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DNSEXT Working Group Levon Esibov
INTERNET-DRAFT Bernard Aboba
Category: Standards Track Dave Thaler
<draft-ietf-dnsext-mdns-33.txt> Microsoft
18 July 2004
Linklocal Multicast Name Resolution (LLMNR)
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 2, 2005.
Copyright Notice
Copyright (C) The Internet Society 2004. All rights reserved.
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.
Esibov, Aboba & Thaler Standards Track [Page 1]
INTERNET-DRAFT LLMNR 18 July 2004
Table of Contents
1. Introduction .......................................... 3
1.1 Requirements .................................... 4
1.2 Terminology ..................................... 4
2. Name resolution using LLMNR ........................... 4
2.1 LLMNR packet format ............................. 6
2.2 Sender behavior ................................. 8
2.3 Responder behavior .............................. 8
2.4 Unicast queries ................................. 11
2.5 Off-link detection .............................. 11
2.6 Responder responsibilities ...................... 12
2.7 Retransmission and jitter ....................... 13
2.8 DNS TTL ......................................... 13
2.9 Use of the authority and additional sections .... 14
3. Usage model ........................................... 14
3.1 LLMNR configuration ............................. 15
4. Conflict resolution ................................... 16
4.1 Considerations for multiple interfaces .......... 18
4.2 API issues ...................................... 19
5. Security considerations ............................... 20
5.1 Scope restriction ............................... 20
5.2 Usage restriction ............................... 21
5.3 Cache and port separation ....................... 22
5.4 Authentication .................................. 22
6. IANA considerations ................................... 22
7. References ............................................ 22
7.1 Normative References ............................ 22
7.2 Informative References .......................... 23
Acknowledgments .............................................. 24
Authors' Addresses ........................................... 25
Intellectual Property Statement .............................. 25
Disclaimer of Validity ....................................... 26
Full Copyright Statement ..................................... 26
Esibov, Aboba & Thaler Standards Track [Page 2]
INTERNET-DRAFT LLMNR 18 July 2004
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 networking scenarios,
or where no DNS server is available that is authoritative for the
names of local hosts, and can support dynamic DNS, such as in
wireless hotspots.
Once we have experience in LLMNR deployment in terms of
administrative issues, usability and impact on the network, it will
be possible to reevaluate which multicast scopes are appropriate for
use with multicast name resolution.
Service discovery in general, as well as discovery of DNS servers
using LLMNR in particular, is outside of the scope of this document,
as is name resolution over non-multicast capable media.
Esibov, Aboba & Thaler Standards Track [Page 3]
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1.1. Requirements
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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 address is considered reachable over a link if either an ARP or
neighbor discovery cache entry exists for the address on the 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.
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
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.
Esibov, Aboba & Thaler Standards Track [Page 4]
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This may occur via any mechanism, including DHCPv4 [RFC2131] or
DHCPv6 [RFC3315].
LLMNR usage MAY be configured manually or automatically on a per
interface basis. By default, LLMNR responders SHOULD be enabled on
all interfaces, at all times. Enabling LLMNR for use in situations
where a DNS server has been configured will result in a change in
default behavior without a simultaneous update to configuration
information. Where this is considered undesirable, LLMNR SHOULD NOT
be enabled by default, so that hosts will neither listen on the link-
scope multicast address, nor will they send queries to that address.
An LLMNR sender may send a request for any name. However, by
default, LLMNR requests SHOULD be sent only when one of the following
conditions are met:
[1] No manual or automatic DNS configuration has been
performed. If an interface has been configured with DNS
server address(es), then LLMNR SHOULD NOT be used as the
primary name resolution mechanism on that interface, although
it MAY be used as a name resolution mechanism of last resort.
[2] DNS servers do not respond.
[3] DNS servers respond to a DNS query with RCODE=3
(Authoritative Name Error) or RCODE=0, and an empty
answer section.
A typical sequence of events for LLMNR usage is as follows:
[a] DNS servers are not configured or do not respond to a
DNS query, or respond with RCODE=3, or RCODE=0 and an
empty answer section.
