draft-ietf-dnsext-mdns-07.txt   draft-ietf-dnsext-mdns-08.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-07.txt> Microsoft <draft-ietf-dnsext-mdns-08.txt> Microsoft
15 November 2001 21 December 2001
Multicast DNS Linklocal Multicast DNS (LMDNS)
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 36 skipping to change at page 1, line 36
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
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 DNS of ad-hoc networks operating without a DNS server. In order to allow
name resolution in such environments, the use of a multicast DNS is name resolution in such environments, Linklocal Multicast DNS (LMDNS) 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 LMDNS ........................... 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 .............................. 8 3.1 LMDNS configuration ............................. 8
4. Sequence of events .................................... 9 4. Sequence of events .................................... 9
5. Conflict resolution ................................... 9 5. Conflict resolution ................................... 9
5.1 Considerations for multiple interfaces .......... 11 5.1 Considerations for multiple interfaces .......... 11
5.2 API issues ...................................... 12 5.2 API issues ...................................... 12
6. IANA considerations ................................... 13 6. Security considerations ............................... 13
7. ARPA domain considerations ............................ 13 7. IANA considerations ................................... 14
8. References ............................................ 14 8. Normative References .................................. 14
9. Security considerations ............................... 15 9. Informative References ................................ 14
Acknowledgments .............................................. 16 Acknowledgments .............................................. 15
Authors' Addresses ........................................... 16 Authors' Addresses ........................................... 15
Intellectual Property Statement .............................. 16 Intellectual Property Statement .............................. 16
Full Copyright Statement ..................................... 17 Full Copyright Statement ..................................... 16
1. Introduction 1. Introduction
Multicast DNS enables DNS name resolution in the scenarios when Linklocal Multicast DNS (LMDNS) enables name resolution in the scenarios
conventional DNS name resolution is not possible. Namely, when there are when conventional DNS name resolution is not possible. Namely, when
no DNS servers available on the network or available DNS servers do not there are no DNS servers available on the network or available DNS
provide name resolution for the names of the hosts on the local network. servers do not provide name resolution for the names of the hosts on the
The latter case, for example, corresponds to a scenario when a network local network. The latter case, for example, corresponds to a scenario
that doesn't have a DNS server is connected to the Internet through an when a network that doesn't have a DNS server is connected to the
ISP and the network hosts are configured with the ISP's DNS server for Internet through an ISP and the network hosts are configured with the
the name resolution. The ISP's DNS server provides the name resolution ISP's DNS server for the name resolution. The ISP's DNS server provides
for the names registered on the Internet, but doesn't provide name the name resolution for the names registered on the Internet, but
resolution for the names of the hosts on the network. doesn't provide name resolution for the names of the hosts on the
network.
This document discusses multicast DNS, an extension to the DNS protocol This document discusses Linklocal Multicast DNS (LMDNS), which operates
which consists of a single change to the method of use, and no change to on a separate port from DNS, with a distinct resolver cache, but does
the format of DNS packets. 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
mDNS in particular is outside of the scope of this document, as is name LMDNS 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 RFC 2119 [1]. described in [RFC2119].
2. Name resolution using Multicast DNS 2. Name resolution using LMDNS
This extension to the DNS protocol consists of a single change to the While operating on a different port with a distinct resolver cache,
method of use, and no change to the current format of DNS packets, It LMDNS makes no change to the current format of DNS packets.
allows multicast DNS queries to be sent to and received on port 53 using
a LINKLOCAL address as specified in "Administratively Scoped IP Linklocal multicast DNS queries are sent to and received on port 5353
Multicast" [2] for IPv4 and the "solicited name" LINKLOCAL multicast using a LINKLOCAL address as specified in "Administratively Scoped IP
addresses for IPv6. The mDNS LINKLOCAL address to be used for IPv4 is Multicast" [RFC2365] for IPv4 and the "solicited name" LINKLOCAL
<TBD>. LINKLOCAL addresses are used to prevent propagation of multicast multicast addresses for IPv6. The LMDNS LINKLOCAL address to be used
DNS traffic across routers, potentially flooding the network. for IPv4 is 224.0.0.251. LINKLOCAL addresses are used to prevent
propagation of LMDNS traffic across routers, potentially flooding the
network.
