draft-ietf-dnsext-mdns-22.txt   draft-ietf-dnsext-mdns-23.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-22.txt> Microsoft <draft-ietf-dnsext-mdns-23.txt> Microsoft
23 July 2003 11 September 2003
Linklocal Multicast Name Resolution (LLMNR) Linklocal Multicast Name Resolution (LLMNR)
This document is an Internet-Draft and is in full conformance with all This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering Task Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet-Drafts. may also distribute working documents as Internet-Drafts.
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2.1 Sender behavior ................................. 4 2.1 Sender behavior ................................. 4
2.2 Responder behavior .............................. 5 2.2 Responder behavior .............................. 5
2.3 Unicast queries ................................. 6 2.3 Unicast queries ................................. 6
2.4 Addressing ...................................... 7 2.4 Addressing ...................................... 7
2.5 Off-link detection .............................. 7 2.5 Off-link detection .............................. 7
2.6 Retransmissions ................................. 8 2.6 Retransmissions ................................. 8
2.7 DNS TTL ......................................... 9 2.7 DNS TTL ......................................... 9
2.8 Use of the authority and additional sections .... 9 2.8 Use of the authority and additional sections .... 9
3. Usage model ........................................... 9 3. Usage model ........................................... 9
3.1 Unqualified names ............................... 10 3.1 Unqualified names ............................... 10
3.2 LLMNR configuration ............................. 10 3.2 LLMNR configuration ............................. 11
4. Conflict resolution ................................... 12 4. Conflict resolution ................................... 12
4.1 Considerations for multiple interfaces .......... 13 4.1 Considerations for multiple interfaces .......... 14
4.2 API issues ...................................... 15 4.2 API issues ...................................... 15
5. Security considerations ............................... 15 5. Security considerations ............................... 15
5.1 Scope restriction ............................... 16 5.1 Scope restriction ............................... 16
5.2 Usage restriction ............................... 16 5.2 Usage restriction ............................... 16
5.3 Cache and port separation ....................... 17 5.3 Cache and port separation ....................... 17
5.4 Authentication .................................. 17 5.4 Authentication .................................. 17
6. IANA considerations ................................... 17 6. IANA considerations ................................... 18
7. Normative References .................................. 18 7. References ............................................ 18
8. Informative References ................................ 18 7.1 Normative References ............................ 18
7.2 Informative References .......................... 18
Acknowledgments .............................................. 19 Acknowledgments .............................................. 19
Authors' Addresses ........................................... 19 Authors' Addresses ........................................... 20
Intellectual Property Statement .............................. 20 Intellectual Property Statement .............................. 20
Full Copyright Statement ..................................... 20 Full Copyright Statement ..................................... 21
1. Introduction 1. Introduction
This document discusses Link Local Multicast Name Resolution (LLMNR), This document discusses Link Local Multicast Name Resolution (LLMNR),
which operates on a separate port from the Domain Name System (DNS), which operates on a separate port from the Domain Name System (DNS),
with a distinct resolver cache, but does not change the format of DNS with a distinct resolver cache, but does not change the format of DNS
packets. LLMNR supports all current and future DNS formats, types and packets. LLMNR supports all current and future DNS formats, types and
classes. However, since LLMNR only operates on the local link, it classes. However, since LLMNR only operates on the local link, it
cannot be considered a substitute for DNS. cannot be considered a substitute for DNS.
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they respond with errors, as described in Section 3. they respond with errors, as described in Section 3.
LLMNR queries are sent to and received on port TBD. Link-scope LLMNR queries are sent to and received on port TBD. Link-scope
multicast addresses are used to prevent propagation of LLMNR traffic multicast addresses are used to prevent propagation of LLMNR traffic
across routers, potentially flooding the network; for details, see across routers, potentially flooding the network; for details, see
Section 2.4. LLMNR queries can also be sent to a unicast address, as Section 2.4. LLMNR queries can also be sent to a unicast address, as
described in Section 2.3. described in Section 2.3.
Propagation of LLMNR packets on the local link is considered sufficient Propagation of LLMNR packets on the local link is considered sufficient
to enable name resolution in small networks. The assumption is that if to enable name resolution in small networks. The assumption is that if
a network has a home gateway, then the network is able to provide DNS a network has a gateway, then the network is able to provide DNS server
server configuration and a DNS server is available that is authoritative configuration. Configuration issues are discussed in Section 3.2.
for the names of local hosts and can support dynamic DNS. Configuration
issues are discussed in Section 3.2.
