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Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-15

Document Type Active Internet-Draft (dnssd WG)
Authors Ted Lemon , Stuart Cheshire
Last updated 2022-09-14
Replaces draft-sctl-service-registration
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Formats
Stream WG state Waiting for WG Chair Go-Ahead
Revised I-D Needed - Issue raised by WG
Document shepherd (None)
IESG IESG state I-D Exists
Consensus boilerplate Yes
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Send notices to (None)
draft-ietf-dnssd-srp-15
Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                               S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: 18 March 2023                                 14 September 2022

     Service Registration Protocol for DNS-Based Service Discovery
                        draft-ietf-dnssd-srp-15

Abstract

   The Service Registration Protocol for DNS-Based Service Discovery
   uses the standard DNS Update mechanism to enable DNS-Based Service
   Discovery using only unicast packets.  This makes it possible to
   deploy DNS Service Discovery without multicast, which greatly
   improves scalability and improves performance on networks where
   multicast service is not an optimal choice, particularly 802.11
   (Wi-Fi) and 802.15.4 (IoT) networks.  DNS-SD Service registration
   uses public keys and SIG(0) to allow services to defend their
   registrations against attack.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 18 March 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Service Registration Protocol . . . . . . . . . . . . . . . .   5
     2.1.  Protocol Variants . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Full-featured Hosts . . . . . . . . . . . . . . . . .   5
       2.1.2.  Constrained Hosts . . . . . . . . . . . . . . . . . .   6
       2.1.3.  Why two variants? . . . . . . . . . . . . . . . . . .   6
     2.2.  Protocol Details  . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.  What to publish . . . . . . . . . . . . . . . . . . .   7
       2.2.2.  Where to publish it . . . . . . . . . . . . . . . . .   8
       2.2.3.  How to publish it . . . . . . . . . . . . . . . . . .   8
         2.2.3.1.  How DNS-SD Service Registration differs from
                 standard RFC2136 DNS Update . . . . . . . . . . . .   9
       2.2.4.  How to secure it  . . . . . . . . . . . . . . . . . .   9
         2.2.4.1.  First-Come First-Served Naming  . . . . . . . . .   9
       2.2.5.  SRP Requestor Behavior  . . . . . . . . . . . . . . .  10
         2.2.5.1.  Public/Private key pair generation and storage  .  10
         2.2.5.2.  Name Conflict Handling  . . . . . . . . . . . . .  10
         2.2.5.3.  Record Lifetimes  . . . . . . . . . . . . . . . .  11
         2.2.5.4.  Compression in SRV records  . . . . . . . . . . .  11
         2.2.5.5.  Removing published services . . . . . . . . . . .  11
     2.3.  Validation and Processing of SRP Updates  . . . . . . . .  13
       2.3.1.  Validation of Adds and Deletes  . . . . . . . . . . .  13
         2.3.1.1.  Service Discovery Instruction . . . . . . . . . .  13
         2.3.1.2.  Service Description Instruction . . . . . . . . .  14
         2.3.1.3.  Host Description Instruction  . . . . . . . . . .  15
       2.3.2.  Valid SRP Update Requirements . . . . . . . . . . . .  15
       2.3.3.  FCFS Name And Signature Validation  . . . . . . . . .  16
       2.3.4.  Handling of Service Subtypes  . . . . . . . . . . . .  16
       2.3.5.  SRP Update response . . . . . . . . . . . . . . . . .  17
       2.3.6.  Optional Behavior . . . . . . . . . . . . . . . . . .  17
   3.  TTL Consistency . . . . . . . . . . . . . . . . . . . . . . .  18
   4.  Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  Cleaning up stale data  . . . . . . . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     5.1.  Source Validation . . . . . . . . . . . . . . . . . . . .  20
     5.2.  SRP Registrar Authentication  . . . . . . . . . . . . . .  21
     5.3.  Required Signature Algorithm  . . . . . . . . . . . . . .  21
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  22
   7.  Delegation of 'service.arpa.' . . . . . . . . . . . . . . . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Registration and Delegation of 'service.arpa' as a
           Special-Use Domain Name . . . . . . . . . . . . . . . . .  22

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     8.2.  Subdomains of 'service.arpa.' . . . . . . . . . . . . . .  22
     8.3.  'dnssd-srp' Service Name  . . . . . . . . . . . . . . . .  23
     8.4.  'dnssd-srp-tls' Service Name  . . . . . . . . . . . . . .  23
     8.5.  Anycast Address . . . . . . . . . . . . . . . . . . . . .  23
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  24
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  25
   12. Informative References  . . . . . . . . . . . . . . . . . . .  27
   Appendix A.  Testing using standard RFC2136-compliant DNS
           servers . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Appendix B.  How to allow SRP requestors to update standard
           RFC2136-compliant servers . . . . . . . . . . . . . . . .  29
   Appendix C.  Sample BIND9 configuration for
           default.service.arpa. . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   DNS-Based Service Discovery [RFC6763] is a component of Zero
   Configuration Networking [RFC6760] [ZC] [ROADMAP].

   This document describes an enhancement to DNS-Based Service Discovery
   [RFC6763] (DNS-SD) that allows servers to register the services they
   offer using the DNS protocol rather than using Multicast DNS
   [RFC6762] (mDNS).  There is already a large installed base of DNS-SD
   clients that can discover services using the DNS protocol.

   This document is intended for three audiences: implementors of
   software that provides services that should be advertised using
   DNS-SD, implementors of DNS servers that will be used in contexts
   where DNS-SD registration is needed, and administrators of networks
   where DNS-SD service is required.  The document is intended to
   provide sufficient information to allow interoperable implementation
   of the registration protocol.

   DNS-Based Service Discovery (DNS-SD) allows services to advertise the
   fact that they provide service, and to provide the information
   required to access that service.  DNS-SD clients can then discover
   the set of services of a particular type that are available.  They
   can then select a service from among those that are available and
   obtain the information required to use it.  Although DNS-SD using the
   DNS protocol (as opposed to mDNS) can be more efficient and
   versatile, it is not common in practice, because of the difficulties
   associated with updating authoritative DNS services with service
   information.

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   Existing practice for updating DNS zones is to either manually enter
   new data, or else use DNS Update [RFC2136].  Unfortunately DNS Update
   requires either that the authoritative DNS server automatically trust
   updates, or else that the DNS Update requestor have some kind of
   shared secret or public key that is known to the DNS server and can
   be used to authenticate the update.  Furthermore, DNS Update can be a
   fairly chatty process, requiring multiple round trips with different
   conditional predicates to complete the update process.

   The SRP protocol adds a set of default heuristics for processing DNS
   updates that eliminates the need for DNS update conditional
   predicates: instead, the SRP registrar (a DNS server that supports
   SRP updates) has a set of default predicates that are applied to the
   update, and the update either succeeds entirely, or fails in a way
   that allows the requestor to know what went wrong and construct a new
   update.

