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

                  Link-Local Multicast Name Resolution (LLMNR)

    Status of This Memo

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

    Copyright Notice

       Copyright (C) The IETF Trust (2007).

    IESG Note

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

    Abstract

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

    Aboba, et al.                Informational                      [Page 1]

    RFC 4795                         LLMNR                      January 2007

    Table of Contents

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

    Aboba, et al.                Informational                      [Page 2]

    RFC 4795                         LLMNR                      January 2007

    1.  Introduction

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

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

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

       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 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].

    Aboba, et al.                Informational                      [Page 3]

    RFC 4795                         LLMNR                      January 2007

    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
                        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.

    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].

    Aboba, et al.                Informational                      [Page 4]

    RFC 4795                         LLMNR                      January 2007

       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 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 Cyclic Redundancy Check (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                    |
          +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

    Aboba, et al.                Informational                      [Page 5]

    RFC 4795                         LLMNR                      January 2007

       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 [RFC4086].

       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 Section 4.2.

       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  [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

    Aboba, et al.                Informational                      [Page 6]

    RFC 4795                         LLMNR                      January 2007

               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.

       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 [RFC2845] 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 [RFC2671] and
               extended RCODE values.

    Aboba, et al.                Informational                      [Page 7]

    RFC 4795                         LLMNR                      January 2007

       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 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) 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.

    Aboba, et al.                Informational                      [Page 8]

    RFC 4795                         LLMNR                      January 2007

    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
       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 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.

    Aboba, et al.                Informational                      [Page 9]

    RFC 4795                         LLMNR                      January 2007

       (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

    Aboba, et al.                Informational                     [Page 10]

    RFC 4795                         LLMNR                      January 2007

       "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.

    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 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.

    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.

    Aboba, et al.                Informational                     [Page 11]

    RFC 4795                         LLMNR                      January 2007

       On receiving an LLMNR query, the responder MUST check whether it was
       sent to an 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
       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().  [RFC3542] 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 [RFC4291].  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.

    Aboba, et al.                Informational                     [Page 12]

    RFC 4795                         LLMNR                      January 2007

       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.

    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
       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.

    Aboba, et al.                Informational                     [Page 13]

    RFC 4795                         LLMNR                      January 2007

       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 Section 4 for details).

    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.

    Aboba, et al.                Informational                     [Page 14]

    RFC 4795                         LLMNR                      January 2007

       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.

    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:

    Aboba, et al.                Informational                     [Page 15]

    RFC 4795                         LLMNR                      January 2007

       (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.

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

    Aboba, et al.                Informational                     [Page 16]

    RFC 4795                         LLMNR                      January 2007

    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
       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.

    Aboba, et al.                Informational                     [Page 17]

    RFC 4795                         LLMNR                      January 2007

       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.

    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).

    Aboba, et al.                Informational                     [Page 18]

    RFC 4795                         LLMNR                      January 2007

       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.

    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)

         - 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.

    Aboba, et al.                Informational                     [Page 19]

    RFC 4795                         LLMNR                      January 2007

       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 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.

    Aboba, et al.                Informational                     [Page 20]

    RFC 4795                         LLMNR                      January 2007

       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.

    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.

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

    Aboba, et al.                Informational                     [Page 21]

    RFC 4795                         LLMNR                      January 2007

       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.

    4.4.  API Issues

       [RFC3493] 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

    Aboba, et al.                Informational                     [Page 22]

    RFC 4795                         LLMNR                      January 2007

       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
       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.

    Aboba, et al.                Informational                     [Page 23]

    RFC 4795                         LLMNR                      January 2007

    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
       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).

    Aboba, et al.                Informational                     [Page 24]

    RFC 4795                         LLMNR                      January 2007

    5.3.  Authentication

       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 [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 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 [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
       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.

    Aboba, et al.                Informational                     [Page 25]

    RFC 4795                         LLMNR                      January 2007

    6.  IANA Considerations

       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 [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)

    Aboba, et al.                Informational                     [Page 26]

    RFC 4795                         LLMNR                      January 2007

    8.  References

    8.1.  Normative References

       [RFC1001]      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, RFC 1001, March 1987.

       [RFC1035]      Mockapetris, P., "Domain names - implementation and
                      specification", STD 13, RFC 1035, November 1987.

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

       [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.

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

       [RFC2845]      Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
                      Wellington, "Secret Key Transaction Authentication for
                      DNS (TSIG)", RFC 2845, May 2000.

       [RFC2931]      Eastlake 3rd, D., "DNS Request and Transaction
                      Signatures ( SIG(0)s )", RFC 2931, September 2000.

       [RFC4291]      Hinden, R. and S. Deering, "IP Version 6 Addressing
                      Architecture", RFC 4291, February 2006.

    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", Work in Progress, October
                      2002.

    Aboba, et al.                Informational                     [Page 27]

    RFC 4795                         LLMNR                      January 2007

       [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", Work in
                      Progress, April 2002.

       [LLMNRSec]     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.

       [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., Postel, J., Neuman, C., Danzig, P., and S.
                      Miller, "Common DNS Implementation Errors and
                      Suggested Fixes", RFC 1536, October 1993.

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

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

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

       [RFC3315]      Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
                      C., and M. Carney, "Dynamic Host Configuration
                      Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

       [RFC3493]      Gilligan, R., Thomson, S., Bound, J., McCann, J., and
                      W. Stevens, "Basic Socket Interface Extensions for
                      IPv6", RFC 3493, February 2003.

       [RFC3542]      Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
                      "Advanced Sockets Application Program Interface (API)
                      for IPv6", RFC 3542, May 2003.

    Aboba, et al.                Informational                     [Page 28]

    RFC 4795                         LLMNR                      January 2007

       [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 IPv4 Link-Local Addresses", RFC 3927,
                      May 2005.

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

       [RFC4086]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                      "Randomness Requirements for Security", BCP 106, RFC
                      4086, June 2005.

    9.  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.

    Aboba, et al.                Informational                     [Page 29]

    RFC 4795                         LLMNR                      January 2007

    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

    Aboba, et al.                Informational                     [Page 30]

    RFC 4795                         LLMNR                      January 2007

    Full Copyright Statement

       Copyright (C) The IETF Trust (2007).

       This document is subject to the rights, licenses and restrictions
       contained in BCP 78, and except as set forth therein, the authors
       retain all their rights.

       This document and the information contained herein are provided on an
       "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
       OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
       THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
       OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
       THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
       WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Intellectual Property

       The IETF takes no position regarding the validity or scope of any
       Intellectual Property Rights or other rights that might be claimed to
       pertain to the implementation or use of the technology described in
       this document or the extent to which any license under such rights
       might or might not be available; nor does it represent that it has
       made any independent effort to identify any such rights.  Information
       on the procedures with respect to rights in RFC documents can be
       found in BCP 78 and BCP 79.

       Copies of IPR disclosures made to the IETF Secretariat and any
       assurances of licenses to be made available, or the result of an
       attempt made to obtain a general license or permission for the use of
       such proprietary rights by implementers or users of this
       specification can be obtained from the IETF on-line IPR repository at
       http://www.ietf.org/ipr.

       The IETF invites any interested party to bring to its attention any
       copyrights, patents or patent applications, or other proprietary
       rights that may cover technology that may be required to implement
       this standard.  Please address the information to the IETF at
       ietf-ipr@ietf.org.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

    Aboba, et al.                Informational                     [Page 31]