draft-ietf-dnsext-mdns-05.txt   draft-ietf-dnsext-mdns-06.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-05.txt> Microsoft <draft-ietf-dnsext-mdns-06.txt> Microsoft
14 September 2001 8 October 2001
Multicast DNS Multicast DNS
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 RFC2026. provisions of Section 10 of RFC2026.
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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of ad-hoc networks operating without a DNS server. In order to allow DNS of ad-hoc networks operating without a DNS server. In order to allow DNS
name resolution in such environments, the use of a multicast DNS is name resolution in such environments, the use of a multicast DNS is
proposed. proposed.
Table of Contents Table of Contents
1. Introduction .......................................... 3 1. Introduction .......................................... 3
2. Name resolution using multicast DNS ................... 3 2. Name resolution using multicast DNS ................... 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 mDNS configuration .............................. 7 3.1 mDNS configuration .............................. 8
4. Sequence of events .................................... 8 4. Sequence of events .................................... 9
5. Conflict resolution ................................... 8 5. Conflict resolution ................................... 9
5.1 Considerations for multiple interfaces .......... 10 5.1 Considerations for multiple interfaces .......... 11
5.2 API issues ...................................... 12 5.2 API issues ...................................... 12
6. IANA considerations ................................... 12 6. IANA considerations ................................... 13
7. ARPA domain considerations ............................ 12 7. ARPA domain considerations ............................ 13
8. References ............................................ 13 8. References ............................................ 14
9. Security considerations ............................... 14 9. Security considerations ............................... 15
ACKNOWLEDGMENTS .............................................. 15 Acknowledgments .............................................. 16
AUTHORS' ADDRESSES ........................................... 15 Authors' Addresses ........................................... 16
Intellectual Property Statement .............................. 16 Intellectual Property Statement .............................. 16
Full Copyright Statement ..................................... 16 Full Copyright Statement ..................................... 17
1. Introduction 1. Introduction
Multicast DNS enables DNS name resolution in the scenarios when Multicast DNS enables DNS name resolution in the scenarios when
conventional DNS name resolution is not possible. Namely, when there are conventional DNS name resolution is not possible. Namely, when there are
no DNS servers available on the network or available DNS servers do not no DNS servers available on the network or available DNS servers do not
provide name resolution for the names of the hosts on the local network. provide name resolution for the names of the hosts on the local network.
The latter case, for example, corresponds to a scenario when a 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 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 ISP and the network hosts are configured with the ISP's DNS server for
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resolution for the names of the hosts on the network. resolution for the names of the hosts on the network.
This document discusses multicast DNS, an extension to the DNS protocol This document discusses multicast DNS, an extension to the DNS protocol
which consists of a single change to the method of use, and no change to which consists of a single change to the method of use, and no change to
the format of DNS packets. 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
mDNS in particular is outside of the scope of this document, as is name mDNS 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 [1]. described in [1].
2. Name resolution using Multicast DNS 2. Name resolution using Multicast DNS
This extension to the DNS protocol consists of a single change to the This extension to the DNS protocol consists of a single change to the
method of use, and no change to the current format of DNS packets. method of use, and no change to the current format of DNS packets.
Namely, this extension allows multicast DNS queries to be sent to and Namely, this extension allows multicast DNS queries to be sent to and
received on port 53. received on port 53.
This extension allows multicast DNS queries to be sent to and received This extension allows multicast DNS queries to be sent to and received
on port 53 using a LINKLOCAL address [2] for IPv4 and the "solicited on port 53 using a LINKLOCAL address as specified in "Administratively
name" LINKLOCAL multicast addresses for IPv6. LINKLOCAL addresses are Scoped IP Multicast" [2] for IPv4 and the "solicited name" LINKLOCAL
used to prevent propagation of multicast DNS traffic across routers, multicast addresses for IPv6. The mDNS LINKLOCAL address to be used for
potentially flooding the network. IPv4 is <TBD>. LINKLOCAL addresses are used to prevent propagation of
multicast DNS traffic across routers, potentially flooding the network.
