rfc9665.original   rfc9665.txt 
Internet Engineering Task Force T. Lemon Internet Engineering Task Force (IETF) T. Lemon
Internet-Draft S. Cheshire Request for Comments: 9665 S. Cheshire
Intended status: Standards Track Apple Inc. Category: Standards Track Apple Inc.
Expires: 5 September 2024 4 March 2024 ISSN: 2070-1721 October 2024
Service Registration Protocol for DNS-Based Service Discovery Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-25
Abstract Abstract
The Service Registration Protocol for DNS-Based Service Discovery The Service Registration Protocol (SRP) for DNS-based Service
uses the standard DNS Update mechanism to enable DNS-Based Service Discovery (DNS-SD) uses the standard DNS Update mechanism to enable
Discovery using only unicast packets. This makes it possible to DNS-SD using only unicast packets. This makes it possible to deploy
deploy DNS Service Discovery without multicast, which greatly DNS-SD without multicast, which greatly improves scalability and
improves scalability and improves performance on networks where improves performance on networks where multicast service is not an
multicast service is not an optimal choice, particularly IEEE 802.11 optimal choice, particularly IEEE 802.11 (Wi-Fi) and IEEE 802.15.4
(Wi-Fi) and IEEE 802.15.4 networks. DNS-SD Service registration uses networks. DNS-SD Service registration uses public keys and SIG(0) to
public keys and SIG(0) to allow services to defend their allow services to defend their registrations.
registrations.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://dnssd-
wg.github.io/draft-ietf-dnssd-srp/draft-ietf-dnssd-srp.html. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-dnssd-srp/.
Discussion of this document takes place on the DNS-SD Working Group
mailing list (mailto:dnssd@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/dnssd/. Subscribe at
https://www.ietf.org/mailman/listinfo/dnssd/.
Source for this draft and an issue tracker can be found at
https://github.com/dnssd-wg/draft-ietf-dnssd-srp.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on 5 September 2024. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9665.
Copyright Notice Copyright Notice
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document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction
2. Conventions and Terminology Used in This Document . . . . . . 6 2. Conventions and Terminology Used in This Document
3. Service Registration Protocol . . . . . . . . . . . . . . . . 6 3. Service Registration Protocol
3.1. Protocol Variants . . . . . . . . . . . . . . . . . . . . 7 3.1. Protocol Variants
3.1.1. Full-featured Hosts . . . . . . . . . . . . . . . . . 7 3.1.1. Full-Featured Hosts
3.1.2. Constrained Hosts . . . . . . . . . . . . . . . . . . 7 3.1.2. Constrained Hosts
3.1.3. Why two variants? . . . . . . . . . . . . . . . . . . 8 3.1.3. Why two variants?
3.2. Protocol Details . . . . . . . . . . . . . . . . . . . . 8 3.2. Protocol Details
3.2.1. What to publish . . . . . . . . . . . . . . . . . . . 8 3.2.1. What to Publish
3.2.2. Where to publish it . . . . . . . . . . . . . . . . . 9 3.2.2. Where to Publish It
3.2.3. How to publish it . . . . . . . . . . . . . . . . . . 10 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 DNS Update as specified in RFC2136 . . . . . . 10 from the DNS Update Specified in RFC 2136
3.2.3.2. Retransmission Strategy . . . . . . . . . . . . . 11 3.2.3.2. Retransmission Strategy
3.2.3.3. Successive Updates . . . . . . . . . . . . . . . 11 3.2.3.3. Successive Updates
3.2.4. How to secure it . . . . . . . . . . . . . . . . . . 11 3.2.4. How to Secure It
3.2.4.1. First-Come First-Served Naming . . . . . . . . . 11 3.2.4.1. FCFS Naming
3.2.5. SRP Requestor Behavior . . . . . . . . . . . . . . . 12 3.2.5. SRP Requestor Behavior
3.2.5.1. Public/Private key pair generation and storage . 12 3.2.5.1. Public/Private Key Pair Generation and Storage
3.2.5.2. Name Conflict Handling . . . . . . . . . . . . . 13 3.2.5.2. Name Conflict Handling
3.2.5.3. Record Lifetimes . . . . . . . . . . . . . . . . 13 3.2.5.3. Record Lifetimes
3.2.5.4. Compression in SRV records . . . . . . . . . . . 13 3.2.5.4. Compression in SRV Records
3.2.5.5. Removing published services . . . . . . . . . . . 14 3.2.5.5. Removing Published Services
3.3. Validation and Processing of SRP Updates . . . . . . . . 15 3.3. Validation and Processing of SRP Updates
3.3.1. Validation of DNS Update Add and Delete RRs . . . . . 15 3.3.1. Validation of DNS Update Add and Delete RRs
3.3.1.1. Service Discovery Instruction . . . . . . . . . . 16 3.3.1.1. Service Discovery Instruction
3.3.1.2. Service Description Instruction . . . . . . . . . 17 3.3.1.2. Service Description Instruction
3.3.1.3. Host Description Instruction . . . . . . . . . . 17 3.3.1.3. Host Description Instruction
3.3.2. Valid SRP Update Requirements . . . . . . . . . . . . 18 3.3.2. Valid SRP Update Requirements
3.3.3. FCFS Name And Signature Validation . . . . . . . . . 18 3.3.3. FCFS Name and Signature Validation
3.3.4. Handling of Service Subtypes . . . . . . . . . . . . 19 3.3.4. Handling of Service Subtypes
3.3.5. SRP Update response . . . . . . . . . . . . . . . . . 20 3.3.5. SRP Update Response
3.3.6. Optional Behavior . . . . . . . . . . . . . . . . . . 20 3.3.6. Optional Behavior
4. TTL Consistency . . . . . . . . . . . . . . . . . . . . . . . 21 4. TTL Consistency
5. Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . 21 5. Maintenance
5.1. Cleaning up stale data . . . . . . . . . . . . . . . . . 22 5.1. Cleaning Up Stale Data
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 6. Security Considerations
6.1. Source Validation . . . . . . . . . . . . . . . . . . . . 24 6.1. Source Validation
