Re: Summary of the LLMNR Last Call

Stuart Cheshire <> Sun, 18 September 2005 22:55 UTC

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Date: Sun, 18 Sep 2005 15:55:32 -0700
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From: Stuart Cheshire <>
To: "Margaret Wasserman" <>, <>
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Subject: Re: Summary of the LLMNR Last Call
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>(3) Separate, but perhaps underlying both of the previous issues, 
>there seems be a fundamental disagreement about what technical 
>approach we should take to link-local name lookup.  LLMNR takes the 
>approach that local lookups should use the same names as global 
>lookups and that upper layers should not care whether a name was 
>resolved in the global DNS or locally, essentially making the local 
>lookup mechanism an extension of the DNS.  mDNS takes the approach 
>that local lookups should be distinguishable from global lookups and 
>accomplishes this through the use of a special local domain (.local).

This claim is one of the bits of misinformation that seems to be spread 
about mDNS for some reason. It's repeated so often that people who 
haven't read the draft assume it's true.

Even on Mac OS 9, five years ago, if you looked up "" and had 
no unicast DNS servers configured, it would look it up via mDNS instead. 
The difference is that we were profoundly nervous about the implications 
of doing this without adequate security, which is why 
draft-cheshire-dnsext-multicastdns.txt allows multicast lookups for 
non-local names, but says:

   (14. Enabling and Disabling Multicast DNS)

   The option to fail-over to Multicast DNS for names not ending
   in ".local." SHOULD be a user-configured option, and SHOULD
   be disabled by default because of the possible security issues
   related to unintended local resolution of apparently global names.

   (24. Security Considerations)

   When DNS queries for *global* DNS names are sent to the mDNS
   multicast address (during network outages which disrupt communication
   with the greater Internet) it is *especially* important to use
   DNSSEC, because the user may have the impression that he or she is
   communicating with some authentic host, when in fact he or she is
   really communicating with some local host that is merely masquerading
   as that name.

The difference between mDNS and LLMNR is not in their lookup of global 
names, but that mDNS *also* designates a special sub-tree of the 
namespace where users explicitly have different security expectations. We 
have an expectation of what means. Our expectation of what 
webserver.local means is different -- we know it's just a local, 
temporary, transient name. We might do on-line banking at, but never at moneybank.local.

Stuart Cheshire <>
 * Wizard Without Portfolio, Apple Computer, Inc.

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