Question

Link-Local Multicast Name Resolution (LLMNR) is a Microsoft tech for service discovery based on multicast DNS. Is it compatible with Bonjour/Zeroconf? If so, where’s the API? The service discovery, I take it, is somehow rolled into vanilla DNS resolution (gethostbyname()?), but what about service advertisement?

Answer 1

LLMNR solves a small subset of the problems that mDNS tackles, as set out in this post. In short, it’s not compatible with Bonjour.

Here is the content of that post, in case archive.org decides to drop the page:

From: Marc Krochmal Date: Fri, 16 Apr 2004 08:54:54
-0700
Hi Tom,

I was reading the August 2003 article in Linux Magazine
(http://www.linux-mag.com/2003-08/zeroconf_01.html) on Zeroconf
environments. Within the article they mention that Link Local
Multicast Name Resolution was adopted by the IETF and that more or
less mDNS was thrown out on its ear, even though it uses existing
standards instead of creating an all new protocol. One of the reasons
they mentioned or implied was that LLMNR was more advanced than mDNS
was ‘includes provisions to keep link-local addresses from
accidentally being propagated into the greater DNS space’ Based on
research I have been doing into Rendezvous environments this is
incorrect (is it not??) since mDNS is aware that domains ending with
‘.local’ are available only on the local link and that a host will
only ever try and resolve a domain name ending with ‘.local’ on the
local link. So have i misread this article or the mDNS white-paper or
is there some truth in this??

LLMNR is much less advanced than mDNS. LLMNR solves a very small
problem. It provides very basic name resolution when the name server
is misconfigured or when it returns an error.

mDNS provides a distributed notification and cache coherency protocol
that facilities ad-hoc DNS-Based Service Discovery. mDNS also provides
reliable local name resolution when a hostname ends with “.local”,
regardless of the state of the DNS server. LLMNR is not a service
discovery protocol. You could say that mDNS is a superset of LLMNR.

After a year and a half of operational experience with mDNS, we have
never seen accidental leakage of link-local addresses outside the
local-link.

There are many people in the IETF DNSEXT working group that would like
to see LLMNR interoperate with mDNS. In fact, some people including
Paul Vixie are saying that LLMNR should be thrown out and replaced
with mDNS. You can read this for yourself in the namedroppers
archives.

http://ops.ietf.org/lists/namedroppers/namedroppers.2004/

Another thing alluded to by this article was that Microsoft is pushing
for LLMNR and as far as I recall this is something provided by Service
Pack 2 in Win XP.

I am not aware of LLMNR being included in Service Pack 2.

Is this not going to take us back to a NetBIOS v AppleTalk days where
instead of working towards a singular standard we now have two for
name resolution in Zeroconf environments???

This is difficult to predict. If Rendezvous continues to gain
momentum, it could easily become the standard in Zeroconf
environments. Pretty much every network enabled printer today ships
with Rendezvous. Rendezvous also enables iTunes Music Sharing on Mac
and Windows which could allow you to potentially share music with many
other devices in the home with zero configuration.

Finally on a different track, the mDNS responder provided by Apple for
OS X handles traditional DNS responses as well as well as link local
mDNS. Is the mDNS responder for Windows and Linux boxes intended to
replace the respective traditional DNS handlers provided by the
Windows OS and Linux OS or does mDNS work in conjunction with their
existing DNS handling (i.e. On a windows box with an mDNS Responder
installed, if a DNS Query (for a global domain) needs to be sent, will
the OS now use the mDNS responder to handle the process or will it use
its ‘out-of-the-box’ DNS responder ???)

Even though mDNSResponder could be used to replace all DNS query
handling, it probably won’t. The name space provider for Windows which
is included in the mDNSResonder project only works for names that end
with “.local”.

Best Regards,

-Marc