Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                        November 5, 2015                        February 4, 2016
Expires: May 8, August 7, 2016

          Hybrid Unicast/Multicast DNS-Based Service Discovery


   Performing DNS-Based Service Discovery using purely link-local
   Multicast DNS enables discovery of services that are on the local
   link, but not (without some kind of proxy or similar special support)
   discovery of services that are outside the local link.  Using a very
   large local link with thousands of hosts facilitates service
   discovery, but at the cost of large 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.  This can be achieved by manual DNS
   configuration (as has been done for many years at IETF meetings to
   advertise the IETF Terminal Room printer) but this is labor
   intensive, error prone, and requires a reasonable degree of DNS
   expertise.  The Unicast DNS namespace can be populated with the
   required data automatically by the devices themselves, but that
   requires configuration of DNS Update keys on the devices offering the
   services, 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.

Status of this Memo

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

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   This Internet-Draft will expire on May 8, August 7, 2016.

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   Copyright (c) 2015 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology Used in this Document  . . . . . .  5
   3.  Compatibility Considerations . . . . . . . . . . . . . . . . .  5
   4.  Hybrid Proxy Operation . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Delegated Subdomain for Service Discovery Records  . . . .  7
     4.2.  Domain Enumeration . . . . . . . . . . . . . . . . . . . .  8
       4.2.1.  Domain Enumeration via Unicast Queries . . . . . . . .  8
       4.2.2.  Domain Enumeration via Multicast Queries . . . . . . .  9
     4.3.  Delegated Subdomain for LDH Host Names . . . . . . . . . . 10
     4.4.  Delegated Subdomain for Reverse Mapping  . . . . . . . . . 12
     4.5.  Data Translation . . . . . . . . . . . . . . . . . . . . . 13
       4.5.1.  DNS TTL limiting . . . . . . . . . . . . . . . . . . . 13
       4.5.2.  Suppressing Unusable Records . . . . . . . . . . . . . 14
       4.5.3.  Application-Specific Data Translation  . . . . . . . . 15
     4.6.  Answer Aggregation . . . . . . . . . . . . . . . . . . . . 16
       4.6.1.  Discovery of LLQ and/or PUSH Notification Service  . . 19
   5.  DNS SOA (Start of Authority) Record  . . . . . . . . . . . . . 20
   6.  Implementation Status  . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Already Implemented and Deployed . . . . . . . . . . . . . 20
     6.2.  Already Implemented  . . . . . . . . . . . . . . . . . . . 21
     6.3.  Partially Implemented  . . . . . . . . . . . . . . . . . . 21
     6.4.  Not Yet Implemented  . . . . . . . . . . . . . . . . . . . 21
   7.  IPv6 Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
     8.1.  Authenticity . . . . . . . . . . . . . . . . . . . . . . . 22
     8.2.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.3.  Denial of Service  . . . . . . . . . . . . . . . . . . . . 23
   9.  Intelectual Property Rights  . . . . . . . . . . . . . . . . . 23
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     12.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25

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 network consisting of just a single link (or several
   physical links bridged together to appear as a single logical link to
   IP) Multicast DNS [RFC6762] is sufficient for client devices to look
   up the dot-local host names of peers on the same home network, and
   perform DNS-Based Service Discovery (DNS-SD) [RFC6763] of 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, do not cross between links.
   (This was a deliberate design choice for Multicast DNS, since even on
   a single link multicast traffic is expensive -- especially on Wi-Fi
   links -- and multiplying the amount of multicast traffic by flooding
   it across multiple links would make that problem even worse.)
   In this environment, Unicast DNS would be preferable to Multicast
   DNS.  (Unicast DNS can be used either with a traditionally assigned
   globally unique domain name, or with a private local unicast domain
   name such as ".home" [HOME].)

   To use Unicast DNS, the names of hosts and services need to be made
   available in the Unicast DNS namespace.  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.

   This document specifies a type of proxy called a Hybrid 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.

2.  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 Hybrid 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 [802.5], 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.

3.  Compatibility Considerations

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

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

   Existing clients that support DNS-Based Service Discovery over
   Unicast DNS (Mac OS X 10.4 and later, including iPhone, iPad, and
   Bonjour for Windows) work with Hybrid Proxy.

4.  Hybrid Proxy Operation

   In a typical configuration, a Hybrid 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 [I-D.ietf-homenet-hncp].

   These three varieties of delegated subdomains (service discovery,
   host names, and reverse mapping) are described below.

4.1.  Delegated Subdomain for Service Discovery Records

   In its simplest form, each physical 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 system.

    | ** |  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 a Hybrid Proxy which serves
   it.  This Hybrid Proxy function could be performed by a router on
   that link, or, with appropriate VLAN configuration, a single Hybrid
   Proxy could have a logical presence on, and serve as the Hybrid Proxy
   for, many links.  In the parent domain, NS records are used to
   delegate ownership of each defined link name
   (e.g., "Building") to the Hybrid Proxy that serves the
   named link.  In other words, the Hybrid Proxy is the authoritative
   name server for that subdomain.