[b] An LLMNR sender sends an LLMNR query to the link-scope
multicast address(es) defined in Section 2, unless a
unicast query is indicated. A sender SHOULD send LLMNR
queries for PTR RRs via unicast, as specified in Section 2.4.
[c] A responder responds to this query only if it is authoritative
for the domain name in the query. A responder responds to a
multicast query by sending a unicast UDP response to the sender.
Unicast queries are responded to as indicated in Section 2.4.
[d] Upon reception of the response, the sender processes it.
Further details of sender and responder behavior are provided in the
sections that follow.
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2.1. LLMNR packet format
LLMNR utilizes the DNS packet format defined in [RFC1035] Section 4
for both queries and responses. LLMNR implementations SHOULD send
UDP queries and responses only as large as are known to be
permissible without causing fragmentation. When in doubt a maximum
packet size of 512 octets SHOULD be used. LLMNR implementations MUST
accept UDP queries and responses as large as permitted by the link
MTU.
2.1.1. LLMNR header format
LLMNR queries and responses utilize the DNS header format defined in
[RFC1035] with exceptions noted below:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z|TC| Z| Z| Z| Z| Z| RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ID A 16 bit identifier assigned by the program that generates any kind
of query. This identifier is copied from the query to the response
and can be used by the sender to match responses to outstanding
queries. The ID field in a query SHOULD be set to a pseudo-random
value.
QR A one bit field that specifies whether this message is an LLMNR
query (0), or an LLMNR response (1).
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.
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LLMNR senders and responders MUST support standard queries (opcode
value of zero). LLMNR queries with unsupported OPCODE values MUST
be silently discarded by responders.
TC TrunCation - specifies that this message was truncated due to
length greater than that permitted on the transmission channel.
The TC bit MUST NOT be set in an LLMNR query and if set is ignored
by an LLMNR responder. If the TC bit is set an LLMNR response,
then the sender MAY use the response if it contains all necessary
information, or the sender MAY discard the response and resend the
LLMNR query over TCP using the unicast address of the responder as
the destination address. See [RFC2181] and Section 2.4 of this
specification for further discussion of the TC bit.
Z Reserved for future use. Implementations of this specification
MUST set these bits to zero in both queries and responses. If
these bits are set in a LLMNR query or response, implementations of
this specification MUST ignore them. Since reserved bits could
conceivably be used for different purposes than in DNS,
implementors are advised not to enable processing of these bits in
an LLMNR implementation starting from a DNS code base.
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 preferrable to
not sending a response, since it enables errors to be diagnosed.
Since LLMNR responders only respond to LLMNR queries for names for
which they are authoritative, LLMNR responders MUST NOT respond
with an RCODE of 3; instead, they should not respond at all.
LLMNR implementations MUST support EDNS0 [RFC2671] and extended
RCODE values.
QDCOUNT
An unsigned 16 bit integer specifying the number of entries in the
question section. A sender MUST place only one question into the
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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.
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.
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
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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
10.1.1.1 and IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6 might be
authoritative for the following records:
host1. IN A 10.1.1.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
host1.example.com. IN A 10.1.1.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
1.1.1.10.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.
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 SHOULD always be sent
from the port to which they were directed.
[c] Responders MUST respond to LLMNR queries for names and addresses
they are authoritative for. This applies to both forward and
reverse lookups.
[d] Responders MUST NOT respond to LLMNR queries for names they are not
authoritative for.
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[e] Responders MUST NOT respond using cached data.
[f] If a DNS server is running on a host that supports LLMNR, the DNS
server MUST respond to LLMNR queries only for the RRSets relating
to the host on which the server is running, but MUST NOT respond
for other records for which the server is authoritative. DNS
servers also MUST NOT send LLMNR queries in order to resolve DNS
queries.
[g] If a responder is authoritative for a name, it MAY respond with
RCODE=0 and an empty answer section, if the type of query does not
match a RR that the responder has.
As an example, a host configured to respond to LLMNR queries for the
name "foo.example.com." is authoritative for the name
"foo.example.com.". On receiving an LLMNR query for an A RR with the
name "foo.example.com." the host authoritatively responds with A
RR(s) that contain IP address(es) in the RDATA of the resource
record. If the responder has a AAAA RR, but no A RR, and an A RR
query is received, the responder would respond with RCODE=0 and an
empty answer section.
