Internet Engineering Task Force (IETF)                          T. Lemon
Internet-Draft
Request for Comments: 9665                                   S. Cheshire
Intended status:
Category: Standards Track                                     Apple Inc.
Expires: 5 September 2024                                   4 March
ISSN: 2070-1721                                             October 2024

     Service Registration Protocol for DNS-Based Service Discovery
                        draft-ietf-dnssd-srp-25

Abstract

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

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://dnssd-
   wg.github.io/draft-ietf-dnssd-srp/draft-ietf-dnssd-srp.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-dnssd-srp/.

   Discussion of this document takes place on the DNS-SD Working Group
   mailing list (mailto:dnssd@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/dnssd/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/dnssd/.

   Source for this draft and an issue tracker can be found at
   https://github.com/dnssd-wg/draft-ietf-dnssd-srp.

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 5 September 2024.
   https://www.rfc-editor.org/info/rfc9665.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Terminology Used in This Document . . . . . .   6
   3.  Service Registration Protocol . . . . . . . . . . . . . . . .   6
     3.1.  Protocol Variants . . . . . . . . . . . . . . . . . . . .   7
       3.1.1.  Full-featured  Full-Featured Hosts . . . . . . . . . . . . . . . . .   7
       3.1.2.  Constrained Hosts . . . . . . . . . . . . . . . . . .   7
       3.1.3.  Why two variants? . . . . . . . . . . . . . . . . . .   8
     3.2.  Protocol Details  . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  What to publish . . . . . . . . . . . . . . . . . . .   8 Publish
       3.2.2.  Where to publish it . . . . . . . . . . . . . . . . .   9 Publish It
       3.2.3.  How to publish it . . . . . . . . . . . . . . . . . .  10 Publish It
         3.2.3.1.  How the DNS-SD Service Registration process differs Process Differs
                 from the DNS Update as specified Specified in RFC2136 . . . . . .  10 RFC 2136
         3.2.3.2.  Retransmission Strategy . . . . . . . . . . . . .  11
         3.2.3.3.  Successive Updates  . . . . . . . . . . . . . . .  11
       3.2.4.  How to secure it  . . . . . . . . . . . . . . . . . .  11 Secure It
         3.2.4.1.  First-Come First-Served  FCFS Naming  . . . . . . . . .  11
       3.2.5.  SRP Requestor Behavior  . . . . . . . . . . . . . . .  12
         3.2.5.1.  Public/Private key pair generation Key Pair Generation and storage  .  12 Storage
         3.2.5.2.  Name Conflict Handling  . . . . . . . . . . . . .  13
         3.2.5.3.  Record Lifetimes  . . . . . . . . . . . . . . . .  13
         3.2.5.4.  Compression in SRV records  . . . . . . . . . . .  13 Records
         3.2.5.5.  Removing published services . . . . . . . . . . .  14 Published Services
     3.3.  Validation and Processing of SRP Updates  . . . . . . . .  15
       3.3.1.  Validation of DNS Update Add and Delete RRs . . . . .  15
         3.3.1.1.  Service Discovery Instruction . . . . . . . . . .  16
         3.3.1.2.  Service Description Instruction . . . . . . . . .  17
         3.3.1.3.  Host Description Instruction  . . . . . . . . . .  17
       3.3.2.  Valid SRP Update Requirements . . . . . . . . . . . .  18
       3.3.3.  FCFS Name And and Signature Validation  . . . . . . . . .  18
       3.3.4.  Handling of Service Subtypes  . . . . . . . . . . . .  19
       3.3.5.  SRP Update response . . . . . . . . . . . . . . . . .  20 Response
       3.3.6.  Optional Behavior . . . . . . . . . . . . . . . . . .  20
   4.  TTL Consistency . . . . . . . . . . . . . . . . . . . . . . .  21
   5.  Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  Cleaning up stale data  . . . . . . . . . . . . . . . . .  22 Up Stale Data
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
     6.1.  Source Validation . . . . . . . . . . . . . . . . . . . .  24
     6.2.  Other DNS updates . . . . . . . . . . . . . . . . . . . .  24 Updates
     6.3.  Risks of allowing arbitrary names Allowing Arbitrary Names to be registered Registered in SRP
           updates . . . . . . . . . . . . . . . . . . . . . . . . .  25
           Updates
     6.4.  Security of local service discovery . . . . . . . . . . .  25 Local Service Discovery
     6.5.  SRP Registrar Authentication  . . . . . . . . . . . . . .  26
     6.6.  Required Signature Algorithm  . . . . . . . . . . . . . .  26
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   8.  Domain Name Reservation Considerations  . . . . . . . . . . .  27
     8.1.  Users . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.2.  Application Software  . . . . . . . . . . . . . . . . . .  27
     8.3.  Name Resolution APIs and Libraries  . . . . . . . . . . .  27
     8.4.  Caching DNS Servers . . . . . . . . . . . . . . . . . . .  28
     8.5.  Authoritative DNS Servers . . . . . . . . . . . . . . . .  29
     8.6.  DNS Server Operators  . . . . . . . . . . . . . . . . . .  29
     8.7.  DNS Registries/Registrars . . . . . . . . . . . . . . . .  29
   9.  Delegation of 'service.arpa.' . . . . . . . . . . . . . . . .  29 "service.arpa."
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Registration and Delegation of 'service.arpa' "service.arpa" as a
            Special-Use Domain Name  . . . . . . . . . . . . . . . .  30
     10.2.  Subdomains of 'service.arpa.'  . . . . . . . . . . . . .  30 "service.arpa."
     10.3.  Service Name registrations . . . . . . . . . . . . . . .  30
     10.4. Registrations
       10.3.1.  'dnssd-srp' Service Name . . . . . . . . . . . . . . . .  31
     10.5.
       10.3.2.  'dnssd-srp-tls' Service Name . . . . . . . . . . . . . .  31
     10.6.
     10.4.  Anycast Address  . . . . . . . . . . . . . . . . . . . .  32
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 33
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 36
   Appendix A.  Testing using standard RFC2136-compliant Using Standard DNS
           servers . . . . . . . . . . . . . . . . . . . . . . . . .  38 Servers Compliant with RFC
           2136
   Appendix B.  How to allow Allow SRP requestors Requestors to update standard
           RFC2136-compliant servers . . . . . . . . . . . . . . . .  39 Update Standard Servers
           Compliant with RFC 2136
   Appendix C.  Sample BIND9 configuration Configuration for
           default.service.arpa. . . . . . . . . . . . . . . . . . .  39 "default.service.arpa."
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   DNS-Based Service Discovery [RFC6763]

   DNS-SD (see [RFC6763]) is a component of Zero Configuration
   Networking [RFC6760] [ZC] [ROADMAP]. (see [RFC6760], [ZC], and [ROADMAP]).

