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                                                         Stuart Cheshire
Document: draft-cheshire-dnsext-multicastdns-00.txt       Apple Computer
Expires 13th January 2002                                 13th July 2001

                 Performing DNS queries via IP Multicast

               <draft-cheshire-dnsext-multicastdns-00.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups.  Note that other groups may
   also distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   Distribution of this memo is unlimited.


Abstract

   Multicast DNS is a really obvious idea, whose time has finally come.
   This draft proposes one possible way of making it work.


1. Acknowledgements

   This work builds upon original work done on Multicast DNS by Bill
   Manning and Bill Woodcock.  The authors gratefully acknowledge their
   contribution to the current specification. Other contributors of
   valuable ideas include Bernard Aboba, Mark Andrews, Randy Bush, Levon
   Esibov, James Gilroy, Olafur Gudmundsson, Erik Guttman, Myron Hattig,
   Thomas Narten, Erik Nordmark and Dave Thaler.

   I apologize humbly to anyone who feels their work has not been
   properly credited and I offer to buy dinner or drinks in
   compensation.







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2. Introduction

   This is a rough first draft. Its purpose is to describe the proposed
   idea well enough for meaningful discussion to take place. As such,
   while feedback concerning typographical mistakes and similar minutiae
   is always appreciated, the reader is advised that it is probably
   unwise to waste a lot of time on such trivia until after we find out
   whether this proposal will even live long enough to become a
   'draft-01'.

   When reading this document, familiarity with the concepts of Zero
   Configuration Networking [ZC] and automatic link-local addressing
   [v4LL] [RFC 2462] is helpful.

   This document proposes no change to the structure of DNS messages,
   and no new operation codes, response codes, resource record types, or
   any other new DNS protocol values. This document simply discusses
   what needs to happen if DNS clients start sending DNS requests to a
   multicast address.

   The primary difference between this document and "draft-ietf-dnsext-
   mdns-01.txt" is the philosophy about how subdomains of the
   "local.arpa." domain are delegated. That document proposes that hosts
   running Multicast DNS Responders each assert an SOA record, thereby
   claiming to be the sole authority for their own little zone within
   the "local.arpa." domain. That approach makes it difficult for
   different hosts to manage two or more resource records with the same
   name, a feature that has some benefits. This document proposes that
   subdomains of the "local.arpa." domain can never be delegated, and
   instead "local.arpa." is managed as a single zone implemented by
   a loose collection of hosts cooperatively executing a distributed
   algorithm. From that philosophical difference, a variety of
   implementation differences emerge.

   There has been discussion of whether "local.arpa." is an appropriate
   domain to use. Perhaps it is not. Perhaps some other domain should,
   by IETF Standards Action, be declared a reserved name in the DNS
   protocol for this particular use. In any case, the text "local.arpa."
   in this document should be taken as a place holder for whatever
   reserved name or "domain" may eventually be allocated for this
   purpose.

   There has been discussion of how much burden Multicast DNS might
   impose on a network. It should be remembered that whenever IPv4 hosts
   communicate they broadcast ARP packets on the network on a regular
   basis, and this is not disastrous. The approximate amount of
   multicast traffic generated by hosts using Multicast DNS is
   anticipated to be roughly the same order of magnitude as the amount
   of broadcast ARP traffic those hosts already generate.



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3. 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" [RFC 2119].

   This document uses the term "host name" in the strict sense to mean a
   fully qualified domain name that has an 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.

4. Multicast DNS Names

   The DNS domain "local.arpa." is (this document proposes) a
   special domain with special semantics, namely that "local.arpa." and
   all its subdomains are link-local, and names within this domain are
   meaningful only on the link where they originate, much as IPv4
   addresses in the 169.254/16 prefix are link-local and meaningful
   only on the link where they originate.

   Any DNS query for a name within the "local.arpa." domain MUST be sent
   to the all-DNS multicast address (224.0.0.251 or its IPv6
   equivalent).