Propagation of multicast DNS packets on the local link is considered Propagation of multicast DNS packets on the local link is considered
sufficient to enable DNS name resolution in small adhoc networks. The sufficient to enable name resolution in small adhoc networks. The
assumption is that if a network has a router, then the network either 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 proxy. has a DNS server 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 all IPv6 hosts also support IPv4, and the DNS proxy network. Where the DNS proxy supports AAAA RRs, resolution for the names
supports AAAA RRs, resolution for the names of dual stack IPv6 hosts on of dual stack IPv6 hosts on the local network can also be provided using
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 multicast DNS need not be networks with a router, it is assumed that LMDNS need not be enabled by
enabled by default. default.
In the future, mDNS may be defined to support greater than LINKLOCAL In the future, multicast DNS may be defined to support greater than
multicast scope. This would occur if LINKLOCAL mDNS deployment is LINKLOCAL multicast scope. This would occur if LMDNS deployment is
successful, the assumption that mDNS is not needed on multiple links successful, the assumption that multicast DNS is not needed on multiple
proves incorrect, and multicast routing becomes ubiquitous. For links 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 LMDNS 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 multicast
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 multicast 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 multicast query is called "responder". A host configured
to be a "responder" may also be a "sender". A host configured to not be to be a "responder" MAY also be a "sender". A host configured not to be
a "responder" cannot be a "sender". "responder" SHOULD NOT be a "sender". While hosts configured only as
senders can detect name conflicts, they cannot notify other senders of
potential conflicts for their name. Thus, implementation of both
responder and sender functionality is encouraged.
2.1.1. Behavior of senders 2.1.1. Behavior of senders
A sender sends multicast DNS query for any legal Type of resource record A sender sends an LMDNS query for any legal Type of resource record
(e.g. A, PTR, etc.) for a name within the ".local.arpa." domain to the (e.g. A, PTR, etc.) to the LINKLOCAL address. The RD (Recursion
LINKLOCAL address. The RD (Recursion Desired) bit MUST NOT be set. If a Desired) bit MUST NOT be set. If a responder receives a query with the
responder receives a query with the header containing RD set bit, the header containing RD set bit, the responder MUST ignore the RD bit.
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 RFC 2535 [15], 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 (see RFC 1321 [16]). record name is then hashed using the MD5 algorithm, described in
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 "solicited name
multicast address". (Note: this procedure is intended to be the same as
that specified in section 3 of "IPv6 Node Information Queries" [14]). A
responder that listens for queries for multiple names will necessarily
listen to multiple of these solicited name multicast addresses. [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
"solicited name multicast address". (Note: this procedure is intended
to be the same as that specified in section 3 of "IPv6 Node Information
Queries" [NodeInfo]). A responder that listens for queries for multiple
names will necessarily listen to multiple of these solicited name
multicast addresses.
If the multicast query is not positively resolved ("positively resolved" If the multicast query is not resolved during a limited amount of time
refers in this document to a response with the RCODE set to 0) during a (LMDNS_TIMEOUT), then a sender MAY repeat the transmission of a query in
limited amount of time, then a sender MAY repeat the transmission of a order to assure themselves that the query has been received by a host
query in order to assure themselves that the query has been received by 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 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
A responder listens on port 53 on the LINKLOCAL address. Responders An LMDNS responder listens on port 5353 on the LINKLOCAL address.
MUST respond to multicast queries to those and only those names for Responders MUST respond to multicast queries to those and only those
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." is authoritative for the domain "host.example.com.".
"host.example.com.local.arpa.". On receiving a multicast DNS A record On receiving a linklocal multicast DNS A record query for the name
query for the name "host.example.com.local.arpa." such a host responds "host.example.com." such a host responds with A record(s) that contain
with A record(s) that contain IP address(es) in the RDATA of the record. 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.local.arpa." is not authoritative for example the host "host.example.com." is not authoritative for the name
the name "child.host.example.com.local.arpa." unless the host is "child.host.example.com." unless the host is configured with multiple
configured with multiple names, including "host.example.com.local.arpa." names, including "host.example.com." and "child.host.example.com.". The
and "child.host.example.com.local.arpa.". The purpose of limiting the purpose of limiting the name authority scope of a responder is to
name authority scope of a responder is to prevent complications that prevent complications that could be caused by coexistence of two or more
could be caused by coexistence of two or more hosts with the names hosts with the names representing child and parent (or grandparent)
representing child and parent (or grandparent) nodes in the DNS tree, nodes in the DNS tree, for example, "host.example.com." and
for example, "host.example.com.local.arpa." and "child.host.example.com.".
