wdiff rfc9665.original rfc9665.txt

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

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

Abstract

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

About This Document

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

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

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

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

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

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Table of Contents

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

1. Introduction

DNS-Based Service Discovery

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

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

This document is intended for three audiences: implementors Implementers of
software that provides services that should be advertised using
DNS-SD, implementors implementers of authoritative 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) Registration Protocol.

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

Existing

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

* that the authoritative DNS server automatically trust
updates, updates or else

* that the DNS Update requestor requester have some kind of shared secret or
public key that is known to the authoritative DNS server and can
be used to authenticate the update.

Furthermore, DNS Update can be a fairly chatty process, requiring
multiple round trips roundtrips with different conditional predicates to complete
the update process.

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

SRP also adds a feature called First-Come, First-Served (FCFS)
Naming, "First Come, First Served Naming" (or
"FCFS Naming"), which allows the requestor to requester to:

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

* authenticate, using SIG(0) [RFC2931], to authenticate both the initial claim (to
ensure it has not been modified in transit) and subsequent updates. updates
(to ensure they come from the same entity that performed the
initial claim).

This prevents name conflicts, since a new service instance from "stealing" a name that is
already in use: A second SRP requestor requester attempting to claim the same an
existing name will not possess the SIG(0) key used by the first requestor
requester to claim it, and
so it. Because of this, its claim will be rejected and the second requestor rejected.
This will have force it 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 SRP
requester sends us. You will notice as you read this document that
we only support adding a very restricted set of records, and the
content of those records is further constrained.

The reason for this is precisely that we have not established trust.
So
So, we can only publish information that we feel safe in publishing
even though we do not have any basis for trusting the requestor. requester. 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 Therefore, you will see in
Section 3.3.1 that a very restricted set of records with a very
restricted set of relationships are allowed. You will also see in
Section 6.1 that we give advice on how to prevent off-network
attacks.

This leads us to the disappointing observation that this protocol is
not a mechanism for adding arbitrary information to DNS zones. We
have not evaluated the security properties of adding, for example, an
SOA record, an MX record, or a CNAME record, and so record; therefore, these are
forbidden. A future protocol Future updates to this specification might include
analyses for other records, records and extend the set of records and/or
record content 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 that is work for a future
document.

Finally, SRP adds the concept of a 'lease,' similar "lease" [RFC9664], analogous to
leases in
Dynamic Host Configuration Protocol DHCP [RFC2131] [RFC8415]. The SRP registration itself has
a lease which that may be on the order of an hour; two hours; if the
requestor requester does
not renew the lease before it has elapsed, the registration is
removed. The claim on the name can have a longer
lease, lease so that
another requestor requester cannot claim the name, even though the registration
has expired.

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

The DNS-SD specification ([RFC6763],

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

RFC6763

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

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

2. Conventions and Terminology Used in This Document

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

Strictly speaking, fully qualified domain names end with a period.
In DNS zone files and other similar contexts, if the final period is
omitted, then a name may be treated incorrectly as relative to some
other parent domain. This document follows the formal DNS
convention, ending fully qualified domain names with a period (".").
When this document mentions domain names such as "local." and
"default.service.arpa.", the final period is part of the domain name
and does not indicate the end of a sentence as it would in normal
prose.

3. Service Registration Protocol

Services that implement SRP use DNS Update [RFC2136] with SIG(0)
[RFC3007] to publish service information in the DNS. Two variants exist, one
exist: One for full-featured hosts, hosts and one for devices designed for "Constrained-
Node Networks"
Constrained-Node Networks (CNNs) [RFC7228]. An SRP registrar is most
likely an authoritative DNS server, server or else is updating an a source of data for one or
more authoritative DNS
server. servers. There is no requirement that the
authoritative DNS server that is receiving SRP updates Updates be the same
authoritative DNS server that is answering queries that return
records that have been registered. For example, an SRP registrar
could be the "hidden primary" that is the source of data for a fleet
of secondary authoritative DNS servers.

3.1. Protocol Variants

3.1.1. Full-featured Full-Featured Hosts

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

Manual configuration

Configuration of the registration domain can be done either either:

* by querying the list of available registration domains
("r._dns-sd._udp") and allowing the user to select one from the
UI, or

* by any other means appropriate to the particular use case being
addressed.

Full-featured devices construct the names of the SRV, TXT, and PTR
records describing their service(s) service or services as subdomains of the
chosen service registration domain. For these names names, they then
discover the zone apex of the closest enclosing DNS zone using SOA
queries as described in Section 6.1 of the DNS Push Notification
specification [RFC8765]. Having discovered the enclosing DNS zone,
they query for the "_dnssd-srp._tcp.<zone>" SRV record to discover
the server SRP registrar to which they can send SRP updates. Updates. Hosts that
support SRP Updates using TLS use the "_dnssd-srp-tls._tcp.<zone>"
SRV record instead.

