DNSEXT Working Group                                        Levon Esibov
INTERNET-DRAFT                                             Bernard Aboba
Category: Standards Track                                    Dave Thaler
<draft-ietf-dnsext-mdns-30.txt>                                Microsoft
20 January
17 March 2004

              Linklocal Multicast Name Resolution (LLMNR)

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

Copyright (C) The Internet Society (2004).  All Rights Reserved.


Today, with the rise of home networking, there are an increasing number
of ad-hoc networks operating without a Domain Name System (DNS) server.
In order to allow name resolution in such environments, Link-Local
Multicast Name Resolution (LLMNR) is proposed.  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.

The goal of LLMNR is to enable name resolution in scenarios in which
conventional DNS name resolution is not possible.  Since LLMNR only
operates on the local link, it cannot be considered a substitute for

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 .................................   10
   2.5       Off-link detection ..............................   11
   2.6       Responder responsibilities ......................   12
   2.7       Retransmission and jitter .......................   13   12
   2.8       DNS TTL .........................................   14   13
   2.9       Use of the authority and additional sections ....   14   13
3.     Usage model ...........................................   14
   3.1       LLMNR configuration .............................   15
4.     Conflict resolution ...................................   16
   4.1       Considerations for multiple interfaces ..........   18
   4.2       API issues ......................................   19
5.     Security considerations ...............................   20   19
   5.1       Scope restriction ...............................   20
   5.2       Usage restriction ...............................   21
   5.3       Cache and port separation .......................   22   21
   5.4       Authentication ..................................   22
6.     IANA considerations ...................................   22
7.     References ............................................   22
   7.1       Normative References ............................   22
   7.2       Informative References ..........................   23
Acknowledgments ..............................................   24
Authors' Addresses ...........................................   25
Intellectual Property Statement ..............................   25
Full Copyright Statement .....................................   26

1.  Introduction

This document discusses Link Local Multicast Name Resolution (LLMNR),
which utilizes 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.

The goal of LLMNR is to enable name resolution in scenarios in which
conventional DNS name resolution is not possible.  These include
scenarios in which hosts are not configured with the address of a DNS
server, where configured DNS servers do not reply to a query, or where
they respond with errors, as described in Section 2.  Since LLMNR only
operates on the local link, it cannot be considered a substitute for

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.  The assumption is that if
a network has a gateway, then 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 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 networking scenarios, or where no
DNS server is available that is authoritative for the names of local
hosts, and can support dynamic DNS, such as in wireless hotspots.

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.

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.  These words are often capitalized.  The key words
NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be

interpreted as described in [RFC2119].

1.2.  Terminology

This document assumes familiarity with DNS terminology defined in
[RFC1035].  Other terminology used in this document includes:

Positively Resolved
     Responses with RCODE set to zero are referred to in this document
     as "positively resolved".

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

     An address is considered reachable over a link if either an ARP or
     neighbor discovery cache entry exists for the address on the link.

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

     A host that sends an LLMNR query.

2.  Name resolution using LLMNR

LLMNR is a peer-to-peer name resolution protocol that is not intended as
a replacement for DNS.  LLMNR queries are sent to and received on port
TBD.  IPv4 administratively scoped multicast usage is specified in
"Administratively Scoped IP Multicast" [RFC2365].  The IPv4 link-scope
multicast address a given responder listens to, and to which a sender
sends queries, is TBD.  The IPv6 link-scope multicast address a given
responder listens to, and to which a sender sends all queries, is TBD.

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 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].

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.  Enabling LLMNR for use in situations where a
DNS server has been configured will result in a change in default
behavior without a simultaneous update to configuration information.

Where this is considered undesirable, LLMNR SHOULD NOT be enabled by
default, so that hosts will neither listen on the link-scope multicast
address, nor will they send queries to that address.

An LLMNR sender may send a request for any name.  However, by default,
LLMNR requests SHOULD be sent only when one of the following conditions
are met:

[1]  No manual or automatic DNS configuration has been performed.  If an
     interface has been configured with DNS server address(es), then
     LLMNR SHOULD NOT be used as the primary name resolution mechanism
     on that interface, although it MAY be used as a name resolution
     mechanism of last resort.

[2]  DNS servers do not respond.

[3]  DNS servers respond to a DNS query with RCODE=3 (Authoritative Name
     Error) or RCODE=0, and an empty answer section.