"child.host.example.com.local.arpa.".
In this example (unless this limitation is introduced) a multicast query In this example (unless this limitation is introduced) a multicast query
for an A record for the name "child.host.example.com.local.arpa." would for an A record for the name "child.host.example.com." would result in
result in two authoritative responses: name error received from two authoritative responses: name error received from
"host.example.com.local.arpa.", and a requested A record - from "host.example.com.", and a requested A record - from
"child.host.example.com.local.arpa.". To prevent this ambiguity, "child.host.example.com.". To prevent this ambiguity, multicast enabled
multicast enabled hosts could perform a dynamic update of the parent (or hosts could perform a dynamic update of the parent (or grandparent) zone
grandparent) zone with a delegation to a child zone. In this example a with a delegation to a child zone. In this example a host
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 "child.host.example.com." would send a dynamic update for the NS and
approach significantly complicates implementation of multicast DNS and glue A record to "host.example.com.", but this approach significantly
would not be acceptable for lightweight hosts. complicates implementation of multicast DNS and 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 RFC 1035 [4]. 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 mDNS query MUST have RCODE set to sender via unicast. A response to an LMDNS query MUST have RCODE set to
zero. mDNS responders may respond only to queries which they can resolve zero. Responses with RCODE set to zero are referred to in this document
positively. as "positively resolved". LMDNS responders may respond only to queries
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. mDNS addressing 2.1.3. LMDNS 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" [18] 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 LINKLOCAL addressing is source/destination address combinations. IPv6 is described in [RFC2460];
described in RFC 2373 [9]. mDNS queries and responses MUST obey the IPv6 LINKLOCAL addressing is described in [RFC2373]. LMDNS queries and
rules laid out in these documents. responses MUST obey the rules laid out in these documents.
In composing an mDNS response, the responder MUST set the Hop Limit In composing an LMDNS 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 LMDNS
multicast DNS response to 255. The sender MUST verify that the Hop Limit response to 255. The sender MUST verify that the Hop Limit field in IPv6
field in IPv6 header and TTL field in IPv4 header of each response to header and TTL field in IPv4 header of each response to the LMDNS query
the multicast DNS query is set to 255. If it is not, then sender MUST 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(). [RFC2292]
[20] specifies similar options for specifying and receiving the IPv6 specifies similar options for specifying and receiving the IPv6 Hop
Hop Limit. Limit.
2.1.4. Use of DNS TTL 2.1.4. Use of DNS TTL
The responder should use a pre-configured TTL value in the records The responder should use a pre-configured TTL value in the records
returned in the multicast DNS query response. Due to the TTL returned in the LMDNS query response. Due to the TTL minimalization
minimalization necessary when caching an RRset, all TTLs in an RRset necessary when caching an RRset, all TTLs in an RRset MUST be set to the
MUST be set to the same value. In the additional and authority section same value. In the additional and authority section of the response the
of the response the responder includes the same records as a DNS server 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
multicast scope, or in the event that no positive non-null responses linklocal multicast scope, or in the event that no positive non-null
exist for the transmitted query. responses exist for the transmitted query. If no positive response is
received, a resolver treats it as a response that no records of the
If no positive response is received, a resolver treats it as a response specified type and class for the specified name exist (NXRRSET).
that no records of the specified 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
multicast query, in the event that several multicast DNS enabled linklocal multicast query, in the event that several linklocal multicast
computers receive the query and respond with valid answers. When this DNS enabled computers receive the query and respond with valid answers.
occurs, the responses MAY first be concatenated, and then treated in the When this occurs, the responses MAY first be concatenated, and then
same manner that multiple RRs received from the same DNS server would, treated in the same manner that multiple RRs received from the same DNS
ordinarily. server would, ordinarily. However, after receiving an initial response,
the sender is not required to wait for LMDNS_TIMEOUT for additional
responses.