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

   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 expected to provide
   sufficient information to allow interoperable implementation of the
   registration protocol.

   DNS-Based Service Discovery (DNS-SD)

   DNS-SD allows services to advertise the fact that they provide service,
   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 Service
   Discovery (DNS-SD) DNS-SD using the DNS protocol (as
   opposed to mDNS) can be more efficient and versatile, it is not
   common in practice, practice because of the difficulties associated with
   updating authoritative DNS services with service information.

   Existing

   The existing practice for updating DNS zones is to either to manually
   enter new data, data or else to use a DNS Update [RFC2136].  Unfortunately (see [RFC2136]).
   Unfortunately, a DNS Update requires either either:

   *  that the authoritative DNS server automatically trust
   updates, 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, the DNS Update can be a fairly chatty process, requiring
   multiple round trips roundtrips with different conditional predicates to complete
   the update process.

   The Service Registration Protocol (SRP) adds a set of default
   heuristics for processing DNS updates that eliminates the need for
   DNS update conditional predicates: instead,
   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, update; and the update either succeeds
   entirely,
   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, "First Come, First Served Naming" (or
   "FCFS Naming"), which allows the requestor to to:

   *  claim a name that is not yet in use, and, and

   *  using SIG(0) [RFC2931], to ([RFC2931]), 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 it: so its claim will be rejected rejected, and the
   second requestor will have to choose a new name.

   It is important to understand that "authenticate" here just means
   that we can tell that an update came from the same source as the
   original registration.  We have not established trust.  This has
   important implications for what we can and can't do with data the
   client sends us.  You will notice as you read this document that we
   only support adding a very restricted set of records, and the content
   of those records is further constrained.

   The reason for this is precisely that we have not established trust.
   So
   So, we can only publish information that we feel safe in publishing
   even though we do not have any basis for trusting the requestor.  We
   reason that mDNS [RFC6762] ([RFC6762]) allows arbitrary hosts on a single IP
   link to advertise services [RFC6763], ([RFC6763]), relying on whatever service
   is advertised to provide authentication as a part of its protocol
   rather than in the service advertisement.

   This is considered reasonably safe because it requires physical
   presence on the network in order to advertise.  An off-network mDNS
   attack is simply not possible.  Our goal with this specification is
   to impose similar constraints.  Because of this  Therefore, you will see in
   Section 3.3.1 that a very restricted set of records with a very
   restricted set of relationships are allowed.  You will also see in
   Section 6.1 that we give advice on how to prevent off-network
   attacks.

   This leads us to the disappointing observation that this protocol is
   not a mechanism for adding arbitrary information to DNS zones.  We
   have not evaluated the security properties of adding, for example, an
   SOA record, an MX record, or a CNAME record, and so record; therefore, these are
   forbidden.  A future protocol specification might include analyses
   for other records, records and extend the set of records that can be
   registered here.  Or it might require establishment of trust, and add
   an authorization model to the authentication model we now have.  But
   this is work for a future document.

   Finally, SRP adds the concept of a 'lease,' "lease", similar to leases in
   Dynamic Host Configuration Protocol [RFC8415]. DHCP
   ([RFC8415]).  The SRP registration itself has a lease which that 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, lease so that another requestor cannot claim
   the name, even though the registration has expired.

   The Service Registration Protocol SRP for DNS-SD (SRP), specified in this
   document, 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], (see Section 10, “Populating the DNS
   with Information”), 10 of [RFC6763] 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.

   RFC6763

   RFC 6763 also allows clients to discover services using the DNS
   protocol [RFC1035]. (see [RFC1035]).  This can be done by having a system
   administrator manually configure service information in the DNS, but DNS;
   however, manually populating DNS authoritative server databases is
   costly and potentially error-prone, error-prone and requires a knowledgeable
   network administrator.  Consequently, although all DNS-SD client
   implementations of which we are aware support DNS-SD using DNS
   queries, in practice practice, it is used much less frequently than mDNS.

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

2.  Conventions and Terminology Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Service Registration Protocol

   Services that implement SRP use DNS Update (see [RFC2136] [RFC3007] and
   [RFC3007]) to publish service information in the DNS.  Two variants exist,
   exist: one for full-featured hosts, hosts and one for devices designed for "Constrained-
   Node Networks" [RFC7228].
   Constrained-Node Networks (CNNs) ([RFC7228]).  An SRP registrar is
   most likely an authoritative DNS server, 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.

3.1.  Protocol Variants

3.1.1.  Full-featured  Full-Featured Hosts

   Full-featured hosts either are configured manually with a
   registration domain, 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 SRP is not discoverable on
   the local network using this mechanism.  Other discovery mechanisms
   are possible, but they are out of scope for this document.

   Manual configuration of the registration domain can be done either either:

   *  by querying the list of available registration domains
      ("r._dns-sd._udp") and allowing the user to select one from the UI, 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) service or services as subdomains of the
   chosen service registration domain.  For these names names, they then
   discover the zone apex of the closest enclosing DNS zone using SOA
   queries (see Section 6.1 of [RFC8765]. [RFC8765]).  Having discovered the
   enclosing DNS zone, they query for the "_dnssd-srp._tcp.<zone>" SRV
   record to discover the server to which they can send SRP updates.
   Hosts that support SRP Updates using TLS use the
   "_dnssd-srp-tls._tcp.<zone>" SRV record instead.

   Examples of full-featured hosts include devices such as home
   computers, laptops, powered peripherals with network connections such
   (such as printers, home routers, and even battery-operated devices
   such as mobile phones that have long battery lives. lives).

3.1.2.  Constrained Hosts

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

   SRP requestors on constrained networks are expected to receive receive, from
   the network network, a list of SRP registrars with which to register.  It is
   the responsibility of a Constrained-Node Network CNN supporting SRP to provide one or more
   registrar addresses.  It is the responsibility of the registrar
   supporting a Constrained-Node Network CNN 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 by the registrar to names
   with broader visibility.

3.1.3.  Why two variants?

   The reason for these different variants is that low-power devices
   that typically use Constrained-Node Networks CNNs 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 network; thus, 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 IP layer.  Nodes connected to such networks can be
   assumed to be able to do DNS-SD service registration domain
   discovery.  Such networks are generally able to provide registration
   domain discovery and routing.  This creates the possibility of off-
   network spoofing, where a device from a foreign network registers a
   service on the local network in order to attack devices on the local
   network.  To prevent such spoofing, TCP is required for such
   networks.

