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Document: draft-cheshire-dnsext-multicastdns-12.txt      Stuart Cheshire
Internet-Draft                                             Marc Krochmal
Category: Standards Track                                     Apple Inc.
Expires: 25 April 2011                                   25 October 2010


                             Multicast DNS

               <draft-cheshire-dnsext-multicastdns-12.txt>

Abstract

   As networked devices become smaller, more portable, and more
   ubiquitous, the ability to operate with less configured
   infrastructure is increasingly important. In particular, the ability
   to look up DNS resource record data types (including, but not limited
   to, host names) in the absence of a conventional managed DNS server
   can be useful.

   Multicast DNS (mDNS) provides the ability to perform DNS-like
   operations on the local link in the absence of any conventional
   unicast DNS server. In addition, mDNS designates a portion of the DNS
   namespace to be free for local use, without the need to pay any
   annual fee, and without the need to set up delegations or otherwise
   configure a conventional DNS server to answer for those names.

   The primary benefits of mDNS names are that (i) they require little
   or no administration or configuration to set them up, (ii) they work
   when no infrastructure is present, and (iii) they work during
   infrastructure failures.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF). Note that other groups may also distribute working
   documents as Internet-Drafts. The list of current Internet-Drafts is
   at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25th April, 2011.








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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   Without obtaining an adequate license from the person(s) controlling
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology Used in this Document  . . . . . .  4
   3.  Multicast DNS Names  . . . . . . . . . . . . . . . . . . . . .  5
   4.  Reverse Address Mapping  . . . . . . . . . . . . . . . . . . .  7
   5.  Querying . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Duplicate Suppression  . . . . . . . . . . . . . . . . . . . . 13
   7.  Responding . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Probing and Announcing on Startup  . . . . . . . . . . . . . . 23
   9.  Conflict Resolution  . . . . . . . . . . . . . . . . . . . . . 29
   10. Resource Record TTL Values and Cache Coherency . . . . . . . . 31
   11. Source Address Check . . . . . . . . . . . . . . . . . . . . . 37
   12. Special Characteristics of Multicast DNS Domains . . . . . . . 38
   13. Multicast DNS for Service Discovery  . . . . . . . . . . . . . 40
   14. Enabling and Disabling Multicast DNS . . . . . . . . . . . . . 40
   15. Considerations for Multiple Interfaces . . . . . . . . . . . . 41
   16. Considerations for Multiple Responders on the Same Machine . . 42
   17. Multicast DNS Character Set  . . . . . . . . . . . . . . . . . 43
   18. Multicast DNS Message Size . . . . . . . . . . . . . . . . . . 45
   19. Multicast DNS Message Format . . . . . . . . . . . . . . . . . 46
   20. Summary of Differences Between Multicast DNS and Unicast
       DNS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
   21. IPv6 Considerations  . . . . . . . . . . . . . . . . . . . . . 50
   22. Security Considerations  . . . . . . . . . . . . . . . . . . . 51
   23. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 52
   24. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 54
   25. References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
     25.1.  Normative References  . . . . . . . . . . . . . . . . . . 55
     25.2.  Informative References  . . . . . . . . . . . . . . . . . 55
   Appendix A.  Design Rationale for Choice of UDP Port Number  . . . 57
   Appendix B.  Design Rationale for Not Using Hashed Multicast
                Addresses . . . . . . . . . . . . . . . . . . . . . . 58
   Appendix C.  Design Rationale for Maximum Multicast DNS Name
                Length  . . . . . . . . . . . . . . . . . . . . . . . 59
   Appendix D.  Benefits of Multicast Responses . . . . . . . . . . . 61
   Appendix E.  Design Rationale for Encoding Negative Responses  . . 63
   Appendix F.  Use of UTF-8  . . . . . . . . . . . . . . . . . . . . 63
   Appendix G.  Governing Standards Body  . . . . . . . . . . . . . . 64
   Appendix H.  Private DNS Namespaces  . . . . . . . . . . . . . . . 65
   Appendix I.  Deployment History  . . . . . . . . . . . . . . . . . 65
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 66









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1.  Introduction

   Multicast DNS and its companion technology DNS Service Discovery
   [DNS-SD] were created to provide IP networking with the ease-of-use
   and autoconfiguration for which AppleTalk was well known [NBP]. When
   reading this document, familiarity with the concepts of Zero
   Configuration Networking [Zeroconf] and automatic link-local
   addressing [RFC3927] [RFC4862] is helpful.

   This document specifies no change to the structure of DNS messages,
   no new operation codes or response codes, and no new resource record
   types. This document describes how clients send DNS-like queries via
   IP multicast, and how a collection of hosts cooperate to collectively
   answer those queries in a useful manner.


2.  Conventions and Terminology Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [RFC2119].

   When this document uses the term "Multicast DNS", it should be taken
   to mean: "Clients performing DNS-like queries for DNS-like resource
   records by sending DNS-like UDP query and response packets over IP
   Multicast to UDP port 5353." The design rationale for selecting UDP
   port 5353 is discussed in Appendix A.

   This document uses the term "host name" in the strict sense to mean a
   fully qualified domain name that has an IPv4 or IPv6 address record.
   It does not use the term "host name" in the commonly used but
   incorrect sense to mean just the first DNS label of a host's fully
   qualified domain name.

   A DNS (or mDNS) packet contains an IP TTL in the IP header, which is
   effectively a hop-count limit for the packet, to guard against
   routing loops. Each Resource Record also contains a TTL, which is the
   number of seconds for which the Resource Record may be cached. This
   document uses the term "IP TTL" to refer to the IP header TTL (hop
   limit), and the term "RR TTL" or just "TTL" to refer to the Resource
   Record TTL (cache lifetime).

   DNS-format messages contain a header, a Question Section, then
   Answer, Authority, and Additional Record Sections. The Answer,
   Authority, and Additional Record Sections all hold resource records
   in the same format. Where this document describes issues that apply
   equally to all three sections, it uses the term "Resource Record



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   Sections" to refer collectively to these three sections.

   This document uses the terms "shared" and "unique" when referring to
   resource record sets [RFC1034]:

   A "shared" resource record set is one where several Multicast DNS
   Responders may have records with the same name, rrtype, and rrclass,
   and several Responders may respond to a particular query.

   A "unique" resource record set is one where all the records with that
   name, rrtype, and rrclass are conceptually under the control or
   ownership of a single Responder, and it is expected that at most one
   Responder should respond to a query for that name, rrtype, and
   rrclass. Before claiming ownership of a unique resource record set, a
   Responder MUST probe to verify that no other Responder already claims
   ownership of that set, as described in Section 8.1 "Probing". (For
   fault-tolerance and other reasons it is permitted sometimes to have
   more than one Responder answering for a particular "unique" resource
   record set, but such cooperating Responders MUST give answers
   containing identical rdata for these records. If they do not give
   answers containing identical rdata then the probing step will reject
   the data as being inconsistent with what is already being advertised
   on the network for those names.)

   Strictly speaking the terms "shared" and "unique" apply to resource
   record sets, not to individual resource records, but it is sometimes
   convenient to talk of "shared resource records" and "unique resource
   records". When used this way, the terms should be understood to mean
   a record that is a member of a "shared" or "unique" resource record
   set, respectively.


3.  Multicast DNS Names

   This document specifies that the DNS top-level domain ".local." is a
   special domain with special semantics, namely that any fully-
   qualified name ending in ".local." is link-local, and names within
   this domain are meaningful only on the link where they originate.
   This is analogous to IPv4 addresses in the 169.254/16 prefix, or IPv6
   addresses in the FE80::/10 prefix, which are link-local and
   meaningful only on the link where they originate.

   Any DNS query for a name ending with ".local." MUST be sent to the
   mDNS multicast address 224.0.0.251 (or its IPv6 equivalent FF02::FB).
   The design rationale for using a fixed multicast address instead of
   selecting from a range of multicast addresses using a hash function
   is discussed in Appendix B. Implementers MAY choose also to look up
   such names concurrently via other mechanisms (e.g. Unicast DNS) and



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   coalesce the results in some fashion. Implementers choosing to do
   this should be aware of the potential for user confusion when a given
   name can produce different results depending on external network
   conditions (such as, but not limited to, which name lookup mechanism
   responds faster).

   It is unimportant whether a name ending with ".local." occurred
   because the user explicitly typed in a fully qualified domain name
   ending in ".local.", or because the user entered an unqualified
   domain name and the host software appended the suffix ".local."
   because that suffix appears in the user's search list. The ".local."
   suffix could appear in the search list because the user manually
   configured it, or because it was received via DHCP [RFC2132], or via
   any other mechanism for configuring the DNS search list. In this
   respect the ".local." suffix is treated no differently to any other
   search domain that might appear in the DNS search list.

   DNS queries for names that do not end with ".local." MAY be sent to
   the mDNS multicast address, if no other conventional DNS server is
   available. This can allow hosts on the same link to continue
   communicating using each other's globally unique DNS names during
   network outages which disrupt communication with the greater
   Internet. When resolving global names via local multicast, it is even
   more important to use DNSSEC [RFC4033] or other security mechanisms
   to ensure that the response is trustworthy. Resolving global names
   via local multicast is a contentious issue, and this document does
   not discuss it further, instead concentrating on the issue of
   resolving local names using DNS packets sent to a multicast address.

   A host that belongs to an organization or individual who has control
   over some portion of the DNS namespace can be assigned a globally
   unique name within that portion of the DNS namespace, such as,
   "cheshire.example.com." For those of us who have this luxury, this
   works very well. However, the majority of home computer users do not
   have easy access to any portion of the global DNS namespace within
   which they have the authority to create names. This leaves the
   majority of home computers effectively anonymous for practical
   purposes.

   To remedy this problem, this document allows any computer user to
   elect to give their computers link-local Multicast DNS host names of
   the form: "single-dns-label.local." For example, a laptop computer
   may answer to the name "MyComputer.local." Any computer user is
   granted the authority to name their computer this way, provided that
   the chosen host name is not already in use on that link. Having named
   their computer this way, the user has the authority to continue using
   that name until such time as a name conflict occurs on the link which
   is not resolved in the user's favor. If this happens, the computer



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   (or its human user) SHOULD cease using the name, and may choose to
   attempt to allocate a new unique name for use on that link. These
   conflicts are expected to be relatively rare for people who choose
   reasonably imaginative names, but it is still important to have a
   mechanism in place to handle them when they happen.

   This document recommends a single flat namespace for dot-local host
   names, (i.e. the names of DNS "A" and "AAAA" records, which map names
   to IPv4 and IPv6 addresses), but other DNS record types (such as
   those used by DNS Service Discovery [DNS-SD]) may contain as many
   labels as appropriate for the desired usage, up to a maximum of 255
   bytes, plus a terminating zero byte at the end. Name length issues
   are discussed further in Appendix C.

   Enforcing uniqueness of host names is probably desirable in the
   common case, but this document does not mandate that. It is
   permissible for a collection of coordinated hosts to agree to
   maintain multiple DNS address records with the same name, possibly
   for load balancing or fault-tolerance reasons. This document does not
   take a position on whether that is sensible. It is important that
   both modes of operation are supported. The Multicast DNS protocol
   allows hosts to verify and maintain unique names for resource records
   where that behavior is desired, and it also allows hosts to maintain
   multiple resource records with a single shared name where that
   behavior is desired. This consideration applies to all resource
   records, not just address records (host names). In summary: It is
   required that the protocol have the ability to detect and handle name
   conflicts, but it is not required that this ability be used for every
   record.


4.  Reverse Address Mapping

   Like ".local.", the IPv4 and IPv6 reverse mapping domains are also
   defined to be link-local:

     Any DNS query for a name ending with "254.169.in-addr.arpa." MUST
     be sent to the IPv4 mDNS multicast address 224.0.0.251 or the IPv6
     mDNS multicast address FF02::FB. Since names under this domain
     correspond to IPv4 link-local addresses, it is logical that the
     local link is the best place to find information pertaining to
     those names.

     Likewise, any DNS query for a name within the reverse mapping
     domains for IPv6 Link-Local addresses ("8.e.f.ip6.arpa.",
     "9.e.f.ip6.arpa.", "a.e.f.ip6.arpa.", and "b.e.f.ip6.arpa.") MUST
     be sent to the IPv6 mDNS link-local multicast address FF02::FB or
     the IPv4 mDNS multicast address 224.0.0.251.



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5.  Querying

   There are three kinds of Multicast DNS Queries, one-shot queries of
   the kind made by conventional DNS clients, one-shot queries
   accumulating multiple responses made by multicast-aware DNS clients,
   and continuous ongoing Multicast DNS Queries used by IP network
   browser software.

   Except in the rare case of a Multicast DNS Responder that is
   advertising only shared resources records and no unique records, a
   Multicast DNS Responder MUST also implement a Multicast DNS Querier
   so that it can first verify the uniqueness of those records before it
   begins answering queries for them.

5.1.  One-Shot Multicast DNS Queries

   The most basic kind of Multicast DNS client may simply send standard
   DNS queries blindly to 224.0.0.251:5353, without necessarily even
   being aware of what a multicast address is. This change can typically
   be implemented with just a few lines of code in an existing DNS
   resolver library. Any time the name being queried for falls within
   one of the reserved mDNS domains (see Section 3 and Section 4) rather
   than using the configured unicast DNS server address, the query is
   instead sent to 224.0.0.251:5353 (or its IPv6 equivalent [FF02::FB]:
   5353). Typically the timeout would also be shortened to two or three
   seconds. It's possible to make a minimal mDNS client with only these
   simple changes. These queries are typically done using a high-
   numbered ephemeral UDP source port, but regardless of whether they
   are sent from a dynamic port or from a fixed port, these queries
   SHOULD NOT be sent using UDP source port 5353, since using UDP source
   port 5353 signals the presence of a fully-compliant Multicast DNS
   client, as described below.

   A simple DNS client like this will typically just take the first
   response it receives. It will not listen for additional UDP
   responses, but in many instances this may not be a serious problem.
   If a user types "http://MyPrinter.local." into their web browser, and
   their simple DNS client just takes the first response it receives,
   and the user gets to see the status and configuration web page for
   their printer, then the protocol has met the user's needs in this
   case.

   While a basic DNS client like this may be adequate for simple host
   name lookup, it may not get ideal behavior in other cases. Additional
   refinements that may be adopted by more sophisticated clients are
   described below.





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5.2.  One-Shot Queries, Accumulating Multiple Responses

   A compliant Multicast DNS client, which implements the rules
   specified in this document, MUST send its Multicast DNS Queries from
   UDP source port 5353 (the well-known port assigned to mDNS), and MUST
   listen for Multicast DNS Replies sent to UDP destination port 5353 at
   the mDNS multicast address (224.0.0.251 and/or its IPv6 equivalent
   FF02::FB).

   As described above, there are some cases, such as looking up the
   address associated with a unique host name, where a single response
   is sufficient, and moreover may be all that is expected. However,
   there are other DNS queries where more than one response is possible
   and useful, and for these queries a more advanced Multicast DNS
   client should include the ability to wait for an appropriate period
   of time to collect multiple responses.

   A naive DNS client retransmits its query only so long as it has
   received no response. A more advanced Multicast DNS client is aware
   that having received one response is not necessarily an indication
   that it might not receive others, and has the ability to retransmit
   its query until it is satisfied with the collection of responses it
   has gathered. When retransmitting, the interval between the first two
   queries SHOULD be at least one second, and the intervals between
   successive queries SHOULD increase by at least a factor of two.

   A Multicast DNS client that is retransmitting a query for which it
   has already received some responses MUST implement Known Answer
   Suppression, as described below in Section 6.1. This indicates to
   Responders who have already replied that their responses have been
   received, and they don't need to send them again in response to this
   repeated query.

5.3.  Continuous Multicast DNS Querying

   In One-Shot Queries, with either single or multiple responses, the
   underlying assumption is that the transaction begins when the
   application issues a query, and ends when the desired responses have
   been received. There is another type of operation which is more akin
   to continuous monitoring.