Examples of full-featured hosts include devices such as home
computers, laptops, powered peripherals with network connections such
(such as printers, printers and home routers, 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] CNNs [RFC7228], some simplifications are
available. Instead of being configured with (or discovering) the
service registration domain, the special-use domain name (see [RFC6761]) "default.service.arpa" [RFC6761]
"default.service.arpa." is used. The details of how SRP registrar(s) registrars
are discovered will be specific to the constrained network, and therefore network;
therefore, we do not suggest a specific mechanism here.

SRP requestors requesters on constrained networks CNNs are expected to receive receive, from the network network, a
list of SRP registrars with which to register. It is the
responsibility of a Constrained-Node Network CNN supporting SRP to provide one or more
registrar addresses. It is the responsibility of the registrar
supporting a Constrained-Node Network CNN to handle the updates appropriately. In some
network environments, updates may be accepted directly into a local "default.service.arpa"
"default.service.arpa." zone, which has only local visibility. In
other network environments, updates for names ending in "default.service.arpa"
"default.service.arpa." may be rewritten by the registrar to names
with broader visibility. Domain name rewriting should be performed
as appropriate for the network environment in question. Some
suggested techniques for how domain names can be translated from a
locally scoped name to a domain name with larger scope can be found
in the discussion of data translation for names in Multicast DNS
answers in Section 5.5 of the Discovery Proxy specification
[RFC8766].

3.1.3. Why two variants?

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

Networks that are not constrained networks CNNs 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 off-network spoofing,
where a device from a foreign network registers a service on the
local network in order to attack devices on the local network. To
prevent such spoofing, TCP is required for such networks.

3.2. Protocol Details

We will discuss several parts to this process:

* how to know what to
publish, publish (Section 3.2.1),
* how to know where to publish it (under what name), name) (Section 3.2.2),
* how to publish it, and it (Section 3.2.3),
* how to secure its publication. In Section 5, we
specify publication (Section 3.2.4), and
* how to maintain the information once published. published (Section 5).

3.2.1. What to publish Publish

SRP Updates are sent by SRP requestors requesters to SRP registrars. Three
types of instructions appear in an SRP update: Update: Service Discovery
instructions, Service Description instructions, and Host Description
instructions. These instructions are made up of DNS Update RRs Resource
Records (RRs) that are either adds or deletes. The types of records
that are added,
updated updated, and removed in each of these instructions,
as well as the constraints that apply to them, are described in
Section 3.3. An SRP Update is a DNS Update message [RFC2136] that is
constructed so as to meet the constraints described in that section.
The following is a brief overview of what is included in a typical
SRP Update:

* Service Discovery PTR Resource Record (RR) RR(s) for services, service(s), which map from a
generic service type (or subtype) name subtype(s)) to a specific Service
Instance
Name. Name [RFC6763].

* For any each Service Instance Name ([RFC6763], Section 4.1), Name, an SRV RR, one or more TXT RRs,
and a KEY RR. Although Although, in principle principle, DNS-SD Service Description
records can include other record types with the same Service
Instance Name, in practice practice, they rarely do. Currently, SRP does
not permit other record types. The KEY RR is used to support FCFS naming,
Naming and has no specific meaning for DNS-SD lookups. SRV
records for all services described in an SRP update Update point to the
same hostname.

* There is never more than always exactly one hostname in a single SRP update. Update. A
DNS Update containing more than one hostname is not an SRP Update.
The hostname has one or more address RRs (AAAA or A) and a KEY RR
(used for FCFS naming). Naming). Depending on the use case, an SRP
requestor
requester 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 requesters. Some suggested
policies for suppressing unusable records can be found in
Section 5.5.2 of the Discovery Proxy specification [RFC8766].

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

3.2.2. Where to publish it Publish It

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

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

Devices made for Constrained-Node Networks CNNs register in the special
use special-use domain name
[RFC6761] "default.service.arpa", "default.service.arpa." and let the SRP registrar handle
rewriting that to a different domain if necessary. necessary, as described in
Section 3.1.2.

3.2.3. How to publish it Publish It

It is possible to issue send a DNS Update message that does several things
at
once; this means that once: For example, it's possible to do all the work of adding in a
PTR resource record single transaction to add or
update a single Host Description while also adding or updating the PTR RRset on
RRs comprising the Service Name, Description(s) for one or more service
instance(s) available on that host and
creating adding or updating the RRs
comprising the Service Instance Name and Host Description,
in a single transaction. Discovery instruction(s) for those service
instance(s).

An SRP Update takes advantage of this: it 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
from normal DNS Updates as defined in [RFC2136]. The [RFC2136] where the update process can could
involve many update attempts: you attempts. You might first attempt to add a name
if it doesn't exist; if that fails, then in a second message you
might update the name if it does exist but matches certain
preconditions. Because the registration protocol Service Registration Protocol described
in this document uses a single transaction, some of this adaptability
is lost.

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

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

DNS-SD Service Registration is based on standard RFC2136 uses the DNS Update, Update specification
[RFC2136] with some differences: additions:

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

* It enforces policy about what updates are allowed.