3.2.  Protocol Details

   We will discuss several parts to this process:

   *  how to know what to
   publish, publish (see Section 3.2.1),

   *  how to know where to publish it (under what name), name) (see
      Section 3.2.2),

   *  how to publish it, and it (see Section 3.2.3),

   *  how to secure its publication.  In publication (see Section 5, we
   specify 3.2.4), and

   *  how to maintain the information once published. published (see Section 5).

3.2.1.  What to publish 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 RRs Resource
   Records (RRs) that are either adds or deletes.  The types of records
   that are added,
   updated updated, and removed in each of these instructions,
   as well as the constraints that apply to them, are described in
   Section 3.3.  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) RR for services, which map from a generic service type (or
      subtype) name to a specific Service Instance
      Name. Name (Section 4.1 of
      [RFC6763]).

   *  For any Service Instance Name ([RFC6763], Section 4.1), Name, an SRV RR, one or more TXT RRs, and
      a KEY RR.  Although  Although, in principle principle, DNS-SD Service Description
      records can include other record types with the same Service
      Instance Name, in practice practice, they rarely do.  SRP does not permit
      other record types.  The KEY RR is used to support FCFS naming, 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].

   [RFC6763] describes the details of what each of these types of RR RRs
   mean, with the exception of the KEY RR, which is defined in
   [RFC2539].  These RFCs should be considered the definitive source sources
   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
   FCFS naming, which we describe later on in this document.

3.2.2.  Where to publish it Publish It

   Multicast DNS (mDNS) uses a single namespace, ".local", which namespace that is valid on the
   local link. link called ".local".  This convenience is not available for
   DNS-SD using the DNS protocol: services must exist in some specific
   DNS namespace that is chosen either by the network operator, operator or
   automatically.

   As described above, full-featured devices are responsible for knowing
   the domain in which to register their services.  Such devices MAY
   optionally support configuration of a registration domain by the
   operator of the device.  However, such devices MUST support
   registration domain discovery as described in Section 11 of
   [RFC6763], "Discovery of Browsing and Registration Domains".
   [RFC6763].

   Devices made for Constrained-Node Networks CNNs register in the special
   use special-use domain name [RFC6761] "default.service.arpa",
   ([RFC6761]) "default.service.arpa" and let the SRP registrar handle
   rewriting that to a different domain if necessary.

3.2.3.  How to publish it Publish It

   It is possible to issue a DNS Update that does several things at
   once; this means
   once: meaning that it's possible to do all the work of adding a PTR resource record
   RR to the PTR RRset on the Service Name, Name and creating or updating the
   Service Instance Name and Host Description, 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] where the update
   process can could involve many update attempts: you 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 described
   in this document uses a single transaction, some of this adaptability
   is lost.

   In order to allow updates to happen in a single transaction, SRP
   Updates do not include update prerequisites.  The requirements
   specified in Section 3.3 are implicit in the processing of SRP
   Updates, and so
   Updates; thus, there is no need for the SRP requestor to put in any
   explicit prerequisites.

3.2.3.1.  How the DNS-SD Service Registration process differs Process Differs from the
          DNS Update as specified Specified in RFC2136 RFC 2136

   DNS-SD Service Registration is based on the standard RFC2136 DNS Update, Update
   specified in [RFC2136], with some differences:

   *  It implements first-come first-served FCFS name allocation, protected using SIG(0) [RFC2931].
      ([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." "default.service.arpa.".

3.2.3.2.  Retransmission Strategy

   The DNS protocol, including DNS updates, can operate over UDP or TCP.
   When using UDP, reliable transmission must be guaranteed by
   retransmitting if a DNS UDP message is not acknowledged in a
   reasonable interval.  Section 4.2.1 of [RFC1035] provides some
   guidance on this topic, as does Section 1 of [RFC1536].
   Section 3.1.3 of [RFC8085] also provides useful guidance that is
   particularly relevant to DNS.

3.2.3.3.  Successive Updates

   Service Registration Protocol

   SRP does not require that every update contain the same information.
   When an SRP requestor needs to send more than one SRP update to the
   SRP registrar, it MUST send these sequentially: until an earlier
   update has been successfully acknowledged, the requestor MUST NOT
   begin sending a subsequent update.

3.2.4.  How to secure it Secure It

   A DNS update update, as described in [RFC2136] [RFC2136], is secured using Secret Key
   Transaction Signatures, [RFC8945], which secret key
   transaction signatures ([RFC8945]) that 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 FCFS naming
   is already an improvement over the security of mDNS.

3.2.4.1.  First-Come First-Served  FCFS Naming

   First-Come First-Serve

   FCFS naming provides a limited degree of security:
   a security.  A server that
   registers its service using the DNS-SD Registration
   protocol Protocol is given
   ownership of a name for an extended period of time based on a lease
   specific to the key used to authenticate the DNS Update, which may be
   longer than the lease associated with the registered records.  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 registration before the KEY lease
   (Section (see Section 4 of [I-D.ietf-dnssd-update-lease])
   [RFC9664]) expires, its name is no longer protected.  FCFS naming is
   used to protect both the Service Description and the Host
   Description.

3.2.5.  SRP Requestor Behavior

3.2.5.1.  Public/Private key pair generation Key Pair Generation and storage Storage

   The requestor generates a public/private key pair (See (see Section 6.6).
   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 preconfigured 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 for the
   former owner to erase the old key pair, which would then require the
   new owner to install a new one.  Therefore, the SRP requestor on the
   device SHOULD provide a mechanism to erase the key, for example key (for example, as
   the result of a "factory reset," reset") and to generate a new key.

   The policy described here for managing keys assumes that the keys are
   only used for SRP.  If a key that is used for SRP is also used for
   other purposes, the policy described here is likely to be
   insufficient.  The policy stated here is NOT RECOMMENDED in such a
   situation: a policy appropriate to the full set of uses for the key
   must be chosen.  Specifying such a policy is out of scope for this
   document.

   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) Extension Mechanisms for DNS (EDNS(0)) Update Lease option [I-D.ietf-dnssd-update-lease].
   (see [RFC9664]).

   The format of the KEY resource record in the SRP Update is defined in
   [RFC3445].  Because the KEY RR used in TSIG is not a zone-signing
   key, the flags field in the KEY RR MUST be all zeroes.

   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.

3.2.5.2.  Name Conflict Handling

   Both

   Adds for both Host Description RR adds RRs and Service Description RR adds RRs 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 SRP registrar to have a conflict
   with an existing name, the registrar will respond to the SRP Update
   with a YXDomain RCODE (Section 2.2 of [RFC2136]).  In this case, the
   requestor MUST choose a new name or give up.

