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DNSEXT Working Group Bernard Aboba
INTERNET-DRAFT Dave Thaler
Category: Standards Track Levon Esibov
<draft-ietf-dnsext-mdns-40.txt> Microsoft
25 May 2005
Linklocal Multicast Name Resolution (LLMNR)
Status of this Memo
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This Internet-Draft will expire on November 22, 2005.
Copyright Notice
Copyright (C) The Internet Society 2005.
Abstract
Today, with the rise of home networking, there are an increasing
number of ad-hoc networks operating without a Domain Name System
(DNS) server. The goal of Link-Local Multicast Name Resolution
(LLMNR) is to enable name resolution in scenarios in which
conventional DNS name resolution is not possible. LLMNR supports all
current and future DNS formats, types and classes, while operating on
a separate port from DNS, and with a distinct resolver cache. Since
LLMNR only operates on the local link, it cannot be considered a
substitute for DNS.
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INTERNET-DRAFT LLMNR 25 May 2005
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 ............................. 6
2.2 Sender Behavior ................................. 9
2.3 Responder Behavior .............................. 9
2.4 Unicast Queries ................................. 11
2.5 Off-link Detection .............................. 12
2.6 Responder Responsibilities ...................... 13
2.7 Retransmission and Jitter ....................... 13
2.8 DNS TTL ......................................... 14
2.9 Use of the Authority and Additional Sections .... 15
3. Usage model ........................................... 15
3.1 LLMNR Configuration ............................. 16
4. Conflict Resolution ................................... 17
4.1 Uniqueness Verification ......................... 18
4.2 Conflict Detection and Defense .................. 19
4.3 Considerations for Multiple Interfaces .......... 20
4.4 API issues ...................................... 21
5. Security Considerations ............................... 21
5.1 Scope Restriction ............................... 22
5.2 Usage Restriction ............................... 23
5.3 Cache and Port Separation ....................... 23
5.4 Authentication .................................. 24
6. IANA considerations ................................... 24
7. Constants ............................................. 24
8. References ............................................ 24
8.1 Normative References ............................ 24
8.2 Informative References .......................... 25
Acknowledgments .............................................. 26
Authors' Addresses ........................................... 27
Intellectual Property Statement .............................. 27
Disclaimer of Validity ....................................... 28
Copyright Statement .......................................... 28
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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 DNS.
Link-scope multicast addresses are used to prevent propagation of
LLMNR traffic across routers, potentially flooding the network.
LLMNR queries can also be sent to a unicast address, as described in
Section 2.4.
Propagation of LLMNR packets on the local link is considered
sufficient to enable name resolution in small networks. 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 arises for
multicast name resolution beyond the link-scope, or multicast routing
becomes ubiquitous. For example, expanded support for multicast name
resolution might be required for mobile ad-hoc networks.
Once we have experience in LLMNR deployment in terms of
administrative issues, usability and impact on the network, it will
be possible to reevaluate which multicast scopes are appropriate for
use with multicast name resolution.
Service discovery in general, as well as discovery of DNS servers
using LLMNR in particular, is outside of the scope of this document,
as is name resolution over non-multicast capable media.
1.1. Requirements
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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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.
Reachable
An LLMNR responder considers one of its addresses reachable over a
link if it will respond to an ARP or Neighbor Discovery query for
that address received on that link.
Responder
A host that listens to LLMNR queries, and responds to those for
which it is authoritative.
Sender
A host that sends an LLMNR query.
UNIQUE
There are some scenarios when multiple responders may respond to
the same query. There are other scenarios when only one responder
may respond to a query. Resource records for which only a single
responder is anticipated are referred to as UNIQUE. Resource
record uniqueness is configured on the responder, and therefore
uniqueness verification is the responder's responsibility.
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 5355. 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 224.0.0.252. The IPv6 link-scope multicast
address a given responder listens to, and to which a sender sends all
queries, is FF02:0:0:0:0:0:1:3.
Typically a host is configured as both an LLMNR sender and a
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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 DNS server address(es) have been
configured, then LLMNR SHOULD NOT be used as the
primary name resolution mechanism, although it MAY
be used as a secondary name resolution mechanism.
For dual stack hosts configured with DNS server
address(es) for one protocol but not another,
this implies that DNS queries SHOULD be sent
over the protocol configured with a DNS
server, prior to sending LLMNR queries.