In conventional DNS terminology a DNS server authoritative for a zone
is authoritative for all the domain names under the zone apex except
for the branches delegated into separate zones. Contrary to
conventional DNS terminology, an LLMNR responder is authoritative
only for the zone 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.".
In this example (unless this limitation is introduced) an LLMNR query
for an A resource record for the name "child.foo.example.com." would
result in two authoritative responses: RCODE=3 (authoritative name
error) received from "foo.example.com.", and a requested A record -
from "child.foo.example.com.". To prevent this ambiguity, LLMNR
enabled hosts could perform a dynamic update of the parent (or
grandparent) zone with a delegation to a child zone. In this example
a host "child.foo.example.com." would send a dynamic update for the
NS and glue A record to "foo.example.com.", but this approach
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significantly complicates implementation of LLMNR and would not be
acceptable for lightweight hosts.
2.4. Unicast queries and responses
Unicast queries SHOULD be sent when:
[a] A sender repeats a query after it received a response
with the TC bit set to the previous LLMNR multicast query, or
[b] The sender queries for a PTR RR of a fully formed IP address
within the "in-addr.arpa" or "ip6.arpa" zones.
Unicast LLMNR queries MUST be done using TCP and the responses MUST
be sent using the same TCP connection as the query. Senders MUST
support sending TCP queries, and responders MUST support listening
for TCP queries. If the sender of a TCP query receives a response to
that query not using TCP, the response MUST be silently discarded.
Unicast UDP queries MUST be silently discarded.
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 an address whose prefix
belongs to a subnet on the local link.
A sender MUST select a source address for LLMNR queries that is "on
link". The destination address of an LLMNR query MUST be a link-
scope multicast address or an "on link" unicast address.
A responder MUST select a source address for responses that is "on
link". The destination address of an LLMNR response MUST be an "on
link" unicast address.
On receiving an LLMNR query, the responder MUST check whether it was
sent to a LLMNR multicast addresses defined in Section 2. If it was
sent to another multicast address, then the query MUST be silently
discarded.
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Section 2.4 discusses use of TCP for LLMNR queries and responses. In
composing an LLMNR query using TCP, the sender MUST set the Hop Limit
field in the IPv6 header and the TTL field in the IPv4 header of the
response to one (1). The responder SHOULD set the TTL or Hop Limit
settings on the TCP listen socket to one (1) so that SYN-ACK packets
will have TTL (IPv4) or Hop Limit (IPv6) set to one (1). This
prevents an incoming connection from off-link since the sender will
not receive a SYN-ACK from the responder.
For UDP queries and responses the Hop Limit field in the IPv6 header,
and the TTL field in the IPV4 header MAY be set to any value.
However, it is RECOMMENDED that the value 255 be used for
compatibility with Apple Rendezvous.
Implementation note:
In the sockets API for IPv4 [POSIX], the IP_TTL and
IP_MULTICAST_TTL socket options are used to set the TTL of
outgoing unicast and multicast packets. The IP_RECVTTL socket
option is available on some platforms to retrieve the IPv4 TTL of
received packets with recvmsg(). [RFC2292] specifies similar
options for setting and retrieving the IPv6 Hop Limit.
2.6. Responder responsibilities
It is the responsibility of the responder to ensure that RRs returned
in LLMNR responses MUST only include values that are valid on the
local interface, such as IPv4 or IPv6 addresses valid on the local
link or names defended using the mechanism described in Section 4.
In 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.
Routable addresses MUST be included first in the response, if
available. This encourages use of routable address(es) for
establishment of new connections.
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2.7. Retransmission and jitter
An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine
when to retransmit an LLMNR query and how long to collect responses
to an LLMNR query.
If an LLMNR query sent over UDP is not resolved within LLMNR_TIMEOUT,
then a sender MAY repeat the transmission of the query in order to
assure that it was received by a host capable of responding to it.
Retransmission of UDP queries SHOULD NOT be attempted more than 3
times. Where LLMNR queries are sent using TCP, retransmission is
handled by the transport layer.