   SRP also adds a feature called First-Come, First-Served (FCFS)
   Naming, which allows the requestor to claim a name that is not yet in
   use, and, using SIG(0) [RFC2931], to authenticate both the initial
   claim and subsequent updates.  This prevents name conflicts, since a
   second SRP requestor attempting to claim the same name will not
   possess the SIG(0) key used by the first requestor to claim it, and
   so its claim will be rejected and the second requestor will have to
   choose a new name.

   Finally, SRP adds the concept of a 'lease,' similar to leases in
   Dynamic Host Configuration Protocol [RFC8415].  The SRP registration
   itself has a lease which may be on the order of an hour; if the
   requestor does not renew the lease before it has elapsed, the
   registration is removed.  The claim on the name can have a longer
   lease, so that another requestor cannot claim the name, even though
   the registration has expired.

   The Service Registration Protocol for DNS-SD (SRP), described in this
   document, provides a reasonably secure mechanism for publishing this
   information.  Once published, these services can be readily
   discovered by DNS-SD clients using standard DNS lookups.

   The DNS-SD specification ([RFC6763], Section 10, "Populating the DNS
   with Information"), briefly discusses ways that servers can publish
   their information in the DNS namespace.  In the case of mDNS, it
   allows servers to publish their information on the local link, using
   names in the ".local" namespace, which makes their services directly
   discoverable by peers attached to that same local link.

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   RFC6763 also allows clients to discover services using the DNS
   protocol [RFC1035].  This can be done by having a system
   administrator manually configure service information in the DNS, but
   manually populating DNS authoritative server databases is costly and
   potentially error-prone, and requires a knowledgable network
   administrator.  Consequently, although all DNS-SD client
   implementations of which we are aware support DNS-SD using DNS
   queries, in practice it is used much less frequently than mDNS.

   The Discovery Proxy [RFC8766] provides one way to automatically
   populate the DNS namespace, but is only appropriate on networks where
   services are easily advertised using mDNS.  This document describes a
   solution more suitable for networks where multicast is inefficient,
   or where sleepy devices are common, by supporting both offering of
   services, and discovery of services, using unicast.

2.  Service Registration Protocol

   Services that implement SRP use DNS Update [RFC2136] [RFC3007] to
   publish service information in the DNS.  Two variants exist, one for
   full-featured hosts, and one for devices designed for "Constrained-
   Node Networks" [RFC7228].  An SRP registrar is most likely an
   authoritative DNS server, or else is updating an authoritative DNS
   server.  There is no requirement that the server that is receiving
   SRP updates be the same server that is answering queries that return
   records that have been registered.

2.1.  Protocol Variants

2.1.1.  Full-featured Hosts

   Full-featured hosts either are configured manually with a
   registration domain, or discover the default registration domain as
   described in Section 11 of [RFC6763].  If this process does not
   produce a default registration domain, the Service Registration
   protocol is not discoverable on the local network using this
   mechanism.  Other discovery mechanisms are possible, but are out of
   scope for this document.

   Manual configuration of the registration domain can be done either by
   querying the list of available registration domains
   ("r._dns-sd._udp") and allowing the user to select one from the UI,
   or by any other means appropriate to the particular use case being
   addressed.  Full-featured devices construct the names of the SRV,
   TXT, and PTR records describing their service(s) as subdomains of the
   chosen service registration domain.  For these names they then
   discover the zone apex of the closest enclosing DNS zone using SOA
   queries Section 6.1 of [RFC8765].  Having discovered the enclosing

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   DNS zone, they query for the "_dnssd-srp._tcp.<zone>" SRV record to
   discover the server to which they should send SRP updates.  Hosts
   that support SRP Updates using TLS use the
   "_dnssd-srp-tls._tcp.<zone>" SRV record instead.

2.1.2.  Constrained Hosts

   For devices designed for Constrained-Node Networks [RFC7228] some
   simplifications are available.  Instead of being configured with (or
   discovering) the service registration domain, the (proposed) special-
   use domain name (see [RFC6761]) "default.service.arpa" is used.  The
   details of how SRP registrar(s) are discovered will be specific to
   the constrained network, and therefore we do not suggest a specific
   mechanism here.

   SRP requestors on constrained networks are expected to receive from
   the network a list of SRP registrars with which to register.  It is
   the responsibility of a Constrained-Node Network supporting SRP to
   provide one or more SRP registrar addresses.  It is the
   responsibility of the SRP registrar supporting a Constrained-Node
   Network to handle the updates appropriately.  In some network
   environments, updates may be accepted directly into a local
   "default.service.arpa" zone, which has only local visibility.  In
   other network environments, updates for names ending in
   "default.service.arpa" may be rewritten internally to names with
   broader visibility.

2.1.3.  Why two variants?

   The reason for these different variants is that low-power devices
   that typically use Constrained-Node Networks may have very limited
   battery storage.  The series of DNS lookups required to discover an
   SRP registrar and then communicate with it will increase the energy
   required to advertise a service; for low-power devices, the
   additional flexibility this provides does not justify the additional
   use of energy.  It is also fairly typical of such networks that some
   network service information is obtained as part of the process of
   joining the network, and so this can be relied upon to provide nodes
   with the information they need.

   Networks that are not constrained networks can have more complicated
   topologies at the Internet layer.  Nodes connected to such networks
   can be assumed to be able to do DNSSD service registration domain
   discovery.  Such networks are generally able to provide registration
   domain discovery and routing.  This creates the possibility of off-
   network spoofing.  To prevent such spoofing, TCP is required for such
   networks.

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2.2.  Protocol Details

   We will discuss several parts to this process: how to know what to
   publish, how to know where to publish it (under what name), how to
   publish it, and how to secure its publication.  In Section 4, we
   specify how to maintain the information once published.

2.2.1.  What to publish

   SRP Updates are sent by SRP requestors to SRP registrars.  Three
   types of instructions appear in an SRP update: Service Discovery
   instructions, Service Description instructions, and Host Description
   instructions.  These instructions are made up of DNS Update adds and
   removes.  The types of records that are updated and removed in each
   of these instructions, as well as the constraints that apply to them,
   are described in Section 2.3.  An SRP Update is a DNS Update message
   that is constructed so as to meet the constraints described in that
   section.  The following is a brief overview of what is included in a
   typical SRP Update:

   *  PTR Resource Record (RR) for services, which map from a generic
      service type (or subtype) name to a specific Service Instance
      Name.
   *  For any Service Instance Name ([RFC6763], Section 4.1), an SRV RR,
      one or more TXT RRs, and a KEY RR.  In principle Service
      Description records can include other record types, with the same
      Service Instance Name, though in practice they rarely do.  SRP
      does not support other record types.  The KEY RR is used to
      support FCFS naming, and has no specific meaning for DNS-SD
      lookups.  SRV records for all services described in an SRP update
      point to the same hostname.
   *  There is never more than one hostname in a single SRP update.  The
      hostname has one or more address RRs (AAAA or A) and a KEY RR
      (used for FCFS naming).  Depending on the use case, an SRP
      requestor may be required to suppress some addresses that would
      not be usable by hosts discovering the service through the SRP
      registrar.  The exact address record suppression behavior required
      may vary for different types of SRP requestors.  An example of
      such advice can be found in Section 5.5.2 of [RFC8766].

