draft-ietf-dnsext-mdns-08.txt   draft-ietf-dnsext-mdns-09.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-08.txt> Microsoft <draft-ietf-dnsext-mdns-09.txt> Microsoft
21 December 2001 February 21 2002
Linklocal Multicast DNS (LMDNS) Link-Local 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.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
skipping to change at page 1, line 37 skipping to change at page 1, line 37
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2001). 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 number
of ad-hoc networks operating without a DNS server. In order to allow of ad-hoc networks operating without a DNS server. In order to allow
name resolution in such environments, Linklocal Multicast DNS (LMDNS) is name resolution in such environments, Link-Local Multicast Name
proposed. Resolution (LLMNR) is proposed.
Table of Contents Table of Contents
1. Introduction .......................................... 3 1. Introduction .......................................... 2
2. Name resolution using LMDNS ........................... 3 2. Name resolution using LLMNR ........................... 3
2.1 Behavior of the sender and responder ............ 4 2.1 Behavior of the sender and responder ............ 4
3. Usage model ........................................... 7 3. Usage model ........................................... 7
3.1 LMDNS configuration ............................. 8 3.1 LLMNR configuration ............................. 7
4. Sequence of events .................................... 9 4. Sequence of events .................................... 8
5. Conflict resolution ................................... 9 5. Conflict resolution ................................... 8
5.1 Considerations for multiple interfaces .......... 11 5.1 Considerations for multiple interfaces .......... 10
5.2 API issues ...................................... 12 5.2 API issues ...................................... 11
6. Security considerations ............................... 13 6. Security considerations ............................... 12
7. IANA considerations ................................... 14 7. IANA considerations ................................... 13
8. Normative References .................................. 14 8. Normative References .................................. 13
9. Informative References ................................ 14 9. Informative References ................................ 13
Acknowledgments .............................................. 15 Acknowledgments .............................................. 14
Authors' Addresses ........................................... 15 Authors' Addresses ........................................... 14
Intellectual Property Statement .............................. 16 Intellectual Property Statement .............................. 15
Full Copyright Statement ..................................... 16 Full Copyright Statement ..................................... 15
1. Introduction 1. Introduction
Linklocal Multicast DNS (LMDNS) enables name resolution in the scenarios Link-Local Multicast Name Resolution (LLMNR) enables name resolution in
when conventional DNS name resolution is not possible. Namely, when the scenarios when conventional DNS name resolution is not possible. The
there are no DNS servers available on the network or available DNS main scenarios that require introduction of a new name resolution
servers do not provide name resolution for the names of the hosts on the mechanism are:
local network. The latter case, for example, corresponds to a scenario
when a network that doesn't have a DNS server is connected to the
Internet through an ISP and the network hosts are configured with the
ISP's DNS server for the name resolution. The ISP's DNS server provides
the name resolution for the names registered on the Internet, but
doesn't provide name resolution for the names of the hosts on the
network.
This document discusses Linklocal Multicast DNS (LMDNS), which operates 1. Multiple computers connected to the same network within the same
on a separate port from DNS, with a distinct resolver cache, but does link-local scope. These computers are not configured with an IP address
not change the format of DNS packets. of any DNS server. Users of these computers need to locate other
computers by their DNS names.
2. Home networks that don't contain a DNS server, but are connected to
the Internet through an ISP. The network hosts are configured with the
ISP's DNS server, which provides the name resolution for the names
registered on the Internet, but doesn't provide name resolution for the
names of the hosts on the network. Users of the computers on the home
network need to locate other computers by their DNS names.
This document discusses Link-Local Multicast Name Resolution (LLMNR),
which operates on a separate port from DNS, with a distinct resolver
cache, but does not change the format of DNS packets.
Service discovery in general, as well as discovery of DNS servers using Service discovery in general, as well as discovery of DNS servers using
LMDNS in particular is outside of the scope of this document, as is name LLMNR in particular is outside of the scope of this document, as is name
resolution over non-multicast capable media. resolution over non-multicast capable media.
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL", In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC2119]. described in [RFC2119].
2. Name resolution using LMDNS 2. Name resolution using LLMNR
While operating on a different port with a distinct resolver cache, While operating on a different port with a distinct resolver cache,
LMDNS makes no change to the current format of DNS packets. LLMNR makes no change to the current format of DNS packets.
Linklocal multicast DNS queries are sent to and received on port 5353 Link-Local Multicast Name Resolution queries are sent to and received on
using a LINKLOCAL address as specified in "Administratively Scoped IP port 5353 using a LINKLOCAL address as specified in "Administratively
Multicast" [RFC2365] for IPv4 and the "solicited name" LINKLOCAL Scoped IP Multicast" [RFC2365] for IPv4 and the "solicited name"
multicast addresses for IPv6. The LMDNS LINKLOCAL address to be used LINKLOCAL multicast addresses for IPv6, and using a unicast addresses in
for IPv4 is 224.0.0.251. LINKLOCAL addresses are used to prevent a few scenarios described below in Section 3. The LLMNR LINKLOCAL
propagation of LMDNS traffic across routers, potentially flooding the address to be used for IPv4 is 224.0.0.251. LINKLOCAL addresses are
network. used to prevent propagation of LLMNR traffic across routers, potentially
flooding the network.
