draft-ietf-dnsext-mdns-30.txt   draft-ietf-dnsext-mdns-31.txt 
DNSEXT Working Group Levon Esibov DNSEXT Working Group Levon Esibov
INTERNET-DRAFT Bernard Aboba INTERNET-DRAFT Bernard Aboba
Category: Standards Track Dave Thaler Category: Standards Track Dave Thaler
<draft-ietf-dnsext-mdns-30.txt> Microsoft <draft-ietf-dnsext-mdns-31.txt> Microsoft
17 March 2004 24 June 2004
Linklocal Multicast Name Resolution (LLMNR) Linklocal Multicast Name Resolution (LLMNR)
This document is an Internet-Draft and is in full conformance with all By submitting this Internet-Draft, I certify that any applicable
provisions of Section 10 of RFC 2026. 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 3667.
Internet-Drafts are working documents of the Internet Engineering Task Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 22, 2004.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
Today, with the rise of home networking, there are an increasing number Today, with the rise of home networking, there are an increasing
of ad-hoc networks operating without a Domain Name System (DNS) server. number of ad-hoc networks operating without a Domain Name System
In order to allow name resolution in such environments, Link-Local (DNS) server. The goal of Link-Local Multicast Name Resolution
Multicast Name Resolution (LLMNR) is proposed. LLMNR supports all (LLMNR) is to enable name resolution in scenarios in which
current and future DNS formats, types and classes, while operating on a conventional DNS name resolution is not possible. LLMNR supports all
separate port from DNS, and with a distinct resolver cache. current and future DNS formats, types and classes, while operating on
a separate port from DNS, and with a distinct resolver cache. Since
The goal of LLMNR is to enable name resolution in scenarios in which LLMNR only operates on the local link, it cannot be considered a
conventional DNS name resolution is not possible. Since LLMNR only substitute for DNS.
operates on the local link, it cannot be considered a substitute for
DNS.
Table of Contents Table of Contents
1. Introduction .......................................... 3 1. Introduction .......................................... 3
1.1 Requirements .................................... 3 1.1 Requirements .................................... 4
1.2 Terminology ..................................... 4 1.2 Terminology ..................................... 4
2. Name resolution using LLMNR ........................... 4 2. Name resolution using LLMNR ........................... 4
2.1 LLMNR packet format ............................. 5 2.1 LLMNR packet format ............................. 6
2.2 Sender behavior ................................. 8 2.2 Sender behavior ................................. 8
2.3 Responder behavior .............................. 8 2.3 Responder behavior .............................. 8
2.4 Unicast queries ................................. 10 2.4 Unicast queries ................................. 11
2.5 Off-link detection .............................. 11 2.5 Off-link detection .............................. 11
2.6 Responder responsibilities ...................... 12 2.6 Responder responsibilities ...................... 12
2.7 Retransmission and jitter ....................... 12 2.7 Retransmission and jitter ....................... 13
2.8 DNS TTL ......................................... 13 2.8 DNS TTL ......................................... 13
2.9 Use of the authority and additional sections .... 13 2.9 Use of the authority and additional sections .... 14
3. Usage model ........................................... 14 3. Usage model ........................................... 14
3.1 LLMNR configuration ............................. 15 3.1 LLMNR configuration ............................. 15
4. Conflict resolution ................................... 16 4. Conflict resolution ................................... 16
4.1 Considerations for multiple interfaces .......... 18 4.1 Considerations for multiple interfaces .......... 18
4.2 API issues ...................................... 19 4.2 API issues ...................................... 19
5. Security considerations ............................... 19 5. Security considerations ............................... 20
5.1 Scope restriction ............................... 20 5.1 Scope restriction ............................... 20
5.2 Usage restriction ............................... 21 5.2 Usage restriction ............................... 21
5.3 Cache and port separation ....................... 21 5.3 Cache and port separation ....................... 22
5.4 Authentication .................................. 22 5.4 Authentication .................................. 22
6. IANA considerations ................................... 22 6. IANA considerations ................................... 22
7. References ............................................ 22 7. References ............................................ 22
7.1 Normative References ............................ 22 7.1 Normative References ............................ 22
7.2 Informative References .......................... 23 7.2 Informative References .......................... 23
Acknowledgments .............................................. 24 Acknowledgments .............................................. 24
Authors' Addresses ........................................... 25 Authors' Addresses ........................................... 25
Intellectual Property Statement .............................. 25 Intellectual Property Statement .............................. 25
Full Copyright Statement ..................................... 26 Full Copyright Statement ..................................... 26
1. Introduction 1. Introduction
This document discusses Link Local Multicast Name Resolution (LLMNR), This document discusses Link Local Multicast Name Resolution (LLMNR),
which utilizes the DNS packet format and supports all current and future which utilizes the DNS packet format and supports all current and
DNS formats, types and classes. LLMNR operates on a separate port from future DNS formats, types and classes. LLMNR operates on a separate
the Domain Name System (DNS), with a distinct resolver cache. 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 The goal of LLMNR is to enable name resolution in scenarios in which
conventional DNS name resolution is not possible. These include conventional DNS name resolution is not possible. These include
scenarios in which hosts are not configured with the address of a DNS 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 server, where configured DNS servers do not reply to a query, or
they respond with errors, as described in Section 2. Since LLMNR only where they respond with errors, as described in Section 2. Since
operates on the local link, it cannot be considered a substitute for LLMNR only operates on the local link, it cannot be considered a
DNS. substitute for DNS.
Link-scope multicast addresses are used to prevent propagation of LLMNR Link-scope multicast addresses are used to prevent propagation of
traffic across routers, potentially flooding the network. LLMNR queries LLMNR traffic across routers, potentially flooding the network.
can also be sent to a unicast address, as described in Section 2.4. 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 Propagation of LLMNR packets on the local link is considered
to enable name resolution in small networks. The assumption is that if sufficient to enable name resolution in small networks. The
a network has a gateway, then the network is able to provide DNS server assumption is that if a network has a gateway, then the network is
configuration. Configuration issues are discussed in Section 3.1. 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 In the future, it may be desirable to consider use of multicast name
resolution with multicast scopes beyond the link-scope. This could resolution with multicast scopes beyond the link-scope. This could
occur if LLMNR deployment is successful, the need for multicast name occur if LLMNR deployment is successful, the need arises for
resolution beyond the link-scope, or multicast routing becomes multicast name resolution beyond the link-scope, or multicast routing
ubiquitous. For example, expanded support for multicast name resolution becomes ubiquitous. For example, expanded support for multicast name
might be required for mobile ad-hoc networking scenarios, or where no resolution might be required for mobile ad-hoc networking scenarios,
DNS server is available that is authoritative for the names of local or where no DNS server is available that is authoritative for the
hosts, and can support dynamic DNS, such as in wireless hotspots. 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 Once we have experience in LLMNR deployment in terms of
issues, usability and impact on the network, it will be possible to administrative issues, usability and impact on the network, it will
reevaluate which multicast scopes are appropriate for use with multicast be possible to reevaluate which multicast scopes are appropriate for
name resolution. use with multicast name resolution.
Service discovery in general, as well as discovery of DNS servers using Service discovery in general, as well as discovery of DNS servers
LLMNR in particular, is outside of the scope of this document, as is using LLMNR in particular, is outside of the scope of this document,
name resolution over non-multicast capable media. as is name resolution over non-multicast capable media.
1.1. Requirements 1.1. Requirements
In this document, several words are used to signify the requirements of In this document, several words are used to signify the requirements
the specification. These words are often capitalized. The key words of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be and "OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
interpreted as described in [RFC2119].
1.2. Terminology 1.2. Terminology
This document assumes familiarity with DNS terminology defined in This document assumes familiarity with DNS terminology defined in
[RFC1035]. Other terminology used in this document includes: [RFC1035]. Other terminology used in this document includes:
Positively Resolved Positively Resolved
Responses with RCODE set to zero are referred to in this document Responses with RCODE set to zero are referred to in this document
as "positively resolved". as "positively resolved".
skipping to change at page 4, line 33 skipping to change at page 4, line 39
Responder Responder
A host that listens to LLMNR queries, and responds to those for A host that listens to LLMNR queries, and responds to those for
which it is authoritative. which it is authoritative.