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   [RFC6763] describes the details of what each of these types of RR
   mean, with the exception of the KEY RR, which is defined in
   [RFC2539].  These RFCs should be considered the definitive source for
   information about what to publish; the reason for summarizing this
   here is to provide the reader with enough information about what will
   be published that the service registration process can be understood
   at a high level without first learning the full details of DNS-SD.
   Also, the "Service Instance Name" is an important aspect of first-
   come, first-serve naming, which we describe later on in this
   document.

2.2.2.  Where to publish it

   Multicast DNS uses a single namespace, ".local", which is valid on
   the local link.  This convenience is not available for DNS-SD using
   the DNS protocol: services must exist in some specific unicast
   namespace.

   As described above, full-featured devices are responsible for knowing
   in what domain they should register their services.  Devices made for
   Constrained-Node Networks register in the (proposed) special use
   domain name [RFC6761] "default.service.arpa", and let the SRP
   registrar handle rewriting that to a different domain if necessary.

2.2.3.  How to publish it

   It is possible to issue a DNS Update that does several things at
   once; this means that it's possible to do all the work of adding a
   PTR resource record to the PTR RRset on the Service Name, and
   creating or updating the Service Instance Name and Host Description,
   in a single transaction.

   An SRP Update takes advantage of this: it is implemented as a single
   DNS Update message that contains a service's Service Discovery
   records, Service Description records, and Host Description records.

   Updates done according to this specification are somewhat different
   than regular DNS Updates as defined in [RFC2136].  The [RFC2136]
   update process can involve many update attempts: you might first
   attempt to add a name if it doesn't exist; if that fails, then in a
   second message you might update the name if it does exist but matches
   certain preconditions.  Because the registration protocol uses a
   single transaction, some of this adaptability is lost.

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   In order to allow updates to happen in a single transaction, SRP
   Updates do not include update prerequisites.  The requirements
   specified in Section 2.3 are implicit in the processing of SRP
   Updates, and so there is no need for the SRP requestor to put in any
   explicit prerequisites.

2.2.3.1.  How DNS-SD Service Registration differs from standard RFC2136
          DNS Update

   DNS-SD Service Registration is based on standard RFC2136 DNS Update,
   with some differences:

   *  It implements first-come first-served name allocation, protected
      using SIG(0) [RFC2931].
   *  It enforces policy about what updates are allowed.
   *  It optionally performs rewriting of "default.service.arpa" to some
      other domain.
   *  It optionally performs automatic population of the address-to-name
      reverse mapping domains.
   *  An SRP registrar is not required to implement general DNS Update
      prerequisite processing.
   *  Constrained-Node SRP requestors are allowed to send updates to the
      generic domain "default.service.arpa."

2.2.4.  How to secure it

   Traditional DNS update is secured using the TSIG protocol, [RFC8945],
   which uses a secret key shared between the DNS Update requestor
   (which issues the update) and the server (which authenticates it).
   This model does not work for automatic service registration.

   The goal of securing the DNS-SD Registration Protocol is to provide
   the best possible security given the constraint that service
   registration has to be automatic.  It is possible to layer more
   operational security on top of what we describe here, but what we
   describe here is an improvement over the security of mDNS.  The goal
   is not to provide the level of security of a network managed by a
   skilled operator.

2.2.4.1.  First-Come First-Served Naming

   First-Come First-Serve naming provides a limited degree of security:
   a server that registers its service using DNS-SD Registration
   protocol is given ownership of a name for an extended period of time
   based on the key used to authenticate the DNS Update.  As long as the
   registration service remembers the name and the key used to register
   that name, no other server can add or update the information
   associated with that.  If the server fails to renew its service

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   registration before the KEY lease (Section 4 of
   [I-D.ietf-dnssd-update-lease]), its name is no longer protected.
   FCFS naming is used to protect both the Service Description and the
   Host Description.

2.2.5.  SRP Requestor Behavior

2.2.5.1.  Public/Private key pair generation and storage

   The requestor generates a public/private key pair (See Section 5.3).
   This key pair MUST be stored in stable storage; if there is no
   writable stable storage on the SRP requestor, the SRP requestor MUST
   be pre-configured with a public/private key pair in read-only storage
   that can be used.  This key pair MUST be unique to the device.  A
   device with rewritable storage should retain this key indefinitely.
   When the device changes ownership, it may be appropriate to erase the
   old key and install a new one.  Therefore, the SRP requestor on the
   device SHOULD provide a mechanism to overwrite the key, for example
   as the result of a "factory reset."

   When sending DNS updates, the requestor includes a KEY record
   containing the public portion of the key in each Host Description
   Instruction and each Service Description Instruction.  Each KEY
   record MUST contain the same public key.  The update is signed using
   SIG(0), using the private key that corresponds to the public key in
   the KEY record.  The lifetimes of the records in the update is set
   using the EDNS(0) Update Lease option [I-D.ietf-dnssd-update-lease].

   The KEY record in Service Description updates MAY be omitted for
   brevity; if it is omitted, the SRP registrar MUST behave as if the
   same KEY record that is given for the Host Description is also given
   for each Service Description for which no KEY record is provided.
   Omitted KEY records are not used when computing the SIG(0) signature.

2.2.5.2.  Name Conflict Handling

   Both Host Description RR adds and Service Description RR adds can
   have names that result in name conflicts.  Service Discovery record
   adds cannot have name conflicts.  If any Host Description or Service
   Description record is found by the registrar to have a conflict with
   an existing name, the registrar will respond to the SRP Update with a
   YXDomain rcode.  In this case, the requestor MUST either abandon the
   service registration attempt, or else choose a new name.

   There is no specific requirement for how this is done; typically,
   however, the requestor will append a number to the preferred name.
   This number could be sequentially increasing, or could be chosen
   randomly.  One existing implementation attempts several sequential

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   numbers before choosing randomly.  So for instance, it might try
   host.default.service.arpa, then host-1.default.service.arpa, then
   host-2.default.service.arpa, then host-31773.default.service.arpa.

2.2.5.3.  Record Lifetimes

   The lifetime of the DNS-SD PTR, SRV, A, AAAA and TXT records
   [RFC6763] uses the LEASE field of the Update Lease option, and is
   typically set to two hours.  This means that if a device is
   disconnected from the network, it does not appear in the user
   interfaces of devices looking for services of that type for too long.

