DNSEXT Working Group                                        Levon Esibov
INTERNET-DRAFT                                             Bernard Aboba
Category: Standards Track                                    Dave Thaler
<draft-ietf-dnsext-mdns-11.txt>                                Microsoft
23 March
20 July 2002

              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 (2002).  All Rights Reserved.


Today, with the rise of home networking, there are an increasing number
of ad-hoc networks operating without a 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.

Table of Contents

1.     Introduction ..........................................    3
   1.1       Requirements ....................................    3
   1.2       Terminology .....................................    3
2.     Name resolution using LLMNR ...........................    3
   2.1       Behavior of the sender and responder ............       Sender behavior .................................    4
   2.2       Responder behavior ..............................    5
   2.3       Addressing ......................................    6
   2.4       TTL .............................................    7
   2.5       No/multiple responses ...........................    7
3.     Usage model ...........................................    7    8
   3.1       LLMNR configuration .............................    8
4.     Sequence of events ....................................    9
5.     Conflict resolution ...................................    9   10
   5.1       Considerations for multiple interfaces ..........   11
   5.2       API issues ......................................   12   13
6.     Security considerations ...............................   13
   6.1       Scope restriction ...............................   13   14
   6.2       Usage restriction ...............................   14
   6.3       Cache and port separation .......................   15
   6.4       Authentication ..................................   15
7.     IANA considerations ...................................   15
8.     Normative References ..................................   15   16
9.     Informative References ................................   16
Acknowledgments ..............................................   17
Authors' Addresses ...........................................   17
Intellectual Property Statement ..............................   18
Full Copyright Statement .....................................   18

1.  Introduction

This document discusses Link-Local Multicast Name Resolution (LLMNR),
which operates on a separate port from 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.

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

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 for IPv4, while remaining unconfigured with a DNS server
suitable for use with IPv6.  Since automatic IPv6 DNS configuration
mechanisms such as [DHCPv6DNS] and [DNSDisc] are not yet widely
deployed, such "partially configuration" may be common in the short
term. However, in the long term, IPv6 DNS configuration will become more
common so that LLMNR will typically be restricted to adhoc networks in
which neither IPv4 nor IPv6 DNS servers are configured.

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, the key words "MAY", "MUST,  "MUST  NOT", "OPTIONAL",
"RECOMMENDED",  "SHOULD",  and  "SHOULD  NOT",  are to be interpreted as
described in [RFC2119].

1.2.  Terminology

Responder      A host that listens to (but not necessarily responds to)
               a LLMNR query is called "responder".

Sender         A host that sends an LLMNR query. The same host may be
               configured as a "sender", but not a "responder" and vice
               versa, i.e. as a "responder", but not a "sender".

2.  Name resolution using LLMNR

While operating on a different port with a distinct resolver cache,
LLMNR makes no change to the current format of DNS packets.

LLMNR queries are sent to and received on port 5353 using a LINKLOCAL
address as specified in "Administratively Scoped IP Multicast" [RFC2365]

for IPv4 and the "solicited name" LINKLOCAL multicast addresses for
IPv6, and using a unicast addresses in a few scenarios described below
in Section 3.  The LLMNR LINKLOCAL address to be used for IPv4 is  LINKLOCAL addresses are used to prevent propagation of
LLMNR traffic across routers, potentially flooding the network.

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 home gateway, then the network either has a DNS server or
the home gateway can function as a DNS proxy.  By implementing DHCPv4 as
well as a DNS proxy and dynamic DNS, home gateways can provide name
resolution for the names of IPv4 hosts on the local network.

For small IPv6 networks, equivalent functionality can be provided by a
home gateway implementing DHCPv6 for DNS configuration [DHCPv6DNS], as
well as a DNS proxy supporting AAAA RRs and dynamic DNS, providing name
resolution for the names of IPv6 hosts on the local network.