Sender Sender
A host that sends an LLMNR query. A host that sends an LLMNR query.
2. Name resolution using LLMNR 2. Name resolution using LLMNR
LLMNR is a peer-to-peer name resolution protocol that is not intended as LLMNR is a peer-to-peer name resolution protocol that is not intended
a replacement for DNS. LLMNR queries are sent to and received on port as a replacement for DNS. LLMNR queries are sent to and received on
TBD. IPv4 administratively scoped multicast usage is specified in port TBD. IPv4 administratively scoped multicast usage is specified
"Administratively Scoped IP Multicast" [RFC2365]. The IPv4 link-scope in "Administratively Scoped IP Multicast" [RFC2365]. The IPv4 link-
multicast address a given responder listens to, and to which a sender scope multicast address a given responder listens to, and to which a
sends queries, is TBD. The IPv6 link-scope multicast address a given sender sends queries, is 224.0.0.252. The IPv6 link-scope multicast
responder listens to, and to which a sender sends all queries, is TBD. 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. Typically a host is configured as both an LLMNR sender and a
A host MAY be configured as a sender, but not a responder. However, a responder. A host MAY be configured as a sender, but not a
host configured as a responder MUST act as a sender to verify the responder. However, a host configured as a responder MUST act as a
uniqueness of names as described in Section 4. This document does not sender to verify the uniqueness of names as described in Section 4.
specify how names are chosen or configured. This may occur via any This document does not specify how names are chosen or configured.
mechanism, including DHCPv4 [RFC2131] or DHCPv6 [RFC3315].
LLMNR usage MAY be configured manually or automatically on a per This may occur via any mechanism, including DHCPv4 [RFC2131] or
interface basis. By default, LLMNR responders SHOULD be enabled on all DHCPv6 [RFC3315].
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 LLMNR usage MAY be configured manually or automatically on a per
default, so that hosts will neither listen on the link-scope multicast interface basis. By default, LLMNR responders SHOULD be enabled on
address, nor will they send queries to that address. 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, An LLMNR sender may send a request for any name. However, by
LLMNR requests SHOULD be sent only when one of the following conditions default, LLMNR requests SHOULD be sent only when one of the following
are met: conditions are met:
[1] No manual or automatic DNS configuration has been performed. If an [1] No manual or automatic DNS configuration has been
interface has been configured with DNS server address(es), then performed. If an interface has been configured with DNS
LLMNR SHOULD NOT be used as the primary name resolution mechanism server address(es), then LLMNR SHOULD NOT be used as the
on that interface, although it MAY be used as a name resolution primary name resolution mechanism on that interface, although
mechanism of last resort. it MAY be used as a name resolution mechanism of last resort.
[2] DNS servers do not respond. [2] DNS servers do not respond.
[3] DNS servers respond to a DNS query with RCODE=3 (Authoritative Name [3] DNS servers respond to a DNS query with RCODE=3
Error) or RCODE=0, and an empty answer section. (Authoritative Name Error) or RCODE=0, and an empty
answer section.
A typical sequence of events for LLMNR usage is as follows: 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 [a] DNS servers are not configured or do not respond to a
respond with RCODE=3, or RCODE=0 and an empty answer section. 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 [b] An LLMNR sender sends an LLMNR query to the link-scope
address(es) defined in Section 2, unless a unicast query is multicast address(es) defined in Section 2, unless a
indicated. A sender SHOULD send LLMNR queries for PTR RRs via unicast query is indicated. A sender SHOULD send LLMNR
unicast, as specified in Section 2.4. 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 [c] A responder responds to this query only if it is authoritative
the domain name in the query. A responder responds to a multicast for the domain name in the query. A responder responds to a
query by sending a unicast UDP response to the sender. Unicast multicast query by sending a unicast UDP response to the sender.
queries are responded to as indicated in Section 2.4. Unicast queries are responded to as indicated in Section 2.4.
[d] Upon reception of the response, the sender processes it. [d] Upon reception of the response, the sender processes it.
Further details of sender and responder behavior are provided in the Further details of sender and responder behavior are provided in the
sections that follow. sections that follow.
2.1. LLMNR packet format 2.1. LLMNR packet format
LLMNR utilizes the DNS packet format defined in [RFC1035] Section 4 for LLMNR utilizes the DNS packet format defined in [RFC1035] Section 4
both queries and responses. LLMNR implementations SHOULD send UDP for both queries and responses. LLMNR implementations SHOULD send
queries and responses only as large as are known to be permissible UDP queries and responses only as large as are known to be
without causing fragmentation. When in doubt a maximum packet size of permissible without causing fragmentation. When in doubt a maximum
512 octets SHOULD be used. LLMNR implementations MUST accept UDP packet size of 512 octets SHOULD be used. LLMNR implementations MUST
queries and responses as large as permitted by the link MTU. accept UDP queries and responses as large as permitted by the link
MTU.
2.1.1. LLMNR header format 2.1.1. LLMNR header format
LLMNR queries and responses utilize the DNS header format defined in LLMNR queries and responses utilize the DNS header format defined in
[RFC1035] with exceptions noted below: [RFC1035] with exceptions noted below:
1 1 1 1 1 1 1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID | | ID |
skipping to change at page 8, line 21 skipping to change at page 8, line 30
An unsigned 16 bit integer specifying the number of resource An unsigned 16 bit integer specifying the number of resource
records in the additional records section. Additional record records in the additional records section. Additional record
section processing is described in Section 2.9. section processing is described in Section 2.9.
2.2. Sender behavior 2.2. Sender behavior
A sender may send an LLMNR query for any legal resource record type 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. (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. 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 Sections 2 and 3 describe the circumstances in which LLMNR queries
be sent. may be sent.
The sender MUST anticipate receiving no replies to some LLMNR queries, The sender MUST anticipate receiving no replies to some LLMNR
in the event that no responders are available within the link-scope or queries, in the event that no responders are available within the
in the event no positive non-null responses exist for the transmitted link-scope or in the event no positive non-null responses exist for
query. If no positive response is received, a resolver treats it as a the transmitted query. If no positive response is received, a
response that no records of the specified type and class exist for the resolver treats it as a response that no records of the specified
specified name (it is treated the same as a response with RCODE=0 and an type and class exist for the specified name (it is treated the same
empty answer section). 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 Since the responder may order the RRs in the response so as to
preference, the sender SHOULD preserve ordering in the response to the indicate preference, the sender SHOULD preserve ordering in the
querying application. response to the querying application.
2.3. Responder behavior 2.3. Responder behavior
An LLMNR response MUST be sent to the sender via unicast. An LLMNR response MUST be sent to the sender via unicast.
Upon configuring an IP address responders typically will synthesize Upon configuring an IP address responders typically will synthesize
corresponding A, AAAA and PTR RRs so as to be able to respond to LLMNR corresponding A, AAAA and PTR RRs so as to be able to respond to
queries for these RRs. An SOA RR is synthesized only when a responder LLMNR queries for these RRs. An SOA RR is synthesized only when a
has another RR as well; the SOA RR MUST NOT be the only RR that a responder has another RR as well; the SOA RR MUST NOT be the only RR
responder has. However, in general whether RRs are manually or that a responder has. However, in general whether RRs are manually
automatically created is an implementation decision. or automatically created is an implementation decision.
For example, a host configured to have computer name "host1" and to be a For example, a host configured to have computer name "host1" and to
member of the "example.com" domain, and with IPv4 address 10.1.1.1 and be a member of the "example.com" domain, and with IPv4 address
IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6 might be authoritative for the 10.1.1.1 and IPv6 address 2001:0DB8::1:2:3:FF:FE:4:5:6 might be
following records: authoritative for the following records:
host1. IN A 10.1.1.1 host1. IN A 10.1.1.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6 IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
host1.example.com. IN A 10.1.1.1 host1.example.com. IN A 10.1.1.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6 IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
1.1.1.10.in-addr.arpa. IN PTR host1. 1.1.1.10.in-addr.arpa. IN PTR host1.