Propagation of multicast DNS packets on the local link is considered Propagation of LLMNR packets on the local link is considered sufficient
sufficient to enable name resolution in small adhoc networks. The to enable name resolution in small adhoc networks. The assumption is
assumption is that if a network has a router, then the network either that if a network has a router, then the network either has a DNS server
has a DNS server or the router can function as a DNS proxy. or the router can function as a DNS proxy.
By implementing DHCPv4 as well as a DNS proxy and dynamic DNS, routers By implementing DHCPv4 as well as a DNS proxy and dynamic DNS, routers
can provide name resolution for the names of IPv4 hosts on the local can provide name resolution for the names of IPv4 hosts on the local
network. Where the DNS proxy supports AAAA RRs, resolution for the names network. Where the DNS proxy supports AAAA RRs, resolution for the names
of dual stack IPv6 hosts on the local network can also be provided using of dual stack IPv6 hosts on the local network can also be provided using
this mechanism. this mechanism.
Within small adhoc IPv6 networks, stateful autoconfiguration is the most Within small adhoc IPv6 networks, stateful autoconfiguration is the most
likely configuration mechanism. If DHCPv6 is not present, then in order likely configuration mechanism. If DHCPv6 is not present, then in order
to support resolution of names of IPv6-only hosts on the local network, to support resolution of names of IPv6-only hosts on the local network,
the DNS proxy will need to support dynamic client update as well as DNS the DNS proxy will need to support dynamic client update as well as DNS
over IPv6. over IPv6.
Given the above mechanisms enabling DNS name resolution in small Given the above mechanisms enabling DNS name resolution in small
networks with a router, it is assumed that LMDNS need not be enabled by networks with a router, it is assumed that LLMNR need not be enabled by
default. default.
In the future, multicast DNS may be defined to support greater than In the future, LLMNR may be defined to support greater than LINKLOCAL
LINKLOCAL multicast scope. This would occur if LMDNS deployment is multicast scope. This would occur if LLMNR deployment is successful,
successful, the assumption that multicast DNS is not needed on multiple the assumption that LLMNR is not needed on multiple links proves
links proves incorrect, and multicast routing becomes ubiquitous. For incorrect, and multicast routing becomes ubiquitous. For example, it is
example, it is not clear that this assumption will be valid in large not clear that this assumption will be valid in large adhoc networking
adhoc networking scenarios. scenarios.
Once we have experience in LMDNS 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 issues, usability and impact on the network it will be possible
reevaluate which multicast scopes are appropriate for use with multicast reevaluate which multicast scopes are appropriate for use with LLMNR.
DNS.
2.1. Behavior of the sender and responder 2.1. Behavior of the sender and responder
For the purpose of this document a host that sends a multicast query is For the purpose of this document a host that sends a LLMNR query is
called a "sender", while a host that listens to (but not necessarily called a "sender", while a host that listens to (but not necessarily
responds to) a multicast query is called "responder". A host configured responds to) a LLMNR query is called "responder". Although the same
to be a "responder" MAY also be a "sender". A host configured not to be host may be configured as a "sender", but not a "responder" and vice
"responder" SHOULD NOT be a "sender". While hosts configured only as versa, i.e. as a "responder", but not a "sender", the host configured as
senders can detect name conflicts, they cannot notify other senders of a "responder" MUST act as a sender by using LLMNR dynamic update
potential conflicts for their name. Thus, implementation of both requests to verify the uniqueness of names as described in Section 5.
responder and sender functionality is encouraged.
2.1.1. Behavior of senders 2.1.1. Behavior of senders
A sender sends an LMDNS query for any legal Type of resource record A sender sends an LLMNR query for any legal Type of resource record
(e.g. A, PTR, etc.) to the LINKLOCAL address. The RD (Recursion (e.g. A, PTR, etc.) to the LINKLOCAL address. Notice that in some
Desired) bit MUST NOT be set. If a responder receives a query with the scenarios described below in Section 3 a sender may also send a unicast
header containing RD set bit, the responder MUST ignore the RD bit. query. The RD (Recursion Desired) bit MUST NOT be set. If a responder
receives a query with the header containing RD set bit, the responder
MUST ignore the RD bit.