   Imagine some hypothetical software which allows users to discover
   network printers. When the user is actively looking for a network
   printer to use, they open a network browsing window which displays
   the list of discovered printers. It would be convenient for the user
   if they could rely on this list of network printers to stay up to
   date as network printers come and go, rather than displaying out-of-
   date stale information, and requiring the user explicitly to click a



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   "refresh" button any time they want to see accurate information
   (which, from the moment it is displayed, is itself already beginning
   to become out-of-date and stale). If we are to display a
   continuously-updated live list like this, we need to be able to do it
   efficiently, without naive constant polling which would be an
   unreasonable burden on the network. It is not expected that all users
   will be browsing to discover new printers all the time, but when a
   user is browsing to discover service instances for an extended
   period, we want to be able to support that operation efficiently.

   Therefore, when retransmitting mDNS queries to implement this kind of
   continuous monitoring, the interval between the first two queries
   SHOULD be at least one second, the intervals between successive
   queries SHOULD increase by at least a factor of two, and the querier
   MUST implement Known Answer Suppression, as described below in
   Section 6.1. When the interval between queries reaches or exceeds 60
   minutes, a querier MAY cap the interval to a maximum of 60 minutes,
   and perform subsequent queries at a steady-state rate of one query
   per hour. To avoid accidental synchronization when for some reason
   multiple clients begin querying at exactly the same moment (e.g.
   because of some common external trigger event), a Multicast DNS
   Querier SHOULD also delay the first query of the series by a
   randomly-chosen amount in the range 20-120ms.

   When a Multicast DNS Querier receives an answer, the answer contains
   a TTL value that indicates for how many seconds this answer is valid.
   After this interval has passed, the answer will no longer be valid
   and SHOULD be deleted from the cache. Before this time is reached, a
   Multicast DNS Querier which has clients with an active interest in
   the state of that record (e.g. a network browsing window displaying a
   list of discovered services to the user) SHOULD re-issue its query to
   determine whether the record is still valid.

   To perform this cache maintenance, a Multicast DNS Querier should
   plan to re-query for records after at least 50% of the record
   lifetime has elapsed. This document recommends the following specific
   strategy:

   The Querier should plan to issue a query at 80% of the record
   lifetime, and then if no answer is received, at 85%, 90% and 95%. If
   an answer is received, then the remaining TTL is reset to the value
   given in the answer, and this process repeats for as long as the
   Multicast DNS Querier has an ongoing interest in the record. If after
   four queries no answer is received, the record is deleted when it
   reaches 100% of its lifetime. A Multicast DNS Querier MUST NOT
   perform this cache maintenance for records for which it has no
   clients with an active interest. If the expiry of a particular record
   from the cache would result in no net effect to any client software



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   running on the Querier device, and no visible effect to the human
   user, then there is no reason for the Multicast DNS Querier to waste
   network bandwidth checking whether the record remains valid.

   To avoid the case where multiple Multicast DNS Queriers on a network
   all issue their queries simultaneously, a random variation of 2% of
   the record TTL should be added, so that queries are scheduled to be
   performed at 80-82%, 85-87%, 90-92% and then 95-97% of the TTL.

   An additional efficiency optimization SHOULD be performed when a
   Multicast DNS response is received containing a unique answer (as
   indicated by the cache flush bit being set, described in Section
   10.3, "Announcements to Flush Outdated Cache Entries"). In this case,
   there is no need for the querier to continue issuing a stream of
   queries with exponentially-increasing intervals, since the receipt of
   a unique answer is a good indication that no other answers will be
   forthcoming. In this case, the Multicast DNS Querier SHOULD plan to
   issue its next query for this record at 80-82% of the record's TTL,
   as described above.

5.4.  Multiple Questions per Query

   Multicast DNS allows a querier to place multiple questions in the
   Question Section of a single Multicast DNS query packet.

   The semantics of a Multicast DNS query packet containing multiple
   questions is identical to a series of individual DNS query packets
   containing one question each. Combining multiple questions into a
   single packet is purely an efficiency optimization, and has no other
   semantic significance.

5.5.  Questions Requesting Unicast Responses

   Sending Multicast DNS responses via multicast has the benefit that
   all the other hosts on the network get to see those responses, and
   can keep their caches up to date, and can detect conflicting
   responses.

   However, there are situations where all the other hosts on the
   network don't need to see every response. Some examples are a laptop
   computer waking from sleep, or the Ethernet cable being connected to
   a running machine, or a previously inactive interface being activated
   through a configuration change. At the instant of wake-up or link
   activation, the machine is a brand new participant on a new network.
   Its Multicast DNS cache for that interface is empty, and it has no
   knowledge of its peers on that link. It may have a significant number
   of questions that it wants answered right away, to discover
   information about its new surroundings and present that information



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   to the user. As a new participant on the network, it has no idea
   whether the exact same questions may have been asked and answered
   just seconds ago. In this case, triggering a large sudden flood of
   multicast responses may impose an unreasonable burden on the network.

   To avoid large floods of potentially unnecessary responses in these
   cases, Multicast DNS defines the top bit in the class field of a DNS
   question as the "unicast response" bit. When this bit is set in a
   question, it indicates that the Querier is willing to accept unicast
   responses instead of the usual multicast responses. These questions
   requesting unicast responses are referred to as "QU" questions, to
   distinguish them from the more usual questions requesting multicast
   responses ("QM" questions). A Multicast DNS Querier sending its
   initial batch of questions immediately on wake from sleep or
   interface activation SHOULD set the "QU" bit in those questions.

   When a question is retransmitted (as described in Section 5.2 and
   Section 5.3) the "QU" bit SHOULD NOT be set in subsequent
   retransmissions of that question. Subsequent retransmissions SHOULD
   be usual "QM" questions. After the first question has received its
   responses, the querier should have a large known-answer list (Section
   6.1) so that subsequent queries should elicit few, if any, further
   responses. Reverting to multicast responses as soon as possible is
   important because of the benefits that multicast responses provide
   (see Appendix D). In addition, the "QU" bit SHOULD be set only for
   questions that are active and ready to be sent the moment of wake
   from sleep or interface activation. New questions issued by clients
   afterwards should be treated as normal "QM" questions and SHOULD NOT
   have the "QU" bit set on the first question of the series.

   When receiving a question with the "unicast response" bit set, a
   Responder SHOULD usually respond with a unicast packet directed back
   to the querier. However, if the Responder has not multicast that
   record recently (within one quarter of its TTL), then the Responder
   SHOULD instead multicast the response so as to keep all the peer
   caches up to date, and to permit passive conflict detection. In the
   case of answering a probe question (Section 8.1) with the "unicast
   response" bit set, the Responder should always generate the requested
   unicast response, but may also send a multicast announcement too if
   the time since the last multicast announcement of that record is more
   than a quarter of its TTL.

   Except when defending a unique name against a probe from another
   host, unicast replies are subject to all the same packet generation
   rules as multicast replies, including the cache flush bit (Section
   10.3) and randomized delays to reduce network collisions (Section 7).





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5.6.  Direct Unicast Queries to port 5353

   In specialized applications there may be rare situations where it
   makes sense for a Multicast DNS Querier to send its query via unicast
   to a specific machine. When a Multicast DNS Responder receives a
   query via direct unicast, it SHOULD respond as it would for a "QU"
   query, as described above in Section 5.5. Since it is possible for a
   unicast query to be received from a machine outside the local link,
   Responders SHOULD check that the source address in the query packet
   matches the local subnet for that link (or, in the case of IPv6, the
   source address has an on-link prefix) and silently ignore the packet
   if not.

   There may be specialized situations, outside the scope of this
   document, where it is intended and desirable to create a Responder
   that does answer queries originating outside the local link. Such a
   Responder would need to ensure that these non-local queries are
   always answered via unicast back to the Querier, since an answer sent
   via link-local multicast would not reach a Querier outside the local
   link.


6.  Duplicate Suppression

   A variety of techniques are used to reduce the amount of redundant
   traffic on the network.

6.1.  Known Answer Suppression

   When a Multicast DNS Querier sends a query to which it already knows
   some answers, it populates the Answer Section of the DNS query
   message with those answers.

   Generally this applies only to Shared records, not Unique records,
   since if a Multicast DNS Querier already has at least one Unique
   record in its cache then it should not be expecting further different
   answers to this question, since the Unique record(s) it already has
   comprise the complete answer, so it has no reason to be sending the
   query at all. In contrast, having some Shared records in its cache
   does not necessarily imply that a Multicast DNS Querier will not
   receive further answers to this query, and it is in this case that it
   is beneficial to use the Known Answer list to suppress repeated
   sending of redundant answers that the Querier already knows.

   A Multicast DNS Responder MUST NOT answer a Multicast DNS Query if
   the answer it would give is already included in the Answer Section
   with an RR TTL at least half the correct value. If the RR TTL of the
   answer as given in the Answer Section is less than half of the true



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   RR TTL as known by the Multicast DNS Responder, the Responder MUST
   send an answer so as to update the Querier's cache before the record
   becomes in danger of expiration.

   Because a Multicast DNS Responder will respond if the remaining TTL
   given in the known answer list is less than half the true TTL, it is
   superfluous for the Querier to include such records in the known
   answer list. Therefore a Multicast DNS Querier SHOULD NOT include
   records in the known answer list whose remaining TTL is less than
   half their original TTL. Doing so would simply consume space in the
   packet without achieving the goal of suppressing responses, and would
   therefore be a pointless waste of network bandwidth.

   A Multicast DNS Querier MUST NOT cache resource records observed in
   the Known Answer Section of other Multicast DNS Queries. The Answer
   Section of Multicast DNS Queries is not authoritative. By placing
   information in the Answer Section of a Multicast DNS Query the
   querier is stating that it *believes* the information to be true. It
   is not asserting that the information *is* true. Some of those
   records may have come from other hosts that are no longer on the
   network. Propagating that stale information to other Multicast DNS
   Queriers on the network would not be helpful.

6.2.  Multi-Packet Known Answer Suppression

   Sometimes a Multicast DNS Querier will already have too many answers
   to fit in the Known Answer Section of its query packets. In this
   case, it should issue a Multicast DNS Query containing a question and
   as many Known Answer records as will fit. It MUST then set the TC
   (Truncated) bit in the header before sending the Query. It MUST then
   immediately follow the packet with another query packet containing no
   questions, and as many more Known Answer records as will fit. If
   there are still too many records remaining to fit in the packet, it
   again sets the TC bit and continues until all the Known Answer
   records have been sent.

   A Multicast DNS Responder seeing a Multicast DNS Query with the TC
   bit set defers its response for a time period randomly selected in
   the interval 400-500ms. This gives the Multicast DNS Querier time to
   send additional Known Answer packets before the Responder responds.
   If the Responder sees any of its answers listed in the Known Answer
   lists of subsequent packets from the querying host, it SHOULD delete
   that answer from the list of answers it is planning to give (provided
   that no other host on the network has also issued a query for that
   record and is waiting to receive an answer).

   If the Responder receives additional Known Answer packets with the TC
   bit set, it SHOULD extend the delay as necessary to ensure a pause of



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   400-500ms after the last such packet before it sends its answer. This
   opens the potential risk that a continuous stream of Known Answer
   packets could, theoretically, prevent a Responder from answering
   indefinitely. In practice answers are never actually delayed
   significantly, and should a situation arise where significant delays
   did happen, that would be a scenario where the network is so
   overloaded that it would be desirable to err on the side of caution.
   The consequence of delaying an answer may be that it takes a user
   longer than usual to discover all the services on the local network;
   in contrast the consequence of incorrectly answering before all the
   Known Answer packets have been received would be wasting bandwidth
   sending unnecessary answers on an already overloaded network. In this
   (rare) situation, sacrificing speed to preserve reliable network
   operation is the right trade-off.

6.3.  Duplicate Question Suppression

   If a host is planning to send a query, and it sees another host on
   the network send a QM query containing the same question, and the
   Known Answer Section of that query does not contain any records which
   this host would not also put in its own Known Answer Section, then
   this host should treat its own query as having been sent. When
   multiple clients on the network are querying for the same resource
   records, there is no need for them to all be repeatedly asking the
   same question.

6.4.  Duplicate Answer Suppression

   If a host is planning to send an answer, and it sees another host on
   the network send a response packet containing the same answer record,
   and the TTL in that record is not less than the TTL this host would
   have given, then this host SHOULD treat its own answer as having been
   sent, and not also send an identical answer itself. When multiple
   Responders on the network have the same data, there is no need for
   all of them to respond.

   This feature is particularly useful when Multicast DNS Proxy Servers
   are in use, where there could be more than one proxy on the network
   giving Multicast DNS answers on behalf of some other host (e.g.
   because that other host is currently asleep and is not itself
   responding to queries).


7.  Responding

   When a Multicast DNS Responder constructs and sends a Multicast DNS
   response packet, the Resource Record Sections of that packet must
   contain only records for which that Responder is explicitly



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   authoritative. These answers may be generated because the record
   answers a question received in a Multicast DNS query packet, or at
   certain other times that the Responder determines than an unsolicited
   announcement is warranted. A Multicast DNS Responder MUST NOT place
   records from its cache, which have been learned from other Responders
   on the network, in the Resource Record Sections of outgoing response
   packets. Only an authoritative source for a given record is allowed
   to issue responses containing that record.

   The determination of whether a given record answers a given question
   is done using the standard DNS rules: The record name must match the
   question name, the record rrtype must match the question qtype unless
   the qtype is "ANY" (255) or the rrtype is "CNAME" (5), and the record
   rrclass must match the question qclass unless the qclass is "ANY"
   (255).

   A Multicast DNS Responder MUST only respond when it has a positive
   non-null response to send, or it authoritatively knows that a
   particular record does not exist. For unique records, where the host
   has already established sole ownership of the name, it MUST return
   negative answers to queries for records that it knows not to exist.
   For example, a host with no IPv6 address, that has claimed sole
   ownership of the name "host.local." for all rrtypes, MUST respond to
   AAAA queries for "host.local." by sending a negative answer
   indicating that no AAAA records exist for that name. See Section 7.1
   "Negative Responses". For shared records, which are owned by no
   single host, the nonexistence of a given record is ascertained by the
   failure of any machine to respond to the Multicast DNS query, not by
   any explicit negative response. NXDOMAIN and other error responses
   MUST NOT be sent.

   Multicast DNS Responses MUST NOT contain any questions in the
   Question Section. Any questions in the Question Section of a received
   Multicast DNS Response MUST be silently ignored. Multicast DNS
   Queriers receiving Multicast DNS Responses do not care what question
   elicited the response; they care only that the information in the
   response is true and accurate.

   A Multicast DNS Responder on Ethernet [IEEE.802.3] and similar shared
   multiple access networks SHOULD have the capability of delaying its
   responses by up to 500ms, as determined by the rules described below.

   If a large number of Multicast DNS Responders were all to respond
   immediately to a particular query, a collision would be virtually
   guaranteed. By imposing a small random delay, the number of
   collisions is dramatically reduced. On a full-sized Ethernet using
   the maximum cable lengths allowed and the maximum number of repeaters
   allowed, an Ethernet frame is vulnerable to collisions during the



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   transmission of its first 256 bits. On 10Mb/s Ethernet, this equates
   to a vulnerable time window of 25.6us. On higher-speed variants of
   Ethernet, the vulnerable time window is shorter.

   In the case where a Multicast DNS Responder has good reason to
   believe that it will be the only Responder on the link that will send
   a response (i.e. because it is able to answer every question in the
   query packet, and for all of those answer records it has previously
   verified that the name, rrtype and rrclass are unique on the link) it
   SHOULD NOT impose any random delay before responding, and SHOULD
   normally generate its response within at most 10ms. In particular,
   this applies to responding to probe queries with the "unicast
   response" bit set. Since receiving a probe query gives a clear
   indication that some other Responder is planning to start using this
   name in the very near future, answering such probe queries to defend
   a unique record is a high priority and needs to be done without
   delay. A probe query can be distinguished from a normal query by the
   fact that a probe query contains a proposed record in the Authority
   Section which answers the question in the Question Section (for more
   details, see Section 8.2, "Simultaneous Probe Tie-Breaking").

