[ntp:questions] NTP Limited Broadcast

Danny Mayer mayer at ntp.isc.org
Thu Mar 15 02:06:01 UTC 2007

Kevin Casey wrote:
> I must say I am humbled by the response to my question. My knowledge of
> IP networking is rudimentary by comparison. I would like to clarify that
> I did not expect to be crossing a gateway boundary using the limited
> broadcast. According to the vendor, because all communication with the
> device is achieved via the broadcast address, its IP address is
> irrelevant and therefore not documented. I only know that the device is
> designed to listen for UDP 123 broadcasts on Nor does
> the vendor require that I use or any particular subnet
> or host address. I just need to make my NTP server broadcast UDP 123 to
> on a connected interface where that interface shares a
> broadcast domain with his device. I suppose this would be similar to the
> behavior of a DHCP client broadcasting DHCPDISCOVER messages on its
> local physical network. However the ntp.conf(5) man page states "Note
> that local broadcast messages go only to the interface associated with
> the subnet specified, but multicast messages go to all interfaces."
> Evidently is not associated with a "broadcast
>" command in ntp.conf.  So I am wondering if there is any
> network interface configuration possible in FreeBSD 6.2 that will permit
> NTPD to broadcast UDP 123 to on that interface. 

There's no problem broadcasting *TO* that address. The problem lies in
*RECEIVING* packets sent to that address. I'm researching ways to
resolve this issue but it may take some time. You could change the code
(specifically ntpd/ntp_io.c) to accept packets on the wildcard sockets.


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