The IPv6 LINKLOCAL address a given responder listens to, and to which a The IPv6 LINKLOCAL address a given responder listens to, and to which a
sender sends, is a link-local multicast address formed as follows: The sender sends, is a link-local multicast address formed as follows: The
name of the resource record in question is expressed in its canonical name of the resource record in question is expressed in its canonical
form (see [RFC2535], section 8.1), which is uncompressed with all form (see [RFC2535], section 8.1), which is uncompressed with all
alphabetic characters in lower case. The first label of the resource alphabetic characters in lower case. The first label of the resource
record name is then hashed using the MD5 algorithm, described in record name is then hashed using the MD5 algorithm, described in
[RFC1321]. The first 32 bits of the resultant 128-bit hash is then [RFC1321]. The first 32 bits of the resultant 128-bit hash is then
appended to the prefix FF02:0:0:0:0:2::/96 to yield the 128-bit appended to the prefix FF02:0:0:0:0:2::/96 to yield the 128-bit
"solicited name multicast address". (Note: this procedure is intended "solicited name multicast address". (Note: this procedure is intended
to be the same as that specified in section 3 of "IPv6 Node Information to be the same as that specified in section 3 of "IPv6 Node Information
Queries" [NodeInfo]). A responder that listens for queries for multiple Queries" [NodeInfo]). A responder that listens for queries for multiple
names will necessarily listen to multiple of these solicited name names will necessarily listen to multiple of these solicited name
multicast addresses. multicast addresses.
If the multicast query is not resolved during a limited amount of time If the LLMNR query is not resolved during a limited amount of time
(LMDNS_TIMEOUT), then a sender MAY repeat the transmission of a query in (LLMNR_TIMEOUT), then a sender MAY repeat the transmission of a query in
order to assure themselves that the query has been received by a host order to assure themselves that the query has been received by a host
capable of responding to the query. capable of responding to the query.
Repetition MUST NOT be attempted more than 3 times and SHOULD NOT be Repetition MUST NOT be attempted more than 3 times and SHOULD NOT be
repeated more often than once per second to reduce unnecessary network repeated more often than once per second to reduce unnecessary network
traffic. The delay between attempts should be randomized so as to avoid traffic. The delay between attempts should be randomized so as to avoid
synchronization effects. synchronization effects.
2.1.2. Behavior of responders 2.1.2. Behavior of responders
An LMDNS responder listens on port 5353 on the LINKLOCAL address. A responder listens on port 5353 on the LINKLOCAL address and on the
Responders MUST respond to multicast queries to those and only those unicast address(es) that could be set as the source address(es) when the
names for which they are authoritative. As an example, computer responder responds to the LLMNR query. Responders MUST respond to
"host.example.com." is authoritative for the domain "host.example.com.".
On receiving a linklocal multicast DNS A record query for the name LLMNR queries to those and only those names for which they are
"host.example.com." such a host responds with A record(s) that contain authoritative. As an example, computer "host.example.com." is
IP address(es) in the RDATA of the record. authoritative for the domain "host.example.com.". On receiving a LLMNR A
record query for the name "host.example.com." such a host responds with
A record(s) that contain IP address(es) in the RDATA of the record.
In conventional DNS terminology a DNS server authoritative for a zone is In conventional DNS terminology a DNS server authoritative for a zone is
authoritative for all the domain names under the zone root except for authoritative for all the domain names under the zone root except for
the branches delegated into separate zones. Contrary to conventional DNS the branches delegated into separate zones. Contrary to conventional DNS
terminology, a responder is authoritative only for the zone root. For terminology, a responder is authoritative only for the zone root. For
example the host "host.example.com." is not authoritative for the name example the host "host.example.com." is not authoritative for the name
"child.host.example.com." unless the host is configured with multiple "child.host.example.com." unless the host is configured with multiple
names, including "host.example.com." and "child.host.example.com.". The names, including "host.example.com." and "child.host.example.com.". The
purpose of limiting the name authority scope of a responder is to purpose of limiting the name authority scope of a responder is to
prevent complications that could be caused by coexistence of two or more prevent complications that could be caused by coexistence of two or more
hosts with the names representing child and parent (or grandparent) hosts with the names representing child and parent (or grandparent)
nodes in the DNS tree, for example, "host.example.com." and nodes in the DNS tree, for example, "host.example.com." and
"child.host.example.com.". "child.host.example.com.".
In this example (unless this limitation is introduced) a multicast query In this example (unless this limitation is introduced) a LLMNR query
for an A record for the name "child.host.example.com." would result in for an A record for the name "child.host.example.com." would result in
two authoritative responses: name error received from two authoritative responses: name error received from
"host.example.com.", and a requested A record - from "host.example.com.", and a requested A record - from
"child.host.example.com.". To prevent this ambiguity, multicast enabled "child.host.example.com.". To prevent this ambiguity, LLMNR enabled
hosts could perform a dynamic update of the parent (or grandparent) zone hosts could perform a dynamic update of the parent (or grandparent) zone
with a delegation to a child zone. In this example a host with a delegation to a child zone. In this example a host
"child.host.example.com." would send a dynamic update for the NS and "child.host.example.com." would send a dynamic update for the NS and
glue A record to "host.example.com.", but this approach significantly glue A record to "host.example.com.", but this approach significantly
complicates implementation of multicast DNS and would not be acceptable complicates implementation of LLMNR and would not be acceptable
for lightweight hosts. for lightweight hosts.
A response to a multicast query is composed in exactly the same manner A response to a LLMNR query is composed in exactly the same manner
as a response to the unicast DNS query as specified in [RFC1035]. as a response to the unicast DNS query as specified in [RFC1035].