Propagation of multicast DNS packets within the local subnet is Propagation of multicast DNS packets within the local subnet is
considered sufficient to enable DNS name resolution in small adhoc considered sufficient to enable DNS name resolution in small adhoc
networks. The assumption is that if a network has a router, then the networks. The assumption is that if a network has a router, then the
network either has a DNS server or the router can function as a DNS network either has a DNS server or the router can function as a DNS
proxy, possibly implementing dynamic DNS. proxy. By implementing DHCP as well as a DNS proxy and dynamic DNS,
routers can provide name resolution for the names of the hosts on the
local network. This functionality is easily provided, and so in such
cases it is assumed that multicast DNS need not be enabled by default.
In the future, mDNS may be defined to support greater than LINKLOCAL In the future, mDNS may be defined to support greater than LINKLOCAL
multicast scope. This would occur if LINKLOCAL mDNS deployment is multicast scope. This would occur if LINKLOCAL mDNS deployment is
successful, the assumption that mDNS is not needed in multiple subnets successful, the assumption that mDNS is not needed in multiple subnets
proves incorrect, and multicast routing becomes ubiquitous. For proves incorrect, and multicast routing becomes ubiquitous. For
example, it is not clear that this assumption will be valid in large example, it is not clear that this assumption will be valid in large
adhoc networking scenarios. adhoc networking scenarios.
Once we have experience in mDNS deployment in terms of administrative Once we have experience in mDNS 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 mDNS. reevaluate which multicast scopes are appropriate for use with mDNS.
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 multicast query is
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responder MUST ignore the RD bit. responder MUST ignore the RD bit.
If the multicast query is not positively resolved ("positively resolved" If the multicast query is not positively resolved ("positively resolved"
refers in this document to a response with the RCODE set to 0) during a refers in this document to a response with the RCODE set to 0) during a
limited amount of time, then a sender MAY repeat the transmission of a limited amount of time, then a sender MAY repeat the transmission of a
query in order to assure themselves that the query has been received by query in order to assure themselves that the query has been received by
a host capable of responding to the query. a host 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 randomised so as to avoid traffic. The delay between attempts should be randomized so as to avoid
synchronisation effects. synchronization effects.
2.1.2. Behavior of responders 2.1.2. Behavior of responders
A responder listens on port 53 on the LINKLOCAL address. The IPv6 A responder listens on port 53 on the LINKLOCAL address. The IPv6
LINKLOCAL address a given responder listens to, and to which a sender LINKLOCAL address a given responder listens to, and to which a sender
sends, is a link-local multicast address formed as follows: The name of sends, is a link-local multicast address formed as follows: The name of
the resource record in question is expressed in its canonical form (see the resource record in question is expressed in its canonical form (see
RFC 2535 [15], section 8.1), which is uncompressed with all alphabetic RFC 2535 [15], section 8.1), which is uncompressed with all alphabetic
characters in lower case. The first label of the resource record name characters in lower case. The first label of the resource record name
is then hashed using the MD5 algorithm (see RFC 1321 [16]). The first is then hashed using the MD5 algorithm (see RFC 1321 [16]). The first
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2.1.2. Behavior of responders 2.1.2. Behavior of responders
A responder listens on port 53 on the LINKLOCAL address. The IPv6 A responder listens on port 53 on the LINKLOCAL address. The IPv6
LINKLOCAL address a given responder listens to, and to which a sender LINKLOCAL address a given responder listens to, and to which a sender
sends, is a link-local multicast address formed as follows: The name of sends, is a link-local multicast address formed as follows: The name of
the resource record in question is expressed in its canonical form (see the resource record in question is expressed in its canonical form (see
RFC 2535 [15], section 8.1), which is uncompressed with all alphabetic RFC 2535 [15], section 8.1), which is uncompressed with all alphabetic
characters in lower case. The first label of the resource record name characters in lower case. The first label of the resource record name
is then hashed using the MD5 algorithm (see RFC 1321 [16]). The first is then hashed using the MD5 algorithm (see RFC 1321 [16]). The first
32 bits of the resultant 128-bit hash is then appended to the prefix 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 "solicited name multicast FF02:0:0:0:0:2::/96 to yield the 128-bit "solicited name multicast
address". (Note: this procedure is intended to be the same as that address". (Note: this procedure is intended to be the same as that
specified in section 3 of [14]) A responder that listens for queries specified in section 3 of "IPv6 Node Information Queries" [14]). A
for multiple names will necessarily listen to multiple of these responder that listens for queries for multiple names will necessarily
listen to multiple of these solicited name multicast addresses.
solicited name multicast addresses.