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 LMDNS "responder" MUST also be configured as
"sender". A host not configured as a "responder" MUST NOT be a "sender". a "sender". A host not configured as a "responder" MUST NOT be a
"sender". An LMDNS "sender" MAY multicast requests for any name. If
Multicast DNS usage is determined by special treatment of the that name is not qualified and does not end in a trailing dot, for the
".local.arpa." namespace. The sender treats queries for ".local.arpa." purposes of LMDNS, the implicit search order is as follows:
as a special case. A sender MUST NOT send a unicast query for names
ending with the ".local.arpa." suffix except when:
a. A sender repeats a query after it received a response
to the previous multicast query with the TC bit set, or
b. The sender's cache contains an NS resource record that enables [1] Request the name with the current domain appended.
the sender to send a query directly to the hosts
authoritative for the name in the query.
It is not expected that a host named "host.example.com." will be [2] Request just the name.
manually configured to have the additional name
"host.example.com.local.arpa." when it is configured to use multicast
DNS. Instead, a responder with a name "host.example.com." configured
with ".local.arpa." suffix in its domain search configuration is
authoritative for the name "host.example.com.local.arpa.". For example,
when a responder with the name "host.example.com." receives an A type
query for the name "host.example.com.local.arpa." it authoritatively
responds to the query.
The same host MAY use multicast DNS queries for the resolution of names This is the behavior suggested by [RFC1536]. LMDNS uses this technique
ending with ".local.arpa.", and unicast DNS queries for resolution of to resolve unqualified host names.
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
query for such a name MUST be multicast and the name SHOULD NOT be
concatenated with any suffix.
If a DNS server is running on a host, then responder MUST NOT listen for If a DNS server is running on a host that supports LMDNS, the DNS server
the multicast DNS queries on the same IP addresses on which the DNS MUST respond to LMDNS queries only for the RRSets owned by the host on
server listens, since otherwise they would intercept DNS queries which the server is running, but MUST NOT respond for the records for
directed to a DNS server. The DNS server MUST respond to the multicast which the server is authoritative.
DNS 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
authoritative.
3.1. mDNS configuration 3.1. LMDNS configuration
Multicast DNS usage can be configured manually or automatically. On LMDNS usage can be configured manually or automatically. On interfaces
interfaces where no manual or automatic configuration has been performed where no manual or automatic configuration has been performed for a
for a given protocol (IPv4 or IPv6), multicast DNS SHOULD be enabled by given protocol (IPv4 or IPv6), LMDNS SHOULD be enabled by default for
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" [19] can be used to discover whether multicast Stateless DNS Discovery" [DNSDisc] can be used to discover whether
DNS should be enabled or disabled on a per-interface basis. linklocal multicast DNS should be enabled or disabled on a per-interface
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 mDNS Enable Option, configure multicast DNS on an interface. The Multicast DNS Enable
described in [6], can be used to explicitly enable or disable use of Option, described in [mDNSEnable], can be used to explicitly enable or
multicast DNS on an interface for a given protocol, as well as to disable use of linklocal multicast DNS on an interface for a given
specify the order in which DNS and multicast DNS is used on that protocol, as well as to specify the order in which DNS and LMDNS is used
interface. on that interface.
The mDNS Enable Option affects only DNS resolver behavior, that is, how The Multicast DNS Enable Option affects only DNS resolver behavior, that
DNS resolution is performed, and whether multicast DNS is used. The is, how DNS resolution is performed, and whether LMDNS is used. The
mDNS Enable Option does not determine whether or in which order DNS Multicast DNS Enable Option does not determine whether or in which order
itself is used for name resolution. This can be specified, for example, DNS itself is used for name resolution. This can be specified, for
using the Name Service Search Option for DHCP, RFC 2937 [3], which can example, using the Name Service Search Option for DHCP, [RFC2937], which
be used to globally determine where DNS is used within the name service can be used to globally determine where DNS is used within the name
search order. service search order.
