[avahi] Unicast Query Requests

Lennart Poettering lennart at poettering.net
Sun Aug 6 05:37:14 PDT 2006

On Wed, 02.08.06 18:07, Alastair Tse (liquidx at gentoo.org) wrote:

> Hi All,
> I was wondering if the developers were planning to implement the option
> in the multicast-dns RFC section 6.5 to allow avahi-daemon to request
> unicast responses rather than the default multicast responses.

No, right now noone of us was planning to implement that.

> The reason I ask this is because on my home network, I have a couple of
> Macs and Linux machines that are all wireless through a Linksys WRT54G
> wireless router. The problem is that multicast is broken on this router
> over the wireless interface.[1] This means, my Linux machine can try to
> make requests to the local network for DAAP shares through avahi, and
> the Mac replies accordingly but the response never reaches the Linux
> machine beacuse (a) multicast packets are not routed properly, and (b)
> promiscious mode does not work on the linux ipw2100 drivers.
> I notice that iTunes on Mac OS X will request multicast dns replies with
> the unicast response flag set for it's query, so on my network there is
> a weird asymmetry where my DAAP server is resolvable by the Mac, but my
> Mac's iTunes DAAP server is not resolvable by the Linux machine.
> I've had a quick run around the code and documentation and can see
> possibly where the changes may be made. I'm just really asking whether
> the developers were considering adding this feature, and if they were
> willing to accept a patch that implements this feature, and if not, why
> not. Or of course, maybe I'm missing something bleedingly obvious in my
> home network setup that is causing this problems, and I'm just jumping
> into the deep end to solve a simple problem.

I am certainly willing to merge such a patch if it is clean. However,
I think that the first priority should be to fix the ipw2100 drivers
for good, instead of adding such a kludge to Avahi. 

> [1] Sidenote: Multicast packets only get forwarded on the wired
> interface even if it originates from the wireless interface by their
> poorly implemented igmprt program. For some reason they've chosen a
> simple incomplete implementation rather than mrouted.

Hmm. mDNS uses a link-local mcast group. It is never routed by
definition. Most WLAN routers bridge the WLAN and the copper LAN and
don't do IP level routing, so there shouldn't be any need for special
multicast routing anyway.

Network drivers with broken multicasting usually have problems only
with recieving multicast packages, not with sending them. On some
drivers you can work around this limitation by setting the ALLMULTI or
PROMISC interface flags using "ifconfig".


Lennart Poettering; lennart [at] poettering [dot] net
ICQ# 11060553; GPG 0x1A015CC4; http://0pointer.net/lennart/

More information about the avahi mailing list