Responders MUST respond to multicast queries to those and only those Responders MUST respond to multicast queries to those and only those
names for which they are authoritative. As an example, computer names for which they are authoritative. As an example, computer
"host.example.com.local.arpa." is authoritative for the domain "host.example.com.local.arpa." is authoritative for the domain
"host.example.com.local.arpa.". On receiving a multicast DNS A record "host.example.com.local.arpa.". On receiving a multicast DNS A record
query for the name "host.example.com.local.arpa." such a host responds query for the name "host.example.com.local.arpa." such a host responds
with A record(s) that contain IP address(es) in the RDATA of the record. 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
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"host.example.com.local.arpa.", and a requested A record - from "host.example.com.local.arpa.", and a requested A record - from
"child.host.example.com.local.arpa.". To prevent this ambiguity, "child.host.example.com.local.arpa.". To prevent this ambiguity,
multicast enabled hosts could perform a dynamic update of the parent (or multicast enabled hosts could perform a dynamic update of the parent (or
grandparent) zone with a delegation to a child zone. In this example a grandparent) zone with a delegation to a child zone. In this example a
host "child.host.example.com.local.arpa." would send a dynamic update host "child.host.example.com.local.arpa." would send a dynamic update
for the NS and glue A record to "host.example.com.local.arpa.", but this for the NS and glue A record to "host.example.com.local.arpa.", but this
approach significantly complicates implementation of multicast DNS and approach significantly complicates implementation of multicast DNS and
would not be acceptable for lightweight hosts. would not be acceptable for lightweight hosts.
A response to a multicast query is composed in exactly the same manner A response to a multicast query is composed in exactly the same manner
as a response to the unicast DNS query as specified in [4]. Responders as a response to the unicast DNS query as specified in RFC 1035 [4].
MUST never respond using cached data, and the AA (Authoritative Answer) Responders MUST never respond using cached data, and the AA
bit MUST be set. The response is sent to the sender via unicast. A (Authoritative Answer) bit MUST be set. The response is sent to the
response to an mDNS query MUST have RCODE set to zero, since mDNS sender via unicast. A response to an mDNS query MUST have RCODE set to
responders MUST NOT send error replies in response to mDNS queries. zero, since mDNS responders MUST NOT send error replies in response to
mDNS queries.
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. mDNS addressing 2.1.3. mDNS addressing
For IPv4 LINKLOCAL addressing, section 2.4 of [18] lays out the rules For IPv4 LINKLOCAL addressing, section 2.4 of "Dynamic Configuration of
with respect to source address selection, TTL settings, and acceptable IPv4 Link-Local Addresses" [18] lays out the rules with respect to
source address selection, TTL settings, and acceptable
source/destination address combinations. IPv6 LINKLOCAL addressing is source/destination address combinations. IPv6 LINKLOCAL addressing is
described in [9]. mDNS queries and responses MUST obey the rules laid described in RFC 2373 [9]. mDNS queries and responses MUST obey the
out in these documents. rules laid out in these documents.
In composing an mDNS response, the responder MUST set the Hop Limit In composing an mDNS response, the responder MUST set the Hop Limit
field in the IPv6 header and the TTL field in IPv4 header of the field in the IPv6 header and the TTL field in IPv4 header of the
multicast DNS response to 255. The sender MUST verify that the Hop Limit multicast DNS 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 field in IPv6 header and TTL field in IPv4 header of each response to
the multicast DNS query is set to 255. If it is not, then sender MUST the multicast DNS query is set to 255. If it is not, then sender MUST
ignore the response. 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(). RFC 2292 to receive the IPv4 TTL of received packets with recvmsg(). RFC 2292
specifies similar options for specifying and receiving the IPv6 Hop [20] specifies similar options for specifying and receiving the IPv6
Limit. Hop Limit.