   The lifetime of the KEY records is set using the KEY-LEASE field of
   the Update Lease Option, and should be set to a much longer time,
   typically 14 days.  The result of this is that even though a device
   may be temporarily unplugged, disappearing from the network for a few
   days, it makes a claim on its name that lasts much longer.

   This means that even if a device is unplugged from the network for a
   few days, and its services are not available for that time, no other
   device can come along and claim its name the moment it disappears
   from the network.  In the event that a device is unplugged from the
   network and permanently discarded, then its name is eventually
   cleaned up and made available for re-use.

2.2.5.4.  Compression in SRV records

   Although [RFC2782] requires that the target name in the SRV record
   not be compressed, an SRP requestor SHOULD compress the target in the
   SRV record.  The motivation for _not_ compressing in [RFC2782] is not
   stated, but is assumed to be because a caching resolver that does not
   understand the format of the SRV record might store it as binary data
   and thus return an invalid pointer in response to a query.  This does
   not apply in the case of SRP: an SRP registrar needs to understand
   SRV records in order to validate the SRP Update.  Compression of the
   target potentially saves substantial space in the SRP Update.  SRP
   registrars MUST support compression of SRV RR targets.

2.2.5.5.  Removing published services

2.2.5.5.1.  Removing all published services

   To remove all the services registered to a particular host, the SRP
   requestor retransmits its most recent update with an Update Lease
   option that has a LEASE value of zero.  If the registration is to be
   permanently removed, KEY-LEASE should also be zero.  Otherwise, it
   should have the same value it had previously; this holds the name in
   reserve for when the SRP requestor is once again able to provide the

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   service.

   SRP requestors are normally expected to remove all service instances
   when removing a host.  However, in some cases a SRP requestor may not
   have retained sufficient state to know that some service instance is
   pointing to a host that it is removing.  This method of removing
   services is intended for the case where the requestor is going
   offline and does not want its services advertised.  Therefore, it is
   sufficient for the requestor to send the Host Description Instruction
   (Section 2.3.1.3).

   To support this, when removing services based on the lease time being
   zero, an SRP registrar MUST remove all service instances pointing to
   a host when a host is removed, even if the SRP requestor doesn't list
   them explicitly.  If the key lease time is nonzero, the SRP registrar
   MUST NOT delete the KEY records for these SRP requestors.

2.2.5.5.2.  Removing some published services

   In some use cases a requestor may need to remove some specific
   service, without removing its other services.  This can be
   accomplished in one of two ways.  To simply remove a specific
   service, the requestor sends a valid SRP Update where the Service
   Discovery Instruction (Section 2.3.1.1) contains a single Delete an
   RR from an RRset ([RFC2136], Section 2.5.4) update that deletes the
   PTR record whose target is the service instance name.  The Service
   Description Instruction (Section 2.3.1.2) in this case contains a
   single Delete all RRsets from a Name ([RFC2136], Section 2.5.3)
   update to the service instance name.

   The second alternative is used when some service is being replaced by
   a different service with a different service instance name.  In this
   case, the old service is deleted as in the first alternative.  The
   new service is added, just as it would be in an update that wasn't
   deleting the old service.  Because both the removal of the old
   service and the add of the new service consist of a valid Service
   Discovery Instruction and a valid Service Description Instruction,
   the update as a whole is a valid SRP Update, and will result in the
   old service being removed and the new one added, or, to put it
   differently, in the old service being replaced by the new service.

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   It is perhaps worth noting that if a service is being updated without
   the service instance name changing, that will look very much like the
   second alternative above.  The difference is that because the target
   for the PTR record in the Service Discovery Instruction is the same
   for both the Delete An RR From An RRset update and the Add To An
   RRSet update, there is no way to tell whether they were intended to
   be one or two Instructions.  The same would be true of the Service
   Description Instruction.

   Whichever of these two alternatives is used, the host lease will be
   updated with the lease time provided in the SRP update.  In neither
   of these cases is it permissible to delete the host.  All services
   must point to a host.  If a host is to be deleted, this must be done
   using the method described in Section 2.2.5.5.1, which deletes the
   host and all services that have that host as their target.

2.3.  Validation and Processing of SRP Updates

2.3.1.  Validation of Adds and Deletes

   The SRP registrar first validates that the DNS Update is a
   syntactically and semantically valid DNS Update according to the
   rules specified in RFC2136.

   SRP Updates consist of a set of _instructions_ that together add or
   remove one or more services.  Each instruction consists of some
   combination of delete updates and add updates.  When an instruction
   contains a delete and an add, the delete MUST precede the add.

   The SRP registrar checks each instruction in the SRP Update to see
   that it is either a Service Discovery Instruction, a Service
   Description Instruction, or a Host Description Instruction.  Order
   matters in DNS updates.  Specifically, deletes must precede adds for
   records that the deletes would affect; otherwise the add will have no
   effect.  This is the only ordering constraint; aside from this
   constraint, updates may appear in whatever order is convenient when
   constructing the update.

   Because the SRP Update is a DNS update, it MUST contain a single
   question that indicates the zone to be updated.  Every delete and
   update in an SRP Update MUST be within the zone that is specified for
   the SRP Update.

2.3.1.1.  Service Discovery Instruction

   An instruction is a Service Discovery Instruction if it contains

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   *  exactly one "Add to an RRSet" ([RFC2136], Section 2.5.1) or
      exactly one "Delete an RR from an RRSet" ([RFC2136],
      Section 2.5.4) RR update,
   *  which updates a PTR RR,
   *  the target of which is a Service Instance Name
   *  for which name a Service Description Instruction is present in the
      SRP Update, and:
      -  if the RR Update is an "Add to an RRSet" instruction, that
         Service Description Instruction contains an "Add to an RRset"
         RR update for the SRV RR describing that service and no other
         "Delete from an RRset" instructions for that Service Instance
         Name; or
      -  if the RR Update is a "Delete an RR from an RRSet" instruction,
         that Service Description Instruction contains a "Delete from an
         RRset" RR update and no other "Add to an RRset" instructions
         for that Service Instance Name.
   *  and contains no other add or delete RR updates for the same name
      as the PTR RR Update.

   Note that there can be more than one Service Discovery Instruction
   for the same name if the SRP requestor is advertising more than one
   service of the same type, or is changing the target of a PTR RR.
   This is also true for SRP subtypes (Section 7.1 of [RFC6763]).  For
   each such PTR RR add or remove, the above constraints must be met.