In the future, LLMNR may be defined to support greater than link-scope In the future, it may be desirable to consider use of multicast name
multicast. This would occur if LLMNR deployment is successful, the resolution with multicast scopes beyond the link-scope. This could
assumption that LLMNR is not needed on multiple links proves incorrect, occur if LLMNR deployment is successful, the need for multicast name
and multicast routing becomes ubiquitous. For example, it is not clear resolution beyond the link-scope, or multicast routing becomes
that this assumption will be valid in large ad hoc networking scenarios. 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 Once we have experience in LLMNR deployment in terms of administrative
issues, usability and impact on the network it will be possible to issues, usability and impact on the network, it will be possible to
reevaluate which multicast scopes are appropriate for use with multicast reevaluate which multicast scopes are appropriate for use with multicast
name resolution mechanisms. 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 using
LLMNR in particular, is outside of the scope of this document, as is LLMNR in particular, is outside of the scope of this document, as is
name resolution over non-multicast capable media. 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 of
the specification. These words are often capitalized. The key words the specification. These words are often capitalized. The key words
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119]. interpreted as described in [RFC2119].
1.2. Terminology 1.2. Terminology
Responder A host that listens to LLMNR queries, and responds to Responder A host that listens to LLMNR queries, and responds to
those for which it is authoritative. those for which it is authoritative.
Sender A host that sends an LLMNR query. Typically a host is Sender A host that sends an LLMNR query. Typically a host is
configured as both a sender and a responder. However, a configured as both a sender and a responder. However, a
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Routable address Routable address
An address other than a Link-Local address. This An address other than a Link-Local address. This
includes globally routable addresses, as well as private includes globally routable addresses, as well as private
addresses. addresses.
2. Name resolution using LLMNR 2. Name resolution using LLMNR
A typical sequence of events for LLMNR usage is as follows: A typical sequence of events for LLMNR usage is as follows:
[1] A sender needs to resolve a query for a name "host.example.com", [1] A sender needs to resolve a query for a name "host.example.com",
so it sends an LLMNR query to the link-scope multicast address. so it sends an LLMNR query to the link-scope multicast address(es)
defined in Section 2.4.
[2] A responder responds to this query only if it is authoritative [2] A responder responds to this query only if it is authoritative
for the domain name "host.example.com". The responder sends for the domain name "host.example.com". The responder sends
a response to the sender via unicast over UDP. a response to the sender via unicast over UDP.
[3] Upon the reception of the response, the sender performs the checks [3] Upon the reception of the response, the sender performs the checks
described in Section 2.5. If these conditions are met, then the described in Section 2.5. If these conditions are met, then the
sender uses and caches the returned response. If not, then the sender uses and caches the returned response. If not, then the
sender ignores the response and continues waiting for the response. sender ignores the response and continues waiting for the response.
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. Sender behavior 2.1. Sender behavior
A sender sends an LLMNR query for any legal resource record type (e.g. A sender sends an LLMNR query for any legal resource record type (e.g.
A/AAAA, SRV, PTR, etc.) to the link-scope multicast address. As A/AAAA, SRV, PTR, etc.) to the link-scope multicast address. As
described in Section 2.3, a sender may also send a unicast query. An described in Section 2.3, a sender may also send a unicast query.
LLMNR sender MAY send a request for any name. Section 3 describes the circumstances in which LLMNR queries may be
sent.
The RD (Recursion Desired) bit MUST NOT be set in a query. If a The RD (Recursion Desired) bit MUST NOT be set in a query. If a
responder receives a query with the header containing RD set bit, the responder receives a query with the header containing RD set bit, the
responder MUST ignore the RD bit. responder MUST ignore the RD bit.
The sender MUST anticipate receiving no replies to some LLMNR queries, 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 that no responders are available within the link-scope or
in the event no positive non-null responses exist for the transmitted 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 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 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 specified name (it is treated the same as a response with RCODE=0 and an
empty answer section). empty answer section).
2.2. Responder behavior 2.2. Responder behavior
A responder MUST listen on UDP port TBD on the link-scope multicast A responder MUST listen on UDP port TBD on the link-scope multicast
address(es) and on UDP and TCP port TBD on the unicast address(es) that address(es) defined in Section 2.4 and on UDP and TCP port TBD on the
could be set as the source address(es) when the responder responds to unicast address(es) that could be set as the source address(es) when the
the LLMNR query. A host configured as a responder MUST act as a sender responder responds to the LLMNR query. A host configured as a responder
to verify the uniqueness of names as described in Section 4. MUST act as a sender to verify the uniqueness of names as described in
Section 4.
