest is documented by client systems via 
IGMP messages, but there there more that one multicast routing protocols
and that's why the routing policy is usually implemented outside the kernel
and the kernel only implements the forwarding mechanism.
> Martin


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