Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                          March                      September 13, 2017
Expires: September 14, 2017 March 17, 2018

       Discovery Proxy for Multicast DNS-Based Service Discovery


   This document specifies a mechanism that uses Multicast DNS to
   automatically populate the wide-area unicast Domain Name System
   namespace with records describing devices and services found on the
   local link.

Status of This Memo

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   This Internet-Draft will expire on September 14, 2017. March 17, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Operational Analogy . . . . . . . . . . . . . . . . . . . . .   6
   3.  Conventions and Terminology Used in this Document . . . . . .   7
   4.  Compatibility Considerations  . . . . . . . . . . . . . . . .   7
   5.  Discovery Proxy Operation . . . . . . . . . . . . . . . . . .   8
     5.1.  Delegated Subdomain for Service Discovery Records . . . .   9
     5.2.  Domain Enumeration  . . . . . . . . . . . . . . . . . . .  11
       5.2.1.  Domain Enumeration via Unicast Queries  . . . . . . .  11
       5.2.2.  Domain Enumeration via Multicast Queries  . . . . . .  13
     5.3.  Delegated Subdomain for LDH Host Names  . . . . . . . . .  14
     5.4.  Delegated Subdomain for Reverse Mapping . . . . . . . . .  16
     5.5.  Data Translation  . . . . . . . . . . . . . . . . . . . .  18
       5.5.1.  DNS TTL limiting  . . . . . . . . . . . . . . . . . .  18
       5.5.2.  Suppressing Unusable Records  . . . . . . . . . . . .  19
       5.5.3.  NSEC and NSEC3 queries  . . . . . . . . . . . . . . .  20
       5.5.4.  No Text Encoding Translation  . . . . . . . . . . . .  20
       5.5.5.  Application-Specific Data Translation . . . . . . . .  21
     5.6.  Answer Aggregation  . . . . . . . . . . . . . . . . . . .  23
   6.  Administrative DNS Records  . . . . . . . . . . . . . . . . .  26
     6.1.  DNS SOA (Start of Authority) Record . . . . . . . . . . .  26
     6.2.  DNS NS Records  . . . . . . . . . . . . . . . . . . . . .  27
     6.3.  DNS SRV Records . . . . . . . . . . . . . . . . . . . . .  27
   7.  DNSSEC Considerations . . . . . . . . . . . . . . . . . . . .  28
     7.1.  On-line signing only  . . . . . . . . . . . . . . . . . .  28
     7.2.  NSEC and NSEC3 Records  . . . . . . . . . . . . . . . . .  28
   8.  IPv6 Considerations . . . . . . . . . . . . . . . . . . . . .  29
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
     9.1.  Authenticity  . . . . . . . . . . . . . . . . . . . . . .  30
     9.2.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .  31
   10. Intelectual Property Rights . . . . . . . . . . . . . . . . .  32
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  33
     13.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .  36
     A.1.  Already Implemented and Deployed  . . . . . . . . . . . .  36
     A.2.  Already Implemented . . . . . . . . . . . . . . . . . . .  36
     A.3.  Partially Implemented . . . . . . . . . . . . . . . . . .  36
     A.4.  Not Yet Implemented . . . . . . . . . . . . . . . . . . .  37
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   Multicast DNS [RFC6762] and its companion technology DNS-based
   Service Discovery [RFC6763] were created to provide IP networking
   with the ease-of-use and autoconfiguration for which AppleTalk was
   well known [RFC6760] [ZC].

   For a small home network consisting of just a single link (or a few
   physical links bridged together to appear as a single logical link
   from the point of view of IP) Multicast DNS [RFC6762] is sufficient
   for client devices to look up the ".local" host names of peers on the
   same home network, and to use Multicast DNS-Based Service Discovery
   (DNS-SD) [RFC6763] to discover services offered on that home network.

   For a larger network consisting of multiple links that are
   interconnected using IP-layer routing instead of link-layer bridging,
   link-local Multicast DNS alone is insufficient because link-local
   Multicast DNS packets, by design, are not propagated onto other

   Using link-local multicast packets for Multicast DNS was a conscious
   design choice [RFC6762].  Even when limited to a single link,
   multicast traffic is still generally considered to be more expensive
   than unicast, because multicast traffic impacts many devices, instead
   of just a single recipient.  In addition, with some technologies like
   Wi-Fi [IEEE-11], multicast traffic is inherently less efficient and
   less reliable than unicast, because Wi-Fi multicast traffic is sent
   using the lower data rates, and is not acknowledged.  Multiplying the
   amount of expensive multicast traffic by flooding it across multiple
   links would make the traffic load even worse.

   Partitioning the network into many small links curtails the spread of
   expensive multicast traffic, but limits the discoverability of
   services.  Using a very large local link with thousands of hosts
   enables better service discovery, but at the cost of larger amounts
   of multicast traffic.

   Performing DNS-Based Service Discovery using purely Unicast DNS is
   more efficient and doesn't require excessively large multicast
   domains, but requires that the relevant data be available in the
   Unicast DNS namespace.  The Unicast DNS namespace in question could
   fall within a traditionally assigned globally unique domain name, or
   could use a private local unicast domain name such as ".home" ""

   In the DNS-SD specification [RFC6763], Section 10 ("Populating the
   DNS with Information") discusses various possible ways that a
   service's PTR, SRV, TXT and address records can make their way into
   the Unicast DNS namespace, including manual zone file configuration
   [RFC1034] [RFC1035], DNS Update [RFC2136] [RFC3007] and proxies of
   various kinds.

   Making the relevant data available in the Unicast DNS namespace by
   manual DNS configuration (as has been done for many years at IETF
   meetings to advertise the IETF Terminal Room printer) is labor
   intensive, error prone, and requires a reasonable degree of DNS

   Populating the Unicast DNS namespace via DNS Update by the devices
   offering the services themselves requires configuration of DNS Update
   keys on those devices, which has proven onerous and impractical for
   simple devices like printers and network cameras.

   Hence, to facilitate efficient and reliable DNS-Based Service
   Discovery, a compromise is needed that combines the ease-of-use of
   Multicast DNS with the efficiency and scalability of Unicast DNS.

   This document specifies a type of proxy called a "Multicast Discovery
   Proxy" (or just "Discovery Proxy") that uses Multicast DNS [RFC6762]
   to discover Multicast DNS records on its local link, and makes
   corresponding DNS records visible in the Unicast DNS namespace.