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

3.2.5.3.  Record Lifetimes

   The lifetime of the DNS-SD PTR, SRV, A, AAAA AAAA, and TXT records
   [RFC6763] (see
   [RFC6763]) uses the LEASE field of the Update Lease option, option and is
   typically set to two hours.  This means that  Thus, 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, Option and SHOULD be set to a much longer time,
   typically 14 days.  The result of this is being that even though a device may be
   temporarily unplugged, unplugged -- disappearing from the network for a few
   days, days
   -- it makes a claim on its name that lasts much longer.

   This means that

   Therefore, 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. reuse.

3.2.5.4.  Compression in SRV records Records

   Although [RFC2782] requires that the target name in the SRV record
   not be compressed, an SRP requestor MAY compress the target in the
   SRV record.  The motivation for _not_ compressing in [RFC2782] is not
   stated,
   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.  An SRP registrar needs to understand
   SRV records in order to validate the SRP Update.  Compression of the
   target can save space in the SRP Update, so we want clients to be
   able to assume that the registrar will handle this.  Therefore, SRP
   registrars MUST support compression of SRV RR targets.

   Note that this does not update [RFC2782]: DNS servers still MUST NOT
   compress SRV record targets.  The requirement to accept compressed
   SRV records in updates only applies to SRP registrars, and SRP
   registrars that are also DNS servers still MUST NOT compress SRV
   record targets in DNS responses.  We note also that [RFC6762]
   recomments
   recommends that SRV records be compressed in mDNS messages, so
   [RFC2782] does not apply to mDNS messages.

   In addition, we note that an implementor of an SRP requestor might
   update existing code that creates SRV records or compresses DNS
   messages so that it compresses the target of an SRV record.  Care
   must be taken if such code is used both in requestors and in DNS
   servers that the code only compresses in the case where a requestor
   is generating an SRP update.

3.2.5.5.  Removing published services Published Services

3.2.5.5.1.  Removing all published services All Published Services

   To remove all the services registered to a particular host, the SRP
   requestor transmits an SRP update for that host 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 be set to the same value it had previously; this holds the
   name in reserve for when the SRP requestor is once again able to
   provide the service.

   SRP requestors are normally expected to remove all service instances
   when removing a host.  However, in some cases cases, an 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
   (see Section 3.3.1.3).

   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.

3.2.5.5.2.  Removing some published services Some Published Services

   In some use cases cases, a requestor may need to remove some a specific
   service, service
   without removing its other services.  This can be accomplished in one
   of two ways. ways:

   1.  To simply remove a specific service, the requestor sends a valid
       SRP Update where the Service Discovery Instruction (Section (see
       Section 3.3.1.1) contains a single Delete an "Delete An RR from an RRset ([RFC2136], Section 2.5.4) From An RRset"
       update (Section 2.5.4 of [RFC2136]) that deletes the PTR record
       whose target is the service instance name.  The  In this case, the
       Service Description Instruction (Section (see Section 3.3.1.2) in this case contains a
       single Delete all "Delete All RRsets from a Name ([RFC2136], Section 2.5.3) From A Name" update (Section 2.5.3 of
       [RFC2136]) to the service instance name.

   The second

   2.  This 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,
       Update and will result in the old service being removed and the
       new one added, added; or, to put it differently, the update will result
       in the old service being replaced by the new service.

   It is perhaps worth noting that that, if a service is being updated
   without the service instance name changing, that situation 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 "Delete An RR From An RRset RRset"
   update and the Add "Add To An
   RRSet update, RRset" update (Section 2.5.1 of [RFC2136]),
   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 3.2.5.5.1, which deletes the
   host and all services that have that host as their target.

3.3.  Validation and Processing of SRP Updates

3.3.1.  Validation of DNS Update Add and Delete RRs

   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 otherwise, the add will have
   no effect.  This is the only ordering constraint; 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.

3.3.1.1.  Service Discovery Instruction

   An instruction is a Service Discovery Instruction if it contains it:

   *  Contains exactly one "Add to an RRSet" ([RFC2136], Section 2.5.1) To An RRset" RR update (Section 2.5.1 of
      [RFC2136]) or exactly one "Delete an An RR from an RRSet" ([RFC2136],
      Section 2.5.4) From An RRset" RR update,
   * update
      (Section 2.5.4 of [RFC2136]), which updates a PTR RR,
   * RR; the target
      of which is a Service Instance Name
   * for which name a Service
      Description Instruction is present in the SRP Update, and: Update.
      Additionally:

      -  if  If the RR Update is an "Add to an RRSet" To An RRset" instruction, that
         Service Description Instruction contains an "Add to an To An RRset"
         RR update for the SRV RR describing that service and no other
         "Delete from an From An RRset" instructions for that Service Instance
         Name; or
         Name.
      -  if  If the RR Update is a "Delete an An RR from an RRSet" From An RRset" instruction,
         that Service Description Instruction contains a "Delete from an From An
         RRset" RR update and no other "Add to an To An RRset" instructions
         for that Service Instance Name.

   *  and contains  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, 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 delete, the above constraints must be met.

3.3.1.2.  Service Description Instruction

   An instruction is a Service Description Instruction if, for the
   appropriate Service Instance Name, the following are true:

   *  It contains exactly one "Delete all All RRsets from a name" From A Name" update for
      the service instance name ([RFC2136], (see Section 2.5.3), 2.5.3 of [RFC2136]).

   *  It contains zero or one "Add to an To An RRset" SRV RR, RR.

   *  It contains zero or one "Add to an 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), Description).

   *  It contains zero or more "Add to an To An RRset" TXT RRs, RRs.

   *  If there is one "Add to an To An RRset" SRV update, there MUST be at
      least one "Add to an To An RRset" TXT update.

   *  The target of the SRV RR Add, if present present, points to a hostname for
      which there is a Host Description Instruction in the SRP Update, Update;
      or
   *  If if there is no "Add to an To An RRset" SRV RR, then either:

      -  the name to which the "Delete all All RRsets from a name" From A Name" applies
         does not exist, or

      -  there is an existing KEY RR on that name, which name that matches the key
         with which the SRP Update was signed.

   *  No other resource records on the Service Instance Name are
      modified.

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

3.3.1.3.  Host Description Instruction

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

   *  exactly one "Delete all All RRsets from a name" From A Name" RR,

   *  one or more "Add to an To An RRset" RRs of type A and/or AAAA, and

   *  exactly one "Add to an 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,
   * message

   Host Description Instructions do not modify any other resource
   records.

   A and/or AAAA records that are not of sufficient scope to be validly
   published in a DNS zone MAY be ignored by the SRP registrar, 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
   registrar might make the records available through a mechanism such
   as an advertising proxy only on the specific link from which the SRP
   update originated; in originated.  In such a situation, locally-scoped locally scoped records are
   still valid.