Responders MUST NOT respond to LLMNR queries for names they are not Responders MUST NOT respond to LLMNR queries for names they are not
authoritative for. Responders SHOULD respond to LLMNR queries for names authoritative for. Responders SHOULD respond to LLMNR queries for names
and addresses they are authoritative for. This applies to both forward and addresses they are authoritative for. This applies to both forward
and reverse lookups. and reverse lookups.
As an example, a computer "host.example.com." configured to respond to As an example, a computer "host.example.com." configured to respond to
LLMNR queries is authoritative for the name "host.example.com.". On LLMNR queries is authoritative for the name "host.example.com.". On
receiving an LLMNR A/AAAA resource record query for the name receiving an LLMNR A/AAAA resource record query for the name
"host.example.com." the host authoritatively responds with A/AAAA "host.example.com." the host authoritatively responds with A/AAAA
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resolved". LLMNR responders may respond only to queries which they can resolved". LLMNR responders may respond only to queries which they can
resolve positively. resolve positively.
2.3. Unicast queries and responses 2.3. 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 a. A sender repeats a query after it received a response
with the TC 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. b. The sender queries for a PTR RR of a fully formed IP address
within the "in-addr.arpa" or "ip6.arpa" zones.
If a TC (truncation) bit is set in the response, then the sender MAY use If a TC (truncation) bit is set in the response, then the sender MAY use
the response if it contains all necessary information, or the sender MAY the response if it contains all necessary information, or the sender MAY
discard the response and resend the query over TCP using the unicast discard the response and resend the query over TCP using the unicast
address of the responder. The RA (Recursion Available) bit in the address of the responder. The RA (Recursion Available) bit in the
header of the response MUST NOT be set. If the RA bit is set in the header of the response MUST NOT be set. If the RA bit is set in the
response header, the sender MUST ignore the RA bit. response header, the sender MUST ignore the RA bit.
Unicast LLMNR queries SHOULD be sent using TCP. Responses to TCP Unicast LLMNR queries SHOULD be sent using TCP. Responses to TCP
unicast LLMNR queries MUST be sent using TCP, using the same connection unicast LLMNR queries MUST be sent using TCP, using the same connection
as the query. If the sender of a TCP query receives a response not as the query. If the sender of a TCP query receives a response not
using TCP, the response MUST be silently discarded. Unicast UDP queries using TCP, the response MUST be silently discarded. Unicast UDP queries
MAY be responded to with an empty answer section and the TC bit set, so MAY be responded to with an empty answer section and the TC bit set, so
as to require the sender to resend the query using TCP. Senders MUST as to require the sender to resend the query using TCP. Senders MUST
support sending TCP queries, and Responders MUST support listening for support sending TCP queries, and Responders MUST support listening for
TCP queries. The Responder SHOULD set the TTL or Hop Limit settings on TCP queries. The Responder SHOULD set the TTL or Hop Limit settings on
the TCP listen socket to one (1) so that SYN-ACK packets will have TTL 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 (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 connection from off-link since the Sender will not receive a SYN-ACK
from the Responder. from the Responder.
If an ICMP "Time Exceeded" message is received in response to a unicast If an ICMP "Time Exceeded" message is received in response to a unicast
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packet, which matches a query that is currently in progress, so as to packet, which matches a query that is currently in progress, so as to
guard against a potential Denial of Service (DoS) attack. If a match guard against a potential Denial of Service (DoS) attack. If a match
cannot be made, then the sender relies on the retransmission and timeout cannot be made, then the sender relies on the retransmission and timeout
behavior described in Section 2.6. behavior described in Section 2.6.
2.4. Addressing 2.4. Addressing
IPv4 administratively scoped multicast usage is specified in IPv4 administratively scoped multicast usage is specified in
"Administratively Scoped IP Multicast" [RFC2365]. The IPv4 link-scope "Administratively Scoped IP Multicast" [RFC2365]. The IPv4 link-scope
multicast address a given responder listens to, and to which a sender multicast address a given responder listens to, and to which a sender
sends queries, is 224.0.0.251. The IPv6 link-scope multicast address a sends queries, is TBD. The IPv6 link-scope multicast address a given
given responder listens to, and to which a sender sends all queries, is responder listens to, and to which a sender sends all queries, is TBD.