   In principle, similar mechanisms could be defined using other local
   service discovery protocols, to discover local information and then
   make corresponding DNS records visible in the Unicast DNS namespace.
   Such mechanisms for other local service discovery protocols could be
   addressed in future documents.

   The design of the Discovery Proxy is guided by the previously
   published Requirements for Scalable DNS-Based Service [RFC7558].

   In simple terms, a descriptive DNS name is chosen for each link in an
   organization.  Using a DNS NS record, responsibility for that DNS
   name is delegated to a Discovery Proxy physically attached to that
   link.  Now, when a remote client issues a unicast query for a name
   falling within the delegated subdomain, the normal DNS delegation
   mechanism results in the unicast query arriving at the Discovery
   Proxy, since it has been declared authoritative for those names.
   Now, instead of consulting a textual zone file on disk to discover
   the answer to the query, as a traditional DNS server would, a
   Discovery Proxy consults its local link, using Multicast DNS, to find
   the answer to the question.

   For fault tolerance reasons there may be more than one Discovery
   Proxy serving a given link.

   Note that the Discovery Proxy uses a "pull" model.  The local link is
   not queried using Multicast DNS until some remote client has
   requested that data.  In the idle state, in the absence of client
   requests, the Discovery Proxy sends no packets and imposes no burden
   on the network.  It operates purely "on demand".

   An alternative proposal that has been suggested is a proxy that
   performs DNS updates to a remote DNS server on behalf of the
   Multicast DNS devices on the local network.  The difficulty of this
   is that the proxy would have to be issuing all possible Multicast DNS
   queries all the time, to discover all the answers it needed to push
   up to the remote DNS server using DNS Update.  It would thus generate
   very high load on the network continuously, even when there were no
   clients with any interest in that data.

   Hence, having a model where the query comes to the Discovery Proxy is
   much more efficient than a model where the Discovery Proxy pushes the
   answers out to some other remote DNS server.

   A client seeking to discover services and other information achieves
   this by sending traditional DNS queries to the Discovery Proxy, or by
   sending DNS Push Notification subscription requests [PUSH].

2.  Operational Analogy

   A Discovery Proxy does not operate as a multicast relay, or multicast
   forwarder.  There is no danger of multicast forwarding loops that
   result in traffic storms, because no multicast packets are forwarded.
   A Discovery Proxy operates as a *proxy* for a remote client,
   performing queries on its behalf and reporting the results back.

   A reasonable analogy would be making a telephone call to a colleague
   at your workplace and saying, "I'm out of the office right now.
   Would you mind bringing up a printer browser window and telling me
   the names of the printers you see?"  That entails no risk of a
   forwarding loop causing a traffic storm, because no multicast packets
   are sent over the telephone call.

   A similar analogy, instead of enlisting another human being to
   initiate the service discovery operation on your behalf, would be to
   log into your own desktop work computer using screen sharing, and
   then run the printer browser yourself to see the list of printers.
   Or log in using ssh and type "dns-sd -B _ipp._tcp" and observe the
   list of discovered printer names.  In neither case is there any risk
   of a forwarding loop causing a traffic storm, because no multicast
   packets are being sent over the screen sharing or ssh connection.

   The Discovery Proxy provides another way of performing remote
   queries, just using a different protocol instead of screen sharing or

   When the Discovery Proxy software performs Multicast DNS operations,
   the exact same Multicast DNS caching mechanisms are applied as when
   any other client software on that Discovery Proxy device performs
   Multicast DNS operations, whether that be running a printer browser
   client locally, or a remote user running the printer browser client
   via a screen sharing connection, or a remote user logged in via ssh
   running a command-line tool like "dns-sd".

3.  Conventions and Terminology Used in this Document

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

   The Discovery Proxy builds on Multicast DNS, which works between
   hosts on the same link.  A set of hosts is considered to be "on the
   same link" if:

   o  when any host A from that set sends a packet to any other host B
      in that set, using unicast, multicast, or broadcast, the entire
      link-layer packet payload arrives unmodified, and

   o  a broadcast sent over that link by any host from that set of hosts
      can be received by every other host in that set

   The link-layer *header* may be modified, such as in Token Ring Source
   Routing [IEEE-5], but not the link-layer *payload*.  In particular,
   if any device forwarding a packet modifies any part of the IP header
   or IP payload then the packet is no longer considered to be on the
   same link.  This means that the packet may pass through devices such
   as repeaters, bridges, hubs or switches and still be considered to be
   on the same link for the purpose of this document, but not through a
   device such as an IP router that decrements the IP TTL or otherwise
   modifies the IP header.

4.  Compatibility Considerations

   No changes to existing devices are required to work with a Discovery

   Existing devices that advertise services using Multicast DNS work
   with Discovery Proxy.

   Existing clients that support DNS-Based Service Discovery over
   Unicast DNS work with Discovery Proxy.  Service Discovery over
   Unicast DNS was introduced in Mac OS X 10.4 in April 2005, as is
   included in Apple products introduced since then, including iPhone
   and iPad, as well as products from other vendors, such as Microsoft
   Windows 10.

5.  Discovery Proxy Operation

   In a typical configuration, a Discovery Proxy is configured to be
   authoritative [RFC1034] [RFC1035] for four DNS subdomains, and
   authority for these subdomains is delegated to it via NS records:

   A DNS subdomain for service discovery records.
      This subdomain name may contain rich text, including spaces and
      other punctuation.  This is because this subdomain name is used
      only in graphical user interfaces, where rich text is appropriate.

   A DNS subdomain for host name records.
      This subdomain name SHOULD be limited to letters, digits and
      hyphens, to facilitate convenient use of host names in command-
      line interfaces.

   A DNS subdomain for IPv6 Reverse Mapping records.
      This subdomain name will be a name that ends in ""

   A DNS subdomain for IPv4 Reverse Mapping records.
      This subdomain name will be a name that ends in ""

   In an enterprise network the naming and delegation of these
   subdomains is typically performed by conscious action of the network
   administrator.  In a home network naming and delegation would
   typically be performed using some automatic configuration mechanism
   such as HNCP [RFC7788].

   These three varieties of delegated subdomains (service discovery,
   host names, and reverse mapping) are described below in sections
   Section 5.1, Section 5.3 and Section 5.4.

   How a client discovers where to issue its service discovery queries
   is described below in section Section 5.2.

