rfc9665v1.txt   rfc9665.txt 
Internet Engineering Task Force (IETF) T. Lemon Internet Engineering Task Force (IETF) T. Lemon
Request for Comments: 9665 S. Cheshire Request for Comments: 9665 S. Cheshire
Category: Standards Track Apple Inc. Category: Standards Track Apple Inc.
ISSN: 2070-1721 October 2024 ISSN: 2070-1721 June 2025
Service Registration Protocol for DNS-Based Service Discovery Service Registration Protocol for DNS-Based Service Discovery
Abstract Abstract
The Service Registration Protocol (SRP) for DNS-based Service The Service Registration Protocol (SRP) for DNS-based Service
Discovery (DNS-SD) uses the standard DNS Update mechanism to enable Discovery (DNS-SD) uses the standard DNS Update mechanism to enable
DNS-SD using only unicast packets. This makes it possible to deploy DNS-SD using only unicast packets. This makes it possible to deploy
DNS-SD without multicast, which greatly improves scalability and DNS-SD without multicast, which greatly improves scalability and
improves performance on networks where multicast service is not an improves performance on networks where multicast service is not an
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received public review and has been approved for publication by the received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841. Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9665. https://www.rfc-editor.org/info/rfc9665.
Copyright Notice Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described Trust Legal Provisions and are provided without warranty as described
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3. Service Registration Protocol 3. Service Registration Protocol
3.1. Protocol Variants 3.1. Protocol Variants
3.1.1. Full-Featured Hosts 3.1.1. Full-Featured Hosts
3.1.2. Constrained Hosts 3.1.2. Constrained Hosts
3.1.3. Why two variants? 3.1.3. Why two variants?
3.2. Protocol Details 3.2. Protocol Details
3.2.1. What to Publish 3.2.1. What to Publish
3.2.2. Where to Publish It 3.2.2. Where to Publish It
3.2.3. How to Publish It 3.2.3. How to Publish It
3.2.3.1. How the DNS-SD Service Registration Process Differs 3.2.3.1. How the DNS-SD Service Registration Process Differs
from the DNS Update Specified in RFC 2136 from DNS Update
3.2.3.2. Retransmission Strategy 3.2.3.2. Retransmission Strategy
3.2.3.3. Successive Updates 3.2.3.3. Successive Updates
3.2.4. How to Secure It 3.2.4. How to Secure It
3.2.4.1. FCFS Naming 3.2.4.1. FCFS Naming
3.2.5. SRP Requestor Behavior 3.2.5. SRP Requester Behavior
3.2.5.1. Public/Private Key Pair Generation and Storage 3.2.5.1. Public/Private Key Pair Generation and Storage
3.2.5.2. Name Conflict Handling 3.2.5.2. Name Conflict Handling
3.2.5.3. Record Lifetimes 3.2.5.3. Record Lifetimes
3.2.5.4. Compression in SRV Records 3.2.5.4. Compression in SRV Records
3.2.5.5. Removing Published Services 3.2.5.5. Removing Published Services
3.3. Validation and Processing of SRP Updates 3.3. Validation and Processing of SRP Updates
3.3.1. Validation of DNS Update Add and Delete RRs 3.3.1. Validation of DNS Update Add and Delete RRs
3.3.1.1. Service Discovery Instruction 3.3.1.1. Service Discovery Instruction
3.3.1.2. Service Description Instruction 3.3.1.2. Service Description Instruction
3.3.1.3. Host Description Instruction 3.3.1.3. Host Description Instruction
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6.3. Risks of Allowing Arbitrary Names to be Registered in SRP 6.3. Risks of Allowing Arbitrary Names to be Registered in SRP
Updates Updates
6.4. Security of Local Service Discovery 6.4. Security of Local Service Discovery
6.5. SRP Registrar Authentication 6.5. SRP Registrar Authentication
6.6. Required Signature Algorithm 6.6. Required Signature Algorithm
7. Privacy Considerations 7. Privacy Considerations
8. Domain Name Reservation Considerations 8. Domain Name Reservation Considerations
8.1. Users 8.1. Users
8.2. Application Software 8.2. Application Software
8.3. Name Resolution APIs and Libraries 8.3. Name Resolution APIs and Libraries
8.4. Caching DNS Servers 8.4. Recursive Resolvers
8.5. Authoritative DNS Servers 8.5. Authoritative DNS Servers
8.6. DNS Server Operators 8.6. DNS Server Operators
8.7. DNS Registries/Registrars 8.7. DNS Registries/Registrars
9. Delegation of "service.arpa." 9. Delegation of "service.arpa."
10. IANA Considerations 10. IANA Considerations
10.1. Registration and Delegation of "service.arpa" as a 10.1. Registration and Delegation of "service.arpa." as a
Special-Use Domain Name Special-Use Domain Name
10.2. Subdomains of "service.arpa." 10.2. Addition of "service.arpa." to the Locally-Served Zones
10.3. Service Name Registrations Registry
10.3.1. 'dnssd-srp' Service Name 10.3. Subdomains of "service.arpa."
10.3.2. 'dnssd-srp-tls' Service Name 10.4. Service Name Registrations
10.4. Anycast Address 10.4.1. "dnssd-srp" Service Name
10.4.2. "dnssd-srp-tls" Service Name
10.5. Anycast Address
11. References 11. References
11.1. Normative References 11.1. Normative References
11.2. Informative References 11.2. Informative References
Appendix A. Testing Using Standard DNS Servers Compliant with RFC Appendix A. Using Standard Authoritative DNS Servers Compliant
2136 with RFC 2136 to Test SRP Requesters
Appendix B. How to Allow SRP Requestors to Update Standard Servers Appendix B. How to Allow SRP Requesters to Update Standard Servers
Compliant with RFC 2136 Compliant with RFC 2136
Appendix C. Sample BIND9 Configuration for "default.service.arpa." Appendix C. Sample BIND 9 Configuration for
"default.service.arpa."
Acknowledgments Acknowledgments
Authors' Addresses Authors' Addresses
1. Introduction 1. Introduction
DNS-SD (see [RFC6763]) is a component of Zero Configuration DNS-SD [RFC6763] is a component of Zero Configuration Networking
Networking (see [RFC6760], [ZC], and [ROADMAP]). [RFC6760] [ZC] [ROADMAP].
This document describes an enhancement to DNS-SD that allows servers This document describes an enhancement to DNS-SD that allows servers
to register the services they offer using the DNS protocol rather to register the services they offer using the DNS protocol over
than using Multicast DNS (mDNS) (see [RFC6762]). There is already a unicast rather than using Multicast DNS (mDNS) [RFC6762]. There is
large installed base of DNS-SD clients that can discover services already a large installed base of DNS-SD clients that can discover
using the DNS protocol (e.g., Android, Windows, Linux, Apple). services using the DNS protocol (e.g., Android, Windows, Linux, Apple
operating systems).
This document is intended for three audiences: implementors of This document is intended for three audiences: Implementers of
software that provides services that should be advertised using software that provides services that should be advertised using
DNS-SD, implementors of DNS servers that will be used in contexts DNS-SD, implementers of authoritative DNS servers that will be used
where DNS-SD registration is needed, and administrators of networks in contexts where DNS-SD registration is needed, and administrators
where DNS-SD is required. The document is expected to provide of networks where DNS-SD service is required. The document is
sufficient information to allow interoperable implementation of the expected to provide sufficient information to allow interoperable
registration protocol. implementation of the Service Registration Protocol.
DNS-SD allows services to advertise the fact that they provide DNS-SD allows servers to publish the information required to access
service and to provide the information required to access that the services they provide. DNS-SD clients can then discover the set
service. DNS-SD clients can then discover the set of services of a of service instances of a particular type that are available. They
particular type that are available. They can then select a service can then select an instance from among those that are available and
from among those that are available and obtain the information obtain the information required to use it. Although DNS-SD using the
required to use it. Although DNS-SD using the DNS protocol (as DNS protocol can be more efficient and versatile than using mDNS, it
opposed to mDNS) can be more efficient and versatile, it is not is not common in practice because of the difficulties associated with
common in practice because of the difficulties associated with
updating authoritative DNS services with service information. updating authoritative DNS services with service information.
The existing practice for updating DNS zones is either to manually The existing practice for updating DNS zones is either to enter new
enter new data or to use a DNS Update (see [RFC2136]). data manually or to use DNS Update [RFC2136]. Unfortunately, DNS
Unfortunately, a DNS Update requires either: Update requires either:
* that the authoritative DNS server automatically trust updates or * that the authoritative DNS server automatically trust updates or
* that the DNS Update requestor have some kind of shared secret or * that the DNS Update requester have some kind of shared secret or
public key that is known to the DNS server and can be used to public key that is known to the authoritative DNS server and can
authenticate the update. be used to authenticate the update.
Furthermore, the DNS Update can be a fairly chatty process, requiring Furthermore, DNS Update can be a fairly chatty process, requiring
multiple roundtrips with different conditional predicates to complete multiple roundtrips with different conditional predicates to complete
the update process. the update process.
The Service Registration Protocol (SRP) adds a set of default The Service Registration Protocol (SRP) adds a set of default
heuristics for processing DNS updates that eliminates the need for heuristics for processing DNS updates that eliminates the need for
DNS-update-conditional predicates. Instead, the SRP registrar (a DNS conditional predicates. Instead, the SRP registrar (an authoritative
server that supports SRP updates) has a set of default predicates DNS server that supports SRP Updates) has a set of default predicates
that are applied to the update; and the update either succeeds that are applied to the update; and the update either succeeds
entirely or fails in a way that allows the requestor to know what entirely or fails in a way that allows the requester to know what
went wrong and construct a new update. went wrong and construct a new update.
SRP also adds a feature called "First Come, First Served Naming" (or SRP also adds a feature called "First Come, First Served Naming" (or
"FCFS Naming"), which allows the requestor to: "FCFS Naming"), which allows the requester to:
* claim a name that is not yet in use, and * claim a name that is not yet in use, and
* using SIG(0) ([RFC2931]), authenticate both the initial claim and * authenticate, using SIG(0) [RFC2931], both the initial claim (to
subsequent updates. ensure it has not been modified in transit) and subsequent updates
(to ensure they come from the same entity that performed the
initial claim).
This prevents name conflicts, since a second SRP requestor attempting This prevents a new service instance from "stealing" a name that is
to claim the same name will not possess the SIG(0) key used by the already in use: A second SRP requester attempting to claim an
first requestor to claim it: so its claim will be rejected, and the existing name will not possess the SIG(0) key used by the first
second requestor will have to choose a new name. requester to claim it. Because of this, its claim will be rejected.
This will force it to choose a new name.
It is important to understand that "authenticate" here just means It is important to understand that "authenticate" here just means
that we can tell that an update came from the same source as the that we can tell that an update came from the same source as the
original registration. We have not established trust. This has original registration. We have not established trust. This has
important implications for what we can and can't do with data the important implications for what we can and can't do with data the SRP
client sends us. You will notice as you read this document that we requester sends us. You will notice as you read this document that
only support adding a very restricted set of records, and the content we only support adding a very restricted set of records, and the
of those records is further constrained. content of those records is further constrained.
The reason for this is precisely that we have not established trust. The reason for this is precisely that we have not established trust.
So, we can only publish information that we feel safe in publishing So, we can only publish information that we feel safe in publishing
even though we do not have any basis for trusting the requestor. We even though we do not have any basis for trusting the requester. We
reason that mDNS ([RFC6762]) allows arbitrary hosts on a single IP reason that mDNS [RFC6762] allows arbitrary hosts on a single IP link
link to advertise services ([RFC6763]), relying on whatever service to advertise services [RFC6763], relying on whatever service is
is advertised to provide authentication as a part of its protocol advertised to provide authentication as a part of its protocol rather
rather than in the service advertisement. than in the service advertisement.
This is considered reasonably safe because it requires physical This is considered reasonably safe because it requires physical
presence on the network in order to advertise. An off-network mDNS presence on the network in order to advertise. An off-network mDNS
attack is simply not possible. Our goal with this specification is attack is simply not possible. Our goal with this specification is
to impose similar constraints. Therefore, you will see in to impose similar constraints. Therefore, you will see in
Section 3.3.1 that a very restricted set of records with a very 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 restricted set of relationships are allowed. You will also see in
Section 6.1 that we give advice on how to prevent off-network Section 6.1 that we give advice on how to prevent off-network
attacks. attacks.
This leads us to the disappointing observation that this protocol is This leads us to the disappointing observation that this protocol is
not a mechanism for adding arbitrary information to DNS zones. We not a mechanism for adding arbitrary information to DNS zones. We
have not evaluated the security properties of adding, for example, an have not evaluated the security properties of adding, for example, an
SOA record, an MX record, or a CNAME record; therefore, these are SOA record, an MX record, or a CNAME record; therefore, these are
forbidden. A future protocol specification might include analyses forbidden. Future updates to this specification might include
for other records and extend the set of records that can be analyses for other records and extend the set of records and/or
registered here. Or it might require establishment of trust, and add record content that can be registered here. Or it might require
an authorization model to the authentication model we now have. But establishment of trust, and add an authorization model to the
this is work for a future document. authentication model we now have. But that is work for a future
document.
Finally, SRP adds the concept of a "lease", similar to leases in DHCP Finally, SRP adds the concept of a "lease" [RFC9664], analogous to
([RFC8415]). The SRP registration itself has a lease that may be on leases in DHCP [RFC2131] [RFC8415]. The SRP registration itself has
the order of an hour; if the requestor does not renew the lease a lease that may be on the order of two hours; if the requester does
before it has elapsed, the registration is removed. The claim on the not renew the lease before it has elapsed, the registration is
name can have a longer lease so that another requestor cannot claim removed. The claim on the name can have a longer lease so that
the name, even though the registration has expired. another requester cannot immediately claim the name, even though the
registration itself has expired.
The SRP for DNS-SD specified in this document provides a reasonably The Service Registration Protocol for DNS-SD specified in this
secure mechanism for publishing this information. Once published, document provides a reasonably secure mechanism for publishing this
these services can be readily discovered by DNS-SD clients using information. Once published, these services can be readily
standard DNS lookups. discovered by DNS-SD clients using standard DNS lookups.
The DNS-SD specification (see Section 10 of [RFC6763] briefly Section 10 of the DNS-SD specification [RFC6763] briefly discusses
discusses ways that servers can publish their information in the DNS ways that servers can advertise the services they provide in the DNS
namespace. In the case of mDNS, it allows servers to publish their namespace. In the case of mDNS, it allows servers to advertise their
information on the local link, using names in the ".local" namespace, services on the local link, using names in the "local." namespace,
which makes their services directly discoverable by peers attached to which makes their services directly discoverable by peers attached to
that same local link. that same local link.