If an interface has been configured for a given protocol via any If an interface has been configured for a given protocol via any
automatic configuration mechanism which is able to supply DNS automatic configuration mechanism which is able to supply DNS
configuration information, then multicast DNS SHOULD NOT be used on that configuration information, then LMDNS SHOULD NOT be used on that
interface for that protocol unless it has been explicitly enabled, interface for that protocol unless it has been explicitly enabled,
whether via that mechanism or any other. This ensures that upgraded whether via that mechanism or any other. This ensures that upgraded
hosts do not change their default behavior, without requiring the source hosts do not change their default behavior, without requiring the source
of the configuration information to be simultaneously updated. This of the configuration information to be simultaneously updated. This
implies that on the interface, the host will neither listen on the DNS implies that on the interface, the host will neither listen on the DNS
LINKLOCAL multicast address, nor will it send queries to that address. 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 Note that it is possible for LMDNS to be enabled for use with IPv6 at
same time it is disabled for IPv4, and vice versa. For example, where a the same time it is disabled for IPv4, and vice versa. For example,
home gateway implements a DNS proxy and DHCPv4, but not DHCPv6 or DNS where a home gateway implements a DNS proxy and DHCPv4, but not DHCPv6
autoconfiguration, there may be no mechanism for allowing IPv6 hosts to or DNS autoconfiguration, there may be no mechanism for allowing IPv6
resolve the names of other IPv6 hosts on the home network. In this hosts to resolve the names of other IPv6 hosts on the home network. In
situation, mDNS is useful for resolution of dynamic names, and it will this situation, LMDNS is useful for resolution of dynamic names, and it
be enabled for use with IPv6, even though it is disabled for use with will be enabled for use with IPv6, even though it is disabled for use
IPv4. 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 LMDNS 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", 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". 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 [18], and IPv6, described in RFC 2373 [9]. If these described in [IPV4Link], and IPv6, described in [RFC2373]. If these
conditions are met, then the sender uses and caches the returned conditions are met, then the sender uses and caches the returned
response. If not, then the sender ignores the response and continues response. If not, then the sender ignores the response and continues
waiting for the response. 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:
- 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 multicast DNS query and/or dynamic Every responder that responds to a linklocal multicast DNS query and/or
update request AND includes a UNIQUE record in the response: 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
multicast DNS query propagation that can return a DNS record LMDNS query propagation that can return a DNS 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 multicast DNS queries on more Where a host is configured to respond to linklocal multicast DNS queries
than one interface, the host MUST verify resource record uniqueness on on more than one interface, the host MUST verify resource record
each interface for each UNIQUE resource record that could be used on uniqueness on each interface for each UNIQUE resource record that could
that interface. To accomplish this, the host MUST multicast a dynamic
DNS update request as specified in RFC 2136 [11] for each new UNIQUE be used on that interface. To accomplish this, the host MUST multicast a
dynamic DNS update request as specified in [RFC2136] for each new UNIQUE
resource record. Uniqueness verification is carried out when the host: resource record. Uniqueness verification is carried out when the host:
- starts up or - starts up or
- is configured to respond to the multicast DNS queries on - is configured to respond to the linklocal multicast DNS queries on
some interface or some interface or
- is configured to respond to the multicast DNS queries using - is configured to respond to the linklocal multicast DNS queries using
additional UNIQUE DNS 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 RFC 2136 [11]. 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
SOA. The zone class in the zone section MUST be set to the class of SOA. The zone class in the zone section MUST be set to the class of
the UNIQUE record. the UNIQUE record.
Prerequisite section Prerequisite section
This section MUST contain a record set whose semantics are This section MUST contain a record set whose semantics are
described in RFC 2136 [11], Section 2.4.3 "RRset Does Not Exist", described in [RFC2136], Section 2.4.3 "RRset Does Not Exist",
requesting that RRs with the NAME and TYPE of the UNIQUE record do requesting that RRs with the NAME and TYPE of the UNIQUE record do
not exist. not exist.
Update section Update section
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 [RFC2136].
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 [RFC2136].
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 check request stating that a UNIQUE resource record does not exist, the host
whether the response arrived on another interface. If this is the case, MUST check whether the response arrived on another interface. If this is
then the client can use the UNIQUE resource record in response to the case, then the client can use the UNIQUE resource record in response
multicast queries and dynamic update requests. If not, then it MUST NOT to multicast queries and dynamic update requests. If not, then it MUST
use the UNIQUE resource record in response to multicast queries and NOT use the UNIQUE resource record in response to linklocal multicast
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 multicast DNS query. In this receives more than one response to its linklocal multicast DNS query.
case, the sender sends the first response that it received to all In this case, the sender sends the first response that it received to
responders that responded to this query except the first one, using all 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 linklocal 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.