5.1.  Delegated Subdomain for Service Discovery Records

   In its simplest form, each link in an organization is assigned a
   unique Unicast DNS domain name, such as "Building" or
   "2nd Floor.Building".  Grouping multiple links under a
   single Unicast DNS domain name is to be specified in a future
   companion document, but for the purposes of this document, assume
   that each link has its own unique Unicast DNS domain name.  In a
   graphical user interface these names are not displayed as strings
   with dots as shown above, but something more akin to a typical file
   browser graphical user interface (which is harder to illustrate in a
   text-only document) showing folders, subfolders and files in a file

    | ** |  Building 1  |  1st Floor  | Alice's printer   |
    |               |  Building 2  | *2nd Floor* | Bob's printer     |
    |               | *Building 3* |  3rd Floor  | Charlie's printer |
    |               |  Building 4  |  4th Floor  |                   |
    |               |  Building 5  |             |                   |
    |               |  Building 6  |             |                   |

                        Figure 1: Illustrative GUI

   Each named link in an organization has one or more Discovery Proxies
   which serve it.  This Discovery Proxy function for each link could be
   performed by a device like a router or switch that is physically
   attached to that link.  In the parent domain, NS records are used to
   delegate ownership of each defined link name
   (e.g., "Building") to the one or more Discovery Proxies
   that serve the named link.  In other words, the Discovery Proxies are
   the authoritative name servers for that subdomain.

   With appropriate VLAN configuration [IEEE-1Q] a single Discovery
   Proxy device could have a logical presence on many links, and serve
   as the Discovery Proxy for all those links.  In such a configuration
   the Discovery Proxy device would have a single physical Ethernet
   [IEEE-3] port, configured as a VLAN trunk port, which would appear to
   software on that device as multiple virtual Ethernet interfaces, one
   connected to each of the VLAN links.

   When a DNS-SD client issues a Unicast DNS query to discover services
   in a particular Unicast DNS subdomain
   (e.g., "_printer._tcp.Building PTR ?") the normal DNS
   delegation mechanism results in that query being forwarded until it
   reaches the delegated authoritative name server for that subdomain,
   namely the Discovery Proxy on the link in question.  Like a
   conventional Unicast DNS server, a Discovery Proxy implements the
   usual Unicast DNS protocol [RFC1034] [RFC1035] over UDP and TCP.
   However, unlike a conventional Unicast DNS server that generates
   answers from the data in its manually-configured zone file, a
   Discovery Proxy generates answers using Multicast DNS.  A Discovery
   Proxy does this by consulting its Multicast DNS cache and/or issuing
   Multicast DNS queries for the corresponding Multicast DNS name, type
   and class, (e.g., in this case, "_printer._tcp.local. PTR ?").  Then,
   from the received Multicast DNS data, the Discovery Proxy synthesizes
   the appropriate Unicast DNS response.  How long the Discovery Proxy
   should wait to accumulate Multicast DNS responses is described below
   in section Section 5.6.

   Naturally, the existing Multicast DNS caching mechanism is used to
   minimize unnecessary Multicast DNS queries on the wire.  The
   Discovery Proxy is acting as a client of the underlying Multicast DNS
   subsystem, and benefits from the same caching and efficiency measures
   as any other client using that subsystem.

5.2.  Domain Enumeration

   A DNS-SD client performs Domain Enumeration [RFC6763] via certain PTR
   queries, using both unicast and multicast.  If it receives a Domain
   Name configuration via DHCP option 15 [RFC2132], then it issues
   unicast queries using this domain.  It issues unicast queries using
   names derived from its IPv6 prefix(es) and IPv4 subnet address(es).
   These are described below in Section 5.2.1.  It also issues multicast
   Domain Enumeration queries in the "local" domain [RFC6762].  These
   are described below in Section 5.2.2.  The results of all the Domain
   Enumeration queries are combined for Service Discovery purposes.

5.2.1.  Domain Enumeration via Unicast Queries

   The administrator creates Domain Enumeration PTR records [RFC6763] to
   inform clients of available service discovery domains, e.g.,:    PTR   Building
                                      PTR   Building
                                      PTR   Building
                                      PTR   Building   PTR   Building   PTR   Building

   The "b" ("browse") records tell the client device the list of
   browsing domains to display for the user to select from and the "db"
   ("default browse") record tells the client device which domain in
   that list should be selected by default.  The "lb" ("legacy browse")
   record tells the client device which domain to automatically browse
   on behalf of applications that don't implement UI for multi-domain
   browsing (which is most of them, as of 2017).  The "lb" domain is
   often the same as the "db" domain, or sometimes the "db" domain plus
   one or more others that should be included in the list of automatic
   browsing domains for legacy clients.

   DNS responses are limited to a maximum size of 65535 bytes.  This
   limits the maximum number of domains that can be returned for a
   Domain Enumeration query, as follows:

   A DNS response header is 12 bytes.  That's typically followed by a
   single qname (up to 256 bytes) plus qtype (2 bytes) and qclass
   (2 bytes), leaving 65275 for the Answer Section.

   An Answer Section Resource Record consists of:

   o  Owner name, encoded as a two-byte compression pointer
   o  Two-byte rrtype (type PTR)
   o  Two-byte rrclass (class IN)
   o  Four-byte ttl
   o  Two-byte rdlength
   o  rdata (domain name, up to 256 bytes)

   This means that each Resource Record in the Answer Section can take
   up to 268 bytes total, which means that the Answer Section can
   contain, in the worst case, no more than 243 domains.

   In a more typical scenario, where the domain names are not all
   maximum-sized names, and there is some similarity between names so
   that reasonable name compression is possible, each Answer
   Section Resource Record may average 140 bytes, which means that the
   Answer Section can contain up to 466 domains.

   It is anticipated that this should be sufficient for even a large
   corporate network or university campus.

5.2.2.  Domain Enumeration via Multicast Queries

   Since a Discovery Proxy exists on many, if not all, the links in an
   enterprise, it offers an additional way to provide Domain Enumeration
   data for clients.

   A Discovery Proxy can be configured to generate Multicast DNS
   responses for the following Multicast DNS Domain Enumeration queries
   issued by clients:

       b._dns-sd._udp.local.    PTR   ?
       db._dns-sd._udp.local.   PTR   ?
       lb._dns-sd._udp.local.   PTR   ?

   This provides the ability for Discovery Proxies to indicate
   recommended browsing domains to DNS-SD clients on a per-link
   granularity.  In some enterprises it may be preferable to provide
   this per-link configuration data in the form of Discovery Proxy
   configuration, rather than populating the Unicast DNS servers with
   the same data (in the "" or "" domains).