IN PTR host1.example.com. 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 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.
skipping to change at page 9, line 17 skipping to change at page 9, line 26
host1.example.com. IN A 10.1.1.1 host1.example.com. IN A 10.1.1.1
IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6 IN AAAA 2001:0DB8::1:2:3:FF:FE:4:5:6
1.1.1.10.in-addr.arpa. IN PTR host1. 1.1.1.10.in-addr.arpa. IN PTR host1.
IN PTR host1.example.com. 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 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.
IN PTR host1.example.com IN PTR host1.example.com
An LLMNR responder might be further manually configured with the name of An LLMNR responder might be further manually configured with the name
a local mail server with an MX RR included in the "host1." and of a local mail server with an MX RR included in the "host1." and
"host1.example.com." records. "host1.example.com." records.
In responding to queries: In responding to queries:
[a] Responders MUST listen on UDP port TBD on the link-scope multicast [a] Responders MUST listen on UDP port TBD on the link-scope multicast
address(es) defined in Section 2, and on UDP and TCP port TBD on address(es) defined in Section 2, and on UDP and TCP port TBD on
the unicast address(es) that could be set as the source address(es) the unicast address(es) that could be set as the source address(es)
when the responder responds to the LLMNR query. when the responder responds to the LLMNR query.
[b] Responders MUST direct responses to the port from which the query [b] Responders MUST direct responses to the port from which the query
skipping to change at page 10, line 9 skipping to change at page 10, line 21
servers also MUST NOT send LLMNR queries in order to resolve DNS servers also MUST NOT send LLMNR queries in order to resolve DNS
queries. queries.
[g] If a responder is authoritative for a name, it MAY respond with [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 RCODE=0 and an empty answer section, if the type of query does not
match a RR that the responder has. match a RR that the responder has.
As an example, a host configured to respond to LLMNR queries for the As an example, a host configured to respond to LLMNR queries for the
name "foo.example.com." is authoritative for the name name "foo.example.com." is authoritative for the name
"foo.example.com.". On receiving an LLMNR query for an A RR with the "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) name "foo.example.com." the host authoritatively responds with A
that contain IP address(es) in the RDATA of the resource record. If the RR(s) that contain IP address(es) in the RDATA of the resource
responder has a AAAA RR, but no A RR, and an A RR query is received, the record. If the responder has a AAAA RR, but no A RR, and an A RR
responder would respond with RCODE=0 and an empty answer section. 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 In conventional DNS terminology a DNS server authoritative for a zone
authoritative for all the domain names under the zone apex except for is authoritative for all the domain names under the zone apex except
the branches delegated into separate zones. Contrary to conventional for the branches delegated into separate zones. Contrary to
DNS terminology, an LLMNR responder is authoritative only for the zone conventional DNS terminology, an LLMNR responder is authoritative
apex. only for the zone apex.
For example the host "foo.example.com." is not authoritative for the For example the host "foo.example.com." is not authoritative for the
name "child.foo.example.com." unless the host is configured with name "child.foo.example.com." unless the host is configured with
multiple names, including "foo.example.com." and multiple names, including "foo.example.com." and
"child.foo.example.com.". As a result, "foo.example.com." cannot reply "child.foo.example.com.". As a result, "foo.example.com." cannot
to an LLMNR query for "child.foo.example.com." with RCODE=3 reply to an LLMNR query for "child.foo.example.com." with RCODE=3
(authoritative name error). The purpose of limiting the name authority (authoritative name error). The purpose of limiting the name
scope of a responder is to prevent complications that could be caused by authority scope of a responder is to prevent complications that could
coexistence of two or more hosts with the names representing child and be caused by coexistence of two or more hosts with the names
parent (or grandparent) nodes in the DNS tree, for example, representing child and parent (or grandparent) nodes in the DNS tree,
"foo.example.com." and "child.foo.example.com.". for example, "foo.example.com." and "child.foo.example.com.".
In this example (unless this limitation is introduced) an LLMNR query In this example (unless this limitation is introduced) an LLMNR query
for an A resource record for the name "child.foo.example.com." would for an A resource record for the name "child.foo.example.com." would
result in two authoritative responses: RCODE=3 (authoritative name result in two authoritative responses: RCODE=3 (authoritative name
error) received from "foo.example.com.", and a requested A record - from error) received from "foo.example.com.", and a requested A record -
"child.foo.example.com.". To prevent this ambiguity, LLMNR enabled from "child.foo.example.com.". To prevent this ambiguity, LLMNR
hosts could perform a dynamic update of the parent (or grandparent) zone enabled hosts could perform a dynamic update of the parent (or
with a delegation to a child zone. In this example a host grandparent) zone with a delegation to a child zone. In this example
"child.foo.example.com." would send a dynamic update for the NS and glue a host "child.foo.example.com." would send a dynamic update for the
A record to "foo.example.com.", but this approach significantly NS and glue A record to "foo.example.com.", but this approach
complicates implementation of LLMNR and would not be acceptable for significantly complicates implementation of LLMNR and would not be
lightweight hosts. acceptable for lightweight hosts.
2.4. Unicast queries and responses 2.4. Unicast queries and responses
Unicast queries SHOULD be sent when: Unicast queries SHOULD be sent when:
[a] A sender repeats a query after it received a response with the TC [a] A sender repeats a query after it received a response
bit set to the previous LLMNR multicast query, or with the TC bit set to the previous LLMNR multicast query, or
[b] The sender queries for a PTR RR of a fully formed IP address within
the "in-addr.arpa" or "ip6.arpa" zones.
A responder receiving a unicast query MUST send the response with a
source address set to the destination address field of the IP header of
the query causing the response.
Unicast LLMNR queries MUST be sent using TCP. Senders MUST support [b] The sender queries for a PTR RR of a fully formed IP address
sending TCP queries, and responders MUST support listening for TCP within the "in-addr.arpa" or "ip6.arpa" zones.
queries.
Responses to TCP unicast LLMNR queries MUST be sent using TCP, using Unicast LLMNR queries MUST be done using TCP and the responses MUST
the same connection as the query. If the sender of a TCP query receives be sent using the same TCP connection as the query. Senders MUST
a response to that query not using TCP, the response MUST be silently support sending TCP queries, and responders MUST support listening
discarded. 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. Unicast UDP queries MUST be silently discarded.
If TCP connection setup cannot be completed in order to send a unicast If TCP connection setup cannot be completed in order to send a
TCP query, this is treated as a response that no records of the unicast TCP query, this is treated as a response that no records of
specified type and class exist for the specified name (it is treated the the specified type and class exist for the specified name (it is
same as a response with RCODE=0 and an empty answer section). treated the same as a response with RCODE=0 and an empty answer
section).
2.5. "Off link" detection 2.5. "Off link" detection
For IPv4, an "on link" address is defined as a link-local address 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 [IPv4Link] or an address whose prefix belongs to a subnet on the
link. For IPv6 [RFC2460] an "on link" address is either a link-local local link. For IPv6 [RFC2460] an "on link" address is either a
address, defined in [RFC2373], or an address whose prefix belongs to a link-local address, defined in [RFC2373], or an address whose prefix
subnet on the local link. belongs to a subnet on the local link.
A sender MUST select a source address for LLMNR queries that is "on 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 link". The destination address of an LLMNR query MUST be a link-
multicast address or an "on link" unicast address. scope multicast address or an "on link" unicast address.
A responder MUST select a source address for responses that is "on 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" link". The destination address of an LLMNR response MUST be an "on
unicast address. link" unicast address.
On receiving an LLMNR query, the responder MUST check whether it was 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 a LLMNR multicast addresses defined in Section 2. If it was
sent to another multicast address, then the query MUST be silently sent to another multicast address, then the query MUST be silently
discarded. discarded.
Section 2.4 discusses use of TCP for LLMNR queries and responses. In 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 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 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 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 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 will have TTL (IPv4) or Hop Limit (IPv6) set to one (1). This
an incoming connection from off-link since the sender will not receive a prevents an incoming connection from off-link since the sender will
not receive a SYN-ACK from the responder.
SYN-ACK from the responder.