This should be adequate as long as home gateways implementing DNS
configuration also support dynamic DNS in some form.  If the home
gateway only supports DNS discovery [DNSDisc] but not DHCPv6 DNS
configuration [DHCPv6DNS] or dynamic client update, then resolution of
the names of IPv6 hosts on the local link will not be possible. Since
IPv6 DNS discovery will configure the DNS server address, LLMNR will not
be enabled by default. Yet without gateway support for client dynamic
update or DHCPv6, dynamic DNS will not be enabled.

In the future, LLMNR may be defined to support greater than LINKLOCAL
multicast scope.  This would occur if LLMNR deployment is successful,
the assumption that LLMNR is not needed on multiple links proves
incorrect, and multicast routing becomes ubiquitous.  For example, it is
not clear that this assumption will be valid in large adhoc networking

Once we have experience in LLMNR deployment in terms of administrative
issues, usability and impact on the network it will be possible
reevaluate which multicast scopes are appropriate for use with multicast
name resolution mechanisms.

2.1.  Behavior of the sender and responder

For the purpose of this document a host that sends a LLMNR query is
called a "sender", while a host that listens to (but not necessarily
responds to) a LLMNR query is called "responder". Although the same host
may be configured as a "sender", but not a "responder" and vice versa,
i.e. as a "responder", but not a "sender", the host configured as a
"responder" MUST act as a sender by using LLMNR dynamic update requests
to verify the uniqueness of names as described in Section 5.

2.1.1.  Behavior of senders  Sender behavior

A sender sends an LLMNR query for any legal Type of resource record
(e.g. A, PTR, etc.) to the LINKLOCAL address. Notice that in some
scenarios described below in Section 3 a sender may also send a unicast
query. The RD (Recursion Desired) bit MUST NOT be set. If a responder
receives a query with the header containing RD set bit, the responder
MUST ignore the RD bit.

The IPv6 LINKLOCAL address a given responder  listens to, and to which a
sender sends, is a link-local multicast address formed as follows: The
name of the resource record in question is expressed in its canonical
form (see [RFC2535], section 8.1), which is uncompressed with all
alphabetic characters in lower case.  The first label of the resource
record name FQDN in the
query is then hashed using the MD5 algorithm, described in [RFC1321].
The first 32 bits of the resultant 128-bit hash is then appended to the
prefix FF02:0:0:0:0:2::/96 to yield the 128-bit "solicited name
multicast address".  (Note: this procedure is intended to be the same as
that specified in section 3 of "IPv6 Node Information Queries"
[NodeInfo]).  A responder that listens for queries for multiple names
will necessarily listen to multiple of these solicited name multicast

If the LLMNR query is not resolved during a limited amount of time
(LLMNR_TIMEOUT), then a sender MAY repeat the transmission of a query in
order to assure themselves that the query has been received by a host
capable of responding to the query. The default value for LLMNR_TIMEOUT
is 1 second.

Repetition MUST NOT be attempted more than 3 times and SHOULD NOT be
repeated more often than once per second to reduce unnecessary network
traffic. The delay between attempts should be randomized so as to avoid
synchronization effects.

2.1.2.  Behavior of responders

2.2.  Responder behavior

A responder listens on port 5353 on the LINKLOCAL address and on the
unicast address(es) that could be set as the source address(es) when the
responder responds to the LLMNR query. The host configured as a
"responder" MUST act as a sender by using LLMNR dynamic update requests
to verify the uniqueness of names as described in Section 5.

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

As an example, assume that computer "host.example.com." is authoritative
for the domain "host.example.com.". On receiving a LLMNR A resource
record query for the name "host.example.com." such a the host responds with A
record(s) that contain IP address(es) in the RDATA of the resource

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, a an LLMNR responder is authoritative only for the zone root.

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) a LLMNR query for
an A record for the name "child.host.example.com." would result in two
authoritative responses: 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 never 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.