   Regardless of how the network operator chooses to provide this
   configuration data, clients will perform Domain Enumeration via both
   unicast and multicast queries, and then combine the results of these

5.3.  Delegated Subdomain for LDH Host Names

   DNS-SD service instance names and domains are allowed to contain
   arbitrary Net-Unicode text [RFC5198], encoded as precomposed UTF-8

   Users typically interact with service discovery software by viewing a
   list of discovered service instance names on a display, and selecting
   one of them by pointing, touching, or clicking.  Similarly, in
   software that provides a multi-domain DNS-SD user interface, users
   view a list of offered domains on the display and select one of them
   by pointing, touching, or clicking.  To use a service, users don't
   have to remember domain or instance names, or type them; users just
   have to be able to recognize what they see on the display and touch
   or click on the thing they want.

   In contrast, host names are often remembered and typed.  Also, host
   names have historically been used in command-line interfaces where
   spaces can be inconvenient.  For this reason, host names have
   traditionally been restricted to letters, digits and hyphens (LDH),
   with no spaces or other punctuation.

   While we still want to allow rich text for DNS-SD service instance
   names and domains, it is advisable, for maximum compatibility with
   existing usage, to restrict host names to the traditional letter-
   digit-hyphen rules.  This means that while a service name
   "My Printer._ipp._tcp.Building" is acceptable and
   desirable (it is displayed in a graphical user interface as an
   instance called "My Printer" in the domain "Building 1" at
   ""), a host name "My-Printer.Building" is
   less desirable (because of the space in "Building 1").

   To accomodate this difference in allowable characters, a Discovery
   Proxy SHOULD support having two separate subdomains delegated to it
   for each link it serves, one whose name is allowed to contain
   arbitrary Net-Unicode text [RFC5198], and a second more constrained
   subdomain whose name is restricted to contain only letters, digits,
   and hyphens, to be used for host name records (names of 'A' and
   'AAAA' address records).

   For example, a Discovery Proxy could have the two subdomains
   "Building" and "" delegated to it.
   The Discovery Proxy would then translate these two Multicast DNS

      My Printer._ipp._tcp.local. SRV 0 0 631 prnt.local.
      prnt.local.                 A

   into Unicast DNS records as follows:

      My Printer._ipp._tcp.Building
                                  SRV 0 0 631     A

   Note that the SRV record name is translated using the rich-text
   domain name ("Building") and the address record name is
   translated using the LDH domain ("").

   A Discovery Proxy MAY support only a single rich text Net-Unicode
   domain, and use that domain for all records, including 'A' and 'AAAA'
   address records, but implementers choosing this option should be
   aware that this choice may produce host names that are awkward to use
   in command-line environments.  Whether this is an issue depends on
   whether users in the target environment are expected to be using
   command-line interfaces.

   A Discovery Proxy MUST NOT be restricted to support only a letter-
   digit-hyphen subdomain, because that results in an unnecessarily poor
   user experience.

5.4.  Delegated Subdomain for Reverse Mapping

   A Discovery Proxy can facilitate easier management of reverse mapping
   domains, particularly for IPv6 addresses where manual management may
   be more onerous than it is for IPv4 addresses.

   To achieve this, in the parent domain, NS records are used to
   delegate ownership of the appropriate reverse mapping domain to the
   Discovery Proxy.  In other words, the Discovery Proxy becomes the
   authoritative name server for the reverse mapping domain.  For fault
   tolerance reasons there may be more than one Discovery Proxy serving
   a given link.

   For example, if a given link is using the
   IPv6 prefix 2001:0DB8:1234:5678/64,
   then the domain ""
   is delegated to the Discovery Proxy for that link.

   If a given link is using the IPv4 subnet 203.0.113/24,
   then the domain ""
   is delegated to the Discovery Proxy for that link.

   When a reverse mapping query arrives at the Discovery Proxy, it
   issues the identical query on its local link as a Multicast DNS
   query.  The mechanism to force an apparently unicast name to be
   resolved using link-local Multicast DNS varies depending on the API
   set being used.  For example, in the "/usr/include/dns_sd.h" APIs
   (available on macOS, iOS, Bonjour for Windows, Linux and Android),
   using kDNSServiceFlagsForceMulticast indicates that the
   DNSServiceQueryRecord() call should perform the query using Multicast
   DNS.  Other APIs sets have different ways of forcing multicast
   queries.  When the host owning that IPv6 or IPv4 address responds
   with a name of the form "something.local", the Discovery Proxy
   rewrites that to use its configured LDH host name domain instead of
   "local", and returns the response to the caller.

   For example, a Discovery Proxy with the two subdomains
   "" and "" delegated to it
   would translate this Multicast DNS record: PTR prnt.local.

   into this Unicast DNS response: PTR

   Subsequent queries for the address record,
   falling as it does within the domain, which is
   delegated to the Discovery Proxy, will arrive at the Discovery Proxy,
   where they are answered by issuing Multicast DNS queries and using
   the received Multicast DNS answers to synthesize Unicast DNS
   responses, as described above.

5.5.  Data Translation

   Generating the appropriate Multicast DNS queries involves,
   at the very least, translating from the configured DNS domain
   (e.g., "Building") on the Unicast DNS side to "local"
   on the Multicast DNS side.

   Generating the appropriate Unicast DNS responses involves translating
   back from "local" to the appropriate configured DNS Unicast domain.

   Other beneficial translation and filtering operations are described

5.5.1.  DNS TTL limiting

   For efficiency, Multicast DNS typically uses moderately high DNS TTL
   values.  For example, the typical TTL on DNS-SD PTR records is 75
   minutes.  What makes these moderately high TTLs acceptable is the
   cache coherency mechanisms built in to the Multicast DNS protocol
   which protect against stale data persisting for too long.  When a
   service shuts down gracefully, it sends goodbye packets to remove its
   PTR records immediately from neighbouring caches.  If a service shuts
   down abruptly without sending goodbye packets, the Passive
   Observation Of Failures (POOF) mechanism described in Section 10.5 of
   the Multicast DNS specification [RFC6762] comes into play to purge
   the cache of stale data.

   A traditional Unicast DNS client on a remote link does not get to
   participate in these Multicast DNS cache coherency mechanisms on the
   local link.  For traditional Unicast DNS queries (those received
   without using Long-Lived Query [LLQ] or DNS Push Notification [PUSH])
   the DNS TTLs reported in the resulting Unicast DNS response SHOULD be
   capped to be no more than ten seconds.

   Similarly, for negative responses, the negative caching TTL indicated
   in the SOA record [RFC2308] should also be ten seconds (Section 6.1).

