[ntp:questions] Multicast and dynamic interfaces

barry bouwsma free_beer_for_all at yahoo.com
Fri Jul 11 13:43:25 UTC 2008

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

barry bouwsma


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