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


              <span class="h1">Link-Local Multicast Name Resolution (LLMNR)</span>

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.
















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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


Table of Contents

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

















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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   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 <a href="#section-2.4">Section 2.4</a>.

   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 <a href="#section-3.1">Section 3.1</a>.

   In the future, it may be desirable to consider use of multicast name
   resolution with multicast scopes beyond the link-scope.  This could
   occur if LLMNR deployment is successful, the need arises for
   multicast name resolution beyond the link-scope, or multicast routing
   becomes ubiquitous.  For example, expanded support for multicast name
   resolution might be required for mobile ad-hoc networks.

   Once we have experience in LLMNR deployment in terms of
   administrative issues, usability, and impact on the network, it will
   be possible to reevaluate which multicast scopes are appropriate for
   use with multicast name resolution.  IPv4 administratively scoped
   multicast usage is specified in "Administratively Scoped IP
   Multicast" [<a href="./rfc2365" title="&quot;Administratively Scoped IP Multicast&quot;">RFC2365</a>].

   Service discovery in general, as well as discovery of DNS servers
   using LLMNR in particular, is outside the scope of this document, as
   is name resolution over non-multicast capable media.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Requirements</span>

   In this document, several words are used to signify the requirements
   of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" in this document are to be interpreted as described in
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].






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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Terminology</span>

   This document assumes familiarity with DNS terminology defined in
   [<a href="./rfc1035" title="&quot;Domain names - implementation and specification&quot;">RFC1035</a>].  Other terminology used in this document includes:

   Routable Address An address other than a link-local address.  This
                    includes globally routable addresses, as well as
                    private addresses.

   Reachable        An LLMNR responder considers one of its addresses
                    reachable over a link if it will respond to an
                    Address Resolution Protocol (ARP) or Neighbor
                    Discovery query for that address received on that
                    link.

   Responder        A host that listens to LLMNR queries, and responds
                    to those for which it is authoritative.

   Sender           A host that sends an LLMNR query.

   UNIQUE           There are some scenarios when multiple responders
                    may respond to the same query.  There are other
                    scenarios when only one responder may respond to a
                    query.  Names for which only a single responder is
                    anticipated are referred to as UNIQUE.  Name
                    uniqueness is configured on the responder, and
                    therefore uniqueness verification is the responder's
                    responsibility.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Name Resolution Using LLMNR</span>

   LLMNR queries are sent to and received on port 5355.  The IPv4 link-
   scope multicast address a given responder listens to, and to which a
   sender sends queries, is 224.0.0.252.  The IPv6 link-scope multicast
   address a given responder listens to, and to which a sender sends all
   queries, is FF02:0:0:0:0:0:1:3.

   Typically, a host is configured as both an LLMNR sender and a
   responder.  A host MAY be configured as a sender, but not a
   responder.  However, a host configured as a responder MUST act as a
   sender, if only to verify the uniqueness of names as described in
   <a href="#section-4">Section 4</a>.  This document does not specify how names are chosen or
   configured.  This may occur via any mechanism, including DHCPv4
   [<a href="./rfc2131" title="&quot;Dynamic Host Configuration Protocol&quot;">RFC2131</a>] or DHCPv6 [<a href="./rfc3315" title="&quot;Dynamic Host Configuration Protocol for IPv6 (DHCPv6)&quot;">RFC3315</a>].







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   A typical sequence of events for LLMNR usage is as follows:

   (a)  An LLMNR sender sends an LLMNR query to the link-scope multicast
        address(es), unless a unicast query is indicated, as specified
        in <a href="#section-2.4">Section 2.4</a>.

   (b)  A responder responds to this query only if it is authoritative
        for the name in the query.  A responder responds to a multicast
        query by sending a unicast UDP response to the sender.  Unicast
        queries are responded to as indicated in <a href="#section-2.4">Section 2.4</a>.

   (c)  Upon reception of the response, the sender processes it.

   The sections that follow provide further details on sender and
   responder behavior.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  LLMNR Packet Format</span>

   LLMNR is based on the DNS packet format defined in [<a href="./rfc1035" title="&quot;Domain names - implementation and specification&quot;">RFC1035</a>] <a href="#section-4">Section</a>
   <a href="#section-4">4</a> for both queries and responses.  LLMNR implementations SHOULD send
   UDP queries and responses only as large as are known to be
   permissible without causing fragmentation.  When in doubt, a maximum
   packet size of 512 octets SHOULD be used.  LLMNR implementations MUST
   accept UDP queries and responses as large as the smaller of the link
   MTU or 9194 octets (Ethernet jumbo frame size of 9KB (9216) minus 22
   octets for the header, VLAN tag and Cyclic Redundancy Check (CRC)).

<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a>.  LLMNR Header Format</span>

   LLMNR queries and responses utilize the DNS header format defined in
   [<a href="./rfc1035" title="&quot;Domain names - implementation and specification&quot;">RFC1035</a>] with exceptions noted below:

                                      1  1  1  1  1  1
        0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |                      ID                       |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |QR|   Opcode  | C|TC| T| Z| Z| Z| Z|   RCODE   |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |                    QDCOUNT                    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |                    ANCOUNT                    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |                    NSCOUNT                    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |                    ARCOUNT                    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+




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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   where:

   ID      A 16-bit identifier assigned by the program that generates
           any kind of query.  This identifier is copied from the query
           to the response and can be used by the sender to match
           responses to outstanding queries.  The ID field in a query
           SHOULD be set to a pseudo-random value.  For advice on
           generation of pseudo-random values, please consult [<a href="./rfc4086" title="&quot;Randomness Requirements for Security&quot;">RFC4086</a>].

   QR      Query/Response.  A 1-bit field, which, if set, indicates that
           the message is an LLMNR response; if clear, then the message
           is an LLMNR query.

   OPCODE  A 4-bit field that specifies the kind of query in this
           message.  This value is set by the originator of a query and
           copied into the response.  This specification defines the
           behavior of standard queries and responses (opcode value of
           zero).  Future specifications may define the use of other
           opcodes with LLMNR.  LLMNR senders and responders MUST
           support standard queries (opcode value of zero).  LLMNR
           queries with unsupported OPCODE values MUST be silently
           discarded by responders.

   C       Conflict.  When set within a query, the 'C'onflict bit
           indicates that a sender has received multiple LLMNR responses
           to this query.  In an LLMNR response, if the name is
           considered UNIQUE, then the 'C' bit is clear; otherwise, it
           is set.  LLMNR senders do not retransmit queries with the 'C'
           bit set.  Responders MUST NOT respond to LLMNR queries with
           the 'C' bit set, but may start the uniqueness verification
           process, as described in <a href="#section-4.2">Section 4.2</a>.

   TC      TrunCation.  The 'TC' bit specifies that this message was
           truncated due to length greater than that permitted on the
           transmission channel.  The 'TC' bit MUST NOT be set in an
           LLMNR query and, if set, is ignored by an LLMNR responder.
           If the 'TC' bit is set in an LLMNR response, then the sender
           SHOULD resend the LLMNR query over TCP using the unicast
           address of the responder as the destination address.  If the
           sender receives a response to the TCP query, then it SHOULD
           discard the UDP response with the TC bit set.  See  [<a href="./rfc2181" title="&quot;Clarifications to the DNS Specification&quot;">RFC2181</a>]
           and <a href="#section-2.4">Section 2.4</a> of this specification for further discussion
           of the 'TC' bit.

