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DNSEXT Working Group                                       Bernard Aboba
INTERNET-DRAFT                                               Dave Thaler
Category: Standards Track                                   Levon Esibov
<draft-ietf-dnsext-mdns-47.txt>                    Microsoft Corporation
13 August 2006

              Link-local Multicast Name Resolution (LLMNR)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 15, 2007.

Copyright Notice

   Copyright (C) The Internet Society 2006.

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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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
   2.2       Sender Behavior .................................    8
   2.3       Responder Behavior ..............................    8
   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 .............................   16
4.     Conflict Resolution ...................................   18
   4.1       Uniqueness Verification .........................   18
   4.2       Conflict Detection and Defense ..................   19
   4.3       Considerations for Multiple Interfaces ..........   20
   4.4       API issues ......................................   22
5.     Security Considerations ...............................   22
   5.1       Denial of Service ...............................   22
   5.2       Spoofing ...............,........................   23
   5.3       Authentication ..................................   24
   5.4       Cache and Port Separation .......................   24
6.     IANA considerations ...................................   25
7.     Constants .............................................   25
8.     References ............................................   25
   8.1       Normative References ............................   25
   8.2       Informative References ..........................   26
Acknowledgments ..............................................   28
Authors' Addresses ...........................................   28
Intellectual Property Statement ..............................   29
Disclaimer of Validity .......................................   29
Copyright Statement ..........................................   29














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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.

   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" [RFC2365].

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

1.1.  Requirements

   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
   [RFC2119].






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1.2.  Terminology

   This document assumes familiarity with DNS terminology defined in
   [RFC1035].  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 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.

2.  Name Resolution Using LLMNR

   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
   Section 4.  This document does not specify how names are chosen or
   configured.  This may occur via any mechanism, including DHCPv4
   [RFC2131] or DHCPv6 [RFC3315].

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

   [a]  An LLMNR sender sends an LLMNR query to the link-scope



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        multicast address(es), unless a unicast query is indicated,
        as specified in Section 2.4.

   [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 Section 2.4.

   [c]  Upon reception of the response, the sender processes it.

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

2.1.  LLMNR Packet Format

   LLMNR is based on the DNS packet format defined in [RFC1035] Section
   4 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 CRC).

2.1.1.  LLMNR Header Format

   LLMNR queries and responses utilize the DNS header format defined in
   [RFC1035] 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                    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   where:





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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 [RFC1750].

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

OPCODE
     A four 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 Section 4.2.

TC   TrunCation - 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  [RFC2181] and
     Section 2.4 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 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 Section 4.1.




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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,
     implementors 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 [RFC2845] include a TSIG RR in an
     unacceptable position (RCODE=1) or a TSIG RR which 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 [RFC2671] and extended
     RCODE values.

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



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     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 Section 2.9.  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 Section 2.9.

2.2.  Sender Behavior

   A sender MAY send an LLMNR query for any legal resource record  type
   (e.g., A, AAAA, PTR, SRV, etc.) to the link-scope multicast address.
   As described in Section 2.4, 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.

2.3.  Responder Behavior

   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



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   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, and 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 Section 2, 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.

[c]  Responders MUST respond to LLMNR queries for names and addresses
     they are authoritative for.  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 they are not
     authoritative for.



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[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 a 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 A
   RR(s) that contain IP address(es) in the RDATA of the resource
   record.  If the responder has a 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
   "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.







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2.4.  Unicast Queries and Responses

   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 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.

2.5.  "Off link" Detection

   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.

   On receiving an LLMNR query, the responder MUST check whether it was
   sent to a LLMNR multicast addresses defined in Section 2.  If it was
   sent to another multicast address, then the query MUST be silently
   discarded.

   Section 2.4 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



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   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 [RFC3927].

   Implementation note:

      In the sockets API for IPv4 [POSIX], 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().  [RFC2292] specifies similar
      options for setting and retrieving the IPv6 Hop Limit.

2.6.  Responder Responsibilities

   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 Section 4.
   IPv4 Link-Local addresses are defined in [RFC3927].  IPv6 Link-Local
   addresses are defined in [RFC2373].  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.

   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,



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       then the responder MUST include a routable address first
       in the response, if available.

2.7.  Retransmission and Jitter

   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 Section
   7.

   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
   assure 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.

   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 delayed by a time randomly selected from
   the interval 0 to JITTER_INTERVAL.  This delay MAY be avoided by
   responders responding with names which they have previously
   determined to be UNIQUE (see Section 4 for details).




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2.8.  RR TTL

   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.

2.9.  Use of the Authority and Additional Sections

   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
   [RFC2308]. 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.

   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.






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3.  Usage Model

   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 Section 5.3) 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 Section 5.3, 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:

   [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.



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   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.

   [RFC1536] describes common DNS implementation errors and fixes.  If
   the proposed fixes are implemented, unnecessary LLMNR queries will be
   reduced substantially, and so implementation of [RFC1536] is
   recommended.