   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 Hybrid Proxy on the link in question.  Like a conventional
   Unicast DNS server, a Hybrid 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 Hybrid Proxy generates
   answers using Multicast DNS.  A Hybrid 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 Hybrid Proxy synthesizes the appropriate
   Unicast DNS response.

   Naturally, the existing Multicast DNS caching mechanism is used to
   avoid issuing unnecessary Multicast DNS queries on the wire.  The
   Hybrid 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.

4.2.  Domain Enumeration

   An DNS-SD client performs Domain Enumeration [RFC6763] via certain
   PTR queries.  It issues unicast Domain Enumeration queries using its
   "home" domain (typically learned via DHCP) and using its IPv6 prefix
   and IPv4 subnet address.  These are described below in Section 4.2.1.
   It also issues multicast Domain Enumeration queries in the "local"
   domain [RFC6762].  These are described below in Section 4.2.2.  The
   results of all Domain Enumeration queries are combined for Service
   Discovery purposes.

4.2.1.  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 2015).  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.

4.2.2.  Domain Enumeration via Multicast Queries

   Since a Hybrid 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 Hybrid Proxy can be configured to generate Multicast DNS responses
   for the following Multicast DNS Domain Enumeration queries issues by

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

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

4.3.  Delegated Subdomain for LDH Host Names

   The traditional rules for host names are more restrictive than those
   for DNS-SD service instance names and domains.

   Users typically interact with DNS-SD by viewing a list of discovered
   service instance names on the 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 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, 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 Hybrid Proxy
   SOULD support having separate subdomains delegated to it, 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 Hybrid Proxy could have the two subdomains
   "Building" and "" delegated to it.
   The Hybrid 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 Hybrid 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 Hybrid Proxy MUST NOT be restricted to support only a letter-digit-
   hyphen subdomain, because that results in an unnecessarily poor user

4.4.  Delegated Subdomain for Reverse Mapping

   A Hybrid 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
   Hybrid Proxy.  In other words, the Hybrid Proxy becomes the
   authoritative name server for the reverse mapping domain.

   For example, if a given link is using the IPv6 prefix 2001:0DB8/32,
   then the domain "" is delegated to the Hybrid
   Proxy for that link.

   If a given link is using the IPv4 subnet 10.1/16, then the domain
   "" is delegated to the Hybrid Proxy for that link.

   When a reverse mapping query arrives at the Hybrid Proxy, it issues
   the identical query on its local link as a Multicast DNS query.
   (In the Apple "/usr/include/dns_sd.h" APIs, using ForceMulticast
   indicates that the DNSServiceQueryRecord() call should perform the
   query using Multicast DNS.)  When the host owning that IPv6 or IPv4
   address responds with a name of the form "something.local", the
   Hybrid Proxy rewrites that to use its configured LDH host name domain
   instead of "local" and returns the response to the caller.

   For example, a Hybrid 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 Hybrid Proxy, will arrive at the Hybrid 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.

4.5.  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 configured DNS Unicast domain.

   Other beneficial translation and filtering operations are described

4.5.1.  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 any Long-Lived Query [I-D.sekar-dns-llq] or DNS Push
   Notification [I-D.ietf-dnssd-push] option) 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 5).

   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
   reasonble 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 contain an LLQ or DNS Push
   Notification option, 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 4.6.

4.5.2.  Suppressing Unusable Records

   A Hybrid Proxy SHOULD suppress Unicast DNS answers for records that
   are not useful outside the local link.  For example, DNS A and AAAA
   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], private addresses
   from one private address realm should not be communicated to clients
   in a different private 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.

4.5.3.  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 Hybrid
   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
   Hybrid 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 dot-
   local hostname needs to be translated to an appropriate name with
   larger scope.  Dot-local names are easily translated 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 Hybrid 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 Hybrid 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.

   As in other similar situations, these

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

4.6.  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 Hybrid 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 Hybrid 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 network.

   Both these dilemmas are solved by use of DNS Long-Lived Queries
   (DNS LLQ) [I-D.sekar-dns-llq] or its newer replacement, DNS Push
   Notifications [I-D.ietf-dnssd-push].  (Clients and Hybrid Proxies can
   support both DNS LLQ and DNS Push, and when talking to a Hybrid 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 Hybrid Proxy receives a query containing a DNS LLQ or DNS Push
   Notification option, 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 Hybrid 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 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 Hybrid 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 included an LLQ
   or DNS Push Notification option (typical with long-lived service
   browsing PTR queries) or not (typical with one-shot operations like
   SRV or address record queries).  Note that queries containing the LLQ
   or PUSH option are received directly from the client (see
   Section 4.6.1).  Queries containing no LLQ or PUSH option are
   generally received via the client's configured recursive (caching)
   name server.

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

   o  No LLQ or PUSH option; 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 an
      NSEC record [RFC6762]), the Hybrid 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.
      DNS TTLs in responses are capped to at most ten seconds.

   o  No LLQ or PUSH option; 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  Query contains LLQ or PUSH option; 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.  (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  Query contains LLQ or PUSH option; 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 Hybrid
   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" name.