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 or using EDNS0 with
larger window using the unicast address of the responder. The RA
(Recursion Available) bit in the header of the response MUST NOT be set.
Even if the RA bit is set in the response header, the sender MUST ignore

2.1.3.  LLMNR addressing

2.3.  Addressing

For IPv4 LINKLOCAL addressing, section 2.4 of "Dynamic Configuration of
IPv4 Link-Local Addresses" [IPV4Link] lays out the rules with respect to
source address selection, TTL settings, and acceptable
source/destination address combinations. IPv6 is described in [RFC2460];
IPv6 LINKLOCAL addressing is described in [RFC2373]. LLMNR queries and
responses MUST obey the rules laid out in these documents.

In composing an LLMNR response, the responder MUST set the Hop Limit
field in the IPv6 header and the TTL field in IPv4 header of the LLMNR
response to 255. The sender MUST verify that the Hop Limit field in IPv6
header and TTL field in IPv4 header of each response to the LLMNR query
is set to 255. If it is not, then sender MUST ignore the response.

   Implementation note:

   In the sockets API for IPv4, the IP_TTL and IP_MULTICAST_TTL socket
   options are used to specify the TTL of outgoing unicast and multicast
   packets. The IP_RECVTTL socket option is available on some platforms
   to receive the IPv4 TTL of received packets with recvmsg(). [RFC2292]
   specifies similar options for specifying and receiving the IPv6 Hop

2.1.4.  Use of LLMNR

2.4.  TTL

The responder should use a pre-configured TTL value in the records
returned in the LLMNR query response. Due to the TTL minimalization
necessary when caching an RRset, all TTLs in an RRset MUST be set to the
same value.  In the additional and authority section of the response the
responder includes the same records as a DNS server would insert in the
response to the unicast DNS query.


2.5.  No/multiple responses

The sender MUST anticipate receiving no replies to some LLMNR queries,
in the event that no responders are available within the linklocal
multicast scope, or in the event that 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 for the specified name exist (NXRRSET).

Since the responder MUST NOT respond to queries for names it is not
authoritative for, a responder MUST NOT respond to an A, A6 or AAAA RR
query with an NXRRSET. However, for other queries, such a response is
possible; for example, if the host has a AAAA RR, but no MX RR, and an
MX RR query is received.

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, ordinarily.
However, after receiving an initial response, the sender is not required
to wait for LLMNR_TIMEOUT for additional responses.

3.  Usage model

The same host may be configured as a "sender", but not a "responder" and
vice versa (as a "responder", but not "sender").  However, the a host
configured as a "responder" MUST at least use "sender"'s a "sender's" capability to
send LLMNR dynamic update requests to verify the uniqueness of the names
names, as described in Section 5. An LLMNR "sender" MAY multicast
requests for any name. If that name is not qualified and does not end in
a trailing dot, for the purposes of LLMNR, the implicit search order is
as follows:

[1]  Request the name with the current domain appended.
[2]  Request just the name.

This is the behavior suggested by [RFC1536].  LLMNR uses this technique
to resolve unqualified host names. The same host MAY use LLMNR queries
for the resolution of unqualified host names, and conventional DNS
queries for resolution of other DNS names.

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 the other records for
which the server is authoritative.

A sender MUST NOT send a unicast LLMNR query except when:

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

  b. The sender's LLMNR cache contains an NS resource record that
     enables the sender to send a query directly to the hosts
     authoritative for the name in the query.

A responder with a name "host.example.com." configured to respond to the
LLMNR queries is authoritative for the name "host.example.com.". For
example, when a responder with the name "host.example.com." receives an
A type LLMNR query for the name "host.example.com." it authoritatively
responds to the query.

The same host MAY use LLMNR queries for the resolution of the local
names, and conventional DNS queries for resolution of other DNS names.

3.1.  LLMNR configuration

LLMNR usage can be configured manually or automatically.  On interfaces
where no manual or automatic DNS configuration has been performed for a
given protocol (IPv4 or IPv6), LLMNR SHOULD be enabled for that

For IPv6, the stateless DNS discovery mechanisms described in "IPv6
Stateless DNS Discovery" [DNSDisc] or "Using DHCPv6 for DNS

Configuration in Hosts" [DHCPv6DNS] can be used to discover whether
LLMNR should be enabled or disabled on a per-interface basis.