6.2. Other DNS updates . . . . . . . . . . . . . . . . . . . . 24 6.2. Other DNS Updates
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 . . . . . . . . . . . . . . . . . . . . . . . . . 25 Updates
6.4. Security of local service discovery . . . . . . . . . . . 25 6.4. Security of Local Service Discovery
6.5. SRP Registrar Authentication . . . . . . . . . . . . . . 26 6.5. SRP Registrar Authentication
6.6. Required Signature Algorithm . . . . . . . . . . . . . . 26 6.6. Required Signature Algorithm
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 26 7. Privacy Considerations
8. Domain Name Reservation Considerations . . . . . . . . . . . 27 8. Domain Name Reservation Considerations
8.1. Users . . . . . . . . . . . . . . . . . . . . . . . . . . 27 8.1. Users
8.2. Application Software . . . . . . . . . . . . . . . . . . 27 8.2. Application Software
8.3. Name Resolution APIs and Libraries . . . . . . . . . . . 27 8.3. Name Resolution APIs and Libraries
8.4. Caching DNS Servers . . . . . . . . . . . . . . . . . . . 28 8.4. Caching DNS Servers
8.5. Authoritative DNS Servers . . . . . . . . . . . . . . . . 29 8.5. Authoritative DNS Servers
8.6. DNS Server Operators . . . . . . . . . . . . . . . . . . 29 8.6. DNS Server Operators
8.7. DNS Registries/Registrars . . . . . . . . . . . . . . . . 29 8.7. DNS Registries/Registrars
9. Delegation of 'service.arpa.' . . . . . . . . . . . . . . . . 29 9. Delegation of "service.arpa."
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 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 . . . . . . . . . . . . . . . . 30 Special-Use Domain Name
10.2. Subdomains of 'service.arpa.' . . . . . . . . . . . . . 30 10.2. Subdomains of "service.arpa."
10.3. Service Name registrations . . . . . . . . . . . . . . . 30 10.3. Service Name Registrations
10.4. 'dnssd-srp' Service Name . . . . . . . . . . . . . . . . 31 10.3.1. 'dnssd-srp' Service Name
10.5. 'dnssd-srp-tls' Service Name . . . . . . . . . . . . . . 31 10.3.2. 'dnssd-srp-tls' Service Name
10.6. Anycast Address . . . . . . . . . . . . . . . . . . . . 32 10.4. Anycast Address
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 32 11. References
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 11.1. Normative References
13. Normative References . . . . . . . . . . . . . . . . . . . . 33 11.2. Informative References
14. Informative References . . . . . . . . . . . . . . . . . . . 36 Appendix A. Testing Using Standard DNS Servers Compliant with RFC
Appendix A. Testing using standard RFC2136-compliant DNS 2136
servers . . . . . . . . . . . . . . . . . . . . . . . . . 38 Appendix B. How to Allow SRP Requestors to Update Standard Servers
Appendix B. How to allow SRP requestors to update standard Compliant with RFC 2136
RFC2136-compliant servers . . . . . . . . . . . . . . . . 39 Appendix C. Sample BIND9 Configuration for "default.service.arpa."
Appendix C. Sample BIND9 configuration for Acknowledgments
default.service.arpa. . . . . . . . . . . . . . . . . . . 39 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction 1. Introduction
DNS-Based Service Discovery [RFC6763] is a component of Zero DNS-SD (see [RFC6763]) is a component of Zero Configuration
Configuration Networking [RFC6760] [ZC] [ROADMAP]. Networking (see [RFC6760], [ZC], and [ROADMAP]).
This document describes an enhancement to DNS-Based Service Discovery This document describes an enhancement to DNS-SD that allows servers
[RFC6763] (DNS-SD) that allows servers to register the services they to register the services they offer using the DNS protocol rather
offer using the DNS protocol rather than using Multicast DNS than using Multicast DNS (mDNS) (see [RFC6762]). There is already a
[RFC6762] (mDNS). There is already a large installed base of DNS-SD large installed base of DNS-SD clients that can discover services
clients that can discover services using the DNS protocol (e.g. using the DNS protocol (e.g., Android, Windows, Linux, Apple).
Android, Windows, Linux, Apple).
This document is intended for three audiences: implementors of This document is intended for three audiences: implementors 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, implementors of DNS servers that will be used in contexts
where DNS-SD registration is needed, and administrators of networks where DNS-SD registration is needed, and administrators of networks
where DNS-SD service is required. The document is expected to where DNS-SD is required. The document is expected to provide
provide sufficient information to allow interoperable implementation sufficient information to allow interoperable implementation of the
of the registration protocol. registration protocol.
DNS-Based Service Discovery (DNS-SD) allows services to advertise the DNS-SD allows services to advertise the fact that they provide
fact that they provide service, and to provide the information service and to provide the information required to access that
required to access that service. DNS-SD clients can then discover service. DNS-SD clients can then discover the set of services of a
the set of services of a particular type that are available. They particular type that are available. They can then select a service
can then select a service 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 Service required to use it. Although DNS-SD using the DNS protocol (as
Discovery (DNS-SD) using the DNS protocol (as opposed to mDNS) can be opposed to mDNS) can be more efficient and versatile, it is not
more efficient and versatile, it is not common in practice, because common in practice because of the difficulties associated with
of the difficulties associated with updating authoritative DNS updating authoritative DNS services with service information.
services with service information.
