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DNSEXT Working Group                                        Levon Esibov
INTERNET-DRAFT                                             Bernard Aboba
Category: Standards Track                                    Dave Thaler
<draft-ietf-dnsext-mdns-24.txt>                                Microsoft
27 September 2003

              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 (2003).  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.

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Table of Contents

1.     Introduction ..........................................    3
   1.1       Requirements ....................................    3
   1.2       Terminology .....................................    4
2.     Name resolution using LLMNR ...........................    4
   2.1       Sender behavior .................................    4
   2.2       Responder behavior ..............................    5
   2.3       Unicast queries .................................    6
   2.4       Addressing ......................................    7
   2.5       Off-link detection ..............................    7
   2.6       Retransmissions .................................    8
   2.7       DNS TTL .........................................    9
   2.8       Use of the authority and additional sections ....    9
3.     Usage model ...........................................    9
   3.1       Unqualified names ...............................   10
   3.2       LLMNR configuration .............................   11
4.     Conflict resolution ...................................   12
   4.1       Considerations for multiple interfaces ..........   13
   4.2       API issues ......................................   15
5.     Security considerations ...............................   15
   5.1       Scope restriction ...............................   16
   5.2       Usage restriction ...............................   17
   5.3       Cache and port separation .......................   17
   5.4       Authentication ..................................   18
6.     IANA considerations ...................................   18
7.     References ............................................   18
   7.1       Normative References ............................   18
   7.2       Informative References ..........................   19
Acknowledgments ..............................................   20
Authors' Addresses ...........................................   20
Intellectual Property Statement ..............................   21
Full Copyright Statement .....................................   21

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1.  Introduction

This document discusses Link Local Multicast Name Resolution (LLMNR),
which operates on a separate port from the Domain Name System (DNS),
with a distinct resolver cache, but does not change the format of DNS
packets.  LLMNR supports all current and future DNS formats, types and
classes.  However, since LLMNR only operates on the local link, it
cannot be considered a substitute for DNS.

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 3.

LLMNR queries are sent to and received on port TBD.  Link-scope
multicast addresses are used to prevent propagation of LLMNR traffic
across routers, potentially flooding the network; for details, see
Section 2.4.  LLMNR queries can also be sent to a unicast address, as
described in Section 2.3.

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 3.2.

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

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NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119].

1.2.  Terminology

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.  Typically a host is
          configured as both a sender and a responder.  However, a host
          may be configured as a sender, but not a responder or as a
          responder, but not a sender.

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

2.  Name resolution using LLMNR

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

[1] A sender needs to resolve a query for a name "host.example.com",
    so it sends an LLMNR query to the link-scope multicast address(es)
    defined in Section 2.4.

[2] A responder responds to this query only if it is authoritative
    for the domain name "host.example.com".  The responder sends
    a response to the sender via unicast over UDP.

[3] Upon the reception of the response, the sender performs the checks
    described in Section 2.5.  If these conditions are met, then the
    sender uses and caches the returned response.  If not, then the
    sender ignores the response and continues waiting for the response.

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

2.1.  Sender behavior

A sender sends an LLMNR query for any legal resource record  type (e.g.
A/AAAA, SRV, PTR, etc.) to the link-scope multicast address.  As
described in Section 2.3, a sender may also send a unicast query.
Section 3 describes the circumstances in which LLMNR queries may be

The RD (Recursion Desired) bit MUST NOT be set in a query.  If a
responder receives a query with the header containing RD set bit, the

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responder MUST ignore the RD bit.

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).

2.2.  Responder behavior

A responder MUST listen on UDP port TBD on the link-scope multicast
address(es) defined in Section 2.4 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.  A host configured as a responder
MUST act as a sender to verify the uniqueness of names as described in
Section 4.

Responders MUST NOT respond to LLMNR queries for names they are not
authoritative for.  Responders SHOULD respond to LLMNR queries for names
and addresses they are authoritative for.  This applies to both forward
and reverse lookups.

As an example, a computer "host.example.com." configured to respond to
LLMNR queries is authoritative for the name "host.example.com.".  On
receiving an LLMNR A/AAAA resource record query for the name
"host.example.com." the host authoritatively responds with A/AAAA
record(s) that contain IP address(es) in the RDATA of the resource

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
owned by the responder.  For example, if the host has a AAAA RR, but no
A RR, and an A RR query is received, the host would respond with RCODE=0
and an empty answer section.

If a DNS server is running on a host that supports LLMNR, the DNS server
MUST respond to LLMNR queries only for the RRSets owned by the host on
which the server is running, but MUST NOT respond for other records for
which the server is authoritative.

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

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For example the host "host.example.com." is not authoritative for the
name "child.host.example.com." unless the host is configured with
multiple names, including "host.example.com."  and
"child.host.example.com.".  As a result, "host" cannot reply to a query
for "child" with NXDOMAIN.  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,
"host.example.com." and "child.host.example.com.".

In this example (unless this limitation is introduced) an LLMNR query
for an A resource record for the name "child.host.example.com." would
result in two authoritative responses: a name error received from
"host.example.com.", and a requested A record - from
"child.host.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.host.example.com." would send a dynamic update for the NS and
glue A record to "host.example.com.", but this approach significantly
complicates implementation of LLMNR and would not be acceptable for
lightweight hosts.

A response to a LLMNR query is composed in exactly the same manner as a
response to the unicast DNS query as specified in [RFC1035].  Responders
MUST NOT respond using cached data, and the AA (Authoritative Answer)
bit MUST be set.  The response is sent to the sender via unicast.  A
response to an LLMNR query MUST have RCODE set to zero.  Responses with
RCODE set to zero are referred to in this document as "positively
resolved".  LLMNR responders may respond only to queries which they can
resolve positively.