2.1.4. Use of DNS TTL 2.1.4. Use of DNS TTL
The responder should use a pre-configured TTL [5] value in the records The responder should use a pre-configured TTL value in the records
returned in the multicast DNS query response. Due to the TTL returned in the multicast DNS query response. Due to the TTL
minimalization necessary when caching an RRset, all TTLs in an RRset minimalization necessary when caching an RRset, all TTLs in an RRset
MUST be set to the same value. In the additional and authority section MUST be set to the same value. In the additional and authority section
of the response the responder includes the same records as a DNS server of the response the responder includes the same records as a DNS server
would insert in the response to the unicast DNS query. would insert in the 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 multicast
queries, in the event that no responders are available within the queries, in the event that no responders are available within the
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3. Usage model 3. Usage model
A host configured to be an mDNS "responder" MUST also be configured as a A host configured to be an mDNS "responder" MUST also be configured as a
"sender". A host not configured as a "responder" MUST NOT be a "sender". "sender". A host not configured as a "responder" MUST NOT be a "sender".
Multicast DNS usage is determined by special treatment of the Multicast DNS usage is determined by special treatment of the
".local.arpa." namespace. The sender treats queries for ".local.arpa." ".local.arpa." namespace. The sender treats queries for ".local.arpa."
as a special case. A sender MUST NOT send a unicast query for names as a special case. A sender MUST NOT send a unicast query for names
ending with the ".local.arpa." suffix except when: ending with the ".local.arpa." suffix except when:
a. A sender repeats a query over TCP after it received a response a. A sender repeats a query after it received a response
to the previous multicast query with the TC bit set, or to the previous multicast query with the TC bit set, or
b. The sender's cache contains an NS resource record that enables b. The sender's cache contains an NS resource record that enables
the sender to send a query directly to the hosts the sender to send a query directly to the hosts
authoritative for the name in the query. authoritative for the name in the query.
It is not expected that a host named "host.example.com." will be It is not expected that a host named "host.example.com." will be
manually configured to have the additional name manually configured to have the additional name
"host.example.com.local.arpa." when it is configured to use multicast "host.example.com.local.arpa." when it is configured to use multicast
DNS. Instead, a responder with a name "host.example.com." configured DNS. Instead, a responder with a name "host.example.com." configured
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The same host MAY use multicast DNS queries for the resolution of names The same host MAY use multicast DNS queries for the resolution of names
ending with ".local.arpa.", and unicast DNS queries for resolution of ending with ".local.arpa.", and unicast DNS queries for resolution of
all other names. When a user or application requests a DNS client to all other names. When a user or application requests a DNS client to
resolve a dot-terminated name that contains a ".local.arpa" suffix, the resolve a dot-terminated name that contains a ".local.arpa" suffix, the
query for such a name MUST be multicast and the name SHOULD NOT be query for such a name MUST be multicast and the name SHOULD NOT be
concatenated with any suffix. concatenated with any suffix.
If a DNS server is running on a host, the host MUST NOT listen for If a DNS server is running on a host, the host MUST NOT listen for
multicast DNS queries, to prevent the host from listening on port 53 and multicast DNS queries, to prevent the host from listening on port 53 and
intercepting DNS queries directed to a DNS server. By default, a DNS intercepting DNS queries directed to a DNS server. By default, a DNS
server MUST NOT listen to multicast DNS queries. server MUST NOT listen to multicast DNS queries.
3.1. mDNS configuration 3.1. mDNS configuration
Multicast DNS usage can be configured manually or automatically. On Multicast DNS usage can be configured manually or automatically. On
interfaces where no manual or automatic configuration has been interfaces where no manual or automatic configuration has been performed
for a given protocol (IPv4 or IPv6), multicast DNS SHOULD be enabled by
performed, multicast DNS is enabled by default. default for that protocol.