* It optionally performs rewriting of "default.service.arpa" "default.service.arpa." to
some other domain.

* It optionally performs automatic population of the address-to-name
reverse mapping domains.

* An SRP registrar is not required to implement general DNS Update
prerequisite processing.

* Constrained-Node CNN SRP requestors requesters are allowed to send updates to the generic
domain "default.service.arpa." "default.service.arpa.".

3.2.3.2. Retransmission Strategy

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

3.2.3.3. Successive Updates

Service Registration Protocol

SRP does not require that every update contain the same information.
When an SRP requestor requester needs to send more than one SRP update Update to the
SRP registrar, it SHOULD combine these into a single SRP Update, when
possible, subject to DNS message size limits and link-specific size
limits (e.g., an IEEE 802.15.4 network will perform poorly when asked
to deliver a packet larger than about 500 bytes). If the updates do
not fit into a single SRP Update, then the SRP requester MUST send these
subsequent SRP Updates sequentially: until Until an earlier update SRP Update has
been successfully acknowledged, the requestor requester MUST NOT begin send any subsequent SRP
Updates. If a configuration change occurs while an outstanding SRP
Update is in flight, the SRP registrar MUST defer sending a subsequent
update. new SRP
Update for that change until the previous SRP Update has completed.

3.2.4. How to secure it Secure It

DNS update as described in [RFC2136] is Update messages can be secured using Secret Key
Transaction Signatures, [RFC8945], which secret key transaction
signatures (TSIG) [RFC8945]. This approach uses a secret key shared
between the DNS Update requestor requester (which issues the update) and the
authoritative DNS 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 Naming
is already an improvement over the security of mDNS.

3.2.4.1. First-Come First-Served FCFS Naming

FCFS Naming

First-Come First-Serve naming provides a limited degree of security:
a security. A server that
registers its service using DNS-SD Registration
protocol SRP 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 SRP Update, which may be longer than the lease
associated with the registered records. RRs. As long as the registration service registrar
remembers the name and the public key corresponding to the private
key used to register RRs on that name, no other server SRP requester can add
or update the information associated with that. that name. If the server SRP
requester fails to renew its service registration before the KEY
lease expires (Section 4 of [I-D.ietf-dnssd-update-lease]) expires, the DNS Update Lease specification
[RFC9664]) its name is no longer protected. FCFS naming Naming is used to
protect both the Service Description and the Host Description.

3.2.5. SRP Requestor Requester Behavior

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

The requestor requester generates a public/private key pair (See Section (Section 6.6).
This key pair MUST be stored in stable storage; if there is no
writable stable storage on the SRP requestor, requester, the SRP requestor requester MUST
be pre-configured preconfigured with a public/private key pair in read-only storage
that can be used. storage.
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 requester on the device SHOULD provide a
mechanism to erase the key, for example key (for example, as the result of a "factory reset,"
reset") and to generate a new key.

Note that when a new key is generated, this will prevent the device
from registering with the name associated with the old key in the
same domain where it had previously registered. So, implicit in the
generation of a new key is the generation of a new name; this can be
done either proactively when regenerating a key or when the SRP
update produces a name conflict.

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

When sending DNS updates, the requestor requester 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 are set
using the EDNS(0) Update Lease option [I-D.ietf-dnssd-update-lease]. [RFC9664].

The format of the KEY resource record in the SRP Update is defined in
[RFC3445].
the IETF specification for DNSSEC Resource Records [RFC4034].
Because the KEY RR used in TSIG SIG(0) is not a zone-signing key, the
flags field in the KEY RR MUST be all zeroes.

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

3.2.5.2. Name Conflict Handling

Both

"Add" operations for both Host Description RR adds RRs and Service
Description RR adds RRs can have names that result in name conflicts.
Service Discovery record
adds "Add" operations 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]). [RFC2136],
indicating that the desired name is already owned by a different
SIG(0) key. In this case, the
requestor SRP requester MUST choose a new name
or give up.

There is no specific requirement for how this is done; typically, the SRP requester should
choose a new name. Typically, however, the requestor requester will append a
number to the preferred name. This number could be sequentially increasing,
increasing or could be chosen randomly. One existing implementation
attempts several sequential numbers before choosing randomly. So for For
instance, it might try
host.default.service.arpa, host.default.service.arpa., then host-1.default.service.arpa,
host-1.default.service.arpa., then
host-2.default.service.arpa, host-2.default.service.arpa., then
host-31773.default.service.arpa.