RFC 6763 also allows clients to discover services using the DNS DNS-SD [RFC6763] also allows clients to discover services by using
protocol (see [RFC1035]). This can be done by having a system the DNS protocol over traditional unicast [RFC1035]. This can be
administrator manually configure service information in the DNS; done by having a system administrator manually configure service
however, manually populating DNS authoritative server databases is information in the DNS; however, manually populating DNS
costly and potentially error-prone and requires a knowledgeable authoritative server databases is costly and potentially error-prone
network administrator. Consequently, although all DNS-SD client and requires a knowledgeable network administrator. Consequently,
implementations of which we are aware support DNS-SD using DNS although all DNS-SD client implementations of which we are aware
queries, in practice, it is used much less frequently than mDNS. support DNS-SD using DNS queries, in practice it is used much less
frequently than mDNS.
The Discovery Proxy (see [RFC8766]) provides one way to automatically The Discovery Proxy [RFC8766] provides one way to automatically
populate the DNS namespace but is only appropriate on networks where populate the DNS namespace but is only appropriate on networks where
services are easily advertised using mDNS. The present document services are easily advertised using mDNS. The present document
describes a solution more suitable for networks where multicast is describes a solution more suitable for networks where multicast is
inefficient or where sleepy devices are common by supporting both the inefficient, or where sleepy devices are common, by supporting the
offering of services and the discovery of services using unicast. use of unicast for both the offering of and the discovery of
services.
2. Conventions and Terminology Used in This Document 2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Strictly speaking, fully qualified domain names end with a dot (".").
In DNS zone files and other similar contexts, if the final dot 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 dot. When
this document mentions domain names such as "local." and
"default.service.arpa.", the final dot is part of the domain name; it
is not a period indicating the end of the sentence.
3. Service Registration Protocol 3. Service Registration Protocol
Services that implement SRP use DNS Update (see [RFC2136] and Services that implement SRP use DNS Update [RFC2136] with SIG(0)
[RFC3007]) to publish service information in the DNS. Two variants [RFC3007] to publish service information in the DNS. Two variants
exist: one for full-featured hosts and one for devices designed for exist: One for full-featured hosts and one for devices designed for
Constrained-Node Networks (CNNs) ([RFC7228]). An SRP registrar is Constrained-Node Networks (CNNs) [RFC7228]. An SRP registrar is most
most likely an authoritative DNS server or is updating an likely an authoritative DNS server or is a source of data for one or
authoritative DNS server. There is no requirement that the server more authoritative DNS servers. There is no requirement that the
that is receiving SRP updates be the same server that is answering authoritative DNS server that is receiving SRP Updates be the same
queries that return records that have been registered. 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. Protocol Variants
3.1.1. Full-Featured Hosts 3.1.1. Full-Featured Hosts
Full-featured hosts either are configured manually with a Full-featured hosts either are configured manually with a
registration domain or discover the default registration domain as registration domain or discover the default registration domain
described in Section 11 of [RFC6763]. If this process does not automatically using the Domain Enumeration process described in
produce a default registration domain, the SRP is not discoverable on Section 11 of the DNS-SD specification [RFC6763]. If this process
the local network using this mechanism. Other discovery mechanisms does not produce a default registration domain, the SRP registrar is
are possible, but they are out of scope for this document. not discoverable on the local network using this mechanism. Other
discovery mechanisms are possible, but they are out of scope for this
document.
Manual configuration of the registration domain can be done either: Configuration of the registration domain can be done either:
* by querying the list of available registration domains * by querying the list of available registration domains
("r._dns-sd._udp") and allowing the user to select one from the UI ("r._dns-sd._udp") and allowing the user to select one from the
or UI, or
* by any other means appropriate to the particular use case being * by any other means appropriate to the particular use case being
addressed. addressed.
Full-featured devices construct the names of the SRV, TXT, and PTR Full-featured devices construct the names of the SRV, TXT, and PTR
records describing their service or services as subdomains of the records describing their service or services as subdomains of the
chosen service registration domain. For these names, they then chosen service registration domain. For these names, they then
discover the zone apex of the closest enclosing DNS zone using SOA discover the zone apex of the closest enclosing DNS zone using SOA
queries (see Section 6.1 of [RFC8765]). Having discovered the queries as described in Section 6.1 of the DNS Push Notification
enclosing DNS zone, they query for the "_dnssd-srp._tcp.<zone>" SRV specification [RFC8765]. Having discovered the enclosing DNS zone,
record to discover the server to which they can send SRP updates. they query for the "_dnssd-srp._tcp.<zone>" SRV record to discover
Hosts that support SRP Updates using TLS use the the SRP registrar to which they can send SRP Updates. Hosts that
"_dnssd-srp-tls._tcp.<zone>" SRV record instead. 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 Examples of full-featured hosts include devices such as home
computers, laptops, powered peripherals with network connections computers, laptops, powered peripherals with network connections
(such as printers, home routers, and even battery-operated devices (such as printers and home routers), and even battery-operated
such as mobile phones that have long battery lives). devices such as mobile phones that have long battery lives.
3.1.2. Constrained Hosts 3.1.2. Constrained Hosts
For devices designed for CNNs ([RFC7228]), some simplifications are For devices designed for CNNs [RFC7228], some simplifications are
available. Instead of being configured with (or discovering) the available. Instead of being configured with (or discovering) the
service registration domain, the special-use domain name (see service registration domain, the special-use domain name [RFC6761]
[RFC6761]) "default.service.arpa" is used. The details of how SRP "default.service.arpa." is used. The details of how SRP registrars
registrars are discovered will be specific to the constrained are discovered will be specific to the constrained network;
network; therefore, we do not suggest a specific mechanism here. therefore, we do not suggest a specific mechanism here.
SRP requestors on constrained networks are expected to receive, from SRP requesters on CNNs are expected to receive, from the network, a
the network, a list of SRP registrars with which to register. It is list of SRP registrars with which to register. It is the
the responsibility of a CNN supporting SRP to provide one or more responsibility of a CNN supporting SRP to provide at least one
registrar addresses. It is the responsibility of the registrar registrar address and port. It is the responsibility of the
supporting a CNN to handle the updates appropriately. In some registrar supporting a CNN to handle the updates appropriately. In
network environments, updates may be accepted directly into a local some network environments, updates may be accepted directly into a
"default.service.arpa" zone, which has only local visibility. In local "default.service.arpa." zone, which has only local visibility.
other network environments, updates for names ending in In other network environments, updates for names ending in
"default.service.arpa" may be rewritten by the registrar to names "default.service.arpa." may be rewritten by the registrar to names
with broader visibility. 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? 3.1.3. Why two variants?
The reason for these different variants is that low-power devices The reason for these different variants is that low-power devices
that typically use CNNs may have very limited battery storage. The that typically use CNNs may have very limited battery capacity. The
series of DNS lookups required to discover an SRP registrar and then series of DNS lookups required to discover an SRP registrar and then
communicate with it will increase the energy required to advertise a communicate with it will increase the energy required to advertise a
service; for low-power devices, the additional flexibility this service; for low-power devices, the additional flexibility this
provides does not justify the additional use of energy. It is also provides does not justify the additional use of energy. It is also
fairly typical of such networks that some network service information fairly typical of such networks that some network service information
is obtained as part of the process of joining the network; thus, this is obtained as part of the process of joining the network; thus, this
can be relied upon to provide nodes with the information they need. can be relied upon to provide nodes with the information they need.
Networks that are not constrained can have more complicated Networks that are not CNNs can have more complicated topologies at
topologies at the IP layer. Nodes connected to such networks can be the IP layer. Nodes connected to such networks can be assumed to be
assumed to be able to do DNS-SD service registration domain able to do DNS-SD service registration domain discovery. Such
discovery. Such networks are generally able to provide registration networks are generally able to provide registration domain discovery
domain discovery and routing. This creates the possibility of off- and routing. This creates the possibility of off-network spoofing,
network spoofing, where a device from a foreign network registers a where a device from a foreign network registers a service on the
service on the local network in order to attack devices on the local local network in order to attack devices on the local network. To
network. To prevent such spoofing, TCP is required for such guard against off-path spoofing, TCP is required for such networks.
networks.
3.2. Protocol Details 3.2. Protocol Details
We will discuss several parts to this process: We will discuss several parts to this process:
* how to know what to publish (see Section 3.2.1), * how to know what to publish (Section 3.2.1),
* how to know where to publish it (under what name) (Section 3.2.2),
* how to know where to publish it (under what name) (see * how to publish it (Section 3.2.3),
Section 3.2.2), * how to secure its publication (Section 3.2.4), and
* how to maintain the information once published (Section 5).
* how to publish it (see Section 3.2.3),
* how to secure its publication (see Section 3.2.4), and
* how to maintain the information once published (see Section 5).
3.2.1. What to Publish 3.2.1. What to Publish
SRP Updates are sent by SRP requestors to SRP registrars. Three SRP Updates are sent by SRP requesters to SRP registrars. Three
types of instructions appear in an SRP update: Service Discovery types of instructions appear in an SRP Update: Service Discovery
instructions, Service Description instructions, and Host Description instructions, Service Description instructions, and Host Description
instructions. These instructions are made up of DNS Update Resource instructions. These instructions are made up of DNS Update Resource
Records (RRs) that are either adds or deletes. The types of records Records (RRs) that are either adds or deletes. The types of records
that are added, updated, and removed in each of these instructions, that are added, updated, and removed in each of these instructions,
as well as the constraints that apply to them, are described in as well as the constraints that apply to them, are described in
Section 3.3. An SRP Update is a DNS Update message that is 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. constructed so as to meet the constraints described in that section.
The following is a brief overview of what is included in a typical The following is a brief overview of what is included in a typical
SRP Update: SRP Update:
* PTR RR for services, which map from a generic service type (or * Service Discovery PTR RR(s) for service(s), which map from a
subtype) name to a specific Service Instance Name (Section 4.1 of generic service type (or subtype(s)) to a specific service
[RFC6763]). instance name [RFC6763].
* For any Service Instance Name, an SRV RR, one or more TXT RRs, and * For each service instance name, an SRV RR, one or more TXT RRs,
a KEY RR. Although, in principle, DNS-SD Service Description and a KEY RR. Although, in principle, DNS-SD Service Description
records can include other record types with the same Service records can include other record types with the same service
Instance Name, in practice, they rarely do. SRP does not permit instance name, in practice, they rarely do. Currently, SRP does
other record types. The KEY RR is used to support FCFS naming and not permit other record types. The KEY RR is used to support FCFS
has no specific meaning for DNS-SD lookups. SRV records for all Naming and has no specific meaning for DNS-SD lookups. SRV
services described in an SRP update point to the same hostname. records for all services described in an SRP Update point to the
same hostname.
* There is never more than one hostname in a single SRP update. The * There is always exactly one hostname in a single SRP Update. A
hostname has one or more address RRs (AAAA or A) and a KEY RR DNS Update containing more than one hostname is not an SRP Update.
(used for FCFS naming). Depending on the use case, an SRP The hostname has one or more address RRs (AAAA or A) and a KEY RR
requestor may be required to suppress some addresses that would (used for FCFS Naming). Depending on the use case, an SRP
requester may be required to suppress some addresses that would
not be usable by hosts discovering the service through the SRP not be usable by hosts discovering the service through the SRP
registrar. The exact address record suppression behavior required registrar. The exact address record suppression behavior required
may vary for different types of SRP requestors. An example of may vary for different types of SRP requesters. Some suggested
such advice can be found in Section 5.5.2 of [RFC8766]. policies for suppressing unusable records can be found in
Section 5.5.2 of the Discovery Proxy specification [RFC8766].
[RFC6763] describes the details of what each of these types of RRs The DNS-Based Service Discovery specification [RFC6763] describes the
mean, with the exception of the KEY RR, which is defined in details of what each of these RR types mean, with the exception of
[RFC2539]. These RFCs should be considered the definitive sources the KEY RR, which is defined in the specification for how to store
for information about what to publish; the reason for summarizing Diffie-Hellman Keys in the DNS [RFC2539]. These specifications
this here is to provide the reader with enough information about what should be considered the definitive sources for information about
will be published that the service registration process can be what to publish; the reason for summarizing this here is to provide
understood at a high level without first learning the full details of the reader with enough information about what will be published that
DNS-SD. Also, the "Service Instance Name" is an important aspect of the service registration process can be understood at a high level
FCFS naming, which we describe later on in this document. without first learning the full details of DNS-SD. Also, the
"service instance name" is an important aspect of FCFS Naming, which
we describe later on in this document.
3.2.2. Where to Publish It 3.2.2. Where to Publish It
Multicast DNS (mDNS) uses a single namespace that is valid on the Multicast DNS (mDNS) uses a single namespace, "local.". Subdomains
local link called ".local". This convenience is not available for of "local." are specific to the local link on which they are
DNS-SD using the DNS protocol: services must exist in some specific advertised. This convenience is not available for DNS-SD using the
DNS namespace that is chosen either by the network operator or DNS protocol: Services must exist in some specific DNS namespace that
automatically. is chosen either by the network operator or automatically.
As described above, full-featured devices are responsible for knowing As described above, full-featured devices are responsible for knowing
the domain in which to register their services. Such devices MAY the domain in which to register their services. Such devices MAY
optionally support configuration of a registration domain by the optionally support configuration of a registration domain by the
operator of the device. However, such devices MUST support operator of the device. However, such devices MUST support
registration domain discovery as described in Section 11 of registration domain discovery as described in Section 11 of the
[RFC6763]. DNS-SD specification [RFC6763].
Devices made for CNNs register in the special-use domain name Devices made for CNNs register in the special-use domain name
([RFC6761]) "default.service.arpa" and let the SRP registrar handle [RFC6761] "default.service.arpa." and let the SRP registrar handle
rewriting that to a different domain if necessary. rewriting that to a different domain if necessary, as mentioned in
Section 3.1.2.
3.2.3. How to Publish It 3.2.3. How to Publish It
It is possible to issue a DNS Update that does several things at It is possible to send a DNS Update message that does several things
once: meaning that it's possible to do all the work of adding a PTR at once: For example, it's possible in a single transaction to add or
RR to the PTR RRset on the Service Name and creating or updating the update a single Host Description while also adding or updating the
Service Instance Name and Host Description in a single transaction. RRs comprising the Service Description(s) for one or more service
instance(s) available on that host and adding or updating the RRs
comprising the Service Discovery instruction(s) for those service
instance(s).
An SRP Update takes advantage of this: it is implemented as a single An SRP Update takes advantage of this: It is implemented as a single
DNS Update message that contains a service's Service Discovery DNS Update message that contains a service's Service Discovery
records, Service Description records, and Host Description records. records, Service Description records, and Host Description records.
Updates done according to this specification are somewhat different Updates done according to this specification are somewhat different
than regular DNS Updates as defined in [RFC2136] where the update from normal DNS Updates [RFC2136] where the update process could
process could involve many update attempts. You might first attempt involve many update attempts. The requester might first attempt to
to add a name if it doesn't exist; if that fails, then in a second 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 message the requester might update the name if it does exist but
certain preconditions. Because the registration protocol described matches certain preconditions. Because the Service Registration
in this document uses a single transaction, some of this adaptability Protocol described in this document uses a single transaction, some
is lost. of this adaptability is lost.