For UDP queries and responses the Hop Limit field in the IPv6 header, 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, and the TTL field in the IPV4 header MAY be set to any value.
it is RECOMMENDED that the value 255 be used for compatibility with However, it is RECOMMENDED that the value 255 be used for
Apple Rendezvous. compatibility with Apple Rendezvous.
Implementation note: Implementation note:
In the sockets API for IPv4 [POSIX], the IP_TTL and IP_MULTICAST_TTL In the sockets API for IPv4 [POSIX], the IP_TTL and
socket options are used to set the TTL of outgoing unicast and IP_MULTICAST_TTL socket options are used to set the TTL of
multicast packets. The IP_RECVTTL socket option is available on some outgoing unicast and multicast packets. The IP_RECVTTL socket
platforms to retrieve the IPv4 TTL of received packets with option is available on some platforms to retrieve the IPv4 TTL of
recvmsg(). [RFC2292] specifies similar options for setting and received packets with recvmsg(). [RFC2292] specifies similar
retrieving the IPv6 Hop Limit. options for setting and retrieving the IPv6 Hop Limit.
2.6. Responder responsibilities 2.6. Responder responsibilities
It is the responsibility of the responder to ensure that RRs returned in It is the responsibility of the responder to ensure that RRs returned
LLMNR responses MUST only include values that are valid on the local in LLMNR responses MUST only include values that are valid on the
interface, such as IPv4 or IPv6 addresses valid on the local link or local interface, such as IPv4 or IPv6 addresses valid on the local
names defended using the mechanism described in Section 4. In link or names defended using the mechanism described in Section 4.
particular: In particular:
[a] If a link-scope IPv6 address is returned in a AAAA RR, that address [a] If a link-scope IPv6 address is returned in a AAAA RR,
MUST be valid on the local link over which LLMNR is used. 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 [b] If an IPv4 address is returned, it MUST be reachable
link over which LLMNR is used. through the link over which LLMNR is used.
[c] If a name is returned (for example in a CNAME, MX or SRV RR), the [c] If a name is returned (for example in a CNAME, MX
name MUST be resolvable on the local link over which LLMNR is used. 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. Routable addresses MUST be included first in the response, if
This encourages use of routable address(es) for establishment of new available. This encourages use of routable address(es) for
connections. establishment of new connections.
2.7. Retransmission and jitter 2.7. Retransmission and jitter
An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine An LLMNR sender uses the timeout interval LLMNR_TIMEOUT to determine
when to retransmit an LLMNR query and how long to collect responses to when to retransmit an LLMNR query and how long to collect responses
an LLMNR query. to an LLMNR query.
If an LLMNR query sent over UDP is not resolved within LLMNR_TIMEOUT, 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 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. 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. Retransmission of UDP queries SHOULD NOT be attempted more than 3
Where LLMNR queries are sent using TCP, retransmission is handled by the times. Where LLMNR queries are sent using TCP, retransmission is
handled by the transport layer.
transport layer.
Because an LLMNR sender cannot know in advance if a query sent using Because an LLMNR sender cannot know in advance if a query sent using
multicast will receive no response, one response, or more than one multicast will receive no response, one response, or more than one
response, the sender SHOULD wait for LLMNR_TIMEOUT in order to collect response, the sender SHOULD wait for LLMNR_TIMEOUT in order to
all possible responses, rather than considering the multicast query collect all possible responses, rather than considering the multicast
answered after the first response is received. A unicast query sender query answered after the first response is received. A unicast query
considers the query answered after the first response is received, so sender considers the query answered after the first response is
that it only waits for LLMNR_TIMEOUT if no response has been received. 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 An LLMNR sender SHOULD dynamically compute the value of LLMNR_TIMEOUT
for each transmission. It is suggested that the computation of for each transmission. It is suggested that the computation of
LLMNR_TIMEOUT be based on the response times for earlier LLMNR queries LLMNR_TIMEOUT be based on the response times for earlier LLMNR
sent on the same interface. queries sent on the same interface.
For example, the algorithms described in RFC 2988 [RFC2988] (including For example, the algorithms described in RFC 2988 [RFC2988]
exponential backoff) compute an RTO, which is used as the value of (including exponential backoff) compute an RTO, which is used as the
LLMNR_TIMEOUT. Smaller values MAY be used for the initial RTO value of LLMNR_TIMEOUT. Smaller values MAY be used for the initial
(discussed in Section 2 of [RFC2988], paragraph 2.1), the minimum RTO RTO (discussed in Section 2 of [RFC2988], paragraph 2.1), the minimum
(discussed in Section 2 of [RFC2988], paragraph 2.4), and the maximum RTO (discussed in Section 2 of [RFC2988], paragraph 2.4), and the
RTO (discussed in Section 2 of [RFC2988], paragraph 2.5). maximum RTO (discussed in Section 2 of [RFC2988], paragraph 2.5).
Recommended values are an initial RTO of 1 second, a minimum RTO of 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 200ms, and a maximum RTO of 5 seconds. In order to avoid
synchronization, the transmission of each LLMNR query and response synchronization, the transmission of each LLMNR query and response
SHOULD delayed by a time randomly selected from the interval 0 to 100 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 ms. This delay MAY be avoided by responders responding with RRs
they have previously determined to be UNIQUE (see Section 4 for which they have previously determined to be UNIQUE (see Section 4 for
details). details).
2.8. DNS TTL 2.8. DNS TTL
The responder should use a pre-configured TTL value in the records The responder should use a pre-configured TTL value in the records
returned an LLMNR response. A default value of 30 seconds is returned an LLMNR response. A default value of 30 seconds is
RECOMMENDED. In highly dynamic environments (such as mobile ad-hoc RECOMMENDED. In highly dynamic environments (such as mobile ad-hoc
networks), the TTL value may need to be reduced. networks), the TTL value may need to be reduced.
Due to the TTL minimalization necessary when caching an RRset, all TTLs Due to the TTL minimalization necessary when caching an RRset, all
in an RRset MUST be set to the same value. TTLs in an RRset MUST be set to the same value.
2.9. Use of the authority and additional sections 2.9. Use of the authority and additional sections
Unlike the DNS, LLMNR is a peer-to-peer protocol and does not have a 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 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 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 delegation semantics as it does in the DNS. Responders MAY have NS
records associated with the names for which they are authoritative, but records associated with the names for which they are authoritative,
they SHOULD NOT include these NS records in the authority sections of but they SHOULD NOT include these NS records in the authority
sections of responses.
responses.
Responders SHOULD insert an SOA record into the authority section of a Responders SHOULD insert an SOA record into the authority section of
negative response, to facilitate negative caching as specified in a negative response, to facilitate negative caching as specified in
[RFC2308]. The owner name of this SOA record MUST be equal to the query [RFC2308]. The owner name of this SOA record MUST be equal to the
name. query name.
Responders SHOULD NOT perform DNS additional section processing, except Responders SHOULD NOT perform DNS additional section processing,
as required for EDNS0 and DNSSEC. except as required for EDNS0 and DNSSEC.
Senders MUST NOT cache RRs from the authority or additional section of a Senders MUST NOT cache RRs from the authority or additional section
response as answers, though they may be used for other purposes such as of a response as answers, though they may be used for other purposes
negative caching. such as negative caching.
3. Usage model 3. Usage model
Since LLMNR is a secondary name resolution mechanism, its usage is in Since LLMNR is a secondary name resolution mechanism, its usage is in
part determined by the behavior of DNS implementations. This document part determined by the behavior of DNS implementations. This
does not specify any changes to DNS resolver behavior, such as document does not specify any changes to DNS resolver behavior, such
searchlist processing or retransmission/failover policy. However, as searchlist processing or retransmission/failover policy. However,
robust DNS resolver implementations are more likely to avoid unnecessary robust DNS resolver implementations are more likely to avoid
LLMNR queries. unnecessary LLMNR queries.
As noted in [DNSPerf], even when DNS servers are configured, a As noted in [DNSPerf], even when DNS servers are configured, a
significant fraction of DNS queries do not receive a response, or result significant fraction of DNS queries do not receive a response, or
in negative responses due to missing inverse mappings or NS records that result in negative responses due to missing inverse mappings or NS
point to nonexistent or inappropriate hosts. This has the potential to records that point to nonexistent or inappropriate hosts. This has
result in a large number of unnecessary LLMNR queries. the potential to result in a large number of unnecessary LLMNR
queries.