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 for DHCP, [RFC2937].

Note that it is possible for LLMNR to be enabled for use with IPv6 at
the same time it is disabled for IPv4, and vice versa. For example, a
home gateway may implement a DNS proxy and DHCPv4, but not DHCPv6 for
DNS configuration [DHCPv6DNS] or stateless DNS discovery [DNSDisc].  In
such a circumstance, IPv6 hosts will not be configured with a DNS
server. Where DHCPv6 is not supported, it will not be possible for the
DNS proxy within the home gateway to dynamically register names learned
via DHCPv6. As a result, unless the DNS proxy supports client update, it
will not be able to respond to AAAA RR queries for local names sent over
IPv4 or IPv6, preventing IPv6 hosts from resolving the names of other
IPv6 hosts on the local link. In this situation, LLMNR enables
resolution of dynamic names, and it will be enabled for use with IPv6,
even though it is disabled for use with IPv4.

4.  Sequence of events

The sequence of events for LLMNR usage is as follows:

1. If a sender needs to resolve a query for a name "host.example.com",
   then it sends a LLMNR query to the LINKLOCAL multicast address.

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 verifies that the Hop
   Limit field in IPv6 header or TTL field in IPv4 header (depending on
   the protocol used) of the response is set to 255. The sender then
   verifies compliance with the addressing requirements for IPv4,
   described in [IPV4Link], and IPv6, described in [RFC2373]. 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.

5.  Conflict resolution

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 a SRV type record
   - multiple hosts may respond to a query for an A 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 type record for
     a hostname.

Every responder that responds to a LLMNR query and/or dynamic update
request 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 respond to LLMNR queries on more than one
interface, the host MUST verify resource record uniqueness on each
interface for each UNIQUE resource record that could be used on that
interface. To accomplish this, the host MUST send a dynamic LLMNR update
request for each new UNIQUE resource record. Format of the dynamic LLMNR
update request is identical to the format of the dynamic DNS update
request specified in [RFC2136]. Uniqueness verification is carried out
when the host:

  - starts up or
  - is configured to respond to the LLMNR queries on some interface or
  - is configured to respond to the LLMNR queries using additional
    UNIQUE resource records.

Below we describe the data to be specified in the dynamic update

Header section
     contains values according to [RFC2136].

Zone section
     The zone name in the zone section MUST be set to the name of the
     UNIQUE record. The zone type in the zone section MUST be set to
     SOA. The zone class in the zone section MUST be set to the class of
     the UNIQUE record.

Prerequisite section
     This section MUST contain a record set whose semantics are
     described in [RFC2136], Section 2.4.3 "RRset Does Not Exist",
     requesting that RRs with the NAME and TYPE of the UNIQUE record do
     not exist.

Update section
     This section MUST be left empty.

Additional section
     This section is set according to [RFC2136].

When a host that owns a UNIQUE record receives a dynamic update request
that requests that the UNIQUE resource record set does not exist, the
host MUST respond via unicast with the YXRRSET error, according to the
rules described in Section 3 of [RFC2136].

After the client receives an YXRRSET response to its dynamic update
request stating that a UNIQUE resource record does not exist, the host
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 and dynamic update requests. If not, then it MUST NOT
use the UNIQUE resource record in response to LLMNR queries and dynamic
update requests.

Note that this name conflict detection mechanism doesn't prevent name
conflicts when previously partitioned segments are connected by a
bridge.  In such a situation, name conflicts are detected when a sender
receives more than one response to its LLMNR query. In this case, the
sender sends the first response that it received to all responders that
responded to this query except the first one, using unicast. A host that
receives a query response containing a UNIQUE resource record that it
owns, even if it didn't send such a query, MUST verify that no other
host within the LLMNR scope is authoritative for the same name, using
the dynamic LLMNR update request mechanism described above.

Based on the result, the host detects whether there is a name conflict
and acts as described above.

5.1.  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 wish 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 5. The situation is illustrated in figure 1 below:

     ----------  ----------
      |      |    |      |
     [A]    [myhost]   [myhost]

   Figure 1. LINKLOCAL 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 below.