[2] DNS servers do not respond. For a dual stack
host, the host SHOULD attempt to reach
DNS servers over all protocols on which
DNS server address(es) are configured, prior
to use of LLMNR.
[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
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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 the smaller of the link
MTU or 8192 octets.
2.1.1. LLMNR Header Format
LLMNR queries and responses utilize the DNS header format defined in
[RFC1035] with exceptions noted below:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | C|TC| T| Z| Z| Z| Z| RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
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ID A 16 bit identifier assigned by the program that generates any kind
of query. This identifier is copied from the query to the response
and can be used by the sender to match responses to outstanding
queries. The ID field in a query SHOULD be set to a pseudo-random
value. For advice on generation of pseudo-random values, please
consult [RFC1750].
QR Query/Response. A one bit field, which if set indicates that the
message is an LLMNR response; if clear then the message is an LLMNR
query.
OPCODE
A four bit field that specifies the kind of query in this message.
This value is set by the originator of a query and copied into the
response. This specification defines the behavior of standard
queries and responses (opcode value of zero). Future
specifications may define the use of other opcodes with LLMNR.
LLMNR senders and responders MUST support standard queries (opcode
value of zero). LLMNR queries with unsupported OPCODE values MUST
be silently discarded by responders.
C Conflict. When set within a request, the 'C'onflict bit indicates
that a sender has received multiple LLMNR responses to this query.
In an LLMNR response, if one or more resource records in the answer
section is UNIQUE, then the 'C' bit is clear, otherwise it is set.
LLMNR senders do not retransmit queries with the 'C' bit set.
Responders MUST NOT respond to LLMNR queries with the 'C' bit set,
but may start the uniqueness verification process, as described in
Section 4.2.
TC TrunCation - specifies that this message was truncated due to
length greater than that permitted on the transmission channel.
The TC bit MUST NOT be set in an LLMNR query and if set is ignored
by an LLMNR responder. If the TC bit is set in an LLMNR response,
then the sender SHOULD 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.
T Tentative. The 'T'entative bit is set in a response if the
responder is authoritative for the name, but has not yet verified
the uniqueness of one or more of the resource record(s) in the
answer section. A responder MUST ignore the 'T' bit in a query, if
set. When a response with the 'T' bit set is received in response
to a uniqueness query, a conflict has been detected and a responder
MUST resolve the conflict as described in Section 4.1.
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Z Reserved for future use. Implementations of this specification
MUST set these bits to zero in both queries and responses. If
these bits are set in a LLMNR query or response, implementations of
this specification MUST ignore them. Since reserved bits could
conceivably be used for different purposes than in DNS,
implementors are advised not to enable processing of these bits in
an LLMNR implementation starting from a DNS code base.
RCODE
Response code -- this 4 bit field is set as part of LLMNR
responses. In an LLMNR query, the 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 preferable 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.
QDCOUNT
An unsigned 16 bit integer specifying the number of entries in the
question section. A sender MUST place only one question into the
question section of an LLMNR query. LLMNR responders MUST silently
discard LLMNR queries with QDCOUNT not equal to one. LLMNR senders
MUST silently discard LLMNR responses with QDCOUNT not equal to
one.
ANCOUNT
An unsigned 16 bit integer specifying the number of resource
records in the answer section. LLMNR responders MUST silently
discard LLMNR queries with ANCOUNT not equal to zero.
NSCOUNT
An unsigned 16 bit integer specifying the number of name server
resource records in the authority records section. Authority
record section processing is described in Section 2.9.
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ARCOUNT
An unsigned 16 bit integer specifying the number of resource
records in the additional records section. Additional record
section processing is described in Section 2.9.
2.2. Sender Behavior
A sender may send an LLMNR query for any legal resource record type
(e.g., A, AAAA, 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.
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 multiple
valid answers are received, they may first be concatenated, and then
treated in the same manner that multiple RRs received from the same
DNS server would.
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
192.0.2.1 and IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6 might be
authoritative for the following records:
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host1. IN A 192.0.2.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
host1.example.com. IN A 192.0.2.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
1.2.0.192.in-addr.arpa. IN PTR host1.
IN PTR host1.example.com.
6.0.5.0.4.0.E.F.F.F.3.0.2.0.1.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.
ip6.arpa IN PTR host1. (line split for formatting reasons)
IN PTR host1.example.com.
An LLMNR responder might be further manually configured with the name
of a local mail server with an MX RR included in the "host1." and
"host1.example.com." records.