3.3.2.  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 Description, or Host Description
   Instructions is not an SRP Update.  A DNS update that contains any
   prerequisites is not an SRP Update.

   An SRP Update MUST include an EDNS(0) Update Lease option
   [I-D.ietf-dnssd-update-lease]. (see
   [RFC9664]).  The LEASE time specified in the Update Lease option MUST
   be less than or equal to the KEY-LEASE time.  A DNS update that does
   not include the Update Lease option, or that includes a KEY-LEASE
   value that is less than the LEASE value, is not an SRP update.

   When an SRP registrar receives a DNS Update that is not an SRP
   update, it MAY process the update as regular RFC2136 updates, updates described in
   [RFC2136], including access control checks and constraint checks, if
   supported.
   Otherwise  Otherwise, the SRP registrar MUST reject the DNS Update
   with the Refused RCODE.

   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 "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, message but may be a valid RFC2136 update. update as described
   in [RFC2136].

3.3.3.  FCFS Name And and Signature Validation

   Assuming that a DNS Update message has been validated with these
   conditions and is a valid SRP Update, the SRP 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 SRP registrar MUST reject the SRP Update with
   the YXDomain RCODE.

   Otherwise, the SRP 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, authentic and is processed according to the
   method described in RFC2136. [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 present.  After the SRP
   Update has been applied, every Service Description that is updated
   MUST have a KEY RR, RR: and it must be the same KEY RR that is present in
   the Host Description to which the Service Description refers.

   [RFC3445] states that the flags field in the KEY RR MUST be zero
   except for bit 7, which can be one in the case of a zone key.
   However, the SRP registrar MUST NOT validate the flags field.

3.3.4.  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.

   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 except for the one to be
   deleted.

3.3.5.  SRP Update response Response

   The status that is returned depends on the result of processing the
   update,
   update and can be either NoError, ServFail, Refused Refused, or YXDomain: all YXDomain.
   All other possible outcomes will already have been accounted for when
   applying the constraints that qualify the update as an SRP Update.
   The meanings of these responses are explained in Section 2.2 of
   [RFC2136].

   In the case of a response other than NoError, Section 3.8 of
   [RFC2136] specifies that the server is permitted to respond either
   with no RRs or to copy the RRs sent by the client into the response.
   The SRP Requestor requestor MUST NOT attempt to validate any RRs that are
   included in the response.  It is possible that a future SRP extension
   may include per-RR indications as to why the update failed, but at
   present
   the time of writing this is not specified, so specified.  So, if a client were to
   attempt to validate the RRs in the response, it might reject such a response,
   response since it would contain RRs, RRs but probably not a set of RRs
   identical to what was sent in the SRP Update.

3.3.6.  Optional Behavior

   The SRP registrar MAY add a Reverse Mapping (Section (see Section 3.5 of
   [RFC1035],
   [RFC1035] and Section 2.5 of [RFC3596]) that corresponds to the Host
   Description.  This is not required because the Reverse Mapping reverse mapping serves
   no protocol function, but it may be useful for debugging, e.g. 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, 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 SRP registrar MAY apply additional criteria when accepting
   updates.  In some networks, it may be possible to do out-of-band
   registration of keys, keys and only accept updates from pre-registered preregistered 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 updates:

   1.  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

   2.  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 implementations described in [RFC2136].  The KEY used to
   sign the SRP Update only allows the SRP requestor to update records
   that refer to its Host Description.  RFC2136 implementations  Implementations specific to
   [RFC2136] do not normally provide a way to enforce a constraint of
   this type.

   The SRP 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 a Refused RCODE.

4.  TTL Consistency

   All RRs within an RRset are required to have the same TTL
   (Clarifications to the DNS Specification [RFC2181], (see
   Section 5.2). 5.2 of [RFC2181]).  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 RRset (for example, when
   processing Service Discovery records, records), the SRP 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 NOT ever
   be longer than the lease time (Section 5.1).  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.

5.  Maintenance

5.1.  Cleaning up stale data Up Stale Data

   Because the DNS-SD registration protocol is automatic, 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]. (see [RFC9664]).  The Update Lease Option
   contains two lease times: the Lease Time and the KEY Lease Time.

   These leases are promises, similar

   Similar to DHCP leases [RFC2131], (see [RFC2131]), these leases are promises
   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 can be assumed to be valid.  The
   KEY lease time represents the time after the update during which the
   KEY record can be assumed to be valid.

   The reasoning behind the different lease times is discussed in the
   section on FCFS naming (Section 3.2.4.1).
   Section 3.2.4.1.  SRP registrars may be configured with limits for
   these values.  A  At the time of writing, 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, ends in case the 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 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, removed since it has
   no key associated with it, 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 being that a requestor may re-register a host with a
   different set of services, 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 because, otherwise, if the SRP
   requestor continues to renew the host, host but never mentions the stale
   service again, the stale service will continue to be advertised.

   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;
   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 transmitted (where N is the lease duration. duration).  SRP
   Registrars
   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.  DNS authoritative servers that allow both SRP
   and non-SRP DNS updates MAY accept updates that don't include leases,
   but they SHOULD differentiate between SRP Updates and other updates, 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 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'.

6.  Security Considerations

6.1.  Source Validation

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

   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 [RFC7413] payloads. payloads (see [RFC7413]).
   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].
   ([RFC2827]).  This would ordinarily be accomplished by measures such
   as
   are those described in Section (Section 4.5 of [RFC7084]. [RFC7084]).  For example, a
   stub router [I-D.ietf-snac-simple] ([SNAC-SIMPLE]) for a constrained network might only
   accept UDP updates from source addresses known to be on-link on that
   stub network, network and might further validate that the UDP update was
   actually received on the stub network interface and not the interface
   connected to the adjacent infrastructure link.

6.2.  Other DNS updates Updates

   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, 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, 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 update records added
   by SRP requestors.

6.3.  Risks of allowing arbitrary names Allowing Arbitrary Names to be registered Registered in SRP updates Updates

   It is possible to set up SRP updates for a zone that is used for non-
   DNSSD services.  For example, imagine that you set up SRP service for
   example.com.  SRP hosts can now register names like "www" or "mail"
   or "smtp" in this domain.  In addition, SRP updates using FCFS naming
   can insert names that are obscene or offensive into the zone.  There
   is no simple solution to these problems.  We  However, we have two
   recommendations to address this problem, however: problem:

   *  Do not provide SRP service in organization-level zones.  Use
      subdomains of the organizational domain for DNS service discovery. DNS-SD.  This does not
      prevent registering names as mentioned above, above but does ensure that
      genuinely important names are not accidentally reserved for SRP
      clients.  So  So, for example, the zone "dnssd.example.com" could be
      used instead of "example.com" for SRP updates.  Because of the way
      that DNS browsing DNS-browsing domains are discovered, there is no need for the
      DNSSD discovery zone that is updated by SRP to have a user-friendly user-
      friendly or important-sounding name.