Existing practice for updating DNS zones is to either manually enter The existing practice for updating DNS zones is either to manually
new data, or else use DNS Update [RFC2136]. Unfortunately DNS Update enter new data or to use a DNS Update (see [RFC2136]).
requires either that the authoritative DNS server automatically trust Unfortunately, a DNS Update requires either:
updates, or else that the DNS Update requestor have some kind of
shared secret or public key that is known to the DNS server and can * that the authoritative DNS server automatically trust updates or
be used to authenticate the update. Furthermore, DNS Update can be a
fairly chatty process, requiring multiple round trips with different * that the DNS Update requestor have some kind of shared secret or
conditional predicates to complete the update process. public key that is known to the DNS server and can be used to
authenticate the update.
Furthermore, the DNS Update can be a fairly chatty process, requiring
multiple roundtrips with different conditional predicates to complete
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 DNS-update-conditional predicates. Instead, the SRP registrar (a DNS
server that supports SRP updates) has a set of default predicates 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 requestor 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 (FCFS) SRP also adds a feature called "First Come, First Served Naming" (or
Naming, which allows the requestor to claim a name that is not yet in "FCFS Naming"), which allows the requestor to:
use, and, using SIG(0) [RFC2931], to authenticate both the initial
claim and subsequent updates. This prevents name conflicts, since a * claim a name that is not yet in use, and
second SRP requestor attempting to claim the same name will not
possess the SIG(0) key used by the first requestor to claim it, and * using SIG(0) ([RFC2931]), authenticate both the initial claim and
so its claim will be rejected and the second requestor will have to subsequent updates.
choose a new name.
This prevents name conflicts, since a second SRP requestor attempting
to claim the same name will not possess the SIG(0) key used by the
first requestor to claim it: so its claim will be rejected, and the
second requestor will have to choose a new name.
It is important to understand that "authenticate" here just means 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
client sends us. You will notice as you read this document that we client sends us. You will notice as you read this document that we
only support adding a very restricted set of records, and the content only support adding a very restricted set of records, and the content
of those records is further constrained. 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 requestor. We
reason that mDNS [RFC6762] allows arbitrary hosts on a single IP link reason that mDNS ([RFC6762]) allows arbitrary hosts on a single IP
to advertise services [RFC6763], relying on whatever service is link to advertise services ([RFC6763]), relying on whatever service
advertised to provide authentication as a part of its protocol rather is advertised to provide authentication as a part of its protocol
than in the service advertisement. rather 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. Because of this 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, and so these are SOA record, an MX record, or a CNAME record; therefore, these are
forbidden. A future protocol specification might include analyses forbidden. A future protocol specification might include analyses
for other records, and extend the set of records that can be for other records and extend the set of records that can be
registered here. Or it might require establishment of trust, and add registered here. Or it might require establishment of trust, and add
an authorization model to the authentication model we now have. But an authorization model to the authentication model we now have. But
this is work for a future document. this is work for a future document.
Finally, SRP adds the concept of a 'lease,' similar to leases in Finally, SRP adds the concept of a "lease", similar to leases in DHCP
Dynamic Host Configuration Protocol [RFC8415]. The SRP registration ([RFC8415]). The SRP registration itself has a lease that may be on
itself has a lease which may be on the order of an hour; if the the order of an hour; if the requestor does not renew the lease
requestor does not renew the lease before it has elapsed, the before it has elapsed, the registration is removed. The claim on the
registration is removed. The claim on the name can have a longer name can have a longer lease so that another requestor cannot claim
lease, so that another requestor cannot claim the name, even though the name, even though the registration has expired.
the registration has expired.
The Service Registration Protocol for DNS-SD (SRP), specified in this The SRP for DNS-SD specified in this document provides a reasonably
document, provides a reasonably secure mechanism for publishing this secure mechanism for publishing this information. Once published,
information. Once published, these services can be readily these services can be readily discovered by DNS-SD clients using
discovered by DNS-SD clients using standard DNS lookups. standard DNS lookups.
The DNS-SD specification ([RFC6763], Section 10, “Populating the DNS The DNS-SD specification (see Section 10 of [RFC6763] briefly
with Information”), briefly discusses ways that servers can publish discusses ways that servers can publish their information in the DNS
their information in the DNS namespace. In the case of mDNS, it namespace. In the case of mDNS, it allows servers to publish their
allows servers to publish their information on the local link, using information on the local link, using names in the ".local" namespace,
names in the ".local" namespace, which makes their services directly which makes their services directly discoverable by peers attached to
discoverable by peers attached to that same local link. that same local link.
RFC6763 also allows clients to discover services using the DNS RFC 6763 also allows clients to discover services using the DNS
protocol [RFC1035]. This can be done by having a system protocol (see [RFC1035]). This can be done by having a system
administrator manually configure service information in the DNS, but administrator manually configure service information in the DNS;
manually populating DNS authoritative server databases is costly and however, manually populating DNS authoritative server databases is
potentially error-prone, and requires a knowledgeable network costly and potentially error-prone and requires a knowledgeable
administrator. Consequently, although all DNS-SD client network administrator. Consequently, although all DNS-SD client
implementations of which we are aware support DNS-SD using DNS implementations of which we are aware support DNS-SD using DNS
queries, in practice it is used much less frequently than mDNS. queries, in practice, it is used much less frequently than mDNS.
The Discovery Proxy [RFC8766] provides one way to automatically The Discovery Proxy (see [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. This document describes a services are easily advertised using mDNS. The present document
solution more suitable for networks where multicast is inefficient, describes a solution more suitable for networks where multicast is
or where sleepy devices are common, by supporting both offering of inefficient or where sleepy devices are common by supporting both the
services, and discovery of services, using unicast. offering of services and the discovery of services using unicast.
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 BCP "OPTIONAL" in this document are to be interpreted as described in
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.
