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Service Registration Protocol for DNS-Based Service Discovery

Document Type Active Internet-Draft (dnssd WG)
Authors Ted Lemon , Stuart Cheshire
Last updated 2024-04-15 (Latest revision 2024-03-04)
Replaces draft-sctl-service-registration
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd David Schinazi
Shepherd write-up Show Last changed 2022-10-13
IESG IESG state RFC Ed Queue
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Éric Vyncke
Send notices to
IANA IANA review state Version Changed - Review Needed
IANA action state On Hold
RFC Editor RFC Editor state EDIT
Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                               S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: 5 September 2024                                   4 March 2024

     Service Registration Protocol for DNS-Based Service Discovery


   The Service Registration Protocol for DNS-Based Service Discovery
   uses the standard DNS Update mechanism to enable DNS-Based Service
   Discovery using only unicast packets.  This makes it possible to
   deploy DNS Service Discovery without multicast, which greatly
   improves scalability and improves performance on networks where
   multicast service is not an optimal choice, particularly 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

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-  Status
   information for this document may be found at

   Discussion of this document takes place on the DNS-SD Working Group
   mailing list (, which is archived at  Subscribe at

   Source for this draft and an issue tracker can be found at

Status of This Memo

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

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

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

   This Internet-Draft will expire on 5 September 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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 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
       3.2.2.  Where to publish it . . . . . . . . . . . . . . . . .   9
       3.2.3.  How to publish it . . . . . . . . . . . . . . . . . .  10  How the DNS-SD Service Registration process differs
                 from DNS Update as specified in RFC2136 . . . . . .  10  Retransmission Strategy . . . . . . . . . . . . .  11  Successive Updates  . . . . . . . . . . . . . . .  11
       3.2.4.  How to secure it  . . . . . . . . . . . . . . . . . .  11  First-Come First-Served Naming  . . . . . . . . .  11
       3.2.5.  SRP Requestor Behavior  . . . . . . . . . . . . . . .  12  Public/Private key pair generation and storage  .  12  Name Conflict Handling  . . . . . . . . . . . . .  13  Record Lifetimes  . . . . . . . . . . . . . . . .  13  Compression in SRV records  . . . . . . . . . . .  13  Removing published services . . . . . . . . . . .  14
     3.3.  Validation and Processing of SRP Updates  . . . . . . . .  15
       3.3.1.  Validation of DNS Update Add and Delete RRs . . . . .  15

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  Service Discovery Instruction . . . . . . . . . .  16  Service Description Instruction . . . . . . . . .  17  Host Description Instruction  . . . . . . . . . .  17
       3.3.2.  Valid SRP Update Requirements . . . . . . . . . . . .  18
       3.3.3.  FCFS Name And Signature Validation  . . . . . . . . .  18
       3.3.4.  Handling of Service Subtypes  . . . . . . . . . . . .  19
       3.3.5.  SRP Update response . . . . . . . . . . . . . . . . .  20
       3.3.6.  Optional Behavior . . . . . . . . . . . . . . . . . .  20
   4.  TTL Consistency . . . . . . . . . . . . . . . . . . . . . . .  21
   5.  Maintenance . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  Cleaning up stale data  . . . . . . . . . . . . . . . . .  22
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
     6.1.  Source Validation . . . . . . . . . . . . . . . . . . . .  24
     6.2.  Other DNS updates . . . . . . . . . . . . . . . . . . . .  24
     6.3.  Risks of allowing arbitrary names to be registered in SRP
           updates . . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.4.  Security of local service discovery . . . . . . . . . . .  25
     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 '' . . . . . . . . . . . . . . . .  29
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Registration and Delegation of '' as a
            Special-Use Domain Name  . . . . . . . . . . . . . . . .  30
     10.2.  Subdomains of ''  . . . . . . . . . . . . .  30
     10.3.  Service Name registrations . . . . . . . . . . . . . . .  30
     10.4.  'dnssd-srp' Service Name . . . . . . . . . . . . . . . .  31
     10.5.  'dnssd-srp-tls' Service Name . . . . . . . . . . . . . .  31
     10.6.  Anycast Address  . . . . . . . . . . . . . . . . . . . .  32
   11. Implementation Status . . . . . . . . . . . . . . . . . . . .  32
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  33
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  33
   14. Informative References  . . . . . . . . . . . . . . . . . . .  36
   Appendix A.  Testing using standard RFC2136-compliant DNS
           servers . . . . . . . . . . . . . . . . . . . . . . . . .  38
   Appendix B.  How to allow SRP requestors to update standard
           RFC2136-compliant servers . . . . . . . . . . . . . . . .  39
   Appendix C.  Sample BIND9 configuration for
  . . . . . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