In responding to queries:
[a] Responders MUST listen on UDP port 5355 on the link-scope multicast
address(es) defined in Section 2, and on UDP and TCP port 5355 on
the unicast address(es) that could be set as the source address(es)
when the responder responds to the LLMNR query.
[b] Responders MUST direct responses to the port from which the query
was sent. When queries are received via TCP this is an inherent
part of the transport protocol. For queries received by UDP the
responder MUST take note of the source port and use that as the
destination port in the response. Responses MUST always be sent
from the port to which they were directed.
[c] Responders MUST respond to LLMNR queries for names and addresses
they are authoritative for. This applies to both forward and
reverse lookups, with the exception of queries with the 'C' bit
set, which do not elicit a response.
[d] Responders MUST NOT respond to LLMNR queries for names they are not
authoritative for.
[e] Responders MUST NOT respond using data from the LLMNR or DNS
resolver cache.
[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
queries.
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[g] If a responder is authoritative for a name, it SHOULD respond with
RCODE=0 and an empty answer section, if the type of query does not
match a RR that the responder has.
As an example, a host configured to respond to LLMNR queries for the
name "foo.example.com." is authoritative for the name
"foo.example.com.". On receiving an LLMNR query for an A RR with the
name "foo.example.com." the host authoritatively responds with A
RR(s) that contain IP address(es) in the RDATA of the resource
record. If the responder has a AAAA RR, but no A RR, and an A RR
query is received, the responder would respond with RCODE=0 and an
empty answer section.
In conventional DNS terminology a DNS server authoritative for a zone
is authoritative for all the domain names under the zone apex except
for the branches delegated into separate zones. Contrary to
conventional DNS terminology, an LLMNR responder is authoritative
only for the zone apex.
For example the host "foo.example.com." is not authoritative for the
name "child.foo.example.com." unless the host is configured with
multiple names, including "foo.example.com." and
"child.foo.example.com.". As a result, "foo.example.com." cannot
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.".
Without the restriction on authority an LLMNR query for an A resource
record for the name "child.foo.example.com." would result in two
authoritative responses: RCODE=3 (authoritative name error) received
from "foo.example.com.", and a requested A record - from
"child.foo.example.com.". To prevent this ambiguity, LLMNR enabled
hosts could perform a dynamic update of the parent (or grandparent)
zone with a delegation to a child zone; for example a host
"child.foo.example.com." could send a dynamic update for the NS and
glue A record to "foo.example.com.". However, this approach
significantly complicates implementation of LLMNR and would not be
acceptable for lightweight hosts.
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
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[b] The sender queries for a PTR RR of a fully formed IP address
within the "in-addr.arpa" or "ip6.arpa" zones.
Unicast LLMNR queries MUST be done using TCP and the responses MUST
be sent using the same TCP connection as the query. Senders MUST
support sending TCP queries, and responders MUST support listening
for TCP queries. If the sender of a TCP query receives a response to
that query not using TCP, the response MUST be silently discarded.
Unicast UDP queries MUST be silently discarded.
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).
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 one belonging to a
prefix that a Router Advertisement indicates is on-link [RFC2461].
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
discarded.
Section 2.4 discusses use of TCP for LLMNR queries and responses. In
composing an LLMNR query using TCP, the sender MUST set the Hop Limit
field in the IPv6 header and the TTL field in the IPv4 header of the
response to one (1). The responder SHOULD set the TTL or Hop Limit
settings on the TCP listen socket to one (1) so that SYN-ACK packets
will have TTL (IPv4) or Hop Limit (IPv6) set to one (1). This
prevents an incoming connection from off-link since the sender will
not receive a SYN-ACK from the responder.
For UDP queries and responses, the Hop Limit field in the IPv6 header
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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 Bonjour [Bonjour].
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 particular:
[a] If a link-scope IPv6 address is returned in a AAAA RR,
that address MUST be valid on the local link over which
LLMNR is used.
[b] If an IPv4 address is returned, it MUST be reachable
through the link over which LLMNR is used.
[c] If a name is returned (for example in a CNAME, MX
or SRV RR), the name MUST be resolvable on the local
link over which LLMNR is used.
Where multiple addresses represent valid responses to a query, the
order in which the addresses are returned is as follows:
[d] If the source address of the query is a link-scope address,
then the responder SHOULD include a link-scope address first
in the response, if available.
[e] If the source address of the query is a routable address,
then the responder MUST include a routable address first
in the response, if available.