   T       Tentative.  The 'T'entative bit is set in a response if the
           responder is authoritative for the name, but has not yet
           verified the uniqueness of the name.  A responder MUST ignore
           the 'T' bit in a query, if set.  A response with the 'T' bit



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           set is silently discarded by the sender, except if it is a
           uniqueness query, in which case, a conflict has been detected
           and a responder MUST resolve the conflict as described in
           <a href="#section-4.1">Section 4.1</a>.

   Z       Reserved for future use.  Implementations of this
           specification MUST set these bits to zero in both queries and
           responses.  If these bits are set in a LLMNR query or
           response, implementations of this specification MUST ignore
           them.  Since reserved bits could conceivably be used for
           different purposes than in DNS, implementers are advised not
           to enable processing of these bits in an LLMNR implementation
           starting from a DNS code base.

   RCODE   Response code.  This 4-bit field is set as part of LLMNR
           responses.  In an LLMNR query, the sender MUST set RCODE to
           zero; the responder ignores the RCODE and assumes it to be
           zero.  The response to a multicast LLMNR query MUST have
           RCODE set to zero.  A sender MUST silently discard an LLMNR
           response with a non-zero RCODE sent in response to a
           multicast query.

           If an LLMNR responder is authoritative for the name in a
           multicast query, but an error is encountered, the responder
           SHOULD send an LLMNR response with an RCODE of zero, no RRs
           in the answer section, and the TC bit set.  This will cause
           the query to be resent using TCP, and allow the inclusion of
           a non-zero RCODE in the response to the TCP query.
           Responding with the TC bit set is preferable to not sending a
           response, since it enables errors to be diagnosed.  This may
           be required, for example, when an LLMNR query includes a TSIG
           RR in the additional section, and the responder encounters a
           problem that requires returning a non-zero RCODE.  TSIG error
           conditions defined in [<a href="./rfc2845" title="&quot;Secret Key Transaction Authentication for DNS (TSIG)&quot;">RFC2845</a>] include a TSIG RR in an
           unacceptable position (RCODE=1) or a TSIG RR that does not
           validate (RCODE=9 with TSIG ERROR 17 (BADKEY) or 16
           (BADSIG)).

           Since LLMNR responders only respond to LLMNR queries for
           names for which they are authoritative, LLMNR responders MUST
           NOT respond with an RCODE of 3; instead, they should not
           respond at all.

           LLMNR implementations MUST support EDNS0 [<a href="./rfc2671" title="&quot;Extension Mechanisms for DNS (EDNS0)&quot;">RFC2671</a>] and
           extended RCODE values.






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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   QDCOUNT An unsigned 16-bit integer specifying the number of entries
           in the question section.  A sender MUST place only one
           question into the question section of an LLMNR query.  LLMNR
           responders MUST silently discard LLMNR queries with QDCOUNT
           not equal to one.  LLMNR senders MUST silently discard LLMNR
           responses with QDCOUNT not equal to one.

   ANCOUNT An unsigned 16-bit integer specifying the number of resource
           records in the answer section.  LLMNR responders MUST
           silently discard LLMNR queries with ANCOUNT not equal to
           zero.

   NSCOUNT An unsigned 16-bit integer specifying the number of name
           server resource records in the authority records section.
           Authority record section processing is described in <a href="#section-2.9">Section</a>
           <a href="#section-2.9">2.9</a>.  LLMNR responders MUST silently discard LLMNR queries
           with NSCOUNT not equal to zero.

   ARCOUNT An unsigned 16-bit integer specifying the number of resource
           records in the additional records section.  Additional record
           section processing is described in <a href="#section-2.9">Section 2.9</a>.

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Sender Behavior</span>

   A sender MAY send an LLMNR query for any legal resource record type
   (e.g., A, AAAA, PTR, SRV) to the link-scope multicast address.  As
   described in <a href="#section-2.4">Section 2.4</a>, a sender MAY also send a unicast query.

   The sender MUST anticipate receiving no responses to some LLMNR
   queries, in the event that no responders are available within the
   link-scope.  If no response is received, a resolver treats it as a
   response that the name does not exist (RCODE=3 is returned).  A
   sender can handle duplicate responses by discarding responses with a
   source IP address and ID field that duplicate a response already
   received.

   When multiple valid LLMNR responses are received with the 'C' bit
   set, they SHOULD be concatenated and treated in the same manner that
   multiple RRs received from the same DNS server would be.  However,
   responses with the 'C' bit set SHOULD NOT be concatenated with
   responses with the 'C' bit clear; instead, only the responses with
   the 'C' bit set SHOULD be returned.  If valid LLMNR response(s) are
   received along with error response(s), then the error responses are
   silently discarded.

   Since the responder may order the RRs in the response so as to
   indicate preference, the sender SHOULD preserve ordering in the
   response to the querying application.



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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Responder Behavior</span>

   An LLMNR response MUST be sent to the sender via unicast.

   Upon configuring an IP address, responders typically will synthesize
   corresponding A, AAAA and PTR RRs so as to be able to respond to
   LLMNR queries for these RRs.  An SOA RR is synthesized only when a
   responder has another RR in addition to the SOA RR;  the SOA RR MUST
   NOT be the only RR that a responder has.  However, in general,
   whether RRs are manually or automatically created is an
   implementation decision.

   For example, a host configured to have computer name "host1" and to
   be a member of the "example.com" domain, with IPv4 address 192.0.2.1
   and IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6, might be authoritative
   for the following records:

   host1. IN A 192.0.2.1
          IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6

   host1.example.com. IN A 192.0.2.1
          IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6

   1.2.0.192.in-addr.arpa. IN PTR host1.
          IN PTR host1.example.com.

   6.0.5.0.4.0.E.F.F.F.3.0.2.0.1.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.
   ip6.arpa IN PTR host1.  (line split for formatting reasons)
            IN PTR host1.example.com.

   An LLMNR responder might be further manually configured with the name
   of a local mail server with an MX RR included in the "host1." and
   "host1.example.com." records.

   In responding to queries:

   (a)  Responders MUST listen on UDP port 5355 on the link-scope
        multicast address(es) defined in <a href="#section-2">Section 2</a>, and on TCP port 5355
        on the unicast address(es) that could be set as the source
        address(es) when the responder responds to the LLMNR query.

   (b)  Responders MUST direct responses to the port from which the
        query was sent.  When queries are received via TCP, this is an
        inherent part of the transport protocol.  For queries received
        by UDP, the responder MUST take note of the source port and use
        that as the destination port in the response.  Responses MUST
        always be sent from the port to which they were directed.




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<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   (c)  Responders MUST respond to LLMNR queries for names and addresses
        for which they are authoritative.  This applies to both forward
        and reverse lookups, with the exception of queries with the 'C'
        bit set, which do not elicit a response.

   (d)  Responders MUST NOT respond to LLMNR queries for names for which
        they are not authoritative.

   (e)  Responders MUST NOT respond using data from the LLMNR or DNS
        resolver cache.

   (f)  If a responder is authoritative for a name, it MUST respond with
        RCODE=0 and an empty answer section, if the type of query does
        not match an RR that the responder has.

   As an example, a host configured to respond to LLMNR queries for the
   name "foo.example.com."  is authoritative for the name
   "foo.example.com.".  On receiving an LLMNR query for an A RR with the
   name "foo.example.com.", the host authoritatively responds with an A
   RR(s) that contain IP address(es) in the RDATA of the resource
   record.  If the responder has an AAAA RR, but no A RR, and an A RR
   query is received, the responder would respond with RCODE=0 and an
   empty answer section.