In order to allow updates to happen in a single transaction, SRP In order to allow updates to happen in a single transaction, SRP
Updates do not include update prerequisites. The requirements Updates do not include update prerequisites. The requirements
specified in Section 3.3 are implicit in the processing of SRP specified in Section 3.3 are implicit in the processing of SRP
Updates; thus, there is no need for the SRP requestor to put in any Updates; thus, there is no need for the SRP requester to put in any
explicit prerequisites. explicit prerequisites.
3.2.3.1. How the DNS-SD Service Registration Process Differs from the 3.2.3.1. How the DNS-SD Service Registration Process Differs from DNS
DNS Update Specified in RFC 2136 Update
DNS-SD Service Registration is based on the standard DNS Update DNS-SD Service Registration uses the DNS Update specification
specified in [RFC2136], with some differences: [RFC2136] with some additions:
* It implements FCFS name allocation, protected using SIG(0) * It implements FCFS Naming, protected using SIG(0) [RFC2931].
([RFC2931]).
* It enforces policy about what updates are allowed. * It enforces policy about what updates are allowed.
* It optionally performs rewriting of "default.service.arpa" to some * It optionally performs rewriting of "default.service.arpa." to
other domain. some other domain.
* It optionally performs automatic population of the address-to-name * It optionally performs automatic population of the address-to-name
reverse mapping domains. reverse mapping domains.
* An SRP registrar is not required to implement general DNS Update * An SRP registrar is not required to implement general DNS Update
prerequisite processing. prerequisite processing.
* Constrained-Node SRP requestors are allowed to send updates to the * CNN SRP requesters are allowed to send updates to the generic
generic domain "default.service.arpa.". domain "default.service.arpa.".
3.2.3.2. Retransmission Strategy 3.2.3.2. Retransmission Strategy
The DNS protocol, including DNS updates, can operate over UDP or TCP. The DNS protocol, including DNS updates, can operate over UDP or TCP.
When using UDP, reliable transmission must be guaranteed by For UDP updates from CNN devices, reliable transmission must be
retransmitting if a DNS UDP message is not acknowledged in a guaranteed by retransmitting when a DNS UDP message is not
reasonable interval. Section 4.2.1 of [RFC1035] provides some acknowledged in a reasonable interval. Section 4.2.1 of the DNS
guidance on this topic, as does Section 1 of [RFC1536]. specification [RFC1035] provides some guidance on this topic, as does
Section 3.1.3 of [RFC8085] also provides useful guidance that is Section 1 of the IETF document describing common DNS implementation
particularly relevant to DNS. 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 3.2.3.3. Successive Updates
SRP does not require that every update contain the same information. SRP does not require that every update contain the same information.
When an SRP requestor needs to send more than one SRP update to the When an SRP requester needs to send more than one SRP Update to the
SRP registrar, it MUST send these sequentially: until an earlier SRP registrar, it SHOULD combine these into a single SRP Update, when
update has been successfully acknowledged, the requestor MUST NOT possible, subject to DNS message size limits and link-specific size
begin sending a subsequent update. 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
subsequent SRP Updates sequentially: Until an earlier SRP Update has
been acknowledged, the requester MUST NOT 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 new SRP
Update for that change until the previous SRP Update has completed.
3.2.4. How to Secure It 3.2.4. How to Secure It
A DNS update, as described in [RFC2136], is secured using secret key DNS Update messages can be secured using secret key transaction
transaction signatures ([RFC8945]) that uses a secret key shared signatures (TSIG) [RFC8945]. This approach uses a secret key shared
between the DNS Update requestor (which issues the update) and the between the DNS Update requester (which issues the update) and the
server (which authenticates it). This model does not work for authoritative DNS server (which authenticates it). This model does
automatic service registration. not work for automatic service registration.
The goal of securing the DNS-SD Registration Protocol is to provide The goal of securing the DNS-SD Registration Protocol is to provide
the best possible security given the constraint that service the best possible security given the constraint that service
registration has to be automatic. It is possible to layer more registration has to be automatic. It is possible to layer more
operational security on top of what we describe here, but FCFS naming operational security on top of what we describe here, but FCFS Naming
is already an improvement over the security of mDNS. is already an improvement over the security of mDNS.
3.2.4.1. FCFS Naming 3.2.4.1. FCFS Naming
FCFS naming provides a limited degree of security. A server that FCFS Naming provides a limited degree of security. A server that
registers its service using the DNS-SD Registration Protocol is given registers its service using SRP is given ownership of a name for an
ownership of a name for an extended period of time based on a lease extended period of time based on a lease specific to the key used to
specific to the key used to authenticate the DNS Update, which may be authenticate the SRP Update, which may be longer than the lease
longer than the lease associated with the registered records. As associated with the registered RRs. As long as the registrar
long as the registration service remembers the name and the key used remembers the name and the public key corresponding to the private
to register that name, no other server can add or update the key used to register RRs on that name, no other SRP requester can add
information associated with that. If the server fails to renew its or update the information associated with that name. If the SRP
service registration before the KEY lease (see Section 4 of requester fails to renew its service registration before the KEY
[RFC9664]) expires, its name is no longer protected. FCFS naming is lease expires (Section 4 of the DNS Update Lease specification
used to protect both the Service Description and the Host [RFC9664]) its name is no longer protected. FCFS Naming is used to
Description. protect both the Service Description and the Host Description.
3.2.5. SRP Requestor Behavior 3.2.5. SRP Requester Behavior
3.2.5.1. Public/Private Key Pair Generation and Storage 3.2.5.1. Public/Private Key Pair Generation and Storage
The requestor generates a public/private key pair (see Section 6.6). The requester generates a public/private key pair (Section 6.6).
This key pair MUST be stored in stable storage; if there is no This key pair MUST be stored in stable storage; if there is no
writable stable storage on the SRP requestor, the SRP requestor MUST writable stable storage on the SRP requester, the SRP requester MUST
be preconfigured with a public/private key pair in read-only storage be preconfigured with a public/private key pair in read-only storage.
that can be used. This key pair MUST be unique to the device. A This key pair MUST be unique to the device. A device with rewritable
device with rewritable storage SHOULD retain this key indefinitely. storage SHOULD retain this key indefinitely. When the device changes
When the device changes ownership, it may be appropriate for the ownership, it may be appropriate for the former owner to erase the
former owner to erase the old key pair, which would then require the old key pair, which would then require the new owner to install a new
new owner to install a new one. Therefore, the SRP requestor on the one. Therefore, the SRP requester on the device SHOULD provide a
device SHOULD provide a mechanism to erase the key (for example, as mechanism to erase the key (for example, as the result of a "factory
the result of a "factory reset") and to generate a new key. 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 only in the event
that the SRP update produces a name conflict.
The policy described here for managing keys assumes that the keys are 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 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 other purposes, the policy described here is likely to be
insufficient. The policy stated here is NOT RECOMMENDED in such a insufficient. The policy stated here is NOT RECOMMENDED in such a
situation: a policy appropriate to the full set of uses for the key 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 must be chosen. Specifying such a policy is out of scope for this
document. document.
When sending DNS updates, the requestor includes a KEY record When sending DNS updates, the requester includes a KEY record
containing the public portion of the key in each Host Description containing the public portion of the key in each Host Description
Instruction and each Service Description Instruction. Each KEY Instruction and each Service Description Instruction. Each KEY
record MUST contain the same public key. The update is signed using 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 SIG(0), using the private key that corresponds to the public key in
the KEY record. The lifetimes of the records in the update is set the KEY record. The lifetimes of the records in the update are set
using the Extension Mechanisms for DNS (EDNS(0)) Update Lease option using the EDNS(0) Update Lease option [RFC9664].
(see [RFC9664]).
The format of the KEY resource record in the SRP Update is defined in The format of the KEY resource record in the SRP Update is defined in
[RFC3445]. Because the KEY RR used in TSIG is not a zone-signing the IETF specification for DNSSEC Resource Records [RFC4034].
key, the flags field in the KEY RR MUST be all zeroes. Because the KEY RR used in 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 The KEY record in Service Description updates MAY be omitted for
brevity; if it is omitted, the SRP registrar MUST behave as if the 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 same KEY record that is given for the Host Description is also given
for each Service Description for which no KEY record is provided. for each Service Description for which no KEY record is provided.
Omitted KEY records are not used when computing the SIG(0) signature. Omitted KEY records are not used when computing the SIG(0) signature.
3.2.5.2. Name Conflict Handling 3.2.5.2. Name Conflict Handling
Adds for both Host Description RRs and Service Description RRs can "Add" operations for both Host Description RRs and Service
have names that result in name conflicts. Service Discovery record Description RRs can have names that result in name conflicts.
adds cannot have name conflicts. If any Host Description or Service Service Discovery record "Add" operations cannot have name conflicts.
Description record is found by the SRP registrar to have a conflict If any Host Description or Service Description record is found by the
with an existing name, the registrar will respond to the SRP Update SRP registrar to have a conflict with an existing name, the registrar
with a YXDomain RCODE (Section 2.2 of [RFC2136]). In this case, the will respond to the SRP Update with a YXDomain RCODE [RFC2136],
requestor MUST choose a new name or give up. indicating that the desired name is already owned by a different
SIG(0) key. In this case, the SRP requester MUST choose a new name
or give up.
There is no specific requirement for how this is done. Typically, There is no specific requirement for how the SRP requester should
however, the requestor will append a number to the preferred name. choose a new name. Typically, however, the requester will append a
This number could be sequentially increasing or could be chosen number to the preferred name. This number could be sequentially
randomly. One existing implementation attempts several sequential increasing or could be chosen randomly. One existing implementation
numbers before choosing randomly. For instance, it might try attempts several sequential numbers before choosing randomly. For
host.default.service.arpa, then host-1.default.service.arpa, then instance, it might try host.default.service.arpa., then
host-2.default.service.arpa, then host-31773.default.service.arpa. host-1.default.service.arpa., then host-2.default.service.arpa., then
host-31773.default.service.arpa.
3.2.5.3. Record Lifetimes 3.2.5.3. Record Lifetimes
The lifetime of the DNS-SD PTR, SRV, A, AAAA, and TXT records (see The lifetime of the DNS-SD PTR, SRV, A, AAAA, and TXT records
[RFC6763]) uses the LEASE field of the Update Lease option and is [RFC6763] uses the LEASE field of the Update Lease option and is
typically set to two hours. Thus, if a device is disconnected from typically set to two hours. Thus, if a device is disconnected from
the network, it does not appear in the user interfaces of devices the network, it does not continue to appear for too long in the user
looking for services of that type for too long. interfaces of devices looking for instances of that service type.
The lifetime of the KEY records is set using the KEY-LEASE field of The lifetime of the KEY records is set using the KEY-LEASE field of
the Update Lease Option and SHOULD be set to a much longer time, the Update Lease Option and SHOULD be set to a much longer time,
typically 14 days. The result being that even though a device may be typically 14 days. The result being that even though a device may be
temporarily unplugged -- disappearing from the network for a few days temporarily disconnected or powered off -- disappearing from the
-- it makes a claim on its name that lasts much longer. network for a few days -- it makes a claim on its name that lasts
much longer.
Therefore, even if a device is unplugged from the network for a few Therefore, even if a device is disconnected from the network for a
days, and its services are not available for that time, no other 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 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 from the network. In the event that a device is disconnected from
network and permanently discarded, then its name is eventually the network and permanently discarded, then its name is eventually
cleaned up and made available for reuse. cleaned up and made available for reuse.
3.2.5.4. Compression in SRV Records 3.2.5.4. Compression in SRV Records
Although [RFC2782] requires that the target name in the SRV record Although the original SRV specification [RFC2782] requires that the
not be compressed, an SRP requestor MAY compress the target in the target hostname in the RDATA of an SRV record not be compressed in
SRV record. The motivation for _not_ compressing in [RFC2782] is not DNS queries and responses, an SRP requester MAY compress the target
stated but is assumed to be because a caching resolver that does not in the SRV record, since an SRP Update is neither a DNS query nor a
understand the format of the SRV record might store it as binary data DNS response. The motivation for _not_ compressing is not stated in
and thus return an invalid pointer in response to a query. This does the SRV specification but is assumed to be because a 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 nonsensical RDATA in response to a query. This concern does
not apply in the case of SRP. An SRP registrar needs to understand not apply in the case of SRP. An SRP registrar needs to understand
SRV records in order to validate the SRP Update. Compression of the SRV records in order to validate the SRP Update. Compression of the
target can save space in the SRP Update, so we want clients to be target can save space in the SRP Update, so we want SRP requesters to
able to assume that the registrar will handle this. Therefore, SRP be able to assume that the registrar will handle this. Therefore,
registrars MUST support compression of SRV RR targets. SRP registrars MUST support compression of SRV RR targets.
Note that this does not update [RFC2782]: DNS servers still MUST NOT Note that this document does not update the SRV specification
compress SRV record targets. The requirement to accept compressed [RFC2782]: Authoritative DNS servers still MUST NOT compress SRV
SRV records in updates only applies to SRP registrars, and SRP record targets. The requirement to accept compressed SRV records in
registrars that are also DNS servers still MUST NOT compress SRV updates only applies to SRP registrars. SRP registrars that are also
record targets in DNS responses. We note also that [RFC6762] authoritative DNS servers still MUST NOT compress SRV record targets
recommends that SRV records be compressed in mDNS messages, so in DNS responses. We note also that Multicast DNS [RFC6762]
[RFC2782] does not apply to mDNS messages. similarly compresses SRV records in mDNS messages.
In addition, we note that an implementor of an SRP requestor might In addition, we note that an implementer of an SRP requester might
update existing code that creates SRV records or compresses DNS update existing code that creates SRV records or compresses DNS
messages so that it compresses the target of an SRV record. Care messages so that it compresses the target of an SRV record. Care
must be taken if such code is used both in requestors and in DNS must be taken if such code is used both in requesters and in
servers that the code only compresses in the case where a requestor authoritative DNS servers that the code only compresses SRV targets
is generating an SRP update. in the case where a requester is generating an SRP Update.
3.2.5.5. Removing Published Services 3.2.5.5. Removing Published Services
3.2.5.5.1. Removing All Published Services 3.2.5.5.1. Removing All Published Services
To remove all the services registered to a particular host, the SRP To remove all the services registered to a particular hostname, the
requestor transmits an SRP update for that host with an Update Lease SRP requester transmits an SRP Update for that hostname with an
option that has a LEASE value of zero. If the registration is to be Update Lease option that has a LEASE value of zero. The SRP Update
permanently removed, KEY-LEASE SHOULD also be zero. Otherwise, it MUST contain exactly one Host Description Instruction that contains
SHOULD be set to the same value it had previously; this holds the exactly one "Delete All RRsets From A Name" instruction for the
name in reserve for when the SRP requestor is once again able to hostname and no "Add to an RRSet" instructions for that hostname. If
provide the service. 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 requester
is once again able to provide the service.