   It is unimportant whether a name within the "local.arpa." domain
   occurred because the user explicitly typed in a fully qualified
   domain name ending in "local.arpa.", or because the user entered an
   unqualified domain name and the host software appended the
   "local.arpa." search domain to it. The "local.arpa." domain could
   appear in the search list because the user manually configured it, or
   because it was received in a DHCP option, or via any other valid
   mechanism for configuring the DNS search list. In this respect the
   "local.arpa." domain is no different to any other search domain that
   might appear in the list.

   DNS queries for a names outside the "local.arpa." domain MAY be sent
   to the all-DNS 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. This is a contentious issue, and this document does not
   discuss it in detail, instead concentrating on the issue of resolving
   local names using DNS packets sent to a multicast address.

   A host which belongs to an organization that owns some portion of the
   DNS namespace can be assigned a globally unique name within that
   portion of the DNS namespace, for example, "cheshire.apple.com."
   Another host, attempting and failing to resolve that name via
   conventional unicast DNS MAY elect to try resolving it via multicast,

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   which may be successful if the two hosts happen to be on the same
   link.

   However, the majority home customers do not have easy access to any
   portion of the global DNS namespace within which they have the
   authority to create names as they wish. This leaves the majority of
   home computers effectively anonymous for practical purposes. These
   users MAY elect to give their computers link-local host names of the
   form: "single-dns-label.local.arpa." For example, my laptop computer
   answers to the name "stu.local.arpa." Any computer user is granted
   the authority to name their computer this way, providing 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 name conflict occurs on the link which
   is not resolved in the user's favour. When this happens, the computer
   (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.

   The point made in the previous paragraph is very important and bears
   repeating. It is easy for those of us in the IETF community who run
   our own name servers at home to forget that the majority of computer
   users do not run their own name server and have no easy way to create
   their own host names. When these users wish to transfer files between
   two laptop computers, they are frequently reduced to typing in
   dotted-decimal IP addresses because they simply have no other way
   for one host to refer to the other by name. This is a sorry state of
   affairs.

   Allowing ad-hoc allocation of single-label names in a single flat
   "local.arpa." namespace may seem to invite chaos. However,
   operational experience with AppleTalk NBP names, which on any given
   link are also effectively single-label names in a flat namespace,
   shows that in practice name collisions happen extremely rarely and
   are not a problem. Groups of computer users from disparate
   organizations bring Macintosh laptop computers to events such as IETF
   Meetings, the Mac Hack conference, the Apple World Wide Developer
   Conference, etc., and complaints at these events about users
   suffering conflicts and being forced to rename their machines have
   never been an issue.

   Enforcing uniqueness of host names (i.e. the names of DNS address
   records mapping names to IP addresses) is probably desirable in the
   common case, but this document does not mandate that. It is also
   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, but it is important that
   the Multicast DNS protocol allows hosts to verify and maintain unique
   names for resource records where that behaviour is desired, and to
   maintain multiple resource records with a single shared name where
   that behaviour is desired. This consideration applies to all resource
   records, not just address records (i.e. host names).

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5. IP TTL Checks

   A host sending a Multicast DNS request to a link-local address MUST
   verify that the TTL in reply packets is 255, and silently discard any
   reply packets where the TTL is not 255. 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.

   There has been some discussion that many current network programming
   APIs to not provide any indication of the TTL on received packets.
   This is unfortunate, and should be fixed for hosts that want to be
   able to guard against spoof packets arriving from off-link.

6. Reverse Address Mapping

   Like "local.arpa." the domain "254.169.in-addr.arpa." is defined to
   be link-local. Any DNS query for a name within the "254.169.in-addr.
   arpa." domain MUST be sent to the all-DNS multicast address
   224.0.0.251.

7. Requesting

   There are three kinds of Multicast DNS Requests, one-shot requests of
   the kind made by today's conventional DNS clients, one-shot requests
   accumulating multiple replies made by multicast-aware DNS clients,
   and continuous ongoing Multicast DNS Requests used by IP network
   browser software.