   *  Configure a dictionary of names that are prohibited.  Dictionaries
      of common obscene and offensive names are no doubt available, available and
      can be augmented with a list of typical "special" names like
      "www", "mail", "smtp" "smtp", and so on.  Lists of names are generally
      available,
      available or can be constructed manually.

6.4.  Security of local service discovery Local Service Discovery

   Local links can be protected by managed services such as RA Router
   Advertisement Guard
   [RFC6105], (see [RFC6105]), but multicast services like DHCP [RFC2131], DHCPv6
   [RFC8415]
   DHCP, DHCPv6, and IPv6 Neighbor Discovery [RFC4861] are (see [RFC2131], [RFC8415],
   and [RFC4861], respectively) are, in most cases cases, not authenticated
   and can't be controlled on unmanaged networks, such as home networks
   and small-office small office networks where no network management staff are
   present.  In such situations, the SRP service has comparatively fewer
   potential security exposures and hence and, hence, is not the weak link.  This
   is discussed in more detail in Section 3.2.4.

   The fundamental protection for networks of this type is the user's
   choice of what devices to add to the network.  Work is being done in
   other working groups and standards bodies to improve the state of the
   art for network on-boarding and device isolation (e.g., [RFC8520]
   provides a means for constraining what behaviors are allowed for a
   device in an automatic way), but such work is out of scope for this
   document.

6.5.  SRP Registrar Authentication

   This specification does not provide a mechanism for validating
   responses from SRP Registrars 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.

   In addition, for DNS-over-TLS connections, out-of-band key pinning as
   described in [RFC7858], Section 4.2 of [RFC7858] could be used for
   authentication of the SRP registrar, e.g. e.g., to prevent man-in-the-middle man-in-the-
   middle attacks.
   However  However, the use of such keys is impractical for an
   unmanaged service registration protocol, and hence protocol; hence, it is out of scope
   for this document.

6.6.  Required Signature Algorithm

   For validation, SRP registrars MUST implement the ECDSAP256SHA256
   signature algorithm.  SRP registrars SHOULD implement the algorithms
   that are specified in [RFC8624], Section 3.1, 3.1 of [RFC8624], in the validation
   column of the table, that are numbered 13 or higher higher, and that 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.

7.  Privacy Considerations

   Because DNS-SD SRP Updates can be sent off-link, the privacy
   implications of SRP are different than for multicast DNS mDNS responses.  Host
   implementations that are using TCP SHOULD also use TLS if available.
   SRP Registrar registrar implementations MUST offer TLS support.  The use of TLS
   with DNS is described in [RFC7858].  Because there is no mechanism
   for sharing keys, validation of DNS-over-TLS keys is not possible;
   DNS-over-TLS is used only as described in [RFC7858], Section 4.1 of [RFC7858].

   Hosts that implement TLS support SHOULD NOT fall back to TCP; since TCP.  Since
   SRP 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.  To avoid DNSSEC validation failures, an SRP registrar
   that signs the zone for DNSSEC but refuses to return a KEY record
   MUST NOT store the KEY record in the zone itself.  Because the KEY
   record isn't in the zone, the nonexistance nonexistence of the KEY record can be
   validated.  If the zone is not signed, the server MAY instead return
   a negative non-error response (either NXDOMAIN or no data).

8.  Domain Name Reservation Considerations

   This section specifies considerations for systems involved in domain
   name resolution when resolving queries for names ending with
   '.service.arpa.'.
   ".service.arpa.".  Each item in this section addresses some aspect of
   the DNS or the process of resolving domain names that would be
   affected by this special-use allocation.  Detailed explanations of
   these items can be found in Section 5 of [RFC6761].

8.1.  Users

   The current proposed use for 'service.arpa' "service.arpa" does not require special
   knowledge on the part of the user.  While the 'default.service.arpa.' "default.service.arpa."
   subdomain is used as a generic name for registration, users are not
   expected to see this name in user interfaces.  In the event that it
   does show up in a user interface, it is just a domain name, name and
   requires no special treatment by the user.  Users are not expected to
   see this name in user interfaces, although it's certainly possible
   that they might.  If they do, they are not expected to treat it
   specially.

8.2.  Application Software

   Application software does not need to handle subdomains of
   'service.arpa'
   "service.arpa" specially.  'service.arpa'  "service.arpa" SHOULD NOT be treated as
   more trustworthy than any other insecure DNS domain, simply because
   it is locally-served locally served (or for any other reason).  It is not possible
   to register a PKI certificate for a subdomain of 'service.arpa.' "service.arpa."
   because it is a locally-served locally served domain name.  So  So, no such subdomain
   can be considered as to be uniquely identifying a particular host, as
   would be required for such a PKI cert certificate to be issued.  If a
   subdomain of
   'service.arpa.' "service.arpa." is returned by an API or entered in an
   input field of an application, PKI authentication of the endpoint
   being identified by the name will not be possible.  Alternative
   methods and practices for authenticating such endpoints are out of
   scope for this document.

8.3.  Name Resolution APIs and Libraries

   Name resolution APIs and libraries MUST NOT recognize names that end
   in '.service.arpa.' "service.arpa." as special and MUST NOT treat them as having
   special significance, except that it may be necessary that such APIs
   not bypass the locally configured recursive resolvers.

   One or more IP addresses for recursive DNS servers will usually be
   supplied to the client through router advertisements or DHCP.  For an
   administrative domain that uses subdomains of 'service.arpa.', "service.arpa.", the
   recursive resolvers provided by that domain will be able to answer
   queries for subdomains of 'service.arpa.'; other "service.arpa.".  Other (non-local)
   resolvers will not, or they will provide answers that are not correct
   within that administrative domain.

   A host that is configured to use a resolver other than one that has
   been provided by the local network may not be unable able to resolve, resolve or may
   receive incorrect results for, for subdomains of 'service.arpa.'. "service.arpa.".  In
   order to avoid this, it is permissible that hosts use the resolvers
   that are locally provided for resolving 'service.arpa.', "service.arpa.", even when
   they are configured to use other resolvers.

8.4.  Caching DNS Servers

   There are three considerations for caching DNS servers that follow
   this specification:

   1.  For correctness, recursive resolvers at sites using
       'service.arpa.' must must, in practice practice, transparently support DNSSEC
       queries: queries for DNSSEC records and queries with the DNSSEC
       OK (DO) bit set (Section 3.2.1 of [RFC4035]).  DNSSEC validation
       is a Best Current Practice [RFC9364]: ([RFC9364]): although validation is
       not required, a caching recursive resolver that does not validate
       answers that can be validated may cache invalid data.  This, in  In turn,
       this would prevent validating stub resolvers from successfully
       validating answers.  Hence, as a practical matter, recursive
       resolvers at sites using 'service.arpa' "service.arpa" should do DNSSEC
       validation.