TBD.
2.5. Off-link detection 2.5. Off-link detection
The source address of LLMNR queries and responses MUST be "on link". A sender MUST select a source address for LLMNR queries that is "on
The destination address of an LLMNR query MUST be a link-scope multicast link". The destination address of an LLMNR query MUST be a link-scope
address or an "on link" unicast address; the destination address of an multicast address or an "on link" unicast address.
LLMNR response MUST be an "on link" unicast address. On receiving an
LLMNR query, the responder MUST check whether it was sent to the LLMNR A responder MUST select a source address for responses that is "on
multicast address; if it was sent to another multicast address, then the link". The destination address of an LLMNR response MUST be an "on
link" unicast address. On receiving an LLMNR query, the responder MUST
check whether it was sent to a LLMNR multicast addresses defined in
Section 2.4. If it was sent to another multicast address, then the
query MUST be silently discarded. query MUST be silently discarded.
For IPv4, an "on link" address is defined as a link-local address or an For IPv4, an "on link" address is defined as a link-local address or an
address whose prefix belongs to a subnet on the local link; for IPv6 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 [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 in [RFC2373], or an address whose prefix belongs to a subnet on the
local link. A sender SHOULD prefer RRs including reachable addresses local link. A sender SHOULD prefer RRs including reachable addresses
where RRs involving both reachable and unreachable addresses are where RRs involving both reachable and unreachable addresses are
returned in response to a query. returned in response to a query.
In composing LLMNR queries, the sender MUST set the Hop Limit field in In composing LLMNR queries, the sender MUST set the Hop Limit field in
the IPv6 header and the TTL field in IPv4 header of the response to one the IPv6 header and the TTL field in IPv4 header of the response to one
(1). Even when LLMNR queries are sent to a link-scope multicast (1). Even when LLMNR queries are sent to a link-scope multicast
address, it is possible that some routers may not properly implement address, it is possible that some routers may not properly implement
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In the sockets API for IPv4, the IP_TTL and IP_MULTICAST_TTL socket In the sockets API for IPv4, the IP_TTL and IP_MULTICAST_TTL socket
options are used to set the TTL of outgoing unicast and multicast options are used to set the TTL of outgoing unicast and multicast
packets. The IP_RECVTTL socket option is available on some platforms packets. The IP_RECVTTL socket option is available on some platforms
to retrieve the IPv4 TTL of received packets with recvmsg(). to retrieve the IPv4 TTL of received packets with recvmsg().
[RFC2292] specifies similar options for setting and retrieving the [RFC2292] specifies similar options for setting and retrieving the
IPv6 Hop Limit. IPv6 Hop Limit.
2.6. Retransmissions 2.6. Retransmissions
In order to avoid synchronization, LLMNR queries and responses are In order to avoid synchronization, LLMNR queries and responses are
delayed by a time uniformly distributed between 0 and 200 ms. delayed by a time randomly selected from the interval 0 to 200 ms.
If an LLMNR query sent over UDP is not resolved within the timeout If an LLMNR query sent over UDP is not resolved within the timeout
interval (LLMNR_TIMEOUT), then a sender MAY repeat the transmission of interval (LLMNR_TIMEOUT), then a sender MAY repeat the transmission of
the query in order to assure that it was received by a host capable of the query in order to assure that it was received by a host capable of
responding to it. Since a multicast query sender cannot know beforehand responding to it. Retransmission of UDP queries SHOULD NOT be attempted
whether it will receive no response, one response, or more than one more than 3 times. Where LLMNR queries are sent using TCP,
response, it SHOULD wait for LLMNR_TIMEOUT in order to collect all retransmission is handled by the transport layer.
possible responses, rather than considering the multicast query answered
after the first response is received. A unicast query sender considers Since a multicast query sender cannot know beforehand whether it will
the query answered after the first response is received, so that it only receive no response, one response, or more than one response, it SHOULD
waits for LLMNR_TIMEOUT if no response has been received. 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.