   A Multicast DNS Responder that is offering records that are intended
   to be unique on the local link MUST also implement a Multicast DNS
   Requester so that it can first verify the uniqueness of those records
   before it begins answering requests for them.

7.1 One-Shot Requests

   An unsophisticated DNS client may simply send its DNS requests
   blindly to the 224.0.0.251 multicast address, without necessarily
   even being aware what a multicast address is. Indeed, certain
   existing DNS clients (e.g. Mac and Windows) can be persuaded to do
   this even today, simply by the user typing in that address as the
   'name server address'.

   Such an unsophisticated DNS client may not get ideal behaviour. Such
   a client may simply take the first response it receives and fail to
   wait to see if there are more, but in many instances this may not be
   a serious problem. If a user types "http://stu.local.arpa." into
   their Web browser and gets to see the page they were hoping for, then
   the protocol has met the user's needs in this case.



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7.2 One-Shot Requests, Accumulating Multiple Replies

   A more sophisticated DNS client should understand that Multicast DNS
   is not exactly the same as unicast DNS, and should modify its
   behaviour in some simple ways.

   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 requests where more than one response is
   possible, and for these requests a more sophisticated 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 request only so long as it has
   received no reply. A more sophisticated 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 request an appropriate number of times at appropriate intervals
   until it is satisfied with the collection of responses it has
   gathered.

   A more sophisticated Multicast DNS client that is retransmitting a
   request for which is has already received some replies, MAY elect to
   implement duplicate suppression, as described below under "Duplicate
   Suppression". 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 request.

   A Multicast DNS Requester MAY place more than one question into the
   Question Section of a Multicast DNS Request.

7.3 Continuous Requesting

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

   Macintosh users are accustomed to opening the "Chooser" window,
   selecting a desired printer, and then closing the Chooser window.
   However, when the desired printer does not appear in the list, the
   user will typically leave the "Chooser" window open while they go and
   check to verify that the printer is plugged in, powered on, connected
   to the Ethernet, etc. While the user jiggles the wires, hits the
   Ethernet hub, and so forth, they keep an eye on the Chooser window,
   and when the printer name appears, they know they have fixed whatever
   the problem was. This can be a useful and intuitive troubleshooting
   technique, but a user who goes home for the weekend leaving the
   Chooser window open places a non-trivial burden on the network.

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   It is important that an IP network browser window displaying
   live information from the network using Multicast DNS, if left
   running for an extended period of time, should generate significantly
   less multicast traffic on the network than the old AppleTalk Chooser.

   A Multicast DNS Requester asking the same question repeatedly for an
   indefinite period of time MUST implement duplicate suppression, as
   described below.


8. Duplicate Suppression

   When a Multicast DNS Requester sends a request to which it already
   knows some answers, it populates the Answer Section of the DNS
   message with those cached resource records whose remaining TTL values
   indicate that they will remain valid for at least the time
   anticipated to send this DNS request, and the next, and the one after
   that. For example, if the Multicast DNS Requester is planning to wait
   four seconds after this request before sending the next, and then
   eight seconds after that, then only resource records with TTL values
   greater than twelve seconds should be included in the answer section.
   This is to ensure that when a resource record's TTL is close to
   expiration, the Multicast DNS Requester has *two* chances to refresh
   it before the cached record expires and has to be removed from the
   list.

   A Multicast DNS Responder SHOULD NOT answer a Multicast DNS Request
   if the answer it would give is already included in the Answer
   Section with a TTL at least half the correct value. If the TTL of the
   answer as given in the Answer Section is less than half of the real
   TTL as known by the Multicast DNS Responder, the responder SHOULD
   send an answer so as to update the Requester's cache before the
   record becomes in danger of expiration.

   A Multicast DNS Requester MUST NOT cache resource records observed in
   the Answer Section of other Multicast DNS Requests. The Answer
   Section of Multicast DNS Requests is not authoritative. By placing
   information in the Answer Section of a Multicast DNS Request the
   requester 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
   Requesters on the network would not be helpful.