Because an LLMNR sender cannot know in advance if a query sent using
multicast will receive no response, one response, or more than one
response, the sender SHOULD wait for LLMNR_TIMEOUT in order to
collect all possible responses, rather than considering the multicast
query answered after the first response is received. A unicast query
sender considers the query answered after the first response is
received, so that it only waits for LLMNR_TIMEOUT if no response has
been received.
An LLMNR sender SHOULD dynamically compute the value of LLMNR_TIMEOUT
for each transmission. It is suggested that the computation of
LLMNR_TIMEOUT be based on the response times for earlier LLMNR
queries sent on the same interface.
For example, the algorithms described in RFC 2988 [RFC2988]
(including exponential backoff) compute an RTO, which is used as the
value of LLMNR_TIMEOUT. Smaller values MAY be used for the initial
RTO (discussed in Section 2 of [RFC2988], paragraph 2.1), the minimum
RTO (discussed in Section 2 of [RFC2988], paragraph 2.4), and the
maximum RTO (discussed in Section 2 of [RFC2988], paragraph 2.5).
Recommended values are an initial RTO of 1 second, a minimum RTO of
200ms, and a maximum RTO of 5 seconds. In order to avoid
synchronization, the transmission of each LLMNR query and response
SHOULD delayed by a time randomly selected from the interval 0 to 100
ms. This delay MAY be avoided by responders responding with RRs
which they have previously determined to be UNIQUE (see Section 4 for
details).
2.8. DNS TTL
The responder should use a pre-configured TTL value in the records
returned an LLMNR response. A default value of 30 seconds is
RECOMMENDED. In highly dynamic environments (such as mobile ad-hoc
networks), the TTL value may need to be reduced.
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Due to the TTL minimalization necessary when caching an RRset, all
TTLs in an RRset MUST be set to the same value.
2.9. Use of the authority and additional sections
Unlike the DNS, LLMNR is a peer-to-peer protocol and does not have a
concept of delegation. In LLMNR, the NS resource record type may be
stored and queried for like any other type, but it has no special
delegation semantics as it does in the DNS. Responders MAY have NS
records associated with the names for which they are authoritative,
but they SHOULD NOT include these NS records in the authority
sections of responses.
Responders SHOULD insert an SOA record into the authority section of
a negative response, to facilitate negative caching as specified in
[RFC2308]. The owner name of this SOA record MUST be equal to the
query name.
Responders SHOULD NOT perform DNS additional section processing,
except as required for EDNS0 and DNSSEC.
Senders MUST NOT cache RRs from the authority or additional section
of a response as answers, though they may be used for other purposes
such as negative caching.
3. Usage model
Since LLMNR is a secondary name resolution mechanism, its usage is in
part determined by the behavior of DNS implementations. This
document does not specify any changes to DNS resolver behavior, such
as searchlist processing or retransmission/failover policy. However,
robust DNS resolver implementations are more likely to avoid
unnecessary LLMNR queries.
As noted in [DNSPerf], even when DNS servers are configured, a
significant fraction of DNS queries do not receive a response, or
result in negative responses due to missing inverse mappings or NS
records that point to nonexistent or inappropriate hosts. This has
the potential to result in a large number of unnecessary LLMNR
queries.
[RFC1536] describes common DNS implementation errors and fixes. If
the proposed fixes are implemented, unnecessary LLMNR queries will be
reduced substantially, and so implementation of [RFC1536] is
recommended.
For example, [RFC1536] Section 1 describes issues with retransmission
and recommends implementation of a retransmission policy based on
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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 AAAA RR queries
sent over IPv4 or IPv6 with an authoritative name error (RCODE=3),
then it will not be possible to resolve the names of IPv6-only hosts.
In this situation, LLMNR over IPv6 can be used for local name
resolution.
Similarly, if a DHCPv4 server is available providing DNS server
configuration, and DNS server(s) exist which are authoritative for
the A RRs of local hosts and support either dynamic client update
over IPv4 or DHCPv4-based dynamic update, then the names of local
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IPv4 hosts can be resolved over IPv4 without LLMNR. However, if no
DNS server is authoritative for the names of local hosts, or the
authoritative DNS server(s) do not support dynamic update, then LLMNR
enables linklocal name resolution over IPv4.