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1.  Introduction

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

   This document describes an enhancement to DNS-Based Service Discovery
   [RFC6763] (DNS-SD) that allows servers to register the services they
   offer using the DNS protocol rather than using Multicast DNS
   [RFC6762] (mDNS).  There is already a large installed base of DNS-SD
   clients that can discover services using the DNS protocol (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) allows services to advertise the
   fact that they provide service, and to provide the information
   required to access that service.  DNS-SD clients can then discover
   the set of services of a particular type that are available.  They
   can then select a service from among those that are available and
   obtain the information required to use it.  Although DNS Service
   Discovery (DNS-SD) using the DNS protocol (as opposed to mDNS) can be
   more efficient and versatile, it is not common in practice, because
   of the difficulties associated with updating authoritative DNS
   services with service information.

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

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

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

   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 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] allows arbitrary hosts on a single IP link
   to advertise services [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 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

   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 these are
   forbidden.  A future protocol specification might include analyses
   for other 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,' similar to leases in
   Dynamic Host Configuration Protocol [RFC8415].  The SRP registration
   itself has a lease which may be on the order of an hour; if the
   requestor does not renew the lease before it has elapsed, the

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   registration is removed.  The claim on the name can have a longer
   lease, so that another requestor cannot claim the name, even though
   the registration has expired.

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

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

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

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

2.  Conventions and Terminology Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in 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 [RFC2136] [RFC3007] to
   publish service information in the DNS.  Two variants exist, one for
   full-featured hosts, and one for devices designed for "Constrained-
   Node Networks" [RFC7228].  An SRP registrar is most likely an
   authoritative DNS server, or else is updating an authoritative DNS
   server.  There is no requirement that the server that is receiving

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   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 Hosts

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

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

3.1.2.  Constrained Hosts

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

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

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   the registrar supporting a Constrained-Node Network to handle the
   updates appropriately.  In some network environments, updates may be
   accepted directly into a local "" zone, which has
   only local visibility.  In other network environments, updates for
   names ending in "" 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 may have very limited
   battery storage.  The series of DNS lookups required to discover an
   SRP registrar and then communicate with it will increase the energy
   required to advertise a service; for low-power devices, the
   additional flexibility this provides does not justify the additional
   use of energy.  It is also fairly typical of such networks that some
   network service information is obtained as part of the process of
   joining the network, and so this can be relied upon to provide nodes
   with the information they need.

   Networks that are not constrained networks can have more complicated
   topologies at the 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

3.2.  Protocol Details

   We will discuss several parts to this process: how to know what to
   publish, how to know where to publish it (under what name), how to
   publish it, and how to secure its publication.  In Section 5, we
   specify how to maintain the information once published.

