Subject: Re: LLMNR is done

From: Bernard Aboba <[email-masked]>
Date: Wed, 28 Jan 2004 15:45:39 -0800 (PST)

>I'm confused. LLMNR is only useful when there is no configured DNS,

LLMNR can be used where there is a configured DNS server as well, but
where the DNS server doesn't answer, or responds with RCODE=3 or RCODE=0
and no answers in the answer section.  See Section 2 for details.

Also note that LLMNR prefers unicast queries for PTR RRs, and does not use
multicast responses, to avoid unnecessary multicast traffic that would
become a problem if the scope were expanded beyond link local. See Section
2.4.

>What sort of scenario are you likely to run in to where there would be no
>unicast DNS but there are routers configured to handle multicast? Where
>is this mentioned as a design goal?

If the user has a router at home that includes support for DHCPv4 only and
no support for IPV6 DNS configuration or resolution of AAAA queries, then
LLMNR can be used to resolve names of IPv6-only hosts on the local
network.  See Section 3.1.

>It seems like LLMNR has a subset of the mDNS functionality.

mDNS is focussed on discovery of stationary services. LLMNR is about name
resolution in environments which may be highly mobile.  They are different
problems and different design choices were made to address them.

>LLMNR focuses on solving the problem of resolving DNS entries when there
>is no DNS server. mDNS attempts to solve the problem of resolving DNS
>names when there is no DNS as well as resolving names even when there is
>a configured DNS server.

LLMNR handles both situations.  LLMNR can be used to resolve any name. See
Section 2.

>If they had private addresses, there would be no point in using dynamic
DNS to register their address. >LLMNR wouldn't work.

LLMNR can resolve a name with any kind of address. LLMNR doesn't rely on
dynamic DNS or support dynamic DNS update op codes.  See Section 2.1.

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From: Joshua Graessley <[email-masked]>
Date: Thu, 29 Jan 2004 14:50:42 -0800

> and no answers in the answer section.  See Section 2 for details.

Counting on DNS to return RCODE=3 or RCODE=0 with no answers is error 
prone. Unless you or someone you know is responsible for a domain, you 
can't guarantee that a query will fail. If you use an unqualified name, 
the resolver may attempt to append a search domain first, which may 
result in a successful unicast DNS query for a name you were not 
expecting. Maybe I'm missing something obvious? Otherwise, it seems 
that LLMNR is unreliable (nearly useless) when there is a configured 
DNS server.

-josh