Responders MUST never respond using cached data, and the AA Responders MUST never respond using cached data, and the AA
(Authoritative Answer) bit MUST be set. The response is sent to the (Authoritative Answer) bit MUST be set. The response is sent to the
sender via unicast. A response to an LMDNS query MUST have RCODE set to sender via unicast. A response to an LLMNR query MUST have RCODE set to
zero. Responses with RCODE set to zero are referred to in this document zero. Responses with RCODE set to zero are referred to in this document
as "positively resolved". LMDNS responders may respond only to queries as "positively resolved". LLMNR responders may respond only to queries
which they can resolve positively. which they can resolve positively.
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 or using EDNS0 with discard the response and resend the query over TCP or using EDNS0 with
larger window using the unicast address of the responder. The RA larger window using the unicast address of the responder. The RA
(Recursion Available) bit in the header of the response MUST NOT be set. (Recursion Available) bit in the header of the response MUST NOT be set.
Even if the RA bit is set in the response header, the sender MUST ignore Even if the RA bit is set in the response header, the sender MUST ignore
it. it.
2.1.3. LMDNS addressing 2.1.3. LLMNR addressing
For IPv4 LINKLOCAL addressing, section 2.4 of "Dynamic Configuration of For IPv4 LINKLOCAL addressing, section 2.4 of "Dynamic Configuration of
IPv4 Link-Local Addresses" [IPV4Link] lays out the rules with respect to IPv4 Link-Local Addresses" [IPV4Link] lays out the rules with respect to
source address selection, TTL settings, and acceptable source address selection, TTL settings, and acceptable
source/destination address combinations. IPv6 is described in [RFC2460]; source/destination address combinations. IPv6 is described in [RFC2460];
IPv6 LINKLOCAL addressing is described in [RFC2373]. LMDNS queries and IPv6 LINKLOCAL addressing is described in [RFC2373]. LLMNR queries and
responses MUST obey the rules laid out in these documents. responses MUST obey the rules laid out in these documents.
In composing an LMDNS response, the responder MUST set the Hop Limit In composing an LLMNR response, the responder MUST set the Hop Limit
field in the IPv6 header and the TTL field in IPv4 header of the LMDNS field in the IPv6 header and the TTL field in IPv4 header of the LLMNR
response to 255. The sender MUST verify that the Hop Limit field in IPv6 response to 255. The sender MUST verify that the Hop Limit field in IPv6
header and TTL field in IPv4 header of each response to the LMDNS query header and TTL field in IPv4 header of each response to the LLMNR query
is set to 255. If it is not, then sender MUST ignore the response. is set to 255. If it is not, then sender MUST ignore the response.
Implementation note: Implementation note:
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 specify the TTL of outgoing unicast and multicast options are used to specify 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 receive the IPv4 TTL of received packets with recvmsg(). [RFC2292] to receive the IPv4 TTL of received packets with recvmsg(). [RFC2292]
specifies similar options for specifying and receiving the IPv6 Hop specifies similar options for specifying and receiving the IPv6 Hop
Limit. Limit.
2.1.4. Use of DNS TTL 2.1.4. Use of LLMNR 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 LMDNS query response. Due to the TTL minimalization returned in the LLMNR query response. Due to the TTL minimalization
necessary when caching an RRset, all TTLs in an RRset MUST be set to the necessary when caching an RRset, all TTLs in an RRset MUST be set to the
same value. In the additional and authority section of the response the same value. In the additional and authority section of the response the
responder includes the same records as a DNS server would insert in the responder includes the same records as a DNS server would insert in the
response to the unicast DNS query. response to the unicast DNS query.
2.1.5. No/multiple responses 2.1.5. No/multiple responses
The sender MUST anticipate receiving no replies to some multicast The sender MUST anticipate receiving no replies to some LLMNR queries,
queries, in the event that no responders are available within the in the event that no responders are available within the linklocal
linklocal multicast scope, or in the event that no positive non-null multicast scope, or in the event that no positive non-null responses
responses exist for the transmitted query. If no positive response is exist for the transmitted query. If no positive response is received,
received, a resolver treats it as a response that no records of the a resolver treats it as a response that no records of the specified
specified type and class for the specified name exist (NXRRSET). type and class for the specified name exist (NXRRSET).
The sender MUST anticipate receiving multiple replies to the same The sender MUST anticipate receiving multiple replies to the same LLMNR
linklocal multicast query, in the event that several linklocal multicast query, in the event that several LLMNR enabled computers receive the
DNS enabled computers receive the query and respond with valid answers. query and respond with valid answers. When this occurs, the responses
When this occurs, the responses MAY first be concatenated, and then MAY first be concatenated, and then treated in the same manner that
treated in the same manner that multiple RRs received from the same DNS multiple RRs received from the same DNS server would, ordinarily.
server would, ordinarily. However, after receiving an initial response, However, after receiving an initial response, the sender is not required
the sender is not required to wait for LMDNS_TIMEOUT for additional to wait for LLMNR_TIMEOUT for additional responses.
responses.