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 LMDNS on only one of its
its active interfaces. In many situations this will be adequate. active interfaces. In many situations this will be adequate. However,
However, should a host wish to configure multicast DNS on more than one should a host wish to configure LMDNS on more than one of its active
of its active interfaces, there are some additional precautions it MUST interfaces, there are some additional precautions it MUST take.
take. Implementers who are not planning to support multicast DNS on Implementers who are not planning to support linklocal 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. 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
skipping to change at page 12, line 4 skipping to change at page 11, line 42
---------- ---------- ---------- ----------
| | | | | | | |
[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 mDNS client sends requests over multiple using the same name. If an LMDNS 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 mDNS on both interfaces, it will If host myhost is configured to use LMDNS on both interfaces, it will
send mDNS queries on both interfaces. When host myhost sends a query send LMDNS 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. Host myhost will then forward a response from the both interfaces.
first responder to the second responder, who will attempt to verify the
uniqueness of host RR for its name, but will not discover a conflict, Host myhost will then forward a response from the first responder to the
since the conflicting host resides on a different link. Therefore it second responder, who will attempt to verify the uniqueness of host RR
will continue using its name. for its name, but will not discover a conflict, since the conflicting
host resides on a different link. Therefore it will continue using its
name.
Indeed, host myhost cannot distinguish between the situation shown in Indeed, host myhost cannot distinguish between the situation shown in
Figure 2, and that shown in Figure 3 where no conflict exists: Figure 2, and that shown in Figure 3 where no conflict exists:
[A] [A]
| | | |
----- ----- ----- -----
| | | |
[myhost] [myhost]
skipping to change at page 12, line 49 skipping to change at page 12, line 48
This illustrates that the proposed name conflict resolution mechanism This illustrates that the proposed name conflict resolution mechanism
does not support detection or resolution of conflicts between hosts on does not support detection or resolution of conflicts between hosts on
different links. This problem can also occur with unicast DNS when a different links. This problem can also occur with unicast DNS when a
multi-homed host is connected to two different networks with separated multi-homed host is connected to two different networks with separated
name spaces. It is not the intent of this document to address the issue name spaces. It is not the intent of this document to address the issue
of uniqueness of names within DNS. of uniqueness of names within DNS.
5.2. API issues 5.2. API issues
RFC 2553 [13] provides an API which can partially solve the name [RFC2553] provides an API which can partially solve the name ambiguity
ambiguity problem for applications written to use this API, since the problem for applications written to use this API, since the sockaddr_in6
structure exposes the scope within which each scoped address exists, and
this structure can be used for both IPv4 (using v4-mapped IPv6
sockaddr_in6 structure exposes the scope within which each scoped addresses) and IPv6 addresses.
address exists, and this structure can be used for both IPv4 (using
v4-mapped IPv6 addresses) and IPv6 addresses.
Following the example in Figure 2, an application on 'myhost' issues the 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. mDNS requests will be sent from both ai_flags=AI_ALL|AI_V4MAPPED. LMDNS 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. IANA Considerations 6. Security Considerations
This specification requires allocation of a link scope IPv4 multicast
addresses for use by multicast DNS.
7. ARPA domain considerations
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
specification requires description and justification.
The 'local.arpa' domain is used to distinguish a local namespace. This
namespace differs from others in the following respects:
- Name servers responding to requests for names in this
domain have different rules concerning authority. As
explained in Section 2.1, mDNS servers have limited
scope of authority, not extending to sub-domains of
domain they are authoritative for.
- DNS servers SHOULD NOT forward or recursively resolve
queries for domain names in the local.arpa domain - if the
server cannot answer the query from its own database,
it MUST NOT reply.
- Hosts may derive their own names in this namespace,
independent of centralized authorization and registration
(as defined in section 3 and section 5).
- There is no delegation or administrative structure to
sub-domains of '.local.arpa'.