For IPv6, the stateless DNS discovery mechanisms described in [19] can For IPv6, the stateless DNS discovery mechanisms described in "IPv6
be used to discover whether multicast DNS is globally enabled or Stateless DNS Discovery" [19] can be used to discover whether multicast
disabled. DNS is globally enabled or disabled.
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 mDNS enable DHCP option, configure multicast DNS on an interface. The mDNS Enable Option,
described in [6], can be used to explicitly enable or disable use of described in [6], can be used to explicitly enable or disable use of
multicast DNS on an interface. The Name Service Search option, described multicast DNS on an interface for a given protocol, as well as to
in RFC 2937 [3], can be used to globally determine where multicast DNS specify the order in which DNS and multicast DNS is used on that
is used within the name service search order. DHCP option codes are used interface.
as RFC 2937 codes signifying name services within the search order. As a
result, to specify multicast DNS usage within the name service search
order, the option code assigned to the mDNS enable option is used.
If an interface has been configured via any automatic configuration The mDNS Enable Option affects only DNS resolver behavior, that is, how
mechanism which is able to supply DNS configuration information, then DNS resolution is performed, and whether multicast DNS is used. The
multicast DNS MUST NOT be used on that interface unless it has been mDNS Enable Option does not determine whether or in which order DNS
explicitly enabled, whether via that mechanism or any other. This itself is used for name resolution. This can be specified, for example,
ensures that upgraded hosts do not change their default behavior, using the Name Service Search Option for DHCP, RFC 2937 [3], which can
without requiring the source of the configuration information to be be used to globally determine where DNS is used within the name service
simultaneously updated. This implies that on the interface, the host search order.
will neither listen on the DNS LINKLOCAL multicast address, nor will it
send queries to that address. For a DNS server, automatic configuration If an interface has been configured for a given protocol via any
mechanisms MUST NOT enable multicast DNS on any interface. automatic configuration mechanism which is able to supply DNS
configuration information, then multicast DNS 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 mDNS to be enabled for use with IPv6 at 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 or DNS
autoconfiguration, there may be no mechanism for allowing IPv6 hosts to
resolve the names of other IPv6 hosts on the home network. In this
situation, mDNS is useful for resolution of dynamic names, and it will
be enabled for use with IPv6, even though it is disabled for use with
IPv4.
4. Sequence of events 4. Sequence of events
The sequence of events for multicast DNS usage is as follows: The sequence of events for multicast DNS 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.local.arpa", then it sends a multicast query to the "host.example.com.local.arpa", then it sends a multicast 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.local.arpa". The responder sends for the domain name "host.example.com.local.arpa". 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 [18] verifies compliance with the addressing requirements for IPv4,
and IPv6 [9]. If these conditions are met, then the sender described in [18], and IPv6, described in RFC 2373 [9]. If these
uses and caches the returned response. If not, then the sender ignores conditions are met, then the sender uses and caches the returned
the response and continues waiting for the response. response. If not, then the sender ignores the response and continues
waiting for the response.
5. Conflict resolution 5. Conflict resolution
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:
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This section MUST be left empty. This section MUST be left empty.
Additional section Additional section
This section is set according to RFC 2136. This section is set according to RFC 2136.
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 RFC 2136 [11]. rules described in Section 3 of RFC 2136 [11].
After client receives an YXRRSET response to its dynamic update request After the client receives an YXRRSET response to its dynamic update
that a UNIQUE resource record does not exist, the host MUST not use the request that a UNIQUE resource record does not exist, the host MUST NOT
UNIQUE resource record in responses to multicast queries and dynamic use the UNIQUE resource record in responses to multicast queries and
update requests. 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 multicast DNS query. In this receives more than one response to its multicast DNS query. In this
case, the sender sends the first response that it received to all case, the sender sends the first response that it received to all
responders that responded to this query except the first one, using responders that responded to this query except the first one, using
unicast. A host that receives a query response containing a UNIQUE unicast. A host that receives a query response containing a UNIQUE
resource record that it owns, even if it didn't send such a query, MUST resource record that it owns, even if it didn't send such a query, MUST
verify that no other host within the multicast DNS scope is verify that no other host within the multicast DNS scope is
authoritative for the same name, using the dynamic DNS update request authoritative for the same name, using the dynamic DNS 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.