   This value of ten seconds is chosen based on user-experience

   For negative caching, suppose a user is attempting to access a remote
   device (e.g., a printer), and they are unsuccessful because that
   device is powered off.  Suppose they then place a telephone call and
   ask for the device to be powered on.  We want the device to become
   available to the user within a reasonable time period.  It is
   reasonable to expect it to take on the order of ten seconds for a
   simple device with a simple embedded operating system to power on.
   Once the device is powered on and has announced its presence on the
   network via Multicast DNS, we would like it to take no more than a
   further ten seconds for stale negative cache entries to expire from
   Unicast DNS caches, making the device available to the user desiring
   to access it.

   Similar reasoning applies to capping positive TTLs at ten seconds.
   In the event of a device moving location, getting a new DHCP address,
   or other renumbering events, we would like the updated information to
   be available to remote clients in a relatively timely fashion.

   However, network administrators should be aware that many recursive
   (caching) DNS servers by default are configured to impose a minimum
   TTL of 30 seconds.  If stale data appears to be persisting in the
   network to the extent that it adversely impacts user experience,
   network administrators are advised to check the configuration of
   their recursive DNS servers.

   For received Unicast DNS queries that use LLQ or DNS Push
   Notification, the Multicast DNS record's TTL SHOULD be returned
   unmodified, because the Push Notification channel exists to inform
   the remote client as records come and go.  For further details about
   Long-Lived Queries, and its newer replacement, DNS Push
   Notifications, see Section 5.6.

5.5.2.  Suppressing Unusable Records

   A Discovery Proxy SHOULD suppress Unicast DNS answers for records
   that are not useful outside the local link.  For example, DNS AAAA
   and A records for IPv6 link-local addresses [RFC4862] and IPv4 link-
   local addresses [RFC3927] SHOULD be suppressed.  Similarly, for sites
   that have multiple private address realms [RFC1918], in cases where
   the Discovery Proxy can determine that the querying client is in a
   different address realm, private addresses MUST NOT be communicated
   to that client.  IPv6 Unique Local Addresses [RFC4193] SHOULD be
   suppressed in cases where the Discovery Proxy can determine that the
   querying client is in a different IPv6 address realm.

   By the same logic, DNS SRV records that reference target host names
   that have no addresses usable by the requester should be suppressed,
   and likewise, DNS PTR records that point to unusable SRV records
   should be similarly be suppressed.

5.5.3.  NSEC and NSEC3 queries

   Since a Discovery Proxy only knows what names exist on the local link
   by issuing queries for them, and since it would be impractical to
   issue queries for every possible name just to find out which names
   exist and which do not, a Discovery Proxy cannot programatically
   generate the traditional NSEC and NSEC3 records which assert the
   nonexistence of a large range of names.

   When queried for an NSEC or NSEC3 record type, the Discovery Proxy
   issues a qtype "ANY" query using Multicast DNS on the local link, and
   then generates an NSEC or NSEC3 response signifying which record
   types do and do not exist just the specific name queried, and no

   Multicast DNS NSEC records received on the local link MUST NOT be
   forwarded unmodified to a unicast querier, because there are slight
   differences in the NSEC record data.  In particular, Multicast DNS
   NSEC records do not have the NSEC bit set in the Type Bit Map,
   whereas conventional Unicast DNS NSEC records do have the NSEC bit

5.5.4.  No Text Encoding Translation

   A Discovery Proxy does no translation between text encodings.
   Specifically, a Discovery Proxy does no translation between Punycode
   and UTF-8, either in the owner name of DNS records, or anywhere in
   the RDATA of DNS records (such as the RDATA of PTR records, SRV
   records, NS records, or other record types like TXT, where it is
   ambiguous whether the RDATA may contain DNS names).  All bytes are
   treated as-is, with no attempt at text encoding translation.  A
   client implementing DNS-based Service Discovery [RFC6763] will use
   UTF-8 encoding for its service discovery queries, which the Discovery
   Proxy passes through without any text encoding translation to the
   Multicast DNS subsystem.  Responses from the Multicast DNS subsystem
   are similarly returned, without any text encoding translation, back
   to the requesting client.

5.5.5.  Application-Specific Data Translation

   There may be cases where Application-Specific Data Translation is

   For example, AirPrint printers tend to advertise fairly verbose
   information about their capabilities in their DNS-SD TXT record.  TXT
   record sizes in the range 500-1000 bytes are not uncommon.  This
   information is a legacy from LPR printing, because LPR does not have
   in-band capability negotiation, so all of this information is
   conveyed using the DNS-SD TXT record instead.  IPP printing does have
   in-band capability negotiation, but for convenience printers tend to
   include the same capability information in their IPP DNS-SD TXT
   records as well.  For local mDNS use this extra TXT record
   information is inefficient, but not fatal.  However, when a Discovery
   Proxy aggregates data from multiple printers on a link, and sends it
   via unicast (via UDP or TCP) this amount of unnecessary TXT record
   information can result in large responses.  A DNS reply over TCP
   carrying information about 70 printers with an average of 700 bytes
   per printer adds up to about 50 kilobytes of data.  Therefore, a
   Discovery Proxy that is aware of the specifics of an application-
   layer protocol such as AirPrint (which uses IPP) can elide
   unnecessary key/value pairs from the DNS-SD TXT record for better
   network efficiency.

   Also, the DNS-SD TXT record for many printers contains an "adminurl"
   key something like "adminurl=http://printername.local/status.html".
   For this URL to be useful outside the local link, the embedded
   ".local" hostname needs to be translated to an appropriate name with
   larger scope.  It is easy to translate ".local" names when they
   appear in well-defined places, either as a record's name, or in the
   rdata of record types like PTR and SRV.  In the printing case, some
   application-specific knowledge about the semantics of the "adminurl"
   key is needed for the Discovery Proxy to know that it contains a name
   that needs to be translated.  This is somewhat analogous to the need
   for NAT gateways to contain ALGs (Application-Specific Gateways) to
   facilitate the correct translation of protocols that embed addresses
   in unexpected places.

   As is the case with NAT ALGs, protocol designers are advised to avoid
   communicating names and addresses in nonstandard locations, because
   those "hidden" names and addresses are at risk of not being
   translated when necessary, resulting in operational failures.  In the
   printing case, the operational failure of failing to translate the
   "adminurl" key correctly is that, when accessed from a different
   link, printing will still work, but clicking the "Admin" UI button
   will fail to open the printer's administration page.  Rather than
   duplicating the host name from the service's SRV record in its
   "adminurl" key, thereby having the same host name appear in two
   places, a better design might have been to omit the host name from
   the "adminurl" key, and instead have the client implicitly substitute
   the target host name from the service's SRV record in place of a
   missing host name in the "adminurl" key.  That way the desired host
   name only appears once, and it is in a well-defined place where
   software like the Discovery Proxy is expecting to find it.