3.2.5.3. Record Lifetimes

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

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

This means that

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

3.2.5.4. Compression in SRV records Records

Although the original SRV specification [RFC2782] requires that the
target name hostname in the rdata of an SRV record not be compressed, compressed in
DNS queries and responses, an SRP requestor requester MAY compress the target
in the SRV record. record, since an SRP Update is neither a DNS query nor a
DNS response. The motivation for _not_ compressing in [RFC2782] is not
stated, stated in
the SRV specification but is assumed to be because a caching recursive
resolver (caching server) that does not understand the format of the
SRV record might store it as binary data without decoding a
compression pointer embedded with the target hostname field and thus
return an invalid pointer nonsensical rdata in response to a query. This concern does
not apply in the case of SRP: an SRP. An SRP registrar needs to understand
SRV records in order to validate the SRP Update. Compression of the
target can save space in the SRP Update, so we want clients SRP requesters to
be able to assume that the registrar will handle this. Therefore,
SRP registrars MUST support compression of SRV RR targets.

Note that this document does not update the SRV specification
[RFC2782]: Authoritative 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 authoritative DNS servers still MUST NOT compress SRV record
targets in DNS responses. We note also that Multicast DNS [RFC6762]
recomments that
similarly compresses SRV records be compressed in mDNS messages, so
[RFC2782] does not apply to mDNS messages.

In addition, we note that an implementor implementer of an SRP requestor requester 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 requesters and in
authoritative DNS servers that the code only compresses in the case
where a requestor requester is generating an SRP update. Update.

3.2.5.5. Removing published services Published Services

3.2.5.5.1. Removing all published services All Published Services

To remove all the services registered to a particular host, hostname, the
SRP
requestor requester transmits an SRP update Update for that host hostname with an
Update Lease option that has a LEASE value of zero. The SRP Update
MUST contain exactly one Host Description Instruction that contains
exactly one "Delete All RRsets From A Name" instruction for the
hostname and no "Add to an RRSet" instructions for that hostname. 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 requester
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
requester is going offline and does not want any of its services advertised. Therefore, it is
sufficient for the requestor to send the Host Description Instruction
(Section 3.3.1.3).
continue being advertised.

To support this, when removing services based on the lease time being
zero, a hostname, an SRP registrar MUST
remove all service instances pointing to
a host when a host is removed, that hostname and all
Service Discovery PTR records pointing to those service instances,
even if the SRP requestor requester 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. requesters.

3.2.5.5.2. Removing some published services Some Published Services

In some use cases cases, a requestor requester may need to remove some a specific
service, service
without removing its other services. For example, a device may shut
down its remote screen access (_rfb._tcp) service while retaining its
command-line login (_ssh._tcp) service. This can be accomplished in
one of two ways. ways:

1. To simply remove a specific service, the requestor requester sends a valid
SRP Update where the with a Service
Discovery Description Instruction
(Section 3.3.1.1) contains 3.3.1.2) containing a single Delete an
RR from an RRset ([RFC2136], Section 2.5.4) "Delete All RRsets From A
Name" update that deletes to the Service Instance Name. The SRP Update SHOULD
include Service Discovery Instructions (Section 3.3.1.1)
consisting of "Delete An RR From An RRset" updates [RFC2136] that
delete any Service Discovery PTR record records whose target is the service instance name. The
Service
Description Instruction (Section 3.3.1.2) Instance Name. However, even in this case contains a
single Delete all RRsets from a Name ([RFC2136], Section 2.5.3)
update the absence of such
Service Discovery Instructions, the SRP registrar MUST delete any
Service Discovery PTR records that point to the deleted Service
Instance Name.

2. When deleting one service instance name.

The second alternative is used when some service is being replaced by while simultaneously creating
a different new service instance with a different service instance name. name, an
alternative is to perform both operations using a single SRP
Update. 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
consists of a valid Service Discovery Instruction and a valid
Service Description Instruction, the update as a whole is a valid
SRP Update, Update and will result in the old service being removed and
the new one added, added; or, to put it differently, the SRP Update will
result 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, Service Instance Name changing (for example, when only the target
port in the SRV record is being updated), then that SRP Update 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 will be the same for both the Delete
"Delete An RR From An RRset RRset" update and the Add "Add To An
RRSet update, there RRset" update
[RFC2136]. Since the removal of the old service and the addition of
the new service are both valid SRP Update operations, the combined
operation is no way a valid SRP Update operation. The SRP registrar does
not need to tell whether they were intended include code to
be one or two Instructions. The same would be true of the Service
Description Instruction. recognize this special case and does not
need to take any special actions to handle it correctly.

Whichever of these two alternatives is used, the host hostname 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. hostname. All
services must point to a host. hostname. If a host hostname is to be deleted,
this must be done using the method described in Section 3.2.5.5.1,
which deletes the
host hostname and all services that have that host hostname
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 message is a
syntactically and semantically valid DNS Update message according to
the usual DNS Update rules specified in [RFC2136].

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

The SRP registrar checks each instruction in the SRP Update to see
that it is either a Service Discovery Instruction, a Service
Description Instruction, or a Host Description Instruction. Order
matters in DNS updates. Specifically, deletes must precede adds for
records that the deletes would affect; otherwise otherwise, the add will have
no effect. This is the only ordering constraint; aside 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
entry in the Zone Section (what would be the Question Section in a
traditional DNS message) that indicates the zone to be updated.
Every delete and update in an SRP Update MUST be within the zone that
is specified for the SRP Update.