   In conventional DNS terminology, a DNS server authoritative for a
   zone is authoritative for all the domain names under the zone apex
   except for the branches delegated into separate zones.  Contrary to
   conventional DNS terminology, an LLMNR responder is authoritative
   only for the zone apex.

   For example, the host "foo.example.com." is not authoritative for the
   name "child.foo.example.com." unless the host is configured with
   multiple names, including "foo.example.com."  and
   "child.foo.example.com.".  As a result, "foo.example.com." cannot
   respond to an LLMNR query for "child.foo.example.com." with RCODE=3
   (authoritative name error).  The purpose of limiting the name
   authority scope of a responder is to prevent complications that could
   be caused by coexistence of two or more hosts with the names
   representing child and parent (or grandparent) nodes in the DNS tree,
   for example, "foo.example.com." and "child.foo.example.com.".

   Without the restriction on authority, an LLMNR query for an A
   resource record for the name "child.foo.example.com." would result in
   two authoritative responses: RCODE=3 (authoritative name error)
   received from "foo.example.com.", and a requested A record from
   "child.foo.example.com.".  To prevent this ambiguity, LLMNR-enabled
   hosts could perform a dynamic update of the parent (or grandparent)
   zone with a delegation to a child zone; for example, a host



<span class="grey">Aboba, et al.                Informational                     [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   "child.foo.example.com." could send a dynamic update for the NS and
   glue A record to "foo.example.com.".  However, this approach
   significantly complicates implementation of LLMNR and would not be
   acceptable for lightweight hosts.

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  Unicast Queries and Responses</span>

   Unicast queries SHOULD be sent when:

   (a) A sender repeats a query after it received a response with the TC
       bit set to the previous LLMNR multicast query, or

   (b) The sender queries for a PTR RR of a fully formed IP address
       within the "in-addr.arpa" or "ip6.arpa" zones.

   Unicast LLMNR queries MUST be done using TCP and the responses MUST
   be sent using the same TCP connection as the query.  Senders MUST
   support sending TCP queries, and responders MUST support listening
   for TCP queries.  If the sender of a TCP query receives a response to
   that query not using TCP, the response MUST be silently discarded.

   Unicast UDP queries MUST be silently discarded.

   A unicast PTR RR query for an off-link address will not elicit a
   response, but instead, an ICMP Time to Live (TTL) or Hop Limit
   exceeded message will be received.  An implementation receiving an
   ICMP message in response to a TCP connection setup attempt can return
   immediately, treating this as a response that no such name exists
   (RCODE=3 is returned).  An implementation that cannot process ICMP
   messages MAY send multicast UDP queries for PTR RRs.  Since TCP
   implementations will not retransmit prior to RTOmin, a considerable
   period will elapse before TCP retransmits multiple times, resulting
   in a long timeout for TCP PTR RR queries sent to an off-link
   destination.

<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>.  "Off-Link" Detection</span>

   A sender MUST select a source address for LLMNR queries that is
   assigned on the interface on which the query is sent.  The
   destination address of an LLMNR query MUST be a link-scope multicast
   address or a unicast address.

   A responder MUST select a source address for responses that is
   assigned on the interface on which the query was received.  The
   destination address of an LLMNR response MUST be a unicast address.






<span class="grey">Aboba, et al.                Informational                     [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   On receiving an LLMNR query, the responder MUST check whether it was
   sent to an LLMNR multicast addresses defined in <a href="#section-2">Section 2</a>.  If it was
   sent to another multicast address, then the query MUST be silently
   discarded.

   <a href="#section-2.4">Section 2.4</a> discusses use of TCP for LLMNR queries and responses.  In
   composing an LLMNR query using TCP, the sender MUST set the Hop Limit
   field in the IPv6 header and the TTL field in the IPv4 header of the
   response to one (1).  The responder SHOULD set the TTL or Hop Limit
   settings on the TCP listen socket to one (1) so that SYN-ACK packets
   will have TTL (IPv4) or Hop Limit (IPv6) set to one (1).  This
   prevents an incoming connection from off-link since the sender will
   not receive a SYN-ACK from the responder.

   For UDP queries and responses, the Hop Limit field in the IPv6 header
   and the TTL field in the IPV4 header MAY be set to any value.
   However, it is RECOMMENDED that the value 255 be used for
   compatibility with early implementations of [<a href="./rfc3927" title="&quot;Dynamic Configuration of IPv4 Link-Local Addresses&quot;">RFC3927</a>].

   Implementation note:

      In the sockets API for IPv4 [<a href="#ref-POSIX" title="Issue 6">POSIX</a>], the IP_TTL and
      IP_MULTICAST_TTL socket options are used to set the TTL of
      outgoing unicast and multicast packets.  The IP_RECVTTL socket
      option is available on some platforms to retrieve the IPv4 TTL of
      received packets with recvmsg().  [<a href="./rfc3542" title="&quot;Advanced Sockets Application Program Interface (API) for IPv6&quot;">RFC3542</a>] specifies similar
      options for setting and retrieving the IPv6 Hop Limit.

<span class="h3"><a class="selflink" id="section-2.6" href="#section-2.6">2.6</a>.  Responder Responsibilities</span>

   It is the responsibility of the responder to ensure that RRs returned
   in LLMNR responses MUST only include values that are valid on the
   local interface, such as IPv4 or IPv6 addresses valid on the local
   link or names defended using the mechanism described in <a href="#section-4">Section 4</a>.
   IPv4 Link-Local addresses are defined in [<a href="./rfc3927" title="&quot;Dynamic Configuration of IPv4 Link-Local Addresses&quot;">RFC3927</a>].  IPv6 Link-Local
   addresses are defined in [<a href="./rfc4291" title="&quot;IP Version 6 Addressing Architecture&quot;">RFC4291</a>].  In particular:

   (a) If a link-scope IPv6 address is returned in a AAAA RR, that
       address MUST be valid on the local link over which LLMNR is used.

   (b) If an IPv4 address is returned, it MUST be reachable through the
       link over which LLMNR is used.

   (c) If a name is returned (for example in a CNAME, MX, or SRV RR),
       the name MUST be resolvable on the local link over which LLMNR is
       used.





<span class="grey">Aboba, et al.                Informational                     [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   Where multiple addresses represent valid responses to a query, the
   order in which the addresses are returned is as follows:

   (d) If the source address of the query is a link-scope address, then
       the responder SHOULD include a link-scope address first in the
       response, if available.

   (e) If the source address of the query is a routable address, then
       the responder MUST include a routable address first in the
       response, if available.

<span class="h3"><a class="selflink" id="section-2.7" href="#section-2.7">2.7</a>.  Retransmission and Jitter</span>

   An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine
   when to retransmit an LLMNR query.  An LLMNR sender SHOULD either
   estimate the LLMNR_TIMEOUT for each interface or set a reasonably
   high initial timeout.  Suggested constants are described in <a href="#section-7">Section</a>
   <a href="#section-7">7</a>.

   If an LLMNR query sent over UDP is not resolved within LLMNR_TIMEOUT,
   then a sender SHOULD repeat the transmission of the query in order to
   ensure that it was received by a host capable of responding to it.
   An LLMNR query SHOULD NOT be sent more than three times.

   Where LLMNR queries are sent using TCP, retransmission is handled by
   the transport layer.  Queries with the 'C' bit set MUST be sent using
   multicast UDP and MUST NOT be retransmitted.