SRP requestors are normally expected to remove all service instances This method of removing services is intended for the case where the
when removing a host. However, in some cases, an SRP requestor may requester is going offline and does not want any of its services to
not have retained sufficient state to know that some service instance continue being advertised.
is pointing to a host that it is removing. This method of removing
services is intended for the case where the requestor is going
offline and does not want its services advertised. Therefore, it is
sufficient for the requestor to send the Host Description Instruction
(see Section 3.3.1.3).
To support this, when removing services based on the lease time being To support this, when removing a hostname, an SRP registrar MUST
zero, an SRP registrar MUST remove all service instances pointing to remove all service instances pointing to that hostname and all
a host when a host is removed, even if the SRP requestor doesn't list Service Discovery PTR records pointing to those service instances,
them explicitly. If the KEY lease time is nonzero, the SRP registrar even if the SRP requester doesn't list them explicitly. If the KEY
MUST NOT delete the KEY records for these SRP requestors. lease time is nonzero, the SRP registrar MUST NOT delete the KEY
records for these service instances.
3.2.5.5.2. Removing Some Published Services 3.2.5.5.2. Removing Some Published Services
In some use cases, a requestor may need to remove a specific service In some use cases, a requester may need to remove a specific service
without removing its other services. This can be accomplished in one instance without removing its other services. For example, a device
of two ways: 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:
1. To simply remove a specific service, the requestor sends a valid 1. To simply remove a specific service instance, the requester sends
SRP Update where the Service Discovery Instruction (see a valid SRP Update with a Service Description Instruction
Section 3.3.1.1) contains a single "Delete An RR From An RRset" (Section 3.3.1.2) containing a single "Delete All RRsets From A
update (Section 2.5.4 of [RFC2136]) that deletes the PTR record Name" update to the service instance name. The SRP Update SHOULD
whose target is the service instance name. In this case, the include Service Discovery Instructions (Section 3.3.1.1)
Service Description Instruction (see Section 3.3.1.2) contains a consisting of "Delete An RR From An RRset" updates [RFC2136] that
single "Delete All RRsets From A Name" update (Section 2.5.3 of delete any Service Discovery PTR records whose target is the
[RFC2136]) to the service instance name. service instance name. However, even in 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. This alternative is used when some service is being replaced by a 2. When deleting one service instance while simultaneously creating
different service with a different service instance name. In a new service instance with a different service instance name, an
this case, the old service is deleted as in the first 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 alternative. The new service is added, just as it would be in an
update that wasn't deleting the old service. Because both the update that wasn't deleting the old service. Because both the
removal of the old service and the add of the new service consist removal of the old service and the add of the new service consist
of a valid Service Discovery Instruction and a valid Service of a valid Service Discovery Instruction and a valid Service
Description Instruction, the update as a whole is a valid SRP Description Instruction, the update as a whole is a valid SRP
Update and will result in the old service being removed and the Update and will result in the old service being removed and the
new one added; or, to put it differently, the update will result new one added; or, to put it differently, the SRP Update will
in the old service being replaced by the new service. result in the old service being replaced by the new service.
It is perhaps worth noting that, if a service is being updated It is perhaps worth noting that if a service is being updated without
without the service instance name changing, that situation will look the service instance name changing (for example, when only the target
very much like the second alternative above. The difference is that port in the SRV record is being updated), then that SRP Update will
because the target for the PTR record in the Service Discovery look very much like the second alternative above. The PTR record in
Instruction is the same for both the "Delete An RR From An RRset" the Service Discovery Instruction will be the same for both the
update and the "Add To An RRset" update (Section 2.5.1 of [RFC2136]), "Delete An RR From An RRset" update and the "Add To An RRset" update
there is no way to tell whether they were intended to be one or two [RFC2136]. Since the removal of the old service and the addition of
Instructions. The same would be true of the Service Description the new service are both valid SRP Update operations, the combined
Instruction. operation is a valid SRP Update operation. The SRP registrar does
not need to include code to 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 lease will be Whichever of these two alternatives is used, the hostname lease will
updated with the lease time provided in the SRP update. In neither be updated with the lease time provided in the SRP update. In
of these cases is it permissible to delete the host. All services neither of these cases is it permissible to delete the hostname. All
must point to a host. If a host is to be deleted, this must be done services must point to a hostname. If a hostname is to be deleted,
using the method described in Section 3.2.5.5.1, which deletes the this must be done using the method described in Section 3.2.5.5.1,
host and all services that have that host as their target. which deletes the hostname and all services that have that hostname
as their target.
3.3. Validation and Processing of SRP Updates 3.3. Validation and Processing of SRP Updates
3.3.1. Validation of DNS Update Add and Delete RRs 3.3.1. Validation of DNS Update Add and Delete RRs
The SRP registrar first validates that the DNS Update is a The SRP registrar first validates that the DNS Update message is a
syntactically and semantically valid DNS Update according to the syntactically and semantically valid DNS Update message according to
rules specified in [RFC2136]. the usual DNS Update rules [RFC2136].
SRP Updates consist of a set of _instructions_ that together add or SRP Updates consist of a set of _instructions_ that together add or
remove one or more services. Each instruction consists of some remove one or more services. Each _instruction_ consists of one or
combination of delete updates and add updates. When an instruction more delete update(s), or one or more add update(s), or some
contains a delete and an add, the delete MUST precede the add. combination of both delete updates and add updates.
The SRP registrar checks each instruction in the SRP Update to see The SRP registrar checks each instruction in the SRP Update to see
that it is either a Service Discovery Instruction, a Service that it is either a Service Discovery Instruction, a Service
Description Instruction, or a Host Description Instruction. Order Description Instruction, or a Host Description Instruction. Order
matters in DNS updates. Specifically, deletes must precede adds for matters in DNS updates. Specifically, deletes must precede adds for
records that the deletes would affect; otherwise, the add will have records that the deletes would affect; otherwise, the add will have
no effect. This is the only ordering constraint: aside from this no effect. This is the only ordering constraint: Aside from this
constraint, updates may appear in whatever order is convenient when constraint, updates may appear in whatever order is convenient when
constructing the update. constructing the update.
Because the SRP Update is a DNS update, it MUST contain a single Because the SRP Update is a DNS update, it MUST contain a single
question that indicates the zone to be updated. Every delete and entry in the Zone Section (what would be the Question Section in a
update in an SRP Update MUST be within the zone that is specified for DNS query or response) that indicates the zone to be updated. Every
the SRP Update. 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 3.3.1.1. Service Discovery Instruction
An instruction is a Service Discovery Instruction if it: An instruction is a Service Discovery Instruction if it:
* Contains exactly one "Add To An RRset" RR update (Section 2.5.1 of * consists of exactly one "Add To An RRSet" or exactly one "Delete
[RFC2136]) or exactly one "Delete An RR From An RRset" RR update An RR From An RRSet" RR update (Section 2.5 of the DNS Update
(Section 2.5.4 of [RFC2136]), which updates a PTR RR; the target specification [RFC2136]),
of which is a Service Instance Name for which name a Service * which updates a PTR RR,
Description Instruction is present in the SRP Update. * the target of which is a service instance name
Additionally: * for which name a Service Description Instruction is present in the
SRP Update, and:
- If the RR Update is an "Add To An RRset" instruction, that - if the Service Discovery Instruction is an "Add To An RRSet"
Service Description Instruction contains an "Add To An RRset" instruction, that Service Description Instruction contains a
RR update for the SRV RR describing that service and no other "Delete All RRsets From A Name" instruction for that service
"Delete From An RRset" instructions for that Service Instance instance name followed by "Add To An RRset" instructions for
Name. the SRV and TXT records describing that service; or
- If the RR Update is a "Delete An RR From An RRset" instruction, - if the Service Discovery Instruction is a "Delete An RR From An
that Service Description Instruction contains a "Delete From An RRSet" instruction, that Service Description Instruction
RRset" RR update and no other "Add To An RRset" instructions contains a "Delete All RRsets From A Name" instruction for that
for that Service Instance Name. service instance name with no following "Add To An RRset"
instructions for the SRV and TXT records describing that
* Contains no other add or delete RR updates for the same name as service. An "Add to an RRset" instruction for the KEY record
the PTR RR Update. here is allowed but not implicit.
Note that there can be more than one Service Discovery Instruction Note that there can be more than one Service Discovery Instruction
for the same name if the SRP requestor is advertising more than one for the same service name (the owner name of the Service Discovery
service of the same type or is changing the target of a PTR RR. This PTR record) if the SRP requester is advertising more than one
is also true for SRP subtypes (Section 7.1 of [RFC6763]). For each instance of the same service type or is changing the target of a PTR
such PTR RR add or delete, the above constraints must be met. RR. When subtypes are being used (Section 7.1 of 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 3.3.1.2. Service Description Instruction
An instruction is a Service Description Instruction if, for the An instruction is a Service Description Instruction if, for the given
appropriate Service Instance Name, the following are true: service instance name, all of the following are true:
* It contains exactly one "Delete All RRsets From A Name" update for * It contains exactly one "Delete All RRsets From A Name" update for
the service instance name (see Section 2.5.3 of [RFC2136]). the service instance name (Section 2.5.3 of the DNS Update
specification [RFC2136]).
* It contains zero or one "Add To An RRset" KEY 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).
* It contains zero or one "Add To An RRset" SRV RR. * It contains zero or one "Add To An RRset" SRV RR.
* If an "Add To An RRSet" update for an SRV RR is present, there
* It contains zero or one "Add To An RRset" KEY RR that, if present, MUST be at least one "Add To An RRset" update for the
contains the public key corresponding to the private key that was corresponding TXT RR, and the target of the SRV RR MUST be the
used to sign the message (if present, the KEY MUST match the KEY hostname given in the Host Description Instruction in the SRP
RR given in the Host Description). Update, or
* If there is no "Add To An RRset" update for an SRV RR, then there
* It contains zero or more "Add To An RRset" TXT RRs. MUST be no "Add To An RRset" updates for the corresponding TXT RR,
and either:
* If there is one "Add To An RRset" SRV update, there MUST be at
least one "Add To An RRset" TXT update.
* The target of the SRV RR Add, if present, points to a hostname for
which there is a Host Description Instruction in the SRP Update;
or if there is no "Add To An RRset" SRV RR, then either:
- the name to which the "Delete All RRsets From A Name" applies - the name to which the "Delete All RRsets From A Name" applies
does not exist, or does not exist, or
- there is an existing KEY RR on that name that matches the key - there is an existing KEY RR on that name that matches the key
with which the SRP Update was signed. with which the SRP Update was signed.
* No other resource records on the Service Instance Name are Service Description Instructions do not add any other resource
modified. records.
An SRP registrar MUST correctly handle compressed names in the SRV An SRP registrar MUST correctly handle compressed names in the SRV
target. target.
3.3.1.3. Host Description Instruction 3.3.1.3. Host Description Instruction
Every SRP Update always contains exactly one Host Description
Instruction.
An instruction is a Host Description Instruction if, for the An instruction is a Host Description Instruction if, for the
appropriate hostname, it contains the following: appropriate hostname, it contains the following:
* exactly one "Delete All RRsets From A Name" RR, * exactly one "Delete All RRsets From A Name" RR
* one or more "Add To An RRset" RRs of type A and/or AAAA, and
* exactly one "Add To An RRset" RR that adds a KEY RR that contains * exactly one "Add To An RRset" RR that adds a KEY RR that contains
the public key corresponding to the private key that was used to the public key corresponding to the private key that was used to
sign the message sign the message
Host Description Instructions do not modify any other resource * zero "Add To An RRset" operations (in the case of deleting a
records. 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 add any other resource records.
A and/or AAAA records that are not of sufficient scope to be validly 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 published in a DNS zone MAY be ignored by the SRP registrar, which
could result in a host description effectively containing zero could result in a Host Description effectively containing zero
reachable addresses even when it contains one or more addresses. reachable addresses even when it contains one or more addresses.
For example, if a link-scope address or IPv4 autoconfiguration For example, if an IPv4 link-local address [RFC3927] or an IPv6 link-
address is provided by the SRP requestor, the SRP registrar could not local address [RFC4862] is provided by the SRP requester, the SRP
publish this in a DNS zone. However, in some situations, the registrar could elect not to publish this in a DNS zone. However, in
registrar might make the records available through a mechanism such some situations, the registrar might make the records available
as an advertising proxy only on the specific link from which the SRP through a mechanism such as an advertising proxy only on the specific
update originated. In such a situation, locally scoped records are link from which the SRP Update originated. In such a situation,
still valid. locally scoped records are still valid.
3.3.2. Valid SRP Update Requirements 3.3.2. Valid SRP Update Requirements
An SRP Update MUST contain exactly one Host Description Instruction. An SRP Update MUST contain exactly one Host Description Instruction.
In addition, there MUST NOT be any Service Description Instruction to Multiple Service Discovery updates and Service Description updates
which no Service Discovery Instruction points. A DNS Update that may be combined into a single single SRP Update along with a single
contains any additional adds or deletes that cannot be identified as Host Description update, as described in Section 3.2.3. A DNS Update
Service Discovery, Service Description, or Host Description message that contains any additional adds or deletes that cannot be
Instructions is not an SRP Update. A DNS update that contains any identified as Service Discovery, Service Description, or Host
prerequisites is not an SRP Update. 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 (see An SRP Update MUST include an EDNS(0) Update Lease option [RFC9664].
[RFC9664]). The LEASE time specified in the Update Lease option MUST The LEASE time specified in the Update Lease option MUST be less than
be less than or equal to the KEY-LEASE time. A DNS update that does or equal to the KEY-LEASE time. A DNS update that does not include
not include the Update Lease option, or that includes a KEY-LEASE the Update Lease option, or that includes a KEY-LEASE value that is
value that is less than the LEASE value, is not an SRP update. less than the LEASE value, is not an SRP Update.
When an SRP registrar receives a DNS Update that is not an SRP When an SRP registrar receives a DNS Update message that is not an
update, it MAY process the update as regular updates described in SRP update, it MAY process the update as normal DNS Update [RFC2136],
[RFC2136], including access control checks and constraint checks, if including access control checks and constraint checks, if supported.
supported. Otherwise, the SRP registrar MUST reject the DNS Update Otherwise, the SRP registrar MUST reject the DNS Update with the
with the Refused RCODE. Refused RCODE.
If the definitions of each of these instructions are followed If the definitions of each of these instructions are followed
carefully and the update requirements are validated correctly, many carefully and the update requirements are validated correctly, many
DNS Updates that look very much like SRP Updates nevertheless will DNS Update messages that look very much like SRP Updates nevertheless
fail to validate. For example, a DNS update that contains an "Add To will fail to validate. For example, a DNS update that contains an
An RRset" instruction for a Service Name and an Add to an RRset "Add To An RRset" instruction for a Service Name and an "Add to an
instruction for a Service Instance Name, where the PTR record added RRset" instruction for a service instance name where the PTR record
to the Service Name does not reference the Service Instance Name, is added to the Service Name does not reference the service instance
not a valid SRP Update message but may be a valid update as described name is not a valid SRP Update but may be a valid DNS Update.
in [RFC2136].