3.3.1.1. Service Discovery Instruction

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

* consists of exactly one "Add to an To An RRSet" ([RFC2136], Section 2.5.1) or exactly one "Delete an
An RR from an From An RRSet" ([RFC2136],
Section 2.5.4) RR update, update (Section 2.5 of the DNS Update
specification [RFC2136]),
* 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 Service Discovery Instruction is an "Add to an To An RRSet"
instruction, that Service Description Instruction contains an a
"Delete All RRsets From A Name" instruction for that Service
Instance Name followed by "Add to an To An RRset"
RR update instructions for
the SRV RR describing that service and no other
"Delete from an RRset" instructions for TXT records describing that Service Instance
Name; service; or
- if the RR Update Service Discovery Instruction is a "Delete an An RR from an From An
RRSet" instruction, that Service Description Instruction
contains a "Delete from an
RRset" RR update and no other "Add to an RRset" instructions All RRsets From A Name" instruction for that
Service Instance Name.
* and contains Name with no other add or delete RR updates following "Add To An RRset"
instructions for the same name
as the PTR RR Update. SRV and TXT records describing that
service.

Note that there can be more than one Service Discovery Instruction
for the same service name (the owner name of the Service Discovery
PTR record) if the SRP requestor requester is advertising more than one
service
instance of the same type, service type or is changing the target of a PTR
RR.
This is also true for SRP When subtypes are being used (Section 7.1 of [RFC6763]). the DNS-SD
specification [RFC6763]), each subtype is a separate Service
Discovery Instruction. For each such PTR RR add or delete, the above
constraints must be met.

3.3.1.2. Service Description Instruction

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

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

Service Description Instructions do not modify any other resource records on the Service Instance Name are
modified.
records.

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

3.3.1.3. Host Description Instruction

Every SRP Update alway contains exactly one Host Description
Instruction.

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

* exactly one "Delete all All RRsets from a name" RR,
* one or more "Add to an RRset" RRs of type From A and/or AAAA, Name" RR

* exactly one "Add to an To An RRset" RR that adds a KEY RR that contains
the public key corresponding to the private key that was used to
sign the message, message

* zero "Add To An RRset" operations (in the case of deleting a
registration) or one or more "Add To An RRset" RRs of type A and/
or AAAA (in the case of creating or updating a registration)

Host Description Instructions do not modify any other resource
records.

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

For example, if a link-scope an IPv4 link-local address [RFC3927] or IPv4 autoconfiguration an IPv6 link-
local address [RFC4862] is provided by the SRP requestor, requester, the SRP
registrar could elect not to 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 Update originated. In such a situation, locally-scoped
locally scoped records are still valid.

3.3.2. Valid SRP Update Requirements

An SRP Update MUST contain exactly one Host Description Instruction.
In addition, there MUST NOT be any Service Description Instruction to
which no
Multiple Service Discovery Instruction points. updates and Service Description updates
may be combined into a single single SRP Update along with a single
Host Description update, as described in Section 3.2.3. A DNS Update
message that contains any additional adds or deletes that cannot be
identified as Service Discovery, Service Description Description, or Host
Description Instructions is not an SRP Update. A DNS update that
contains any prerequisites is not an SRP Update.

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

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

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

3.3.3. FCFS Name And and Signature Validation

Assuming that the SRP registrar has confirmed that a DNS Update
message has been validated with these
conditions and is a valid SRP Update, the SRP registrar Update (Section 3.3.2), it then checks that
the name in the Host Description Instruction exists. exists in the zone being
updated. If so, then the registrar checks to see if the KEY record
on that name is the same as the KEY record in the Host Description
Instruction. The registrar performs the same check for the KEY
records in any Service Description Instructions. For KEY records
that were omitted from Service Description Instructions, the KEY from
the Host Description Instruction is used. If any existing KEY record
corresponding to a KEY record in the SRP Update does not match the
KEY record in the SRP Update (whether provided or taken from the Host
Description Instruction), then the SRP registrar MUST reject the SRP
Update with
the an YXDomain RCODE.

Otherwise, RCODE indicating that the desired name is
already owned by a different SIG(0) key. This informs the SRP
requester that it should select a different name and try again.

If the SRP Update is not in conflict with existing data in the zone
being updated, 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 SRP Update is
malformed, and the registrar MUST reject the SRP Update with the
Refused RCODE. Otherwise, the SRP Update is considered valid and authentic,
authentic and is processed according to the
method described in RFC2136. as for a normal DNS Update [RFC2136].

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

[RFC3445]

The IETF specification for DNSSEC Resource Records [RFC4034] 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, SRP requesters
implementing this version of the SRP registrar specification MUST NOT validate set the flags
field in the KEY RR to all zeroes. SRP registrars implementing this
version of the SRP specification MUST accept and store the flags field.
field in the KEY RR as received, without checking or modifying its
value.