2.7. Retransmission and Jitter
An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine
when to retransmit an LLMNR query and how long to collect responses
to an LLMNR query.
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If an LLMNR query sent over UDP is not resolved within LLMNR_TIMEOUT,
then a sender SHOULD repeat the transmission of the query in order to
assure that it was received by a host capable of responding to it.
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. Queries with the 'C' bit set MUST be
sent over multicast UDP and MUST NOT be retransmitted.
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. For example, the algorithms described in RFC
2988 [RFC2988] (including exponential backoff) 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).
In order to avoid synchronization, the transmission of each LLMNR
query and response SHOULD delayed by a time randomly selected from
the interval 0 to JITTER_INTERVAL. This delay MAY be avoided by
responders responding with RRs which they have previously determined
to be UNIQUE (see Section 4 for details).
Recommended values for constants (including LLMNR_TIMEOUT if it is
set statically) are given in Section 7.
2.8. DNS TTL
The responder should insert a pre-configured TTL value in the records
returned in an LLMNR response. A default value of 30 seconds is
RECOMMENDED. In highly dynamic environments (such as mobile ad-hoc
networks), the TTL value may need to be reduced.
Due to the TTL minimalization necessary when caching an RRset, all
TTLs in an RRset MUST be set to the same value.
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2.9. Use of the Authority and Additional Sections
Unlike the DNS, LLMNR is a peer-to-peer protocol and does not have a
concept of delegation. In LLMNR, the NS resource record type may be
stored and queried for like any other type, but it has no special
delegation semantics as it does in the DNS. Responders MAY have NS
records associated with the names for which they are authoritative,
but they SHOULD NOT include these NS records in the authority
sections of responses.
Responders SHOULD insert an SOA record into the authority section of
a negative response, to facilitate negative caching as specified in
[RFC2308]. The owner name of this SOA record MUST be equal to the
query name.
In LLMNR, the additional section is primarily intended for use by
EDNS0, TSIG and SIG(0). As a result, unless the 'C' bit is set,
senders MAY only include pseudo RR-types in the additional section of
a query; unless the 'C' bit is set, responders MUST ignore the
additional section of queries containing other RR types.
In queries where the 'C' bit is set, the sender SHOULD include the
conflicting RRs in the additional section. Since conflict
notifications are advisory, responders SHOULD log information from
the additional section, but otherwise MUST ignore the additional
section.
Senders MUST NOT cache RRs from the authority or additional section
of a response as answers, though they may be used for other purposes
such as negative caching.
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
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the proposed fixes are implemented, unnecessary LLMNR queries will be
reduced substantially, and so implementation of [RFC1536] is
recommended.
For example, [RFC1536] Section 1 describes issues with retransmission
and recommends implementation of a retransmission policy based on
round trip estimates, with exponential 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 name.
For example, if the configured DNS server responds to a AAAA RR query
sent over IPv4 or IPv6 with an authoritative name error (RCODE=3) or
RCODE=0 and an empty answer section, then a AAAA RR query sent using
LLMNR over IPv6 may be successful in resolving the name of an
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IPv6-only host on the local link.
Similarly, if a DHCPv4 server is available providing DNS server
configuration, and DNS server(s) exist which are authoritative for
the A RRs of local hosts and support either dynamic client update
over IPv4 or DHCPv4-based dynamic update, then the names of local
IPv4 hosts can be resolved over IPv4 without LLMNR. However, if no
DNS server is authoritative for the names of local hosts, or the
authoritative DNS server(s) do not support dynamic update, then LLMNR
enables 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.
An outage in the DNS configuration mechanism may result in hosts
continuing to use LLMNR even once the outage is repaired. Since
LLMNR only enables linklocal name resolution, this represents a
degradation in capabilities. As a result, hosts without a configured
DNS server may wish to periodically attempt to obtain DNS
configuration if permitted by the configuration mechanism in use. In
the absence of other guidance, a default retry interval of one (1)
minute is RECOMMENDED.
4. Conflict Resolution
The uniqueness of a resource record MAY depend on the nature of the
name in the query and type of the query. For example, 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). By default, a
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responder SHOULD be configured to behave as though all RRs are UNIQUE
on each interface on which LLMNR is enabled.
To detect duplicate use of a name, an administrator can use a name
resolution utility which employs LLMNR and lists both responses and
responders. This would allow an administrator to diagnose behavior
and potentially to intervene and reconfigure LLMNR responders who
should not be configured to respond to the same name.