3. Usage model 3. Usage model
A host configured to be an LMDNS "responder" MUST also be configured as Although the same host may be configured as a "sender", but not a
a "sender". A host not configured as a "responder" MUST NOT be a "responder" and vice versa, i.e. as a "responder", but not "sender", the
"sender". An LMDNS "sender" MAY multicast requests for any name. If host configured as a "responder" MUST at least use "sender"'s capability
that name is not qualified and does not end in a trailing dot, for the to send LLMNR dynamic update requests to verify the uniqueness of the
purposes of LMDNS, the implicit search order is as follows: names as it is described in Section 5. An LLMNR "sender" MAY multicast
requests for any name. If that name is not qualified and does not end in
a trailing dot, for the purposes of LLMNR, the implicit search order is
as follows:
[1] Request the name with the current domain appended. [1] Request the name with the current domain appended.
[2] Request just the name. [2] Request just the name.
This is the behavior suggested by [RFC1536]. LMDNS uses this technique This is the behavior suggested by [RFC1536]. LLMNR uses this technique
to resolve unqualified host names. to resolve unqualified host names.
If a DNS server is running on a host that supports LMDNS, the DNS server If a DNS server is running on a host that supports LLMNR, the DNS server
MUST respond to LMDNS queries only for the RRSets owned by the host on MUST respond to LLMNR queries only for the RRSets owned by the host on
which the server is running, but MUST NOT respond for the records for which the server is running, but MUST NOT respond for the records for
which the server is authoritative. which the server is authoritative.
3.1. LMDNS configuration A sender MUST NOT send a unicast LLMNR query except when:
a. A sender repeats a query after it received a response
to the previous LLMNR query with the TC bit set, or
LMDNS usage can be configured manually or automatically. On interfaces b. The sender's LLMNR cache contains an NS resource record that
enables the sender to send a query directly to the hosts
authoritative for the name in the query.
A responder with a name "host.example.com." configured to respond to the
LLMNR queries is authoritative for the name "host.example.com.". For
example, when a responder with the name "host.example.com." receives an
A type LLMNR query for the name "host.example.com." it authoritatively
responds to the query.
The same host MAY use LLMNR queries for the resolution of the local
names, and conventional DNS queries for resolution of other DNS names.
3.1. LLMNR configuration
LLMNR usage can be configured manually or automatically. On interfaces
where no manual or automatic configuration has been performed for a where no manual or automatic configuration has been performed for a
given protocol (IPv4 or IPv6), LMDNS SHOULD be enabled by default for given protocol (IPv4 or IPv6), LLMNR SHOULD be enabled by default for
that protocol. that protocol.
For IPv6, the stateless DNS discovery mechanisms described in "IPv6 For IPv6, the stateless DNS discovery mechanisms described in "IPv6
Stateless DNS Discovery" [DNSDisc] can be used to discover whether Stateless DNS Discovery" [DNSDisc] can be used to discover whether
linklocal multicast DNS should be enabled or disabled on a per-interface LLMNR should be enabled or disabled on a per-interface basis.
basis.
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 multicast DNS on an interface. The Multicast DNS Enable configure LLMNR on an interface. The LLMNR Enable Option, described in
Option, described in [mDNSEnable], can be used to explicitly enable or [mDNSEnable], can be used to explicitly enable or disable use of LLMNR
disable use of linklocal multicast DNS on an interface for a given on an interface. The LLMNR Enable Option does not determine whether or
protocol, as well as to specify the order in which DNS and LMDNS is used in which order DNS itself is used for name resolution. The order in
on that interface. which various name resolution mechanisms should be used can be specified
using the Name Service Search Option for DHCP, [RFC2937].
The Multicast DNS Enable Option affects only DNS resolver behavior, that
is, how DNS resolution is performed, and whether LMDNS is used. The
Multicast DNS Enable Option does not determine whether or in which order
DNS itself is used for name resolution. This can be specified, for
example, using the Name Service Search Option for DHCP, [RFC2937], which
can be used to globally determine where DNS is used within the name
service search order.
If an interface has been configured for a given protocol via any
automatic configuration mechanism which is able to supply DNS
configuration information, then LMDNS SHOULD NOT be used on that
interface for that protocol unless it has been explicitly enabled,
whether via that mechanism or any other. This ensures that upgraded
hosts do not change their default behavior, without requiring the source
of the configuration information to be simultaneously updated. This
implies that on the interface, the host will neither listen on the DNS
LINKLOCAL multicast address, nor will it send queries to that address.
Note that it is possible for LMDNS to be enabled for use with IPv6 at Note that it is possible for LLMNR to be enabled for use with IPv6 at
the same time it is disabled for IPv4, and vice versa. For example, the same time it is disabled for IPv4, and vice versa. For example,
where a home gateway implements a DNS proxy and DHCPv4, but not DHCPv6 where a home gateway implements a DNS proxy and DHCPv4, but not DHCPv6
or DNS autoconfiguration, there may be no mechanism for allowing IPv6 or DNS autoconfiguration, there may be no mechanism for allowing
hosts to resolve the names of other IPv6 hosts on the home network. In IPv6-only hosts to resolve the names of other IPv6-only hosts on the
this situation, LMDNS is useful for resolution of dynamic names, and it home network. In this situation, LLMNR is useful for resolution of
will be enabled for use with IPv6, even though it is disabled for use dynamic names, and it will be enabled for use with IPv6, even though it
with IPv4. is disabled for use with IPv4.