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5.1. Considerations for Multiple Interfaces 5.1. Considerations for Multiple Interfaces
A multi-homed host may elect to configure multicast DNS on only one of A multi-homed host may elect to configure multicast DNS on only one of
its active interfaces. In many situations this will be adequate. its active interfaces. In many situations this will be adequate.
However, should a host wish to configure multicast DNS on more than one However, should a host wish to configure multicast DNS on more than one
of its active interfaces, there are some additional precautions it MUST of its active interfaces, there are some additional precautions it MUST
take. Implementers who are not planning to support multicast DNS on take. Implementers who are not planning to support multicast DNS on
multiple interfaces simultaneously may skip this section. multiple 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. in Section 5. The situation is illustrated in figure 1 below:
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, and as a result, the multi-homed myhost interface, but not the other. The multi-homed myhost will not be able to
will not be able to respond with a host RR for "myhost". respond with a host RR for "myhost" on the interface on the right (see
Figure 1). The multi-homed host may, however, be configured to use the
"myhost" name on the interface on the left.
Since names are only unique per-link, hosts on different links could be Since names are only unique per-link, hosts on different links could be
using the same name. If an mDNS client sends requests over multiple using the same name. If an mDNS 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]
skipping to change at page 12, line 32 skipping to change at page 13, line 7
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. IANA Considerations 6. IANA Considerations
Authors will contact IANA to reserve LINKLOCAL IPv4 and IPv6 addresses. This specification requires allocation of a link scope IPv4 multicast
addresses for use by multicast DNS.
7. ARPA domain considerations 7. ARPA domain considerations
This document specifies the use of a new sub-domain of the "ARPA" This document specifies the use of a new sub-domain of the "ARPA"
domain. According to Section 2.1 of the ARPA Guidelines [12], this domain. According to Section 2.1 of the ARPA Guidelines [12], this
specification requires description and justification. specification requires description and justification.
The 'local.arpa' domain is used to distinguish a local namespace. This The 'local.arpa' domain is used to distinguish a local namespace. This
namespace differs from others in the following respects: namespace differs from others in the following respects:
- Name servers responding to requests for names in this - Name servers responding to requests for names in this
domain have different rules concerning authority. As domain have different rules concerning authority. As
explained in Section 2.1, mDNS servers have limited explained in Section 2.1, mDNS servers have limited
scope of authority, not extending to sub-domains of scope of authority, not extending to sub-domains of
domain they are authoritative for. domain they are authoritative for.
- DNS servers SHOULD NOT forward queries for domain names - DNS servers SHOULD NOT forward or recursively resolve
in the local.arpa domain - if the server cannot answer queries for domain names in the local.arpa domain - if the
the query from its own database, it should reply with a server cannot answer the query from its own database,
non-authoritative NXDOMAIN. it MUST NOT reply.
- Hosts may derive their own names in this namespace, - Hosts may derive their own names in this namespace,
independent of centralized authorization and registration independent of centralized authorization and registration
(as defined in section 3 and section 5). (as defined in section 3 and section 5).
- There is no delegation or administrative structure to - There is no delegation or administrative structure to
sub-domains of '.local.arpa'. sub-domains of '.local.arpa'.
How protocol objects are mapped into lookup keys: How protocol objects are mapped into lookup keys:
skipping to change at page 14, line 36 skipping to change at page 15, line 22
progress), draft-aboba-dhc-domsearch-06.txt, August 2001. progress), draft-aboba-dhc-domsearch-06.txt, August 2001.
[18] Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4 Link-Local [18] Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4 Link-Local
Addresses", Internet draft (work in progress), draft-ietf-zeroconf- Addresses", Internet draft (work in progress), draft-ietf-zeroconf-
ipv4-linklocal-05.txt, September 2001. ipv4-linklocal-05.txt, September 2001.