   An LLMNR sender cannot know in advance if a query sent using
   multicast will receive no response, one response, or more than one
   response.  An LLMNR sender MUST wait for LLMNR_TIMEOUT if no response
   has been received, or if it is necessary to collect all potential
   responses, such as if a uniqueness verification query is being made.
   Otherwise, an LLMNR sender SHOULD consider a multicast query answered
   after the first response is received, if that response has the 'C'
   bit clear.

   However, if the first response has the 'C' bit set, then the sender
   SHOULD wait for LLMNR_TIMEOUT + JITTER_INTERVAL in order to collect
   all possible responses.  When multiple valid answers are received,
   they may first be concatenated, and then treated in the same manner
   that multiple RRs received from the same DNS server would.  A unicast
   query sender considers the query answered after the first response is
   received.







<span class="grey">Aboba, et al.                Informational                     [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   Since it is possible for a response with the 'C' bit clear to be
   followed by a response with the 'C' bit set, an LLMNR sender SHOULD
   be prepared to process additional responses for the purposes of
   conflict detection, even after it has considered a query answered.

   In order to avoid synchronization, the transmission of each LLMNR
   query and response SHOULD be delayed by a time randomly selected from
   the interval 0 to JITTER_INTERVAL.  This delay MAY be avoided by
   responders responding with names that they have previously determined
   to be UNIQUE (see <a href="#section-4">Section 4</a> for details).

<span class="h3"><a class="selflink" id="section-2.8" href="#section-2.8">2.8</a>.  RR TTL</span>

   The responder should insert a pre-configured TTL value in the records
   returned in an LLMNR response.  A default value of 30 seconds is
   RECOMMENDED.  In highly dynamic environments (such as mobile ad-hoc
   networks), the TTL value may need to be reduced.

   Due to the TTL minimalization necessary when caching an RRset, all
   TTLs in an RRset MUST be set to the same value.

<span class="h3"><a class="selflink" id="section-2.9" href="#section-2.9">2.9</a>.  Use of the Authority and Additional Sections</span>

   Unlike the DNS, LLMNR is a peer-to-peer protocol and does not have a
   concept of delegation.  In LLMNR, the NS resource record type may be
   stored and queried for like any other type, but it has no special
   delegation semantics as it does in the DNS.  Responders MAY have NS
   records associated with the names for which they are authoritative,
   but they SHOULD NOT include these NS records in the authority
   sections of responses.

   Responders SHOULD insert an SOA record into the authority section of
   a negative response, to facilitate negative caching as specified in
   [<a href="./rfc2308" title="&quot;Negative Caching of DNS Queries (DNS NCACHE)&quot;">RFC2308</a>].  The TTL of this record is set from the minimum of the
   MINIMUM field of the SOA record and the TTL of the SOA itself, and
   indicates how long a resolver may cache the negative answer.  The
   owner name of the SOA record (MNAME) MUST be set to the query name.
   The RNAME, SERIAL, REFRESH, RETRY, and EXPIRE values MUST be ignored
   by senders.  Negative responses without SOA records SHOULD NOT be
   cached.

   In LLMNR, the additional section is primarily intended for use by
   EDNS0, TSIG, and SIG(0).  As a result, unless the 'C' bit is set,
   senders MAY only include pseudo RR-types in the additional section of
   a query; unless the 'C' bit is set, responders MUST ignore the
   additional section of queries containing other RR types.





<span class="grey">Aboba, et al.                Informational                     [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   In queries where the 'C' bit is set, the sender SHOULD include the
   conflicting RRs in the additional section.  Since conflict
   notifications are advisory, responders SHOULD log information from
   the additional section, but otherwise MUST ignore the additional
   section.

   Senders MUST NOT cache RRs from the authority or additional section
   of a response as answers, though they may be used for other purposes,
   such as negative caching.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Usage Model</span>

   By default, an LLMNR sender SHOULD send LLMNR queries only for
   single-label names.  Stub resolvers supporting both DNS and LLMNR
   SHOULD avoid sending DNS queries for single-label names, in order to
   reduce unnecessary DNS queries.  An LLMNR sender SHOULD NOT be
   enabled to send a query for any name, except where security
   mechanisms (described in <a href="#section-5.3">Section 5.3</a>) can be utilized.  An LLMNR
   query SHOULD only be sent for the originally requested name; a
   searchlist is not used to form additional LLMNR queries.

   LLMNR is a peer-to-peer name resolution protocol that is not intended
   as a replacement for DNS; rather, it enables name resolution in
   scenarios in which conventional DNS name resolution is not possible.
   Where LLMNR security is not enabled as described in <a href="#section-5.3">Section 5.3</a>, if
   LLMNR is given higher priority than DNS among the enabled name
   resolution mechanisms, this would allow the LLMNR cache, once
   poisoned, to take precedence over the DNS cache.  As a result, use of
   LLMNR as a primary name resolution mechanism is NOT RECOMMENDED.

   Instead, it is recommended that LLMNR be utilized as a secondary name
   resolution mechanism, for use in situations where hosts are not
   configured with the address of a DNS server, where the DNS server is
   unavailable or unreachable, where there is no DNS server
   authoritative for the name of a host, or where the authoritative DNS
   server does not have the desired RRs.

   When LLMNR is configured as a secondary name resolution mechanism,
   LLMNR queries SHOULD only be sent when all of the following
   conditions are met:











<span class="grey">Aboba, et al.                Informational                     [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   (1) No manual or automatic DNS configuration has been performed.  If
       DNS server address(es) have been configured, a host SHOULD
       attempt to reach DNS servers over all protocols on which DNS
       server address(es) are configured, prior to sending LLMNR
       queries.  For dual-stack hosts configured with DNS server
       address(es) for one protocol but not another, this implies that
       DNS queries SHOULD be sent over the protocol configured with a
       DNS server, prior to sending LLMNR queries.

   (2) All attempts to resolve the name via DNS on all interfaces have
       failed after exhausting the searchlist.  This can occur because
       DNS servers did not respond, or because they responded to DNS
       queries with RCODE=3 (Authoritative Name Error) or RCODE=0, and
       an empty answer section.  Where a single resolver call generates
       DNS queries for A and AAAA RRs, an implementation MAY choose not
       to send LLMNR queries if any of the DNS queries is successful.

   Where LLMNR is used as a secondary name resolution mechanism, its
   usage is in part determined by the behavior of DNS resolver
   implementations; robust resolver implementations are more likely to
   avoid unnecessary LLMNR queries.

   [<a id="ref-RFC1536">RFC1536</a>] describes common DNS implementation errors and fixes.  If
   the proposed fixes are implemented, unnecessary LLMNR queries will be
   reduced substantially, so implementation of [<a href="./rfc1536" title="&quot;Common DNS Implementation Errors and Suggested Fixes&quot;">RFC1536</a>] is recommended.

   For example, <a href="./rfc1536#section-1">[RFC1536] Section&nbsp;1</a> describes issues with retransmission
   and recommends implementation of a retransmission policy based on
   round trip estimates, with exponential back-off.  <a href="./rfc1536#section-4">[RFC1536] Section&nbsp;4</a>
   describes issues with failover, and recommends that resolvers try
   another server when they don't receive a response to a query.  These
   policies are likely to avoid unnecessary LLMNR queries.

   [<a id="ref-RFC1536">RFC1536</a>] <a href="#section-3">Section 3</a> describes zero answer bugs, which if addressed
   will also reduce unnecessary LLMNR queries.