3.3.4. Handling of Service Subtypes

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

Similarly, there

There is intentionally no mechanism for deleting subtypes. a single subtype
individually. A delete of a service deletes all of its subtypes. To
delete an individual
subtype, a single subtype individually, an SRP Update must be
constructed that contains the service type and all subtypes for that
service except for the one subtype(s) to be deleted.

3.3.5. SRP Update response Response

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

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

3.3.6. Optional Behavior

The SRP registrar MAY add a Reverse Mapping (Section PTR record (described for
IPv4 in Section 3.5 of
[RFC1035], [RFC1035] of the DNS specification [RFC1035]
and for IPv6 in Section 2.5 of [RFC3596] of the later document
updating DNS for IPv6 [RFC3596]) that corresponds to the Host
Description. This is not required optional because the Reverse Mapping reverse mapping PTR record
serves no essential protocol function, but it may be useful for
debugging, e.g. for example, in annotating network packet traces or logs.
In order for the registrar to do a reverse mapping update, it must be
authoritative for the zone that would need to be updated, updated or have
credentials to do the update. The SRP requestor requester MAY also do a
reverse mapping update if it has credentials to do so.

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

There When use of managed keys is
desired, there are at least two benefits to doing this in conjunction
with SRP rather than simply performing traditional DNS Updates using normal
SIG(0) DNS updates. First, the keys:

1. The same over-the-air registration protocol can be is used in both
cases, so both use cases can be addressed by the same SRP requestor
requester implementation. Second, the
registration protocol

2. The Service Registration Protocol includes maintenance
functionality not present with normal DNS updates.

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

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

4. TTL Consistency

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

Requestors

Requesters sending SRP Updates MUST use consistent TTLs in all RRs
within the each RRset contained within an SRP Update.

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

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

TTLs sent in SRP Updates are advisory: they indicate the SRP
requestor's
requester'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 DNS server, and also increases network load, which
may be an issue for constrained networks. CNNs. Longer TTLs will increase the likelihood
that data in caches will be stale. TTL minimums and maximums SHOULD
be configurable by the operator of the SRP registrar.

5. Maintenance

5.1. Cleaning up stale data Up Stale Data

Because the DNS-SD registration protocol Service Registration Protocol is automatic, automatic and not
managed by humans, some additional bookkeeping is required. When an
update is constructed by the SRP requestor, requester, it MUST include an
EDNS(0) Update Lease Option [I-D.ietf-dnssd-update-lease]. [RFC9664]. The Update Lease Option
contains two lease times: the Lease Time and the KEY Lease Time.

These leases are promises, similar

Similar to DHCP leases [RFC2131], these leases are promises from the
SRP requestor requester 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 duration 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 duration after the update
during which the KEY record can be assumed to be valid. The
reasoning behind the different lease times is discussed in the
section on FCFS naming (Section 3.2.4.1). Sections
3.2.4.1 and 3.2.5.3.

SRP registrars may be configured with limits for these values. A At
the time of writing, a default limit of two hours for the Lease and
14 days for the SIG(0) KEY are currently thought to be good choices. Constrained devices 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 requesters that are going to continue to use
names on which they hold leases SHOULD update refresh them well before the
lease ends, ends in case the registrar is temporarily unavailable or under
heavy load.

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

SRP registrars MUST also track a lease time per service instance.
The reason for doing this is being that a requestor requester may re-register a host hostname with a
different set of services, services and not remember that some different
service instance had previously been registered. In this case, when
that service instance lease expires, the SRP registrar MUST remove
the service instance instance, and any associated Service Discovery PTR
records pointing to that 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 it avoids stale service will continue
services continuing to be advertised. advertised after the SRP requester has
forgotten about them.

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.
response. The requestor requester MUST check for the EDNS(0) Update Lease
option in the response response, and when deciding when to renew its
registration the requester MUST use the lease times from that
received option in place of the options lease times that it sent to originally
requested from the registrar when deciding when to renew its registration. registrar. The times may be shorter or longer
than those specified in the SRP
Update; the Update. The SRP requestor requester must honor
them in either case.

SRP requestors requesters SHOULD assume that each lease ends N seconds after the
update was first transmitted, where transmitted (where N is the granted lease duration. duration).
SRP
Registrars registrars SHOULD assume that each lease ends N seconds after the
update that was successfully processed was received. Because the
registrar will always receive the update after the SRP requestor requester sent
it, this avoids the possibility of misunderstandings. a race condition where the SRP
registrar prematurely removes a service when the SRP requester thinks
the lease has not yet expired. In addition, the SRP requester MUST
begin attempting to renew its lease in advance of the expected
expiration time, as required by the DNS Update Lease specification
[RFC9664], to accomodate the situation where the clocks on the SRP
requester and the SRP registrar do not run at precisely the same
rate.