2.3.1.2.  Service Description Instruction

   An instruction is a Service Description Instruction if, for the
   appropriate Service Instance Name, it contains

   *  exactly one "Delete all RRsets from a name" update for the service
      instance name ([RFC2136], Section 2.5.3),
   *  zero or one "Add to an RRset" SRV RR,
   *  zero or one "Add to an RRset" KEY RR that, if present, contains
      the public key corresponding to the private key that was used to
      sign the message (if present, the KEY MUST match the KEY RR given
      in the Host Description),
   *  zero or more "Add to an RRset" TXT RRs,
   *  If there is one "Add to an RRset" SRV update, there MUST be at
      least one "Add to an RRset" TXT update.
   *  the target of the SRV RR Add, if present points to a hostname for
      which there is a Host Description Instruction in the SRP Update,
      or
   *  if there is no "Add to an RRset" SRV RR, then either
      -  the name to which the "Delete all RRsets from a name" applies
         does not exist, or

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      -  there is an existing KEY RR on that name, which matches the key
         with which the SRP Update was signed.
   *  Service Descriptions Instructions do not modify any other resource
      records.

   An SRP registrar MUST correctly handle compressed names in the SRV
   target.

2.3.1.3.  Host Description Instruction

   An instruction is a Host Description Instruction if, for the
   appropriate hostname, it contains

   *  exactly one "Delete all RRsets from a name" RR,
   *  one or more "Add to an RRset" RRs of type A and/or AAAA,
   *  exactly one "Add to an RRset" RR that adds a KEY RR that contains
      the public key corresponding to the private key that was used to
      sign the message,
   *  Host Description Instructions do not modify any other resource
      records.

   A and/or AAAA records that are not of of sufficient scope to be
   validly published in a DNS zone can be ignored by the SRP server,
   which could result in a host description effectively containing zero
   reachable addresses even when it contains one or more addresses.

   For example, if a link-scope address or IPv4 autoconfiguration
   address is provided by the SRP requestor, the SRP registrar could not
   publish this in a DNS zone.  However, in some situations, the SRP
   registrar may make the records available through a mechanism such as
   an advertising proxy only on the specific link from which the SRP
   update originated; in such a situation, locally-scoped records are
   still valid.

2.3.2.  Valid SRP Update Requirements

   An SRP Update MUST contain exactly one Host Description Instruction.
   In addition, there MUST NOT be any Service Description Instruction to
   which no Service Discovery Instruction points.  A DNS Update that
   contains any additional adds or deletes that cannot be identified as
   Service Discovery, Service Description or Host Description
   Instructions is not an SRP Update.  A DNS update that contains any
   prerequisites is not an SRP Update.  An SRP registrar MAY either
   process such messages are either processed as regular RFC2136
   updates, including access control checks and constraint checks, if
   supported, or MAY reject them with Refused RCODE.

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   If the definitions of each of these instructions are followed
   carefully and the update requirements are validated correctly, many
   DNS Updates that look very much like SRP Updates nevertheless will
   fail to validate.  For example, a DNS update that contains an Add to
   an RRset instruction for a Service Name and an Add to an RRset
   instruction for a Service Instance Name, where the PTR record added
   to the Service Name does not reference the Service Instance Name, is
   not a valid SRP Update message, but may be a valid RFC2136 update.

2.3.3.  FCFS Name And Signature Validation

   Assuming that a DNS Update message has been validated with these
   conditions and is a valid SRP Update, the registrar checks that the
   name in the Host Description Instruction exists.  If so, then the
   registrar checks to see if the KEY record on that name is the same as
   the KEY record in the Host Description Instruction.  The registrar
   performs the same check for the KEY records in any Service
   Description Instructions.  For KEY records that were omitted from
   Service Description Instructions, the KEY from the Host Description
   Instruction is used.  If any existing KEY record corresponding to a
   KEY record in the SRP Update does not match the KEY record in the SRP
   Update (whether provided or taken from the Host Description
   Instruction), then the registrar MUST reject the SRP Update with the
   YXDomain RCODE.

   Otherwise, the registrar validates the SRP Update using SIG(0)
   against the public key in the KEY record of the Host Description
   Instruction.  If the validation fails, the registrar MUST reject the
   SRP Update with the Refused RCODE.  Otherwise, the SRP Update is
   considered valid and authentic, and is processed according to the
   method described in RFC2136.

   KEY record updates omitted from Service Description Instruction are
   processed as if they had been explicitly present: every Service
   Description that is updated MUST, after the SRP Update has been
   applied, have a KEY RR, and it must be the same KEY RR that is
   present in the Host Description to which the Service Description
   refers.

2.3.4.  Handling of Service Subtypes

   SRP registrars MUST treat the update instructions for a service type
   and all its subtypes as atomic.  That is, when a service and its
   subtypes are being updated, whatever information appears in the SRP
   Update is the entirety of information about that service and its
   subtypes.  If any subtype appeared in a previous update but does not
   appear in the current update, then the SRP registrar MUST remove that
   subtype.

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   Similarly, there is no mechanism for deleting subtypes.  A delete of
   a service deletes all of its subtypes.  To delete an individual
   subtype, an SRP Update must be constructed that contains the service
   type and all subtypes for that service.

2.3.5.  SRP Update response

   The status that is returned depends on the result of processing the
   update, and can be either NoError or ServFail: all other possible
   outcomes should already have been accounted for when applying the
   constraints that qualify the update as an SRP Update.

2.3.6.  Optional Behavior

   The registrar MAY add a Reverse Mapping (Section 3.5 of [RFC1035],
   Section 2.5 of [RFC3596]) that corresponds to the Host Description.
   This is not required because the Reverse Mapping serves no protocol
   function, but it may be useful for debugging, e.g. in annotating
   network packet traces or logs.  In order for the registrar to do a
   reverse mapping update, it must be authoritative for the zone that
   would need to be updated, or have credentials to do the update.  The
   SRP requestor MAY also do a reverse mapping update if it has
   credentials to do so.

   The registrar MAY apply additional criteria when accepting updates.
   In some networks, it may be possible to do out-of-band registration
   of keys, and only accept updates from pre-registered keys.  In this
   case, an update for a key that has not been registered should be
   rejected with the Refused RCODE.

   There are at least two benefits to doing this rather than simply
   using normal SIG(0) DNS updates.  First, the same registration
   protocol can be used in both cases, so both use cases can be
   addressed by the same SRP requestor implementation.  Second, the
   registration protocol includes maintenance functionality not present
   with normal DNS updates.

   Note that the semantics of using SRP in this way are different than
   for typical RFC2136 implementations: the KEY used to sign the SRP
   Update only allows the SRP requestor to update records that refer to
   its Host Description.  RFC2136 implementations do not normally
   provide a way to enforce a constraint of this type.

   The registrar could also have a dictionary of names or name patterns
   that are not permitted.  If such a list is used, updates for Service
   Instance Names that match entries in the dictionary are rejected with
   Refused.

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3.  TTL Consistency

   All RRs within an RRset are required to have the same TTL
   (Clarifications to the DNS Specification [RFC2181], Section 5.2).  In
   order to avoid inconsistencies, SRP places restrictions on TTLs sent
   by requestors and requires that SRP registrars enforce consistency.

   Requestors sending SRP Updates MUST use consistent TTLs in all RRs
   within the SRP Update.