Where DHCPv4 or DHCPv6 is implemented, DHCP options can be used to
configure LLMNR on an interface. The LLMNR Enable Option, described
in [LLMNREnable], can be used to explicitly enable or disable use of
LLMNR on an interface. The LLMNR Enable Option does not determine
whether or in which order DNS itself is used for name resolution.
The order in which various name resolution mechanisms should be used
can be specified using the Name Service Search Option (NSSO) for DHCP
[RFC2937], using the LLMNR Enable Option code carried in the NSSO
data.
It is possible that DNS configuration mechanisms will go in and out
of service. In these circumstances, it is possible for hosts within
an administrative domain to be inconsistent in their DNS
configuration.
For example, where DHCP is used for configuring DNS servers, one or
more DHCP servers can fail. As a result, hosts configured prior to
the outage will be configured with a DNS server, while hosts
configured after the outage will not. Alternatively, it is possible
for the DNS configuration mechanism to continue functioning while
configured DNS servers fail.
Unless unconfigured hosts periodically retry configuration, an outage
in the DNS configuration mechanism will result in hosts continuing to
use LLMNR even once the outage is repaired. Since LLMNR only enables
linklocal name resolution, this represents an unnecessary degradation
in capabilities. As a result, it is recommended that hosts without a
configured DNS server periodically attempt to obtain DNS
configuration. For example, where DHCP is used for DNS
configuration, [RFC2131] recommends a maximum retry interval of 64
seconds. In the absence of other guidance, a default retry interval
of one (1) minute is RECOMMENDED.
4. Conflict resolution
The sender MUST anticipate receiving multiple replies to the same
LLMNR query, in the event that several LLMNR enabled computers
receive the query and respond with valid answers. When this occurs,
the responses may first be concatenated, and then treated in the same
manner that multiple RRs received from the same DNS server would; the
sender perceives no inherent conflict in the receipt of multiple
responses.
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There are some scenarios when multiple responders MAY respond to the
same query. There are other scenarios when only one responder MAY
respond to a query. Resource records for which the latter queries
are submitted are referred as UNIQUE throughout this document. The
uniqueness of a resource record depends on a nature of the name in
the query and type of the query. For example it is expected that:
- multiple hosts may respond to a query for an SRV type record
- multiple hosts may respond to a query for an A or AAAA type
record for a cluster name (assigned to multiple hosts in
the cluster)
- only a single host may respond to a query for an A or AAAA
type record for a name.
Every responder that responds to an LLMNR query AND includes a UNIQUE
record in the response:
[1] MUST verify that there is no other host within the
scope of the LLMNR query propagation that can return
a resource record for the same name, type and class.
[2] MUST NOT include a UNIQUE resource record in the
response without having verified its uniqueness.
Where a host is configured to issue LLMNR queries on more than one
interface, each interface should have its own independent LLMNR
cache. For each UNIQUE resource record in a given interface's
configuration, the host MUST verify resource record uniqueness on
that interface. To accomplish this, the host MUST send an LLMNR
query for each UNIQUE resource record.
By default, a host SHOULD be configured to behave as though all RRs
are UNIQUE. Uniqueness verification is carried out when the host:
- starts up or is rebooted
- wakes from sleep (if the network interface was inactive during sleep)
- is configured to respond to the LLMNR queries on an interface
enabled for transmission and reception of IP traffic
- is configured to respond to the LLMNR queries using additional
UNIQUE resource records
- detects that an interface is connected and is usable
(e.g. an IEEE 802 hardware link-state change indicating
that a cable was attached or completion of authentication
(and if needed, association) with a wireless base station
or adhoc network
When a host that has a UNIQUE record receives an LLMNR query for that
record, the host MUST respond. After the client receives a response,
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it MUST check whether the response arrived on an interface different
from the one on which the query was sent. If the response arrives on
a different interface, the client can use the UNIQUE resource record
in response to LLMNR queries. If not, then it MUST NOT use the
UNIQUE resource record in response to LLMNR queries.
The name conflict detection mechanism doesn't prevent name conflicts
when previously partitioned segments are connected by a bridge. In
order to minimize the chance of conflicts in such a situation, it is
recommended that steps be taken to ensure name uniqueness. For
example, the name could be chosen randomly from a large pool of
potential names, or the name could be assigned via a process designed
to guarantee uniqueness.