3.2.1.  What to publish

   SRP Updates are sent by SRP requestors to SRP registrars.  Three
   types of instructions appear in an SRP update: Service Discovery
   instructions, Service Description instructions, and Host Description
   instructions.  These instructions are made up of DNS Update RRs that
   are either adds or deletes.  The types of records that are added,
   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

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   overview of what is included in a typical SRP Update:

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

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

3.2.2.  Where to publish it

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

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

3.2.3.  How to publish it

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

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

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

   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 there is no need for the SRP requestor to put in any
   explicit prerequisites.  How the DNS-SD Service Registration process differs from DNS
          Update as specified in RFC2136

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

   *  It implements first-come first-served name allocation, protected
      using SIG(0) [RFC2931].
   *  It enforces policy about what updates are allowed.
   *  It optionally performs rewriting of "" 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 ""

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   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.  Successive Updates

   Service Registration Protocol 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

3.2.4.  How to secure it

   DNS update as described in [RFC2136] is secured using Secret Key
   Transaction Signatures, [RFC8945], which uses a secret key shared
   between the DNS Update requestor (which issues the update) and the
   server (which authenticates it).  This model does not work for
   automatic service registration.

   The goal of securing the DNS-SD Registration Protocol is to provide
   the best possible security given the constraint that service
   registration has to be automatic.  It is possible to layer more
   operational security on top of what we describe here, but FCFS naming
   is already an improvement over the security of mDNS.  First-Come First-Served Naming

   First-Come First-Serve naming provides a limited degree of security:
   a server that registers its service using DNS-SD Registration
   protocol is given ownership of a name for an extended period of time
   based on 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 4 of [I-D.ietf-dnssd-update-lease]) expires, its name is no
   longer protected.  FCFS naming is used to protect both the Service
   Description and the Host Description.

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3.2.5.  SRP Requestor Behavior  Public/Private key pair generation and storage

   The requestor generates a public/private key pair (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 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 as
   the result of a "factory 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

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

   The 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.

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Internet-Draft        Service Registration Protocol           March 2024  Name Conflict Handling

   Both Host Description RR adds and Service Description RR adds can
   have names that result in name conflicts.  Service Discovery record
   adds cannot have name conflicts.  If any Host Description or Service
   Description record is found by the 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,
   however, the requestor will append a number to the preferred name.
   This number could be sequentially increasing, or could be chosen
   randomly.  One existing implementation attempts several sequential
   numbers before choosing randomly.  So for instance, it might try, then, then, then  Record Lifetimes

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

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

   This means that even if a device is unplugged from the network for a
   few days, and its services are not available for that time, no other
   device can come along and claim its name the moment it disappears
   from the network.  In the event that a device is unplugged from the
   network and permanently discarded, then its name is eventually
   cleaned up and made available for re-use.  Compression in SRV 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, but is assumed to be because a caching resolver that does not
   understand the format of the SRV record might store it as binary data
   and thus return an invalid pointer in response to a query.  This does
   not apply in the case of SRP: an SRP registrar needs to understand

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   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 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.  Removing published services  Removing 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 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

   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.

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Internet-Draft        Service Registration Protocol           March 2024  Removing some published services

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

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

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

   Whichever of these two alternatives is used, the host lease will be
   updated with the lease time provided in the SRP update.  In neither
   of these cases is it permissible to delete the host.  All services
   must point to a host.  If a host is to be deleted, this must be done
   using the method described in Section, 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].

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

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

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

   An instruction is a Service Discovery Instruction if it contains

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

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

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

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

      -  there is an existing KEY RR on that name, which matches the key
         with which the SRP Update was signed.
   *  No other resource records on the Service Instance Name are

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

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

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

   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.

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   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 such a situation, 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 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].  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,
   including access control checks and constraint checks, if supported.
   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 instruction for a Service Name and an Add to an RRset
   instruction for a Service Instance Name, where the PTR record added
   to the Service Name does not reference the Service Instance Name, is
   not a valid SRP Update message, but may be a valid RFC2136 update.

3.3.3.  FCFS Name And Signature Validation

   Assuming that a DNS Update message has been validated with these
   conditions and is a valid SRP Update, the 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

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   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, and is processed according to the
   method described in RFC2136.

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

   [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

   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

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3.3.5.  SRP Update response

   The status that is returned depends on the result of processing the
   update, and can be either NoError, ServFail, Refused or 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

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

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

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

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

   The 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], Section 5.2).  In
   order to avoid inconsistencies, SRP places restrictions on TTLs sent
   by requestors and requires that SRP registrars enforce consistency.