   Note that this kind of Application-Specific Data Translation is
   expected to be very rare.  It is the exception, rather than the rule.
   This is an example of a common theme in computing.  It is frequently
   the case that it is wise to start with a clean, layered design, with
   clear boundaries.  Then, in certain special cases, those layer
   boundaries may be violated, where the performance and efficiency
   benefits outweigh the inelegance of the layer violation.

   These layer violations are optional.  They are done primarily for
   efficiency reasons, and generally should not be required for correct
   operation.  A Discovery Proxy MAY operate solely at the mDNS layer,
   without any knowledge of semantics at the DNS-SD layer or above.

5.6.  Answer Aggregation

   In a simple analysis, simply gathering multicast answers and
   forwarding them in a unicast response seems adequate, but it raises
   the question of how long the Discovery Proxy should wait to be sure
   that it has received all the Multicast DNS answers it needs to form a
   complete Unicast DNS response.  If it waits too little time, then it
   risks its Unicast DNS response being incomplete.  If it waits too
   long, then it creates a poor user experience at the client end.  In
   fact, there may be no time which is both short enough to produce a
   good user experience and at the same time long enough to reliably
   produce complete results.

   Similarly, the Discovery Proxy -- the authoritative name server for
   the subdomain in question -- needs to decide what DNS TTL to report
   for these records.  If the TTL is too long then the recursive
   (caching) name servers issuing queries on behalf of their clients
   risk caching stale data for too long.  If the TTL is too short then
   the amount of network traffic will be more than necessary.  In fact,
   there may be no TTL which is both short enough to avoid undesirable
   stale data and at the same time long enough to be efficient on the

   Both these dilemmas are solved by use of DNS Long-Lived Queries
   (DNS LLQ) [LLQ] or its newer replacement, DNS Push Notifications

   Clients supporting unicast DNS Service Discovery SHOULD implement DNS
   Push Notifications [PUSH] for improved user experience.

   Clients and Discovery Proxies MAY support both DNS LLQ and DNS Push,
   and when talking to a Discovery Proxy that supports both, the client
   may use either protocol, as it chooses, though it is expected that
   only DNS Push will continue to be supported in the long run.

   When a Discovery Proxy receives a query using DNS LLQ or DNS Push
   Notification, it responds immediately using the Multicast DNS records
   it already has in its cache (if any).  This provides a good client
   user experience by providing a near-instantaneous response.
   Simultaneously, the Discovery Proxy issues a Multicast DNS query on
   the local link to discover if there are any additional Multicast DNS
   records it did not already know about.  Should additional Multicast
   DNS responses be received, these are then delivered to the client
   using additional DNS LLQ or DNS Push Notification update messages.
   The timeliness of such update messages is limited only by the
   timeliness of the device responding to the Multicast DNS query.  If
   the Multicast DNS device responds quickly, then the update message is
   delivered quickly.  If the Multicast DNS device responds slowly, then
   the update message is delivered slowly.  The benefit of using update
   messages is that the Discovery Proxy can respond promptly because it
   doesn't have to delay its unicast response to allow for the expected
   worst-case delay for receiving all the Multicast DNS responses.  Even
   if a proxy were to try to provide reliability by assuming an
   excessively pessimistic worst-case time (thereby giving a very poor
   user experience) there would still be the risk of a slow Multicast
   DNS device taking even longer than that (e.g., a device that is not
   even powered on until ten seconds after the initial query is
   received) resulting in incomplete responses.  Using update message
   solves this dilemma: even very late responses are not lost; they are
   delivered in subsequent update messages.

   There are two factors that determine specifically how responses are

   The first factor is whether the query from the client used LLQ or DNS
   Push Notification (typical with long-lived service browsing PTR
   queries) or not (typical with one-shot operations like SRV or address
   record queries).  Note that queries using LLQ or DNS Push
   Notification are received directly from the client.  Queries not
   using LLQ or DNS Push Notification are generally received via the
   client's configured recursive (caching) name server.

   The second factor is whether the Discovery Proxy already has at least
   one record in its cache that positively answers the question.

   o  Not using LLQ or Push Notification; no answer in cache:
      Issue an mDNS query, exactly as a local client would issue an mDNS
      query on the local link for the desired record name, type and
      class, including retransmissions, as appropriate, according to the
      established mDNS retransmission schedule [RFC6762].  As soon as
      any Multicast DNS response packet is received that contains one or
      more positive answers to that question (with or without the Cache
      Flush bit [RFC6762] set), or a negative answer (signified via a
      Multicast DNS NSEC record [RFC6762]), the Discovery Proxy
      generates a Unicast DNS response packet containing the
      corresponding (filtered and translated) answers and sends it to
      the remote client.  If after six seconds no Multicast DNS answers
      have been received, return a negative response to the remote
      client.  Six seconds is enough time to transmit three mDNS
      queries, and allow some time for responses to arrive.
      DNS TTLs in responses are capped to at most ten seconds.

   o  Not using LLQ or Push Notification; at least one answer in cache:

      Send response right away to minimise delay.
      DNS TTLs in responses are capped to at most ten seconds.
      No local mDNS queries are performed.
      (Reasoning: Given RRSet TTL harmonisation, if the proxy has one
      Multicast DNS answer in its cache, it can reasonably assume that
      it has all of them.)

   o  Using LLQ or Push Notification; no answer in cache:
      As in the case above with no answer in the cache, perform mDNS
      querying for six seconds, and send a response to the remote client
      as soon as any relevant mDNS response is received.
      If after six seconds no relevant mDNS response has been received,
      return negative response to the remote client (for LLQ; not
      applicable for PUSH).
      (Reasoning: We don't need to rush to send an empty answer.)
      Whether or not a relevant mDNS response is received within six
      seconds, the query remains active for as long as the client
      maintains the LLQ or PUSH state, and if mDNS answers are received
      later, LLQ or PUSH update messages are sent.
      DNS TTLs in responses are returned unmodified.

   o  Using LLQ or Push Notification; at least one answer in cache:
      As in the case above with at least one answer in cache, send
      response right away to minimise delay.
      The query remains active for as long as the client maintains the
      LLQ or PUSH state, and if additional mDNS answers are received
      later, LLQ or PUSH update messages are sent.
      (Reasoning: We want UI that is displayed very rapidly, yet
      continues to remain accurate even as the network environment
      DNS TTLs in responses are returned unmodified.