When name conflicts are detected, they SHOULD be logged. To detect
duplicate use of a name, an administrator can use a name resolution
utility which employs LLMNR and lists both responses and responders.
This would allow an administrator to diagnose behavior and
potentially to intervene and reconfigure LLMNR responders who should
not be configured to respond to the same name.
4.1. Considerations for Multiple Interfaces
A multi-homed host may elect to configure LLMNR on only one of its
active interfaces. In many situations this will be adequate.
However, should a host need to configure LLMNR on more than one of
its active interfaces, there are some additional precautions it MUST
take. Implementers who are not planning to support LLMNR on multiple
interfaces simultaneously may skip this section.
A multi-homed host checks the uniqueness of UNIQUE records as
described in Section 4. The situation is illustrated in figure 1.
---------- ----------
| | | |
[A] [myhost] [myhost]
Figure 1. Link-scope name conflict
In this situation, the multi-homed myhost will probe for, and defend,
its host name on both interfaces. A conflict will be detected on one
interface, but not the other. The multi-homed myhost will not be
able to respond with a host RR for "myhost" on the interface on the
right (see Figure 1). The multi-homed host may, however, be
configured to use the "myhost" name on the interface on the left.
Since names are only unique per-link, hosts on different links could
be using the same name. If an LLMNR client sends requests over
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multiple interfaces, and receives replies from more than one, the
result returned to the client is defined by the implementation. The
situation is illustrated in figure 2.
---------- ----------
| | | |
[A] [myhost] [A]
Figure 2. Off-segment name conflict
If host myhost is configured to use LLMNR on both interfaces, it will
send LLMNR queries on both interfaces. When host myhost sends a
query for the host RR for name "A" it will receive a response from
hosts on both interfaces.
Host myhost cannot distinguish between the situation shown in Figure
2, and that shown in Figure 3 where no conflict exists.
[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.
4.2. 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
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of both hosts responding to the name 'A'. Link-local addresses will
have a sin6_scope_id value that disambiguates which interface is used
to reach the address. Of course, to the application, Figures 2 and 3
are still indistinguishable, but this API allows the application to
communicate successfully with any address in the list.
5. Security Considerations
LLMNR is by nature a peer-to-peer name resolution protocol. It is
therefore inherently more vulnerable than DNS, since existing DNS
security mechanisms are difficult to apply to LLMNR. While tools
exist to alllow an attacker to spoof a response to a DNS query,
spoofing a response to an LLMNR query is easier since the query is
sent to a link-scope multicast address, where every host on the
logical link will be made aware of it.
In order to address the security vulnerabilities, the following
mechanisms are contemplated:
[1] Scope restrictions.
[2] Usage restrictions.
[3] Cache and port separation.
[4] Authentication.
These techniques are described in the following sections.
5.1. Scope restriction
With LLMNR it is possible that hosts will allocate conflicting names
for a period of time, or that attackers will attempt to deny service
to other hosts by allocating the same name. Such attacks also allow
hosts to receive packets destined for other hosts.
Since LLMNR is typically deployed in situations where no trust model
can be assumed, it is likely that LLMNR queries and responses will be
unauthenticated. In the absence of authentication, LLMNR reduces the
exposure to such threats by utilizing UDP queries sent to a link-
scope multicast address, as well as setting the TTL (IPv4) or Hop
Limit (IPv6) fields to one (1) on 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
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possible for an attacker to spoof a response to a frequent query
(such as an A or AAAA query for a popular Internet host), and by
using a TTL or Hop Limit field larger than one (1), for the forged
response to reach the LLMNR sender.
When LLMNR queries are sent to a link-scope multicast address, it is
possible that some routers may not properly implement link-scope
multicast, or that link-scope multicast addresses may leak into the
multicast routing system.
Setting the IPv6 Hop Limit or IPv4 TTL field to a value larger than
one in an LLMNR UDP response may enable denial of service attacks
across the Internet. However, since LLMNR responders only respond to
queries for which they are authoritative, and LLMNR does not provide
wildcard query support, it is believed that this threat is minimal.