   For example, [RFC1536] Section 1 describes issues with retransmission
   and recommends implementation of a retransmission policy based on
   round trip estimates, with exponential back-off.  [RFC1536] Section 4
   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.

   [RFC1536] Section 3 describes zero answer bugs, which if addressed
   will also reduce unnecessary LLMNR queries.

   [RFC1536] Section 6 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 [DNSPerf] 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.

3.1.  LLMNR Configuration

   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 [LLMNREnable], 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
   [RFC2937], using the LLMNR Enable Option code carried in the NSSO



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   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 [RFC3315] and [DNSDisc]) 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 [RFC3315],
   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.

   For example, if the configured DNS server responds to a AAAA RR query
   sent over IPv4 or IPv6 with an authoritative name error (RCODE=3) or
   RCODE=0 and an empty answer section, then a 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.




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   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.

4.  Conflict Resolution

   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).

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

4.1.  Uniqueness Verification

   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)



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     - 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 Section 2.7.  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.

   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.

4.2.  Conflict Detection and Defense

   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 which 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



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   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.

   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 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.

4.3.  Considerations for Multiple Interfaces

   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 Section 4.  The situation is illustrated in Figure 1.




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        ----------  ----------
         |      |    |      |
        [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.

   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]    [myhost]   [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]
               |   |
           -----   -----
               |   |
              [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.



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4.4.  API Issues

   [RFC2553] provides an API which 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
   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.

5.  Security Considerations

   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 [IEEE-802.11i]
   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.

5.1.  Denial of Service

   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



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   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.

5.2.  Spoofing

   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)" [RFC3833] 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



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   response will never be received.

   This vulnerability can be reduced by limiting use of LLMNR to
   resolution of single-label names as described in Section 3, or by
   implementation of authentication (see Section 5.3).

5.3.  Authentication

   LLMNR is a peer-to-peer name resolution protocol, and as a result,
   it 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 [RFC2845] and/or SIG(0)
     [RFC2931] security mechanisms.  "DNS Name Service based on Secure
     Multicast DNS for IPv6 Mobile Ad Hoc Networks" [LLMNRSec] 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 ESP with 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 [RFC4033].  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.

5.4.  Cache and Port Separation

   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



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   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.

6.  IANA Considerations

   This specification creates a new name space: the LLMNR namespace.

   In order to 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 a FQDN within LLMNR should first acquire the
   rights to use the corresponding FQDN within DNS.  Using a 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
   name space, first defined in [RFC1001].  Since single-label names are
   not unique, no registration process is required.

7.  Constants

   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)

8.  References

8.1.  Normative References

[RFC1001] Auerbach, K. and A. Aggarwal, "Protocol Standard for a NetBIOS
          Service on a TCP/UDP Transport: Concepts and Methods", RFC
          1001, March 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and
          Specification", RFC 1035, November 1987.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.




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[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
          Specification", RFC 2181, July 1997.

[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
          RFC 2308, March 1998.

[RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing
          Architecture", RFC 2373, July 1998.

[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
          Considerations Section in RFCs", BCP 26, RFC 2434, October
          1998.

[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
          August 1999.

[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
          "Secret Key Transaction Authentication for DNS (TSIG)", RFC
          2845, May 2000.

[RFC2931] Eastlake, D., "DNS Request and Transaction Signatures
          (SIG(0)s)", RFC 2931, September 2000.

8.2.  Informative References

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

[DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site local
          unicast addresses to communicate with recursive DNS servers",
          Internet draft (work in progress), draft-ietf-ipv6-dns-
          discovery-07.txt, October 2002.

[IEEE-802.11i]
          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.

[LLMNREnable]
          Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work
          in progress), draft-guttman-mdns-enable-02.txt, April 2002.

[LLMNRSec]
          Jeong, J., Park, J. and H. Kim, "DNS Name Service based on



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          Secure Multicast DNS for IPv6 Mobile Ad Hoc Networks", ICACT
          2004, Phoenix Park, Korea, February 9-11, 2004.

[POSIX]   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.  http://www.opengroup.org/austin

[RFC1536] Kumar, A., et. al., "DNS Implementation Errors and Suggested
          Fixes", RFC 1536, October 1993.

[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
          Recommendations for Security", RFC 1750, December 1994.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
          March 1997.

[RFC2292] Stevens, W. and M. Thomas, "Advanced Sockets API for IPv6",
          RFC 2292, February 1998.

[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
          2365, July 1998.

[RFC2553] Gilligan, R., Thomson, S., Bound, J. and W. Stevens, "Basic
          Socket Interface Extensions for IPv6", RFC 2553, March 1999.

[RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC
          2937, September 2000.

[RFC3315] Droms, R., et al., "Dynamic Host Configuration Protocol for
          IPv6 (DHCPv6)", RFC 3315, July 2003.

[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name
          System (DNS)", RFC 3833, August 2004.

[RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
          of Link-Local IPv4 Addresses", RFC 3927, October 2004.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
          "DNS Security Introduction and Requirement", RFC 4033, March
          2005.










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Acknowledgments

   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.

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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