[RFC1536] describes common DNS implementation errors and fixes. If the [RFC1536] describes common DNS implementation errors and fixes. If
proposed fixes are implemented, unnecessary LLMNR queries will be the proposed fixes are implemented, unnecessary LLMNR queries will be
reduced substantially, and so implementation of [RFC1536] is reduced substantially, and so implementation of [RFC1536] is
recommended. recommended.
For example, [RFC1536] Section 1 describes issues with retransmission For example, [RFC1536] Section 1 describes issues with retransmission
and recommends implementation of a retransmission policy based on round and recommends implementation of a retransmission policy based on
trip estimates, with exponential backoff. [RFC1536] Section 4 describes round trip estimates, with exponential backoff. [RFC1536] Section 4
issues with failover, and recommends that resolvers try another server describes issues with failover, and recommends that resolvers try
when they don't receive a response to a query. These policies are another server when they don't receive a response to a query. These
likely to avoid unnecessary LLMNR queries. policies are likely to avoid unnecessary LLMNR queries.
[RFC1536] Section 3 describes zero answer bugs, which if addressed will [RFC1536] Section 3 describes zero answer bugs, which if addressed
also reduce unnecessary LLMNR queries. will also reduce unnecessary LLMNR queries.
[RFC1536] Section 6 describes name error bugs and recommended searchlist [RFC1536] Section 6 describes name error bugs and recommended
processing that will reduce unnecessary RCODE=3 (authoritative name) searchlist processing that will reduce unnecessary RCODE=3
errors, thereby also reducing unnecessary LLMNR queries. (authoritative name) errors, thereby also reducing unnecessary LLMNR
queries.
3.1. LLMNR configuration 3.1. LLMNR configuration
Since IPv4 and IPv6 utilize distinct configuration mechanisms, it is Since IPv4 and IPv6 utilize distinct configuration mechanisms, it is
possible for a dual stack host to be configured with the address of a 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 DNS server over IPv4, while remaining unconfigured with a DNS server
suitable for use over IPv6. suitable for use over IPv6.
In these situations, a dual stack host will send AAAA queries to the In these situations, a dual stack host will send AAAA queries to the
configured DNS server over IPv4. However, an IPv6-only host configured DNS server over IPv4. However, an IPv6-only host
unconfigured with a DNS server suitable for use over IPv6 will be unable unconfigured with a DNS server suitable for use over IPv6 will be
to resolve names using DNS. Automatic IPv6 DNS configuration mechanisms unable to resolve names using DNS. Automatic IPv6 DNS configuration
(such as [RFC3315] and [DNSDisc]) are not yet widely deployed, and not mechanisms (such as [RFC3315] and [DNSDisc]) are not yet widely
all DNS servers support IPv6. Therefore lack of IPv6 DNS configuration deployed, and not all DNS servers support IPv6. Therefore lack of
may be a common problem in the short term, and LLMNR may prove useful in IPv6 DNS configuration may be a common problem in the short term, and
enabling linklocal name resolution over IPv6. LLMNR may prove useful in enabling linklocal name resolution over
IPv6.
Where a DHCPv4 server is available but not a DHCPv6 server [RFC3315], 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 IPv6-only hosts may not be configured with a DNS server. Where there
no DNS server authoritative for the name of a host or the authoritative is no DNS server authoritative for the name of a host or the
DNS server does not support dynamic client update over IPv6 or authoritative DNS server does not support dynamic client update over
DHCPv6-based dynamic update, then an IPv6-only host will not be able to IPv6 or DHCPv6-based dynamic update, then an IPv6-only host will not
do DNS dynamic update, and other hosts will not be able to resolve its be able to do DNS dynamic update, and other hosts will not be able to
name. resolve its name.
For example, if the configured DNS server responds to AAAA RR queries 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 sent over IPv4 or IPv6 with an authoritative name error (RCODE=3),
it will not be possible to resolve the names of IPv6-only hosts. In then it will not be possible to resolve the names of IPv6-only hosts.
this situation, LLMNR over IPv6 can be used for local name resolution. In this situation, LLMNR over IPv6 can be used for local name
resolution.
Similarly, if a DHCPv4 server is available providing DNS server Similarly, if a DHCPv4 server is available providing DNS server
configuration, and DNS server(s) exist which are authoritative for the A configuration, and DNS server(s) exist which are authoritative for
RRs of local hosts and support either dynamic client update over IPv4 or the A RRs of local hosts and support either dynamic client update
DHCPv4-based dynamic update, then the names of local IPv4 hosts can be over IPv4 or DHCPv4-based dynamic update, then the names of local
resolved over IPv4 without LLMNR. However, if no DNS server is IPv4 hosts can be resolved over IPv4 without LLMNR. However, if no
authoritative for the names of local hosts, or the authoritative DNS DNS server is authoritative for the names of local hosts, or the
server(s) do not support dynamic update, then LLMNR enables linklocal authoritative DNS server(s) do not support dynamic update, then LLMNR
name resolution over IPv4. enables linklocal name resolution over IPv4.
Where DHCPv4 or DHCPv6 is implemented, DHCP options can be used to Where DHCPv4 or DHCPv6 is implemented, DHCP options can be used to
configure LLMNR on an interface. The LLMNR Enable Option, described in configure LLMNR on an interface. The LLMNR Enable Option, described
[LLMNREnable], can be used to explicitly enable or disable use of LLMNR in [LLMNREnable], can be used to explicitly enable or disable use of
on an interface. The LLMNR Enable Option does not determine whether or LLMNR on an interface. The LLMNR Enable Option does not determine
in which order DNS itself is used for name resolution. The order in whether or in which order DNS itself is used for name resolution.
which various name resolution mechanisms should be used can be specified The order in which various name resolution mechanisms should be used
using the Name Service Search Option (NSSO) for DHCP [RFC2937], using can be specified using the Name Service Search Option (NSSO) for DHCP
the LLMNR Enable Option code carried in the NSSO data. [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 It is possible that DNS configuration mechanisms will go in and out
administrative domain to be inconsistent in their DNS configuration. 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 For example, where DHCP is used for configuring DNS servers, one or
DHCP servers can fail. As a result, hosts configured prior to the more DHCP servers can fail. As a result, hosts configured prior to
outage will be configured with a DNS server, while hosts configured the outage will be configured with a DNS server, while hosts
after the outage will not. Alternatively, it is possible for the DNS configured after the outage will not. Alternatively, it is possible
configuration mechanism to continue functioning while configured DNS for the DNS configuration mechanism to continue functioning while
servers fail. configured DNS servers fail.
Unless unconfigured hosts periodically retry configuration, an outage in Unless unconfigured hosts periodically retry configuration, an outage
the DNS configuration mechanism will result in hosts continuing to use in the DNS configuration mechanism will result in hosts continuing to
LLMNR even once the outage is repaired. Since LLMNR only enables use LLMNR even once the outage is repaired. Since LLMNR only enables
linklocal name resolution, this represents an unnecessary degradation in linklocal name resolution, this represents an unnecessary degradation
capabilities. As a result, it is recommended that hosts without a in capabilities. As a result, it is recommended that hosts without a
configured DNS server periodically attempt to obtain DNS configuration. configured DNS server periodically attempt to obtain DNS
For example, where DHCP is used for DNS configuration, [RFC2131] configuration. For example, where DHCP is used for DNS
recommends a maximum retry interval of 64 seconds. In the absence of configuration, [RFC2131] recommends a maximum retry interval of 64
other guidance, a default retry interval of one (1) minute is seconds. In the absence of other guidance, a default retry interval
RECOMMENDED. of one (1) minute is RECOMMENDED.