LLMNR implementations SHOULD dynamically estimate the timeout value LLMNR implementations SHOULD dynamically estimate the timeout value
(LLMNR_TIMEOUT) based on the last response received, on a per-interface (LLMNR_TIMEOUT) based on the last response received for a query, on a
basis. The algorithms described in [RFC2988] are suggested, with a per-interface basis. The algorithms described in [RFC2988] are
minimum timeout value of 300 ms. Retransmission of UDP queries SHOULD suggested (including exponential backoff). Smaller values of
NOT be attempted more than 3 times. Where LLMNR queries are sent using RTOinitial, RTOmin and RTOmax MAY be used. Recommended values are
TCP, retransmission is handled by the transport layer. RTOinitial=1 second, RTOmin=200ms, RTOmax=20 seconds.
2.7. DNS TTL 2.7. 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 in the LLMNR query response. Due to the TTL minimalization returned in the LLMNR query response. A default value of 0 is
necessary when caching an RRset, all TTLs in an RRset MUST be set to the recommended in highly dynamic environments (such as mobile ad-hoc
same value. networks). In less dynamic environments, LLMNR traffic can be reduced
by setting the TTL to a higher value.
Due to the TTL minimalization necessary when caching an RRset, all TTLs
in an RRset MUST be set to the same value.
2.8. Use of the authority and additional sections 2.8. 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, but
they SHOULD NOT include these NS records in the authority sections of 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 a
negative response, to facilitate negative caching as specified in negative response, to facilitate negative caching as specified in
RFC2308. The owner name of of this SOA record MUST be equal to the [RFC2308]. The owner name of of this SOA record MUST be equal to the
query name. query name.
Responders SHOULD NOT perform DNS additional section processing. Responders SHOULD NOT perform DNS additional section processing.
Senders MUST NOT cache RRs from the authority or additional section of a 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 response as answers, though they may be used for other purposes such as
negative caching. negative caching.
3. Usage model 3. Usage model
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 as
a replacement for DNS. By default, LLMNR requests SHOULD be sent only a replacement for DNS. By default, LLMNR requests SHOULD be sent only
when no manual or automatic DNS configuration has been performed, when when no manual or automatic DNS configuration has been performed, when
DNS servers do not respond, or when they respond to a query with RCODE=3 DNS servers do not respond, or when they respond to a query with RCODE=3
(Authoritative Name Error) or RCODE=0, and an empty answer section.
(Authoritative Name Error) or RCODE=0, and an empty answer section. An
LLMNR sender may send a request for any name.
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 result
in negative responses due to missing inverse mappings or NS records that in negative responses due to missing inverse mappings or NS records that
point to nonexistent or inappropriate hosts. Given this, support for point to nonexistent or inappropriate hosts. Given this, support for
LLMNR as a secondary name resolution mechanism has the potential to LLMNR as a secondary name resolution mechanism has the potential to
result in a large number of inappropriate queries without the following result in a large number of inappropriate queries without the following
additional restrictions: additional restrictions:
[1] If a DNS query does not receive a response, prior to falling [1] If a DNS query does not receive a response, prior to falling
back to LLMNR, the query SHOULD be retransmitted at least back to LLMNR, the query SHOULD be retransmitted at least
once. once.
[2] Where a DNS server is configured, by default a sender [2] A responder with both link-local and routable addresses
SHOULD send LLMNR queries only for names that are either
unqualified or exist within the default domain. Where no
DNS server is configured, an LLMNR query MAY be sent for
any name.
[3] A responder with both link-local and routable addresses
MUST respond to LLMNR queries for A/AAAA RRs only with MUST respond to LLMNR queries for A/AAAA RRs only with
routable address(es). This encourages use of routable routable address(es). This encourages use of routable
address(es) for establishment of new connections. address(es) for establishment of new connections.
[4] A sender SHOULD send LLMNR queries for PTR RRs [3] A sender SHOULD send LLMNR queries for PTR RRs
via unicast, as specified in Section 2.3. via unicast, as specified in Section 2.3.
RRs returned in LLMNR responses MUST only include values that are valid It is the responsibility of the responder to ensure that RRs returned in
on the local interface, such as IPv4 or IPv6 addresses valid on the LLMNR responses MUST only include values that are valid on the local
local link or names defended using the mechanism described in Section 4. interface, such as IPv4 or IPv6 addresses valid on the local link or
In particular: names defended using the mechanism described in Section 4. In
particular:
[1] If a link-scope IPv6 address is returned in a AAAA RR, that [1] If a link-scope IPv6 address is returned in a AAAA RR, that
address MUST be valid on the local link over which LLMNR is address MUST be valid on the local link over which LLMNR is
used. used.
[2] If an IPv4 address is returned, it must be reachable through [2] If an IPv4 address is returned, it MUST be reachable through
the link over which LLMNR is used. the link over which LLMNR is used.