   Responding without delay is appropriate for records like the address
   record for a particular host name, when the host name has been
   previously verified unique. Responding without delay is *not*
   appropriate for things like looking up PTR records used for DNS
   Service Discovery [DNS-SD], where a large number of responses may be
   anticipated.

   In any case where there may be multiple responses, such as queries
   where the answer is a member of a shared resource record set, each
   Responder SHOULD delay its response by a random amount of time
   selected with uniform random distribution in the range 20-120ms. The
   reason for requiring that the delay be at least 20ms is to
   accommodate the situation where two or more query packets are sent
   back-to-back, because in that case we want a Responder with answers
   to more than one of those queries to have the opportunity to
   aggregate all of its answers into a single response packet.

   In the case where the query has the TC (truncated) bit set,
   indicating that subsequent known answer packets will follow,
   Responders SHOULD delay their responses by a random amount of time
   selected with uniform random distribution in the range 400-500ms, to
   allow enough time for all the known answer packets to arrive, as
   described in Section 6.2 "Multi-Packet Known Answer Suppression".

   The source UDP port in all Multicast DNS Responses MUST be 5353 (the
   well-known port assigned to mDNS). Multicast DNS implementations MUST
   silently ignore any Multicast DNS Responses they receive where the



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   source UDP port is not 5353.

   The destination UDP port in all Multicast DNS Responses MUST be 5353
   and the destination address must be the multicast address 224.0.0.251
   or its IPv6 equivalent FF02::FB, except when a unicast response has
   been explicitly requested:

    * via the "unicast response" bit,
    * by virtue of being a Legacy Query (Section 7.6), or
    * by virtue of being a direct unicast query.

   The benefits of sending Responses via multicast are discussed in
   Appendix D.

   To protect the network against excessive packet flooding due to
   software bugs or malicious attack, a Multicast DNS Responder MUST NOT
   (except in the one special case of answering probe queries) multicast
   a record on a given interface until at least one second has elapsed
   since the last time that record was multicast on that particular
   interface. A legitimate client on the network should have seen the
   previous transmission and cached it. A client that did not receive
   and cache the previous transmission will retry its request and
   receive a subsequent response. In the special case of answering probe
   queries, because of the limited time before the probing host will
   make its decision about whether or not to use the name, a Multicast
   DNS Responder MUST respond quickly. In this special case only, when
   responding via multicast to a probe, a Multicast DNS Responder is
   only required to delay its transmission as necessary to ensure an
   interval of at least 250ms since the last time the record was
   multicast on that interface.

7.1.  Negative Responses

   In the early design of Multicast DNS it was assumed that explicit
   negative responses would never be needed. Hosts can assert the
   existence of the set of records which that host claims to exist, and
   the union of all such sets on a link is the set of Multicast DNS
   records that exist on that link. Asserting the non-existence of every
   record in the complement of that set -- i.e. all possible Multicast
   DNS records that could exist on this link but do not at this moment
   -- was felt to be impractical and unnecessary. The non-existence of a
   record would be ascertained by a client querying for it and failing
   to receive a response from any of the hosts currently attached to the
   link.

   However, operational experience showed that explicit negative
   responses can sometimes be valuable. One such example is when a
   client is querying for a AAAA record, and the host name in question



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   has no associated IPv6 addresses. In this case the responding host
   knows it currently has exclusive ownership of that name, and it knows
   that it currently does not have any IPv6 addresses, so an explicit
   negative response is preferable to the client having to retransmit
   its query multiple times and eventually give up with a timeout before
   it can conclude that a given AAAA record does not exist.

   Any time a Responder receives a query for a name for which it has
   verified exclusive ownership, for a type for which that name has no
   records, the Responder MUST (except as allowed in (a) below) respond
   asserting the nonexistence of that record using a DNS NSEC record
   [RFC4034]. In the case of Multicast DNS the NSEC record is not being
   used for its usual DNSSEC [RFC4033] security properties, but simply
   as a way of expressing which records do or do not exist with a given
   name.

   Implementers working with devices with sufficient memory and CPU
   resources MAY choose to implement code to handle the full generality
   of the DNS NSEC record [RFC4034], including bitmaps up to 65,536 bits
   long. To facilitate use by clients with limited memory and CPU
   resources, Multicast DNS clients are only REQUIRED to be able to
   parse a restricted form of the DNS NSEC record. All compliant
   Multicast DNS implementations MUST at least correctly generate and
   parse the restricted DNS NSEC record format described below:

    o The 'Next Domain Name' field contains the record's own name. When
      used with name compression, this means that the 'Next Domain Name'
      field always takes exactly two bytes in the packet.

    o The Type Bit Map block number is 0.

    o The Type Bit Map block length byte is a value in the range 1-32.

    o The Type Bit Map data is 1-32 bytes, as indicated by length byte.

   Because this restricted form of the DNS NSEC record is limited to
   Type Bit Map block number zero, it cannot express the existence of
   rrtypes above 255. Because of this, if a Multicast DNS Responder were
   to have records with rrtypes above 255, it MUST NOT generate these
   restricted-form NSEC records for those names, since to do so would
   imply that the name has no records with rrtypes above 255, which
   would be false. In such cases a Multicast DNS Responder MUST either
   (a) emit no NSEC record for that name, or (b) emit a full NSEC record
   containing the appropriate Type Bit Map block(s) with the correct
   bits set for all the record types that exist. In practice this is not
   a significant limitation, since rrtypes above 255 are not currently
   in widespread use.




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   If a Multicast DNS implementation receives an NSEC record where the
   'Next Domain Name' field is not the record's own name, then the
   implementation SHOULD ignore the 'Next Domain Name' field and process
   the remainder of the NSEC record as usual. In Multicast DNS the 'Next
   Domain Name' field is not currently used, but it could be used in a
   future version of this protocol, which is why a Multicast DNS
   implementation MUST NOT reject or ignore an NSEC record it receives
   just because it finds an unexpected value in the 'Next Domain Name'
   field.

   If a Multicast DNS implementation receives an NSEC record containing
   more than one Type Bit Map, or where the Type Bit Map block number is
   not zero, or where the block length is not in the range 1-32, then
   the Multicast DNS implementation MAY silently ignore the entire NSEC
   record. A Multicast DNS implementation MUST NOT ignore an entire
   packet just because that packet contains one or more NSEC record(s)
   that the Multicast DNS implementation cannot parse. This provision is
   to allow future enhancements to the protocol to be introduced in a
   backwards-compatible way that does not break compatibility with older
   Multicast DNS implementations.

   To help differentiate these synthesized NSEC records (generated
   programmatically on-the-fly) from conventional Unicast DNS NSEC
   records (which actually exist in a signed DNS zone) the synthesized
   Multicast DNS NSEC records MUST NOT have the 'NSEC' bit set in the
   Type Bit Map, whereas conventional Unicast DNS NSEC records do have
   the 'NSEC' bit set.

   The TTL of the NSEC record indicates the intended lifetime of the
   negative cache entry. In general, the TTL given for an NSEC record
   SHOULD be the same as the TTL that the record would have had, had it
   existed. For example, the TTL for address records in Multicast DNS is
   typically 120 seconds, so the negative cache lifetime for an address
   record that does not exist should also be 120 seconds.

   A Responder should only generate negative responses to queries for
   which it has legitimate ownership of the name/rrtype/rrclass in
   question, and can legitimately assert that no record with that name/
   rrtype/rrclass exists. A Responder can assert that a specified rrtype
   does not exist for one of its names if it knows a priori that it has
   exclusive ownership of that name (e.g. for reverse address mapping
   PTR records) or if it previously claimed unique ownership of that
   name using probe queries for rrtype "ANY". (If it were to use probe
   queries for a specific rrtype, then it would only own the name for
   that rrtype, and could not assert that other rrtypes do not exist.)

   On receipt of a question for a particular name/rrtype/rrclass which a
   Responder knows not to exist (due to a priori knowledge, or as a



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   result of successful probing), the Responder MUST send a response
   packet containing the appropriate NSEC record, if it can do so using
   the restricted form of the NSEC record described above. If a
   legitimate restricted-form NSEC record cannot be created (because
   rrtypes above 255 exist for that name) the Responder MAY emit a full
   NSEC record, or it MAY emit no NSEC record, at the implementer's
   discretion.

   On receipt of a question for a particular name/rrtype/rrclass for
   which a Responder does have one or more unique answers, the Responder
   MAY also include an NSEC record in the additional section indicating
   the non-existence of other rrtypes for that name.

   The design rationale for this mechanism for encoding Negative
   Responses is discussed further in Appendix E.

7.2.  Responding to Address Queries

   In Multicast DNS, whenever a Responder places an IPv4 or IPv6 address
   record (rrtype "A" or "AAAA") into a response packet, it SHOULD also
   place the corresponding other address type into the additional
   section, if there is space in the packet.

   This is to provide fate sharing, so that all a device's addresses are
   delivered atomically in a single packet, to reduce the risk that
   packet loss could cause a querier to receive only the IPv4 addresses
   and not the IPv6 addresses, or vice versa.

   In the event that a device has only IPv4 addresses but no IPv6
   addresses, or vice versa, then the appropriate NSEC record SHOULD be
   placed into the additional section, so that queriers can know with
   certainty that the device has no addresses of that kind.

   Some Multicast DNS Responders treat a physical interface with both
   IPv4 and IPv6 address as a single interface with two addresses. Other
   Multicast DNS Responders treat this case as logically two interfaces,
   each with one address, but Responders that operate this way MUST NOT
   put the corresponding automatic NSEC records in replies they send
   (i.e. a negative IPv4 assertion in their IPv6 responses, and a
   negative IPv6 assertion in their IPv4 responses) because this would
   cause incorrect operation in Responders on the network that work the
   former way.

7.3.  Responding to Multi-Question Queries

   Multicast DNS Responders MUST correctly handle DNS query packets
   containing more than one question, by answering any or all of the
   questions to which they have answers. Any (non-defensive) answers



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   generated in response to query packets containing more than one
   question SHOULD be randomly delayed in the range 20-120ms, or 400-
   500ms if the TC (truncated) bit is set, as described above. (Answers
   defending a name, in response to a probe for that name, are not
   subject to this delay rule and are still sent immediately.)

7.4.  Response Aggregation

   When possible, a Responder SHOULD, for the sake of network
   efficiency, aggregate as many responses as possible into a single
   Multicast DNS response packet. For example, when a Responder has
   several responses it plans to send, each delayed by a different
   interval, then earlier responses SHOULD be delayed by up to an
   additional 500ms if that will permit them to be aggregated with other
   responses scheduled to go out a little later.

7.5.  Wildcard Queries (qtype "ANY" and qclass "ANY")

   When responding to queries using qtype "ANY" (255) and/or qclass
   "ANY" (255), a Multicast DNS Responder MUST respond with *ALL* of its
   records that match the query. This is subtly different to how qtype
   "ANY" and qclass "ANY" work in Unicast DNS.

   A common misconception is that a Unicast DNS query for qtype "ANY"
   will elicit a response containing all matching records. This is
   incorrect. If there are any records that match the query, the
   response is required only to contain at least one of them, not
   necessarily all of them.

   This somewhat surprising behavior is commonly seen with caching (i.e.
   "recursive") name servers. If a caching server receives a qtype "ANY"
   query for which it has at least one valid answer, it is allowed to
   return only those matching answers it happens to have already in its
   cache, and is not required to reconsult the authoritative name server
   to check if there are any more records that also match the qtype
   "ANY" query.

   For example, one might imagine that a query for qtype "ANY" for name
   "host.example.com" would return both the IPv4 (A) and the IPv6 (AAAA)
   address records for that host. In reality what happens is that it
   depends on the history of what queries have been previously received
   by intervening caching servers. If a caching server has no records
   for "host.example.com" then it will consult another server (usually
   the authoritative name server for the name in question) and in that
   case it will typically return all IPv4 and IPv6 address records. If
   however some other host has recently done a query for qtype "A" for
   name "host.example.com", so that the caching server already has IPv4
   address records for "host.example.com" in its cache, but no IPv6



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   address records, then it will return only the IPv4 address records it
   already has cached, and no IPv6 address records.

   Multicast DNS does not share this property that qtype "ANY" and
   qclass "ANY" queries return some undefined subset of the matching
   records. When responding to queries using qtype "ANY" (255) and/or
   qclass "ANY" (255), a Multicast DNS Responder MUST respond with *ALL*
   of its records that match the query.

7.6.  Legacy Unicast Responses

   If the source UDP port in a received Multicast DNS Query is not port
   5353, this indicates that the client originating the query is a
   simple client that does not fully implement all of Multicast DNS. In
   this case, the Multicast DNS Responder MUST send a UDP response
   directly back to the client, via unicast, to the query packet's
   source IP address and port. This unicast response MUST be a
   conventional unicast response as would be generated by a conventional
   unicast DNS server; for example, it MUST repeat the query ID and the
   question given in the query packet. In addition, the "cache flush"
   bit described in Section 10.3 "Announcements to Flush Outdated Cache
   Entries" is specific to Multicast DNS, and MUST NOT be set in legacy
   unicast responses.

   The resource record TTL given in a legacy unicast response SHOULD NOT
   be greater than ten seconds, even if the true TTL of the Multicast
   DNS resource record is higher. This is because Multicast DNS
   Responders that fully participate in the protocol use the cache
   coherency mechanisms described in Section 10 "Resource Record TTL
   Values and Cache Coherency" to update and invalidate stale data. Were
   unicast responses sent to legacy clients to use the same high TTLs,
   these legacy clients, which do not implement these cache coherency
   mechanisms, could retain stale cached resource record data long after
   it is no longer valid.

   Having sent this unicast response, if the Responder has not sent this
   record in any multicast response recently, it SHOULD schedule the
   record to be sent via multicast as well, to facilitate passive
   conflict detection. "Recently" in this context means "if the time
   since the record was last sent via multicast is less than one quarter
   of the record's TTL".


8.  Probing and Announcing on Startup

   Typically a Multicast DNS Responder should have, at the very least,
   address records for all of its active interfaces. Creating and
   advertising an HINFO record on each interface as well can be useful



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   to network administrators.

   Whenever a Multicast DNS Responder starts up, wakes up from sleep,
   receives an indication of an Ethernet "Link Change" event, or has any
   other reason to believe that its network connectivity may have
   changed in some relevant way, it MUST perform the two startup steps
   below: Probing (Section 8.1) and Announcing (Section 8.3).

8.1.  Probing

   The first startup step is that for all those resource records that a
   Multicast DNS Responder desires to be unique on the local link, it
   MUST send a Multicast DNS Query asking for those resource records, to
   see if any of them are already in use. The primary example of this is
   a host's address records which map its unique host name to its unique
   IPv4 and/or IPv6 addresses. All Probe Queries SHOULD be done using
   the desired resource record name and query type "ANY" (255), to
   elicit answers for all types of records with that name. This allows a
   single question to be used in place of several questions, which is
   more efficient on the network. It also allows a host to verify
   exclusive ownership of a name for all rrtypes, which is desirable in
   most cases. It would be confusing, for example, if one host owned the
   "A" record for "myhost.local.", but a different host owned the "AAAA"
   record for that name.

   The ability to place more than one question in a Multicast DNS Query
   is useful here, because it can allow a host to use a single packet to
   probe for all of its resource records instead of needing a separate
   packet for each. For example, a host can simultaneously probe for
   uniqueness of its "A" record and all its SRV records [DNS-SD] in the
   same query packet.

   When ready to send its mDNS probe packet(s) the host should first
   wait for a short random delay time, uniformly distributed in the
   range 0-250ms. This random delay is to guard against the case where a
   group of devices are powered on simultaneously, or a group of devices
   are connected to an Ethernet hub which is then powered on, or some
   other external event happens that might cause a group of hosts to all
   send synchronized probes.

   250ms after the first query the host should send a second, then 250ms
   after that a third. If, by 250ms after the third probe, no
   conflicting Multicast DNS responses have been received, the host may
   move to the next step, announcing. (Note that probing is the one
   exception from the normal rule that there should be at least one
   second between repetitions of the same question, and the interval
   between subsequent repetitions should at least double.)