4. Conflict resolution 4. Conflict resolution
The sender MUST anticipate receiving multiple replies to the same LLMNR The sender MUST anticipate receiving multiple replies to the same
query, in the event that several LLMNR enabled computers receive the LLMNR query, in the event that several LLMNR enabled computers
query and respond with valid answers. When this occurs, the responses receive the query and respond with valid answers. When this occurs,
may first be concatenated, and then treated in the same manner that the responses may first be concatenated, and then treated in the same
multiple RRs received from the same DNS server would; the sender manner that multiple RRs received from the same DNS server would; the
perceives no inherent conflict in the receipt of multiple responses. sender perceives no inherent conflict in the receipt of multiple
responses.
There are some scenarios when multiple responders MAY respond to the There are some scenarios when multiple responders MAY respond to the
same query. There are other scenarios when only one responder MAY same query. There are other scenarios when only one responder MAY
respond to a query. Resource records for which the latter queries are respond to a query. Resource records for which the latter queries
submitted are referred as UNIQUE throughout this document. The are submitted are referred as UNIQUE throughout this document. The
uniqueness of a resource record depends on a nature of the name in the uniqueness of a resource record depends on a nature of the name in
query and type of the query. For example it is expected that: 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 SRV type record
- multiple hosts may respond to a query for an A or AAAA type - multiple hosts may respond to a query for an A or AAAA type
record for a cluster name (assigned to multiple hosts in record for a cluster name (assigned to multiple hosts in
the cluster) the cluster)
- only a single host may respond to a query for an A or AAAA - only a single host may respond to a query for an A or AAAA
type record for a name. type record for a name.
Every responder that responds to an LLMNR query AND includes a UNIQUE Every responder that responds to an LLMNR query AND includes a UNIQUE
record in the response: record in the response:
[1] MUST verify that there is no other host within the scope of the [1] MUST verify that there is no other host within the
LLMNR query propagation that can return a resource record for the scope of the LLMNR query propagation that can return
same name, type and class. a resource record for the same name, type and class.
[2] MUST NOT include a UNIQUE resource record in the response without [2] MUST NOT include a UNIQUE resource record in the
having verified its uniqueness. response without having verified its uniqueness.
Where a host is configured to issue LLMNR queries on more than one Where a host is configured to issue LLMNR queries on more than one
interface, each interface should have its own independent LLMNR cache. interface, each interface should have its own independent LLMNR
For each UNIQUE resource record in a given interface's configuration, cache. For each UNIQUE resource record in a given interface's
the host MUST verify resource record uniqueness on that interface. To configuration, the host MUST verify resource record uniqueness on
accomplish this, the host MUST send an LLMNR query for each UNIQUE that interface. To accomplish this, the host MUST send an LLMNR
resource record. query for each UNIQUE resource record.
By default, a host SHOULD be configured to behave as though all RRs are By default, a host SHOULD be configured to behave as though all RRs
UNIQUE. Uniqueness verification is carried out when the host: are UNIQUE. Uniqueness verification is carried out when the host:
- starts up or is rebooted - starts up or is rebooted
- wakes from sleep (if the network interface was inactive during sleep) - wakes from sleep (if the network interface was inactive during sleep)
- is configured to respond to the LLMNR queries on an interface - is configured to respond to the LLMNR queries on an interface
enabled for transmission and reception of IP traffic enabled for transmission and reception of IP traffic
- is configured to respond to the LLMNR queries using additional - is configured to respond to the LLMNR queries using additional
UNIQUE resource records UNIQUE resource records
- detects that an interface is connected and is usable - detects that an interface is connected and is usable
(e.g. an IEEE 802 hardware link-state change indicating (e.g. an IEEE 802 hardware link-state change indicating
that a cable was attached or completion of authentication that a cable was attached or completion of authentication
(and if needed, association) with a wireless base station (and if needed, association) with a wireless base station
or adhoc network or adhoc network
When a host that has a UNIQUE record receives an LLMNR query for that When a host that has a UNIQUE record receives an LLMNR query for that
record, the host MUST respond. After the client receives a response, it record, the host MUST respond. After the client receives a response,
MUST check whether the response arrived on an interface different from it MUST check whether the response arrived on an interface different
the one on which the query was sent. If the response arrives on a from the one on which the query was sent. If the response arrives on
different interface, the client can use the UNIQUE resource record in a different interface, the client can use the UNIQUE resource record
response to LLMNR queries. If not, then it MUST NOT use the UNIQUE in response to LLMNR queries. If not, then it MUST NOT use the
resource record in response to LLMNR queries. UNIQUE resource record in response to LLMNR queries.
The name conflict detection mechanism doesn't prevent name conflicts The name conflict detection mechanism doesn't prevent name conflicts
when previously partitioned segments are connected by a bridge. In order when previously partitioned segments are connected by a bridge. In
to minimize the chance of conflicts in such a situation, it is order to minimize the chance of conflicts in such a situation, it is
recommended that steps be taken to ensure name uniqueness. For example, recommended that steps be taken to ensure name uniqueness. For
the name could be chosen randomly from a large pool of potential names, example, the name could be chosen randomly from a large pool of
or the name could be assigned via a process designed to guarantee potential names, or the name could be assigned via a process designed
uniqueness. to guarantee uniqueness.
When name conflicts are detected, they SHOULD be logged. To detect When name conflicts are detected, they SHOULD be logged. To detect
duplicate use of a name, an administrator can use a name resolution duplicate use of a name, an administrator can use a name resolution
utility which employs LLMNR and lists both responses and responders. utility which employs LLMNR and lists both responses and responders.
This would allow an administrator to diagnose behavior and
This would allow an administrator to diagnose behavior and potentially potentially to intervene and reconfigure LLMNR responders who should
to intervene and reconfigure LLMNR responders who should not be not be configured to respond to the same name.
configured to respond to the same name.
4.1. Considerations for Multiple Interfaces 4.1. Considerations for Multiple Interfaces
A multi-homed host may elect to configure LLMNR on only one of its A multi-homed host may elect to configure LLMNR on only one of its
active interfaces. In many situations this will be adequate. However, active interfaces. In many situations this will be adequate.
should a host need to configure LLMNR on more than one of its active However, should a host need to configure LLMNR on more than one of
interfaces, there are some additional precautions it MUST take. its active interfaces, there are some additional precautions it MUST
Implementers who are not planning to support LLMNR on multiple take. Implementers who are not planning to support LLMNR on multiple
interfaces simultaneously may skip this section. interfaces simultaneously may skip this section.
A multi-homed host checks the uniqueness of UNIQUE records as described A multi-homed host checks the uniqueness of UNIQUE records as
in Section 4. The situation is illustrated in figure 1. described in Section 4. The situation is illustrated in figure 1.
---------- ---------- ---------- ----------
| | | | | | | |
[A] [myhost] [myhost] [A] [myhost] [myhost]
Figure 1. Link-scope name conflict Figure 1. Link-scope name conflict
In this situation, the multi-homed myhost will probe for, and defend, 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 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 interface, but not the other. The multi-homed myhost will not be
to respond with a host RR for "myhost" on the interface on the right able to respond with a host RR for "myhost" on the interface on the
(see Figure 1). The multi-homed host may, however, be configured to use right (see Figure 1). The multi-homed host may, however, be
the "myhost" name on the interface on the left. 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 Since names are only unique per-link, hosts on different links could
using the same name. If an LLMNR client sends requests over multiple be using the same name. If an LLMNR client sends requests over
interfaces, and receives replies from more than one, the result returned multiple interfaces, and receives replies from more than one, the
to the client is defined by the implementation. The situation is result returned to the client is defined by the implementation. The
illustrated in figure 2. situation is illustrated in figure 2.
---------- ---------- ---------- ----------
| | | | | | | |
[A] [myhost] [A] [A] [myhost] [A]
Figure 2. Off-segment name conflict Figure 2. Off-segment name conflict
If host myhost is configured to use LLMNR on both interfaces, it will 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 send LLMNR queries on both interfaces. When host myhost sends a
for the host RR for name "A" it will receive a response from hosts on query for the host RR for name "A" it will receive a response from
both interfaces. 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. Host myhost cannot distinguish between the situation shown in Figure
2, and that shown in Figure 3 where no conflict exists.