3. Service Registration Protocol 3. Service Registration Protocol
Services that implement SRP use DNS Update [RFC2136] [RFC3007] to Services that implement SRP use DNS Update (see [RFC2136] and
publish service information in the DNS. Two variants exist, one for [RFC3007]) to publish service information in the DNS. Two variants
full-featured hosts, and one for devices designed for "Constrained- exist: one for full-featured hosts and one for devices designed for
Node Networks" [RFC7228]. An SRP registrar is most likely an Constrained-Node Networks (CNNs) ([RFC7228]). An SRP registrar is
authoritative DNS server, or else is updating an authoritative DNS most likely an authoritative DNS server or is updating an
server. There is no requirement that the server that is receiving authoritative DNS server. There is no requirement that the server
SRP updates be the same server that is answering queries that return that is receiving SRP updates be the same server that is answering
records that have been registered. queries that return records that have been registered.
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 as
described in Section 11 of [RFC6763]. If this process does not described in Section 11 of [RFC6763]. If this process does not
produce a default registration domain, the Service Registration produce a default registration domain, the SRP is not discoverable on
protocol is not discoverable on the local network using this the local network using this mechanism. Other discovery mechanisms
mechanism. Other discovery mechanisms are possible, but are out of are possible, but they are out of scope for this document.
scope for this document.
Manual configuration of the registration domain can be done either by Manual configuration of the registration domain can be done either:
querying the list of available registration domains
("r._dns-sd._udp") and allowing the user to select one from the UI, * by querying the list of available registration domains
or by any other means appropriate to the particular use case being ("r._dns-sd._udp") and allowing the user to select one from the UI
addressed. Full-featured devices construct the names of the SRV, or
TXT, and PTR records describing their service(s) as subdomains of the
chosen service registration domain. For these names they then * by any other means appropriate to the particular use case being
addressed.
Full-featured devices construct the names of the SRV, TXT, and PTR
records describing their service or services as subdomains of the
chosen service registration domain. For these names, they then
discover the zone apex of the closest enclosing DNS zone using SOA discover the zone apex of the closest enclosing DNS zone using SOA
queries Section 6.1 of [RFC8765]. Having discovered the enclosing queries (see Section 6.1 of [RFC8765]). Having discovered the
DNS zone, they query for the "_dnssd-srp._tcp.<zone>" SRV record to enclosing DNS zone, they query for the "_dnssd-srp._tcp.<zone>" SRV
discover the server to which they can send SRP updates. Hosts that record to discover the server to which they can send SRP updates.
support SRP Updates using TLS use the "_dnssd-srp-tls._tcp.<zone>" Hosts that support SRP Updates using TLS use the
SRV record instead. "_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 such computers, laptops, powered peripherals with network connections
as printers, home routers, and even battery-operated devices such as (such as printers, home routers, and even battery-operated devices
mobile phones that have long battery lives. such as mobile phones that have long battery lives).
3.1.2. Constrained Hosts 3.1.2. Constrained Hosts
For devices designed for Constrained-Node Networks [RFC7228] some For devices designed for CNNs ([RFC7228]), some simplifications are
simplifications are available. Instead of being configured with (or available. Instead of being configured with (or discovering) the
discovering) the service registration domain, the special-use domain service registration domain, the special-use domain name (see
name (see [RFC6761]) "default.service.arpa" is used. The details of [RFC6761]) "default.service.arpa" is used. The details of how SRP
how SRP registrar(s) are discovered will be specific to the registrars are discovered will be specific to the constrained
constrained network, and therefore we do not suggest a specific network; therefore, we do not suggest a specific mechanism here.
mechanism here.
SRP requestors on constrained networks are expected to receive from SRP requestors on constrained networks are expected to receive, from
the network a list of SRP registrars with which to register. It is the network, a list of SRP registrars with which to register. It is
the responsibility of a Constrained-Node Network supporting SRP to the responsibility of a CNN supporting SRP to provide one or more
provide one or more registrar addresses. It is the responsibility of registrar addresses. It is the responsibility of the registrar
the registrar supporting a Constrained-Node Network to handle the supporting a CNN to handle the updates appropriately. In some
updates appropriately. In some network environments, updates may be network environments, updates may be accepted directly into a local
accepted directly into a local "default.service.arpa" zone, which has "default.service.arpa" zone, which has only local visibility. In
only local visibility. In other network environments, updates for other network environments, updates for names ending in
names ending in "default.service.arpa" may be rewritten by the "default.service.arpa" may be rewritten by the registrar to names
registrar to names with broader visibility. with broader visibility.
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 Constrained-Node Networks may have very limited that typically use CNNs may have very limited battery storage. The
battery storage. The series of DNS lookups required to discover an series of DNS lookups required to discover an SRP registrar and then
SRP registrar and then communicate with it will increase the energy communicate with it will increase the energy required to advertise a
required to advertise a service; for low-power devices, the service; for low-power devices, the additional flexibility this
additional flexibility this provides does not justify the additional provides does not justify the additional use of energy. It is also
use of energy. It is also fairly typical of such networks that some fairly typical of such networks that some network service information
network service information is obtained as part of the process of is obtained as part of the process of joining the network; thus, this
joining the network, and so this can be relied upon to provide nodes can be relied upon to provide nodes with the information they need.
with the information they need.
Networks that are not constrained networks can have more complicated Networks that are not constrained can have more complicated
topologies at the IP layer. Nodes connected to such networks can be topologies at the IP layer. Nodes connected to such networks can be
assumed to be able to do DNS-SD service registration domain assumed to be able to do DNS-SD service registration domain
discovery. Such networks are generally able to provide registration discovery. Such networks are generally able to provide registration
domain discovery and routing. This creates the possibility of off- domain discovery and routing. This creates the possibility of off-
network spoofing, where a device from a foreign network registers a network spoofing, where a device from a foreign network registers a
service on the local network in order to attack devices on the local service on the local network in order to attack devices on the local
network. To prevent such spoofing, TCP is required for such network. To prevent such spoofing, TCP is required for such
networks. networks.