2.3.  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.

If a TC (truncation) bit is set in the response, then the sender MAY use
the response if it contains all necessary information, or the sender MAY
discard the response and resend the query over TCP using the unicast
address of the responder.  The RA (Recursion Available) bit in the
header of the response MUST NOT be set.  If the RA bit is set in the
response header, the sender MUST ignore the RA bit.

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Unicast LLMNR queries SHOULD be sent using 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 not
using TCP, the response MUST be silently discarded.  Unicast UDP queries
MAY be responded to with an empty answer section and the TC bit set, so
as to require the sender to resend the query using TCP.  Senders MUST
support sending TCP queries, and Responders MUST support listening for
TCP queries.  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.

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 2.6.

2.4.  Addressing

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.

2.5.  Off-link detection

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 2.4.  If it was sent to another multicast address, then the
query MUST be silently discarded.

For IPv4, an "on link" address is defined as a link-local address or an
address whose prefix belongs to a subnet on the local link.  For IPv6

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[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 SHOULD prefer RRs including reachable addresses
where RRs involving both reachable and unreachable addresses are
returned in response to a query.

In composing LLMNR queries, the sender MUST set the Hop Limit field in
the IPv6 header and the TTL field in IPv4 header of the response to one
(1).  Even 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.  Therefore setting the IPv6 Hop Limit
or IPv4 TTL field to one provides an additional precaution against
leakage of LLMNR queries.

In composing a response to an LLMNR query, the responder MUST set the
Hop Limit field in the IPv6 header and the TTL field in 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.

Implementation note:

   In the sockets API for IPv4, 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.  Retransmissions

In order to avoid synchronization, LLMNR queries and responses are
delayed by a time randomly selected from the interval 0 to 200 ms.

If an LLMNR query sent over UDP is not resolved within the timeout
interval (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.

Since a multicast query sender cannot know beforehand whether it will
receive no response, one response, or more than one response, it 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.

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LLMNR implementations SHOULD dynamically estimate the timeout value
(LLMNR_TIMEOUT) based on the last response received for a query, on a
per-interface basis.  The algorithms described in [RFC2988] are
suggested (including exponential backoff).  Smaller values of
RTOinitial, RTOmin and RTOmax MAY be used. Recommended values are
RTOinitial=1 second, RTOmin=200ms, RTOmax=20 seconds.

2.7.  DNS TTL

The responder should use a pre-configured TTL value in the records
returned in the LLMNR query response.  A default value of 0 is
recommended in highly dynamic environments (such as mobile ad-hoc
networks).  In less dynamic environments, LLMNR traffic can be reduced
by setting the TTL to a higher value.

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

2.8.  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 of this SOA record MUST be equal to the
query name.

Responders SHOULD NOT perform DNS additional section processing.

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

LLMNR is a peer-to-peer name resolution protocol that is not intended as
a replacement for DNS. By default, LLMNR requests SHOULD be sent only
when no manual or automatic DNS configuration has been performed, when
DNS servers do not respond, or when they respond to a query with RCODE=3
(Authoritative Name Error) or RCODE=0, and an empty answer section.  An
LLMNR sender may send a request for any name.

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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. Given this, support for
LLMNR as a secondary name resolution mechanism has the potential to
result in a large number of inappropriate queries without the following
additional restrictions:

[1] If a DNS query does not receive a response, prior to falling
    back to LLMNR, the query SHOULD be retransmitted at least

[2] A sender SHOULD send LLMNR queries for PTR RRs
    via unicast, as specified in Section 2.3.

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

[1] 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

[2] If an IPv4 address is returned, it MUST be reachable through
    the link over which LLMNR is used.

[3] If a name is returned (for example in a CNAME, MX
    or SRV RR), the name MUST be valid on the local interface.

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

3.1.  Unqualified names

If a name is not qualified, for the purposes of LLMNR the implicit
search order is as follows:

[1]  Request the name with the current domain appended.
[2]  Request the name with the root domain (".") appended.

This is the behavior suggested by [RFC1536].

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3.2.  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.

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 [DHCPv6DNS] 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 [DHCPv6DNS],
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

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using the Name Service Search Option for DHCP [RFC2937].

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.
A default retry interval of one (1) minute is RECOMMENDED.

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 hostname.

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

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       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, as described in Section 2.6.

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

When a host that owns 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 another interface.  If this
is the case, then 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 hostname uniqueness. For
example, the hostname could be chosen randomly from a large pool of
potential names, or the hostname could be assigned via a process
designed to guarantee uniqueness.

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,

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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.

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

   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, which can propagate to multiple switch ports.

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

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[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 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 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/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. 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.

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Link-layer security can be of assistance against these threats if it is

5.2.  Usage restriction

As noted in Section 3, LLMNR is intended for usage in a limited set of

While LLMNR can be used to resolve any name, 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.

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.

Note: enabling LLMNR for use in situations where a DNS server has been
configured will result in upgraded hosts changing their 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.

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

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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 does not require use of DNSSEC, 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

6.  IANA Considerations

This specification does not create any new name spaces for IANA
administration.  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 as well
as a link-scope multicast IPv6 address TBD.

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.

[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.

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[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.

[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.

[RFC2136] Vixie, P., et al., "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.

          Droms, R., "A Guide to Implementing Stateless DHCPv6 Service",
          Internet draft (work in progress), draft-droms-
          dhcpv6-stateless-guide-01.txt, October 2002.

[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

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          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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Open Issues

Open issues with this specification are tracked on the following web


Expiration Date

This memo is filed as <draft-ietf-dnsext-mdns-24.txt>,  and  expires
February 22, 2004.

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