   2.  Unless configured otherwise, recursive resolvers and DNS proxies
       MUST behave as described in Locally Served Zones, Section Zones (Section 3 of
       [RFC6303].
       [RFC6303]).  That is, queries for 'service.arpa.' "service.arpa." and subdomains
       of 'service.arpa.' "service.arpa."  MUST NOT be forwarded, with one important
       exception: a query for a DS record with the DO bit set MUST
       return the correct answer for that question, including correct
       information in the authority section that proves that the record
       is nonexistent.

       So, for example, a query for the NS record for 'service.arpa.' "service.arpa."
       MUST NOT result in that query being forwarded to an upstream
       cache nor to the authoritative DNS server for '.arpa.'. ".arpa.".  However,
       as necessary to provide accurate authority information, a query
       for the DS record MUST result in forwarding whatever queries are
       necessary; typically,
       necessary.  Typically, this will just be a query for the DS
       record,
       record since the necessary authority information will be included
       in the authority section of the response if the DO bit is set.

8.5.  Authoritative DNS Servers

   No special processing of 'service.arpa.' "service.arpa." is required for
   authoritative DNS server implementations.  It is possible that an
   authoritative DNS server might attempt to check the authoritative
   servers for 'service.arpa.' "service.arpa." for a delegation beneath that name before
   answering authoritatively for such a delegated name.  In such a case,
   because the name always has only local significance, there will be no
   such delegation in the 'service.arpa.' zone, and so "service.arpa." zone; therefore, the server
   would refuse to answer authoritatively for such a zone.  A server
   that implements this sort of check MUST be configurable so that
   either it does not do this check for the 'service.arpa.' "service.arpa." domain or it
   ignores the results of the check.

8.6.  DNS Server Operators

   DNS server operators MAY configure an authoritative server for
   'service.arpa.'
   "service.arpa." for use with SRP.  The operator for the DNS servers
   authoritative for 'service.arpa.' "service.arpa." in the global DNS will configure
   any such servers as described in Section 9.

8.7.  DNS Registries/Registrars

   'service.arpa.'

   "service.arpa." is a subdomain of the 'arpa' "arpa" top-level domain, which
   is operated by IANA under the authority of the Internet Architecture
   Board (IAB) according to the rules established in [RFC3172].  There
   are no other DNS registrars for '.arpa'. ".arpa".

9.  Delegation of 'service.arpa.' "service.arpa."

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

10.  IANA Considerations

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

   IANA is requested to record has recorded the domain name 'service.arpa.' "service.arpa." in the
   Special-Use "Special-Use
   Domain Names Names" registry [SUDN]. (see [SUDN]).  IANA is requested, with
   the approval of IAB, to implement has implemented the
   delegation requested in Section 9.

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

10.2.  Subdomains of 'service.arpa.' "service.arpa."

   This document only makes use of the 'default.service.arpa' "default.service.arpa" subdomain
   of 'service.arpa.' "service.arpa."  Other subdomains are reserved for future use by
   DNS-SD or related work.  The  IANA is requested to create a registry, has created the "service.arpa
   Subdomain" registry. registry (see [SUB]).  The IETF shall have has change control for
   this registry.  New entries may be added either as a result of
   Standards Action Section (Section 4.9 of [RFC8126] [RFC8126]) or with IESG
   approval Section Approval
   (Section 4.10 of [RFC8126], [RFC8126]), provided that a specification exists Section
   (Section 4.6 of [RFC8126].

   The [RFC8126]).

   IANA shall group has grouped the "service.arpa Subdomain" registry with the
   "Locally-Served DNS Zones" registry. group.  The registry shall be is a table with three
   columns: the subdomain name (expressed as a fully- fully qualified domain
   name), a brief description of how it is used, and a reference to the
   document that describes its use in detail.

   This initial contents of this registry shall begin are as the following table: follows:

          +=======================+=================+===========+
          | Subdomain Name        | Description     | reference Reference |
          +=======================+=================+===========+
          | default.service.arpa. | Default domain  | [THIS RFC 9665  |
          |                       | for SRP updates | DOCUMENT]           |
          +-----------------------+-----------------+-----------+

                                  Table 1

10.3.  Service Name registrations Registrations

   IANA is requested to add has added two new entries to the Service Names "Service Name and Transport
   Protocol Port Numbers registry. Number Registry" (see [PORT]).  The following sections
   subsections contain tables with the fields required by Section 8.1.1
   of [RFC6335].

10.4.

10.3.1.  'dnssd-srp' Service Name

           +--------------------+-----------------------------+

           +====================+=============================+
           | Field Name         | Value                       |
           +--------------------+-----------------------------+
           +====================+=============================+
           | Service Name       | dnssd-srp                   |
           +--------------------+-----------------------------+
           | Transport Protocol | TCP tcp                         |
           +--------------------+-----------------------------+
           | Assignee           | IESG <iesg@ietf.org>        |
           +--------------------+-----------------------------+
           | Contact            | IETF Chair <chair@ietf.org> |
           +--------------------+-----------------------------+
           | Description        | DNS-SD Service Registration Discovery    |
           +--------------------+-----------------------------+
           | Reference          | this document RFC 9665                    |
           +--------------------+-----------------------------+
           | Port Number        | None                        |
           +--------------------+-----------------------------+
           | Service Code       | None                        |
           +--------------------+-----------------------------+

                                 Table 2

10.5.

10.3.2.  'dnssd-srp-tls' Service Name

        +--------------------+-----------------------------------+

          +====================+================================+
          | Field Name         | Value                          |
        +--------------------+-----------------------------------+
          +====================+================================+
          | Service Name       | dnssd-srp-tls                  |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Transport Protocol | TCP tcp                            |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Assignee           | IESG <iesg@ietf.org>           |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Contact            | IETF Chair Chair<chair@ietf.org>     |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Description        | DNS-SD Service Registration Discovery (TLS) |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Reference          | this document RFC 9665                       |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Port Number        | None                           |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+
          | Service Code       | None                           |
        +--------------------+-----------------------------------+
          +--------------------+--------------------------------+

                                  Table 3

10.6.