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   When sending probe queries, a host MUST NOT consult its cache for
   potential answers. Only conflicting Multicast DNS responses received
   "live" from the network are considered valid for the purposes of
   determining whether probing has succeeded or failed.

   In order to allow services to announce their presence without
   unreasonable delay, the time window for probing is intentionally set
   quite short. As a result of this, from the time the first probe
   packet is sent, another device on the network using that name has
   just 750ms to respond to defend its name. On networks that are slow,
   or busy, or both, it is possible for round-trip latency to account
   for a few hundred milliseconds, and software delays in slow devices
   can add additional delay. For this reason, it is important that when
   a device receives a probe query for a name that it is currently using
   it SHOULD generate its response to defend that name immediately and
   send it as quickly as possible. The usual rules about random delays
   before responding, to avoid sudden bursts of simultaneous answers
   from different hosts, do not apply here since normally at most one
   host should ever respond to a given probe question. Even when a
   single DNS query packet contains multiple probe questions, it would
   be unusual for that packet to elicit a defensive response from more
   than one other host. Because of the mDNS multicast rate limiting
   rules, the first two probes SHOULD be sent as "QU" questions with the
   "unicast response" bit set, to allow a defending host to respond
   immediately via unicast, instead of potentially having to wait before
   replying via multicast. At the present time, this document recommends
   that the third probe SHOULD be sent as a standard "QM" question, for
   backwards compatibility with the small number of old devices still in
   use that don't implement unicast responses.

   If during probing, from the time the first probe packet is sent until
   250ms after the third probe, any conflicting Multicast DNS response
   is received, then the probing host MUST defer to the existing host,
   and MUST choose new names for some or all of its resource records as
   appropriate. Apparently conflicting Multicast DNS responses received
   *before* the first probe packet is sent should be silently ignored
   (see discussion of stale probe packets in Section 8.2 "Simultaneous
   Probe Tie-Breaking" below). In the case of a host probing using query
   type "ANY" as recommended above, any answer containing a record with
   that name, of any type, MUST be considered a conflicting response and
   handled accordingly.

   If fifteen failures occur within any ten-second period, then the host
   MUST wait at least five seconds before each successive additional
   probe attempt. This is to help ensure that in the event of software
   bugs or other unanticipated problems, errant hosts do not flood the
   network with a continuous stream of multicast traffic. For very
   simple devices, a valid way to comply with this requirement is to



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   always wait five seconds after any failed probe attempt before trying
   again.

   If a Responder knows by other means, with absolute certainty, that
   its unique resource record set name, rrtype and rrclass cannot
   already be in use by any other Responder on the network, then it MAY
   skip the probing step for that resource record set. For example, when
   creating the reverse address mapping PTR records, the host can
   reasonably assume that no other host will be trying to create those
   same PTR records, since that would imply that the two hosts were
   trying to use the same IP address, and if that were the case, the two
   hosts would be suffering communication problems beyond the scope of
   what Multicast DNS is designed to solve.

8.2.  Simultaneous Probe Tie-Breaking

   The astute reader will observe that there is a race condition
   inherent in the previous description. If two hosts are probing for
   the same name simultaneously, neither will receive any response to
   the probe, and the hosts could incorrectly conclude that they may
   both proceed to use the name. To break this symmetry, each host
   populates the Query packets's Authority Section with the record or
   records with the rdata that it would be proposing to use, should its
   probing be successful. The Authority Section is being used here in a
   way analogous to the way it is used as the "Update Section" in a DNS
   Update packet [RFC2136].

   When a host is probing for a group of related records with the same
   name (e.g. the SRV and TXT record describing a DNS-SD service), only
   a single question need be placed in the Question Section, since query
   type "ANY" (255) is used, which will elicit answers for all records
   with that name. However, for tie-breaking to work correctly in all
   cases, the Authority Section must contain *all* the records and
   proposed rdata being probed for uniqueness.

   When a host that is probing for a record sees another host issue a
   query for the same record, it consults the Authority Section of that
   query. If it finds any resource record(s) there which answers the
   query, then it compares the data of that (those) resource record(s)
   with its own tentative data. We consider first the simple case of a
   host probing for a single record, receiving a simultaneous probe from
   another host also probing for a single record. The two records are
   compared and the lexicographically later data wins. This means that
   if the host finds that its own data is lexicographically later, it
   simply ignores the other host's probe. If the host finds that its own
   data is lexicographically earlier, then it defers to the winning host
   by waiting one second, and then begins probing for this record again.
   The logic for waiting one second and then trying again is to guard



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   against stale probe packets on the network (possibly even stale probe
   packets sent moments ago by this host itself, before some
   configuration change, which may be echoed back after a short delay by
   some Ethernet switches and some 802.11 base stations). If the winning
   simultaneous probe was from a real other host on the network, then
   after one second it will have completed its probing, and will answer
   our subsequent probes. If the apparently winning simultaneous probe
   was in fact just an old stale packet on the network (maybe from
   ourself), then when we retry our probing in one second, our probes
   will go unanswered, and we will successfully claim the name we want.

   The determination of "lexicographically later" is performed by first
   comparing the record class (excluding the cache flush bit described
   in Section 10.3), then the record type, then raw comparison of the
   binary content of the rdata without regard for meaning or structure.
   If the record classes differ, then the numerically greater class is
   considered "lexicographically later". Otherwise, if the record types
   differ, then the numerically greater type is considered
   "lexicographically later". If the rrtype and rrclass both match then
   the rdata is compared.

   In the case of resource records containing rdata that is subject to
   name compression [RFC1035], the names MUST be uncompressed before
   comparison. (The details of how a particular name is compressed is an
   artifact of how and where the record is written into the DNS message;
   it is not an intrinsic property of the resource record itself.)

   The bytes of the raw uncompressed rdata are compared in turn,
   interpreting the bytes as eight-bit UNSIGNED values, until a byte is
   found whose value is greater than that of its counterpart (in which
   case the rdata whose byte has the greater value is deemed
   lexicographically later) or one of the resource records runs out of
   rdata (in which case the resource record which still has remaining
   data first is deemed lexicographically later).

   The following is an example of a conflict:

   MyPrinter.local. A 169.254.99.200
   MyPrinter.local. A 169.254.200.50

   In this case 169.254.200.50 is lexicographically later (the third
   byte, with value 200, is greater than its counterpart with value 99),
   so it is deemed the winner.

   Note that it is vital that the bytes are interpreted as UNSIGNED
   values in the range 0-255, or the wrong outcome may result. In the
   example above, if the byte with value 200 had been incorrectly
   interpreted as a signed eight-bit value then it would be interpreted



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   as value -56, and the wrong address record would be deemed the
   winner.

8.2.1.  Simultaneous Probe Tie-Breaking for Multiple Records

   When a host is probing for a set of records with the same name, or a
   packet is received containing multiple tie-breaker records answering
   a given probe question in the Question Section, the host's records
   and the tie-breaker records from the packet are each sorted into
   order, and then compared pairwise, using the same comparison
   technique described above, until a difference is found.

   The records are sorted using the same lexicographical order as
   described above, that is: if the record classes differ, the record
   with the lower class number comes first. If the classes are the same
   but the rrtypes differ, the record with the lower rrtype number comes
   first. If the class and rrtype match, then the rdata is compared
   bytewise until a difference is found. For example, in the common case
   of advertising DNS-SD services with a TXT record and an SRV record,
   the TXT record comes first (the rrtype value for TXT is 16) and the
   SRV record comes second (the rrtype value for SRV is 33).

   When comparing the records, if the first records match perfectly,
   then the second records are compared, and so on. If either list of
   records runs out of records before any difference is found, then the
   list with records remaining is deemed to have won the tie-break. If
   both lists run out of records at the same time without any difference
   being found, then this indicates that two devices are advertising
   identical sets of records, as is sometimes done for fault tolerance,
   and there is in fact no conflict.

8.3.  Announcing

   The second startup step is that the Multicast DNS Responder MUST send
   a gratuitous Multicast DNS Response containing, in the Answer
   Section, all of its newly registered resource records (both shared
   records, and unique records that have completed the probing step). If
   there are too many resource records to fit in a single packet,
   multiple packets should be used.

   In the case of shared records (e.g. the PTR records used by DNS
   Service Discovery [DNS-SD]), the records are simply placed as-is into
   the Answer Section of the DNS Response.

   In the case of records that have been verified to be unique in the
   previous step, they are placed into the Answer Section of the DNS
   Response with the most significant bit of the rrclass set to one. The
   most significant bit of the rrclass for a record in the Answer



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   Section of a response packet is the mDNS "cache flush" bit and is
   discussed in more detail below in Section 10.3 "Announcements to
   Flush Outdated Cache Entries".

   The Multicast DNS Responder MUST send at least two gratuitous
   responses, one second apart. A Responder MAY send up to eight
   gratuitous Responses, provided that the interval between gratuitous
   responses increases by at least a factor of two with every response
   sent.

   A Multicast DNS Responder MUST NOT send announcements in the absence
   of information that its network connectivity may have changed in some
   relevant way. In particular, a Multicast DNS Responder MUST NOT send
   regular periodic announcements as a matter of course.

   Whenever a Multicast DNS Responder receives any Multicast DNS
   response (gratuitous or otherwise) containing a conflicting resource
   record, the conflict MUST be resolved as described below in "Conflict
   Resolution".

8.4.  Updating

   At any time, if the rdata of any of a host's Multicast DNS records
   changes, the host MUST repeat the Announcing step described above to
   update neighboring caches. For example, if any of a host's IP
   addresses change, it MUST re-announce those address records.

   In the case of shared records, a host MUST send a "goodbye"
   announcement with RR TTL zero (see Section 10.2 "Goodbye Packets")
   for the old rdata, to cause it to be deleted from peer caches, before
   announcing the new rdata. In the case of unique records, a host
   SHOULD omit the "goodbye" announcement, since the cache flush bit on
   the newly announced records will cause old rdata to be flushed from
   peer caches anyway.

   A host may update the contents of any of its records at any time,
   though a host SHOULD NOT update records more frequently than ten
   times per minute. Frequent rapid updates impose a burden on the
   network. If a host has information to disseminate which changes more
   frequently than ten times per minute, then it may be more appropriate
   to design a protocol for that specific purpose.


9.  Conflict Resolution

   A conflict occurs when a Multicast DNS Responder has a unique record
   for which it is currently authoritative, and it receives a Multicast
   DNS response packet containing a record with the same name, rrtype



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   and rrclass, but inconsistent rdata. What may be considered
   inconsistent is context sensitive, except that resource records with
   identical rdata are never considered inconsistent, even if they
   originate from different hosts. This is to permit use of proxies and
   other fault-tolerance mechanisms that may cause more than one
   Responder to be capable of issuing identical answers on the network.

   A common example of a resource record type that is intended to be
   unique, not shared between hosts, is the address record that maps a
   host's name to its IP address. Should a host witness another host
   announce an address record with the same name but a different IP
   address, then that is considered inconsistent, and that address
   record is considered to be in conflict.

   Whenever a Multicast DNS Responder receives any Multicast DNS
   response (gratuitous or otherwise) containing a conflicting resource
   record in any of the Resource Record Sections, the Multicast DNS
   Responder MUST immediately reset its conflicted unique record to
   probing state, and go through the startup steps described above in
   Section 8, "Probing and Announcing on Startup". The protocol used in
   the Probing phase will determine a winner and a loser, and the loser
   MUST cease using the name, and reconfigure.

   It is very important that any host receiving a resource record that
   conflicts with one of its own MUST take action as described above. In
   the case of two hosts using the same host name, where one has been
   configured to require a unique host name and the other has not, the
   one that has not been configured to require a unique host name will
   not perceive any conflict, and will not take any action. By reverting
   to Probing state, the host that desires a unique host name will go
   through the necessary steps to ensure that a unique host name is
   obtained.

   The recommended course of action after probing and failing is as
   follows:

   1. Programmatically change the resource record name in an attempt to
      find a new name that is unique. This could be done by adding some
      further identifying information (e.g. the model name of the
      hardware) if it is not already present in the name, or appending
      the digit "2" to the name, or incrementing a number at the end of
      the name if one is already present.

   2. Probe again, and repeat as necessary until a unique name is found.

   3. Once an available unique name has been determined, by probing
      without receiving any conflicting response, record this newly
      chosen name in persistent storage so that the device will use the



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      same name the next time it is power-cycled.

   4. Display a message to the user or operator informing them of the
      name change. For example:

        The name "Bob's Music" is in use by another music
        server on the network. Your music has been renamed to
        "Bob's Music (2)". If you want to change this name, use
        [describe appropriate menu item or preference dialog here].

   5. If after one minute of probing the Multicast DNS Responder has
      been unable to find any unused name, it should display a message
      to the user or operator informing them of this fact. This
      situation should never occur in normal operation. The only
      situations that would cause this to happen would be either a
      deliberate denial-of-service attack, or some kind of very obscure
      hardware or software bug that acts like a deliberate denial-of-
      service attack.

      How the user or operator is informed depends on context. A desktop
      computer with a screen might put up a dialog box. A headless
      server in the closet may write a message to a log file, or use
      whatever mechanism (email, SNMP trap, etc.) it uses to inform the
      administrator of error conditions. On the other hand a headless
      server in the closet may not inform the user at all -- if the user
      cares, they will notice the name has changed, and connect to the
      server in the usual way (e.g. via web browser) to configure a new
      name.

   These considerations apply to address records (i.e. host names) and
   to all resource records where uniqueness (or maintenance of some
   other defined constraint) is desired.


10.  Resource Record TTL Values and Cache Coherency

   As a general rule, the recommended TTL value for Multicast DNS
   resource records with a host name as the resource record's name (e.g.
   A, AAAA, HINFO, etc.) or a host name contained within the resource
   record's rdata (e.g. SRV, reverse mapping PTR record, etc.) is 120
   seconds.

   The recommended TTL value for other Multicast DNS resource records is
   75 minutes.

   A client with an active outstanding query will issue a query packet
   when one or more of the resource record(s) in its cache is (are) 80%
   of the way to expiry. If the TTL on those records is 75 minutes, this



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   ongoing cache maintenance process yields a steady-state query rate of
   one query every 60 minutes.

   Any distributed cache needs a cache coherency protocol. If Multicast
   DNS resource records follow the recommendation and have a TTL of 75
   minutes, that means that stale data could persist in the system for a
   little over an hour. Making the default RR TTL significantly lower
   would reduce the lifetime of stale data, but would produce too much
   extra traffic on the network. Various techniques are available to
   minimize the impact of such stale data.

10.1.  Cooperating Multicast DNS Responders

   If a Multicast DNS Responder ("A") observes some other Multicast DNS
   Responder ("B") send a Multicast DNS Response packet containing a
   resource record with the same name, rrtype and rrclass as one of A's
   resource records, but different rdata, then:

   o If A's resource record is intended to be a shared resource record,
     then this is no conflict, and no action is required.

   o If A's resource record is intended to be a member of a unique
     resource record set owned solely by that Responder, then this is a
     conflict and MUST be handled as described in Section 9 "Conflict
     Resolution".

   If a Multicast DNS Responder ("A") observes some other Multicast DNS
   Responder ("B") send a Multicast DNS Response packet containing a
   resource record with the same name, rrtype and rrclass as one of A's
   resource records, and identical rdata, then:

   o If the TTL of B's resource record given in the packet is at least
     half the true TTL from A's point of view, then no action is
     required.

   o If the TTL of B's resource record given in the packet is less than
     half the true TTL from A's point of view, then A MUST mark its
     record to be announced via multicast. Clients receiving the record
     from B would use the TTL given by B, and hence may delete the
     record sooner than A expects. By sending its own multicast response
     correcting the TTL, A ensures that the record will be retained for
     the desired time.

   These rules allow multiple Multicast DNS Responders to offer the same
   data on the network (perhaps for fault tolerance reasons) without
   conflicting with each other.