   A Multicast DNS Responder that implements duplicate suppression
   SHOULD implement EDNS0 [RFC 2671] to allow larger-sized requests and
   replies.





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9. Responding

   A Multicast DNS Responder MUST only reply when it has a positive
   non-null response to send. Error responses must never be sent. The
   non-existence of any name in a Multicast DNS Domain is ascertained by
   the failure of any machine to respond to the Multicast DNS query, not
   by NXDOMAIN errors.

   A Multicast DNS Responder on Ethernet [IEEE802] and similar shared
   multiple access networks SHOULD delay its responses by a random
   amount of time selected with uniform random distribution in the range
   0-10ms. If multiple Multicast DNS Responders were all to immediately
   reply to a particular request, a collision would be virtually
   guaranteed. By imposing a small random delay, the number of
   collisions is dramatically reduced. 10ms is a short enough time that
   it is not perceptible to a human user, but long enough to
   significantly reduce the risk of Ethernet collisions. 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 transmission of its first 256 bits. On 10Mb/s
   Ethernet, this equates to a vulnerable time window of 25.6us.

   In the case where a Multicast DNS Responder has good reason to
   believe that it will be the only responder on the link with a
   positive non-null response, it MAY reply immediately, without the
   random delay. To do this safely, it MUST have previously verified
   that the requested name type and class in the DNS query are unique on
   this link. This may be appropriate for things like looking up the
   address record for a particular host name, when the host name has
   been previously verified unique. This is *not* appropriate for things
   like looking up PTR records used for DNS Service Discovery [NIAS],
   where a large number of responses may be anticipated.

   Multicast DNS Responses MUST be sent to UDP port 53 (the well-known
   port assigned to DNS) on the 224.0.0.251 multicast address. Operating
   in a Zeroconf environment requires constant vigilance. Just because a
   name has been previously verified unique does not mean 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 the source UDP port in a received Multicast DNS Request is not
   port 53, this suggests that the client originating the request is an
   old naive client that is not entirely aware that it is using a
   multicast address. (The host OS needs to understand what an IP
   multicast address is in order to hash it to the correct Ethernet
   multicast address, but the user-level DNS client software does not
   need to know anything about multicast to blindly send a UDP packet to
   the IP address 224.0.0.251.) In this case, after sending the usual


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   Multicast DNS Response to 224.0.0.251 port 53, the Multicast DNS
   Responder MUST also send a second identical UDP reply to the client
   via unicast to the request packet's source IP address and port.

   Multicast DNS Responders MUST correctly handle DNS request packets
   containing more than one question, by answering any or all of the
   questions to which they have answers.

   Multicast DNS Responders SHOULD implement EDNS0 [RFC 2671] to allow
   larger-sized requests and replies. Larger-sized requests are useful
   to allow longer duplicate suppression lists in the Answer Section.

10. Startup Procedure

   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 two startup steps.

   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
   its address record which maps its unique host name to its unique IP
   address. The ability to place more than one question in a Multicast
   DNS Request is useful here, because it can allow a host to use a
   single packet for all of its resource records instead of needing a
   separate packet for each. If any conflicting Multicast DNS replies
   are received, then the host MUST defer to the other host already
   using those names, and MUST select new names for its conflicting
   records which need to be unique. One second after the first query it
   should send a second, then two seconds after that a third. If, after
   a total of seven seconds, no conflicting Multicast DNS replies have
   been received, the host may move to the second step.

   The second startup step is that the Multicast DNS Responder SHOULD
   send a gratuitous Multicast DNS Response containing, in the Answer
   Section, all those resource records that may be of interest to other
   hosts on the link. One example of this is the PTR records used by DNS
   Service Discovery [NIAS]. Since other hosts running Multicast DNS
   Requesters may have network browser windows open using an extremely
   long interval between Multicast DNS Request packets, the reception of
   a gratuitous Multicast DNS Response from a new device starting up
   allows the browser window to update immediately instead of having to
   wait until the next request is sent.