There also are scenarios such as public "hotspots" where attackers
can be present on the same link. These threats are most serious in
wireless networks such as 802.11, since attackers on a wired network
will require physical access to the home network, while wireless
attackers may reside outside the home. Link-layer security can be of
assistance against these threats if it is available.
5.2. Usage restriction
As noted in Sections 2 and 3, LLMNR is intended for usage in a
limited set of scenarios.
If an LLMNR query is sent whenever a DNS server does not respond in a
timely way, then an attacker can poison the LLMNR cache by responding
to the query with incorrect information. To some extent, these
vulnerabilities exist today, since DNS response spoofing tools are
available that can allow an attacker to respond to a query more
quickly than a distant DNS server.
Since LLMNR queries are sent and responded to on the local-link, an
attacker will need to respond more quickly to provide its own
response prior to arrival of the response from a legitimate
responder. If an LLMNR query is sent for an off-link host, spoofing a
response in a timely way is not difficult, since a legitimate
response will never be 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,
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eliminating the benefits of cache separation. As a result, LLMNR is
only used as a name resolution mechanism of last resort.
5.3. Cache and port separation
In order to prevent responses to LLMNR queries from polluting the DNS
cache, LLMNR implementations MUST use a distinct, isolated cache for
LLMNR on each interface. The use of separate caches is most effective
when LLMNR is used as a name resolution mechanism of last resort,
since this minimizes the opportunities for poisoning the LLMNR cache,
and decreases reliance on it.
LLMNR operates on a separate port from DNS, reducing the likelihood
that a DNS server will unintentionally respond to an LLMNR query.
5.4. Authentication
LLMNR implementations may not support DNSSEC or TSIG, and as a
result, responses to LLMNR queries may be unauthenticated. If
authentication is desired, and a pre-arranged security configuration
is possible, then IPsec ESP with a null-transform MAY be used to
authenticate LLMNR responses. In a small network without a
certificate authority, this can be most easily accomplished through
configuration of a group pre-shared key for trusted hosts.
6. IANA Considerations
This specification creates one new name space: the reserved bits in
the LLMNR header. These are allocated by IETF Consensus, in
accordance with BCP 26 [RFC2434].
LLMNR requires allocation of 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. References
7.1. Normative References
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", RFC 1035, November 1987.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
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INTERNET-DRAFT LLMNR 18 July 2004
[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.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
7.2. Informative References
[RFC1536] Kumar, A., et. al., "DNS Implementation Errors and Suggested
Fixes", RFC 1536, October 1993.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.
[RFC2292] Stevens, W. and M. Thomas, "Advanced Sockets API for IPv6",
RFC 2292, February 1998.
[RFC2553] Gilligan, R., Thomson, S., Bound, J. and W. Stevens, "Basic
Socket Interface Extensions for IPv6", RFC 2553, March 1999.
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[RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC
2937, September 2000.
[RFC3315] Droms, R., et al., "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[DNSPerf] Jung, J., et al., "DNS Performance and the Effectiveness of
Caching", IEEE/ACM Transactions on Networking, Volume 10,
Number 5, pp. 589, October 2002.
[DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site local
unicast addresses to communicate with recursive DNS servers",
Internet draft (work in progress), draft-ietf-ipv6-dns-
discovery-07.txt, October 2002.
[IPV4Link]
Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
of IPv4 Link-Local Addresses", Internet draft (work in
progress), draft-ietf-zeroconf-ipv4-linklocal-15.txt, May
2004.
[POSIX] IEEE Std. 1003.1-2001 Standard for Information Technology --
Portable Operating System Interface (POSIX). Open Group
Technical Standard: Base Specifications, Issue 6, December
2001. ISO/IEC 9945:2002. http://www.opengroup.org/austin
[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 has
also been received from Mark Andrews, Stuart Cheshire, Randy Bush,
Robert Elz, Rob Austein, James Gilroy, Olafur Gudmundsson, Erik
Guttman, Myron Hattig, Thomas Narten, Christian Huitema, Erik
Nordmark, Sander Van-Valkenburg, Tomohide Nagashima, Brian Zill,
Keith Moore and Markku Savela.
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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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Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Open Issues
Open issues with this specification are tracked on the following web
site:
http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html
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