[A] [A]
| | | |
----- ----- ----- -----
| | | |
[myhost] [myhost]
Figure 3. Multiple paths to same host Figure 3. Multiple paths to same host
This illustrates that the proposed name conflict resolution mechanism This illustrates that the proposed name conflict resolution mechanism
does not support detection or resolution of conflicts between hosts on does not support detection or resolution of conflicts between hosts
different links. This problem can also occur with unicast DNS when a on different links. This problem can also occur with unicast DNS
multi-homed host is connected to two different networks with separated when a multi-homed host is connected to two different networks with
name spaces. It is not the intent of this document to address the issue separated name spaces. It is not the intent of this document to
of uniqueness of names within DNS. address the issue of uniqueness of names within DNS.
4.2. API issues 4.2. API issues
[RFC2553] provides an API which can partially solve the name ambiguity [RFC2553] provides an API which can partially solve the name
problem for applications written to use this API, since the sockaddr_in6 ambiguity problem for applications written to use this API, since the
structure exposes the scope within which each scoped address exists, and sockaddr_in6 structure exposes the scope within which each scoped
this structure can be used for both IPv4 (using v4-mapped IPv6 address exists, and this structure can be used for both IPv4 (using
addresses) and IPv6 addresses. v4-mapped IPv6 addresses) and IPv6 addresses.
Following the example in Figure 2, an application on 'myhost' issues the Following the example in Figure 2, an application on 'myhost' issues
request getaddrinfo("A", ...) with ai_family=AF_INET6 and the request getaddrinfo("A", ...) with ai_family=AF_INET6 and
ai_flags=AI_ALL|AI_V4MAPPED. LLMNR requests will be sent from both ai_flags=AI_ALL|AI_V4MAPPED. LLMNR requests will be sent from both
interfaces and the resolver library will return a list containing interfaces and the resolver library will return a list containing
multiple addrinfo structures, each with an associated sockaddr_in6 multiple addrinfo structures, each with an associated sockaddr_in6
structure. This list will thus contain the IPv4 and IPv6 addresses of structure. This list will thus contain the IPv4 and IPv6 addresses
both hosts responding to the name 'A'. Link-local addresses will have a of both hosts responding to the name 'A'. Link-local addresses will
sin6_scope_id value that disambiguates which interface is used to reach have a sin6_scope_id value that disambiguates which interface is used
the address. Of course, to the application, Figures 2 and 3 are still to reach the address. Of course, to the application, Figures 2 and 3
indistinguishable, but this API allows the application to communicate are still indistinguishable, but this API allows the application to
successfully with any address in the list. communicate successfully with any address in the list.
5. Security Considerations 5. Security Considerations
LLMNR is by nature a peer-to-peer name resolution protocol. It is LLMNR is by nature a peer-to-peer name resolution protocol. It is
therefore inherently more vulnerable than DNS, since existing DNS therefore inherently more vulnerable than DNS, since existing DNS
security mechanisms are difficult to apply to LLMNR. While tools exist security mechanisms are difficult to apply to LLMNR. While tools
to alllow an attacker to spoof a response to a DNS query, spoofing a exist to alllow an attacker to spoof a response to a DNS query,
response to an LLMNR query is easier since the query is sent to a link- spoofing a response to an LLMNR query is easier since the query is
scope multicast address, where every host on the logical link will be sent to a link-scope multicast address, where every host on the
made aware of it. logical link will be made aware of it.
In order to address the security vulnerabilities, the following In order to address the security vulnerabilities, the following
mechanisms are contemplated: mechanisms are contemplated:
[1] Scope restrictions. [1] Scope restrictions.
[2] Usage restrictions. [2] Usage restrictions.
[3] Cache and port separation. [3] Cache and port separation.
[4] Authentication. [4] Authentication.
These techniques are described in the following sections. These techniques are described in the following sections.
5.1. Scope restriction 5.1. Scope restriction
With LLMNR it is possible that hosts will allocate conflicting names for With LLMNR it is possible that hosts will allocate conflicting names
a period of time, or that attackers will attempt to deny service to for a period of time, or that attackers will attempt to deny service
other hosts by allocating the same name. Such attacks also allow hosts to other hosts by allocating the same name. Such attacks also allow
to receive packets destined for other hosts. hosts to receive packets destined for other hosts.
Since LLMNR is typically deployed in situations where no trust model can Since LLMNR is typically deployed in situations where no trust model
be assumed, it is likely that LLMNR queries and responses will be can be assumed, it is likely that LLMNR queries and responses will be
unauthenticated. In the absence of authentication, LLMNR reduces the unauthenticated. In the absence of authentication, LLMNR reduces the
exposure to such threats by utilizing UDP queries sent to a link-scope exposure to such threats by utilizing UDP queries sent to a link-
multicast address, as well as setting the TTL (IPv4) or Hop Limit (IPv6) scope multicast address, as well as setting the TTL (IPv4) or Hop
fields to one (1) on TCP queries and responses. 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 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 unicast LLMNR query reduces the likelihood of both denial of service
attacks and spoofed responses. Checking that an LLMNR query is sent to attacks and spoofed responses. Checking that an LLMNR query is sent
a link-scope multicast address should prevent spoofing of multicast to a link-scope multicast address should prevent spoofing of
queries by off-link attackers. multicast queries by off-link attackers.
While this limits the ability of off-link attackers to spoof LLMNR While this limits the ability of off-link attackers to spoof LLMNR
queries and responses, it does not eliminate it. For example, it is queries and responses, it does not eliminate it. For example, it is
possible for an attacker to spoof a response to a frequent query (such possible for an attacker to spoof a response to a frequent query
as an A or AAAA query for a popular Internet host), and by using a TTL (such as an A or AAAA query for a popular Internet host), and by
or Hop Limit field larger than one (1), for the forged response to reach using a TTL or Hop Limit field larger than one (1), for the forged
the LLMNR sender. response to reach the LLMNR sender.
When LLMNR queries are sent to a link-scope multicast address, it is When LLMNR queries are sent to a link-scope multicast address, it is
possible that some routers may not properly implement link-scope possible that some routers may not properly implement link-scope
multicast, or that link-scope multicast addresses may leak into the multicast, or that link-scope multicast addresses may leak into the
multicast routing system. multicast routing system.
Setting the IPv6 Hop Limit or IPv4 TTL field to a value larger than one Setting the IPv6 Hop Limit or IPv4 TTL field to a value larger than
in an LLMNR UDP response may enable denial of service attacks across the one in an LLMNR UDP response may enable denial of service attacks
Internet. However, since LLMNR responders only respond to queries for across the Internet. However, since LLMNR responders only respond to
queries for which they are authoritative, and LLMNR does not provide
which they are authoritative, and LLMNR does not provide wildcard query wildcard query support, it is believed that this threat is minimal.
support, it is believed that this threat is minimal.
There also are scenarios such as public "hotspots" where attackers can There also are scenarios such as public "hotspots" where attackers
be present on the same link. These threats are most serious in wireless can be present on the same link. These threats are most serious in
networks such as 802.11, since attackers on a wired network will require wireless networks such as 802.11, since attackers on a wired network
physical access to the home network, while wireless attackers may reside will require physical access to the home network, while wireless
outside the home. Link-layer security can be of assistance against attackers may reside outside the home. Link-layer security can be of
these threats if it is available. assistance against these threats if it is available.
5.2. Usage restriction 5.2. Usage restriction
As noted in Sections 2 and 3, LLMNR is intended for usage in a limited As noted in Sections 2 and 3, LLMNR is intended for usage in a
set of scenarios. limited set of scenarios.
If an LLMNR query is sent whenever a DNS server does not respond in a 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 timely way, then an attacker can poison the LLMNR cache by responding
the query with incorrect information. To some extent, these to the query with incorrect information. To some extent, these
vulnerabilities exist today, since DNS response spoofing tools are vulnerabilities exist today, since DNS response spoofing tools are
available that can allow an attacker to respond to a query more quickly available that can allow an attacker to respond to a query more
than a distant DNS server. quickly than a distant DNS server.
Since LLMNR queries are sent and responded to on the local-link, an 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 attacker will need to respond more quickly to provide its own
prior to arrival of the response from a legitimate responder. If an response prior to arrival of the response from a legitimate
LLMNR query is sent for an off-link host, spoofing a response in a responder. If an LLMNR query is sent for an off-link host, spoofing a
timely way is not difficult, since a legitimate response will never be response in a timely way is not difficult, since a legitimate
received. response will never be received.