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

[a]  DNS servers are not configured or do not respond to a DNS query, or
     respond with RCODE=3, or RCODE=0 and an empty answer section.

[b]  An LLMNR sender sends an LLMNR query to the link-scope multicast
     address(es) defined in Section 2, unless a unicast query is
     indicated.  A sender SHOULD send LLMNR queries for PTR RRs via
     unicast, as specified in Section 2.4.

[c]  A responder responds to this query only if it is authoritative for
     the domain 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.

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

Further details of sender and responder behavior are provided in the
sections that follow.

2.1.  LLMNR packet format

LLMNR utilizes 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 permitted by the link MTU.

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  | Z|TC| Z| Z| Z| Z| Z|   RCODE   |
|                    QDCOUNT                    |
|                    ANCOUNT                    |
|                    NSCOUNT                    |
|                    ARCOUNT                    |


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

QR   A one bit field that specifies whether this message is an LLMNR
     query (0), or an LLMNR response (1).

     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.

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 an LLMNR response,
     then the sender MAY use the response if it contains all necessary
     information, or the sender MAY discard the response and resend the
     LLMNR query over TCP using the unicast address of the responder as
     the destination address.  See  [RFC2181] and Section 2.4 of this
     specification for further discussion of the TC bit.

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.

     Response code -- this 4 bit field is set as part of LLMNR
     responses.  In an LLMNR query, the RCODE MUST be zero, and is
     ignored by the responder.  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 preferrable to
     not sending a response, since it enables errors to be diagnosed.

     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.

     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

     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.

     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.

     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, SRV, etc.) to the link-scope multicast address.

As described in Section 2.4, a sender may also send a unicast query.
Sections 2 and 3 describe the circumstances in which LLMNR queries may
be sent.

The sender MUST anticipate receiving no replies to some LLMNR queries,
in the event that no responders are available within the link-scope or
in the event no positive non-null responses exist for the transmitted
query.  If no positive response is received, a resolver treats it as a
response that no records of the specified type and class exist for the
specified name (it is treated the same as a response with RCODE=0 and an
empty answer section).

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 responder
has another RR as well;  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 and
IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6 might be authoritative for the
following records:

host1. IN A

IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6

host1.example.com. IN A
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6 IN PTR host1.
IN PTR host1.example.com.
IN PTR host1.
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 TBD on the link-scope multicast
     address(es) defined in Section 2, and on UDP and TCP port TBD 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 SHOULD 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.

[d]  Responders MUST NOT respond to LLMNR queries for names they are not
     authoritative for.

[e]  Responders MUST NOT respond using cached data.

[f]  If a DNS server is running on a host that supports LLMNR, the DNS
     server 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 server is authoritative.  DNS
     servers also MUST NOT send LLMNR queries in order to resolve DNS

[g]  If a responder is authoritative for a name, it MAY 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

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 reply
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.".

In this example (unless this limitation is introduced) 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.  In this example a host
"child.foo.example.com." would send a dynamic update for the NS and glue
A record to "foo.example.com.", but 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.

A responder receiving a unicast query MUST send the response with a
source address set to the destination address field of the IP header of
the query causing the response.

Unicast LLMNR queries SHOULD MUST be sent using TCP.  Senders MUST support
sending TCP queries, and responders MUST support listening for TCP

Responses to TCP unicast LLMNR queries MUST be sent using TCP,  using
the same connection as the query.  If the sender of a TCP query receives
a response to that query not using TCP, the response MUST be silently

Unicast UDP queries MAY MUST be responded to with a UDP response containing
an empty answer section and the TC bit set, so as to require the sender
to resend the query using TCP. silently discarded.

If an ICMP "Time Exceeded" message is received in response to a unicast
UDP query, or if TCP connection setup cannot be completed in order to send a unicast
TCP query, this is treated as a response that no records of the
specified type and class exist for the specified name (it is treated the
same as a response with RCODE=0 and an empty answer section).  The UDP sender receiving an ICMP "Time Exceeded" message
SHOULD verify that the ICMP error payload contains a valid LLMNR query
packet, which matches a query that is currently in progress, so as to
guard against a potential Denial of Service (DoS) attack.  If a match
cannot be made, then the sender relies on the retransmission and timeout
behavior described in Section 2.7.

2.5.  "Off link" detection

For IPv4, an "on link" address is defined as a link-local address
[IPv4Link] or an address whose prefix belongs to a subnet on the local
link.  For IPv6 [RFC2460] an "on link" address is either a link-local
address, defined in [RFC2373], or an address whose prefix belongs to a
subnet on the local link.