3.2. Protocol Details 3.2. Protocol Details
We will discuss several parts to this process: how to know what to We will discuss several parts to this process:
publish, how to know where to publish it (under what name), how to
publish it, and how to secure its publication. In Section 5, we
specify how to maintain the information once published.
3.2.1. What to publish * how to know what to publish (see Section 3.2.1),
* how to know where to publish it (under what name) (see
Section 3.2.2),
* 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
SRP Updates are sent by SRP requestors to SRP registrars. Three SRP Updates are sent by SRP requestors 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 RRs that instructions. These instructions are made up of DNS Update Resource
are either adds or deletes. The types of records that are added, Records (RRs) that are either adds or deletes. The types of records
updated and removed in each of these instructions, as well as the that are added, updated, and removed in each of these instructions,
constraints that apply to them, are described in Section 3.3. An SRP as well as the constraints that apply to them, are described in
Update is a DNS Update message that is constructed so as to meet the Section 3.3. An SRP Update is a DNS Update message that is
constraints described in that section. The following is a brief constructed so as to meet the constraints described in that section.
overview of what is included in a typical SRP Update: The following is a brief overview of what is included in a typical
SRP Update:
* PTR RR for services, which map from a generic service type (or
subtype) name to a specific Service Instance Name (Section 4.1 of
[RFC6763]).
* For any Service Instance Name, an SRV RR, one or more TXT RRs, and
a KEY RR. Although, in principle, DNS-SD Service Description
records can include other record types with the same Service
Instance Name, in practice, they rarely do. SRP does not permit
other record types. The KEY RR is used to support FCFS naming and
has no specific meaning for DNS-SD lookups. SRV records for all
services described in an SRP update point to the same hostname.
* PTR Resource Record (RR) for services, which map from a generic
service type (or subtype) name to a specific Service Instance
Name.
* For any Service Instance Name ([RFC6763], Section 4.1), an SRV RR,
one or more TXT RRs, and a KEY RR. Although in principle DNS-SD
Service Description records can include other record types with
the same Service Instance Name, in practice they rarely do. SRP
does not permit other record types. The KEY RR is used to support
FCFS naming, and has no specific meaning for DNS-SD lookups. SRV
records for all services described in an SRP update point to the
same hostname.
* There is never more than one hostname in a single SRP update. The * There is never more than one hostname in a single SRP update. The
hostname has one or more address RRs (AAAA or A) and a KEY RR hostname has one or more address RRs (AAAA or A) and a KEY RR
(used for FCFS naming). Depending on the use case, an SRP (used for FCFS naming). Depending on the use case, an SRP
requestor may be required to suppress some addresses that would requestor 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 requestors. An example of
such advice can be found in Section 5.5.2 of [RFC8766]. such advice can be found in Section 5.5.2 of [RFC8766].
[RFC6763] describes the details of what each of these types of RR [RFC6763] describes the details of what each of these types of RRs
mean, with the exception of the KEY RR, which is defined in mean, with the exception of the KEY RR, which is defined in
[RFC2539]. These RFCs should be considered the definitive source for [RFC2539]. These RFCs should be considered the definitive sources
information about what to publish; the reason for summarizing this for information about what to publish; the reason for summarizing
here is to provide the reader with enough information about what will this here is to provide the reader with enough information about what
be published that the service registration process can be understood will be published that the service registration process can be
at a high level without first learning the full details of DNS-SD. understood at a high level without first learning the full details of
Also, the "Service Instance Name" is an important aspect of FCFS DNS-SD. Also, the "Service Instance Name" is an important aspect of
naming, which we describe later on in this document. 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 uses a single namespace, ".local", which is valid on Multicast DNS (mDNS) uses a single namespace that is valid on the
the local link. This convenience is not available for DNS-SD using local link called ".local". This convenience is not available for
the DNS protocol: services must exist in some specific DNS namespace DNS-SD using the DNS protocol: services must exist in some specific
that is chosen either by the network operator, or automatically. DNS namespace that 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
[RFC6763], "Discovery of Browsing and Registration Domains". [RFC6763].
Devices made for Constrained-Node Networks register in the special Devices made for CNNs register in the special-use domain name
use domain name [RFC6761] "default.service.arpa", and let the SRP ([RFC6761]) "default.service.arpa" and let the SRP registrar handle
registrar handle rewriting that to a different domain if necessary. rewriting that to a different domain if necessary.
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 issue a DNS Update that does several things at
once; this means that it's possible to do all the work of adding a once: meaning that it's possible to do all the work of adding a PTR
PTR resource record to the PTR RRset on the Service Name, and RR to the PTR RRset on the Service Name and creating or updating the
creating or updating the Service Instance Name and Host Description, Service Instance Name and Host Description in a single transaction.
in a single transaction.
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]. The [RFC2136] than regular DNS Updates as defined in [RFC2136] where the update
update process can involve many update attempts: you might first process could involve many update attempts. You might first attempt
attempt to add a name if it doesn't exist; if that fails, then in a to add a name if it doesn't exist; if that fails, then in a second
second message you might update the name if it does exist but matches message you might update the name if it does exist but matches
certain preconditions. Because the registration protocol uses a certain preconditions. Because the registration protocol described
single transaction, some of this adaptability is lost. in this document uses a single transaction, some of this adaptability
is lost.
In order to allow updates to happen in a single transaction, SRP 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, and so there is no need for the SRP requestor to put in any Updates; thus, there is no need for the SRP requestor to put in any
explicit prerequisites. explicit prerequisites.