   Up to ten of gratuitous Multicast DNS Responses may be sent,
   providing that the interval between gratuitous responses doubles
   with every response sent, and the interval between the first two
   gratuitous responses is not less than one second.


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   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".

   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.


11. Conflict Resolution

   A conflict occurs when two resource records with the same name, type
   and class have 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. In the case of a host desiring to
   have a unique host name, another address record with the same name
   but a different IP address is considered inconsistent.

   Whenever a Multicast DNS Responder receives any Multicast DNS
   response (gratuitous or otherwise) containing a conflicting resource
   record, the Multicast DNS Responder must cease using that record
   and potentially reconfigure.

   In the case of a typical laptop or desktop computer with a human
   user, reconfiguration is achieved by displaying an error message to
   the user and suggesting that they choose a new name. In the case of a
   device with no human operator, reconfiguration is achieved by its
   software programmatically generating a new name. In either case, the
   host must then test the new name for uniqueness as described above in
   "Startup Procedure".

   It is important that the host that believes there is a conflict be
   the one to take action. 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 configured to require a unique host name
   must be the one to reconfigure, since the other one doesn't view the
   sharing of address records as a conflict and hence sees no reason why
   it should reconfigure. This algorithm could result in situations
   where both hosts reconfigure, but this will be rare. The uniqueness
   check described above in "Startup Procedure" helps reduces resource
   record conflicts to only those cases where two separate links are
   connected together, or a previously partitioned link is re-joined.

   The examples in this section focus on address records (i.e. host
   names), but the same considerations apply to all resource records
   where uniqueness or some other defined constraint is desired.



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12. Special Characteristics of Multicast DNS Domains

   Unlike conventional DNS, the DNS domains "local.arpa." and "254.169.
   in-addr.arpa." have only local significance. Conventional 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. (However, split views,
   firewalls, intranets and the like have somewhat interfered with this
   goal of DNS representing a single universal truth.) In contrast, each
   IP link has its own private "local.arpa." and "254.169.in-addr.arpa."
   namespaces, and the answer to any query for a name within those
   domains depends on where that query is asked.

   Multicast DNS Domains are not delegated from their parent domain via
   use of NS records. Instead, all Multicast DNS Domains are delegated
   to the IP address 224.0.0.251 by (potential) IETF Standards Action
   (i.e. this document, should it become a standard). There are no NS
   records anywhere in Multicast DNS Domains.

   The name server for a Multicast DNS Domain is 224.0.0.251. This is a
   multicast address; therefore it identifies 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 cluster of loosely
   cooperating CPUs rather than as a single process running on a single
   CPU (or tightly coupled multiprocessor).

   No delegation is performed within Multicast DNS Domains. Because the
   cluster of loosely coordinated CPUs is cooperating to administer a
   single zone, no delegation is necessary or desirable. Just because a
   particular host on the network may answer queries for a particular
   record type with the name "example.local.arpa." does not imply
   anything about whether that host will answer for the name
   "child.example.local.arpa.", or indeed for other record types with
   the "example.local.arpa."

   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.

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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 support) the browsing and service
   discovery mechanisms proposed in "Discovering Named Instances of
   Abstract Services using DNS" [NIAS].

   This document places few limitations on what DNS record types may be
   looked up in the "local.arpa." domain. In particular, a Multicast DNS
   request for the SRV record named "_dns._udp.local.arpa." may yield
   the port number and host name (and thence IP address) of a
   conventional DNS server willing to perform general recursive DNS
   lookups. The benefit of using this mechanism rather than a DHCP
   option to configure a host's DNS server address is that using DHCP is
   an outward-looking solution that makes DNS dependent on another
   protocol, which may not be running on every network (e.g. an IPv6
   network using stateless address autoconfiguration [RFC 2462]).
   Locating a recursive DNS server using Multicast DNS is a self-
   sufficient solution that reduces DNS's need for support from other
   protocols. This possibility is not discussed futher here.