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

A responder MUST select a source address for responses that is "on
link". The destination address of an LLMNR response MUST be an "on link"
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

In composing LLMNR queries, the sender MUST set the Hop Limit field in
the IPv6 header and the TTL field in IPv4 header

Section 2.4 discusses use of the response to one
(1).  Even when TCP for LLMNR queries are sent to a link-scope multicast
address, 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.  Therefore setting the IPv6 Hop Limit
or IPv4 TTL field to one provides an additional precaution against
leakage of LLMNR queries. and responses.  In
composing a response to an LLMNR query, query using TCP, the responder 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).  This is done so as to prevent the use of LLMNR
for denial of service attacks across the Internet.

Section 2.4 discusses use of TCP for LLMNR queries and responses.  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
Apple Rendezvous.

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.  In

[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.

Routable addresses MUST be included first in the response, if available.
This encourages use of routable address(es) for establishment of new

2.7.  Retransmission and jitter

An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine
when to retransmit an LLMNR query and how long to collect responses to
an LLMNR query.

If an LLMNR query sent over UDP is not resolved within LLMNR_TIMEOUT,
then a sender MAY repeat the transmission of the query in order to
assure that it was received by a host capable of responding to it.
Retransmission of UDP queries SHOULD NOT be attempted more than 3 times.
Where LLMNR queries are sent using TCP, retransmission is handled by the

transport layer.

Because an LLMNR sender cannot know in advance if a query sent using
multicast will receive no response, one response, or more than one
response, the sender SHOULD wait for LLMNR_TIMEOUT in order to collect
all possible responses, rather than considering the multicast query
answered after the first response is received. A unicast query sender
considers the query answered after the first response is received, so
that it only waits for LLMNR_TIMEOUT if no response has been received.

An LLMNR sender SHOULD dynamically compute the value of LLMNR_TIMEOUT
for each transmission. It is suggested that the computation of
LLMNR_TIMEOUT be based on the response times for earlier LLMNR queries
sent on the same interface.

For example, the algorithms described in RFC 2988 [RFC2988] (including
exponential backoff) to compute an RTO, which is used as the value of
LLMNR_TIMEOUT.  Smaller values MAY be used for the initial RTO
(discussed in Section 2 of [RFC2988], paragraph 2.1), the minimum RTO
(discussed in Section 2 of [RFC2988], paragraph 2.4), and the maximum
RTO (discussed in Section 2 of [RFC2988], paragraph 2.5).

Recommended values are an initial RTO of 1 second, a minimum RTO of
200ms, and a maximum RTO of 5 seconds.  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 100
ms.  This delay MAY be avoided by responders responding with RRs which
they have previously determined to be UNIQUE (see Section 4 for

2.8.  DNS TTL

The responder should use a pre-configured TTL value in the records
returned 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


Responders SHOULD insert an SOA record into the authority section of a
negative response, to facilitate negative caching as specified in
[RFC2308].  The owner name of this SOA record MUST be equal to the query

Responders SHOULD NOT perform DNS additional section processing, except
as required for EDNS0 and DNSSEC.

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

Since LLMNR is a secondary name resolution mechanism, its usage is in
part determined by the behavior of DNS implementations.  This document
does not specify any changes to DNS resolver behavior, such as
searchlist processing or retransmission/failover policy.  However,
robust DNS resolver implementations are more likely to avoid unnecessary
LLMNR queries.

As noted in [DNSPerf], even when DNS servers are configured, 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.  This has the potential to
result in a large number of 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

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

3.1.  LLMNR configuration

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 linklocal 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

For example, if the configured DNS server responds to AAAA RR queries
sent over IPv4 or IPv6 with an authoritative name error (RCODE=3), then
it will not be possible to resolve the names of IPv6-only hosts.  In
this situation, LLMNR over IPv6 can be used for local name resolution.

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 linklocal
name resolution over IPv4.

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.

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.