3.3.3. FCFS Name and Signature Validation 3.3.3. FCFS Name and Signature Validation
Assuming that a DNS Update message has been validated with these Assuming that the SRP registrar has confirmed that a DNS Update
conditions and is a valid SRP Update, the SRP registrar checks that message is a valid SRP Update (Section 3.3.2), it then checks that
the name in the Host Description Instruction exists. If so, then the the name in the Host Description Instruction exists in the zone being
registrar checks to see if the KEY record on that name is the same as updated. If so, then the registrar checks to see if the KEY record
the KEY record in the Host Description Instruction. The registrar on that name is the same as the KEY record in the Host Description
performs the same check for the KEY records in any Service Instruction. The registrar performs the same check for the KEY
Description Instructions. For KEY records that were omitted from records in any Service Description Instructions. For KEY records
Service Description Instructions, the KEY from the Host Description that were omitted from Service Description Instructions, the KEY from
Instruction is used. If any existing KEY record corresponding to a the Host Description Instruction is used. If any existing KEY record
KEY record in the SRP Update does not match the KEY record in the SRP corresponding to a KEY record in the SRP Update does not match the
Update (whether provided or taken from the Host Description KEY record in the SRP Update (whether provided or taken from the Host
Instruction), then the SRP registrar MUST reject the SRP Update with Description Instruction), then the SRP registrar MUST reject the SRP
the YXDomain RCODE. Update with a YXDomain 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.
Otherwise, the SRP registrar validates the SRP Update using SIG(0) If the SRP Update is not in conflict with existing data in the zone
against the public key in the KEY record of the Host Description being updated, the SRP registrar validates the SRP Update using
Instruction. If the validation fails, the registrar MUST reject the SIG(0) against the public key in the KEY record of the Host
SRP Update with the Refused RCODE. Otherwise, the SRP Update is Description Instruction. If the validation fails, the SRP Update is
considered valid and authentic and is processed according to the malformed, and the registrar MUST reject the SRP Update with the
method described in [RFC2136]. Refused RCODE. Otherwise, the SRP Update is considered valid and
authentic and is processed as for a normal DNS Update [RFC2136].
KEY record updates omitted from Service Description Instruction are KEY record updates omitted from Service Description Instruction(s)
processed as if they had been explicitly present. After the SRP are processed as if they had been explicitly present. After the SRP
Update has been applied, every Service Description that is updated Update has been applied, every Service Description that is updated
MUST have a KEY RR: and it must be the same KEY RR that is present in MUST have a KEY RR, which MUST have the same value as the KEY RR that
the Host Description to which the Service Description refers. is present in the Host Description to which the Service Description
refers.
[RFC3445] states that the flags field in the KEY RR MUST be zero The IETF specification for DNSSEC Resource Records [RFC4034] states
except for bit 7, which can be one in the case of a zone key. that the flags field in the KEY RR MUST be zero except for bit 7,
However, the SRP registrar MUST NOT validate the flags field. which can be one in the case of a zone key. SRP requesters
implementing this version of the SRP specification MUST 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 in the KEY RR as received, without checking or modifying its
value.
3.3.4. Handling of Service Subtypes 3.3.4. Handling of Service Subtypes
SRP registrars MUST treat the update instructions for a service type SRP registrars MUST treat the update instructions for a service type
and all its subtypes as atomic. That is, when a service and its and all its subtypes as atomic. That is, when a service and its
subtypes are being updated, whatever information appears in the SRP subtypes are being updated, whatever information appears in the SRP
Update is the entirety of information about that service and its Update is the entirety of information about that service and its
subtypes. If any subtype appeared in a previous update but does not subtypes. If any subtype appeared in a previous update but does not
appear in the current update, then the SRP registrar MUST remove that appear in the current update, then the SRP registrar MUST remove that
subtype. subtype.
Similarly, there is no mechanism for deleting subtypes. A delete of There is intentionally no mechanism for deleting a single subtype
a service deletes all of its subtypes. To delete an individual individually. A delete of a service deletes all of its subtypes. To
subtype, an SRP Update must be constructed that contains the service delete a single subtype individually, an SRP Update must be
type and all subtypes for that service except for the one to be constructed that contains the service type and all subtypes for that
deleted. service except for the subtype(s) to be deleted.
3.3.5. SRP Update Response 3.3.5. SRP Update Response
The status that is returned depends on the result of processing the The status that is returned depends on the result of processing the
update and can be either NoError, ServFail, Refused, or YXDomain. update and can be either NoError, ServFail, Refused, or YXDomain.
All other possible outcomes will already have been accounted for when All other possible outcomes will already have been accounted for when
applying the constraints that qualify the update as an SRP Update. applying the constraints that qualify the update as an SRP Update.
The meanings of these responses are explained in Section 2.2 of The meanings of these responses are explained in Section 2.2 of the
[RFC2136]. DNS Update specification [RFC2136].
In the case of a response other than NoError, Section 3.8 of In the case of a response other than NoError, Section 3.8 of the DNS
[RFC2136] specifies that the server is permitted to respond either Update specification [RFC2136] states that the authoritative DNS
with no RRs or to copy the RRs sent by the client into the response. server is permitted to respond either with no RRs or to copy the RRs
The SRP requestor MUST NOT attempt to validate any RRs that are sent by the DNS Update client into the response. The SRP requester
included in the response. It is possible that a future SRP extension MUST NOT attempt to validate any RRs that are included in the
may include per-RR indications as to why the update failed, but at response. It is possible that a future SRP extension may include
the time of writing this is not specified. So, if a client were to per-RR indications as to why the update failed, but at the time of
writing this is not specified. So, if an SRP requester were to
attempt to validate the RRs in the response, it might reject such a attempt to validate the RRs in the response, it might reject such a
response since it would contain RRs but probably not a set of RRs response, since it would contain RRs but probably not a set of RRs
identical to what was sent in the SRP Update. identical to what was sent in the SRP Update.
3.3.6. Optional Behavior 3.3.6. Optional Behavior
The SRP registrar MAY add a Reverse Mapping (see Section 3.5 of The SRP registrar MAY add a Reverse Mapping PTR record (described for
[RFC1035] and Section 2.5 of [RFC3596]) that corresponds to the Host IPv4 in Section 3.5 of the DNS specification [RFC1035] and for IPv6
Description. This is not required because the reverse mapping serves in Section 2.5 of the later document updating DNS for IPv6 [RFC3596])
no protocol function, but it may be useful for debugging, e.g., in that corresponds to the Host Description. This is optional: The
annotating network packet traces or logs. In order for the registrar reverse mapping PTR record serves no essential protocol function.
One reason to provide reverse mappings is that they can be used to
annotate logs and network packet traces. In order for the registrar
to do a reverse mapping update, it must be authoritative for the zone to do a reverse mapping update, it must be authoritative for the zone
that would need to be updated or have credentials to do the update. that would need to be updated or have credentials to do the update.
The SRP requestor MAY also do a reverse mapping update if it has The SRP requester MAY also do a reverse mapping update if it has
credentials to do so. credentials to do so.
The SRP registrar MAY apply additional criteria when accepting The SRP registrar MAY apply additional criteria when accepting
updates. In some networks, it may be possible to do out-of-band updates. In some networks, it may be possible to do out-of-band
registration of keys and only accept updates from preregistered keys. registration of keys and only accept updates from preregistered keys.
In this case, an update for a key that has not been registered SHOULD In this case, an update for a key that has not been registered SHOULD
be rejected with the Refused RCODE. be rejected with the Refused RCODE. When use of managed keys is
desired, there are at least two benefits to doing this in conjunction
There are at least two benefits to doing this rather than simply with SRP rather than simply performing traditional DNS Updates using
using normal SIG(0) DNS updates: SIG(0) keys:
1. The same registration protocol can be used in both cases, so both 1. The same over-the-air registration protocol is used in both
use cases can be addressed by the same SRP requestor cases, so both use cases can be addressed by the same SRP
implementation. requester implementation.
2. The registration protocol includes maintenance functionality not 2. The Service Registration Protocol includes maintenance
present with normal DNS updates. functionality not present with normal DNS updates.
Note that the semantics of using SRP in this way are different than Note that the semantics of using SRP in this way are different from
for typical implementations described in [RFC2136]. The KEY used to the semantics of typical implementations of DNS Update. The KEY used
sign the SRP Update only allows the SRP requestor to update records to sign the SRP Update only allows the SRP requester to update
that refer to its Host Description. Implementations specific to records that refer to its Host Description. Implementations of
[RFC2136] do not normally provide a way to enforce a constraint of traditional DNS Update [RFC2136] do not normally provide a way to
this type. enforce a constraint of this type.
The SRP registrar could also have a dictionary of names or name 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 patterns that are not permitted. If such a list is used, updates for
Service Instance Names that match entries in the dictionary are service instance names that match entries in the dictionary are
rejected with a Refused RCODE. rejected with a Refused RCODE.
4. TTL Consistency 4. TTL Consistency
All RRs within an RRset are required to have the same TTL (see All RRs within an RRset are required to have the same TTL (required
Section 5.2 of [RFC2181]). In order to avoid inconsistencies, SRP by Section 5.2 of the DNS Clarifications document [RFC2181]). In
places restrictions on TTLs sent by requestors and requires that SRP order to avoid inconsistencies, SRP places restrictions on TTLs sent
registrars enforce consistency. by requesters and requires that SRP registrars enforce consistency.
Requestors sending SRP Updates MUST use consistent TTLs in all RRs Requesters sending SRP Updates MUST use consistent TTLs in all RRs
within the SRP Update. within each RRset contained within an SRP Update.
SRP registrars MUST check that the TTLs for all RRs within the SRP SRP registrars MUST check that the TTLs for all RRs within each RRset
Update are the same. If they are not, the SRP update MUST be contained within an SRP Update are the same. If they are not, the
rejected with a Refused RCODE. SRP update MUST be rejected with a Refused RCODE.
Additionally, when adding RRs to an RRset (for example, when Additionally, when adding RRs to an RRset (for example, when
processing Service Discovery records), the SRP registrar MUST use the processing Service Discovery records), the SRP registrar MUST use the
same TTL on all RRs in the RRset. How this consistency is enforced same TTL on all RRs in the RRset. How this consistency is enforced
is up to the implementation. is up to the implementation.
TTLs sent in SRP Updates are advisory: they indicate the SRP TTLs sent in SRP Updates are advisory: they indicate the SRP
requestor's guess as to what a good TTL would be. SRP registrars may requester's guess as to what a good TTL would be. SRP registrars may
override these TTLs. SRP registrars SHOULD ensure that TTLs are override these TTLs. SRP registrars SHOULD ensure that TTLs are
reasonable: neither too long nor too short. The TTL SHOULD NOT ever reasonable: neither too long nor too short. The TTL SHOULD NOT ever
be longer than the lease time (Section 5.1). Shorter TTLs will be longer than the lease time (Section 5.1). Shorter TTLs will
result in more frequent data refreshes; this increases latency on the result in more frequent data refreshes; this increases latency on the
DNS-SD client side, increases load on any caching resolvers and on DNS-SD client side, increases load on any caching resolvers and on
the authoritative server, and also increases network load, which may the authoritative DNS server, and also increases network load, which
be an issue for constrained networks. Longer TTLs will increase the may be an issue for CNNs. Longer TTLs will increase the likelihood
likelihood that data in caches will be stale. TTL minimums and that data in caches will be stale. TTL minimums and maximums SHOULD
maximums SHOULD be configurable by the operator of the SRP registrar. be configurable by the operator of the SRP registrar.
5. Maintenance 5. Maintenance
5.1. Cleaning Up Stale Data 5.1. Cleaning Up Stale Data
Because the DNS-SD registration protocol is automatic and not managed Because the DNS-SD Service Registration Protocol is automatic and not
by humans, some additional bookkeeping is required. When an update managed by humans, some additional bookkeeping is required. When an
is constructed by the SRP requestor, it MUST include an EDNS(0) update is constructed by the SRP requester, it MUST include an
Update Lease Option (see [RFC9664]). The Update Lease Option EDNS(0) Update Lease Option [RFC9664]. The Update Lease Option
contains two lease times: the Lease Time and the KEY Lease Time. contains two lease times: the Lease Time and the KEY Lease Time.
Similar to DHCP leases (see [RFC2131]), these leases are promises Similar to DHCP leases [RFC2131], these leases are promises from the
from the SRP requestor that it will send a new update for the service SRP requester that it will send a new update for the service
registration before the lease time expires. The Lease time is chosen registration before the lease time expires. The Lease time is chosen
to represent the time after the update during which the registered to represent the duration after the update during which the
records other than the KEY record can be assumed to be valid. The registered records other than the KEY record can be assumed to be
KEY lease time represents the time after the update during which the valid. The KEY lease time represents the duration after the update
KEY record can be assumed to be valid. during which the KEY record can be assumed to be valid. The
reasoning behind the different lease times is discussed in Sections
3.2.4.1 and 3.2.5.3.
The reasoning behind the different lease times is discussed in SRP registrars may be configured with limits for these values. At
Section 3.2.4.1. SRP registrars may be configured with limits for the time of writing, a default limit of two hours for the Lease and
these values. At the time of writing, a default limit of two hours 14 days for the SIG(0) KEY are thought to be good choices. Devices
for the Lease and 14 days for the SIG(0) KEY are thought to be good with limited battery that wake infrequently are likely to request
choices. Constrained devices with limited battery that wake longer leases; registrars that support such devices may need to set
infrequently are likely to request longer leases; registrars that higher limits. SRP requesters that are going to continue to use
support such devices may need to set higher limits. SRP requestors names on which they hold leases SHOULD refresh them well before the
that are going to continue to use names on which they hold leases lease ends in case the registrar is temporarily unavailable or under
SHOULD update well before the lease ends in case the registrar is heavy load.
unavailable or under heavy load.
The lease time applies specifically to the host. All service The lease time applies specifically to the hostname. All service
instances, and all service entries for such service instances, depend instances, and all service entries for such service instances, depend
on the host. When the lease on a host expires, the host and all on the hostname. When the lease on a hostname expires, the hostname
services that reference it MUST be removed at the same time: it is and all services that reference it MUST be removed at the same time:
never valid for a service instance to remain when the host it It is never valid for a service instance to remain when the hostname
references has been removed. If the KEY record for the host is to it references has been removed. If the KEY record for the hostname
remain, the KEY record for any services that reference it MUST also is to remain, the KEY record for any services that reference it MUST
remain. However, the service PTR record MUST be removed since it has also remain. However, the Service Discovery PTR record MUST be
no key associated with it and since it is never valid to have a removed since it has no key associated with it and since it is never
service PTR record for which there is no service instance on the valid to have a Service Discovery PTR record for which there is no
target of the PTR record. service instance on the target of the PTR record.
SRP registrars MUST also track a lease time per service instance. SRP registrars MUST also track a lease time per service instance.