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

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

   Additionally, when adding RRs to an RRset, for example when
   processing Service Discovery records, the 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

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5.1.  Cleaning up stale data

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

   These leases are promises, similar to DHCP leases [RFC2131], from the
   SRP requestor that it will send a new update for the service
   registration before the lease time expires.  The Lease time is chosen
   to represent the time after the update during which the registered
   records other than the KEY record 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  SRP registrars may be
   configured with limits for these values.  A default limit of two
   hours for the Lease and 14 days for the SIG(0) KEY are currently
   thought to be good choices.  Constrained devices with limited battery
   that wake infrequently are likely to request longer leases;
   registrars that support such devices may need to set higher limits.
   SRP requestors that are going to continue to use names on which they
   hold leases SHOULD update well before the lease ends, in case the
   registrar is unavailable or under heavy load.

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

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

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

   SRP requestors SHOULD assume that each lease ends N seconds after the
   update was first transmitted, where N is the lease duration.  SRP
   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 SHOULD differentiate between SRP Updates and other updates, and
   MUST reject updates that would otherwise be SRP Updates if they do
   not include leases.

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

6.  Security Considerations

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6.1.  Source Validation

   SRP Updates have no authorization semantics other than FCFS.  This
   means that if an attacker from outside of the administrative domain
   of the SRP registrar knows the registrar's IP address, it can in
   principle send updates to the registrar that will be processed
   successfully.  SRP 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.  If the
   network infrastructure allows it, an SRP registrar MAY accept TCP
   Fast Open payloads if all such packets are validated along the path,
   and the network is able to reject this type of spoofing at all
   ingress points.

   For UDP updates from constrained devices, spoofing would have to be
   prevented with appropriate source address filtration on routers
   [RFC2827].  This would ordinarily be accomplished by measures such as
   are described in Section 4.5 of [RFC7084].  For example, a stub
   router [I-D.ietf-snac-simple] for a constrained network might only
   accept UDP updates from source addresses known to be on-link on that
   stub 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

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

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

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6.3.  Risks of allowing arbitrary names to be registered in SRP 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  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 have two recommendations
   to address this problem, however:

   *  Do not provide SRP service in organization-level zones.  Use
      subdomains of the organizational domain for DNS service discovery.
      This does not prevent registering names as mentioned above, but
      does ensure that genuinely important names are not accidentally
      reserved for SRP clients.  So for example, the zone
      "" could be used instead of "" for SRP
      updates.  Because of the way that DNS browsing domains are
      discovered, there is no need for the DNSSD discovery zone that is
      updated by SRP to have a 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, and
      can be augmented with a list of typical "special" names like
      "www", "mail", "smtp" and so on.  Lists of names are generally
      available, or can be constructed manually.

6.4.  Security of local service discovery

   Local links can be protected by managed services such as RA Guard
   [RFC6105], but multicast services like DHCP [RFC2131], DHCPv6
   [RFC8415] and IPv6 Neighbor Discovery [RFC4861] are in most cases not
   authenticated and can't be controlled on unmanaged networks, such as
   home networks and small-office networks where no network management
   staff are present.  In such situations, the SRP service has
   comparatively fewer potential security exposures 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

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6.5.  SRP Registrar Authentication

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

   In addition, for DNS-over-TLS connections, out-of-band key pinning as
   described in [RFC7858], Section 4.2 could be used for authentication
   of the SRP registrar, e.g. to prevent man-in-the-middle attacks.
   However the use of such keys is impractical for an unmanaged service
   registration protocol, and hence 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
   specified in [RFC8624], Section 3.1, in the validation column of the
   table, that are numbered 13 or higher and have a "MUST",
   "RECOMMENDED", or "MAY" designation in the validation column of the
   table.  SRP requestors MUST NOT assume that any algorithm numbered
   lower than 13 is available for use in validating SIG(0) signatures.