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10.2.  Goodbye Packets

   In the case where a host knows that certain resource record data is
   about to become invalid (for example when the host is undergoing a
   clean shutdown) the host SHOULD send a gratuitous announcement mDNS
   response packet, giving the same resource record name, rrtype,
   rrclass and rdata, but an RR TTL of zero. This has the effect of
   updating the TTL stored in neighboring hosts' cache entries to zero,
   causing that cache entry to be promptly deleted.

   Clients receiving a Multicast DNS Response with a TTL of zero SHOULD
   NOT immediately delete the record from the cache, but instead record
   a TTL of 1 and then delete the record one second later. In the case
   of multiple Multicast DNS Responders on the network described in
   Section 10.1 above, if one of the Responders shuts down and
   incorrectly sends goodbye packets for its records, it gives the other
   cooperating Responders one second to send out their own response to
   "rescue" the records before they expire and are deleted.

10.3.  Announcements to Flush Outdated Cache Entries

   Whenever a host has a resource record with new data, or with what
   might potentially be new data (e.g. after rebooting, waking from
   sleep, connecting to a new network link, changing IP address, etc.),
   the host needs to inform peers of that new data. In cases where the
   host has not been continuously connected and participating on the
   network link, it MUST first Probe to re-verify uniqueness of its
   unique records, as described above in Section 8.1 "Probing".

   Having completed the Probing step if necessary, the host MUST then
   send a series of gratuitous announcements to update cache entries in
   its neighbor hosts. In these gratuitous announcements, if the record
   is one that has been verified unique, the host sets the most
   significant bit of the rrclass field of the resource record. This
   bit, the "cache flush" bit, tells neighboring hosts that this is not
   a shared record type. Instead of merging this new record additively
   into the cache in addition to any previous records with the same
   name, rrtype and rrclass, all old records with that name, type and
   class that were received more than one second ago are declared
   invalid, and marked to expire from the cache in one second.

   The semantics of the cache flush bit are as follows: Normally when a
   resource record appears in a Resource Record Section of the DNS
   Response, it means, "This is an assertion that this information is
   true." When a resource record appears in a Resource Record Section of
   the DNS Response with the "cache flush" bit set, it means, "This is
   an assertion that this information is the truth and the whole truth,
   and anything you may have heard more than a second ago regarding



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   records of this name/rrtype/rrclass is no longer true".

   To accommodate the case where the set of records from one host
   constituting a single unique RRSet is too large to fit in a single
   packet, only cache records that are more than one second old are
   flushed. This allows the announcing host to generate a quick burst of
   packets back-to-back on the wire containing all the members of the
   RRSet. When receiving records with the "cache flush" bit set, all
   records older than one second are marked to be deleted one second in
   the future. One second after the end of the little packet burst, any
   records not represented within that packet burst will then be expired
   from all peer caches.

   Any time a host sends a response packet containing some members of a
   unique RRSet, it SHOULD send the entire RRSet, preferably in a single
   packet, or if the entire RRSet will not fit in a single packet, in a
   quick burst of packets sent as close together as possible. The host
   SHOULD set the cache flush bit on all members of the unique RRSet. In
   the event that for some reason the host chooses not to send the
   entire unique RRSet in a single packet or a rapid packet burst, it
   MUST NOT set the cache flush bit on any of those records.

   The reason for waiting one second before deleting stale records from
   the cache is to accommodate bridged networks. For example, a host's
   address record announcement on a wireless interface may be bridged
   onto a wired Ethernet, and cause that same host's Ethernet address
   records to be flushed from peer caches. The one-second delay gives
   the host the chance to see its own announcement arrive on the wired
   Ethernet, and immediately re-announce its Ethernet interface's
   address records so that both sets remain valid and live in peer
   caches.

   These rules, about when to set the cache flush bit and about sending
   the entire rrset, apply regardless of *why* the response packet is
   being generated. They apply to startup announcements as described in
   Section 8.3 "Announcing", and to responses generated as a result of
   receiving query packets.

   The "cache flush" bit is only set in records in the Resource Record
   Sections of Multicast DNS responses sent to UDP port 5353.

   The "cache flush" bit MUST NOT be set in any resource records in a
   response packet sent in legacy unicast responses to UDP ports other
   than 5353.

   The "cache flush" bit MUST NOT be set in any resource records in the
   known-answer list of any query packet.




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   The "cache flush" bit MUST NOT ever be set in any shared resource
   record. To do so would cause all the other shared versions of this
   resource record with different rdata from different Responders to be
   immediately deleted from all the caches on the network.

   The "cache flush" bit does *not* apply to questions listed in the
   Question Section of a Multicast DNS packet. The top bit of the
   rrclass field in questions is used for an entirely different purpose
   (see Section 5.5, "Questions Requesting Unicast Responses").

   Note that the "cache flush" bit is NOT part of the resource record
   class. The "cache flush" bit is the most significant bit of the
   second 16-bit word of a resource record in a Resource Record Section
   of an mDNS packet (the field conventionally referred to as the
   rrclass field), and the actual resource record class is the least-
   significant fifteen bits of this field. There is no mDNS resource
   record class 0x8001. The value 0x8001 in the rrclass field of a
   resource record in an mDNS response packet indicates a resource
   record with class 1, with the "cache flush" bit set. When receiving a
   resource record with the "cache flush" bit set, implementations
   should take care to mask off that bit before storing the resource
   record in memory, or otherwise ensure that it is given the correct
   semantic interpretation.

   The re-use of the top bit of the rrclass field only applies to
   conventional Resource Record types that are subject to caching, not
   to pseudo-RRs like OPT [RFC2671], TSIG [RFC2845], TKEY [RFC2930],
   SIG0 [RFC2931], etc., that pertain only to a particular transport
   level message and not to any actual DNS data. Since pseudo-RRs should
   never go into the mDNS cache, the concept of a "cache flush" bit for
   these types is not applicable. In particular the rrclass field of an
   OPT records encodes the sender's UDP payload size, and should be
   interpreted as a 16-bit length value in the range 0-65535, not a one-
   bit flag and a 15-bit length.

10.4.  Cache Flush on Topology change

   If the hardware on a given host is able to indicate physical changes
   of connectivity, then when the hardware indicates such a change, the
   host should take this information into account in its mDNS cache
   management strategy. For example, a host may choose to immediately
   flush all cache records received on a particular interface when that
   cable is disconnected. Alternatively, a host may choose to adjust the
   remaining TTL on all those records to a few seconds so that if the
   cable is not reconnected quickly, those records will expire from the
   cache.

   Likewise, when a host reboots, or wakes from sleep, or undergoes some



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   other similar discontinuous state change, the cache management
   strategy should take that information into account.

10.5.  Cache Flush on Failure Indication

   Sometimes a cache record can be determined to be stale when a client
   attempts to use the rdata it contains, and finds that rdata to be
   incorrect.

   For example, the rdata in an address record can be determined to be
   incorrect if attempts to contact that host fail, either because (for
   an IPv4 address on a local subnet) ARP requests for that address go
   unanswered, because (for an IPv6 address with an on-link prefix) ND
   requests for that address go unanswered, or because (for an address
   on a remote network) a router returns an ICMP "Host Unreachable"
   error.

   The rdata in an SRV record can be determined to be incorrect if
   attempts to communicate with the indicated service at the host and
   port number indicated are not successful.

   The rdata in a DNS-SD PTR record can be determined to be incorrect if
   attempts to look up the SRV record it references are not successful.

   In any such case, the software implementing the mDNS resource record
   cache should provide a mechanism so that clients detecting stale
   rdata can inform the cache.

   When the cache receives this hint that it should reconfirm some
   record, it MUST issue two or more queries for the resource record in
   question. If no response is received in a reasonable amount of time,
   then, even though its TTL may indicate that it is not yet due to
   expire, that record SHOULD be promptly flushed from the cache.

   The end result of this is that if a printer suffers a sudden power
   failure or other abrupt disconnection from the network, its name may
   continue to appear in DNS-SD browser lists displayed on users'
   screens. Eventually that entry will expire from the cache naturally,
   but if a user tries to access the printer before that happens, the
   failure to successfully contact the printer will trigger the more
   hasty demise of its cache entries. This is a sensible trade-off
   between good user-experience and good network efficiency. If we were
   to insist that printers should disappear from the printer list within
   30 seconds of becoming unavailable, for all failure modes, the only
   way to achieve this would be for the client to poll the printer at
   least every 30 seconds, or for the printer to announce its presence
   at least every 30 seconds, both of which would be an unreasonable
   burden on most networks.



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10.6.  Passive Observation of Failures (POOF)

   A host observes the multicast queries issued by the other hosts on
   the network. One of the major benefits of also sending responses
   using multicast is that it allows all hosts to see the responses (or
   lack thereof) to those queries.

   If a host sees queries, for which a record in its cache would be
   expected to be given as an answer in a multicast response, but no
   such answer is seen, then the host may take this as an indication
   that the record may no longer be valid.

   After seeing two or more of these queries, and seeing no multicast
   response containing the expected answer within a reasonable amount of
   time, then even though its TTL may indicate that it is not yet due to
   expire, that record MAY be flushed from the cache. The host SHOULD
   NOT perform its own queries to re-confirm that the record is truly
   gone. If every host on a large network were to do this, it would
   cause a lot of unnecessary multicast traffic. If host A sends
   multicast queries that remain unanswered, then there is no reason to
   suppose that host B or any other host is likely to be any more
   successful.

   The previous section, "Cache Flush on Failure Indication", describes
   a situation where a user trying to print discovers that the printer
   is no longer available. By implementing the passive observation
   described here, when one user fails to contact the printer, all hosts
   on the network observe that failure and update their caches
   accordingly.


11.  Source Address Check

   All Multicast DNS responses (including responses sent via unicast)
   SHOULD be sent with IP TTL set to 255. This is recommended to provide
   backwards-compatibility with older Multicast DNS clients that check
   the IP TTL on reception to determine whether the packet originated on
   the local link. These older clients discard all packets with TTLs
   other than 255.

   A host sending Multicast DNS queries to a link-local destination
   address (including the 224.0.0.251 and FF02::FB link-local multicast
   addresses) MUST only accept responses to that query that originate
   from the local link, and silently discard any other response packets.
   Without this check, it could be possible for remote rogue hosts to
   send spoof answer packets (perhaps unicast to the victim host) which
   the receiving machine could misinterpret as having originated on the
   local link.



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   The test for whether a response originated on the local link is done
   in two ways:

   * All responses received with a destination address in the IP header
     which is the link-local multicast address 224.0.0.251 or FF02::FB
     are necessarily deemed to have originated on the local link,
     regardless of source IP address. This is essential to allow devices
     to work correctly and reliably in unusual configurations, such as
     multiple logical IP subnets overlayed on a single link, or in cases
     of severe misconfiguration, where devices are physically connected
     to the same link, but are currently misconfigured with completely
     unrelated IP addresses and subnet masks.

   * For responses received with a unicast destination address in the IP
     header, the source IP address in the packet is checked to see if it
     is an address on a local subnet. An IPv4 source address is
     determined to be on a local subnet if, for (one of) the address(es)
     configured on the interface receiving the packet, (I & M) == (P &
     M), where I and M are the interface address and subnet mask
     respectively, P is the source IP address from the packet, '&'
     represents the bitwise logical 'and' operation, and '==' represents
     a bitwise equality test. An IPv6 source address is determined to be
     on the local link if, for any of the on-link IPv6 prefixes on the
     interface receiving the packet (learned via IPv6 router
     advertisements or otherwise configured on the host), the first 'n'
     bits of the IPv6 source address match the first 'n' bits of the
     prefix address, where 'n' is the length of the prefix being
     considered.

   Since queriers will ignore responses apparently originating outside
   the local subnet, a Responder SHOULD avoid generating responses that
   it can reasonably predict will be ignored. This applies particularly
   in the case of overlayed subnets. If a Responder receives a query
   addressed to the link-local multicast address 224.0.0.251, from a
   source address not apparently on the same subnet as the Responder
   (or, in the case of IPv6, from a source IPv6 address for which the
   Responder does not have any address with the same prefix on that
   interface) then even if the query indicates that a unicast response
   is preferred (see Section 5.5, "Questions Requesting Unicast
   Responses"), the Responder SHOULD elect to respond by multicast
   anyway, since it can reasonably predict that a unicast response with
   an apparently non-local source address will probably be ignored.


12.  Special Characteristics of Multicast DNS Domains

   Unlike conventional DNS names, names that end in ".local." have only
   local significance. The same is true of names within the IPv4 Link-



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   Local reverse mapping domain "254.169.in-addr.arpa." and the IPv6
   Link-Local reverse mapping domains "8.e.f.ip6.arpa.",
   "9.e.f.ip6.arpa.", "a.e.f.ip6.arpa.", and "b.e.f.ip6.arpa."

   These names function primarily as protocol identifiers, rather than
   as user-visible identifiers. Even though they may occasionally be
   visible to end users, that is not their primary purpose. As such
   these names should be treated as opaque identifiers. In particular,
   the string "local" should not be translated or localized into
   different languages, much as the name "localhost" is not translated
   or localized into different languages.

   Conventional Unicast DNS seeks to provide a single unified namespace,
   where a given DNS query yields the same answer no matter where on the
   planet it is performed or to which recursive DNS server the query is
   sent. In contrast, each IP link has its own private ".local.",
   "254.169.in-addr.arpa." and IPv6 Link-Local reverse mapping
   namespaces, and the answer to any query for a name within those
   domains depends on where that query is asked. (This characteristic is
   not unique to Multicast DNS. Although the original concept of DNS was
   a single global namespace, in recent years split views, firewalls,
   intranets, and the like have increasingly meant that the answer to a
   given DNS query has become dependent on the location of the querier.)

   The IPv4 name server for a Multicast DNS Domain is 224.0.0.251. The
   IPv6 name server for a Multicast DNS Domain is FF02::FB. These are
   multicast addresses; therefore they identify not a single host but a
   collection of hosts, working in cooperation to maintain some
   reasonable facsimile of a competently managed DNS zone. Conceptually
   a Multicast DNS Domain is a single DNS zone, however its server is
   implemented as a distributed process running on a cluster of loosely
   cooperating CPUs rather than as a single process running on a single
   CPU.

   Multicast DNS Domains are not delegated from their parent domain via
   use of NS records, and there is also no concept of delegation of
   subdomains within a Multicast DNS Domain. Just because a particular
   host on the network may answer queries for a particular record type
   with the name "example.local." does not imply anything about whether
   that host will answer for the name "child.example.local.", or indeed
   for other record types with the name "example.local."

   There are no NS records anywhere in Multicast DNS Domains. Instead,
   the Multicast DNS Domains are reserved by IANA and there is
   effectively an implicit delegation of all Multicast DNS Domains to
   the 224.0.0.251:5353 and [FF02::FB]:5353, by virtue of client
   software implementing the protocol rules specified in this document.




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   Multicast DNS Zones have no SOA record. A conventional DNS zone's SOA
   record contains information such as the email address of the zone
   administrator and the monotonically increasing serial number of the
   last zone modification. There is no single human administrator for
   any given Multicast DNS Zone, so there is no email address. Because
   the hosts managing any given Multicast DNS Zone are only loosely
   coordinated, there is no readily available monotonically increasing
   serial number to determine whether or not the zone contents have
   changed. A host holding part of the shared zone could crash or be
   disconnected from the network at any time without informing the other
   hosts. There is no reliable way to provide a zone serial number that
   would, whenever such a crash or disconnection occurred, immediately
   change to indicate that the contents of the shared zone had changed.

   Zone transfers are not possible for any Multicast DNS Zone.


13.  Multicast DNS for Service Discovery

   This document does not describe using Multicast DNS for network
   browsing or service discovery. However, the mechanisms this document
   describes are compatible with, and enable, the browsing and service
   discovery mechanisms specified in "DNS-Based Service Discovery"
   [DNS-SD].


14.  Enabling and Disabling Multicast DNS

   The option to fail-over to Multicast DNS for names not ending in
   ".local." SHOULD be a user-configured option, and SHOULD be disabled
   by default because of the possible security issues related to
   unintended local resolution of apparently global names.

