[ntp:questions] Multicast and dynamic interfaces

Danny Mayer mayer at ntp.isc.org
Sun Jul 13 16:21:11 UTC 2008

barry bouwsma wrote:
> Sorry for line-mangling...
> --- On Fri, 7/11/08, Danny Mayer <mayer at ntp.isc.org> wrote:
>>> Should ntpd be able to listen for multicasts on
>> interfaces that
>>> get added/configured after ntpd starts?
>>> For example, I just brought up and configured this
>> interface:
>>> Jul  9 21:12:46 localhost ntpd[3422]: Listening on
>> interface #8 eth3, fe80::200:e8ff:fe00:fe0#123 Enabled
>>> Jul  9 21:12:46 localhost ntpd[3422]: Listening on
>> interface #9 eth3, Enabled
>>> Is this a case of missing or non-functioning code, or
>> is this
>>> something that really isn't possible?
>> It sounds like a bug, possibly 2 bugs. Please file a bug
>> report. The 
>> multicast option should reconfigure itself after a change
>> of IP address 
>> and rebind itself to the new address. If the address has
>> gone away then 
>> the multicast socket won't be able to receive any
>> multicast packets. Are 
>> you using DHCP provided addresses or static ones?
> This is a case of connecting after boot a USB ethernet adapter
> and manually assigning an IP address based on to which net I'm
> attaching myself and with which machines I wish to speak (though
> that shouldn't matter for multicast).
> I've also seen the same when I've manually reconfigured a WLAN
> interface to associate with a different network, over which I
> broadcast the name quoted multicast that I'm trying to hear via
> the USB adapter above.
> The IPv6 multicast seems to require no user intervention, either
> thanks to, in my case, the Linux kernel, or something else, while
> I have to set the route to 224.x.x.x in order for IPv4 multicast
> to be received.  In the example I posted, I had failed to set
> that route before restarting ntpd, while the IPv6 m'cast was heard
> loud and clear, and the needed handshake took place.
> I start ntpd at boot; network configuration takes place later (WLAN
> depending on to which network I wish to associate, not all of which
> have NTP m'casts) by hand, though I've a template at boot that sets
> most ifconfig needed for IP address and such (interface doesn't go
> RUNNING until I actually associate with a particular WLAN).
> Hmm, maybe I need to test if the WLAN interface, which has the IPv6
> ff00::/8 route on it, can pick up m'cast after switching WLANs...
> Nope, didn't help

It's almost certainly a bug. It should always be able to associate the 
multicast address with the correct local address and it needs to be 
reassociated if the local address changes or is not there in the first 

> thanks,
> barry bouwsma

More information about the questions mailing list