The reason being that a requestor may re-register a host with a The reason being that a requester may re-register a hostname with a
different set of services and not remember that some different different set of services and not remember that some different
service instance had previously been registered. In this case, when service instance had previously been registered. In this case, when
that service instance lease expires, the SRP registrar MUST remove that service instance lease expires, the SRP registrar MUST remove
the service instance (although the KEY record for the service the service instance, and any associated Service Discovery PTR
instance SHOULD be retained until the KEY lease on that service records pointing to that service instance, (although the KEY record
expires). This is beneficial because, otherwise, if the SRP for the service instance SHOULD be retained until the KEY lease on
requestor continues to renew the host but never mentions the stale that service expires). This is beneficial because it avoids stale
service again, the stale service will continue to be advertised. services continuing to be advertised after the SRP requester has
forgotten about them.
The SRP registrar MUST include an EDNS(0) Update Lease option in the The SRP registrar MUST include an EDNS(0) Update Lease option in the
response if the lease time proposed by the requestor has been response. The requester MUST check for the EDNS(0) Update Lease
shortened or lengthened by the registrar. The requestor MUST check option in the response, and when deciding when to renew its
for the EDNS(0) Update Lease option in the response and MUST use the registration the requester MUST use the lease times from the Update
lease times from that option in place of the options that it sent to Lease option in the response in place of the lease times that it
the registrar when deciding when to renew its registration. The originally requested from the registrar. The times may be shorter or
times may be shorter or longer than those specified in the SRP longer than those specified in the SRP Update. The SRP requester
Update: the SRP requestor must honor them in either case. must honor them in either case.
SRP requestors SHOULD assume that each lease ends N seconds after the SRP requesters SHOULD assume that each lease ends N seconds after the
update was first transmitted (where N is the lease duration). SRP update was first transmitted (where N is the granted lease duration).
registrars SHOULD assume that each lease ends N seconds after the SRP registrars SHOULD assume that each lease ends N seconds after the
update that was successfully processed was received. Because the update that was successfully processed was received. Because the
registrar will always receive the update after the SRP requestor sent registrar will always receive the update after the SRP requester sent
it, this avoids the possibility of misunderstandings. it, this avoids the possibility of 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 accommodate 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) SRP registrars MUST reject updates that do not include an EDNS(0)
Update Lease option. DNS authoritative servers that allow both SRP Update Lease option. DNS authoritative servers that allow both SRP
and non-SRP DNS updates MAY accept updates that don't include leases, and non-SRP DNS updates MAY accept updates that don't include leases,
but they SHOULD differentiate between SRP Updates and other updates but they SHOULD differentiate between SRP Updates and other updates
and MUST reject updates that would otherwise be SRP Updates if they and MUST reject updates that would otherwise be SRP Updates if they
do not include leases. do not include leases.
Lease times have a completely different function than TTLs. On an The function of Lease times and the function of TTLs are completely
authoritative DNS server, the TTL on a resource record is a constant. different. On an authoritative DNS server, the TTL on a resource
Whenever that RR is served in a DNS response, the TTL value sent in record is a constant. Whenever that RR is served in a DNS response,
the answer is the same. The lease time is never sent as a TTL; its the TTL value sent in the answer is the same. The lease time is
sole purpose is to determine when the authoritative DNS server will never sent as a TTL; its sole purpose is to determine when the
delete stale records. It is not an error to send a DNS response with authoritative DNS server will delete stale records. It is not an
a TTL of 'n' when the remaining time on the lease is less than 'n'. error to send a DNS response with a TTL of M when the remaining time
on the lease is less than M.
6. Security Considerations 6. Security Considerations
6.1. Source Validation 6.1. Source Validation
SRP Updates have no authorization semantics other than FCFS. Thus, SRP Updates have no authorization semantics other than "First Come,
if an attacker from outside the administrative domain of the SRP First Served" (FCFS). Thus, if an attacker from outside the
registrar knows the registrar's IP address, it can, in principle, administrative domain of the SRP registrar knows the registrar's IP
send updates to the registrar that will be processed successfully. address, it can, in principle, send updates to the registrar that
Therefore, SRP registrars SHOULD be configured to reject updates from will be processed successfully. Therefore, SRP registrars SHOULD be
source addresses outside of the administrative domain of the configured to reject updates from source addresses outside of the
registrar. administrative domain of the registrar.
For TCP updates, the initial SYN-SYN+ACK handshake prevents updates For TCP updates, the initial SYN-SYN+ACK handshake prevents updates
being forged by an off-network attacker. In order to ensure that being forged by an off-path attacker. In order to ensure that this
this handshake happens, SRP registrars relying on three-way-handshake handshake happens, SRP registrars relying on three-way-handshake
validation MUST NOT accept TCP Fast Open payloads (see [RFC7413]). validation MUST NOT accept TCP Fast Open payloads [RFC7413]. If the
If the network infrastructure allows it, an SRP registrar MAY accept network infrastructure allows it, an SRP registrar MAY accept TCP
TCP Fast Open payloads if all such packets are validated along the Fast Open payloads if all such packets are validated along the path,
path, and the network is able to reject this type of spoofing at all and the network is able to reject this type of spoofing at all
ingress points. ingress points.
For UDP updates from constrained devices, spoofing would have to be For UDP updates from CNN devices, spoofing would have to be prevented
prevented with appropriate source address filtration on routers with appropriate source address filtering on routers [RFC2827]. This
([RFC2827]). This would ordinarily be accomplished by measures such would ordinarily be accomplished by measures such as those described
as those described in (Section 4.5 of [RFC7084]). For example, a in Section 4.5 of the IPv6 CE Router Requirements document [RFC7084].
stub router ([SNAC-SIMPLE]) for a constrained network might only For example, a stub router [SNAC-SIMPLE] for a CNN might only accept
accept UDP updates from source addresses known to be on-link on that UDP updates from source addresses known to be on-link on that stub
stub network and might further validate that the UDP update was network and might further validate that the UDP update was actually
actually received on the stub network interface and not the interface received on the stub network interface and not the interface
connected to the adjacent infrastructure link. connected to the adjacent infrastructure link.
6.2. Other DNS Updates 6.2. Other DNS Updates
Note that these rules only apply to the validation of SRP Updates. A Note that these rules only apply to the validation of SRP Updates.
server that accepts updates from SRP requestors may also accept other An authoritative DNS server that accepts updates from SRP requesters
DNS updates, and those DNS updates may be validated using different may also accept other DNS Update messages, and those DNS Update
rules. However, in the case of a DNS server that accepts SRP messages may be validated using different rules. However, in the
updates, the intersection of the SRP Update rules and whatever other case of an authoritative DNS server that accepts SRP updates, the
update rules are present must be considered very carefully. intersection of the SRP Update rules and whatever other update rules
are present must be considered very carefully.
For example, a normal authenticated DNS update to any RR that was For example, a normal authenticated DNS update to any RR that was
added using SRP, but that is authenticated using a different key, added using SRP, but is authenticated using a different key, could be
could be used to override a promise made by the SRP registrar to an used to override a promise made by the SRP registrar to an SRP
SRP requestor by replacing all or part of the service registration requester by replacing all or part of the service registration
information with information provided by an authenticated DNS update information with information provided by an authenticated DNS update
requestor. An implementation that allows both kinds of updates requester. An implementation that allows both kinds of updates
SHOULD NOT allow DNS Update requestors that are using different SHOULD NOT allow DNS Update requesters that are using different
authentication and authorization credentials to update records added authentication and authorization credentials to update records added
by SRP requestors. by SRP requesters.
6.3. Risks of Allowing Arbitrary Names to be Registered in SRP Updates 6.3. Risks of Allowing Arbitrary Names to be Registered in SRP Updates
It is possible to set up SRP updates for a zone that is used for non- It is possible to set up SRP Updates for a zone that is also used for
DNSSD services. For example, imagine that you set up SRP service for non-DNS-SD records. For example, imagine that you set up SRP service
example.com. SRP hosts can now register names like "www" or "mail" for "example.com". SRP requesters can now register names like "www"
or "smtp" in this domain. In addition, SRP updates using FCFS naming or "mail" or "smtp" in this domain. In addition, SRP Updates using
can insert names that are obscene or offensive into the zone. There FCFS Naming can insert names that are obscene or offensive into the
is no simple solution to these problems. However, we have two zone. There is no simple solution to these problems. However, we
recommendations to address this problem: have two recommendations to address this problem:
* Do not provide SRP service in organization-level zones. Use * Do not provide SRP service in organization-level zones. Use
subdomains of the organizational domain for DNS-SD. This does not subdomains of the organizational domain for DNS-SD. This does not
prevent registering names as mentioned above but does ensure that prevent registering names as mentioned above but does ensure that
genuinely important names are not accidentally reserved for SRP genuinely important names are not accidentally claimed by SRP
clients. So, for example, the zone "dnssd.example.com" could be requesters. So, for example, the zone "dnssd.example.com." could
used instead of "example.com" for SRP updates. Because of the way be used instead of "example.com." for SRP Updates. Because of the
that DNS-browsing domains are discovered, there is no need for the way that DNS-browsing domains are discovered, there is no need for
DNSSD discovery zone that is updated by SRP to have a user- the DNS-SD discovery zone that is updated by SRP to have a user-
friendly or important-sounding name. friendly or important-sounding name.
* Configure a dictionary of names that are prohibited. Dictionaries * Configure a dictionary of names that are prohibited. Dictionaries
of common obscene and offensive names are no doubt available and of common obscene and offensive names are no doubt available and
can be augmented with a list of typical "special" names like can be augmented with a list of typical "special" names like
"www", "mail", "smtp", and so on. Lists of names are generally "www", "mail", "smtp", and so on. Lists of names are generally
available or can be constructed manually. 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 6.4. Security of Local Service Discovery
Local links can be protected by managed services such as Router Local links can be protected by managed services such as RA Guard
Advertisement Guard (see [RFC6105]), but multicast services like [RFC6105], but multicast services like DHCP [RFC2131], DHCPv6
DHCP, DHCPv6, and IPv6 Neighbor Discovery (see [RFC2131], [RFC8415], [RFC8415], and IPv6 Neighbor Discovery [RFC4861] are, in most cases,
and [RFC4861], respectively) are, in most cases, not authenticated not authenticated and can't be controlled on unmanaged networks, such
and can't be controlled on unmanaged networks, such as home networks as home networks and small office networks where no network
and small office networks where no network management staff are management staff are present. In such situations, the SRP service
present. In such situations, the SRP service has comparatively fewer has comparatively fewer potential security exposures and, hence, is
potential security exposures and, hence, is not the weak link. This not the weak link. This is discussed in more detail in
is discussed in more detail in Section 3.2.4. Section 3.2.4.
The fundamental protection for networks of this type is the user's 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 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 other working groups and standards bodies to improve the state of the
art for network on-boarding and device isolation (e.g., [RFC8520] art for network on-boarding and device isolation (e.g., Manufacturer
provides a means for constraining what behaviors are allowed for a Usage Descriptions [RFC8520] provide a means for constraining what
device in an automatic way), but such work is out of scope for this behaviors are allowed for a device in an automatic way), but such
document. work is out of scope for this document.
6.5. SRP Registrar Authentication 6.5. SRP Registrar Authentication
This specification does not provide a mechanism for validating This specification does not provide a mechanism for validating
responses from SRP registrars to SRP requestors. In principle, a KEY responses from SRP registrars to SRP requesters. In principle, a KEY
RR could be used by a non-constrained SRP requestor to validate RR could be used by a non-CNN SRP requester to validate responses
responses from the registrar, but this is not required, nor do we from the registrar, but this is not required, nor do we specify a
specify a mechanism for determining which key to use. mechanism for determining which key to use.
In addition, for DNS-over-TLS connections, out-of-band key pinning as In addition, for DNS-over-TLS connections, out-of-band key pinning as
described in Section 4.2 of [RFC7858] could be used for described in Section 4.2 of the DNS-over-TLS specification [RFC7858]
authentication of the SRP registrar, e.g., to prevent man-in-the- could be used for authentication of the SRP registrar, e.g., to
middle attacks. However, the use of such keys is impractical for an prevent man-in-the-middle attacks. However, the use of such keys is
unmanaged service registration protocol; hence, it is out of scope impractical for an unmanaged service registration protocol; hence, it
for this document. is out of scope for this document.
6.6. Required Signature Algorithm 6.6. Required Signature Algorithm
For validation, SRP registrars MUST implement the ECDSAP256SHA256 For validation, SRP registrars MUST implement the ECDSAP256SHA256
signature algorithm. SRP registrars SHOULD implement the algorithms signature algorithm. SRP registrars SHOULD implement the algorithms
that are specified in Section 3.1 of [RFC8624], in the validation that are listed in Section 3.1 of the DNSSEC Cryptographic Algorithms
column of the table, that are numbered 13 or higher, and that have a specification [RFC8624], in the validation column of the table, that
"MUST", "RECOMMENDED", or "MAY" designation in the validation column are numbered 13 or higher and that have a "MUST", "RECOMMENDED", or
of the table. SRP requestors MUST NOT assume that any algorithm "MAY" designation in the validation column of the table. SRP
numbered lower than 13 is available for use in validating SIG(0) requesters MUST NOT assume that any algorithm numbered lower than 13
signatures. is available for use in validating SIG(0) signatures.
7. Privacy Considerations 7. Privacy Considerations
Because DNS-SD SRP Updates can be sent off-link, the privacy Because DNS-SD SRP Updates can be sent off-link, the privacy
implications of SRP are different than for mDNS responses. Host implications of SRP are different from those for mDNS responses. SRP
implementations that are using TCP SHOULD also use TLS if available. Requester implementations that are using TCP SHOULD also use DNS-
SRP registrar implementations MUST offer TLS support. The use of TLS over-TLS [RFC7858] if available. SRP registrar implementations MUST
with DNS is described in [RFC7858]. Because there is no mechanism offer TLS support. Because there is no mechanism for sharing keys,
for sharing keys, validation of DNS-over-TLS keys is not possible; validation of DNS-over-TLS keys is not possible; DNS-over-TLS is used
DNS-over-TLS is used only as described in Section 4.1 of [RFC7858]. only for Opportunistic Privacy, as documented in Section 4.1 of the
DNS-over-TLS specification [RFC7858].
Hosts that implement TLS support SHOULD NOT fall back to TCP. Since SRP requesters that are able to use TLS SHOULD NOT fall back to TCP.
SRP registrars are required to support TLS, it is entirely up to the Since all SRP registrars are required to support TLS, whether to use
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 Public keys can be used as identifiers to track hosts. SRP
registrars MAY elect not to return KEY records for queries for SRP registrars MAY elect not to return KEY records for queries for SRP
registrations. To avoid DNSSEC validation failures, an SRP registrar registrations. To avoid DNSSEC validation failures, an SRP registrar
that signs the zone for DNSSEC but refuses to return a KEY record 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 MUST NOT store the KEY record in the zone itself. Because the KEY
record isn't in the zone, the nonexistence of the KEY record can be record isn't in the zone, the nonexistence of the KEY record can be
validated. If the zone is not signed, the server MAY instead return validated. If the zone is not signed, the authoritative DNS server
a negative non-error response (either NXDOMAIN or no data). MAY instead return a negative response (either NXDOMAIN or no data).