     ----------  ----------
      |      |    |     |
     [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 will then forward a response from the first responder to the
second responder, who will attempt to verify the uniqueness of host RR
for its name, but will not discover a conflict, since the conflicting
host resides on a different link.  Therefore it will continue using its

Indeed, 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.

5.2.  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.

6.  Security Considerations

LLMNR is by nature a peer to peer name resolution protocol, for use in
situations when a DNS server is not configured.  It is therefore
inherently more vulnerable than DNS, since existing DNS security
mechanisms are difficult to apply to LLMNR and an attacker only needs to
be misconfigured to answer an LLMNR query with incorrect information.

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.

6.1.  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.

In the absence of authentication, LLMNR reduces the exposure to such
threats by ignoring LLMNR query response packets received from off-link
senders.  In all received responses, the Hop Limit field in IPv6 and the
TTL field in IPv4 are verified to contain 255, the maximum legal value.
Since routers decrement the Hop Limit on all packets they forward,
received packets containing a Hop Limit of 255 must have originated from
a neighbor.

While restricting ignoring packets received from off-link senders
reduces the level of vulnerability, it does not eliminate it. There 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.

6.2.  Usage restriction

As noted in Section 3.1, LLMNR is intended for usage in scenarios where
a DNS server is not configured.  If an interface has been configured for
a given protocol via any automatic configuration mechanism which is able
to supply DNS configuration information, then LLMNR SHOULD NOT be used
on that interface for that protocol unless it has been explicitly
enabled, whether via that mechanism or any other. This ensures that
upgraded hosts do not change their default behavior, without requiring
the source of the configuration information to be simultaneously
updated.  This implies that on the interface, the host will neither
listen on the LINKLOCAL multicast address, nor will it send queries to
that address.

Violation of this guideline can significantly increases security
vulnerabilities.  For example, if an LLMNR query were to be sent
whenever a DNS server did not respond in a timely way, then an attacker
could execute a denial of service attack on the DNS server(s) and then
poison the LLMNR cache by responding to the resulting LLMNR queries with
incorrect information.

The vulnerability would be even greater 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 best thought of as a name resolution mechanism of
last resort, useful only in situations where a DNS server is not
configured. Where resilience against DNS server failure is desired,
configuration of additional DNS servers or DNS server clustering is
recommended; LLMNR is not an appropriate "failsafe" mechanism.

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

LLMNR operates on a separate port (5353) from DNS, reducing the
likelihood that a DNS server will unintentionally respond to an LLMNR

6.4.  Authentication

LLMNR does not require use of DNSSEC, and as a result, responses to
LLMNR queries MAY NOT be authenticated.  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

7.  IANA Considerations

This specification does not create any new name spaces for IANA
administration.  Since it uses a port (5353) and link scope multicast

IPv4 address ( previously allocated for use with LLMNR, no
additional IANA allocations are required.

8.  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.

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

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

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

[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.

[IPV4Link]     Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4
               Link-Local Addresses", Internet draft (work in progress),
               draft-ietf-zeroconf-ipv4-linklocal-05.txt, November 2001.

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

9.  Informative References

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

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

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

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

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

[DHCPv6DNS]    Droms, R., Narten, T., and Aboba, B. "Using DHCPv6 for
               DNS Configuration in Hosts", draft-droms-dnsconfig-
               dhcpv6-01.txt, Internet draft (work in progress), March

[DNSDisc]      Thaler, D., Hagino, I., "IPv6 Stateless DNS Discovery",
               Internet draft (work in progress), draft-ietf-ipngwg-dns-
               discovery-03.txt, November 2001.

[NodeInfo]     Crawford, Matt, "IPv6 Node Information Queries", Internet
               draft (work in progress), draft-ietf-ipn-gwg-icmp-name-
               lookups-08.txt, July 2001.


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, Robert Elz, Rob
Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig,
Thomas Narten, Erik Nordmark, Sander Van-Valkenburg, Tomohide Nagashima
and Brian Zill.

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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