[3] If a name is returned (for example in a CNAME, MX [3] If a name is returned (for example in a CNAME, MX
or SRV RR), the name MUST be valid on the local interface. or SRV RR), the name MUST be valid on the local interface.
3.1. Unqualified names 3.1. Unqualified names
The same host MAY use LLMNR queries for the resolution of unqualified If a name is not qualified, for the purposes of LLMNR the implicit
host names, and conventional DNS queries for resolution of other DNS search order is as follows:
names.
If a name is not qualified and does not end in a trailing dot, for the
purposes of LLMNR, the implicit search order is as follows:
[1] Request the name with the current domain appended. [1] Request the name with the current domain appended.
[2] Request just the name. [2] Request the name with the root domain (".") appended.
This is the behavior suggested by [RFC1536]. LLMNR uses this technique This is the behavior suggested by [RFC1536].
to resolve unqualified host names.
3.2. LLMNR configuration 3.2. LLMNR configuration
LLMNR usage MAY be configured manually or automatically on a per LLMNR usage MAY be configured manually or automatically on a per
interface basis. By default, LLMNR Responders SHOULD be enabled on all interface basis. By default, LLMNR Responders SHOULD be enabled on all
interfaces, at all times. interfaces, at all times.
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 unable
skipping to change at page 11, line 23 skipping to change at page 11, line 27
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 unable
to resolve names using DNS. Automatic IPv6 DNS configuration mechanisms to resolve names using DNS. Automatic IPv6 DNS configuration mechanisms
(such as [DHCPv6DNS] and [DNSDisc]) are not yet widely deployed, and not (such as [DHCPv6DNS] and [DNSDisc]) are not yet widely deployed, and not
all DNS servers support IPv6. Therefore lack of IPv6 DNS configuration 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 may be a common problem in the short term, and LLMNR may prove useful in
enabling linklocal name resolution over IPv6. enabling linklocal name resolution over IPv6.
Where a DHCPv4 server is available but not a DHCPv6 server [DHCPv6DNS], Where a DHCPv4 server is available but not a DHCPv6 server [DHCPv6DNS],
IPv6-only hosts may not be configured with a DNS server. Where there is IPv6-only hosts may not be configured with a DNS server. Where there is
no DNS server authoritative for the names of hosts on the local network no DNS server authoritative for the name of a host or the authoritative
or the authoritative DNS server does not support dynamic client update DNS server does not support dynamic client update over IPv6 or
over IPv6 or DHCPv6-based dynamic update, hosts on the home network will DHCPv6-based dynamic update, then an IPv6-only host will not be able to
not be able to dynamically register or resolve the names of local IPv6 do DNS dynamic update, and other hosts will not be able to resolve its
hosts. For example, if the configured DNS server responds to AAAA RR name.
queries sent over IPv4 or IPv6 with an authoritative name error
(RCODE=3), then it will not be possible to resolve the names of For example, if the configured DNS server responds to AAAA RR queries
IPv6-only hosts. In this situation, LLMNR over IPv6 can be used for sent over IPv4 or IPv6 with an authoritative name error (RCODE=3), then
local name resolution. 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 Similarly, if a DHCPv4 server is available providing DNS server
configuration, and the DNS server authoritative for the A RRs of local configuration, and DNS server(s) exist which are authoritative for the A
hosts also supports either dynamic client update over IPv4 or RRs of local hosts and support either dynamic client update over IPv4 or
DHCPv4-based dynamic update, then resolution of the names of local IPv4 DHCPv4-based dynamic update, then the names of local IPv4 hosts can be
hosts can be provided over IPv4 without LLMNR. However, if there is no resolved over IPv4 without LLMNR. However, if no DNS server is
DNS server authoritative for the names of local hosts, or the authoritative for the names of local hosts, or the authoritative DNS
authoritative DNS server does not support dynamic update, then LLMNR may server(s) do not support dynamic update, then LLMNR enables linklocal
prove useful in enabling linklocal name resoltion over IPv4. 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 in
[LLMNREnable], can be used to explicitly enable or disable use of LLMNR [LLMNREnable], can be used to explicitly enable or disable use of LLMNR
on an interface. The LLMNR Enable Option does not determine whether or 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 in which order DNS itself is used for name resolution. The order in
which various name resolution mechanisms should be used can be specified which various name resolution mechanisms should be used can be specified
using the Name Service Search Option for DHCP [RFC2937]. using the Name Service Search Option for DHCP [RFC2937].