   The option to lookup unqualified (relative) names by appending
   ".local." (or not) is controlled by whether ".local." appears (or
   not) in the client's DNS search list.

   No special control is needed for enabling and disabling Multicast DNS
   for names explicitly ending with ".local." as entered by the user.
   The user doesn't need a way to disable Multicast DNS for names ending
   with ".local.", because if the user doesn't want to use Multicast
   DNS, they can achieve this by simply not using those names. If a user
   *does* enter a name ending in ".local.", then we can safely assume
   the user's intention was probably that it should work. Having user
   configuration options that can be (intentionally or unintentionally)
   set so that local names don't work is just one more way of
   frustrating the user's ability to perform the tasks they want,
   perpetuating the view that, "IP networking is too complicated to



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   configure and too hard to use."


15.  Considerations for Multiple Interfaces

   A host SHOULD defend its dot-local host name on all active interfaces
   on which it is answering Multicast DNS queries.

   In the event of a name conflict on *any* interface, a host should
   configure a new host name, if it wishes to maintain uniqueness of its
   host name.

   A host may choose to use the same name for all of its address records
   on all interfaces, or it may choose to manage its Multicast DNS host
   name(s) independently on each interface, potentially answering to
   different names on different interfaces.

   When answering a Multicast DNS query, a multi-homed host with a link-
   local address (or addresses) SHOULD take care to ensure that any
   address going out in a Multicast DNS response is valid for use on the
   interface on which the response is going out.

   Just as the same link-local IP address may validly be in use
   simultaneously on different links by different hosts, the same link-
   local host name may validly be in use simultaneously on different
   links, and this is not an error. A multi-homed host with connections
   to two different links may be able to communicate with two different
   hosts that are validly using the same name. While this kind of name
   duplication should be rare, it means that a host that wants to fully
   support this case needs network programming APIs that allow
   applications to specify on what interface to perform a link-local
   Multicast DNS query, and to discover on what interface a Multicast
   DNS response was received.

   There is one other special precaution that multi-homed hosts need to
   take. It's common with today's laptop computers to have an Ethernet
   connection and an 802.11 [IEEE.802.11] wireless connection active at
   the same time. What the software on the laptop computer can't easily
   tell is whether the wireless connection is in fact bridged onto the
   same network segment as its Ethernet connection. If the two networks
   are bridged together, then packets the host sends on one interface
   will arrive on the other interface a few milliseconds later, and care
   must be taken to ensure that this bridging does not cause problems:

   When the host announces its host name (i.e. its address records) on
   its wireless interface, those announcement records are sent with the
   cache-flush bit set, so when they arrive on the Ethernet segment,
   they will cause all the peers on the Ethernet to flush the host's



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   Ethernet address records from their caches. The mDNS protocol has a
   safeguard to protect against this situation: when records are
   received with the cache-flush bit set, other records are not deleted
   from peer caches immediately, but are marked for deletion in one
   second. When the host sees its own wireless address records arrive on
   its Ethernet interface, with the cache-flush bit set, this one-second
   grace period gives the host time to respond and re-announce its
   Ethernet address records, to reinstate those records in peer caches
   before they are deleted.

   As described, this solves one problem, but creates another, because
   when those Ethernet announcement records arrive back on the wireless
   interface, the host would again respond defensively to reinstate its
   wireless records, and this process would continue forever,
   continuously flooding the network with traffic. The mDNS protocol has
   a second safeguard, to solve this problem: the cache-flush bit does
   not apply to records received very recently, within the last second.
   This means that when the host sees its own Ethernet address records
   arrive on its wireless interface, with the cache-flush bit set, it
   knows there's no need to re-announce its wireless address records
   again because it already sent them less than a second ago, and this
   makes them immune from deletion from peer caches.


16.  Considerations for Multiple Responders on the Same Machine

   It is possible to have more than one Multicast DNS Responder and/or
   Querier implementation coexist on the same machine, but there are
   some known issues.

16.1.  Receiving Unicast Responses

   In most operating systems, incoming *multicast* packets can be
   delivered to *all* open sockets bound to the right port number,
   provided that the clients take the appropriate steps to allow this.
   For this reason, all Multicast DNS implementations SHOULD use the
   SO_REUSEPORT and/or SO_REUSEADDR options (or equivalent as
   appropriate for the operating system in question) so they will all be
   able to bind to UDP port 5353 and receive incoming multicast packets
   addressed to that port. However, unlike multicast packets, incoming
   unicast UDP packets are typically delivered only to the first socket
   to bind to that port. This means that "QU" responses and other
   packets sent via unicast will be received only by the first Multicast
   DNS Responder and/or Querier on a system. This limitation can be
   partially mitigated if Multicast DNS implementations detect when they
   are not the first to bind to port 5353, and in that case they do not
   request "QU" responses. One way to detect if there is another
   Multicast DNS implementation already running is to attempt binding to



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   port 5353 without using SO_REUSEPORT and/or SO_REUSEADDR, and if that
   fails it indicates that some other socket is already bound to this
   port.

16.2.  Multi-Packet Known-Answer lists

   When a Multicast DNS Querier issues a query with too many known
   answers to fit into a single packet, it divides the known answer list
   into two or more packets. Multicast DNS Responders associate the
   initial truncated query with its continuation packets by examining
   the source IP address in each packet. Since two independent Multicast
   DNS Queriers running on the same machine will be sending packets with
   the same source IP address, from an outside perspective they appear
   to be a single entity. If both Queriers happened to send the same
   multi-packet query at the same time, with different known answer
   lists, then they could each end up suppressing answers that the other
   needs.

16.3.  Efficiency

   If different clients on a machine were to each have their own
   separate independent Multicast DNS implementation, they would lose
   certain efficiency benefits. Apart from the unnecessary code
   duplication, memory usage, and CPU load, the clients wouldn't get the
   benefit of a shared system-wide cache, and they would not be able to
   aggregate separate queries into single packets to reduce network
   traffic.

16.4.  Recommendation

   Because of these issues, this document encourages implementers to
   design systems with a single Multicast DNS implementation that
   provides Multicast DNS services shared by all clients on that
   machine, much as most operating systems today have a single TCP
   implementation, which is shared between all clients on that machine.
   Due to engineering constraints, there may be situations where
   embedding a "user level" Multicast DNS implementation in the client
   application software is the most expedient solution, and while this
   will usually work in practice, implementers should be aware of the
   issues outlined in this section.


17.  Multicast DNS Character Set

   Historically, unicast DNS has been plagued by the lack of any support
   for non-US characters. Indeed, conventional DNS is usually limited to
   just letters, digits and hyphens, not even allowing spaces or other
   punctuation. Attempts to remedy this for unicast DNS have been badly



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   constrained by the perceived need to accommodate old buggy legacy DNS
   implementations. In reality, the DNS specification itself actually
   imposes no limits on what characters may be used in names, and good
   DNS implementations handle any arbitrary eight-bit data without
   trouble. "Clarifications to the DNS Specification" [RFC2181] directly
   discusses the subject of allowable character set in Section 11 ("Name
   syntax"), and explicitly states that DNS names may contain arbitrary
   eight-bit data. However, the old rules for ARPANET host names back in
   the 1980s required host names to be just letters, digits, and hyphens
   [RFC1034], and since the predominant use of DNS is to store host
   address records, many have assumed that the DNS protocol itself
   suffers from the same limitation. It might be accurate to say that
   there could be hypothetical bad implementations that do not handle
   eight-bit data correctly, but it would not be accurate to say that
   the protocol doesn't allow names containing eight-bit data.

   Multicast DNS is a new protocol and doesn't (yet) have old buggy
   legacy implementations to constrain the design choices. Accordingly,
   it adopts the simple obvious elegant solution: all names in Multicast
   DNS are encoded using precomposed UTF-8 [RFC3629]. The characters
   SHOULD conform to Unicode Normalization Form C (NFC) [UAX15]: Use
   precomposed characters instead of combining sequences where possible,
   e.g. use U+00C4 ("Latin capital letter A with diaeresis") instead of
   U+0041 U+0308 ("Latin capital letter A", "combining diaeresis").

   Some users of 16-bit Unicode have taken to stuffing a "zero-width
   non-breaking space" character (U+FEFF) at the start of each UTF-16
   file, as a hint to identify whether the data is big-endian or little-
   endian, and calling it a "Byte Order Mark" (BOM). Since there is only
   one possible byte order for UTF-8 data, a BOM is neither necessary
   nor permitted. Multicast DNS names MUST NOT contain a "Byte Order
   Mark". Any occurrence of the Unicode character U+FEFF at the start or
   anywhere else in a Multicast DNS name MUST be interpreted as being an
   actual intended part of the name, representing (just as for any other
   legal unicode value) an actual literal instance of that character (in
   this case a zero-width non-breaking space character).

   For names that are restricted to letters, digits and hyphens, the
   UTF-8 encoding is identical to the US-ASCII encoding, so this is
   entirely compatible with existing host names. For characters outside
   the US-ASCII range, UTF-8 encoding is used.

   Multicast DNS implementations MUST NOT use any other encodings apart
   from precomposed UTF-8 (US-ASCII being considered a compatible subset
   of UTF-8). The reasons for selecting UTF-8 instead of Punycode
   [RFC3492] are discussed further in Appendix F.

   The simple rules for case-insensitivity in Unicast DNS also apply in



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   Multicast DNS; that is to say, in name comparisons, the lower-case
   letters "a" to "z" (0x61 to 0x7A) match their upper-case equivalents
   "A" to "Z" (0x41 to 0x5A). Hence, if a client issues a query for an
   address record with the name "myprinter.local.", then a Responder
   having an address record with the name "MyPrinter.local." should
   issue a response. No other automatic equivalences should be assumed.
   In particular all UTF-8 multi-byte characters (codes 0x80 and higher)
   are compared by simple binary comparison of the raw byte values.
   Accented characters are *not* defined to be automatically equivalent
   to their unaccented counterparts. Where automatic equivalences are
   desired, this may be achieved through the use of programmatically-
   generated CNAME records. For example, if a Responder has an address
   record for an accented name Y, and a client issues a query for a name
   X, where X is the same as Y with all the accents removed, then the
   Responder may issue a response containing two resource records: A
   CNAME record "X CNAME Y", asserting that the requested name X
   (unaccented) is an alias for the true (accented) name Y, followed by
   the address record for Y.


18.  Multicast DNS Message Size

   RFC 1035 restricts DNS Messages carried by UDP to no more than 512
   bytes (not counting the IP or UDP headers) [RFC1035]. For UDP packets
   carried over the wide-area Internet in 1987, this was appropriate.
   For link-local multicast packets on today's networks, there is no
   reason to retain this restriction. Given that the packets are by
   definition link-local, there are no Path MTU issues to consider.

   Multicast DNS Messages carried by UDP may be up to the IP MTU of the
   physical interface, less the space required for the IP header (20
   bytes for IPv4; 40 bytes for IPv6) and the UDP header (8 bytes).

   In the case of a single mDNS Resource Record which is too large to
   fit in a single MTU-sized multicast response packet, a Multicast DNS
   Responder SHOULD send the Resource Record alone, in a single IP
   datagram, sent using multiple IP fragments. Resource Records this
   large SHOULD be avoided, except in the very rare cases where they
   really are the appropriate solution to the problem at hand.
   Implementers should be aware that many simple devices do not re-
   assemble fragmented IP datagrams, so large Resource Records SHOULD
   NOT be used except in specialized cases where the implementer knows
   that all receivers implement reassembly.

   A Multicast DNS packet larger than the interface MTU, which is sent
   using fragments, MUST NOT contain more than one Resource Record.

   Even when fragmentation is used, a Multicast DNS packet, including IP



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   and UDP headers, MUST NOT exceed 9000 bytes. 9000 bytes is the
   maximum payload size of an Ethernet "Jumbo" packet, which makes it a
   convenient upper limit to specify for the maximum Multicast DNS
   packet size.

   In practice Ethernet "Jumbo" packets are not widely used, so it is
   advantageous to keep packets under 1500 bytes whenever possible. Even
   on hosts that normally handle Ethernet "Jumbo" packets and IP
   fragment reassembly, it is becoming more common for these hosts to
   implement power-saving modes where the main CPU goes to sleep and
   hands off packet reception tasks to a more limited processor in the
   network interface hardware, which may not support Ethernet "Jumbo"
   packets or IP fragment reassembly.


19.  Multicast DNS Message Format

   This section describes specific rules pertaining to the allowable
   values for the header fields of a Multicast DNS message, and other
   message format considerations.

19.1.  ID (Query Identifier)

   Multicast DNS clients SHOULD listen for gratuitous responses issued
   by hosts booting up (or waking up from sleep or otherwise joining the
   network). Since these gratuitous responses may contain a useful
   answer to a question for which the client is currently awaiting an
   answer, Multicast DNS clients SHOULD examine all received Multicast
   DNS response messages for useful answers, without regard to the
   contents of the ID field or the Question Section. In Multicast DNS,
   knowing which particular query message (if any) is responsible for
   eliciting a particular response message is less interesting than
   knowing whether the response message contains useful information.

   Multicast DNS clients MAY cache any or all Multicast DNS response
   messages they receive, for possible future use, provided of course
   that normal TTL aging is performed on these cached resource records.

   In multicast query messages, the Query ID SHOULD be set to zero on
   transmission.

   In multicast responses, including gratuitous multicast responses, the
   Query ID MUST be set to zero on transmission, and MUST be ignored on
   reception.

   In unicast response messages generated specifically in response to a
   particular (unicast or multicast) query, the Query ID MUST match the
   ID from the query message.



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19.2.  QR (Query/Response) Bit

   In query messages, MUST be zero.
   In response messages, MUST be one.

19.3.  OPCODE

   In both multicast query and multicast response messages, MUST be zero
   (only standard queries are currently supported over multicast).

19.4.  AA (Authoritative Answer) Bit

   In query messages, the Authoritative Answer bit MUST be zero on
   transmission, and MUST be ignored on reception.

   In response messages for Multicast Domains, the Authoritative Answer
   bit MUST be set to one (not setting this bit would imply there's some
   other place where "better" information may be found) and MUST be
   ignored on reception.

19.5.  TC (Truncated) Bit

   In query messages, if the TC bit is set, it means that additional
   Known Answer records may be following shortly. A Responder SHOULD
   record this fact, and wait for those additional Known Answer records,
   before deciding whether to respond. If the TC bit is clear, it means
   that the querying host has no additional Known Answers.

   In multicast response messages, the TC bit MUST be zero on
   transmission, and MUST be ignored on reception.

   In legacy unicast response messages, the TC bit has the same meaning
   as in conventional unicast DNS: it means that the response was too
   large to fit in a single packet, so the client SHOULD re-issue its
   query using TCP in order to receive the larger response.

19.6.  RD (Recursion Desired) Bit

   In both multicast query and multicast response messages, the
   Recursion Desired bit SHOULD be zero on transmission, and MUST be
   ignored on reception.

19.7.  RA (Recursion Available) Bit

   In both multicast query and multicast response messages, the
   Recursion Available bit MUST be zero on transmission, and MUST be
   ignored on reception.




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19.8.  Z (Zero) Bit

   In both query and response messages, the Zero bit MUST be zero on
   transmission, and MUST be ignored on reception.

19.9.  AD (Authentic Data) Bit

   In both multicast query and multicast response messages the Authentic
   Data bit [RFC2535] MUST be zero on transmission, and MUST be ignored
   on reception.

19.10.  CD (Checking Disabled) Bit

   In both multicast query and multicast response messages, the Checking
   Disabled bit [RFC2535] MUST be zero on transmission, and MUST be
   ignored on reception.

19.11.  RCODE (Response Code)

   In both multicast query and multicast response messages, the Response
   Code MUST be zero on transmission. Multicast DNS messages received
   with non-zero Response Codes MUST be silently ignored.

19.12.  Repurposing of top bit of qclass in Question Section

   In the Question Section of a Multicast DNS Query, the top bit of the
   qclass field is used to indicate that unicast responses are preferred
   for this particular question.