   Note that the "negative responses" referred to above are "no error no
   answer" negative responses, not NXDOMAIN.  This is because the
   Discovery Proxy cannot know all the Multicast DNS domain names that
   may exist on a link at any given time, so any name with no answers
   may have child names that do exist, making it an "empty nonterminal"

6.  Administrative DNS Records

6.1.  DNS SOA (Start of Authority) Record

   The MNAME field SHOULD contain the host name of the Discovery Proxy
   device (i.e., the same domain name as the rdata of the NS record
   delegating the relevant zone(s) to this Discovery Proxy device).

   The RNAME field SHOULD contain the mailbox of the person responsible
   for administering this Discovery Proxy device.

   The SERIAL field MUST be zero.

   Zone transfers are undefined for Discovery Proxy zones, and
   consequently the REFRESH, RETRY and EXPIRE fields have no useful
   meaning for Discovery Proxy zones.  These fields SHOULD contain
   reasonable default values.  The RECOMMENDED values are: REFRESH 7200,
   RETRY 3600, EXPIRE 86400.

   The MINIMUM field (used to control the lifetime of negative cache
   entries) SHOULD contain the value 10.  The value of ten seconds is
   chosen based on user-experience considerations (see Section 5.5.1).

   In the event that there are multiple Discovery Proxy devices on a
   link for fault tolerance reasons, this will result in clients
   receiving inconsistent SOA records (different MNAME, and possibly
   RNAME) depending on which Discovery Proxy answers their SOA query.
   However, since clients generally have no reason to use the MNAME or
   RNAME data, this is unlikely to cause any problems.

6.2.  DNS NS Records

   In the event that there are multiple Discovery Proxy devices on a
   link for fault tolerance reasons, the parent zone MUST be configured
   with glue records giving the names and addresses of all the Discovery
   Proxy devices on the link.

   Each Discovery Proxy device MUST be configured with its own NS
   record, and with the NS records of its fellow Discovery Proxy devices
   on the same link, so that it can return the correct answers for NS

6.3.  DNS SRV Records

   In the event that a Discovery Proxy implements Long-Lived Queries
   [LLQ] and/or DNS Push Notifications [PUSH] (as most SHOULD) they MUST
   generate answers for the appropriate corresponding
   _dns-llq._udp.<zone> and/or _dns-push-tls._tcp.<zone> SRV record
   queries.  These records are conceptually inserted into the namespace
   of the corresponding zones.  They do not exist in the ".local"
   namespace of the local link.

7.  DNSSEC Considerations

7.1.  On-line signing only

   The Discovery Proxy acts as the authoritative name server for
   designated subdomains, and if DNSSEC is to be used, the Discovery
   Proxy needs to possess a copy of the signing keys, in order to
   generate authoritative signed data from the local Multicast DNS
   responses it receives.  Off-line signing not applicable to Discovery

7.2.  NSEC and NSEC3 Records

   In DNSSEC, NSEC and NSEC3 records are used to assert the nonexistence
   of certain names, also described as "authenticated denial of

   Since a Discovery Proxy only knows what names exist on the local link
   by issuing queries for them, and since it would be impractical to
   issue queries for every possible name just to find out which names
   exist and which do not, a Discovery Proxy cannot programatically
   synthesize the traditional NSEC and NSEC3 records which assert the
   nonexistence of a large range names.  Instead, when generating a
   negative response, a Discovery Proxy programatically synthesizes a
   single NSEC record assert the nonexistence of just the specific name
   queried, and no others.  Since the Discovery Proxy has the zone
   signing key, it can do this on demand.  Since the NSEC record asserts
   the nonexistence of only a single name, zone walking is not a
   concern, so NSEC3 is not necessary.

   Note that this applies only to traditional immediate DNS queries,
   which may return immediate negative answers when no immediate
   positive answer is available.  When used with a DNS Push Notification
   subscription [PUSH] there are no negative answers, merely the absence
   of answers so far, which may change in the future if answers become

8.  IPv6 Considerations

   An IPv6-only host and an IPv4-only host behave as "ships that pass in
   the night".  Even if they are on the same Ethernet [IEEE-3], neither
   is aware of the other's traffic.  For this reason, each link may have
   *two* unrelated ".local." zones, one for IPv6 and one for IPv4.
   Since for practical purposes, a group of IPv6-only hosts and a group
   of IPv4-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.

   It will be desirable to have a mechanism to 'stitch' together these
   two unrelated ".local." zones so that they appear as one.  Such
   mechanism will need to be able to differentiate between a dual-stack
   (v4/v6) host participating in both ".local." zones, and two different
   hosts, one IPv6-only and the other IPv4-only, which are both trying
   to use the same name(s).  Such a mechanism will be specified in a
   future companion document.

   At present, it is RECOMMENDED that a Discovery Proxy be configured
   with a single domain name for both the IPv4 and IPv6 ".local." zones
   on the local link, and when a unicast query is received, it should
   issue Multicast DNS queries using both IPv4 and IPv6 on the local
   link, and then combine the results.

9.  Security Considerations

9.1.  Authenticity

   A service proves its presence on a link by its ability to answer
   link-local multicast queries on that link.  If greater security is
   desired, then the Discovery Proxy mechanism should not be used, and
   something with stronger security should be used instead, such as
   authenticated secure DNS Update [RFC2136] [RFC3007].

9.2.  Privacy

   The Domain Name System is, generally speaking, a global public
   database.  Records that exist in the Domain Name System name
   hierarchy can be queried by name from, in principle, anywhere in the
   world.  If services on a mobile device (like a laptop computer) are
   made visible via the Discovery Proxy mechanism, then when those
   services become visible in a domain such as "My"
   that might indicate to (potentially hostile) observers that the
   mobile device is in my house.  When those services disappear from
   "My" that change could be used by observers to
   infer when the mobile device (and possibly its owner) may have left
   the house.  The privacy of this information may be protected using
   techniques like firewalls, split-view DNS, and Virtual Private
   Networks (VPNs), as are customarily used today to protect the privacy
   of corporate DNS information.

   The Discovery Proxy could also provide sensitive records only to
   authenticated users.  This is a general DNS problem, not specific to
   the Discovery Proxy.  Work is underway in the IETF to tackle this
   problem [RFC7626].