It is possible that DNS configuration mechanisms will go in and out of It is possible that DNS configuration mechanisms will go in and out of
service. In these circumstances, it is possible for hosts within an service. In these circumstances, it is possible for hosts within an
administrative domain to be inconsistent in their DNS configuration. 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 more
DHCP servers can fail. As a result, hosts configured prior to the DHCP servers can fail. As a result, hosts configured prior to the
outage will be configured with a DNS server, while hosts configured outage will be configured with a DNS server, while hosts configured
after the outage will not. Alternatively, it is possible for the DNS after the outage will not. Alternatively, it is possible for the DNS
skipping to change at page 12, line 18 skipping to change at page 12, line 24
after the outage will not. Alternatively, it is possible for the DNS after the outage will not. Alternatively, it is possible for the DNS
configuration mechanism to continue functioning while configured DNS configuration mechanism to continue functioning while configured DNS
servers fail. servers fail.
Unless unconfigured hosts periodically retry configuration, an outage in Unless unconfigured hosts periodically retry configuration, an outage in
the DNS configuration mechanism will result in hosts continuing to use the DNS configuration mechanism will result in hosts continuing to use
LLMNR even once the outage is repaired. Since LLMNR only enables 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 in
capabilities. As a result, it is recommended that hosts without a 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 configuration.
A default retry interval of two (2) minutes is RECOMMENDED. A default retry interval 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 LLMNR
query, in the event that several LLMNR enabled computers receive the query, in the event that several LLMNR enabled computers receive the
query and respond with valid answers. When this occurs, the responses query and respond with valid answers. When this occurs, the responses
MAY first be concatenated, and then treated in the same manner that MAY first be concatenated, and then treated in the same manner that
multiple RRs received from the same DNS server would. multiple RRs received from the same DNS server would; the sender
perceives no inherent conflict in the receipt of multiple responses.
There are some scenarios when multiple responders MAY respond to the 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 are
submitted are referred as UNIQUE throughout this document. The 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 the
query and type of the query. For example it is expected that: 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
skipping to change at page 12, line 51 skipping to change at page 13, line 9
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 scope of the
LLMNR query propagation that can return a resource record LLMNR query propagation that can return a resource record
for the same name, type and class. for the same name, type and class.
2. MUST NOT include a UNIQUE resource record in the 2. MUST NOT include a UNIQUE resource record in the
response without having verified its uniqueness. response without having verified its uniqueness.
Where a host is configured to respond to 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 cache.
For each UNIQUE resource record in a given interface's configuration, For each UNIQUE resource record in a given interface's configuration,
the host MUST verify resource record uniqueness on that interface. To the host MUST verify resource record uniqueness on that interface. To
accomplish this, the host MUST send an LLMNR query for each UNIQUE accomplish this, the host MUST send an LLMNR query for each UNIQUE
resource record. resource record, as described in Section 2.6.
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 are
UNIQUE. Uniqueness verification is carried out when the host: 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
- 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
(e.g. an IEEE 802 hardware link-state change indicating that a
cable was attached or that an association has occurred with a
wireless base station and that any required authentication has
completed)
When a host that owns a UNIQUE record receives an LLMNR query for that When a host that owns a UNIQUE record receives an LLMNR query for that
record, the host MUST respond. After the client receives a response, it record, the host MUST respond. After the client receives a response, it
MUST check whether the response arrived on another interface. If this MUST check whether the response arrived on another interface. If this
is the case, then the client can use the UNIQUE resource record in is the case, then the client can use the UNIQUE resource record in
response to LLMNR queries. If not, then it MUST NOT use the UNIQUE response to LLMNR queries. If not, then it MUST NOT use the UNIQUE
resource record in response to LLMNR queries. 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 order
skipping to change at page 17, line 46 skipping to change at page 18, line 10
null-transform MAY be used to authenticate LLMNR responses. In a small null-transform MAY be used to authenticate LLMNR responses. In a small
network without a certificate authority, this can be most easily network without a certificate authority, this can be most easily
accomplished through configuration of a group pre-shared key for trusted accomplished through configuration of a group pre-shared key for trusted
hosts. hosts.
6. IANA Considerations 6. IANA Considerations
This specification does not create any new name spaces for IANA This specification does not create any new name spaces for IANA
administration. LLMNR requires allocation of a port TBD for both TCP administration. LLMNR requires allocation of a port TBD for both TCP
and UDP. Assignment of the same port for both transports is requested. and UDP. Assignment of the same port for both transports is requested.