   SRP registrars MUST check that the TTLs for all RRs within the SRP
   Update are the same.  If they are not, the SRP update MUST be
   rejected with a Refused RCODE.

   Additionally, when adding RRs to an RRset, for example when
   processing Service Discovery records, the registrar MUST use the same
   TTL on all RRs in the RRset.  How this consistency is enforced is up
   to the implementation.

   TTLs sent in SRP Updates are advisory: they indicate the SRP
   requestor's guess as to what a good TTL would be.  SRP registrars may
   override these TTLs.  SRP registrars SHOULD ensure that TTLs are
   reasonable: neither too long nor too short.  The TTL should never be
   longer than the lease time (Section 4.1).  Shorter TTLs will result
   in more frequent data refreshes; this increases latency on the DNS-SD
   client side, increases load on any caching resolvers and on the
   authoritative server, and also increases network load, which may be
   an issue for constrained networks.  Longer TTLs will increase the
   likelihood that data in caches will be stale.  TTL minimums and
   maximums SHOULD be configurable by the operator of the SRP registrar.

4.  Maintenance

4.1.  Cleaning up stale data

   Because the DNS-SD registration protocol is automatic, and not
   managed by humans, some additional bookkeeping is required.  When an
   update is constructed by the SRP requestor, it MUST include an
   EDNS(0) Update Lease Option [I-D.ietf-dnssd-update-lease].  The
   Update Lease Option contains two lease times: the Lease Time and the
   Key Lease Time.

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   These leases are promises, similar to DHCP leases [RFC2131], from the
   SRP requestor that it will send a new update for the service
   registration before the lease time expires.  The Lease time is chosen
   to represent the time after the update during which the registered
   records other than the KEY record should be assumed to be valid.  The
   Key Lease time represents the time after the update during which the
   KEY record should be assumed to be valid.

   The reasoning behind the different lease times is discussed in the
   section on first-come, first-served naming (Section 2.2.4.1).  SRP
   registrars may be configured with limits for these values.  A default
   limit of two hours for the Lease and 14 days for the SIG(0) KEY are
   currently thought to be good choices.  Constrained devices with
   limited battery that wake infrequently are likely to request longer
   leases; registrars that support such devices may need to set higher
   limits.  SRP requestors that are going to continue to use names on
   which they hold leases should update well before the lease ends, in
   case the SRP registrar is unavailable or under heavy load.

   The lease time applies specifically to the host.  All service
   instances, and all service entries for such service instances, depend
   on the host.  When the lease on a host expires, the host and all
   services that reference it MUST be removed at the same time-it is
   never valid for a service instance to remain when the host it
   references has been removed.  If the KEY record for the host is to
   remain, the KEY record for any services that reference it MUST also
   remain.  However, the service PTR record MUST be removed, since it
   has no key associated with it, and since it is never valid to have a
   service PTR record for which there is no service instance on the
   target of the PTR record.

   SRP registrars MUST also track a lease time per service instance.
   The reason for doing this is that a requestor may re-register a host
   with a different set of services, and not remember that some
   different service instance had previously been registered.  In this
   case, when that service instance lease expires, the SRP registrar
   MUST remove the service instance (although the KEY record for the
   service instance SHOULD be retained until the key lease on that
   service expires).  This is beneficial because otherwise if the SRP
   requestor continues to renew the host, but never mentions the stale
   service again, the stale service will continue to be advertised.

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   The SRP registrar MUST include an EDNS(0) Update Lease option in the
   response if the lease time proposed by the requestor has been
   shortened or lengthened by the registrar.  The requestor MUST check
   for the EDNS(0) Update Lease option in the response and MUST use the
   lease times from that option in place of the options that it sent to
   the registrar when deciding when to renew its registration.  The
   times may be shorter or longer than those specified in the SRP
   Update; the SRP requestor must honor them in either case.

   SRP requestors should assume that each lease ends N seconds after the
   update was first transmitted, where N is the lease duration.
   Registrars should assume that each lease ends N seconds after the
   update that was successfully processed was received.  Because the
   registrar will always receive the update after the SRP requestor sent
   it, this avoids the possibility of misunderstandings.

   SRP registrars MUST reject updates that do not include an EDNS(0)
   Update Lease option.  Dual-use servers MAY accept updates that don't
   include leases, but SHOULD differentiate between SRP Updates and
   other updates, and MUST reject updates that would otherwise be SRP
   Updates if they do not include leases.

   Lease times have a completely different function than TTLs.  On an
   authoritative DNS server, the TTL on a resource record is a constant:
   whenever that RR is served in a DNS response, the TTL value sent in
   the answer is the same.  The lease time is never sent as a TTL; its
   sole purpose is to determine when the authoritative DNS server will
   delete stale records.  It is not an error to send a DNS response with
   a TTL of 'n' when the remaining time on the lease is less than 'n'.

5.  Security Considerations

5.1.  Source Validation

   SRP Updates have no authorization semantics other than first-come,
   first-served.  This means that if an attacker from outside of the
   administrative domain of the registrar knows the registrar's IP
   address, it can in principle send updates to the registrar that will
   be processed successfully.  Registrars should therefore be configured
   to reject updates from source addresses outside of the administrative
   domain of the registrar.

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   For TCP updates, the initial SYN-SYN+ACK handshake prevents updates
   being forged by an off-network attacker.  In order to ensure that
   this handshake happens, SRP registrars relying on three-way-handshake
   validation MUST NOT accept TCP Fast Open payloads.  If the network
   infrastructure allows it, an SRP registrar MAY accept TCP Fast Open
   payloads if all such packets are validated along the path, and the
   network is able to reject this type of spoofing at all ingress
   points.

   For UDP updates from constrained devices, spoofing would have to be
   prevented with appropriate source address filtration on routers
   [RFC2827].  This would ordinarily be accomplished by measures such as
   are described in Section 4.5 of [RFC7084]

   Note that these rules only apply to the validation of SRP Updates.  A
   server that accepts updates from SRP requestors may also accept other
   DNS updates, and those DNS updates may be validated using different
   rules.  However, in the case of a DNS server that accepts SRP
   updates, the intersection of the SRP Update rules and whatever other
   update rules are present must be considered very carefully.

   For example, a normal, authenticated DNS update to any RR that was
   added using SRP, but that is authenticated using a different key,
   could be used to override a promise made by the SRP registrar to an
   SRP requestor, by replacing all or part of the service registration
   information with information provided by an authenticated DNS update
   requestor.  An implementation that allows both kinds of updates
   should not allow DNS Update requestors that are using different
   authentication and authorization credentials to to update records
   added by SRP requestors.

5.2.  SRP Registrar Authentication

   This specification does not provide a mechanism for validating
   responses from SRP Registrars to SRP requestors.  In principle, a KEY
   RR could be used by a non-constrained SRP requestor to validate
   responses from the registrar, but this is not required, nor do we
   specify a mechanism for determining which key to use.