10.4.  Anycast Address

   IANA is requested to allocate has allocated an IPv6 Anycast address from the "IANA IPv6
   Special-Purpose Address Registry, Registry" (see [IPv6]), similar to the Port
   Control Protocol anycast address, address: 2001:1::1.  The value TBD is to be replaced
   with the actual allocation in the table that follows.  The purpose of this
   allocation is to provide a fixed anycast address that can be commonly
   used as a destination for SRP updates when no SRP registrar is
   explicitly configured.  The initial values for the registry are:

          +----------------------+-----------------------------+ are as
   follows:

          +======================+=============================+
          | Attribute            | value Value                       |
          +----------------------+-----------------------------+
          +======================+=============================+
          | Address Block        | 2001:1::TBD/128 2001:1::3/128               |
          +----------------------+-----------------------------+
          | Name                 | DNS-SD Service Registration |
          |                      | Protocol Anycast Address    |
          +----------------------+-----------------------------+
          | RFC                  | [this document] RFC 9665                    |
          +----------------------+-----------------------------+
          | Allocation Date      | [date of allocation] 2024-04                     |
          +----------------------+-----------------------------+
          | Termination Date     | N/A                         |
          +----------------------+-----------------------------+
          | Source               | True                        |
          +----------------------+-----------------------------+
          | Destination          | True                        |
          +----------------------+-----------------------------+
          | Forwardable          | True                        |
          +----------------------+-----------------------------+
          | Global Globally Reachable   | True                        |
          +----------------------+-----------------------------+
          | Reserved-by-protocol Reserved-by-Protocol | False                       |
          +----------------------+-----------------------------+

                                 Table 4

11.  References

11.1.  Normative References

   [I-D.ietf-dnssd-update-lease]
              Cheshire, S. and T. Lemon, "An EDNS(0) option to negotiate
              Leases on DNS Updates", Work in Progress, Internet-Draft,
              draft-ietf-dnssd-update-lease-08, 7 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
              update-lease-08>.

   [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>.

   [RFC1536]  Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
              Miller, "Common DNS Implementation Errors and Suggested
              Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
              <https://www.rfc-editor.org/info/rfc1536>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC3445]  Massey, D. and S. Rose, "Limiting the Scope of the KEY
              Resource Record (RR)", RFC 3445, DOI 10.17487/RFC3445,
              December 2002, <https://www.rfc-editor.org/info/rfc3445>.

   [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>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC6303]  Andrews, M., "Locally Served DNS Zones", BCP 163,
              RFC 6303, DOI 10.17487/RFC6303, July 2011,
              <https://www.rfc-editor.org/info/rfc6303>.

   [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>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

   [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>.

   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,
              <https://www.rfc-editor.org/info/rfc9364>.

   [RFC9664]  Cheshire, S. and T. Lemon, "An EDNS(0) Option to Negotiate
              Leases on DNS Updates", RFC 9664, DOI 10.17487/RFC9664,
              October 2024, <https://www.rfc-editor.org/info/rfc9664>.

11.2.  Informative References

   [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/>.

   [IPv6]     IANA, "IANA IPv6 Special-Purpose Address Registry",
              <https://www.iana.org/assignments/iana-ipv6-special-
              registry>.

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

   [PORT]     IANA, "Service Name and Transport Protocol Port Number
              Registry", <https://www.iana.org/assignments/service-
              names-port-numbers>.

   [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>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              DOI 10.17487/RFC6105, February 2011,
              <https://www.rfc-editor.org/info/rfc6105>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [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>.

   [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>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [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>.

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,
              <https://www.rfc-editor.org/info/rfc8520>.

   [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://datatracker.ietf.org/doc/html/
              draft-cheshire-dnssd-roadmap-03>.

   [AP]       Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
              Service Registration Protocol", Work in Progress,
              July 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-dnssd-advertising-proxy-03>.

   [I-D.ietf-snac-simple]

   [SNAC-SIMPLE]
              Lemon, T. and J. Hui, "Automatically Connecting Stub
              Networks to Unmanaged Infrastructure", Work in Progress,
              Internet-Draft, draft-ietf-snac-simple-03, 30 January draft-ietf-snac-simple-05, 8 July 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              snac-simple-03>.
              <https://datatracker.ietf.org/doc/html/draft-ietf-snac-
              simple-05>.

   [SUB]      IANA, "service.arpa Subdomain",
              <https://www.iana.org/assignments/locally-served-dns-
              zones/locally-served-dns-zones>.

   [SUDN]     IANA, "Special-Use Domain Names Registry", July 2012, Names",
              <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/>.
              names>.

   [ZC]       Cheshire, S. and D.H.       Steinberg, D.H. and S. Cheshire, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. , Inc.,
              ISBN 0-596-10100-7, 9780596101008, December 2005.

Appendix A.  Testing using standard RFC2136-compliant Using Standard DNS servers Servers Compliant with RFC 2136

   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, address or by 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", "default.service.arpa" and
   to accept updates from hosts on local networks for names under
   "default.service.arpa" without authentication, authentication since such servers will
   not have support for FCFS authentication (Section 3.2.4.1).

   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 applied; this means
   that the test server will accept internally inconsistent SRP
   Updates, Updates
   and will not stop two SRP Updates, Updates sent by different
   services, services that
   claim the same name(s), name or names 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, 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 Allow SRP requestors Requestors to update standard
             RFC2136-compliant servers

   Ordinarily Update Standard Servers
             Compliant with RFC 2136

   Ordinarily, SRP Updates will fail when sent to an RFC 2136-compliant a server compliant
   with [RFC2136] that does not implement SRP because the zone being
   updated is
   "default.service.arpa", "default.service.arpa" and because no DNS server that is
   not an SRP registrar would 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, SRP but
   does support RFC2136, [RFC2136] because the RCODE will either be NotZone,
   NotAuth
   NotAuth, or Refused, Refused or because there is no response to the update
   request (when using the anycast address) address).

   In this case case, a requestor MAY attempt to register itself using
   regular
   RFC2136 DNS updates. updates described in [RFC2136].  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 it is expected to 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, itself and
   only attempt to use RFC2136 backwards compatibility capability with [RFC2136] 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 Configuration for default.service.arpa. "default.service.arpa."

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

                 Figure 1: Zone Configuration in named.conf

 $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 0 3 13 (
                            qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
                            9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
                         ); alg = ECDSAP256SHA256 ; key id = 15008
                         AAAA    ::1

                      Figure 2: Example Zone file File

Acknowledgments

   Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
   Dong
   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 an 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 Last Call
   review.  Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
   Dong, Martin Turon, and Michael Cowan for their detailed second last
   call reviews.  Thanks to Patrik Fältström, Dhruv Dhody, David Dong,
   Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective
   directorate reviews.  Thanks to Paul Wouters for a _really_ detailed
   IESG review!  Thanks also to the other IESG members who provided
   comments or simply took the time to review the document.

Authors' Addresses

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

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