4.6.1.  Discovery of LLQ and/or PUSH Notification Service

   To issue LLQ or PUSH queries, clients need to communicate directly
   with the authoritative Hybrid Proxy.  The procedure by which the
   client locates the authoritative Hybrid Proxy is described in the LLQ
   specification [I-D.sekar-dns-llq] and the DNS Push Notifications
   specification [I-D.ietf-dnssd-push].

   Briefly, the procedure is as follows:

   To discover the LLQ service for a given domain name, a client first
   performs DNS zone apex discovery, and then, having discovered <apex>,
   the client then issues a DNS query for the SRV record with the name
   _dns-llq._udp.<apex> to find the target host and port for the LLQ
   service for that zone.  By default LLQ service runs on UDP port 5352,
   but since SRV records are used, the LLQ service can be offered on any

   To discover the DNS Push Notification service for a given domain
   name, a client first performs DNS zone apex discovery, and then,
   having discovered <apex>, the client then issues a DNS query for the
   SRV record with the name _dns-push-tls._tcp.<apex> to find the target
   host and port for the DNS Push Notification service for that zone.
   By default DNS Push Notification service runs on TCP port 5352, but
   since SRV records are used, the DNS Push Notification service can be
   offered on any port.

   A client performs DNS zone apex discovery using the procedure below:

   1.  The client issues a DNS query for the SOA record with the given
       domain name.

   2.  A conformant recursive (caching) name server will either send a
       positive response, or a negative response containing the SOA
       record of the zone apex in the Authority Section.

   3.  If the name server sends a negative response that does not
       contain the SOA record of the zone apex, the client trims the
       first label off the given domain name and returns to step 1 to
       try again.

   By this method, the client iterates until it learns the name of the
   zone apex, or (in pathological failure cases) reaches the root and
   gives up.

   Normal DNS caching is used to avoid repetitive queries on the wire.

5.  DNS SOA (Start of Authority) Record

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

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

   The SERIAL field SHOULD contain a sequence number that increments
   each time the Hybrid Proxy returns an SOA record to any client.
   [Author's note: Or maybe it could just be zero?]

   Since zone transfers are undefined for Hybrid Proxy zones, the
   REFRESH, RETRY and EXPIRE fields have no useful meaning for Hybrid
   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 4.5.1).

   [Author's note: Discussion of these recommendations is requested.]

6.  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.

6.1.  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 you Press Cmd-P on your Mac, or
   select AirPrint on your iPad or iPhone, and the Terminal room
   printers appear, that is because your client is sending unicast DNS
   queries to the IETF DNS servers.

6.2.  Already Implemented

   A minimal portable Hybrid 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.

6.3.  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 Hybrid
   Proxy deployment becomes common, then application writers will have a
   reason to provide better UI.  Existing applications will work with
   the Hybrid 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 [I-D.sekar-dns-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
   [I-D.ietf-dnssd-push].  The pragmatic short-term deployment approach
   is for vendors to produce Hybrid Proxies that implement both the
   deployed Long-Lived Query mechanism [I-D.sekar-dns-llq] (for today's
   clients) and the new DNS Push Notifications mechanism
   [I-D.ietf-dnssd-push] as the preferred long-term direction.

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

6.4.  Not Yet Implemented

   Client implementations of the new DNS Push Notifications mechanism
   [I-D.ietf-dnssd-push] are currently underway.

   A mechanism to 'stitch' together multiple ".local." zones so that
   they appear as one.  Such a mechanism will be specified in a future
   companion document.

7.  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, neither is aware
   of the other's traffic.  For this reason, each physical 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.

8.  Security Considerations

8.1.  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 Hybrid Proxy mechanism should not be used, and
   something with stronger security should be used instead, such as
   authenticated secure DNS Update [RFC2136] [RFC3007].

8.2.  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 Hybrid Proxy mechanism, then when those services
   become visibile 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 and split-view DNS, as are customarily used
   today to protect the privacy of corporate DNS information.

8.3.  Denial of Service

   A remote attacker could use a rapid series of unique Unicast DNS
   queries to induce a Hybrid Proxy to generate a rapid series of
   corresponding Multicast DNS queries on one or more of its local
   links.  Multicast traffic is expensive -- especially on Wi-Fi links
   -- which makes this attack particularly serious.  To limit the damage
   that can be caused by such attacks, a Hybrid 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 Wi-Fi links the Multicast DNS subsystem SHOULD NOT
   issue more than 20 Multicast DNS query packets per second.  On other
   link technologies like Gigabit Ethernet higher limits may be

9.  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].

10.  IANA Considerations

   This document has no IANA Considerations.

11.  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 and
   Andrew Yourtchenko for
   comments about privacy issues. their comments.  [Partial list; more names to
   be added.]

12.  References

12.1.  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., J. 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,

   [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.

              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-03 (work in progress),
              November 2015.

12.2.  Informative References

   [HOME]     Cheshire, S., "Special Use Top Level Domain 'home'",
              draft-cheshire-homenet-dot-home (work in progress),
              November 2015.

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

   [ohp]      "Hybrid Proxy implementation for OpenWrt",

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

              Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", draft-ietf-homenet-hncp-09 (work in
              progress), August 2015.

   [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,

   [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.

Author's Address

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

   Phone: +1 408 974 3207