9.3.  Denial of Service

   A remote attacker could use a rapid series of unique Unicast DNS
   queries to induce a Discovery Proxy to generate a rapid series of
   corresponding Multicast DNS queries on one or more of its local
   links.  Multicast traffic is generally more expensive than unicast
   traffic -- especially on Wi-Fi links -- which makes this attack
   particularly serious.  To limit the damage that can be caused by such
   attacks, a Discovery Proxy (or the underlying Multicast DNS subsystem
   which it utilizes) MUST implement Multicast DNS query rate limiting
   appropriate to the link technology in question.  For today's
   802.11b/g/n/ac Wi-Fi links (for which approximately 200 multicast
   packets per second is sufficient to consume approximately 100% of the
   wireless spectrum) a limit of 20 Multicast DNS query packets per
   second is RECOMMENDED.  On other link technologies like Gigabit
   Ethernet higher limits may be appropriate.  A consequence of this
   rate limiting is that a rogue remote client could issue an excessive
   number of queries, resuling in denial of service to other remote
   clients attempting to use that Discovery Proxy.  However, this is
   preferable to a rogue remote client being able to inflict even
   greater harm on the local network, which could impact the correct
   operation of all local clients on that network.

10.  Intelectual Property Rights

   Apple has submitted an IPR disclosure concerning the technique
   proposed in this document.  Details are available on the IETF IPR
   disclosure page [IPR2119].

11.  IANA Considerations

   This document has no IANA Considerations.

12.  Acknowledgments

   Thanks to Markus Stenberg for helping develop the policy regarding
   the four styles of unicast response according to what data is
   immediately available in the cache.  Thanks to Anders Brandt, Tim
   Chown, Ralph Droms, Ray Hunter, Ted Lemon, Tom Pusateri, Markus
   Stenberg, Dave Thaler, and Andrew Yourtchenko for their comments.

13.  References

13.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <>.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              DOI 10.17487/RFC3927, May 2005,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              December 2012.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, December 2012.

   [PUSH]     Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-12 (work in progress), October 2016. July 2017.

13.2.  Informative References

   [HOME]     Cheshire, S.,     Pfister, P. and T. Lemon, "Special Use Top Level Domain 'home'",
              ''", draft-ietf-homenet-dot-07 (work in
              November 2015. June 2017.

   [IPR2119]  "Apple Inc.'s Statement about IPR related to Hybrid
              Unicast/Multicast DNS-Based Service Discovery",

   [ohp]      "Discovery Proxy (Hybrid Proxy) implementation for
              OpenWrt", <>.

   [LLQ]      Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
              llq-01 (work in progress), August 2006.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC7558]  Lynn, K., Cheshire, S., Blanchet, M., and D. Migault,
              "Requirements for Scalable DNS-Based Service Discovery
              (DNS-SD) / Multicast DNS (mDNS) Extensions", RFC 7558,
              DOI 10.17487/RFC7558, July 2015,

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,

   [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
              2016, <>.

   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, December 2012.

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

   [IEEE-1Q]  "IEEE Standard for Local and metropolitan area networks --
              Bridges and Bridged Networks", IEEE Std 802.1Q-2014,
              November 2014, <

   [IEEE-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,

   [IEEE-5]   Institute of Electrical and Electronics Engineers,
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              5: Token ring access method and physical layer
              specification", IEEE Std 802.5-1998, 1995.

   [IEEE-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, <>.

Appendix A.  Implementation Status

   Some aspects of the mechanism specified in this document already
   exist in deployed software.  Some aspects are new.  This section
   outlines which aspects already exist and which are new.

A.1.  Already Implemented and Deployed

   Domain enumeration by the client (the "b._dns-sd._udp" queries) is
   already implemented and deployed.

   Unicast queries to the indicated discovery domain is already
   implemented and deployed.

   These are implemented and deployed in Mac OS X 10.4 and later
   (including all versions of Apple iOS, on all iPhone and iPads), in
   Bonjour for Windows, and in Android 4.1 "Jelly Bean" (API Level 16)
   and later.

   Domain enumeration and unicast querying have been used for several
   years at IETF meetings to make Terminal Room printers discoverable
   from outside the Terminal room.  When an IETF attendee presses Cmd-P
   on a Mac, or selects AirPrint on an iPad or iPhone, and the Terminal
   room printers appear, that is because the client is sending unicast
   DNS queries to the IETF DNS servers.

A.2.  Already Implemented

   A minimal portable Discovery Proxy implementation has been produced
   by Markus Stenberg and Steven Barth, which runs on OS X and several
   Linux variants including OpenWrt [ohp].  It was demonstrated at the
   Berlin IETF in July 2013.

   Tom Pusateri also has an implementation that runs on any Unix/Linux.
   It has a RESTful interface for management and an experimental demo
   CLI and web interface.

A.3.  Partially Implemented

   The current APIs make multiple domains visible to client software,
   but most client UI today lumps all discovered services into a single
   flat list.  This is largely a chicken-and-egg problem.  Application
   writers were naturally reluctant to spend time writing domain-aware
   UI code when few customers today would benefit from it.  If Discovery
   Proxy deployment becomes common, then application writers will have a
   reason to provide better UI.  Existing applications will work with
   the Discovery Proxy, but will show all services in a single flat
   list.  Applications with improved UI will group services by domain.

   The Long-Lived Query mechanism [LLQ] referred to in this
   specification exists and is deployed, but has not been standardized
   by the IETF.  The IETF is considering standardizing a superior Long-
   Lived Query mechanism called DNS Push Notifications [PUSH].  The
   pragmatic short-term deployment approach is for vendors to produce
   Discovery Proxies that implement both the deployed Long-Lived Query
   mechanism [LLQ] (for today's clients) and the new DNS Push
   Notifications mechanism [PUSH] as the preferred long-term direction.

   The translating/filtering Discovery Proxy specified in this document.
   Implementations are under development, and operational experience
   with these implementations has guided updates to this document.

A.4.  Not Yet Implemented

   Client implementations of the new DNS Push Notifications mechanism
   [PUSH] are currently underway.

   A mechanism to 'stitch' together multiple ".local." zones so that
   they appear as one.  Such a stitching mechanism will be specified in
   a future companion document.  This stitching mechanism addresses the
   issue that if a printer is physically moved from one link to another,
   then conceptually the old service has disappeared from the DNS
   namespace, and a new service with a similar name has appeared.  This
   stitching mechanism will allow a service to change its point of
   attachment without changing the name by which it can be found.

Author's Address

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

   Phone: +1 408 974 3207