4.1. Uniqueness Verification
Prior to including a UNIQUE resource record in a response with the
'T' bit clear, for each UNIQUE resource record in a given interface's
configuration, the host MUST verify that there is no other host
within the scope of LLMNR query propagation that can return a
resource record for the same name, type and class on that interface.
Once a responder has verified the uniqueness of a UNIQUE resource
record, if it receives an LLMNR query for that resource record, with
the 'C' bit clear, it MUST respond, with the 'T' bit clear. Prior to
verifying uniqueness, a responder MUST set the 'T' bit in responses.
Uniqueness verification is carried out when the host:
- starts up or is rebooted
- wakes from sleep (if the network interface was inactive
during sleep)
- is configured to respond to LLMNR queries on an interface
enabled for transmission and reception of IP traffic
- is configured to respond to LLMNR queries using additional
UNIQUE resource records
- verifies the acquisition of a new IP address and configuration
on an interface
To verify uniqueness, a responder MUST send an LLMNR query with the
'C' bit clear, over all protocols on which it responds to LLMNR
queries (IPv4 and/or IPv6). It is RECOMMENDED that responders verify
uniqueness of a name by sending a query for the name with type='ANY'.
If no response is received, the sender retransmits the query, as
specified in Section 2.7. If a response is received with the 'T' bit
clear, the responder MUST NOT use the name in response to LLMNR
queries received over any protocol (IPv4 or IPv6). If a response is
received with the 'T' bit set, the responder MUST check if the source
IP address in the response, interpreted as an unsigned integer, is
less than the source IP address in the query. If so, the responder
MUST NOT use the name in response to LLMNR queries received over any
protocol (IPv4 or IPv6). For the purpose of uniqueness verification,
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the contents of the answer section in a response is irrelevant.
Periodically carrying out uniqueness verification in an attempt to
detect name conflicts is not necessary, wastes network bandwidth, and
may actually be detrimental. For example, if network links are
joined only briefly, and are separated again before any new
communication is initiated, temporary conflicts are benign and no
forced reconfiguration is required. LLMNR responders SHOULD NOT
periodically attempt uniqueness verification.
4.2. Conflict Detection and Defense
Hosts on disjoint network links may configure the same name for use
with LLMNR. If these separate network links are later joined or
bridged together, then there may be multiple hosts which are now on
the same link, trying to use the same name.
In order to enable ongoing detection of name conflicts, when an LLMNR
sender receives multiple LLMNR responses to a query, it MUST check if
the 'C' bit is clear in any of the responses. If so, the sender
SHOULD send another query for the same name, type and class, this
time with the 'C' bit set, with the potentially conflicting resource
records included in the additional section.
Queries with the 'C' bit set are considered advisory and responders
MUST verify the existence of a conflict before acting on it. A
responder receiving a query with the 'C' bit set MUST NOT respond.
If the query is for UNIQUE resource record(s), then the responder
MUST send its own query for the same name, type and class, with the
'C' bit clear. If a response is received, then a conflict has been
detected.
An LLMNR responder MUST NOT ignore conflicts once detected and SHOULD
log them. Upon detecting a conflict, an LLMNR responder MUST
immediately stop using the conflicting name in response to LLMNR
queries received over any supported protocol, if the source IP
address in the response, interpreted as an unsigned integer, is less
than the source IP address in the uniqueness verification query.
After stopping the use of a name, the responder MAY elect to
configure a new name. However, since name reconfiguration may be
disruptive, this is not required, and a responder may have been
configured to respond to multiple names so that alternative names may
already be available.
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4.3. Considerations for Multiple Interfaces
A multi-homed host may elect to configure LLMNR on only one of its
active interfaces. In many situations this will be adequate.
However, should a host need to configure LLMNR on more than one of
its active interfaces, there are some additional precautions it MUST
take. Implementers who are not planning to support LLMNR on multiple
interfaces simultaneously may skip this section.
Where a host is configured to issue LLMNR queries on more than one
interface, each interface maintains its own independent LLMNR
resolver cache, containing the responses to LLMNR queries.
A multi-homed host checks the uniqueness of UNIQUE records as
described in Section 4. The situation is illustrated in figure 1.