How protocol objects are mapped into lookup keys:
Names are associated with resources which can be requested This draft does not prescribe a means of securing the LMDNS mechanism.
according to the DNS protocol. However, recursive lookup It is possible that hosts will allocate conflicting names for a period
is impossible. Further, mDNS specifies only the use of of time, or that non-conforming hosts will attempt to deny service to
multicast to transmit these requests. other hosts by allocating the same name. Such attacks also allow nodes
to receive packets destined for other nodes. The protocol reduces the
exposure to such threats in the absence of authentication by ignoring
LMDNS query response packets received from off-link senders.
8. References In order to prevent responses to LMDNS queries from polluting the DNS
cache, LMDNS implementations MUST use a distinct, isolated cache for
LMDNS.
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement In all received responses, the Hop Limit field in IPv6 and the TTL field
Levels", BCP 14, RFC 2119, March 1997. in IPv4 are verified to contain 255, the maximum legal value. Since
routers decrement the Hop Limit on all packets they forward, received
packets containing a Hop Limit of 255 must have originated from a
neighbor.
[2] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC These threats are most serious in wireless networks such as 802.11,
2365, July 1998. since attackers on a wired network will require physical access to the
home network, while wireless attackers may reside outside the home.
Link-layer security will serve to secure LMDNS against the above threats
if it is available. For example, where 802.11 "Wired Equivalency
Privacy" (WEP) [IEEE80211] is implemented, a casual attacker is likely
to be deterred from gaining access to the home network.
[3] Smith, C., "The Name Service Search Option for DHCP", RFC 2937, The mechanism specified in this draft does not require use of DNSSEC.
September 2000. As a result, responses to LMDNS queries MAY NOT be authenticated. If
authentication is desired, and a pre-arranged security configuration is
possible, then IPsec ESP with a null-transform MAY be used to
authenticate LMDNS responses. In a small network without a certificate
authority, this can be most easily accomplished through configuration of
[4] Mockapetris, P., "Domain Names - Implementation and Specification", a group pre-shared key for trusted hosts.
RFC 1035, November 1987.
[5] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC 7. IANA Considerations
1034, November, 1987.
[6] Guttman, E., "DHCP mDNS Enable Option", Internet draft (work in This specification does not create any new name spaces for IANA
progress), draft-guttman-mdns-enable-01.txt, July 2001. administration. Since it uses a port (5353) and link scope multicast
IPv4 address (224.0.0.251) previously allocated for use with LMDNS, no
additional IANA allocations are required.
[7] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA 8. Normative References
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[8] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) [RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", RFC 2460, December 1998. Specification", RFC 1035, November 1987.
[9] Hinden, R., Deering, S., "IP Version 6 Addressing Architecture", [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
RFC 2373, July 1998. Requirement Levels", BCP 14, RFC 2119, March 1997.
[10] Information technology - Telecommunications and information [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., Bound, J., "Dynamic
exchange between systems - Local and metropolitan area networks - Updates in the Domain Name System (DNS UPDATE)", RFC
Specific Requirements Part 11: Wireless LAN Medium Access Control 2136, April 1997.
(MAC) and Physical Layer (PHY) Specifications, IEEE Std.
802.11-1997, 1997.
[11] Vixie, P., Thomson, S., Rekhter, Y., Bound, J., "Dynamic Updates in [RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP
the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. 23, RFC 2365, July 1998.
[12] Huston, G., "Management Guidelines & Operational Requirements for [RFC2373] Hinden, R., Deering, S., "IP Version 6 Addressing
the Internet Infrastructure Domain ("ARPA")", Internet draft (work Architecture", RFC 2373, July 1998.
in progress), draft-iab-arpa-03.txt, July 2001.
[13] Gilligan, R., Thomson, S., Bound, J., Stevens, W., "Basic Socket [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
Interface Extensions for IPv6", RFC 2553, March 1999. (IPv6) Specification", RFC 2460, December 1998.
[14] Crawford, Matt, "IPv6 Node Information Queries", Internet draft [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
(work in progress), draft-ietf-ipn-gwg-icmp-name-lookups-07.txt, RFC 2535, March 1999.
August 2000.