   [<a id="ref-RFC1536">RFC1536</a>] <a href="#section-6">Section 6</a> describes name error bugs and recommended
   searchlist processing that will reduce unnecessary RCODE=3
   (authoritative name) errors, thereby also reducing unnecessary LLMNR
   queries.

   As noted in [<a href="#ref-DNSPerf" title="&quot;DNS Performance and the Effectiveness of Caching&quot;">DNSPerf</a>], a significant fraction of DNS queries do not
   receive a response, or result in negative responses due to missing
   inverse mappings or NS records that point to nonexistent or
   inappropriate hosts.  Therefore, a reduction in missing records can
   prevent many unnecessary LLMNR queries.





<span class="grey">Aboba, et al.                Informational                     [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  LLMNR Configuration</span>

   LLMNR usage MAY be configured manually or automatically on a per-
   interface basis.  By default, LLMNR responders SHOULD be enabled on
   all interfaces, at all times.  Where this is considered undesirable,
   LLMNR SHOULD be disabled, so that hosts will neither listen on the
   link-scope multicast address, nor will they send queries to that
   address.

   Where DHCPv4 or DHCPv6 is implemented, DHCP options can be used to
   configure LLMNR on an interface.  The LLMNR Enable Option, described
   in [<a href="#ref-LLMNREnable" title="&quot;DHCP LLMNR Enable Option&quot;">LLMNREnable</a>], can be used to explicitly enable or disable use of
   LLMNR on an interface.  The LLMNR Enable Option does not determine
   whether, or in which order, DNS itself is used for name resolution.
   The order in which various name resolution mechanisms should be used
   can be specified using the Name Service Search Option (NSSO) for DHCP
   [<a href="./rfc2937" title="&quot;The Name Service Search Option for DHCP&quot;">RFC2937</a>], using the LLMNR Enable Option code carried in the NSSO
   data.

   In situations where LLMNR is configured as a secondary name
   resolution protocol on a dual-stack host, behavior will be governed
   by both IPv4 and IPv6 configuration mechanisms.  Since IPv4 and IPv6
   utilize distinct configuration mechanisms, it is possible for a
   dual-stack host to be configured with the address of a DNS server
   over IPv4, while remaining unconfigured with a DNS server suitable
   for use over IPv6.

   In these situations, a dual-stack host will send AAAA queries to the
   configured DNS server over IPv4.  However, an IPv6-only host
   unconfigured with a DNS server suitable for use over IPv6 will be
   unable to resolve names using DNS.  Automatic IPv6 DNS configuration
   mechanisms (such as [<a href="./rfc3315" title="&quot;Dynamic Host Configuration Protocol for IPv6 (DHCPv6)&quot;">RFC3315</a>] and [<a href="#ref-DNSDisc" title="&quot;Well known site local unicast addresses to communicate with recursive DNS servers&quot;">DNSDisc</a>]) are not yet widely
   deployed, and not all DNS servers support IPv6.  Therefore, lack of
   IPv6 DNS configuration may be a common problem in the short term, and
   LLMNR may prove useful in enabling link-local name resolution over
   IPv6.

   Where a DHCPv4 server is available but not a DHCPv6 server [<a href="./rfc3315" title="&quot;Dynamic Host Configuration Protocol for IPv6 (DHCPv6)&quot;">RFC3315</a>],
   IPv6-only hosts may not be configured with a DNS server.  Where there
   is no DNS server authoritative for the name of a host or the
   authoritative DNS server does not support dynamic client update over
   IPv6 or DHCPv6-based dynamic update, then an IPv6-only host will not
   be able to do DNS dynamic update, and other hosts will not be able to
   resolve its name.







<span class="grey">Aboba, et al.                Informational                     [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   For example, if the configured DNS server responds to an AAAA RR
   query sent over IPv4 or IPv6 with an authoritative name error
   (RCODE=3) or RCODE=0 and an empty answer section, then an AAAA RR
   query sent using LLMNR over IPv6 may be successful in resolving the
   name of an IPv6-only host on the local link.

   Similarly, if a DHCPv4 server is available providing DNS server
   configuration, and DNS server(s) exist which are authoritative for
   the A RRs of local hosts and support either dynamic client update
   over IPv4 or DHCPv4-based dynamic update, then the names of local
   IPv4 hosts can be resolved over IPv4 without LLMNR.  However, if no
   DNS server is authoritative for the names of local hosts, or the
   authoritative DNS server(s) do not support dynamic update, then LLMNR
   enables link-local name resolution over IPv4.

   It is possible that DNS configuration mechanisms will go in and out
   of service.  In these circumstances, it is possible for hosts within
   an administrative domain to be inconsistent in their DNS
   configuration.

   For example, where DHCP is used for configuring DNS servers, one or
   more DHCP servers can fail.  As a result, hosts configured prior to
   the outage will be configured with a DNS server, while hosts
   configured after the outage will not.  Alternatively, it is possible
   for the DNS configuration mechanism to continue functioning while
   configured DNS servers fail.

   An outage in the DNS configuration mechanism may result in hosts
   continuing to use LLMNR even once the outage is repaired.  Since
   LLMNR only enables link-local name resolution, this represents a
   degradation in capabilities.  As a result, hosts without a configured
   DNS server may wish to periodically attempt to obtain DNS
   configuration if permitted by the configuration mechanism in use.  In
   the absence of other guidance, a default retry interval of one (1)
   minute is RECOMMENDED.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Conflict Resolution</span>

   By default, a responder SHOULD be configured to behave as though its
   name is UNIQUE on each interface on which LLMNR is enabled.  However,
   it is also possible to configure multiple responders to be
   authoritative for the same name.  For example, multiple responders
   MAY respond to a query for an A or AAAA type record for a cluster
   name (assigned to multiple hosts in the cluster).







<span class="grey">Aboba, et al.                Informational                     [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   To detect duplicate use of a name, an administrator can use a name
   resolution utility that employs LLMNR and lists both responses and
   responders.  This would allow an administrator to diagnose behavior
   and potentially intervene and reconfigure LLMNR responders that
   should not be configured to respond to the same name.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Uniqueness Verification</span>

   Prior to sending an LLMNR response with the 'T' bit clear, a
   responder configured with a UNIQUE name MUST verify that there is no
   other host within the scope of LLMNR query propagation that is
   authoritative for the same name on that interface.

   Once a responder has verified that its name is UNIQUE, if it receives
   an LLMNR query for that name with the 'C' bit clear, it MUST respond
   with the 'T' bit clear.  Prior to verifying that its name is UNIQUE,
   a responder MUST set the 'T' bit in responses.

   Uniqueness verification is carried out when the host:

     - starts up or is rebooted

     - wakes from sleep (if the network interface was inactive during
       sleep)

     - is configured to respond to LLMNR queries on an interface enabled
       for transmission and reception of IP traffic

     - is configured to respond to LLMNR queries using additional UNIQUE
       resource records

     - verifies the acquisition of a new IP address and configuration on
       an interface

   To verify uniqueness, a responder MUST send an LLMNR query with the
   'C' bit clear, over all protocols on which it responds to LLMNR
   queries (IPv4 and/or IPv6).  It is RECOMMENDED that responders verify
   uniqueness of a name by sending a query for the name with type='ANY'.

   If no response is received, the sender retransmits the query, as
   specified in <a href="#section-2.7">Section 2.7</a>.  If a response is received, the sender MUST
   check if the source address matches the address of any of its
   interfaces; if so, then the response is not considered a conflict,
   since it originates from the sender.  To avoid triggering conflict
   detection, a responder that detects that it is connected to the same
   link on multiple interfaces SHOULD set the 'C' bit in responses.