4. Sequence of events 4. Sequence of events
The sequence of events for LMDNS usage is as follows: The sequence of events for LLMNR usage is as follows:
1. If a sender needs to resolve a query for a name 1. If a sender needs to resolve a query for a name "host.example.com",
"host.example.com", then it sends a multicast query to the then it sends a LLMNR query to the LINKLOCAL multicast address.
LINKLOCAL multicast address.
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 verifies that the Hop 3. Upon the reception of the response, the sender verifies that the Hop
Limit field in IPv6 header or TTL field in IPv4 header (depending on Limit field in IPv6 header or TTL field in IPv4 header (depending on
the protocol used) of the response is set to 255. The sender then the protocol used) of the response is set to 255. The sender then
verifies compliance with the addressing requirements for IPv4, verifies compliance with the addressing requirements for IPv4,
described in [IPV4Link], and IPv6, described in [RFC2373]. If these described in [IPV4Link], and IPv6, described in [RFC2373]. If these
skipping to change at page 9, line 41 skipping to change at page 9, line 5
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 a SRV type record - multiple hosts may respond to a query for a SRV type record
- multiple hosts may respond to a query for an A type record for a - multiple hosts may respond to a query for an A type record for a
cluster name (assigned to multiple hosts in the cluster) cluster name (assigned to multiple hosts in the cluster)
- only a single host may respond to a query for an A type record for - only a single host may respond to a query for an A type record for
a hostname. a hostname.
Every responder that responds to a linklocal multicast DNS query and/or Every responder that responds to a LLMNR
dynamic update request AND includes a UNIQUE record in the response: query and/or dynamic update request AND includes a UNIQUE record in the
response:
1. MUST verify that there is no other host within the scope of the 1. MUST verify that there is no other host within the scope of the
LMDNS query propagation that can return a DNS 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 linklocal multicast DNS queries Where a host is configured to respond to LLMNR queries on more than one
on more than one interface, the host MUST verify resource record interface, the host MUST verify resource record uniqueness on each
uniqueness on each interface for each UNIQUE resource record that could interface for each UNIQUE resource record that could be used on that
interface. To accomplish this, the host MUST send a dynamic LLMNR update
be used on that interface. To accomplish this, the host MUST multicast a request for each new UNIQUE resource record. Format of the dynamic LLMNR
dynamic DNS update request as specified in [RFC2136] for each new UNIQUE update request is identical to the format of the dynamic DNS update
resource record. Uniqueness verification is carried out when the host: request specified in [RFC2136]. Uniqueness verification is carried out
when the host:
- starts up or - starts up or
- is configured to respond to the linklocal multicast DNS queries on - is configured to respond to the LLMNR queries on some interface or
some interface or - is configured to respond to the LLMNR queries using additional
- is configured to respond to the linklocal multicast DNS queries using UNIQUE resource records.
additional UNIQUE DNS records.
Below we describe the data to be specified in the dynamic update Below we describe the data to be specified in the dynamic update
request: request:
Header section Header section
contains values according to [RFC2136]. contains values according to [RFC2136].
Zone section Zone section
The zone name in the zone section MUST be set to the name of the The zone name in the zone section MUST be set to the name of the
UNIQUE record. The zone type in the zone section MUST be set to UNIQUE record. The zone type in the zone section MUST be set to
skipping to change at page 10, line 48 skipping to change at page 10, line 9
When a host that owns a UNIQUE record receives a dynamic update request When a host that owns a UNIQUE record receives a dynamic update request
that requests that the UNIQUE resource record set does not exist, the that requests that the UNIQUE resource record set does not exist, the
host MUST respond via unicast with the YXRRSET error, according to the host MUST respond via unicast with the YXRRSET error, according to the
rules described in Section 3 of [RFC2136]. rules described in Section 3 of [RFC2136].
After the client receives an YXRRSET response to its dynamic update After the client receives an YXRRSET response to its dynamic update
request stating that a UNIQUE resource record does not exist, the host request stating that a UNIQUE resource record does not exist, the host
MUST check whether the response arrived on another interface. If this is MUST check whether the response arrived on another interface. If this is
the case, then the client can use the UNIQUE resource record in response the case, then the client can use the UNIQUE resource record in response
to multicast queries and dynamic update requests. If not, then it MUST to LLMNR queries and dynamic update requests. If not, then it MUST
NOT use the UNIQUE resource record in response to linklocal multicast NOT use the UNIQUE resource record in response to LLMNR
queries and dynamic update requests. queries and dynamic update requests.