LLMNR utilizes a link-scope multicast IPv4 address (224.0.0.251) that LLMNR requires allocation of a link-scope multicast IPv4 address as well
has been previously allocated to LLMNR by IANA. It also requires as a link-scope multicast IPv6 address TBD.
allocation of a link-scope multicast IPv6 address.
7. Normative References 7. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., et al., "Dynamic Updates in the Domain Name [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
System (DNS UPDATE)", RFC 2136, April 1997. NCACHE)", RFC 2308, March 1998.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP [RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP
23, RFC 2365, July 1998. 23, RFC 2365, July 1998.
[RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing [RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998. 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 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998. (IPv6) Specification", RFC 2460, December 1998.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions", [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999. RFC 2535, March 1999.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000. Timer", RFC 2988, November 2000.
8. Informative References 7.2. Informative References
[RFC1536] Kumar, A., et. al., "DNS Implementation Errors and [RFC1536] Kumar, A., et. al., "DNS Implementation Errors and
Suggested Fixes", RFC 1536, October 1993. Suggested Fixes", RFC 1536, October 1993.
[RFC2136] Vixie, P., et al., "Dynamic Updates in the Domain Name
System (DNS UPDATE)", RFC 2136, April 1997.
[RFC2292] Stevens, W. and M. Thomas, "Advanced Sockets API for [RFC2292] Stevens, W. and M. Thomas, "Advanced Sockets API for
IPv6", RFC 2292, February 1998. IPv6", RFC 2292, February 1998.
[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2553] Gilligan, R., Thomson, S., Bound, J. and W. Stevens, [RFC2553] Gilligan, R., Thomson, S., Bound, J. and W. Stevens,
"Basic Socket Interface Extensions for IPv6", RFC 2553, "Basic Socket Interface Extensions for IPv6", RFC 2553,
March 1999. March 1999.
[RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC [RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC
2937, September 2000. 2937, September 2000.
[DHCPv6DNS] Droms, R., "A Guide to Implementing Stateless DHCPv6 [DHCPv6DNS] Droms, R., "A Guide to Implementing Stateless DHCPv6
Service", Internet draft (work in progress), draft-droms- Service", Internet draft (work in progress), draft-droms-
dhcpv6-stateless-guide-01.txt, October 2002. dhcpv6-stateless-guide-01.txt, October 2002.
skipping to change at page 19, line 21 skipping to change at page 19, line 34
10, Number 5, pp. 589, October 2002. 10, Number 5, pp. 589, October 2002.
[DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site [DNSDisc] Durand, A., Hagino, I. and D. Thaler, "Well known site
local unicast addresses to communicate with recursive DNS local unicast addresses to communicate with recursive DNS
servers", Internet draft (work in progress), draft-ietf- servers", Internet draft (work in progress), draft-ietf-
ipv6-dns-discovery-07.txt, October 2002. ipv6-dns-discovery-07.txt, October 2002.
[IPV4Link] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic [IPV4Link] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", Internet Configuration of IPv4 Link-Local Addresses", Internet
draft (work in progress), draft-ietf-zeroconf- draft (work in progress), draft-ietf-zeroconf-
ipv4-linklocal-08.txt, June 2003. ipv4-linklocal-09.txt, September 2003.
[LLMNREnable] Guttman, E., "DHCP LLMNR Enable Option", Internet draft [LLMNREnable] Guttman, E., "DHCP LLMNR Enable Option", Internet draft
(work in progress), draft-guttman-mdns-enable-02.txt, (work in progress), draft-guttman-mdns-enable-02.txt,
April 2002. April 2002.
[NodeInfo] Crawford, M., "IPv6 Node Information Queries", Internet [NodeInfo] Crawford, M., "IPv6 Node Information Queries", Internet
draft (work in progress), draft-ietf-ipn-gwg-icmp-name- draft (work in progress), draft-ietf-ipn-gwg-icmp-name-
lookups-09.txt, May 2002. lookups-09.txt, May 2002.
Acknowledgments Acknowledgments
skipping to change at page 21, line 27 skipping to change at page 21, line 39
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-22.txt>, and expires This memo is filed as <draft-ietf-dnsext-mdns-23.txt>, and expires
January 22, 2004. February 22, 2004.
 End of changes. 

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