Unless unconfigured hosts periodically retry configuration, an outage in
the DNS configuration mechanism will result in hosts continuing to use
LLMNR even once the outage is repaired.  Since LLMNR only enables
linklocal name resolution, this represents an unnecessary degradation in
capabilities.  As a result, it is recommended that hosts without a
configured DNS server periodically attempt to obtain DNS configuration.
For example, where DHCP is used for DNS configuration, [RFC2131]
recommends a maximum retry interval of 64 seconds.  In the absence of
other guidance, a default retry interval of one (1) minute is

4.  Conflict resolution

The sender MUST anticipate receiving multiple replies to the same LLMNR
query, in the event that several LLMNR enabled computers receive the
query and respond with valid answers.  When this occurs, the responses
may first be concatenated, and then treated in the same manner that
multiple RRs received from the same DNS server would; the sender
perceives no inherent conflict in the receipt of multiple responses.

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.  Resource records for which the latter queries are
submitted are referred as UNIQUE throughout this document.  The
uniqueness of a resource record depends on a nature of the name in the
query and type of the query.  For example it is expected that:

   - multiple hosts may respond to a query for an SRV type record
   - multiple hosts may respond to a query for an A or AAAA type
     record for a cluster name (assigned to multiple hosts in
     the cluster)
   - only a single host may respond to a query for an A or AAAA
     type record for a name.

Every responder that responds to an LLMNR query AND includes a UNIQUE
record in the response:

[1]  MUST verify that there is no other host within the scope of the
     LLMNR query propagation that can return a resource record for the
     same name, type and class.

[2]  MUST NOT include a UNIQUE resource record in the response without
     having verified its uniqueness.

Where a host is configured to issue LLMNR queries on more than one
interface, each interface should have its own independent LLMNR cache.
For each UNIQUE resource record in a given interface's configuration,
the host MUST verify resource record uniqueness on that interface.  To
accomplish this, the host MUST send an LLMNR query for each UNIQUE
resource record.

By default, a host SHOULD be configured to behave as though all RRs are
UNIQUE.  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 the LLMNR queries on an interface
    enabled for transmission and reception of IP traffic
  - is configured to respond to the LLMNR queries using additional
    UNIQUE resource records
  - detects that an interface is connected and is usable
    (e.g. an IEEE 802 hardware link-state change indicating
    that a cable was attached or that an association has occurred completion of authentication
    (and if needed, association) with a wireless base station and that any required authentication
    has completed)
    or adhoc network

When a host that has a UNIQUE record receives an LLMNR query for that
record, the host MUST respond.  After the client receives a response, it
MUST check whether the response arrived on an interface different from
the one on which the query was sent.  If the response arrives on a
different interface, the client can use the UNIQUE resource record in
response to LLMNR queries.  If not, then it MUST NOT use the UNIQUE
resource record in response to LLMNR queries.

The name conflict detection mechanism doesn't prevent name conflicts
when previously partitioned segments are connected by a bridge. In order
to minimize the chance of conflicts in such a situation, it is
recommended that steps be taken to ensure name uniqueness. For example,
the name could be chosen randomly from a large pool of potential names,
or the name could be assigned via a process designed to guarantee

When name conflicts are detected, they SHOULD be logged.  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.  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.

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.

Since names are only unique per-link, hosts on different links could be
using the same name.  If an LLMNR client sends requests over multiple
interfaces, and receives replies 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.

            |   |
        -----   -----
            |   |

   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 unicast 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.

4.2.  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 requests 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 by nature a peer-to-peer name resolution protocol. It is
therefore inherently more vulnerable than DNS, since existing DNS
security mechanisms are difficult to apply to LLMNR. While tools exist
to alllow an attacker to spoof a response to a DNS query, spoofing a
response to an LLMNR query is easier since the query is sent to a link-
scope multicast address, where every host on the logical link will be
made aware of it.

In order to address the security vulnerabilities, the following
mechanisms are contemplated:

[1]  Scope restrictions.

[2]  Usage restrictions.

[3]  Cache and port separation.

[4]  Authentication.

These techniques are described in the following sections.

5.1.  Scope restriction

With LLMNR it is possible that hosts will allocate conflicting names for
a period of time, or that attackers will attempt to deny service to
other hosts by allocating the same name. Such attacks also allow hosts
to receive packets destined for other hosts.

Since LLMNR is typically deployed in situations where no trust model can
be assumed, it is likely that LLMNR queries and responses will be
unauthenticated.  In the absence of authentication, LLMNR reduces the
exposure to such threats by utilizing UDP queries sent to a link-scope
multicast address, as well as setting the TTL (IPv4) or Hop Limit (IPv6)
fields to one (1) on both TCP queries and responses.

A TTL of one (1) was chosen so as to limit the likelihood that LLMNR can
be used to launch denial of service attacks. For example, were the TTL
of an LLMNR Response to be set to a value larger than one (1), an
attacker could send a large volume of queries from a spoofed source
address, causing an off-link target to be deluged with responses.