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

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

6. Security Considerations

6.1. Source Validation

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

For TCP updates, the initial SYN-SYN+ACK handshake prevents updates
being forged by an off-network off-path attacker. In order to ensure that this
handshake happens, SRP registrars relying on three-way-handshake
validation MUST NOT accept TCP Fast Open [RFC7413] payloads. payloads [RFC7413]. If the
network infrastructure allows it, an SRP registrar MAY accept TCP
Fast Open payloads if all such packets are validated along the path,
and the network is able to reject this type of spoofing at all
ingress points.

For UDP updates from constrained CNN devices, spoofing would have to be prevented
with appropriate source address filtration filtering on routers [RFC2827]. This
would ordinarily be accomplished by measures such as
are those described
in Section 4.5 of the IPv6 CE Router Requirements document [RFC7084].
For example, a stub router [I-D.ietf-snac-simple] [SNAC-SIMPLE] for a constrained network CNN might only accept
UDP updates from source addresses known to be on-link on that stub network,
network and might further validate that the UDP update was actually
received on the stub network interface and not the interface
connected to the adjacent infrastructure link.

6.2. Other DNS updates Updates

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

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

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

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

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

* Configure a dictionary of names that are prohibited. Dictionaries
of common obscene and offensive names are no doubt available, available and
can be augmented with a list of typical "special" names like
"www", "mail", "smtp" "smtp", and so on. Lists of names are generally
available,
available or can be constructed manually. Names rejected due to
this should return a Refused RCODE, indicating to the SRP
requester that it should not append or increment a number at the
end of the name and then try again, since this would likely result
in an infinite loop. If a name is considered unacceptable because
it is obscene or offensive, adding a number on the end is unlikely
to make the name acceptable.

6.4. Security of local service discovery Local Service Discovery

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

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

6.5. SRP Registrar Authentication

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

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

6.6. Required Signature Algorithm

For validation, SRP registrars MUST implement the ECDSAP256SHA256
signature algorithm. SRP registrars SHOULD implement the algorithms
specified
that are listed in [RFC8624], Section 3.1, 3.1 of the DNSSEC Cryptographic Algorithms
specification [RFC8624], in the validation column of the table, that
are numbered 13 or higher and that have a "MUST", "RECOMMENDED", or
"MAY" designation in the validation column of the table. SRP requestors
requesters 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 from those for multicast DNS mDNS responses.
Host SRP
Requester implementations that are using TCP SHOULD also use TLS DNS-
over-TLS [RFC7858] if available. SRP Registrar registrar implementations MUST
offer TLS support.
The use of TLS with DNS is described in [RFC7858]. Because there is no mechanism for sharing keys,
validation of DNS-over-TLS keys is not possible; DNS-over-TLS is used
only for Opportunistic Privacy, as described documented in [RFC7858], Section 4.1

Hosts of the
DNS-over-TLS specification [RFC7858].

SRP requesters that implement are able to use TLS support SHOULD NOT fall back to TCP; since TCP.
Since all SRP registrars are required to support TLS, it is entirely up to the
host implementation whether to use it.
TLS is entirely the decision of the SRP requester.

Public keys can be used as identifiers to track hosts. SRP
registrars MAY elect not to return KEY records for queries for SRP
registrations. To avoid DNSSEC validation failures, an SRP registrar
that signs the zone for DNSSEC but refuses to return a KEY record
MUST NOT store the KEY record in the zone itself. Because the KEY
record isn't in the zone, the nonexistance nonexistence of the KEY record can be
validated. If the zone is not signed, the authoritative DNS server
MAY instead return a negative non-error response (either NXDOMAIN or
no data).

8. Domain Name Reservation Considerations

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

8.1. Users

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

8.2. Application Software

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

8.3. Name Resolution APIs and Libraries

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

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

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

8.4. Caching DNS Servers Recursive Resolvers

There are three two considerations for caching recursive resolvers (also known as
"caching DNS servers servers" or "recursive DNS servers") that follow this
specification:

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

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

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

8.5. Authoritative DNS Servers

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

8.6. DNS Server Operators

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

8.7. DNS Registries/Registrars

'service.arpa.'

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

9. Delegation of 'service.arpa.'

In order to be fully functional, the "service.arpa."

The owner of the 'arpa.' zone must
add "arpa." zone, at the time of writing the IAB
[IAB-ARPA], has added a delegation of 'service.arpa.' "service.arpa." in the '.arpa.' "arpa."
zone [RFC3172].
This delegation is to be set up as was done for 'home.arpa', as a
result of [RFC3172], following the specification guidance provided in Section 7 of [RFC8375]. This is
currently the responsibility of the IAB [IAB-ARPA]
"home.arpa." specification [RFC8375].

10. IANA Considerations

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

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

IANA is further requested

10.2. Addition of "service.arpa." to add the Locally-Served Zones Registry

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

10.2.