<span class="grey">Aboba, et al.                Informational                     [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   If a response is received with the 'T' bit clear, the responder MUST
   NOT use the name in response to LLMNR queries received over any
   protocol (IPv4 or IPv6).  If a response is received with the 'T' bit
   set, the responder MUST check if the source IP address in the
   response is lexicographically smaller than the source IP address in
   the query.  If so, the responder MUST NOT use the name in response to
   LLMNR queries received over any protocol (IPv4 or IPv6).  For the
   purpose of uniqueness verification, the contents of the answer
   section in a response is irrelevant.

   Periodically carrying out uniqueness verification in an attempt to
   detect name conflicts is not necessary, wastes network bandwidth, and
   may actually be detrimental.  For example, if network links are
   joined only briefly, and are separated again before any new
   communication is initiated, temporary conflicts are benign and no
   forced reconfiguration is required.  LLMNR responders SHOULD NOT
   periodically attempt uniqueness verification.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Conflict Detection and Defense</span>

   Hosts on disjoint network links may configure the same name for use
   with LLMNR.  If these separate network links are later joined or
   bridged together, then there may be multiple hosts that are now on
   the same link, trying to use the same name.

   In order to enable ongoing detection of name conflicts, when an LLMNR
   sender receives multiple LLMNR responses to a query, it MUST check if
   the 'C' bit is clear in any of the responses.  If so, the sender

   SHOULD send another query for the same name, type, and class, this
   time with the 'C' bit set, with the potentially conflicting resource
   records included in the additional section.

   Queries with the 'C' bit set are considered advisory, and responders
   MUST verify the existence of a conflict before acting on it.  A
   responder receiving a query with the 'C' bit set MUST NOT respond.

   If the query is for a UNIQUE name, then the responder MUST send its
   own query for the same name, type, and class, with the 'C' bit clear.
   If a response is received, the sender MUST check if the source
   address matches the address of any of its interfaces; if so, then the
   response is not considered a conflict, since it originates from the
   sender.  To avoid triggering conflict detection, a responder that
   detects that it is connected to the same link on multiple interfaces
   SHOULD set the 'C' bit in responses.






<span class="grey">Aboba, et al.                Informational                     [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   An LLMNR responder MUST NOT ignore conflicts once detected, and
   SHOULD log them.  Upon detecting a conflict, an LLMNR responder MUST
   immediately stop using the conflicting name in response to LLMNR
   queries received over any supported protocol, if the source IP
   address in the response is lexicographically smaller than the source
   IP address in the uniqueness verification query.

   After stopping the use of a name, the responder MAY elect to
   configure a new name.  However, since name reconfiguration may be
   disruptive, this is not required, and a responder may have been
   configured to respond to multiple names so that alternative names may
   already be available.  A host that has stopped the use of a name may
   attempt uniqueness verification again after the expiration of the TTL
   of the conflicting response.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Considerations for Multiple Interfaces</span>

   A multi-homed host may elect to configure LLMNR on only one of its
   active interfaces.  In many situations, this will be adequate.
   However, should a host need to configure LLMNR on more than one of
   its active interfaces, there are some additional precautions it MUST
   take.  Implementers who are not planning to support LLMNR on multiple
   interfaces simultaneously may skip this section.

   Where a host is configured to issue LLMNR queries on more than one
   interface, each interface maintains its own independent LLMNR
   resolver cache, containing the responses to LLMNR queries.

   A multi-homed host checks the uniqueness of UNIQUE records as
   described in <a href="#section-4">Section 4</a>.  The situation is illustrated in Figure 1.

                       ----------  ----------
                        |      |    |      |
                       [<a href="#ref-A">A</a>]    [myhost]   [myhost]

                  Figure 1.  Link-scope name conflict

   In this situation, the multi-homed myhost will probe for, and defend,
   its host name on both interfaces.  A conflict will be detected on one
   interface, but not the other.  The multi-homed myhost will not be
   able to respond with a host RR for "myhost" on the interface on the
   right (see Figure 1).  The multi-homed host may, however, be
   configured to use the "myhost" name on the interface on the left.








<span class="grey">Aboba, et al.                Informational                     [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   Since names are only unique per link, hosts on different links could
   be using the same name.  If an LLMNR client sends queries over
   multiple interfaces, and receives responses from more than one, the
   result returned to the client is defined by the implementation.  The
   situation is illustrated in Figure 2.

                       ----------  ----------
                        |      |    |     |
                       [<a href="#ref-A">A</a>]    [myhost]   [<a href="#ref-A">A</a>]

               Figure 2.  Off-segment name conflict

   If host myhost is configured to use LLMNR on both interfaces, it will
   send LLMNR queries on both interfaces.  When host myhost sends a
   query for the host RR for name "A", it will receive a response from
   hosts on both interfaces.

   Host myhost cannot distinguish between the situation shown in Figure
   2, and that shown in Figure 3, where no conflict exists.

                                [<a id="ref-A">A</a>]
                               |   |
                           -----   -----
                               |   |
                              [myhost]

               Figure 3.  Multiple paths to same host

   This illustrates that the proposed name conflict-resolution mechanism
   does not support detection or resolution of conflicts between hosts
   on different links.  This problem can also occur with DNS when a
   multi-homed host is connected to two different networks with
   separated name spaces.  It is not the intent of this document to
   address the issue of uniqueness of names within DNS.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  API Issues</span>

   [<a id="ref-RFC3493">RFC3493</a>] provides an API that can partially solve the name ambiguity
   problem for applications written to use this API, since the
   sockaddr_in6 structure exposes the scope within which each scoped
   address exists, and this structure can be used for both IPv4 (using
   v4-mapped IPv6 addresses) and IPv6 addresses.

   Following the example in Figure 2, an application on 'myhost' issues
   the request getaddrinfo("A", ...) with ai_family=AF_INET6 and
   ai_flags=AI_ALL|AI_V4MAPPED.  LLMNR queries will be sent from both
   interfaces, and the resolver library will return a list containing
   multiple addrinfo structures, each with an associated sockaddr_in6



<span class="grey">Aboba, et al.                Informational                     [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   structure.  This list will thus contain the IPv4 and IPv6 addresses
   of both hosts responding to the name 'A'.  Link-local addresses will
   have a sin6_scope_id value that disambiguates which interface is used
   to reach the address.  Of course, to the application, Figures 2 and 3
   are still indistinguishable, but this API allows the application to
   communicate successfully with any address in the list.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Security Considerations</span>

   LLMNR is a peer-to-peer name resolution protocol designed for use on
   the local link.  While LLMNR limits the vulnerability of responders
   to off-link senders, it is possible for an off-link responder to
   reach a sender.

   In scenarios such as public "hotspots", attackers can be present on
   the same link.  These threats are most serious in wireless networks,
   such as IEEE 802.11, since attackers on a wired network will require
   physical access to the network, while wireless attackers may mount
   attacks from a distance.  Link-layer security, such as
   [<a href="#ref-IEEE-802.11i" title="&quot;Supplement to Standard for Telecommunications and Information Exchange Between Systems - LAN/MAN Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Specification for Enhanced Security&quot;">IEEE-802.11i</a>], can be of assistance against these threats if it is
   available.

   This section details security measures available to mitigate threats
   from on and off-link attackers.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Denial of Service</span>

   Attackers may take advantage of LLMNR conflict detection by
   allocating the same name, denying service to other LLMNR responders,
   and possibly allowing an attacker to receive packets destined for
   other hosts.  By logging conflicts, LLMNR responders can provide
   forensic evidence of these attacks.