Utilizing a TTL of one (1) in LLMNR responses ensures that they will not
be forwarded off-link.

Using a TTL of one (1) to set up a TCP connection in order to send a
unicast LLMNR query reduces the likelihood of both denial of service
attacks and spoofed responses.  Checking that an LLMNR query is sent to
a link-scope multicast address should prevent spoofing of multicast
queries by off-link attackers.

While this limits the ability of off-link attackers to spoof LLMNR
queries and responses, it does not eliminate it. For example, it is
possible for an attacker to spoof a response to a frequent 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.

When LLMNR queries are sent to a link-scope multicast address, 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.

Setting the IPv6 Hop Limit or IPv4 TTL field to a value larger than one
in an LLMNR UDP response may enable denial of service attacks across the
Internet.  However, since LLMNR responders only respond to queries for

which they are authoritative, and LLMNR does not provide wildcard query
support, it is believed that this threat is minimal.

There also are scenarios such as public "hotspots" where attackers can
be present on the same link.  These threats are most serious in wireless
networks such as 802.11, since attackers on a wired network will require
physical access to the home network, while wireless attackers may reside
outside the home.  Link-layer security can be of assistance against
these threats if it is available.

5.2.  Usage restriction

As noted in Sections 2 and 3, LLMNR is intended for usage in a limited
set of scenarios.

If an LLMNR query is sent whenever a DNS server does not respond in a
timely way, then an attacker can poison the LLMNR cache by responding to
the query with incorrect information.  To some extent, these
vulnerabilities exist today, since DNS response spoofing tools are
available that can allow an attacker to respond to a query more quickly
than a distant DNS server.

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

The vulnerability is more serious if LLMNR is given higher priority than
DNS among the enabled name resolution mechanisms. In such a
configuration, a denial of service attack on the DNS server would not be
necessary in order to poison the LLMNR cache, since LLMNR queries would
be sent even when the DNS server is available. In addition, the LLMNR
cache, once poisoned, would take precedence over the DNS cache,
eliminating the benefits of cache separation. As a result, LLMNR is only
used as a name resolution mechanism of last resort.

5.3.  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. The use of separate caches is most effective
when LLMNR is used as a name resolution mechanism of last resort, since
this minimizes the opportunities for poisoning the LLMNR cache, and
decreases reliance on it.

LLMNR operates on a separate port from DNS, reducing the likelihood that

a DNS server will unintentionally respond to an LLMNR query.

5.4.  Authentication

LLMNR implementations may not support DNSSEC or TSIG, and as a result,
responses to LLMNR queries may be unauthenticated.  If authentication is
desired, and a pre-arranged security configuration is possible, then
IPsec ESP with a null-transform MAY be used to authenticate LLMNR
responses.  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.

6.  IANA Considerations

This specification creates one new name space:  the reserved bits in the
LLMNR header.  These are allocated by IETF Consensus, in accordance with
BCP 26 [RFC2434].

LLMNR requires allocation of a port TBD for both TCP and UDP.
Assignment of the same port for both transports is requested.

LLMNR requires allocation of a link-scope multicast IPv4 address TBD.
LLMNR also requires allocation of a link-scope multicast IPv6 address

7.  References

7.1.  Normative References

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

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
          April 1992.

[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.

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

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
          (IPv6) Specification", RFC 2460, December 1998.

[RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC
          2535, March 1999.

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

[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
          Timer", RFC 2988, November 2000.

7.2.  Informative References

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

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

[RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
          Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
          April 1997.

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

[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.

          Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
          of IPv4 Link-Local Addresses", Internet draft (work in
          progress), draft-ietf-zeroconf-ipv4-linklocal-10.txt, October
          2003. draft-ietf-zeroconf-ipv4-linklocal-14.txt, April

[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

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

          Crawford, M., "IPv6 Node Information Queries", Internet draft
          (work in progress), draft-ietf-ipn-gwg-icmp-name-
          lookups-09.txt, May 2002.


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, Stuart Cheshire, Randy Bush, Robert
Elz, Rob Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron
Hattig, Thomas Narten, Christian Huitema, Erik Nordmark, Sander Van-
Valkenburg, Tomohide Nagashima, Brian Zill, Keith Moore and Markku

Authors' Addresses

Levon Esibov
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: levone@microsoft.com

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

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