---------- ----------
| | | |
[A] [myhost] [myhost]
Figure 1. Link-scope name conflict
In this situation, the multi-homed myhost will probe for, and defend,
its host name on both interfaces. A conflict will be detected on one
interface, but not the other. The multi-homed myhost will not be
able to respond with a host RR for "myhost" on the interface on the
right (see Figure 1). The multi-homed host may, however, be
configured to use the "myhost" name on the interface on the left.
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
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2, and that shown in Figure 3 where no conflict exists.
[A]
| |
----- -----
| |
[myhost]
Figure 3. Multiple paths to same host
This illustrates that the proposed name conflict resolution mechanism
does not support detection or resolution of conflicts between hosts
on different links. This problem can also occur with 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.
4.4. API Issues
[RFC2553] provides an API which can partially solve the name
ambiguity problem for applications written to use this API, since the
sockaddr_in6 structure exposes the scope within which each scoped
address exists, and this structure can be used for both IPv4 (using
v4-mapped IPv6 addresses) and IPv6 addresses.
Following the example in Figure 2, an application on 'myhost' issues
the request getaddrinfo("A", ...) with ai_family=AF_INET6 and
ai_flags=AI_ALL|AI_V4MAPPED. LLMNR 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 allow 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
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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 TCP queries and responses.
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 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
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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 received.
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.
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5.4. Authentication
LLMNR implementations MAY support TSIG and/or SIG(0) security
mechanisms. Since LLMNR does not support "delegated trust" (CD or AD
bits), and LLMNR senders are unlikely to be DNSSEC-aware, in practice
LLMNR is not compatible with DNSSEC.
Since LLMNR implementations MAY NOT support TSIG or SIG(0), 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 unicast
LLMNR queries and responses or LLMNR responses to multicast queries.
In a small network without a certificate authority, this can be most
easily accomplished through configuration of a group pre-shared key
for trusted hosts.
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 port 5355 for both TCP and UDP.
LLMNR requires allocation of link-scope multicast IPv4 address
224.0.0.252, as well as link-scope multicast IPv6 address
FF02:0:0:0:0:0:1:3.
7. Constants
The following timing constants are used in this protocol; they are
not intended to be user configurable.
JITTER_INTERVAL 100 ms
LLMNR_TIMEOUT 1 second (if set statically)
RTOinit 500 ms (initial value of LLMNR_TIMEOUT)
RTOmax 5 seconds (maximum value of LLMNR_TIMEOUT)
RTOmin 100 ms (minimum value of LLMNR_TIMEOUT)
8. References
8.1. 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.
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[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
1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, 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.
8.2. Informative References
[RFC1536] Kumar, A., et. al., "DNS Implementation Errors and Suggested
Fixes", RFC 1536, October 1993.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.
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[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.
[RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
of Link-Local IPv4 Addresses", RFC 3927, October 2004.
[Bonjour] Cheshire, S. and M. Krochmal, "Multicast DNS", Internet draft
(work in progress), draft-cheshire-dnsext-multicastdns-04.txt,
February 2004.
[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.
[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
[LLMNREnable]
Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work
in progress), draft-guttman-mdns-enable-02.txt, April 2002.
[NodeInfo]
Crawford, M., "IPv6 Node Information Queries", Internet draft
(work in progress), draft-ietf-ipn-gwg-icmp-name-
lookups-09.txt, May 2002.
Acknowledgments
This work builds upon original work done on multicast DNS by Bill
Manning and Bill Woodcock. Bill Manning's work was funded under
DARPA grant #F30602-99-1-0523. The authors gratefully acknowledge
their contribution to the current specification. Constructive input
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has also been received from Mark Andrews, Rob Austein, Randy Bush,
Stuart Cheshire, Ralph Droms, Robert Elz, James Gilroy, Olafur
Gudmundsson, Andreas Gustafsson, Erik Guttman, Myron Hattig,
Christian Huitema, Olaf Kolkman, Mika Liljeberg, Keith Moore,
Tomohide Nagashima, Thomas Narten, Erik Nordmark, Markku Savela, Mike
St. Johns, Sander Van-Valkenburg, and Brian Zill.
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 6605
EMail: bernarda@microsoft.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
Levon Esibov
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: levone@microsoft.com
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specification can be obtained from the IETF on-line IPR repository at
Aboba, Thaler & Esibov Standards Track [Page 27]
INTERNET-DRAFT LLMNR 25 May 2005
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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Open Issues
Open issues with this specification are tracked on the following web
site:
http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html
Aboba, Thaler & Esibov Standards Track [Page 28]
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