10.3. Subdomains of 'service.arpa.' "service.arpa."

This document only makes use of the 'default.service.arpa' "default.service.arpa." subdomain
of 'service.arpa.' "service.arpa." Other subdomains are reserved for future use by
DNS-SD or related work. The IANA is requested to create a registry, has created the "service.arpa "service.arpa.
Subdomain" registry. registry [SUB]. The IETF shall have has 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], Approval, provided that a specification exists Section 4.6 of
[RFC8126].

The

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

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

Table 1

10.3.

10.4. Service Name registrations Registrations

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

10.4. 'dnssd-srp'

10.4.1. "dnssd-srp" Service Name

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

Table 2

10.5. 'dnssd-srp-tls'

10.4.2. "dnssd-srp-tls" Service Name

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

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

Table 3

10.6.

10.5. Anycast Address

IANA is requested to allocate has allocated an IPv6 Anycast anycast address from the "IANA IPv6
Special-Purpose Address Registry, Registry" [IPv6], similar to the Port Control
Protocol [RFC6887] anycast address, 2001:1::1. The value TBD is to be replaced
with the actual allocation in the table that follows. address [RFC7723]. The purpose of this
allocation is to provide a fixed anycast address that can be commonly
used as a destination for SRP updates Updates when no SRP registrar is
explicitly configured. The initial values for the registry are:

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

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

Table 4

11. Implementation Status

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

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

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

There are two known independent implementations of SRP requestors:

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

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

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

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. References

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

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

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

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

[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.

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

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

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

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

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

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

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

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.

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

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

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

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

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

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

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

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

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

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

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

14.

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

11.2. Informative References

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

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

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

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

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

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

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

[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<https://www.rfc-editor.org/info/rfc3927>.

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

[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.

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

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

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

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

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

[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.

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

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

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

[RFC7723] Kiesel, S. and R. Penno, "Port Control Protocol (PCP)
Anycast Addresses", RFC 7723, DOI 10.17487/RFC7723,
January 2016, <https://www.rfc-editor.org/info/rfc7723>.

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

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

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

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

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

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

[I-D.ietf-snac-simple]

[SNAC-SIMPLE]
Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-03, 30 January draft-ietf-snac-simple-06, 4 November
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
snac-simple-03>.
snac-simple-06>.

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

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

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

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

Appendix A. Testing using standard RFC2136-compliant Using Standard Authoritative DNS servers

It Servers Compliant with RFC
2136 to Test SRP Requesters

For testing, 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
authoritative DNS server to listen on the anycast address, address or by
advertising it in the
_dnssd-srp._tcp.<zone> SRV "_dnssd-srp._tcp.<zone>" and _dnssd-srp-tls._tcp.<zone> record.
"_dnssd-srp-tls._tcp.<zone>" SRV records. It must be configured to
be authoritative for "default.service.arpa", "default.service.arpa." and to accept updates
from hosts on local networks for names under
"default.service.arpa" "default.service.arpa."
without authentication, authentication since such authoritative DNS servers will not
have support for FCFS authentication (Section 3.2.4.1).

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

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

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

Ordinarily Update Standard Servers
Compliant with RFC 2136

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

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

Appendix C. Sample BIND9 configuration BIND 9 Configuration for default.service.arpa. "default.service.arpa."

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

Figure 1: Zone Configuration in named.conf
$ORIGIN .

$TTL 57600 ; 16 hours
default.service.arpa
@ IN SOA ns3.default.service.arpa.
postmaster.default.service.arpa. ns postmaster (
2951053287 ; serial
3600 ; refresh (1 hour)
1800 ; retry (30 minutes)
604800 ; expire (1 week)
3600 ; minimum (1 hour)
)
NS ns3.default.service.arpa.
SRV 0 0 53 ns3.default.service.arpa.
$ORIGIN default.service.arpa. ns
ns AAAA 2001:db8:0:2::1

$TTL 3600 ; 1 hour
_ipps._tcp PTR demo._ipps._tcp
$ORIGIN _ipps._tcp.default.service.arpa.
demo TXT "0"

; Autoconguration bootstrap records
_dnssd-srp._tcp SRV 0 0 9992 demo.default.service.arpa.
$ORIGIN _udp.default.service.arpa.
$TTL 3600 53 ns
_dnssd-srp-tls._tcp SRV 0 0 853 ns

; 1 hour
_dns-update PTR ns3.default.service.arpa.
$ORIGIN _tcp.default.service.arpa.
_dnssd-srp Service Discovery Instruction
_ipps._tcp PTR ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 300 demo._ipps._tcp

; 5 minutes
ns3 AAAA 2001:db8:0:1::1
$TTL 3600 Service Description Instruction
demo._ipps._tcp SRV 0 0 631 demohost
TXT ""

; 1 hour
demo Host Description Instruction
demohost AAAA 2001:db8:0:2::1 2001:db8:0:2::2
KEY 0 3 13 (
qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
); alg = ECDSAP256SHA256 ; key id = 15008
AAAA ::1 14495

Figure 2: Example Zone file File

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
requester 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.

Authors' Addresses

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

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