8. Domain Name Reservation Considerations 8. Domain Name Reservation Considerations
This section specifies considerations for systems involved in domain This section specifies considerations for systems involved in domain
name resolution when resolving queries for names ending with name resolution when resolving queries for names ending with
".service.arpa.". Each item in this section addresses some aspect of ".service.arpa.". Each item in this section addresses some aspect of
the DNS or the process of resolving domain names that would be the DNS or the process of resolving domain names that would be
affected by this special-use allocation. Detailed explanations of affected by this special-use allocation. Detailed explanations of
these items can be found in Section 5 of [RFC6761]. these items can be found in Section 5 of the Special-Use Domain Names
specification [RFC6761].
8.1. Users 8.1. Users
The current proposed use for "service.arpa" does not require special The current proposed use for "service.arpa." does not require special
knowledge on the part of the user. While the "default.service.arpa." knowledge on the part of the user. While the "default.service.arpa."
subdomain is used as a generic name for registration, users are not 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 expected to see this name in user interfaces. In the event that it
does show up in a user interface, it is just a domain name and does show up in a user interface, it is just a domain name and
requires no special treatment by the user. Users are not expected to requires no special treatment by the user.
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 8.2. Application Software
Application software does not need to handle subdomains of Application software does not need to handle subdomains of
"service.arpa" specially. "service.arpa" SHOULD NOT be treated as "service.arpa." specially. "service.arpa." SHOULD NOT be treated as
more trustworthy than any other insecure DNS domain, simply because more trustworthy than any other insecure DNS domain, simply because
it is locally served (or for any other reason). It is not possible it is locally served (or for any other reason). It is not possible
to register a PKI certificate for a subdomain of "service.arpa." to register a PKI certificate for a subdomain of "service.arpa."
because it is a locally served domain name. So, no such subdomain because it is a locally served domain name. So, no such subdomain
can be considered to be uniquely identifying a particular host, as can be considered to be uniquely identifying a particular host, as
would be required for such a PKI certificate to be issued. If a would be required for such a PKI certificate to be issued. If a
subdomain of "service.arpa." is returned by an API or entered in an subdomain of "service.arpa." is returned by an API or entered in an
input field of an application, PKI authentication of the endpoint input field of an application, PKI authentication of the endpoint
being identified by the name will not be possible. Alternative being identified by the name will not be possible. Alternative
methods and practices for authenticating such endpoints are out of methods and practices for authenticating such endpoints are out of
scope for this document. scope for this document.
8.3. Name Resolution APIs and Libraries 8.3. Name Resolution APIs and Libraries
Name resolution APIs and libraries MUST NOT recognize names that end Name resolution APIs and libraries MUST NOT recognize names that end
in "service.arpa." as special and MUST NOT treat them as having in "service.arpa." as special and MUST NOT treat them as having
special significance, except that it may be necessary that such APIs special significance, except that it may be necessary that such APIs
not bypass the locally configured recursive resolvers. not bypass the locally discovered recursive resolvers.
One or more IP addresses for recursive DNS servers will usually be One or more IP addresses for recursive resolvers will usually be
supplied to the client through router advertisements or DHCP. For an supplied to the SRP requester through router advertisements or DHCP.
administrative domain that uses subdomains of "service.arpa.", the For an administrative domain that uses subdomains of "service.arpa.",
recursive resolvers provided by that domain will be able to answer the recursive resolvers provided by that domain will be able to
queries for subdomains of "service.arpa.". Other (non-local) answer queries for subdomains of "service.arpa.". Other (non-local)
resolvers will not, or they will provide answers that are not correct resolvers will not, or they will provide answers that are not correct
within that administrative domain. within that administrative domain.
A host that is configured to use a resolver other than one that has A host that is configured to use a resolver other than one that has
been provided by the local network may not be able to resolve or may been provided by the local network may not be able to resolve or may
receive incorrect results for subdomains of "service.arpa.". In receive incorrect results for subdomains of "service.arpa.". In
order to avoid this, it is permissible that hosts use the resolvers order to avoid this, hosts SHOULD use the resolvers that are locally
that are locally provided for resolving "service.arpa.", even when provided for resolving "service.arpa." names, even when they are
they are configured to use other resolvers. configured to use other resolvers for other names.
8.4. Caching DNS Servers 8.4. Recursive Resolvers
There are three considerations for caching DNS servers that follow There are two considerations for recursive resolvers (also known as
this specification: "caching DNS servers" or "recursive DNS servers") that follow this
specification:
1. For correctness, recursive resolvers at sites using 1. For correctness, recursive resolvers at sites using
'service.arpa.' must, in practice, transparently support DNSSEC 'service.arpa.' must, in practice, transparently support DNSSEC
queries: queries for DNSSEC records and queries with the DNSSEC queries: queries for DNSSEC records and queries with the DNSSEC
OK (DO) bit set (Section 3.2.1 of [RFC4035]). DNSSEC validation OK (DO) bit set (Section 3.2.1 of the DNSSEC specification
is a Best Current Practice ([RFC9364]): although validation is [RFC4035]). DNSSEC validation [RFC9364] is a best current
not required, a caching recursive resolver that does not validate practice: Although validation is not required, a caching
answers that can be validated may cache invalid data. In turn, recursive resolver that does not validate answers that can be
this would prevent validating stub resolvers from successfully validated may cache invalid data. In turn, this would prevent
validating answers. Hence, as a practical matter, recursive validating stub resolvers from successfully validating answers.
resolvers at sites using "service.arpa" should do DNSSEC Hence, as a practical matter, recursive resolvers at sites using
validation. "service.arpa." should do DNSSEC validation.
2. Unless configured otherwise, recursive resolvers and DNS proxies 2. Unless configured otherwise, recursive resolvers and DNS proxies
MUST behave as described in Locally Served Zones (Section 3 of MUST behave following the rules prescribed for Iterative
[RFC6303]). That is, queries for "service.arpa." and subdomains Resolvers in Section 3 of the IETF Locally Served DNS Zones
of "service.arpa." MUST NOT be forwarded, with one important document [RFC6303]. That is, queries for "service.arpa." and
exception: a query for a DS record with the DO bit set MUST subdomains of "service.arpa." MUST NOT be forwarded, with one
return the correct answer for that question, including correct important exception: a query for a DS record with the DO bit set
information in the authority section that proves that the record MUST return the correct answer for that question, including
is nonexistent. 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." So, for example, a query for the NS record for "service.arpa."
MUST NOT result in that query being forwarded to an upstream MUST NOT result in that query being forwarded to an upstream
cache nor to the authoritative DNS server for ".arpa.". However, cache nor to the authoritative DNS server for ".arpa.". However,
as necessary to provide accurate authority information, a query to provide accurate authority information, a query for the DS
for the DS record MUST result in forwarding whatever queries are record MUST result in forwarding whatever queries are necessary.
necessary. Typically, this will just be a query for the DS Typically, this will just be a query for the DS record since the
record since the necessary authority information will be included necessary authority information will be included in the authority
in the authority section of the response if the DO bit is set. section of the response if the DO bit is set.
8.5. Authoritative DNS Servers 8.5. Authoritative DNS Servers
No special processing of "service.arpa." is required for No special processing of "service.arpa." is required for
authoritative DNS server implementations. It is possible that an authoritative DNS server implementations. It is possible that an
authoritative DNS server might attempt to check the authoritative authoritative DNS server might attempt to check the authoritative DNS
servers for "service.arpa." for a delegation beneath that name before servers for "service.arpa." for a delegation beneath that name before
answering authoritatively for such a delegated name. In such a case, answering authoritatively for such a delegated name. In such a case,
because the name always has only local significance, there will be no because the name always has only local significance, there will be no
such delegation in the "service.arpa." zone; therefore, the server such delegation in the "service.arpa." zone; therefore, the
would refuse to answer authoritatively for such a zone. A server authoritative DNS server would refuse to answer authoritatively for
that implements this sort of check MUST be configurable so that such a zone. An authoritative DNS server that implements this sort
either it does not do this check for the "service.arpa." domain or it of check MUST be configurable so that either it does not do this
ignores the results of the check. check for the "service.arpa." domain or it ignores the results of the
check.
8.6. DNS Server Operators 8.6. DNS Server Operators
DNS server operators MAY configure an authoritative server for DNS server operators MAY configure an authoritative DNS server for
"service.arpa." for use with SRP. The operator for the DNS servers "service.arpa." for use with SRP. The operator for the DNS servers
authoritative for "service.arpa." in the global DNS will configure that are authoritative for "service.arpa." in the global DNS will
any such servers as described in Section 9. configure any such DNS servers as described in Section 9.
8.7. DNS Registries/Registrars 8.7. DNS Registries/Registrars
"service.arpa." is a subdomain of the "arpa" top-level domain, which "service.arpa." is a subdomain of the "arpa." top-level domain, which
is operated by IANA under the authority of the Internet Architecture is operated by IANA under the authority of the Internet Architecture
Board (IAB) according to the rules established in [RFC3172]. There Board (IAB) [RFC3172]. There are no other DNS registrars for
are no other DNS registrars for ".arpa". "arpa.".
9. Delegation of "service.arpa." 9. Delegation of "service.arpa."
In order to be fully functional, the owner of the "arpa." zone must The owner of the "arpa." zone, at the time of writing the IAB
add a delegation of "service.arpa." in the ".arpa." zone (see [IAB-ARPA], has added a delegation of "service.arpa." in the "arpa."
[RFC3172]). This delegation is to be set up as was done for zone [RFC3172], following the guidance provided in Section 7 of the
"home.arpa", as a result of the specification in Section 7 of "home.arpa." specification [RFC8375].
[RFC8375]. This is currently the responsibility of the IAB (see
[IAB-ARPA]).
10. IANA Considerations 10. IANA Considerations
10.1. Registration and Delegation of "service.arpa" as a Special-Use 10.1. Registration and Delegation of "service.arpa." as a Special-Use
Domain Name Domain Name
IANA has recorded the domain name "service.arpa." in the "Special-Use IANA has recorded the domain name "service.arpa." in the "Special-Use
Domain Names" registry (see [SUDN]). IANA has implemented the Domain Names" registry [SUDN]. IANA has implemented the delegation
delegation requested in Section 9. requested in Section 9.
10.2. Addition of "service.arpa." to the Locally-Served Zones Registry
IANA has also added a new entry to the "Transport-Independent IANA has also added a new entry to the "Transport-Independent
Locally-Served Zones Registry" registry of the "Locally-Served DNS Locally-Served Zones Registry" registry of the "Locally-Served DNS
Zones" group (see [LSDZ]). The entry is for the domain Zones" group [LSDZ]. The entry is for the domain "SERVICE.ARPA."
"SERVICE.ARPA" with the description "DNS-SD Service Registration with the description "DNS-SD Service Registration Protocol Special-
Protocol Special-Use Domain" and lists this document as the Use Domain" and lists this document as the reference.
reference.
10.2. Subdomains of "service.arpa." 10.3. Subdomains of "service.arpa."
This document only makes use of the "default.service.arpa" subdomain This document only makes use of the "default.service.arpa." subdomain
of "service.arpa." Other subdomains are reserved for future use by of "service.arpa." Other subdomains are reserved for future use by
DNS-SD or related work. IANA has created the "service.arpa DNS-SD or related work. IANA has created the "service.arpa.
Subdomain" registry (see [SUB]). The IETF has change control for Subdomain" registry [SUB]. The IETF has change control for this
this registry. New entries may be added either as a result of registry. New entries may be added either as a result of Standards
Standards Action (Section 4.9 of [RFC8126]) or with IESG Approval Action (Section 4.9 of the IANA Guidelines) or with IESG Approval
(Section 4.10 of [RFC8126]), provided that a specification exists (Section 4.10 of the IANA Guidelines) [RFC8126], provided that the
(Section 4.6 of [RFC8126]). values and their meanings are documented in a permanent and readily
available public specification, in sufficient detail so that
interoperability between independent implementations is possible.
IANA has grouped the "service.arpa Subdomain" registry with the IANA has grouped the "service.arpa. Subdomain" registry with the
"Locally-Served DNS Zones" group. The registry is a table with three "Locally-Served DNS Zones" group. The registry is a table with three
columns: the subdomain name (expressed as a fully qualified domain columns: the subdomain name (expressed as a fully qualified domain
name), a brief description of how it is used, and a reference to the name), a brief description of how it is used, and a reference to the
document that describes its use in detail. document that describes its use in detail.
This initial contents of this registry are as follows: This initial contents of this registry are as follows:
+=======================+=================+===========+ +=======================+=================+===========+
| Subdomain Name | Description | Reference | | Subdomain Name | Description | Reference |
+=======================+=================+===========+ +=======================+=================+===========+
| default.service.arpa. | Default domain | RFC 9665 | | default.service.arpa. | Default domain | RFC 9665 |
| | for SRP updates | | | | for SRP Updates | |
+-----------------------+-----------------+-----------+ +-----------------------+-----------------+-----------+
Table 1 Table 1
10.3. Service Name Registrations 10.4. Service Name Registrations
IANA has added two new entries to the "Service Name and Transport IANA has added two new entries to the "Service Name and Transport
Protocol Port Number Registry" (see [PORT]). The following Protocol Port Number Registry" [PORT]. The following subsections
subsections contain tables with the fields required by Section 8.1.1 contain tables with the fields required by Section 8.1.1 of IANA's
of [RFC6335]. Procedures for Service Name allocation [RFC6335].
10.3.1. 'dnssd-srp' Service Name 10.4.1. "dnssd-srp" Service Name
+====================+=============================+ +====================+=============================+
| Field Name | Value | | Field Name | Value |
+====================+=============================+ +====================+=============================+
| Service Name | dnssd-srp | | Service Name | dnssd-srp |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Transport Protocol | tcp | | Transport Protocol | tcp |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Assignee | IESG <iesg@ietf.org> | | Assignee | IESG <iesg@ietf.org> |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
skipping to change at line 1439 skipping to change at line 1540
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Reference | RFC 9665 | | Reference | RFC 9665 |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Port Number | None | | Port Number | None |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Service Code | None | | Service Code | None |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
Table 2 Table 2
10.3.2. 'dnssd-srp-tls' Service Name 10.4.2. "dnssd-srp-tls" Service Name
+====================+================================+ +====================+================================+
| Field Name | Value | | Field Name | Value |
+====================+================================+ +====================+================================+
| Service Name | dnssd-srp-tls | | Service Name | dnssd-srp-tls |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Transport Protocol | tcp | | Transport Protocol | tcp |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Assignee | IESG <iesg@ietf.org> | | Assignee | IESG <iesg@ietf.org> |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Contact | IETF Chair<chair@ietf.org> | | Contact | IETF Chair <chair@ietf.org> |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Description | DNS-SD Service Discovery (TLS) | | Description | DNS-SD Service Discovery (TLS) |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Reference | RFC 9665 | | Reference | RFC 9665 |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Port Number | None | | Port Number | None |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
| Service Code | None | | Service Code | None |
+--------------------+--------------------------------+ +--------------------+--------------------------------+
Table 3 Table 3
10.4. Anycast Address 10.5. Anycast Address
IANA has allocated an IPv6 Anycast address from the "IANA IPv6 IANA has allocated an IPv6 anycast address from the "IANA IPv6
Special-Purpose Address Registry" (see [IPv6]), similar to the Port Special-Purpose Address Registry" [IPv6], similar to the Port Control
Control Protocol anycast address: 2001:1::1. The purpose of this Protocol [RFC6887] anycast address [RFC7723]. The purpose of this
allocation is to provide a fixed anycast address that can be commonly allocation is to provide a fixed anycast address that can be commonly
used as a destination for SRP updates when no SRP registrar is used as a destination for SRP Updates when no SRP registrar is
explicitly configured. The initial values for the registry are as explicitly configured. The initial values for the registry are as
follows: follows:
+======================+=============================+ +======================+=============================+
| Attribute | Value | | Attribute | Value |
+======================+=============================+ +======================+=============================+
| Address Block | 2001:1::3/128 | | Address Block | 2001:1::3/128 |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Name | DNS-SD Service Registration | | Name | DNS-SD Service Registration |
| | Protocol Anycast Address | | | Protocol Anycast Address |
skipping to change at line 1545 skipping to change at line 1646
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
2000, <https://www.rfc-editor.org/info/rfc2931>. 2000, <https://www.rfc-editor.org/info/rfc2931>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational [RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/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, [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88, "DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003, RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>. <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. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>. <https://www.rfc-editor.org/info/rfc4035>.