Note that this name conflict detection mechanism doesn't prevent name Note that this name conflict detection mechanism doesn't prevent name
conflicts when previously partitioned segments are connected by a conflicts when previously partitioned segments are connected by a
bridge. In such a situation, name conflicts are detected when a sender bridge. In such a situation, name conflicts are detected when a sender
receives more than one response to its linklocal multicast DNS query. receives more than one response to its LLMNR query. In this case, the
In this case, the sender sends the first response that it received to sender sends the first response that it received to all responders that
all responders that responded to this query except the first one, using responded to this query except the first one, using unicast. A host that
unicast. A host that receives a query response containing a UNIQUE receives a query response containing a UNIQUE resource record that it
resource record that it owns, even if it didn't send such a query, MUST owns, even if it didn't send such a query, MUST verify that no other
verify that no other host within the linklocal multicast DNS scope is host within the LLMNR scope is authoritative for the same name, using
authoritative for the same name, using the dynamic DNS update request the dynamic LLMNR update request mechanism described above.
mechanism described above.
Based on the result, the host detects whether there is a name conflict Based on the result, the host detects whether there is a name conflict
and acts as described above. and acts as described above.
5.1. Considerations for Multiple Interfaces 5.1. Considerations for Multiple Interfaces
A multi-homed host may elect to configure LMDNS 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. However,
should a host wish to configure LMDNS on more than one of its active should a host wish to configure LLMNR on more than one of its active
interfaces, there are some additional precautions it MUST take. interfaces, there are some additional precautions it MUST take.
Implementers who are not planning to support linklocal multicast DNS on Implementers who are not planning to support LLMNR on multiple
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 described
in Section 5. The situation is illustrated in figure 1 below: in Section 5. The situation is illustrated in figure 1 below:
---------- ---------- ---------- ----------
| | | | | | | |
[A] [myhost] [myhost] [A] [myhost] [myhost]
Figure 1. LINKLOCAL name conflict Figure 1. LINKLOCAL 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 to interface, but not the other. The multi-homed myhost will not be able to
respond with a host RR for "myhost" on the interface on the right (see respond with a host RR for "myhost" on the interface on the right (see
Figure 1). The multi-homed host may, however, be configured to use the Figure 1). The multi-homed host may, however, be configured to use the
"myhost" name on the interface on the left. "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 be
using the same name. If an LMDNS client sends requests over multiple using the same name. If an LLMNR client sends requests over multiple
interfaces, and receives replies from more than one, the result returned interfaces, and receives replies from more than one, the result returned
to the client is defined by the implementation. The situation is to the client is defined by the implementation. The situation is
illustrated in figure 2 below. illustrated in figure 2 below.
---------- ---------- ---------- ----------
| | | | | | | |
[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 LMDNS on both interfaces, it will If host myhost is configured to use LLMNR on both interfaces, it will
send LMDNS queries on both interfaces. When host myhost sends a query send LLMNR queries on both interfaces. When host myhost sends a query
for the host RR for name "A" it will receive a response from hosts on for the host RR for name "A" it will receive a response from hosts on
both interfaces. both interfaces.
Host myhost will then forward a response from the first responder to the Host myhost will then forward a response from the first responder to the
second responder, who will attempt to verify the uniqueness of host RR second responder, who will attempt to verify the uniqueness of host RR
for its name, but will not discover a conflict, since the conflicting for its name, but will not discover a conflict, since the conflicting
host resides on a different link. Therefore it will continue using its host resides on a different link. Therefore it will continue using its
name. name.
Indeed, host myhost cannot distinguish between the situation shown in Indeed, host myhost cannot distinguish between the situation shown in
skipping to change at page 13, line 9 skipping to change at page 12, line 9
[RFC2553] provides an API which can partially solve the name ambiguity [RFC2553] provides an API which can partially solve the name ambiguity
problem for applications written to use this API, since the sockaddr_in6 problem for applications written to use this API, since the sockaddr_in6
structure exposes the scope within which each scoped address exists, and structure exposes the scope within which each scoped address exists, and
this structure can be used for both IPv4 (using v4-mapped IPv6 this structure can be used for both IPv4 (using v4-mapped IPv6
addresses) and IPv6 addresses. 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 the
request getaddrinfo("A", ...) with ai_family=AF_INET6 and request getaddrinfo("A", ...) with ai_family=AF_INET6 and
ai_flags=AI_ALL|AI_V4MAPPED. LMDNS 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 of
both hosts responding to the name 'A'. Link-local addresses will have a both hosts responding to the name 'A'. Link-local addresses will have a
sin6_scope_id value that disambiguates which interface is used to reach sin6_scope_id value that disambiguates which interface is used to reach
the address. Of course, to the application, Figures 2 and 3 are still the address. Of course, to the application, Figures 2 and 3 are still
indistinguishable, but this API allows the application to communicate indistinguishable, but this API allows the application to communicate
successfully with any address in the list. successfully with any address in the list.
6. Security Considerations 6. Security Considerations
This draft does not prescribe a means of securing the LMDNS mechanism. This draft does not prescribe a means of securing the LLMNR mechanism.