14. Resource Record TTL Values

   Multicast DNS resource records used in typical 'One-Shot' requests
   should generally have fairly low TTL values, on the order of seconds,
   rather than hours or days. The transient nature of Zeroconf networks
   [ZC] [v4LL] means that information can change at any time, and a host
   caching ancient stale resource records with unreasonably long TTL
   values could be left trying to work with hopelessly out-of-date
   information.

   Having hosts send gratuitous responses when configuration changes
   occur can somewhat mitigate this problem, but in the event of a
   network partition, or temporary signal fade in a wireless network, it
   is not safe to assume that all hosts will necessarily see all
   gratuitous responses.

   The one exception to this recommendation is resource records expected
   to be used to populate network browser lists, such as the PTR records
   used for DNS Service Discovery [NIAS]. Using short TTL values here
   would force the network browser to be continuously sending Multicast
   DNS Requests to refresh records before they expired from the list.
   In this case, the harm done by stale data due to high TTL values is
   relatively mild. The appearance of names in the network browser list
   is merely an assertion that the name exists now or has existed in the
   recent past. In order to actually use any named service, the client
   has to perform another DNS request to find the IP address, and in the
   case where the service has been forced to reconfigure to a new IP
   address (or has left the network entirely), the client will quickly
   discover that.

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15. Enabling and Disabling Multicast DNS

   The option to fail-over to Multicast DNS for names outside the
   "local.arpa." domain 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 in the
   "local.arpa." domain (or not) is controlled by whether or not
   "local.arpa." appears in the client's DNS search list.

   No special control is needed for enabling and disabling Multicast DNS
   for names within the "local.arpa." domain. The user doesn't need a
   way to disable Multicast DNS for names within the "local.arpa."
   domain, because if the user doesn't want to use Multicast DNS, they
   can achieve this by simply not using names that end in ".local.arpa."
   If a user *does* enter a name ending in ".local.arpa." into their Web
   browser, then we can safely assume their intention was probably that
   it should work. Having user configuration options that can be
   (intentionally or unintentionally) set so that this doesn'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 configure and too hard to use." This in turn
   perpetuates the continued use of protocols like AppleTalk, and
   there's no DHCP option to disable that! If we want to retire
   AppleTalk, we need to offer users equivalent IP functionality that
   they can rely on to, "always work, like AppleTalk." A little
   Multicast DNS traffic may be a burden on the network, but it is an
   insignificant burden compared to continued widespread use of
   AppleTalk.


16. Considerations for Multiple Interfaces

   A host should defend its host name (FQDN) on all active interfaces on
   which it is answering Multicast DNS requests.

   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.

   When answering a Multicast DNS request, 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 reply is valid for use
   on the interface on which the reply is going out.

   Just as the same link-local IP address may validly be in use
   simultaneously on different links, 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


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   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 which 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 request and/or
   on what interface a Multicast DNS reply was received.


17. DNS Message Format

   This section describes specific restrictions on the allowable
   values for the header fields of a Multicast DNS message.


17.1. 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, providing of course
   that normal TTL aging is performed on these cashed 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.


17.2. QR (Query/Response) Bit

   In query messages, MUST be zero.

   In response messages, MUST be one.




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17.3. OPCODE

   In both multicast query and multicast response messages, MUST be zero
   (only standard queries are currently supported over multicast, unless
   other queries are allowed by future IETF Standards Action).

17.4. 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 one -- not setting this bit implies there's some other
   place where 'better' information may be found.

17.5. TC (Truncated) Bit

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

   In response messages, if the message does not contain all the data
   the requester was looking for, the requester SHOULD open a TCP
   connection to the responder and repeat the query.

17.6. RD (Recursion Desired) Bit

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

17.7. 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.

17.8. Z (Zero) Bit

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

17.9. AD (Authentic Data) Bit [RFC 2535]

   In query messages the Authentic Data bit MUST be zero on
   transmission, and MUST be ignored on reception.