[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163, [RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
RFC 6303, DOI 10.17487/RFC6303, July 2011, RFC 6303, DOI 10.17487/RFC6303, July 2011,
<https://www.rfc-editor.org/info/rfc6303>. <https://www.rfc-editor.org/info/rfc6303>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
skipping to change at line 1604 skipping to change at line 1706
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications", [RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020, RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/info/rfc8765>. <https://www.rfc-editor.org/info/rfc8765>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237, [RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023, RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/info/rfc9364>. <https://www.rfc-editor.org/info/rfc9364>.
[RFC9664] Cheshire, S. and T. Lemon, "An EDNS(0) Option to Negotiate [RFC9664] Cheshire, S. and T. Lemon, "An EDNS(0) Option to Negotiate
Leases on DNS Updates", RFC 9664, DOI 10.17487/RFC9664, Leases on DNS Updates", RFC 9664, DOI 10.17487/RFC9664,
October 2024, <https://www.rfc-editor.org/info/rfc9664>. June 2025, <https://www.rfc-editor.org/info/rfc9664>.
11.2. Informative References 11.2. Informative References
[IAB-ARPA] "Internet Architecture Board statement on the registration [IAB-ARPA] "Internet Architecture Board statement on the registration
of special use names in the ARPA domain", March 2017, of special use names in the ARPA domain", March 2017,
<https://www.iab.org/documents/correspondence-reports- <https://www.iab.org/documents/correspondence-reports-
documents/2017-2/iab-statement-on-the-registration-of- documents/2017-2/iab-statement-on-the-registration-of-
special-use-names-in-the-arpa-domain/>. special-use-names-in-the-arpa-domain/>.
[IPv6] IANA, "IANA IPv6 Special-Purpose Address Registry", [IPv6] IANA, "IANA IPv6 Special-Purpose Address Registry",
skipping to change at line 1639 skipping to change at line 1741
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>. May 2000, <https://www.rfc-editor.org/info/rfc2827>.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic [RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, DOI 10.17487/RFC3007, November 2000, Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
<https://www.rfc-editor.org/info/rfc3007>. <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, [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007, DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>. <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. [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
DOI 10.17487/RFC6105, February 2011, DOI 10.17487/RFC6105, February 2011,
<https://www.rfc-editor.org/info/rfc6105>. <https://www.rfc-editor.org/info/rfc6105>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA) Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011, RFC 6335, DOI 10.17487/RFC6335, August 2011,
skipping to change at line 1669 skipping to change at line 1781
<https://www.rfc-editor.org/info/rfc6760>. <https://www.rfc-editor.org/info/rfc6760>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names", [RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013, RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>. <https://www.rfc-editor.org/info/rfc6761>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013, DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>. <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 [RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084, Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013, DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>. <https://www.rfc-editor.org/info/rfc7084>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014, DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>. <https://www.rfc-editor.org/info/rfc7228>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>. <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., [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters, Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018, RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>. <https://www.rfc-editor.org/info/rfc8415>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage [RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520, Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019, DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>. <https://www.rfc-editor.org/info/rfc8520>.
skipping to change at line 1712 skipping to change at line 1833
<https://www.rfc-editor.org/info/rfc8945>. <https://www.rfc-editor.org/info/rfc8945>.
[ROADMAP] Cheshire, S., "Service Discovery Road Map", Work in [ROADMAP] Cheshire, S., "Service Discovery Road Map", Work in
Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03, Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
23 October 2018, <https://datatracker.ietf.org/doc/html/ 23 October 2018, <https://datatracker.ietf.org/doc/html/
draft-cheshire-dnssd-roadmap-03>. draft-cheshire-dnssd-roadmap-03>.
[SNAC-SIMPLE] [SNAC-SIMPLE]
Lemon, T. and J. Hui, "Automatically Connecting Stub Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress, Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-05, 8 July 2024, Internet-Draft, draft-ietf-snac-simple-06, 4 November
<https://datatracker.ietf.org/doc/html/draft-ietf-snac- 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
simple-05>. snac-simple-06>.
[SUB] IANA, "service.arpa Subdomain", [SUB] IANA, "service.arpa Subdomain",
<https://www.iana.org/assignments/locally-served-dns- <https://www.iana.org/assignments/locally-served-dns-
zones/locally-served-dns-zones>. zones/locally-served-dns-zones>.
[SUDN] IANA, "Special-Use Domain Names", [SUDN] IANA, "Special-Use Domain Names",
<https://www.iana.org/assignments/special-use-domain- <https://www.iana.org/assignments/special-use-domain-
names>. names>.
[ZC] Steinberg, D.H. and S. Cheshire, "Zero Configuration [ZC] Steinberg, D.H. and S. Cheshire, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc., Networking: The Definitive Guide", O'Reilly Media, Inc.,
ISBN 9780596101008, December 2005. ISBN 9780596101008, December 2005.
Appendix A. Testing Using Standard DNS Servers Compliant with RFC 2136 Appendix A. Using Standard Authoritative DNS Servers Compliant with RFC
2136 to Test SRP Requesters
It may be useful to set up an authoritative DNS server for testing For testing, it may be useful to set up an authoritative DNS server
that does not implement SRP. This can be done by configuring the that does not implement SRP. This can be done by configuring the
server to listen on the anycast address or by advertising it in the authoritative DNS server to listen on the anycast address or by
_dnssd-srp._tcp.<zone> SRV and _dnssd-srp-tls._tcp.<zone> record. It advertising it in the "_dnssd-srp._tcp.<zone>" and
must be configured to be authoritative for "default.service.arpa" and "_dnssd-srp-tls._tcp.<zone>" SRV records. It must be configured to
to accept updates from hosts on local networks for names under be authoritative for "default.service.arpa." and to accept updates
"default.service.arpa" without authentication since such servers will from hosts on local networks for names under "default.service.arpa."
not have support for FCFS authentication (Section 3.2.4.1). without 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 An authoritative DNS server configured in this way will be able to
successfully accept and process SRP Updates from requestors that send successfully accept and process SRP Updates from requesters that send
SRP updates. However, no prerequisites will be applied; this means SRP updates. However, no prerequisites will be applied; this means
that the test server will accept internally inconsistent SRP Updates that the test authoritative DNS server will accept internally
and will not stop two SRP Updates sent by different services that inconsistent SRP Updates and will not stop two SRP Updates sent by
claim the same name or names from overwriting each other. different services that claim the same name or names from overwriting
each other.
Since SRP Updates are signed with keys, validation of the SIG(0) Since SRP Updates are signed with keys, validation of the SIG(0)
algorithm used by the requestor can be done by manually installing algorithm used by the requester can be done by manually installing
the requestor's public key on the DNS server that will be receiving the requester's public key on the authoritative DNS server that will
the updates. The key can then be used to authenticate the SRP update be receiving the updates. The key can then be used to authenticate
and can be used as a requirement for the update. An example the SRP Update and can be used as a requirement for the update. An
configuration for testing SRP using BIND 9 is given in Appendix C. example configuration for testing SRP using BIND 9 is given in
Appendix C.
Appendix B. How to Allow SRP Requestors to Update Standard Servers Appendix B. How to Allow SRP Requesters to Update Standard Servers
Compliant with RFC 2136 Compliant with RFC 2136
Ordinarily, SRP Updates will fail when sent to a server compliant Ordinarily, CNN SRP Updates sent to an authoritative DNS server that
with [RFC2136] that does not implement SRP because the zone being implements standard DNS Update [RFC2136] but not SRP will fail
updated is "default.service.arpa" and because no DNS server that is because the zone being updated is "default.service.arpa." and because
not an SRP registrar would normally be configured to be authoritative no authoritative DNS server that is not an SRP registrar would
for "default.service.arpa". Therefore, a requestor that sends an SRP normally be configured to be authoritative for
Update can tell that the receiving server does not support SRP but "default.service.arpa.". Therefore, a requester that sends an SRP
does support [RFC2136] because the RCODE will either be NotZone, Update can tell that the receiving authoritative DNS server does not
NotAuth, or Refused or because there is no response to the update support SRP but does support standard DNS Update [RFC2136] because
request (when using the anycast address). the RCODE will either be NotZone, NotAuth, or Refused or because
there is no response to the update request (when using the anycast
address).
In this case, a requestor MAY attempt to register itself using In this case, a requester MAY attempt to register itself using normal
regular DNS updates described in [RFC2136]. To do so, it must DNS updates [RFC2136]. To do so, it must discover the default
discover the default registration zone and the DNS server designated registration zone and the authoritative DNS server designated to
to receive updates for that zone, as described earlier, using the receive updates for that zone, as described earlier, using the
_dns-update._udp SRV record. It can then send the update to the port _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 and host pointed to by the SRV record, and it is expected to use
appropriate prerequisites to avoid overwriting competing records. appropriate prerequisites to avoid overwriting competing records.
Such updates are out of scope for SRP, and a requestor that Such updates are out of scope for SRP, and a requester that
implements SRP MUST first attempt to use SRP to register itself and implements SRP MUST first attempt to use SRP to register itself and
only attempt to use backwards capability with [RFC2136] if that only attempt to use backwards capability with normal DNS Update
fails. Although the owner name for the SRV record specifies UDP for [RFC2136] if that fails. Although the owner name of the SRV record
updates, it is also possible to use TCP, and TCP SHOULD be required for DNS Update (_dns-update._udp) specifies UDP, it is also possible
to prevent spoofing. to use TCP, and TCP SHOULD be required to prevent spoofing.
Appendix C. Sample BIND9 Configuration for "default.service.arpa." Appendix C. Sample BIND 9 Configuration for "default.service.arpa."
zone "default.service.arpa." { zone "default.service.arpa." {
type primary; type primary;
file "/etc/bind/primary/service.db"; file "/etc/bind/primary/service.db";
allow-update { key demo.default.service.arpa.; }; allow-update { key demo.default.service.arpa.; };
}; };
Figure 1: Zone Configuration in named.conf Figure 1: Zone Configuration in named.conf
$ORIGIN . $TTL 57600 ; 16 hours
$TTL 57600 ; 16 hours @ IN SOA ns postmaster (
default.service.arpa IN SOA ns3.default.service.arpa. 2951053287 ; serial
postmaster.default.service.arpa. ( 3600 ; refresh (1 hour)
2951053287 ; serial 1800 ; retry (30 minutes)
3600 ; refresh (1 hour) 604800 ; expire (1 week)
1800 ; retry (30 minutes) 3600 ; minimum (1 hour)
604800 ; expire (1 week) )
3600 ; minimum (1 hour) NS ns
) ns AAAA 2001:db8:0:2::1
NS ns3.default.service.arpa.
SRV 0 0 53 ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 3600 ; 1 hour
_ipps._tcp PTR demo._ipps._tcp
$ORIGIN _ipps._tcp.default.service.arpa.
demo TXT "0"
SRV 0 0 9992 demo.default.service.arpa.
$ORIGIN _udp.default.service.arpa.
$TTL 3600 ; 1 hour
_dns-update PTR ns3.default.service.arpa.
$ORIGIN _tcp.default.service.arpa.
_dnssd-srp PTR ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 300 ; 5 minutes
ns3 AAAA 2001:db8:0:1::1
$TTL 3600 ; 1 hour
demo AAAA 2001:db8:0:2::1
KEY 0 3 13 (
qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
); alg = ECDSAP256SHA256 ; key id = 15008
AAAA ::1
Figure 2: Example Zone File $TTL 3600 ; 1 hour
; Autoconguration bootstrap records
_dnssd-srp._tcp SRV 0 0 53 ns
_dnssd-srp-tls._tcp SRV 0 0 853 ns
; Service Discovery Instruction
_ipps._tcp PTR demo._ipps._tcp
; Service Description Instruction
demo._ipps._tcp SRV 0 0 631 demohost
TXT ""
; Host Description Instruction
demohost AAAA 2001:db8:0:2::2
KEY 0 3 13 (
qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
); alg = ECDSAP256SHA256 ; key id = 14495
Figure 2: Example Zone File
Acknowledgments Acknowledgments
Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
Dong, and Abtin Keshavarzian for their thorough technical reviews. Dong, and Abtin Keshavarzian for their thorough technical reviews.
Thanks to Kangping and Abtin as well for testing the document by Thanks to Kangping and Abtin as well for testing the document by
doing an independent implementation. Thanks to Tamara Kemper for doing an independent implementation. Thanks to Tamara Kemper for
doing a nice developmental edit, Tim Wattenberg for doing an SRP doing a nice developmental edit, Tim Wattenberg for doing an SRP
requestor proof-of-concept implementation at the Montreal Hackathon requester proof-of-concept implementation at the Montreal Hackathon
at IETF 102, and Tom Pusateri for reviewing during the hackathon and at IETF 102, and Tom Pusateri for reviewing during the hackathon and
afterwards. Thanks to Esko for a really thorough second Last Call afterwards. Thanks to Esko for a really thorough second Last Call
review. Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping review. Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
Dong, Martin Turon, and Michael Cowan for their detailed second last Dong, Martin Turon, and Michael Cowan for their detailed second last
call reviews. Thanks to Patrik Fältström, Dhruv Dhody, David Dong, call reviews. Thanks to Patrik Fältström, Dhruv Dhody, David Dong,
Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective
directorate reviews. Thanks to Paul Wouters for a _really_ detailed directorate reviews. Thanks to Paul Wouters for a _really_ detailed
IESG review! Thanks also to the other IESG members who provided IESG review! Thanks also to the other IESG members who provided
comments or simply took the time to review the document. comments or simply took the time to review the document.
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