3.2.3.1. How the DNS-SD Service Registration process differs from DNS 3.2.3.1. How the DNS-SD Service Registration Process Differs from the
Update as specified in RFC2136 DNS Update Specified in RFC 2136
DNS-SD Service Registration is based on standard RFC2136 DNS Update, DNS-SD Service Registration is based on the standard DNS Update
with some differences: specified in [RFC2136], with some differences:
* It implements FCFS name allocation, protected using SIG(0)
([RFC2931]).
* It implements first-come first-served name allocation, protected
using SIG(0) [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 some
other domain. 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 * Constrained-Node SRP requestors are allowed to send updates to the
generic domain "default.service.arpa." generic 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 When using UDP, reliable transmission must be guaranteed by
retransmitting if a DNS UDP message is not acknowledged in a retransmitting if a DNS UDP message is not acknowledged in a
reasonable interval. Section 4.2.1 of [RFC1035] provides some reasonable interval. Section 4.2.1 of [RFC1035] provides some
guidance on this topic, as does Section 1 of [RFC1536]. guidance on this topic, as does Section 1 of [RFC1536].
Section 3.1.3 of [RFC8085] also provides useful guidance that is Section 3.1.3 of [RFC8085] also provides useful guidance that is
particularly relevant to DNS. particularly relevant to DNS.
3.2.3.3. Successive Updates 3.2.3.3. Successive Updates
Service Registration Protocol does not require that every update SRP does not require that every update contain the same information.
contain the same information. When an SRP requestor needs to send When an SRP requestor needs to send more than one SRP update to the
more than one SRP update to the SRP registrar, it MUST send these SRP registrar, it MUST send these sequentially: until an earlier
sequentially: until an earlier update has been successfully update has been successfully acknowledged, the requestor MUST NOT
acknowledged, the requestor MUST NOT begin sending a subsequent begin sending a subsequent update.
update.
3.2.4. How to secure it 3.2.4. How to Secure It
DNS update as described in [RFC2136] is secured using Secret Key A DNS update, as described in [RFC2136], is secured using secret key
Transaction Signatures, [RFC8945], which uses a secret key shared transaction signatures ([RFC8945]) that uses a secret key shared
between the DNS Update requestor (which issues the update) and the between the DNS Update requestor (which issues the update) and the
server (which authenticates it). This model does not work for server (which authenticates it). This model does not work for
automatic service registration. 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. First-Come First-Served Naming 3.2.4.1. FCFS Naming
First-Come First-Serve naming provides a limited degree of security: FCFS naming provides a limited degree of security. A server that
a server that registers its service using DNS-SD Registration registers its service using the DNS-SD Registration Protocol is given
protocol is given ownership of a name for an extended period of time ownership of a name for an extended period of time based on a lease
based on a lease specific to the key used to authenticate the DNS specific to the key used to authenticate the DNS Update, which may be
Update, which may be longer than the lease associated with the longer than the lease associated with the registered records. As
registered records. As long as the registration service remembers long as the registration service remembers the name and the key used
the name and the key used to register that name, no other server can to register that name, no other server can add or update the
add or update the information associated with that. If the server information associated with that. If the server fails to renew its
fails to renew its service registration before the KEY lease service registration before the KEY lease (see Section 4 of
(Section 4 of [I-D.ietf-dnssd-update-lease]) expires, its name is no [RFC9664]) expires, its name is no longer protected. FCFS naming is
longer protected. FCFS naming is used to protect both the Service used to protect both the Service Description and the Host
Description and the Host Description. Description.
3.2.5. SRP Requestor Behavior 3.2.5. SRP Requestor 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 requestor generates a public/private key pair (see 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 requestor, the SRP requestor MUST
be pre-configured 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 that can be used. This key pair MUST be unique to the device. A
device with rewritable storage SHOULD retain this key indefinitely. device with rewritable storage SHOULD retain this key indefinitely.
When the device changes ownership, it may be appropriate for the When the device changes ownership, it may be appropriate for the
former owner to erase the old key pair, which would then require the former owner to erase the old key pair, which would then require the
new owner to install a new one. Therefore, the SRP requestor on the new owner to install a new one. Therefore, the SRP requestor on the
device SHOULD provide a mechanism to erase the key, for example as device SHOULD provide a mechanism to erase the key (for example, as
the result of a "factory reset," and to generate a new key. the result of a "factory reset") and to generate a new key.
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 requestor 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 is set
using the EDNS(0) Update Lease option [I-D.ietf-dnssd-update-lease]. using the Extension Mechanisms for DNS (EDNS(0)) Update Lease option
(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 [RFC3445]. Because the KEY RR used in TSIG is not a zone-signing
key, the flags field in the KEY RR MUST be all zeroes. 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
Both Host Description RR adds and Service Description RR adds can Adds for both Host Description RRs and Service Description RRs can
have names that result in name conflicts. Service Discovery record have names that result in name conflicts. Service Discovery record
adds cannot have name conflicts. If any Host Description or Service adds cannot have name conflicts. If any Host Description or Service
Description record is found by the SRP registrar to have a conflict Description record is found by the SRP registrar to have a conflict
with an existing name, the registrar will respond to the SRP Update with an existing name, the registrar will respond to the SRP Update
with a YXDomain RCODE (Section 2.2 of [RFC2136]). In this case, the with a YXDomain RCODE (Section 2.2 of [RFC2136]). In this case, the
requestor MUST choose a new name or give up. requestor 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 this is done. Typically,
however, the requestor will append a number to the preferred name. however, the requestor will append a number to the preferred name.