It is possible that hosts will allocate conflicting names for a period It is possible that hosts will allocate conflicting names for a period
of time, or that non-conforming hosts will attempt to deny service to of time, or that non-conforming hosts will attempt to deny service to
other hosts by allocating the same name. Such attacks also allow nodes other hosts by allocating the same name. Such attacks also allow nodes
to receive packets destined for other nodes. The protocol reduces the to receive packets destined for other nodes. The protocol reduces the
exposure to such threats in the absence of authentication by ignoring exposure to such threats in the absence of authentication by ignoring
LMDNS query response packets received from off-link senders. LLMNR query response packets received from off-link senders.
In order to prevent responses to LMDNS queries from polluting the DNS In order to prevent responses to LLMNR queries from polluting the DNS
cache, LMDNS implementations MUST use a distinct, isolated cache for cache, LLMNR implementations MUST use a distinct, isolated cache for
LMDNS. LLMNR.
In all received responses, the Hop Limit field in IPv6 and the TTL field In all received responses, the Hop Limit field in IPv6 and the TTL field
in IPv4 are verified to contain 255, the maximum legal value. Since in IPv4 are verified to contain 255, the maximum legal value. Since
routers decrement the Hop Limit on all packets they forward, received routers decrement the Hop Limit on all packets they forward, received
packets containing a Hop Limit of 255 must have originated from a packets containing a Hop Limit of 255 must have originated from a
neighbor. neighbor.
These threats are most serious in wireless networks such as 802.11, These threats are most serious in wireless networks such as 802.11,
since attackers on a wired network will require physical access to the since attackers on a wired network will require physical access to the
home network, while wireless attackers may reside outside the home. home network, while wireless attackers may reside outside the home.
Link-layer security will serve to secure LMDNS against the above threats Link-layer security will serve to secure LLMNR against the above threats
if it is available. For example, where 802.11 "Wired Equivalency if it is available. For example, where 802.11 "Wired Equivalency
Privacy" (WEP) [IEEE80211] is implemented, a casual attacker is likely Privacy" (WEP) [IEEE80211] is implemented, a casual attacker is likely
to be deterred from gaining access to the home network. to be deterred from gaining access to the home network.
The mechanism specified in this draft does not require use of DNSSEC. The mechanism specified in this draft does not require use of DNSSEC.
As a result, responses to LMDNS queries MAY NOT be authenticated. If As a result, responses to LLMNR queries MAY NOT be authenticated. If
authentication is desired, and a pre-arranged security configuration is authentication is desired, and a pre-arranged security configuration is
possible, then IPsec ESP with a null-transform MAY be used to possible, then IPsec ESP with a null-transform MAY be used to
authenticate LMDNS responses. In a small network without a certificate authenticate LLMNR responses. In a small network without a certificate
authority, this can be most easily accomplished through configuration of authority, this can be most easily accomplished through configuration of
a group pre-shared key for trusted hosts. a group pre-shared key for trusted hosts.
7. IANA Considerations 7. 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. Since it uses a port (5353) and link scope multicast administration. Since it uses a port (5353) and link scope multicast
IPv4 address (224.0.0.251) previously allocated for use with LMDNS, no IPv4 address (224.0.0.251) previously allocated for use with LLMNR, no
additional IANA allocations are required. additional IANA allocations are required.
8. Normative References 8. 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.
[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.
skipping to change at page 14, line 45 skipping to change at page 13, line 45
[RFC2535] Eastlake, D., "Domain Name System Security Extensions", [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999. RFC 2535, 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.
[IPV4Link] Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4 [IPV4Link] Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4
Link-Local Addresses", Internet draft (work in progress), Link-Local Addresses", Internet draft (work in progress),
draft-ietf-zeroconf-ipv4-linklocal-05.txt, November 2001. draft-ietf-zeroconf-ipv4-linklocal-05.txt, November 2001.
[mDNSEnable] Guttman, E., "DHCP Multicast DNS Enable Option", Internet [mDNSEnable] Guttman, E., "DHCP mDNS Enable Option", Internet
draft (work in progress), draft-guttman-mdns- draft (work in progress), draft-guttman-mdns-
enable-01.txt, July 2001. enable-01.txt, July 2001.
9. Informative References 9. Informative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992. April 1992.
[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.
skipping to change at page 15, line 42 skipping to change at page 14, line 42
lookups-07.txt, August 2000. lookups-07.txt, August 2000.
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 also
been received from Mark Andrews, Stuart Cheshire, Robert Elz, Rob been received from Mark Andrews, Stuart Cheshire, Robert Elz, Rob
Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig,
Thomas Narten, Erik Nordmark, Sander Van-Valkenburg and Tomohide Thomas Narten, Erik Nordmark, Sander Van-Valkenburg, Tomohide
Nagashima. Nagashima and Brian Zill.
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 17, line 20 skipping to change at page 16, line 20
perpetual and will not be revoked by the Internet Society or its perpetual and will not be revoked by the Internet Society or its
successors or assigns. This document and the information contained successors or assigns. This document and the information contained
herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Expiration Date Expiration Date
This memo is filed as <draft-ietf-dnsext-mdns-08.txt>, and expires This memo is filed as <draft-ietf-dnsext-mdns-09.txt>, and expires
June 22, 2002. August 21, 2002.
 End of changes. 

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