[Dnsmasq-discuss] Multicast dns

Jan 'RedBully' Seiffert redbully at cc.fh-luh.de
Mon Apr 23 20:37:20 BST 2007

Simon Kelley wrote:
> Tom Fanning wrote:
>>> OK, my understanding (which might be wrong) is this:
>> <snip>
>>> On the other hand, just editing /etc/nsswitch.conf to remove the mDNS
>>> resolver would do the same thing.
>> ... the vital remaining part of the puzzle that I couldn't find. Thank you.
>>> Hmm, difficult.
>> Point taken, I'll go away now. It was very late last night when I posted.
>> Sorry for the noise.
> Not at all, It's a reasonable suggestion that dnsmasq should be
> configurable to listen for mDNS and reply to everything: Even with the
> nsswitch.conf fix, that has to be fixed on every new Ubuntu Feisty
> install,
It's not only Ubunty...
More than half a year ago my brother visited me. I also had the TLD
.local . He plugged his laptop in, dnsmasq served him "his" dhcp entry
and *poof* his network was dead. Or no, he was sending his DNS-Queries
via multicast.
What happened? Before he came to me, he updated his Debian testing to
the latest versions and got mDNS/zeroconf/avahy behind his back.

So he deinstalled that avahi-stuff, and *plop* network worked again :).

Since then i switched my TLD to .lan. (Was a PITA. Services not running
anymore because you configured them to connect to foo.bar.local, the IP
would have been more stable...)

On Gentoo there was also a time, when they installed mDNS for you by
default (with glibc), now it is an extra package, you have to install

> a dnsmasq configuration tweak would do it for  a whole network
> and if mDNS was not otherwise in use, nothing is lost.
My problem was, which really made me mad, that suddenly ALL DNS queries
seemed to go over mDNS, not only those local zeroconf foo.
But i had no "for outer World DNS"-mDNS-server on the multicast address,
even with dnsmasq, the little swiss army knife (and i don't wanted to
blow up my router setup with adding a mDNS server).

> OTOH, against doing this is that it breaks "real mDNS" so it's  a bug to
> fix another bug.
But could be an interresting feature:
If my memorie serves me right, there are special request types in the
sense of "give me the network default printer" or "who answers DNS
queries here" (service discovery) and at least the last should be
answered by dnsmasq (or registering on avahi? how does this deamon
interact with this?). Thats how these zeroconf networks (ip 169....) are
suposed to work.
With the possiblity to add maunal entries into the dnsamsq conf one
could for example redirect "Got Printer?"-queries to his cups (until
cups grows this feature itself)

> Also it's possibly bloat for dnsmasq
Yup, hmmm.
Basically it's just another socket, hmmm, but some "special parsing", if
i'm correct.

But with a nice ./configure-option?

> and of minor real
> world use for most people.
IMHO it's an emerging thing, all those network-appliances grow this
feature (Net-Printer, NAS, Media-streaming boxes, VoIP-Phones?? (again,
service discovery)), and "Out-Of-The-Box"-Distros seem to enable it (and
again, service discovery, so Joe Average gets a nice "local Printers in
range" tab in his printing dialog).
Oh, and i think in IPv6-land there are some basic services (DHCP?)
changed to multicast, so maybe dnsmasq needs a multicast socket anyway.

> A judgement call, like most possible
> enhancements.
I personly wouldn't need it (nor have any testcase, since i notoriously
disable auto-foobar), and would label it as "nice to have when you need
it" thing.
So, personal POV: hacking away on some mDNS code ASAP, no (some more
thoughts/research first).
Some mDNS awareness (<joke> Simon reads some RFC's and enlightens us :D
</joke>) esp. not how does the packet look like, but what could be done
with it and has to be done about it, in FAQ, Manual, HOWTOs (code?),
maybe yes.

> Cheers,
> Simon.


Fun things to slip into your budged:
Traffic shaping on the loopback interface.

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