5.3.  Required Signature Algorithm

   For validation, SRP registrars MUST implement the ECDSAP256SHA256
   signature algorithm.  SRP registrars SHOULD implement the algorithms
   specified in [RFC8624], Section 3.1, in the validation column of the
   table, that are numbered 13 or higher and have a "MUST",
   "RECOMMENDED", or "MAY" designation in the validation column of the
   table.  SRP requestors MUST NOT assume that any algorithm numbered
   lower than 13 is available for use in validating SIG(0) signatures.

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6.  Privacy Considerations

   Because DNSSD SRP Updates can be sent off-link, the privacy
   implications of SRP are different than for multicast DNS responses.
   Host implementations that are using TCP SHOULD also use TLS if
   available.  Registrar implementations MUST offer TLS support.  The
   use of TLS with DNS is described in [RFC7858].

   Hosts that implement TLS support SHOULD NOT fall back to TCP; since
   registrars are required to support TLS, it is entirely up to the host
   implementation whether to use it.

   Public keys can be used as identifiers to track hosts.  SRP
   registrars MAY elect not to return KEY records for queries for SRP
   registrations.

7.  Delegation of 'service.arpa.'

   In order to be fully functional, the owner of the 'arpa.' zone must
   add a delegation of 'service.arpa.' in the '.arpa.' zone [RFC3172].
   This delegation should be set up as was done for 'home.arpa', as a
   result of the specification in Section 7 of [RFC8375].  This is
   currently the responsibility of the IAB [IAB-ARPA]

8.  IANA Considerations

8.1.  Registration and Delegation of 'service.arpa' as a Special-Use
      Domain Name

   IANA is requested to record the domain name 'service.arpa.' in the
   Special-Use Domain Names registry [SUDN].  IANA is requested, with
   the approval of IAB, to implement the delegation requested in
   Section 7.

   IANA is further requested to add a new entry to the "Transport-
   Independent Locally-Served Zones" subregistry of the the "Locally-
   Served DNS Zones" registry [LSDZ].  The entry will be for the domain
   'service.arpa.' with the description "DNS-SD Service Registration
   Protocol Special-Use Domain", listing this document as the reference.

8.2.  Subdomains of 'service.arpa.'

   This document only makes use of the 'default.service.arpa' subdomain
   of 'service.arpa.'  Other subdomains are reserved for future use by
   DNS-SD or related work.  The IANA is requested to create a registry,
   the "service.arpa Subdomain Registry".  This registry shall begin as
   the following table:

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   +================+================================+=================+
   | Subdomain Name | Description                    | reference       |
   +================+================================+=================+
   | default        | Default domain for SRP updates | [THIS           |
   |                |                                | DOCUMENT]       |
   +----------------+--------------------------------+-----------------+

                                  Table 1

8.3.  'dnssd-srp' Service Name

   IANA is also requested to add a new entry to the Service Names and
   Port Numbers registry for dnssd-srp with a transport type of tcp.  No
   port number is to be assigned.  The reference should be to this
   document, and the Assignee and Contact information should reference
   the authors of this document.  The Description should be "DNS-SD
   Service Registration."

8.4.  'dnssd-srp-tls' Service Name

   IANA is also requested to add a new entry to the Service Names and
   Port Numbers registry for dnssd-srp-tls with a transport type of tcp.
   No port number is to be assigned.  The reference should be to this
   document, and the Assignee and Contact information should reference
   the authors of this document.  The Description should be "DNS-SD
   Service Registration (TLS).

8.5.  Anycast Address

   IANA is requested to allocate an IPv6 Anycast address from the IPv6
   Special-Purpose Address Registry, similar to the Port Control
   Protocol anycast address, 2001:1::1.  The value TBD should be
   replaced with the actual allocation in the table that follows.  The
   values for the registry are:

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          +----------------------+-----------------------------+
          | Attribute            | value                       |
          +----------------------+-----------------------------+
          | Address Block        | 2001:1::TBD/128             |
          +----------------------+-----------------------------+
          | Name                 | DNS-SD Service Registration |
          |                      | Protocol Anycast Address    |
          +----------------------+-----------------------------+
          | RFC                  | [this document]             |
          +----------------------+-----------------------------+
          | Allocation Date      | [date of allocation]        |
          +----------------------+-----------------------------+
          | Termination Date     | N/A                         |
          +----------------------+-----------------------------+
          | Source               | True                        |
          +----------------------+-----------------------------+
          | Destination          | True                        |
          +----------------------+-----------------------------+
          | Forwardable          | True                        |
          +----------------------+-----------------------------+
          | Global               | True                        |
          +----------------------+-----------------------------+
          | Reserved-by-protocol | False                       |
          +----------------------+-----------------------------+

                                 Table 2

9.  Implementation Status

   [Note to the RFC Editor: please remove this section prior to
   publication.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation

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   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   There are two known independent implementations of SRP requestors:

   *  SRP Client for OpenThread:
      https://github.com/openthread/openthread/pull/6038

   *  mDNSResponder open source project: https://github.com/Abhayakara/
      mdnsresponder

   There are two related implementations of an SRP registrar.  One acts
   as a DNS Update proxy, taking an SRP Update and applying it to the
   specified DNS zone using DNS update.  The other acts as an
   Advertising Proxy [AP].  Both are included in the mDNSResponder open
   source project mentioned above.

10.  Acknowledgments

   Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
   Dong and Abtin Keshavarzian for their thorough technical reviews.
   Thanks to Kangping and Abtin as well for testing the document by
   doing an independent implementation.  Thanks to Tamara Kemper for
   doing a nice developmental edit, Tim Wattenberg for doing a SRP
   requestor proof-of-concept implementation at the Montreal Hackathon
   at IETF 102, and Tom Pusateri for reviewing during the hackathon and
   afterwards.  Thanks to Esko for a really thorough second last call
   review.  Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
   Dong, Martin Turon and Michael Cowan for their detailed second last
   call reviews.

11.  Normative References

   [I-D.ietf-dnssd-update-lease]
              Cheshire, S. and T. Lemon, "An EDNS0 option to negotiate
              Leases on DNS Updates", Work in Progress, Internet-Draft,
              draft-ietf-dnssd-update-lease-02, 11 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-dnssd-update-
              lease-02.txt>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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   [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,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2539]  Eastlake 3rd, D., "Storage of Diffie-Hellman Keys in the
              Domain Name System (DNS)", RFC 2539, DOI 10.17487/RFC2539,
              March 1999, <https://www.rfc-editor.org/info/rfc2539>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

   [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational
              Requirements for the Address and Routing Parameter Area
              Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
              September 2001, <https://www.rfc-editor.org/info/rfc3172>.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", STD 88,
              RFC 3596, DOI 10.17487/RFC3596, October 2003,
              <https://www.rfc-editor.org/info/rfc3596>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8375]  Pfister, P. and T. Lemon, "Special-Use Domain
              'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
              <https://www.rfc-editor.org/info/rfc8375>.