19.13.  Repurposing of top bit of rrclass in Resource Record Sections

   In the Resource Record Sections of a Multicast DNS Response, the top
   bit of the rrclass field is used to indicate that the record is a
   member of a unique RRSet, and the entire RRSet has been sent together
   (in the same packet, or in consecutive packets if there are too many
   records to fit in a single packet).

19.14.  Name Compression

   When generating Multicast DNS packets, implementations SHOULD use
   name compression wherever possible to compress the names of resource
   records, by replacing some or all of the resource record name with a
   compact two-byte reference to an appearance of that data somewhere
   earlier in the packet [RFC1035].

   This applies not only to Multicast DNS Responses, but also to
   Queries. When a Query contains more than one question, successive
   questions in the same message often contain similar names, and



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   consequently name compression SHOULD be used, to save bytes. In
   addition, Queries may also contain Known Answers in the Answer
   Section, or probe tie-breaking data in the Authority Section, and
   these names SHOULD similarly be compressed for network efficiency.

   In addition to compressing the *names* of resource records, names
   that appear within the *rdata* of the following rrtypes SHOULD also
   be compressed in all Multicast DNS packets:

     NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC

   Until future IETF Standards Action specifying that names in the rdata
   of other types should be compressed, names that appear within the
   rdata of any type not listed above MUST NOT be compressed.

   Implementations receiving Multicast DNS packets MUST correctly decode
   compressed names appearing in the Question Section, and compressed
   names of resource records appearing in other sections.

   In addition, implementations MUST correctly decode compressed names
   appearing within the *rdata* of the rrtypes listed above. Where
   possible, implementations SHOULD also correctly decode compressed
   names appearing within the *rdata* of other rrtypes known to the
   implementers at the time of implementation, because such forward-
   thinking planning helps facilitate the deployment of future
   implementations that may have reason to compress those rrtypes. It is
   possible that no future IETF Standards Action will be created which
   mandates or permits the compression of rdata in new types, but having
   implementations designed such that they are capable of decompressing
   all known types known helps keep future options open.

   One specific difference between Unicast DNS and Multicast DNS is that
   Unicast DNS does not allow name compression for the target host in an
   SRV record, because Unicast DNS implementations before the first SRV
   specification in 1996 [RFC2052] may not decode these compressed
   records properly. Since all Multicast DNS implementations were
   created after 1996, all Multicast DNS implementations are REQUIRED to
   decode compressed SRV records correctly.

   In legacy unicast responses generated to answer legacy queries, name
   compression MUST NOT be performed on SRV records.


20.  Summary of Differences Between Multicast DNS and Unicast DNS

   The value of Multicast DNS is that it shares, as much as possible,
   the familiar APIs, naming syntax, resource record types, etc., of
   Unicast DNS. There are of course necessary differences by virtue of



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   it using multicast, and by virtue of it operating in a community of
   cooperating peers, rather than a precisely defined hierarchy
   controlled by a strict chain of formal delegations from the root.
   These differences are summarized below:

   Multicast DNS...
   * uses multicast
   * uses UDP port 5353 instead of port 53
   * operates in well-defined parts of the DNS namespace
   * uses UTF-8, and only UTF-8, to encode resource record names
   * allows names up to 255 bytes plus a terminating zero byte
   * allows name compression in rdata for SRV and other record types
   * allows larger UDP packets
   * allows more than one question in a query packet
   * defines consistent results for qtype "ANY" and qclass "ANY" queries
   * uses the Answer Section of a query to list Known Answers
   * uses the TC bit in a query to indicate additional Known Answers
   * uses the Authority Section of a query for probe tie-breaking
   * ignores the Query ID field (except for generating legacy responses)
   * doesn't require the question to be repeated in the response packet
   * uses gratuitous responses to announce new records to the peer group
   * uses NSEC records to signal non-existence of records
   * defines a "unicast response" bit in the rrclass of query questions
   * defines a "cache flush" bit in the rrclass of response answers
   * uses DNS RR TTL 0 to indicate that a record has been deleted
   * recommends AAAA records in the additional section when responding
     to rrtype "A" queries, and vice versa
   * monitors queries to perform Duplicate Question Suppression
   * monitors responses to perform Duplicate Answer Suppression...
   * ... and Ongoing Conflict Detection
   * ... and Opportunistic Caching


21.  IPv6 Considerations

   An IPv4-only host and an IPv6-only host behave as "ships that pass in
   the night". Even if they are on the same Ethernet, neither is aware
   of the other's traffic. For this reason, each physical link may have
   *two* unrelated ".local." zones, one for IPv4 and one for IPv6. Since
   for practical purposes, a group of IPv4-only hosts and a group of
   IPv6-only hosts on the same Ethernet act as if they were on two
   entirely separate Ethernet segments, it is unsurprising that their
   use of the ".local." zone should occur exactly as it would if they
   really were on two entirely separate Ethernet segments.

   A dual-stack (v4/v6) host can participate in both ".local." zones,
   and should register its name(s) and perform its lookups both using
   IPv4 and IPv6. This enables it to reach, and be reached by, both



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   IPv4-only and IPv6-only hosts. In effect this acts like a multi-homed
   host, with one connection to the logical "IPv4 Ethernet segment", and
   a connection to the logical "IPv6 Ethernet segment". When such a host
   generates NSEC records, if it is using the same hostname for its IPv4
   addresses and its IPv6 addresses on that network interface, its NSEC
   records should indicate that the hostname has both 'A' and AAAA
   records.


22.  Security Considerations

   The algorithm for detecting and resolving name conflicts is, by its
   very nature, an algorithm that assumes cooperating participants. Its
   purpose is to allow a group of hosts to arrive at a mutually disjoint
   set of host names and other DNS resource record names, in the absence
   of any central authority to coordinate this or mediate disputes. In
   the absence of any higher authority to resolve disputes, the only
   alternative is that the participants must work together cooperatively
   to arrive at a resolution.

   In an environment where the participants are mutually antagonistic
   and unwilling to cooperate, other mechanisms are appropriate, like
   manually configured DNS.

   In an environment where there is a group of cooperating participants,
   but there may be other antagonistic participants on the same physical
   link, the cooperating participants need to use IPSEC signatures
   and/or DNSSEC [RFC4033] signatures so that they can distinguish mDNS
   messages from trusted participants (which they process as usual) from
   mDNS messages from untrusted participants (which they silently
   discard).

   When DNS queries for *global* DNS names are sent to the mDNS
   multicast address (during network outages which disrupt communication
   with the greater Internet) it is *especially* important to use
   DNSSEC, because the user may have the impression that he or she is
   communicating with some authentic host, when in fact he or she is
   really communicating with some local host that is merely masquerading
   as that name. This is less critical for names ending with ".local.",
   because the user should be aware that those names have only local
   significance and no global authority is implied.

   Most computer users neglect to type the trailing dot at the end of a
   fully qualified domain name, making it a relative domain name (e.g.
   "www.example.com"). In the event of network outage, attempts to
   positively resolve the name as entered will fail, resulting in
   application of the search list, including ".local.", if present. A
   malicious host could masquerade as "www.example.com." by answering



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   the resulting Multicast DNS query for "www.example.com.local." To
   avoid this, a host MUST NOT append the search suffix ".local.", if
   present, to any relative (partially qualified) host name containing
   two or more labels. Appending ".local." to single-label relative host
   names is acceptable, since the user should have no expectation that a
   single-label host name will resolve as-is. However, users who have
   both "example.com" and "local" in their search lists should be aware
   that if they type "www" into their web browser, it may not be
   immediately clear to them whether the page that appears is
   "www.example.com" or "www.local".

   Multicast DNS uses UDP port 5353. On operating systems where only
   privileged processes are allowed to use ports below 1024, no such
   privilege is required to use port 5353.


23.  IANA Considerations

   IANA has allocated the IPv4 link-local multicast address 224.0.0.251
   for the use described in this document.

   IANA has allocated the IPv6 multicast address set FF0X::FB for the
   use described in this document. Only address FF02::FB (Link-Local
   Scope) is currently in use by deployed software, but it is possible
   that in future implementers may experiment with Multicast DNS using
   larger-scoped addresses, such as FF05::FB (Site-Local Scope)
   [RFC4291].

   When this document is published, IANA should designate a list of
   domains which are deemed to have only link-local significance, as
   described in Section 12 of this document ("Special Characteristics of
   Multicast DNS Domains"). For discussion of why maintaining this list
   of reserved domains is an IANA function rather than an ICANN
   function, see Appendix G. For discussion of other "private" DNS
   Namespaces see Appendix H.

   Specifically, the designated link-local domains are:

      local.
      254.169.in-addr.arpa.
      8.e.f.ip6.arpa.
      9.e.f.ip6.arpa.
      a.e.f.ip6.arpa.
      b.e.f.ip6.arpa.

   These domains, and any of their subdomains (e.g. "MyPrinter.local.",
   "34.12.254.169.in-addr.arpa.", "Ink-Jet._pdl-datastream._tcp.local.")
   are special in the following ways:



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   1. Users may use these names as they would other DNS names, entering
      them anywhere that they would otherwise enter a conventional DNS
      name, or a dotted decimal IPv4 address, or a literal IPv6 address.

      Since there is no central authority responsible for assigning dot-
      local names, and all devices on the local network are equally
      entitled to claim any dot-local name, users SHOULD be aware of
      this and SHOULD exercise appropriate caution. In an untrusted or
      unfamiliar network environment, users SHOULD be aware that using a
      name like "www.local" may not actually connect them to the web
      site they expected, and could easily connect them to a different
      web page, or even a fake or spoof of their intended web site,
      designed to trick them into revealing confidential information. As
      always with networking, end-to-end cryptographic security can be a
      useful tool. For example, when connecting with ssh, the ssh host
      key verification process will inform the user if it detects that
      the identity of the entity they are communicating with has changed
      since the last time they connected to that name.

   2. Application software may use these names as they would other
      similar DNS names, and is not required to recognize the names and
      treat them specially. Due to the relative ease of spoofing dot-
      local names, end-to-end cryptographic security remains important
      when communicating across a local network, as it is when
      communicating across the global Internet.

   3. Name resolutions APIs and libraries SHOULD recognize these names
      as special and SHOULD NOT send queries for these names to their
      configured (unicast) caching DNS server(s). This is to avoid
      unnecessary load on the root name servers and other name servers,
      caused by queries for which those name servers do not have useful
      non-negative answers to give, and will not ever have useful non-
      negative answers to give.

   4. Caching DNS servers SHOULD recognize these names as special and
      SHOULD NOT attempt to look up NS records for them, or otherwise
      query authoritative DNS servers in an attempt to resolve these
      names. Instead, caching DNS servers SHOULD generate immediate
      NXDOMAIN responses for all such queries they may receive (from
      misbehaving name resolver libraries). This is to avoid unnecessary
      load on the root name servers and other name servers.

   5. Authoritative DNS servers SHOULD NOT by default be configurable to
      answer queries for these names, and, like caching DNS servers,
      SHOULD generate immediate NXDOMAIN responses for all such queries
      they may receive. DNS server software MAY provide a configuration
      option to override this default, for testing purposes or other
      specialized uses.



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   6. DNS server operators SHOULD NOT attempt to configure authoritative
      DNS servers to act as authoritative for any of these names.
      Configuring an authoritative DNS server to act as authoritative
      for any of these names may not, in many cases, yield the expected
      result, since name resolver libraries and caching DNS servers
      SHOULD NOT send queries for those names (see 3 and 4 above), so
      such queries SHOULD be suppressed before they even reach the
      authoritative DNS server in question, and consequently it will not
      even get an opportunity to answer them.

   7. DNS Registrars MUST NOT allow any of these names to be registered
      in the normal way to any person or entity. These names are
      reserved protocol identifiers with special meaning and fall
      outside the set of names available for allocation by registrars.
      Attempting to allocate one of these names as if it were a normal
      DNS domain name will probably not work as desired, for reasons 3,
      4 and 6 above.

   The re-use of the top bit of the rrclass field in the Question and
   Resource Record Sections means that Multicast DNS can only carry DNS
   records with classes in the range 0-32767. Classes in the range 32768
   to 65535 are incompatible with Multicast DNS. IANA is requested to
   take note of this fact, and if IANA receives a request to allocate a
   DNS class value above 32767, IANA should make sure the requester is
   aware of this implication before proceeding. This does not mean that
   allocations of DNS class values above 32767 should not be allowed,
   only that they should not be allowed until the requester has
   indicated that they are aware of how this allocation will interact
   with Multicast DNS. However, since to-date only three DNS classes
   have been assigned by IANA (1, 3 and 4), and only one (1, "Internet")
   is actually in widespread use, this issue is likely to remain a
   purely theoretical one.

   No other IANA services are required by this document.


24.  Acknowledgments

   The concepts described in this document have been explored, developed
   and implemented with help from Freek Dijkstra, Erik Guttman, Paul
   Vixie, Bill Woodcock, and others. Special thanks go to Bob Bradley,
   Josh Graessley, Scott Herscher, Rory McGuire, Roger Pantos and Kiren
   Sekar for their significant contributions.


25.  References





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25.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
              Specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [UAX15]    "Unicode Normalization Forms",
              <http://www.unicode.org/reports/tr15/>.

25.2.  Informative References

   [B4W]      "Bonjour for Windows",
              <http://en.wikipedia.org/wiki/Bonjour_(software)>.

   [DNS-SD]   Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-07 (work in
              progress), October 2010.

   [IEEE.802.3]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              3: Carrier Sense Multiple Access with Collision Detection
              (CMSA/CD) Access Method and Physical Layer
              Specifications", IEEE Std 802.3-2008, December 2008,
              <http://standards.ieee.org/getieee802/802.3.html>.

   [IEEE.802.11]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              11: Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications", IEEE Std 802.11-2007,
              June 2007,
              <http://standards.ieee.org/getieee802/802.11.html>.




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   [NBP]      Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace AppleTalk NBP", draft-cheshire-dnsext-nbp-09
              (work in progress), October 2010.

   [RFC2052]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
              location of services (DNS SRV)", RFC 2052, October 1996.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, May 2000.

   [RFC2860]  Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
              Understanding Concerning the Technical Work of the
              Internet Assigned Numbers Authority", RFC 2860, June 2000.

   [RFC2930]  Eastlake, D., "Secret Key Establishment for DNS (TKEY
              RR)", RFC 2930, September 2000.

   [RFC2931]  Eastlake, D., "DNS Request and Transaction Signatures (
              SIG(0)s)", RFC 2931, September 2000.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.




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   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [Zeroconf]
              Cheshire, S. and D. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, December 2005.


Appendix A.  Design Rationale for Choice of UDP Port Number

   Arguments were made for and against using Multicast on UDP port 53.
   The final decision was to use UDP port 5353. Some of the arguments
   for and against are given below.

   Arguments for using UDP port 53:

   * This is "just DNS", so it should be the same port.

   * There is less work to be done updating old clients to do simple
     mDNS queries. Only the destination address need be changed. In some
     cases, this can be achieved without any code changes, just by
     adding the address 224.0.0.251 to a configuration file.

   Arguments for using a different port (UDP port 5353):

   * This is not "just DNS". This is a DNS-like protocol, but different.

   * Changing client code to use a different port number is not hard.

   * Using the same port number makes it hard to run an mDNS Responder
     and a conventional unicast DNS server on the same machine. If a
     conventional unicast DNS server wishes to implement mDNS as well,
     it can still do that, by opening two sockets. Having two different
     port numbers allows this flexibility.





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   * Some VPN software hijacks all outgoing traffic to port 53 and
     redirects it to a special DNS server set up to serve those VPN
     clients while they are connected to the corporate network. It is
     questionable whether this is the right thing to do, but it is
     common, and redirecting link-local multicast DNS packets to a
     remote server rarely produces any useful results. It does mean, for
     example, that a user of such VPN software becomes unable to access
     their local network printer sitting on their desk right next to
     their computer. Using a different UDP port helps avoid this
     particular problem.