7.  Privacy Considerations

   Because DNS-SD SRP Updates can be sent off-link, the privacy
   implications of SRP are different than for multicast DNS responses.
   Host implementations that are using TCP SHOULD also use TLS if
   available.  SRP 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

   Hosts that implement TLS support SHOULD NOT fall back to 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 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).

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8.  Domain Name Reservation Considerations

   This section specifies considerations for systems involved in domain
   name resolution when resolving queries for names ending with
   ''.  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 '' does not require special
   knowledge on the part of the user.  While the ''
   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, 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

8.2.  Application Software

   Application software does not need to handle subdomains of
   '' specially.  '' SHOULD NOT be treated as
   more trustworthy than any other insecure DNS domain, simply because
   it is locally-served (or for any other reason).  It is not possible
   to register a PKI certificate for a subdomain of ''
   because it is a locally-served domain name.  So no such subdomain can
   be considered as uniquely identifying a particular host, as would be
   required for such a PKI cert to be issued.  If a subdomain of
   '' 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 '' 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.

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   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 '', the
   recursive resolvers provided by that domain will be able to answer
   queries for subdomains of ''; 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 be unable to resolve, or may
   receive incorrect results for, subdomains of ''.  In
   order to avoid this, it is permissible that hosts use the resolvers
   that are locally provided for resolving '', 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
       '' must in 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]: although validation is not
       required, a caching recursive resolver that does not validate
       answers that can be validated may cache invalid data.  This, in
       turn, would prevent validating stub resolvers from successfully
       validating answers.  Hence, as a practical matter, recursive
       resolvers at sites using '' should do DNSSEC

   2.  Unless configured otherwise, recursive resolvers and DNS proxies
       MUST behave as described in Locally Served Zones, Section 3 of
       [RFC6303].  That is, queries for '' and subdomains
       of ''  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.

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       So, for example, a query for the NS record for ''
       MUST NOT result in that query being forwarded to an upstream
       cache nor to the authoritative DNS server for '.arpa.'.  However,
       as necessary to provide accurate authority information, a query
       for the DS record MUST result in forwarding whatever queries are
       necessary; typically, this will just be a query for the DS
       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 '' is required for
   authoritative DNS server implementations.  It is possible that an
   authoritative DNS server might attempt to check the authoritative
   servers for '' 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 '' zone, and so 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 '' domain or it ignores
   the results of the check.

8.6.  DNS Server Operators

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

8.7.  DNS Registries/Registrars

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

9.  Delegation of ''

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

10.  IANA Considerations

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

   IANA is requested to record the domain name '' in the
   Special-Use Domain Names registry [SUDN].  IANA is requested, with
   the approval of IAB, to implement the delegation requested in
   Section 9.

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

10.2.  Subdomains of ''

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

   The IANA shall group the " Subdomain" registry with the
   "Locally-Served DNS Zones" registry.  The registry shall be a table
   with three columns: the subdomain name (expressed as a 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 registry shall begin as the following table:

          | Subdomain Name        | Description     | reference |
          | | Default domain  | [THIS     |
          |                       | for SRP updates | DOCUMENT] |

                                  Table 1

10.3.  Service Name registrations

   IANA is requested to add two new entries to the Service Names and
   Port Numbers registry.  The following sections contain tables with
   the fields required by Section 8.1.1 of [RFC6335].

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10.4.  'dnssd-srp' Service Name

           | Field Name         | Value                       |
           | Service Name       | dnssd-srp                   |
           | Transport Protocol | TCP                         |
           | Assignee           | IESG <>        |
           | Contact            | IETF Chair <> |
           | Description        | DNS-SD Service Registration |
           | Reference          | this document               |
           | Port Number        | None                        |
           | Service Code       | None                        |

                                 Table 2

10.5.  'dnssd-srp-tls' Service Name

        | Field Name         | Value                             |
        | Service Name       | dnssd-srp-tls                     |
        | Transport Protocol | TCP                               |
        | Assignee           | IESG                              |
        | Contact            | IETF Chair                        |
        | Description        | DNS-SD Service Registration (TLS) |
        | Reference          | this document                     |
        | Port Number        | None                              |
        | Service Code       | None                              |