This number could be sequentially increasing, or could be chosen This number could be sequentially increasing or could be chosen
randomly. One existing implementation attempts several sequential randomly. One existing implementation attempts several sequential
numbers before choosing randomly. So for instance, it might try numbers before choosing randomly. For instance, it might try
host.default.service.arpa, then host-1.default.service.arpa, then host.default.service.arpa, then host-1.default.service.arpa, then
host-2.default.service.arpa, then host-31773.default.service.arpa. 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 The lifetime of the DNS-SD PTR, SRV, A, AAAA, and TXT records (see
[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. This means that if a device is typically set to two hours. Thus, if a device is disconnected from
disconnected from the network, it does not appear in the user the network, it does not appear in the user interfaces of devices
interfaces of devices looking for services of that type for too long. looking for services of that type for too long.
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 of this is that even though a device typically 14 days. The result being that even though a device may be
may be temporarily unplugged, disappearing from the network for a few temporarily unplugged -- disappearing from the network for a few days
days, it makes a claim on its name that lasts much longer. -- it makes a claim on its name that lasts much longer.
This means that even if a device is unplugged from the network for a Therefore, even if a device is unplugged from the network for a few
few days, and its services are not available for that time, no other 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 unplugged from the
network and permanently discarded, then its name is eventually network and permanently discarded, then its name is eventually
cleaned up and made available for re-use. 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 [RFC2782] requires that the target name in the SRV record
not be compressed, an SRP requestor MAY compress the target in the not be compressed, an SRP requestor MAY compress the target in the
SRV record. The motivation for _not_ compressing in [RFC2782] is not SRV record. The motivation for _not_ compressing in [RFC2782] is not
stated, but is assumed to be because a caching resolver that does not stated but is assumed to be because a caching resolver that does not
understand the format of the SRV record might store it as binary data understand the format of the SRV record might store it as binary data
and thus return an invalid pointer in response to a query. This does and thus return an invalid pointer in response to a query. This does
not apply in the case of SRP: an SRP registrar needs to understand 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 clients to be
able to assume that the registrar will handle this. Therefore, SRP able to assume that the registrar will handle this. Therefore, SRP
registrars MUST support compression of SRV RR targets. registrars MUST support compression of SRV RR targets.
Note that this does not update [RFC2782]: DNS servers still MUST NOT Note that this does not update [RFC2782]: DNS servers still MUST NOT
compress SRV record targets. The requirement to accept compressed compress SRV record targets. The requirement to accept compressed
SRV records in updates only applies to SRP registrars, and SRP SRV records in updates only applies to SRP registrars, and SRP
registrars that are also DNS servers still MUST NOT compress SRV registrars that are also DNS servers still MUST NOT compress SRV
record targets in DNS responses. We note also that [RFC6762] record targets in DNS responses. We note also that [RFC6762]
recomments that SRV records be compressed in mDNS messages, so recommends that SRV records be compressed in mDNS messages, so
[RFC2782] does not apply to mDNS messages. [RFC2782] does not apply to mDNS messages.
In addition, we note that an implementor of an SRP requestor might In addition, we note that an implementor of an SRP requestor 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 requestors and in DNS
servers that the code only compresses in the case where a requestor servers that the code only compresses in the case where a requestor
is generating an SRP update. 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 host, the SRP
requestor transmits an SRP update for that host with an Update Lease requestor transmits an SRP update for that host with an Update Lease
option that has a LEASE value of zero. If the registration is to be option that has a LEASE value of zero. If the registration is to be
permanently removed, KEY-LEASE SHOULD also be zero. Otherwise, it permanently removed, KEY-LEASE SHOULD also be zero. Otherwise, it
SHOULD be set to the same value it had previously; this holds the SHOULD be set to the same value it had previously; this holds the
name in reserve for when the SRP requestor is once again able to name in reserve for when the SRP requestor is once again able to
provide the service. provide the service.
SRP requestors are normally expected to remove all service instances SRP requestors are normally expected to remove all service instances
when removing a host. However, in some cases an SRP requestor may when removing a host. However, in some cases, an SRP requestor may
not have retained sufficient state to know that some service instance not have retained sufficient state to know that some service instance
is pointing to a host that it is removing. This method of removing is pointing to a host that it is removing. This method of removing
services is intended for the case where the requestor is going services is intended for the case where the requestor is going
offline and does not want its services advertised. Therefore, it is offline and does not want its services advertised. Therefore, it is
sufficient for the requestor to send the Host Description Instruction sufficient for the requestor to send the Host Description Instruction
(Section 3.3.1.3). (see Section 3.3.1.3).
To support this, when removing services based on the lease time being To support this, when removing services based on the lease time being
zero, an SRP registrar MUST remove all service instances pointing to zero, an SRP registrar MUST remove all service instances pointing to
a host when a host is removed, even if the SRP requestor doesn't list a host when a host is removed, even if the SRP requestor doesn't list
them explicitly. If the KEY lease time is nonzero, the SRP registrar them explicitly. If the KEY lease time is nonzero, the SRP registrar
MUST NOT delete the KEY records for these SRP requestors. MUST NOT delete the KEY records for these SRP requestors.
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 some specific In some use cases, a requestor may need to remove a specific service
service, without removing its other services. This can be without removing its other services. This can be accomplished in one
accomplished in one of two ways. To simply remove a specific of two ways:
service, the requestor sends a valid SRP Update where the Service
Discovery Instruction (Section 3.3.1.1) contains a single Delete an
RR from an RRset ([RFC2136], Section 2.5.4) update that deletes the
PTR record whose target is the service instance name. The Service
Description Instruction (Section 3.3.1.2) in this case contains a
single Delete all RRsets from a Name ([RFC2136], Section 2.5.3)
update to the service instance name.