   In response messages, the Authentic Data bit MAY be set. Resolvers
   receiving response messages with the AD bit set MUST NOT trust the AD
   bit unless they trust the source of the message and either have a
   secure path to it or use DNS transaction security.


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17.10. CD (Checking Disabled) Bit [RFC 2535]

   In query messages, a resolver willing to do cryptography SHOULD set
   the Checking Disabled bit to permit it to impose its own policies.

   In response messages, the Checking Disabled bit MUST be zero on
   transmission, and MUST be ignored on reception.

17.11. 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.

18. 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.arpa." 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.arpa." 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.arpa."
   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 IPv4-only and IPv6-only hosts.

   There has been discussion of the proposal that in the IPv6 case, the
   all-DNS multicast address should not be a single address, but instead
   a range of addresses selected using a hash function of the name being
   looked for. There are some issues with this:

   1. The hash function must work correctly with both normal
   (case-insensitive) DNS labels and binary labels [RFC 2673].

   2. This may prevent more than one question being put into a single
   packet, since the different questions may hash to different multicast
   addresses.

   3. This impedes the ability to use a single multicast reply packet to
   answer the client and simultaneously facilitate ongoing conflict
   monitoring, because every client would have to listen on every
   multicast address in the range (or rapidly join and leave multicast
   groups on demand for each request) in order to receive the reply.

   4. This limits the ability to gain certain useful functionality out
   of old resolver software by configuring it with a single All-DNS
   multicast address to which it can send its queries.

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19. Security Considerations

   DNSSEC [RFC 2535] should be used where the authenticity of
   information is important.

   When DNS queries for names outside the "local.arpa." domain are sent
   to the all-DNS multicast address (during of 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
   within the "local.arpa." domain, because within this domain the user
   can be aware that 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.arpa.", if present.
   A malicious host could masquerade as "www.example.com" by answering
   the resulting Multicast DNS request for "www.example.com.local.arpa."
   To avoid this, a host MUST NOT append the search domain
   "local.arpa.", if present, to any relative (partially qualified)
   domain name containing two or more labels. Appending "local.arpa." to
   single-label relative domain names is acceptable, since the user
   should have no expectation that a single-label domain name will
   resolve as-is.

   [Lots more work to be done here!]


20. IANA Considerations

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

   We'd like the IANA to designate the DNS domain "local.arpa." a
   "Multicast Domain" with special semantics, namely that "local.arpa."
   and its subdomains are link-local, and names within this domain are
   meaningful only on the link where they originate, much as IPv4
   addresses in the 169.254/16 prefix are link-local and meaningful only
   on the link where they originate. Likewise we'd like the IANA to
   designate the DNS domain "254.169.in-addr.arpa." to be similarly
   link-local and non-delegated.

   No other IANA services are required by this document.




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21. Copyright

   Copyright (C) The Internet Society 8th March 2000.
   All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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22. References

   [IEEE802]  IEEE Standards for Local and Metropolitan Area Networks:
              Overview and Architecture.
              Institute of Electrical and Electronic Engineers,
              IEEE Standard 802, 1990.

   [NIAS]     S. Cheshire, "Discovering Named Instances of Abstract
              Services using DNS", Internet-Draft (work in progress),
              draft-cheshire-dnsext-nias-00.txt, July 2001.

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

   [RFC 2462] S. Thomson and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

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

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

   [RFC 2673] M. Crawford, "Binary Labels in the Domain Name System",
              RFC 2673, August 1999.

   [v4LL]     S. Cheshire and B. Aboba, "Dynamic Configuration of IPv4
              Link-Local Addresses", Internet-Draft (work in progress),
              draft-ietf-zeroconf-ipv4-linklocal-03.txt, June 2001.

   [ZC]       M. Hattig, "Zeroconf Requirements", Internet-Draft (work
              in progress), draft-ietf-zeroconf-reqts-08.txt, May 2001.


23. Author's Address

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

   Phone: +1 408 974 3207
   EMail: rfc@stuartcheshire.org







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