   * On many operating systems, unprivileged clients may not send or
     receive packets on low-numbered ports. This means that any client
     sending or receiving mDNS packets on port 53 would have to run as
     "root", which is an undesirable security risk. Using a higher-
     numbered UDP port avoids this restriction.


Appendix B.  Design Rationale for Not Using Hashed Multicast Addresses

   Some discovery protocols use a range of multicast addresses, and
   determine the address to be used by a hash function of the name being
   sought. Queries are sent via multicast to the address as indicated by
   the hash function, and responses are returned to the querier via
   unicast. Particularly in IPv6, where multicast addresses are
   extremely plentiful, this approach is frequently advocated. For
   example, IPv6 Neighbor Discovery [RFC4861] sends Neighbor
   Solicitation messages to the "solicited-node multicast address",
   which is computed as a function of the solicited IPv6 address.

   There are some disadvantages to using hashed multicast addresses like
   this in a service discovery protocol:

   * When a host has a large number of records with different names, the
     host may have to join a large number of multicast groups. This can
     place undue burden on the Ethernet hardware, which typically
     supports a limited number of multicast addresses efficiently. When
     this number is exceeded, the Ethernet hardware may have to resort
     to receiving all multicasts and passing them up to the host
     networking code for filtering in software, thereby defeating much
     of the point of using a multicast address range in the first place.

   * Multiple questions cannot be placed in one packet if they don't all
     hash to the same multicast address.







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   * Duplicate Question Suppression doesn't work if queriers are not
     seeing each other's queries.

   * Duplicate Answer Suppression doesn't work if Responders are not
     seeing each other's responses.

   * Opportunistic Caching doesn't work.

   * Ongoing Conflict Detection doesn't work.


Appendix C.  Design Rationale for Maximum Multicast DNS Name Length

   Multicast DNS domain names may be up to 255 bytes long, not counting
   the terminating zero byte at the end.

   "Domain Names - Implementation and Specification" [RFC1035] says:

     Various objects and parameters in the DNS have size limits.
     They are listed below.  Some could be easily changed, others
     are more fundamental.

     labels          63 octets or less

     names           255 octets or less

     ...

     the total length of a domain name (i.e., label octets and
     label length octets) is restricted to 255 octets or less.

   This text does not state whether this 255-byte limit includes the
   terminating zero at the end of every name.

   Several factors lead us to conclude that the 255-byte limit does
   *not* include the terminating zero:

   o It is common in software engineering to have size limits that are a
     power of two, or a multiple of a power of two, for efficiency. For
     example, an integer on a modern processor is typically 2, 4, or 8
     bytes, not 3 or 5 bytes. The number 255 is not a power of two, nor
     is it to most people a particularly noteworthy number. It is
     noteworthy to computer scientists for only one reason -- because it
     is exactly one *less* than a power of two. When a size limit is
     exactly one less than a power of two, that suggests strongly that
     the one extra byte is being reserved for some specific reason -- in
     this case reserved perhaps to leave room for a terminating zero at
     the end.



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   o In the case of DNS label lengths, the stated limit is 63 bytes. As
     with the total name length, this limit is exactly one less than a
     power of two. This label length limit also excludes the label
     length byte at the start of every label. Including that extra byte,
     a 63-byte label takes 64 bytes of space in memory or in a DNS
     packet.

   o It is common in software engineering for the semantic "length" of
     an object to be one less than the number of bytes it takes to store
     that object. For example, in C, strlen("foo") is 3, but
     sizeof("foo") (which includes the terminating zero byte at the end)
     is 4.

   o The text describing the total length of a domain name mentions
     explicitly that label length and data octets are included, but does
     not mention the terminating zero at the end. The zero byte at the
     end of a domain name is not a label length. Indeed, the value zero
     is chosen as the terminating marker precisely because it is not a
     legal length byte value -- DNS prohibits empty labels. For example,
     a name like "bad..name." is not a valid domain name because it
     contains a zero-length label in the middle, which cannot be
     expressed in a DNS packet, because software parsing the packet
     would misinterpret a zero label-length byte as being a zero "end of
     name" marker instead.

   Finally, "Clarifications to the DNS Specification" [RFC2181] offers
   additional confirmation that in the context of DNS specifications the
   stated "length" of a domain name does not include the terminating
   zero byte at the end. That document refers to the root name, which is
   typically written as "." and is represented in a DNS packet by a
   single lone zero byte (i.e. zero bytes of data plus a terminating
   zero), as the "zero length full name":

     The zero length full name is defined as representing the root of
     the DNS tree, and is typically written and displayed as ".".
















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   This wording supports the interpretation that, in a DNS context, when
   talking about lengths of names, the terminating zero byte at the end
   is not counted. If the root name (".") is considered to be zero
   length, then to be consistent, the length (for example) of "org" has
   to be 4 and the length of "ietf.org" has to be 9, as shown below:

                                                  ------
                                                 | 0x00 |   length = 0
                                                  ------

                             ------------------   ------
                            | 0x03 | o | r | g | | 0x00 |   length = 4
                             ------------------   ------

      -----------------------------------------   ------
     | 0x04 | i | e | t | f | 0x03 | o | r | g | | 0x00 |   length = 9
      -----------------------------------------   ------

   This means that the maximum length of a domain name, as represented
   in a Multicast DNS packet, up to but not including the final
   terminating zero, must not exceed 255 bytes.

   However, many unicast DNS implementers have read these RFCs
   differently, and argue that the 255-byte limit does include the
   terminating zero, and that the "Clarifications to the DNS
   Specification" [RFC2181] statement that "." is the "zero length full
   name" was simply a mistake.

   Hence, implementers should be aware that other unicast DNS
   implementations may limit the maximum domain name to 254 bytes plus a
   terminating zero, depending on how that implementer interpreted the
   DNS specifications.

   Compliant Multicast DNS implementations must support names up to 255
   bytes plus a terminating zero, i.e. 256 bytes total.


Appendix D.  Benefits of Multicast Responses

   Some people have argued that sending responses via multicast is
   inefficient on the network. In fact using multicast responses can
   result in a net lowering of overall multicast traffic for a variety
   of reasons, and provides other benefits too:

   * Opportunistic Caching. One multicast response can update the caches
     on all machines on the network. If another machine later wants to
     issue the same query, it already has the answer in its cache, so it
     may not need to even transmit that multicast query on the network



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     at all.

   * Duplicate Query Suppression. When more than one machine has the
     same ongoing long-lived query running, every machine does not have
     to transmit its own independent query. When one machine transmits a
     query, all the other hosts see the answers, so they can suppress
     their own queries.

   * Passive Observation Of Failures (POOF). When a host sees a
     multicast query, but does not see the corresponding multicast
     response, it can use this information to promptly delete stale data
     from its cache. To achieve the same level of user-interface quality
     and responsiveness without multicast responses would require lower
     cache lifetimes and more frequent network polling, resulting in a
     higher packet rate.

   * Passive Conflict Detection. Just because a name has been previously
     verified unique does not guarantee it will continue to be so
     indefinitely. By allowing all Multicast DNS Responders to
     constantly monitor their peers' responses, conflicts arising out of
     network topology changes can be promptly detected and resolved. If
     responses were not sent via multicast, some other conflict
     detection mechanism would be needed, imposing its own additional
     burden on the network.

   * Use on devices with constrained memory resources: When using
     delayed responses to reduce network collisions, clients need to
     maintain a list recording to whom each answer should be sent. The
     option of multicast responses allows clients with limited storage,
     which cannot store an arbitrarily long list of response addresses,
     to choose to fail-over to a single multicast response in place of
     multiple unicast responses, when appropriate.

   * Overlayed Subnets. In the case of overlayed subnets, multicast
     responses allow a receiver to know with certainty that a response
     originated on the local link, even when its source address may
     apparently suggest otherwise.

   * Robustness in the face of misconfiguration: Link-local multicast
     transcends virtually every conceivable network misconfiguration.
     Even if you have a collection of devices where every device's IP
     address, subnet mask, default gateway, and DNS server address are
     all wrong, packets sent by any of those devices addressed to a
     link-local multicast destination address will still be delivered to
     all peers on the local link. This can be extremely helpful when
     diagnosing and rectifying network problems, since it facilitates a
     direct communication channel between client and server that works
     without reliance on ARP, IP routing tables, etc. Being able to



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     discover what IP address a device has (or thinks it has) is
     frequently a very valuable first step in diagnosing why it is
     unable to communicate on the local network.


Appendix E.  Design Rationale for Encoding Negative Responses

   Alternative methods of asserting nonexistence were considered, such
   as using an NXDOMAIN response, or emitting a resource record with
   zero-length rdata.

   Using an NXDOMAIN response does not work well with Multicast DNS. A
   Unicast DNS NXDOMAIN response applies to the entire packet, but for
   efficiency Multicast DNS allows (and encourages) multiple responses
   in a single packet. If the error code in the header were NXDOMAIN, it
   would not be clear to which name(s) that error code applied.

   Asserting nonexistence by emitting a resource record with zero-length
   rdata would mean that there would be no way to differentiate between
   a record that doesn't exist, and a record that does exist, with zero-
   length rdata. By analogy, most file systems today allow empty files,
   so a file that exists with zero bytes of data is not considered
   equivalent to a filename that does not exist.

   A benefit of asserting nonexistence through NSEC records instead of
   through NXDOMAIN responses is that NSEC records can be added to the
   Additional Section of a DNS Response to offer additional information
   beyond what the client explicitly requested. For example, in a
   response to an SRV query, a Responder should include 'A' record(s)
   giving its IPv4 addresses in the Additional Section, and an NSEC
   record indicating which other types it does or does not have for this
   name. If the Responder is running on an host that does not support
   IPv6 (or does support IPv6 but currently has no IPv6 address on that
   interface) then this NSEC record in the Additional Section will
   indicate this absence of AAAA records. In effect, the Responder is
   saying, "Here's my SRV record, and here are my IPv4 addresses, and
   no, I don't have any IPv6 addresses, so don't waste your time
   asking." Without this information in the Additional Section it would
   take the client an additional round-trip to perform an additional
   Query to ascertain that the target host has no AAAA records.
   (Arguably Unicast DNS could also benefit from this ability to express
   nonexistence in the Additional Section, but that is outside the scope
   of this document.)


Appendix F.  Use of UTF-8

   After many years of debate, as a result of the perceived need to



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   accommodate certain DNS implementations that apparently couldn't
   handle any character that's not a letter, digit or hyphen (and
   apparently never would be updated to remedy this limitation) the
   unicast DNS community settled on an extremely baroque encoding called
   "Punycode" [RFC3492]. Punycode is a remarkably ingenious encoding
   solution, but it is complicated, hard to understand, and hard to
   implement, using sophisticated techniques including insertion unsort
   coding, generalized variable-length integers, and bias adaptation.
   The resulting encoding is remarkably compact given the constraints,
   but it's still not as good as simple straightforward UTF-8, and it's
   hard even to predict whether a given input string will encode to a
   Punycode string that fits within DNS's 63-byte limit, except by
   simply trying the encoding and seeing whether it fits. Indeed, the
   encoded size depends not only on the input characters, but on the
   order they appear, so the same set of characters may or may not
   encode to a legal Punycode string that fits within DNS's 63-byte
   limit, depending on the order the characters appear. This is
   extremely hard to present in a user interface that explains to users
   why one name is allowed, but another name containing the exact same
   characters is not. Neither Punycode nor any other of the "Ascii
   Compatible Encodings" proposed for Unicast DNS may be used in
   Multicast DNS packets. Any text being represented internally in some
   other representation must be converted to canonical precomposed UTF-8
   before being placed in any Multicast DNS packet.


Appendix G.  Governing Standards Body

   Note that this use of the ".local." suffix falls under IETF/IANA
   jurisdiction, not ICANN jurisdiction. DNS is an IETF network
   protocol, governed by protocol rules defined by the IETF. These IETF
   protocol rules dictate character set, maximum name length, packet
   format, etc. ICANN determines additional rules that apply when the
   IETF's DNS protocol is used on the public Internet. In contrast,
   private uses of the DNS protocol on isolated private networks are not
   governed by ICANN. Since this change is a change to the core DNS
   protocol rules, it affects everyone, not just those machines using
   the public Internet. Hence this change falls into the category of an
   IETF protocol rule, not an ICANN usage rule.












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   This allocation of responsibility is formally established in
   "Memorandum of Understanding Concerning the Technical Work of the
   Internet Assigned Numbers Authority" [RFC2860]. Exception (a) of
   clause 4.3 states that the IETF has the authority to instruct IANA to
   reserve pseudo-TLDs as required for protocol design purposes. For
   example, "Reserved Top Level DNS Names" [RFC2606] defines the
   following pseudo-TLDs:

      .test
      .example
      .invalid
      .localhost


Appendix H.  Private DNS Namespaces

   The special treatment of names ending in ".local." has been
   implemented in Macintosh computers since the days of Mac OS 9, and
   continues today in Mac OS X. There are also implementations for
   Microsoft Windows [B4W], Linux, and other platforms. Operators
   setting up private internal networks ("intranets") are advised that
   their lives may be easier if they avoid using the suffix ".local." in
   names in their private internal DNS server. Alternative possibilities
   include:

      .intranet
      .internal
      .private
      .corp
      .home
      .lan

   At sites where the DNS operator has decided to use the suffix
   ".local." for private internal names, clients can be configured to
   send both Multicast and Unicast DNS queries in parallel for these
   names. This allows names to be looked up both ways, but it is NOT
   RECOMMENDED because it results in additional network traffic and
   additional delays in name resolution, as well as potentially creating
   user confusion when it is not clear whether any given result was
   received via link-local multicast from a peer on the same link, or
   from the configured unicast name server.


Appendix I.  Deployment History

   Internet Draft "draft-cheshire-dnsext-multicastdns-00.txt" was
   published in July 2001, and later that same year an update to Mac OS
   9 added client support for Multicast DNS. If the user typed a name



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   such as "MyPrinter.local." into any piece of networking software that
   used the standard Mac OS 9 name lookup APIs, then those name lookup
   APIs would recognize the name as a dot-local name and query for it by
   sending simple one-shot Multicast DNS Queries to 224.0.0.251:5353.
   This enabled the user to, for example, enter the name
   "MyPrinter.local." into their web browser in order to view a
   printer's status and configuration web page, or enter the name
   "MyPrinter.local." into the printer setup utility to create a print
   queue for printing documents on that printer.

   Multicast DNS Responder software first began shipping to end users in
   volume with the launch of Mac OS X 10.2 Jaguar in August 2002, and
   network printer makers (who had historically supported AppleTalk in
   their network printers, and were receptive to IP-based technologies
   that could offer them similar ease-of-use) started adopting Multicast
   DNS shortly thereafter.

   In September 2002 Apple released the source code for the
   mDNSResponder daemon as Open Source under Apple's standard Apple
   Public Source License (APSL).

   Multicast DNS Responder software became available for Microsoft
   Windows users in June 2004 with the launch of Apple's "Rendezvous for
   Windows" (now "Bonjour for Windows"), both in executable form (a
   downloadable installer for end users) and as Open Source (one of the
   supported platforms within Apple's body of cross-platform code in the
   publicly-accessible mDNSResponder CVS source code repository) [B4W].

   In August 2006, Apple re-licensed the cross-platform mDNSResponder
   source code under the Apache License, Version 2.0.

   In addition to desktop and laptop computers running Mac OS X and
   Microsoft Windows, Multicast DNS is implemented in a wide range of
   hardware devices, such as Apple's "AirPort" wireless base stations,
   iPhone and iPad, and in home gateways from other vendors, network
   printers, network cameras, TiVo DVRs, etc.

   The Open Source community has produced many independent
   implementations of Multicast DNS, some in C like Apple's
   mDNSResponder daemon, and others in a variety of different languages
   including Java, Python, Perl, and C#/Mono.










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Authors' Addresses

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino
   California  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com


   Marc Krochmal
   Apple Inc.
   1 Infinite Loop
   Cupertino
   California  95014
   USA

   Phone: +1 408 974 4368
   Email: marc@apple.com





























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