                                 Table 3

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10.6.  Anycast Address

   IANA is requested to allocate an IPv6 Anycast address from the IPv6
   Special-Purpose Address Registry, similar to the Port Control
   Protocol anycast address, 2001:1::1.  The value TBD 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 values for the registry are:

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

                                 Table 4

11.  Implementation Status

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

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation

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   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   There are two known independent implementations of SRP requestors:

   *  SRP Client for OpenThread:

   *  mDNSResponder open source project:

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

12.  Acknowledgments

   Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
   Dong and Abtin Keshavarzian for their thorough technical reviews.
   Thanks to Kangping and Abtin as well for testing the document by
   doing an independent implementation.  Thanks to Tamara Kemper for
   doing a nice developmental edit, Tim Wattenberg for doing 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
   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.

13.  Normative References

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              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,

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

   [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,

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

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

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

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

   [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,

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <>.

   [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, <>.

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   [RFC3445]  Massey, D. and S. Rose, "Limiting the Scope of the KEY
              Resource Record (RR)", RFC 3445, DOI 10.17487/RFC3445,
              December 2002, <>.

   [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,

   [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,

   [RFC6303]  Andrews, M., "Locally Served DNS Zones", BCP 163,
              RFC 6303, DOI 10.17487/RFC6303, July 2011,

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [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, <>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8375]  Pfister, P. and T. Lemon, "Special-Use Domain
              ''", RFC 8375, DOI 10.17487/RFC8375, May 2018,

   [RFC8624]  Wouters, P. and O. Sury, "Algorithm Implementation
              Requirements and Usage Guidance for DNSSEC", RFC 8624,
              DOI 10.17487/RFC8624, June 2019,

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   [RFC8765]  Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              RFC 8765, DOI 10.17487/RFC8765, June 2020,

   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,

14.  Informative References

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [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, <>.

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

   [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,

   [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,

   [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,

   [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,

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

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   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [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,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [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,

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,

   [RFC8766]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
              2020, <>.

   [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,

   [ROADMAP]  Cheshire, S., "Service Discovery Road Map", Work in
              Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
              23 October 2018, <

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   [AP]       Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
              Service Registration Protocol", Work in Progress,
              Internet-Draft, draft-ietf-dnssd-advertising-proxy-03, 28
              July 2023, <

              Lemon, T. and J. Hui, "Automatically Connecting Stub
              Networks to Unmanaged Infrastructure", Work in Progress,
              Internet-Draft, draft-ietf-snac-simple-03, 30 January
              2024, <

   [SUDN]     "Special-Use Domain Names Registry", July 2012,

   [LSDZ]     "Locally-Served DNS Zones Registry", July 2011,

   [IAB-ARPA] "Internet Architecture Board statement on the registration
              of special use names in the ARPA domain", March 2017,

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

Appendix A.  Testing using standard RFC2136-compliant DNS servers

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

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

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

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

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

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

Appendix C.  Sample BIND9 configuration for

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

                 Figure 1: Zone Configuration in named.conf

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 $TTL 57600  ; 16 hours IN SOA
                 2951053287 ; serial
                 3600       ; refresh (1 hour)
                 1800       ; retry (30 minutes)
                 604800     ; expire (1 week)
                 3600       ; minimum (1 hour)
                         SRV 0 0 53
 $TTL 3600   ; 1 hour
 _ipps._tcp              PTR          demo._ipps._tcp
 demo                    TXT          "0"
                         SRV 0 0 9992
 $TTL 3600   ; 1 hour
 _dns-update             PTR
 _dnssd-srp              PTR
 $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 (
                         ); alg = ECDSAP256SHA256 ; key id = 15008
                         AAAA    ::1

                      Figure 2: Example Zone file

Authors' Addresses

   Ted Lemon
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

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

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