Link-local Multicast Name Resolution (LLMNR)
RFC 4795

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Network Working Group                                           B. Aboba
Request for Comments: 4795                                     D. Thaler
Category: Informational                                        L. Esibov
                                                   Microsoft Corporation
                                                            January 2007

              Link-Local Multicast Name Resolution (LLMNR)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

IESG Note

   This document was originally intended for advancement as a Proposed
   Standard, but the IETF did not achieve consensus on the approach.
   The document has had significant review and input.  At time of
   publication, early versions were implemented and deployed.

Abstract

   The goal of Link-Local Multicast Name Resolution (LLMNR) is to enable
   name resolution in scenarios in which conventional DNS name
   resolution is not possible.  LLMNR supports all current and future
   DNS formats, types, and classes, while operating on a separate port
   from DNS, and with a distinct resolver cache.  Since LLMNR only
   operates on the local link, it cannot be considered a substitute for
   DNS.

Aboba, et al.                Informational                      [Page 1]
RFC 4795                         LLMNR                      January 2007

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements ...............................................3
      1.2. Terminology ................................................4
   2. Name Resolution Using LLMNR .....................................4
      2.1. LLMNR Packet Format ........................................5
           2.1.1. LLMNR Header Format .................................5
      2.2. Sender Behavior ............................................8
      2.3. Responder Behavior .........................................9
      2.4. Unicast Queries and Responses .............................11
      2.5. "Off-Link" Detection ......................................11
      2.6. Responder Responsibilities ................................12
      2.7. Retransmission and Jitter .................................13
      2.8. RR TTL ....................................................14
      2.9. Use of the Authority and Additional Sections ..............14
   3. Usage Model ....................................................15
      3.1. LLMNR Configuration .......................................17
   4. Conflict Resolution ............................................18
      4.1. Uniqueness Verification ...................................19
      4.2. Conflict Detection and Defense ............................20
      4.3. Considerations for Multiple Interfaces ....................21
      4.4. API Issues ................................................22
   5. Security Considerations ........................................23
      5.1. Denial of Service .........................................23
      5.2. Spoofing ..................................................24
      5.3. Authentication ............................................25
      5.4. Cache and Port Separation .................................25
   6. IANA Considerations ............................................26
   7. Constants ......................................................26
   8. References .....................................................27
      8.1. Normative References ......................................27
      8.2. Informative References ....................................27
   9. Acknowledgments ................................................29

Aboba, et al.                Informational                      [Page 2]
RFC 4795                         LLMNR                      January 2007

1.  Introduction

   This document discusses Link-Local Multicast Name Resolution (LLMNR),
   which is based on the DNS packet format and supports all current and
   future DNS formats, types, and classes.  LLMNR operates on a separate
   port from the Domain Name System (DNS), with a distinct resolver
   cache.

   Since LLMNR only operates on the local link, it cannot be considered
   a substitute for DNS.  Link-scope multicast addresses are used to
   prevent propagation of LLMNR traffic across routers, potentially
   flooding the network.  LLMNR queries can also be sent to a unicast
   address, as described in Section 2.4.

   Propagation of LLMNR packets on the local link is considered
   sufficient to enable name resolution in small networks.  In such
   networks, if a network has a gateway, then typically the network is
   able to provide DNS server configuration.  Configuration issues are
   discussed in Section 3.1.