[15] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, [RFC2937] Smith, C., "The Name Service Search Option for DHCP", RFC
March 1999. 2937, September 2000.
[16] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April [IPV4Link] Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4
1992. Link-Local Addresses", Internet draft (work in progress),
draft-ietf-zeroconf-ipv4-linklocal-05.txt, November 2001.
[17] Aboba, B., "DHCP Domain Search Option", Internet draft (work in [mDNSEnable] Guttman, E., "DHCP Multicast DNS Enable Option", Internet
progress), draft-aboba-dhc-domsearch-08.txt, November 2001. draft (work in progress), draft-guttman-mdns-
enable-01.txt, July 2001.
[18] Cheshire, S., Aboba, B., "Dynamic Configuration of IPv4 Link-Local 9. Informative References
Addresses", Internet draft (work in progress), draft-ietf-zeroconf-
ipv4-linklocal-05.txt, November 2001.
[19] Thaler, D., Hagino, I., "IPv6 Stateless DNS Discovery", Internet [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
draft (work in progress), draft-ietf-ipngwg-dns-discovery-02.txt, April 1992.
July 2001.
[20] Stevens, W., Thomas, M., "Advanced Sockets API for IPv6", RFC 2292, [RFC1536] Kumar, A., et. al. "DNS Implementation Errors and
February 1998. Suggested Fixes", RFC 1536, October 1993.
9. Security Considerations [RFC2292] Stevens, W., Thomas, M., "Advanced Sockets API for IPv6",
RFC 2292, February 1998.
This draft does not prescribe a means of securing the multicast DNS [RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
mechanism. It is possible that hosts will allocate conflicting names for IANA Considerations Section in RFCs", BCP 26, RFC 2434,
a period of time, or that non-conforming hosts will attempt to deny October 1998.
service to other hosts by allocating the same name. Such attacks also
allow nodes to receive packets destined for other nodes. The protocol
reduces the exposure to such threats in the absence of authentication by
ignoring multicast DNS query response packets received from off-link
senders.
In all received responses, the Hop Limit field in IPv6 and the TTL field [RFC2553] Gilligan, R., Thomson, S., Bound, J., Stevens, W., "Basic
in IPv4 are verified to contain 255, the maximum legal value. Since Socket Interface Extensions for IPv6", RFC 2553, March
routers decrement the Hop Limit on all packets they forward, received 1999.
packets containing a Hop Limit of 255 must have originated from a
neighbor.
These threats are most serious in wireless networks such as 802.11, [IEEE80211] Information technology - Telecommunications and
since attackers on a wired network will require physical access to the information exchange between systems - Local and
home network, while wireless attackers may reside outside the home. metropolitan area networks - Specific Requirements Part
Link-layer security will serve to secure mDNS against the above threats 11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications, IEEE Std.
802.11-1997, 1997.
if it is available. For example, where 802.11 "Wired Equivalency [DNSDisc] Thaler, D., Hagino, I., "IPv6 Stateless DNS Discovery",
Privacy" (WEP) [10] is implemented, a casual attacker is likely to be Internet draft (work in progress), draft-ietf-ipngwg-dns-
deterred from gaining access to the home network. discovery-02.txt, July 2001.
The mechanism specified in this draft does not require use of DNSSEC. [NodeInfo] Crawford, Matt, "IPv6 Node Information Queries", Internet
As a result, responses to multicast DNS queries MAY NOT be draft (work in progress), draft-ietf-ipn-gwg-icmp-name-
authenticated. If a network contains a "signed key distribution center" lookups-07.txt, August 2000.
for some of the DNS zones that the responders are authoritative for, and
senders on that network are configured with the key for the top zone
"local.arpa." (hosted by "signed keys distribution center"), then
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, Rob
Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig, Thomas Narten, Austein, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig,
Erik Nordmark, Sander Van-Valkenburg and Tomohide Nagashima. Thomas Narten, Erik Nordmark, Sander Van-Valkenburg and Tomohide
Nagashima.
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
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: +1 425 706 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
skipping to change at page 18, line 5 skipping to change at page 17, 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-07.txt>, and expires May This memo is filed as <draft-ietf-dnsext-mdns-08.txt>, and expires
22, 2002. June 22, 2002.
 End of changes. 

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