[19] Thaler, D., Hagino, I., "IPv6 Stateless DNS Discovery", Internet [19] Thaler, D., Hagino, I., "IPv6 Stateless DNS Discovery", Internet
draft (work in progress), draft-ietf-ipngwg-dns-discovery-02.txt, draft (work in progress), draft-ietf-ipngwg-dns-discovery-02.txt,
July 2001. July 2001.
[20] Stevens, W., Thomas, M., "Advanced Sockets API for IPv6", RFC 2292,
February 1998.
9. Security Considerations 9. Security Considerations
This draft does not prescribe a means of securing the multicast DNS This draft does not prescribe a means of securing the multicast DNS
mechanism. It is possible that hosts will allocate conflicting names for mechanism. It is possible that hosts will allocate conflicting names for
a period of time, or that non-conforming hosts will attempt to deny a period of time, or that non-conforming hosts will attempt to deny
service to other hosts by allocating the same name. Such attacks also service to other hosts by allocating the same name. Such attacks also
allow nodes to receive packets destined for other nodes. The protocol allow nodes to receive packets destined for other nodes. The protocol
reduces the exposure to such threats in the absence of authentication by reduces the exposure to such threats in the absence of authentication by
ignoring multicast DNS query response packets received from off-link ignoring multicast DNS query response packets received from off-link
senders. senders.
skipping to change at page 15, line 13 skipping to change at page 15, line 50
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 mDNS against the above threats Link-layer security will serve to secure mDNS 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) [10] is implemented, a casual attacker is likely to be Privacy" (WEP) [10] is implemented, a casual attacker is likely to be
deterred from gaining access to the home network. deterred from gaining access to the home network.
The mechanism specified in this draft does not require use of the The mechanism specified in this draft does not require use of DNSSEC.
DNSSEC, which means that the responses to the multicast DNS queries may As a result, responses to multicast DNS queries MAY NOT be
not be authenticated. If a network contains a "signed key distribution authenticated. If a network contains a "signed key distribution center"
center" for all (or at least some) of the DNS zones that the responders
are authoritative for, then hosts on such a network are configured with for some of the DNS zones that the responders are authoritative for, and
the key for the top zone, "local.arpa." (hosted by "signed keys senders on that network are configured with the key for the top zone
distribution center") and may use DNSSEC for authentication of the "local.arpa." (hosted by "signed keys distribution center"), then
responders using DNSSEC. senders MAY authenticate the responses using DNSSEC.
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, James been received from Mark Andrews, Stuart Cheshire, Robert Elz, James
Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Thomas Narten, Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Thomas Narten,
Erik Nordmark, Sander Van-Valkenburg and Tomohide Nagashima. Erik Nordmark, Sander Van-Valkenburg and Tomohide Nagashima.
skipping to change at page 15, line 46 skipping to change at page 16, line 34
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
EMail: levone@microsoft.com EMail: levone@microsoft.com
Bernard Aboba Bernard Aboba
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
Phone: +1 (425) 936-6605 Phone: +1 425 936 6605
EMail: bernarda@microsoft.com EMail: bernarda@microsoft.com
Dave Thaler Dave Thaler
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
Phone: +1 (425) 703-8835 Phone: +1 425 703 8835
EMail: dthaler@microsoft.com EMail: dthaler@microsoft.com
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to pertain intellectual property or other rights that might be claimed to pertain
to the implementation or use of the technology described in this to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or document or the extent to which any license under such rights might or
might not be available; neither does it represent that it has made any might not be available; neither does it represent that it has made any
effort to identify any such rights. Information on the IETF's effort to identify any such rights. Information on the IETF's
skipping to change at page 17, line 4 skipping to change at page 17, line 37
on all such copies and derivative works. However, this document itself on all such copies and derivative works. However, this document itself
may not be modified in any way, such as by removing the copyright notice may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations, or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into Standards process must be followed, or as required to translate it into
languages other than English. The limited permissions granted above are languages other than English. The limited permissions granted above are
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-05.txt>, and expires This memo is filed as <draft-ietf-dnsext-mdns-06.txt>, and expires
March 22, 2002. April 22, 2002.
 End of changes. 

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