The vulnerability is more serious if LLMNR is given higher priority than The vulnerability is more serious if LLMNR is given higher priority
DNS among the enabled name resolution mechanisms. In such a than DNS among the enabled name resolution mechanisms. In such a
configuration, a denial of service attack on the DNS server would not be configuration, a denial of service attack on the DNS server would not
necessary in order to poison the LLMNR cache, since LLMNR queries would be necessary in order to poison the LLMNR cache, since LLMNR queries
be sent even when the DNS server is available. In addition, the LLMNR would be sent even when the DNS server is available. In addition, the
cache, once poisoned, would take precedence over the DNS cache, LLMNR cache, once poisoned, would take precedence over the DNS cache,
eliminating the benefits of cache separation. As a result, LLMNR is only eliminating the benefits of cache separation. As a result, LLMNR is
used as a name resolution mechanism of last resort. only used as a name resolution mechanism of last resort.
5.3. Cache and port separation 5.3. Cache and port separation
In order to prevent responses to LLMNR queries from polluting the DNS In order to prevent responses to LLMNR queries from polluting the DNS
cache, LLMNR implementations MUST use a distinct, isolated cache for cache, LLMNR implementations MUST use a distinct, isolated cache for
LLMNR on each interface. The use of separate caches is most effective 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 when LLMNR is used as a name resolution mechanism of last resort,
this minimizes the opportunities for poisoning the LLMNR cache, and since this minimizes the opportunities for poisoning the LLMNR cache,
decreases reliance on it. 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. 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 5.4. Authentication
LLMNR implementations may not support DNSSEC or TSIG, and as a result, LLMNR implementations may not support DNSSEC or TSIG, and as a
responses to LLMNR queries may be unauthenticated. If authentication is result, responses to LLMNR queries may be unauthenticated. If
desired, and a pre-arranged security configuration is possible, then authentication is desired, and a pre-arranged security configuration
IPsec ESP with a null-transform MAY be used to authenticate LLMNR is possible, then IPsec ESP with a null-transform MAY be used to
responses. In a small network without a certificate authority, this can authenticate LLMNR responses. In a small network without a
be most easily accomplished through configuration of a group pre-shared certificate authority, this can be most easily accomplished through
key for trusted hosts. configuration of a group pre-shared key for trusted hosts.
6. IANA Considerations 6. IANA Considerations
This specification creates one new name space: the reserved bits in the This specification creates one new name space: the reserved bits in
LLMNR header. These are allocated by IETF Consensus, in accordance with the LLMNR header. These are allocated by IETF Consensus, in
BCP 26 [RFC2434]. accordance with BCP 26 [RFC2434].
LLMNR requires allocation of a port TBD for both TCP and UDP. LLMNR requires allocation of a port TBD for both TCP and UDP.
Assignment of the same port for both transports is requested. Assignment of the same port for both transports is requested.
LLMNR requires allocation of a link-scope multicast IPv4 address TBD. LLMNR requires allocation of link-scope multicast IPv4 address
LLMNR also requires allocation of a link-scope multicast IPv6 address 224.0.0.252. LLMNR also requires allocation of link-scope multicast
TBD. IPv6 address FF02:0:0:0:0:0:1:3.
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC1035] Mockapetris, P., "Domain Names - Implementation and [RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", RFC 1035, November 1987. Specification", RFC 1035, November 1987.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992. April 1992.
skipping to change at page 24, line 13 skipping to change at page 24, line 23
Number 5, pp. 589, October 2002. Number 5, pp. 589, October 2002.
[DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site local [DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site local
unicast addresses to communicate with recursive DNS servers", unicast addresses to communicate with recursive DNS servers",
Internet draft (work in progress), draft-ietf-ipv6-dns- Internet draft (work in progress), draft-ietf-ipv6-dns-
discovery-07.txt, October 2002. discovery-07.txt, October 2002.
[IPV4Link] [IPV4Link]
Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
of IPv4 Link-Local Addresses", Internet draft (work in of IPv4 Link-Local Addresses", Internet draft (work in
progress), draft-ietf-zeroconf-ipv4-linklocal-14.txt, April progress), draft-ietf-zeroconf-ipv4-linklocal-15.txt, May
2004. 2004.
[POSIX] IEEE Std. 1003.1-2001 Standard for Information Technology -- [POSIX] IEEE Std. 1003.1-2001 Standard for Information Technology --
Portable Operating System Interface (POSIX). Open Group Portable Operating System Interface (POSIX). Open Group
Technical Standard: Base Specifications, Issue 6, December Technical Standard: Base Specifications, Issue 6, December
2001. ISO/IEC 9945:2002. http://www.opengroup.org/austin 2001. ISO/IEC 9945:2002. http://www.opengroup.org/austin
[LLMNREnable] [LLMNREnable]
Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work Guttman, E., "DHCP LLMNR Enable Option", Internet draft (work
in progress), draft-guttman-mdns-enable-02.txt, April 2002. in progress), draft-guttman-mdns-enable-02.txt, April 2002.
skipping to change at page 24, line 35 skipping to change at page 24, line 45
[NodeInfo] [NodeInfo]
Crawford, M., "IPv6 Node Information Queries", Internet draft Crawford, M., "IPv6 Node Information Queries", Internet draft
(work in progress), draft-ietf-ipn-gwg-icmp-name- (work in progress), draft-ietf-ipn-gwg-icmp-name-
lookups-09.txt, May 2002. lookups-09.txt, May 2002.
Acknowledgments Acknowledgments
This work builds upon original work done on multicast DNS by Bill This work builds upon original work done on multicast DNS by Bill
Manning and Bill Woodcock. Bill Manning's work was funded under DARPA Manning and Bill Woodcock. Bill Manning's work was funded under DARPA
grant #F30602-99-1-0523. The authors gratefully acknowledge their grant #F30602-99-1-0523. The authors gratefully acknowledge their
contribution to the current specification. Constructive input has also contribution to the current specification. Constructive input has
been received from Mark Andrews, Stuart Cheshire, Randy Bush, Robert also been received from Mark Andrews, Stuart Cheshire, Randy Bush,
Elz, Rob Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Robert Elz, Rob Austein, James Gilroy, Olafur Gudmundsson, Erik
Hattig, Thomas Narten, Christian Huitema, Erik Nordmark, Sander Van- Guttman, Myron Hattig, Thomas Narten, Christian Huitema, Erik
Valkenburg, Tomohide Nagashima, Brian Zill, Keith Moore and Markku Nordmark, Sander Van-Valkenburg, Tomohide Nagashima, Brian Zill,
Savela. Keith Moore and Markku Savela.
Authors' Addresses Authors' Addresses
Levon Esibov Levon Esibov
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
EMail: levone@microsoft.com EMail: levone@microsoft.com
skipping to change at page 25, line 33 skipping to change at page 25, line 33
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
Phone: +1 425 703 8835 Phone: +1 425 703 8835
EMail: dthaler@microsoft.com EMail: dthaler@microsoft.com
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procedures with respect to rights in standards-track and standards- IETF's procedures with respect to rights in standards-track and
related documentation can be found in BCP-11. Copies of claims of standards- related documentation can be found in BCP-11. Copies of
rights made available for publication and any assurances of licenses to claims of rights made available for publication and any assurances of
be made available, or the result of an attempt made to obtain a general licenses to be made available, or the result of an attempt made to
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implementors or users of this specification can be obtained from the proprietary rights by implementors or users of this specification can
IETF Secretariat. be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
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which case the procedures for copyrights defined in the Internet developing Internet standards in which case the procedures for
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Open Issues Open Issues
Open issues with this specification are tracked on the following web Open issues with this specification are tracked on the following web
site: site:
http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html
Expiration Date Expiration Date
This memo is filed as <draft-ietf-dnsext-mdns-30.txt>, and expires This memo is filed as <draft-ietf-dnsext-mdns-31.txt>, and expires
October 4, 2004. November 22, 2004.
 End of changes. 

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