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   [RFC8624]  Wouters, P. and O. Sury, "Algorithm Implementation
              Requirements and Usage Guidance for DNSSEC", RFC 8624,
              DOI 10.17487/RFC8624, June 2019,
              <https://www.rfc-editor.org/info/rfc8624>.

   [RFC8765]  Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              RFC 8765, DOI 10.17487/RFC8765, June 2020,
              <https://www.rfc-editor.org/info/rfc8765>.

   [SUDN]     "Special-Use Domain Names Registry", July 2012,
              <https://www.iana.org/assignments/special-use-domain-
              names/special-use-domain-names.xhtml>.

   [LSDZ]     "Locally-Served DNS Zones Registry", July 2011,
              <https://www.iana.org/assignments/locally-served-dns-
              zones/locally-served-dns-zones.xhtml>.

   [IAB-ARPA] "Internet Architecture Board statement on the registration
              of special use names in the ARPA domain", March 2017,
              <https://www.iab.org/documents/correspondence-reports-
              documents/2017-2/iab-statement-on-the-registration-of-
              special-use-names-in-the-arpa-domain/>.

12.  Informative References

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, DOI 10.17487/RFC6760, February 2013,
              <https://www.rfc-editor.org/info/rfc6760>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

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   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,
              <https://www.rfc-editor.org/info/rfc7084>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8766]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
              2020, <https://www.rfc-editor.org/info/rfc8766>.

   [RFC8945]  Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
              Gudmundsson, O., and B. Wellington, "Secret Key
              Transaction Authentication for DNS (TSIG)", STD 93,
              RFC 8945, DOI 10.17487/RFC8945, November 2020,
              <https://www.rfc-editor.org/info/rfc8945>.

   [ROADMAP]  Cheshire, S., "Service Discovery Road Map", Work in
              Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
              23 October 2018, <https://www.ietf.org/archive/id/draft-
              cheshire-dnssd-roadmap-03.txt>.

   [AP]       Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
              Service Registration Protocol", Work in Progress,
              Internet-Draft, draft-ietf-dnssd-advertising-proxy-01, 11
              July 2022, <https://www.ietf.org/archive/id/draft-ietf-
              dnssd-advertising-proxy-01.txt>.

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

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Appendix A.  Testing using standard RFC2136-compliant DNS servers

   It may be useful to set up an authoritative DNS server for testing
   that does not implement SRP.  This can be done by configuring the
   server to listen on the anycast address, or advertising it in the
   _dnssd-srp._tcp.<zone> SRV and _dnssd-srp-tls._tcp.<zone> record.  It
   must be configured to be authoritative for "default.service.arpa",
   and to accept updates from hosts on local networks for names under
   "default.service.arpa" without authentication, since such servers
   will not have support for FCFS authentication (Section 2.2.4.1).

   An authoritative DNS server configured in this way will be able to
   successfully accept and process SRP Updates from requestors that send
   SRP updates.  However, no prerequisites will be applied, and this
   means that the test server will accept internally inconsistent SRP
   Updates, and will not stop two SRP Updates, sent by different
   services, that claim the same name(s), from overwriting each other.

   Since SRP Updates are signed with keys, validation of the SIG(0)
   algorithm used by the requestor can be done by manually installing
   the requestor's public key on the DNS server that will be receiving
   the updates.  The key can then be used to authenticate the SRP
   update, and can be used as a requirement for the update.  An example
   configuration for testing SRP using BIND 9 is given in Appendix C.

Appendix B.  How to allow SRP requestors to update standard
             RFC2136-compliant servers

   Ordinarily SRP Updates will fail when sent to an RFC 2136-compliant
   server that does not implement SRP because the zone being updated is
   "default.service.arpa", and no DNS server that is not an SRP
   registrar should normally be configured to be authoritative for
   "default.service.arpa".  Therefore, a requestor that sends an SRP
   Update can tell that the receiving server does not support SRP, but
   does support RFC2136, because the RCODE will either be NotZone,
   NotAuth or Refused, or because there is no response to the update
   request (when using the anycast address)

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   In this case a requestor MAY attempt to register itself using regular
   RFC2136 DNS updates.  To do so, it must discover the default
   registration zone and the DNS server designated to receive updates
   for that zone, as described earlier, using the _dns-update._udp SRV
   record.  It can then send the update to the port and host pointed to
   by the SRV record, and should use appropriate prerequisites to avoid
   overwriting competing records.  Such updates are out of scope for
   SRP, and a requestor that implements SRP MUST first attempt to use
   SRP to register itself, and should only attempt to use RFC2136
   backwards compatibility if that fails.  Although the owner name for
   the SRV record specifies the UDP protocol for updates, it is also
   possible to use TCP, and TCP should be required to prevent spoofing.

Appendix C.  Sample BIND9 configuration for default.service.arpa.

   zone "default.service.arpa." {
     type master;
     file "/etc/bind/master/service.db";
     allow-update { key demo.default.service.arpa.; };
   };

                 Figure 1: Zone Configuration in named.conf

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 $ORIGIN .
 $TTL 57600  ; 16 hours
 default.service.arpa IN SOA          ns3.default.service.arpa.
                                      postmaster.default.service.arpa. (
                 2951053287 ; serial
                 3600       ; refresh (1 hour)
                 1800       ; retry (30 minutes)
                 604800     ; expire (1 week)
                 3600       ; minimum (1 hour)
 )
                         NS           ns3.default.service.arpa.
                         SRV 0 0 53   ns3.default.service.arpa.
 $ORIGIN default.service.arpa.
 $TTL 3600   ; 1 hour
 _ipps._tcp              PTR          demo._ipps._tcp
 $ORIGIN _ipps._tcp.default.service.arpa.
 demo                    TXT          "0"
                         SRV 0 0 9992 demo.default.service.arpa.
 $ORIGIN _udp.default.service.arpa.
 $TTL 3600   ; 1 hour
 _dns-update             PTR          ns3.default.service.arpa.
 $ORIGIN _tcp.default.service.arpa.
 _dnssd-srp              PTR          ns3.default.service.arpa.
 $ORIGIN default.service.arpa.
 $TTL 300    ; 5 minutes
 ns3                     AAAA         2001:db8:0:1::1
 $TTL 3600   ; 1 hour
 demo                    AAAA         2001:db8:0:2::1
                         KEY 513 3 13 (
                            qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
                            9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
                         ); alg = ECDSAP256SHA256 ; key id = 15008
                         AAAA    ::1

                      Figure 2: Example Zone file

Authors' Addresses

   Ted Lemon
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America
   Email: mellon@fugue.com

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   Stuart Cheshire
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America
   Phone: +1 408 974 3207
   Email: cheshire@apple.com

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