   An attacker may spoof LLMNR queries from a victim's address in order
   to mount a denial of service attack.  Responders setting the IPv6 Hop
   Limit or IPv4 TTL field to a value larger than one in an LLMNR UDP
   response may be able to reach the victim across the Internet.

   While LLMNR responders only respond to queries for which they are
   authoritative, and LLMNR does not provide wildcard query support, an
   LLMNR response may be larger than the query, and an attacker can
   generate multiple responses to a query for a name used by multiple
   responders.  A sender may protect itself against unsolicited
   responses by silently discarding them.







<span class="grey">Aboba, et al.                Informational                     [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Spoofing</span>

   LLMNR is designed to prevent reception of queries sent by an off-link
   attacker.  LLMNR requires that responders receiving UDP queries check
   that they are sent to a link-scope multicast address.  However, it is
   possible that some routers may not properly implement link-scope
   multicast, or that link-scope multicast addresses may leak into the
   multicast routing system.  To prevent successful setup of TCP
   connections by an off-link sender, responders receiving a TCP SYN
   reply with a TCP SYN-ACK with TTL set to one (1).

   While it is difficult for an off-link attacker to send an LLMNR query
   to a responder, it is possible for an off-link attacker to spoof a
   response to a query (such as an A or AAAA query for a popular
   Internet host), and by using a TTL or Hop Limit field larger than one
   (1), for the forged response to reach the LLMNR sender.  Since the
   forged response will only be accepted if it contains a matching ID
   field, choosing a pseudo-random ID field within queries provides some
   protection against off-link responders.

   When LLMNR is utilized as a secondary name resolution service,
   queries can be sent when DNS server(s) do not respond.  An attacker
   can execute a denial of service attack on the DNS server(s), and then
   poison the LLMNR cache by responding to an LLMNR query with incorrect
   information.  As noted in "Threat Analysis of the Domain Name System
   (DNS)" [<a href="./rfc3833" title="&quot;Threat Analysis of the Domain Name System (DNS)&quot;">RFC3833</a>], these threats also exist with DNS, since DNS-
   response spoofing tools are available that can allow an attacker to
   respond to a query more quickly than a distant DNS server.  However,
   while switched networks or link-layer security may make it difficult
   for an on-link attacker to snoop unicast DNS queries, multicast LLMNR
   queries are propagated to all hosts on the link, making it possible
   for an on-link attacker to spoof LLMNR responses without having to
   guess the value of the ID field in the query.

   Since LLMNR queries are sent and responded to on the local link, an
   attacker will need to respond more quickly to provide its own
   response prior to arrival of the response from a legitimate
   responder.  If an LLMNR query is sent for an off-link host, spoofing
   a response in a timely way is not difficult, since a legitimate
   response will never be received.

   This vulnerability can be reduced by limiting use of LLMNR to
   resolution of single-label names as described in <a href="#section-3">Section 3</a>, or by
   implementation of authentication (see <a href="#section-5.3">Section 5.3</a>).







<span class="grey">Aboba, et al.                Informational                     [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Authentication</span>

   LLMNR is a peer-to-peer name resolution protocol and, as a result, is
   often deployed in situations where no trust model can be assumed.
   Where a pre-arranged security configuration is possible, the
   following security mechanisms may be used:

   (a)  LLMNR implementations MAY support TSIG [<a href="./rfc2845" title="&quot;Secret Key Transaction Authentication for DNS (TSIG)&quot;">RFC2845</a>] and/or SIG(0)
        [<a href="./rfc2931" title="&quot;DNS Request and Transaction Signatures ( SIG(0)s )&quot;">RFC2931</a>] security mechanisms.  "DNS Name Service based on
        Secure Multicast DNS for IPv6 Mobile Ad Hoc Networks" [<a href="#ref-LLMNRSec" title="&quot;DNS Name Service based on Secure Multicast DNS for IPv6 Mobile Ad Hoc Networks&quot;">LLMNRSec</a>]
        describes the use of TSIG to secure LLMNR, based on group keys.
        While group keys can be used to demonstrate membership in a
        group, they do not protect against forgery by an attacker that
        is a member of the group.

   (b)  IPsec Encapsulating Security Payload (ESP) with a NULL
        encryption algorithm MAY be used to authenticate unicast LLMNR
        queries and responses, or LLMNR responses to multicast queries.
        In a small network without a certificate authority, this can be
        most easily accomplished through configuration of a group pre-
        shared key for trusted hosts.  As with TSIG, this does not
        protect against forgery by an attacker with access to the group
        pre-shared key.

   (c)  LLMNR implementations MAY support DNSSEC [<a href="./rfc4033" title="&quot;DNS Security Introduction and Requirements&quot;">RFC4033</a>].  In order to
        support DNSSEC, LLMNR implementations MAY be configured with
        trust anchors, or they MAY make use of keys obtained from DNS
        queries.  Since LLMNR does not support "delegated trust" (CD or
        AD bits), LLMNR implementations cannot make use of DNSSEC unless
        they are DNSSEC-aware and support validation.  Unlike approaches
        [a] or [b], DNSSEC permits a responder to demonstrate ownership
        of a name, not just membership within a trusted group.  As a
        result, it enables protection against forgery.

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Cache and Port Separation</span>

   In order to prevent responses to LLMNR queries from polluting the DNS
   cache, LLMNR implementations MUST use a distinct, isolated cache for
   LLMNR on each interface.  LLMNR operates on a separate port from DNS,
   reducing the likelihood that a DNS server will unintentionally
   respond to an LLMNR query.

   If a DNS server is running on a host that supports LLMNR, the LLMNR
   responder on that host MUST respond to LLMNR queries only for the
   RRSets relating to the host on which the server is running, but MUST
   NOT respond for other records for which the DNS server is
   authoritative.  DNS servers MUST NOT send LLMNR queries in order to
   resolve DNS queries.



<span class="grey">Aboba, et al.                Informational                     [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  IANA Considerations</span>

   This specification creates a new namespace: the LLMNR namespace.

   In order to avoid creating any new administrative procedures,
   administration of the LLMNR namespace will piggyback on the
   administration of the DNS namespace.

   The rights to use a fully qualified domain name (FQDN) within LLMNR
   are obtained by acquiring the rights to use that name within DNS.
   Those wishing to use an FQDN within LLMNR should first acquire the
   rights to use the corresponding FQDN within DNS.  Using an FQDN
   within LLMNR without ownership of the corresponding name in DNS
   creates the possibility of conflict and therefore is discouraged.

   LLMNR responders may self-allocate a name within the single-label
   namespace first defined in [<a href="./rfc1001" title="&quot;Protocol standard for a NetBIOS service on a TCP/UDP transport: Concepts and methods&quot;">RFC1001</a>].  Since single-label names are
   not unique, no registration process is required.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Constants</span>

   The following timing constants are used in this protocol; they are
   not intended to be user configurable.

   JITTER_INTERVAL    100 ms
   LLMNR_TIMEOUT      1 second (if set statically on all interfaces)
                      100 ms (IEEE 802 media, including IEEE 802.11)
























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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  References</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Normative References</span>

   [<a id="ref-RFC1001">RFC1001</a>]      NetBIOS Working Group in the Defense Advanced Research
                  Projects Agency, Internet Activities Board, and End-
                  to-End Services Task Force, "Protocol standard for a
                  NetBIOS service on a TCP/UDP transport: Concepts and
                  methods", STD 19, <a href="./rfc1001">RFC 1001</a>, March 1987.

   [<a id="ref-RFC1035">RFC1035</a>]      Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, <a href="./rfc1035">RFC 1035</a>, November 1987.

   [<a id="ref-RFC2119">RFC2119</a>]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC2181">RFC2181</a>]      Elz, R. and R. Bush, "Clarifications to the DNS
                  Specification", <a href="./rfc2181">RFC 2181</a>, July 1997.

   [<a id="ref-RFC2308">RFC2308</a>]      Andrews, M., "Negative Caching of DNS Queries (DNS
                  NCACHE)", <a href="./rfc2308">RFC 2308</a>, March 1998.

   [<a id="ref-RFC2671">RFC2671</a>]      Vixie, P., "Extension Mechanisms for DNS (EDNS0)", <a href="./rfc2671">RFC</a>
                  <a href="./rfc2671">2671</a>, August 1999.

   [<a id="ref-RFC2845">RFC2845</a>]      Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
                  Wellington, "Secret Key Transaction Authentication for
                  DNS (TSIG)", <a href="./rfc2845">RFC 2845</a>, May 2000.

   [<a id="ref-RFC2931">RFC2931</a>]      Eastlake 3rd, D., "DNS Request and Transaction
                  Signatures ( SIG(0)s )", <a href="./rfc2931">RFC 2931</a>, September 2000.

   [<a id="ref-RFC4291">RFC4291</a>]      Hinden, R. and S. Deering, "IP Version 6 Addressing
                  Architecture", <a href="./rfc4291">RFC 4291</a>, February 2006.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Informative References</span>

   [<a id="ref-DNSPerf">DNSPerf</a>]      Jung, J., et al., "DNS Performance and the
                  Effectiveness of Caching", IEEE/ACM Transactions on
                  Networking, Volume 10, Number 5, pp. 589, October
                  2002.

   [<a id="ref-DNSDisc">DNSDisc</a>]      Durand, A., Hagino, I., and D. Thaler, "Well known
                  site local unicast addresses to communicate with
                  recursive DNS servers", Work in Progress, October
                  2002.





<span class="grey">Aboba, et al.                Informational                     [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   [<a id="ref-IEEE-802.11i">IEEE-802.11i</a>] Institute of Electrical and Electronics Engineers,
                  "Supplement to Standard for Telecommunications and
                  Information Exchange Between Systems - LAN/MAN
                  Specific Requirements - Part 11: Wireless LAN Medium
                  Access Control (MAC) and Physical Layer (PHY)
                  Specifications: Specification for Enhanced Security",
                  IEEE 802.11i, July 2004.

   [<a id="ref-LLMNREnable">LLMNREnable</a>]  Guttman, E., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22DHCP+LLMNR+Enable+Option%22'>"DHCP LLMNR Enable Option"</a>, Work in
                  Progress, April 2002.

   [<a id="ref-LLMNRSec">LLMNRSec</a>]     Jeong, J., Park, J. and H. Kim, "DNS Name Service
                  based on Secure Multicast DNS for IPv6 Mobile Ad Hoc
                  Networks", ICACT 2004, Phoenix Park, Korea, February
                  9-11, 2004.

   [<a id="ref-POSIX">POSIX</a>]        IEEE Std. 1003.1-2001 Standard for Information
                  Technology -- Portable Operating System Interface
                  (POSIX). Open Group Technical Standard: Base
                  Specifications, Issue 6, December 2001.  ISO/IEC
                  9945:2002.  <a href="http://www.opengroup.org/austin">http://www.opengroup.org/austin</a>

   [<a id="ref-RFC1536">RFC1536</a>]      Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
                  Miller, "Common DNS Implementation Errors and
                  Suggested Fixes", <a href="./rfc1536">RFC 1536</a>, October 1993.

   [<a id="ref-RFC2131">RFC2131</a>]      Droms, R., "Dynamic Host Configuration Protocol", <a href="./rfc2131">RFC</a>
                  <a href="./rfc2131">2131</a>, March 1997.

   [<a id="ref-RFC2365">RFC2365</a>]      Meyer, D., "Administratively Scoped IP Multicast", <a href="https://www.rfc-editor.org/bcp/bcp23">BCP</a>
                  <a href="https://www.rfc-editor.org/bcp/bcp23">23</a>, <a href="./rfc2365">RFC 2365</a>, July 1998.

   [<a id="ref-RFC2937">RFC2937</a>]      Smith, C., "The Name Service Search Option for DHCP",
                  <a href="./rfc2937">RFC 2937</a>, September 2000.

   [<a id="ref-RFC3315">RFC3315</a>]      Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
                  C., and M. Carney, "Dynamic Host Configuration
                  Protocol for IPv6 (DHCPv6)", <a href="./rfc3315">RFC 3315</a>, July 2003.

   [<a id="ref-RFC3493">RFC3493</a>]      Gilligan, R., Thomson, S., Bound, J., McCann, J., and
                  W. Stevens, "Basic Socket Interface Extensions for
                  IPv6", <a href="./rfc3493">RFC 3493</a>, February 2003.

   [<a id="ref-RFC3542">RFC3542</a>]      Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
                  "Advanced Sockets Application Program Interface (API)
                  for IPv6", <a href="./rfc3542">RFC 3542</a>, May 2003.





<span class="grey">Aboba, et al.                Informational                     [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


   [<a id="ref-RFC3833">RFC3833</a>]      Atkins, D. and R. Austein, "Threat Analysis of the
                  Domain Name System (DNS)", <a href="./rfc3833">RFC 3833</a>, August 2004.

   [<a id="ref-RFC3927">RFC3927</a>]      Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
                  Configuration of IPv4 Link-Local Addresses", <a href="./rfc3927">RFC 3927</a>,
                  May 2005.

   [<a id="ref-RFC4033">RFC4033</a>]      Arends, R., Austein, R., Larson, M., Massey, D., and
                  S. Rose, "DNS Security Introduction and Requirements",
                  <a href="./rfc4033">RFC 4033</a>, March 2005.

   [<a id="ref-RFC4086">RFC4086</a>]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                  "Randomness Requirements for Security", <a href="https://www.rfc-editor.org/bcp/bcp106">BCP 106</a>, <a href="./rfc4086">RFC</a>
                  <a href="./rfc4086">4086</a>, June 2005.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Acknowledgments</span>

   This work builds upon original work done on multicast DNS by Bill
   Manning and Bill Woodcock.  Bill Manning's work was funded under
   DARPA grant #F30602-99-1-0523.  The authors gratefully acknowledge
   their contribution to the current specification.  Constructive input
   has also been received from Mark Andrews, Rob Austein, Randy Bush,
   Stuart Cheshire, Ralph Droms, Robert Elz, James Gilroy, Olafur
   Gudmundsson, Andreas Gustafsson, Erik Guttman, Myron Hattig,
   Christian Huitema, Olaf Kolkman, Mika Liljeberg, Keith Moore,
   Tomohide Nagashima, Thomas Narten, Erik Nordmark, Markku Savela, Mike
   St. Johns, Sander van Valkenburg, and Brian Zill.
























<span class="grey">Aboba, et al.                Informational                     [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 706 6605
   EMail: bernarda@microsoft.com


   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 703 8835
   EMail: dthaler@microsoft.com


   Levon Esibov
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: levone@microsoft.com

























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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc4795">RFC 4795</a>                         LLMNR                      January 2007</span>


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
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Aboba, et al.                Informational                     [Page 31]
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