The second alternative is used when some service is being replaced by 1. To simply remove a specific service, the requestor sends a valid
a different service with a different service instance name. In this SRP Update where the Service Discovery Instruction (see
case, the old service is deleted as in the first alternative. The Section 3.3.1.1) contains a single "Delete An RR From An RRset"
new service is added, just as it would be in an update that wasn't update (Section 2.5.4 of [RFC2136]) that deletes the PTR record
deleting the old service. Because both the removal of the old whose target is the service instance name. In this case, the
service and the add of the new service consist of a valid Service Service Description Instruction (see Section 3.3.1.2) contains a
Discovery Instruction and a valid Service Description Instruction, single "Delete All RRsets From A Name" update (Section 2.5.3 of
the update as a whole is a valid SRP Update, and will result in the [RFC2136]) to the service instance name.
old service being removed and the new one added, or, to put it
differently, in the old service being replaced by the new service.
It is perhaps worth noting that if a service is being updated without 2. This alternative is used when some service is being replaced by a
the service instance name changing, that will look very much like the different service with a different service instance name. In
second alternative above. The difference is that because the target this case, the old service is deleted as in the first
for the PTR record in the Service Discovery Instruction is the same alternative. The new service is added, just as it would be in an
for both the Delete An RR From An RRset update and the Add To An update that wasn't deleting the old service. Because both the
RRSet update, there is no way to tell whether they were intended to removal of the old service and the add of the new service consist
be one or two Instructions. The same would be true of the Service of a valid Service Discovery Instruction and a valid Service
Description Instruction. Description Instruction, the update as a whole is a valid SRP
Update and will result in the old service being removed and the
new one added; or, to put it differently, the update will result
in the old service being replaced by the new service.
It is perhaps worth noting that, if a service is being updated
without the service instance name changing, that situation will look
very much like the second alternative above. The difference is that
because the target for the PTR record in the Service Discovery
Instruction is the same for both the "Delete An RR From An RRset"
update and the "Add To An RRset" update (Section 2.5.1 of [RFC2136]),
there is no way to tell whether they were intended to be one or two
Instructions. The same would be true of the Service Description
Instruction.
Whichever of these two alternatives is used, the host lease will be Whichever of these two alternatives is used, the host lease will be
updated with the lease time provided in the SRP update. In neither updated with the lease time provided in the SRP update. In neither
of these cases is it permissible to delete the host. All services of these cases is it permissible to delete the host. All services
must point to a host. If a host is to be deleted, this must be done must point to a host. If a host is to be deleted, this must be done
using the method described in Section 3.2.5.5.1, which deletes the using the method described in Section 3.2.5.5.1, which deletes the
host and all services that have that host as their target. host and all services that have that host as their target.
3.3. Validation and Processing of SRP Updates 3.3. Validation and Processing of SRP Updates
skipping to change at page 16, line 14 skipping to change at line 728
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 some
combination of delete updates and add updates. When an instruction combination of delete updates and add updates. When an instruction
contains a delete and an add, the delete MUST precede the add. contains a delete and an add, the delete MUST precede the add.
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 no records that the deletes would affect; otherwise, the add will have
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 question that indicates the zone to be updated. Every delete and
update in an SRP Update MUST be within the zone that is specified for update in an SRP Update MUST be within the zone that is specified for
the SRP Update. 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 contains An instruction is a Service Discovery Instruction if it:
* exactly one "Add to an RRSet" ([RFC2136], Section 2.5.1) or * Contains exactly one "Add To An RRset" RR update (Section 2.5.1 of
exactly one "Delete an RR from an RRSet" ([RFC2136], [RFC2136]) or exactly one "Delete An RR From An RRset" RR update
Section 2.5.4) RR update, (Section 2.5.4 of [RFC2136]), which updates a PTR RR; the target
* which updates a PTR RR, of which is a Service Instance Name for which name a Service
* the target of which is a Service Instance Name Description Instruction is present in the SRP Update.
* for which name a Service Description Instruction is present in the Additionally:
SRP Update, and:
- if the RR Update is an "Add to an RRSet" instruction, that - If the RR Update is an "Add To An RRset" instruction, that
Service Description Instruction contains an "Add to an RRset" Service Description Instruction contains an "Add To An RRset"
RR update for the SRV RR describing that service and no other RR update for the SRV RR describing that service and no other
"Delete from an RRset" instructions for that Service Instance "Delete From An RRset" instructions for that Service Instance
Name; or Name.
- if the RR Update is a "Delete an RR from an RRSet" instruction, - If the RR Update is a "Delete An RR From An RRset" instruction,
that Service Description Instruction contains a "Delete from an that Service Description Instruction contains a "Delete From An
RRset" RR update and no other "Add to an RRset" instructions RRset" RR update and no other "Add To An RRset" instructions
for that Service Instance Name. for that Service Instance Name.
* and contains no other add or delete RR updates for the same name
as the PTR RR Update. * Contains no other add or delete RR updates for the same name as
the PTR RR Update.
Note that there can be more than one Service Discovery Instruction 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 name if the SRP requestor is advertising more than one
service of the same type, or is changing the target of a PTR RR. service of the same type or is changing the target of a PTR RR. This
This is also true for SRP subtypes (Section 7.1 of [RFC6763]). For is also true for SRP subtypes (Section 7.1 of [RFC6763]). For each
each such PTR RR add or delete, the above constraints must be met. 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
appropriate Service Instance Name, the following are true: appropriate Service Instance Name, 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 ([RFC2136], Section 2.5.3), the service instance name (see Section 2.5.3 of [RFC2136]).
* It contains zero or one "Add to an RRset" SRV RR,
* It contains zero or one "Add to an RRset" KEY RR that, if present, * It contains zero or one "Add To An RRset" SRV RR.
* It contains zero or one "Add To An RRset" KEY RR that, if present,
contains the public key corresponding to the private key that was contains the public key corresponding to the private key that was
used to sign the message (if present, the KEY MUST match the KEY used to sign the message (if present, the KEY MUST match the KEY
RR given in the Host Description), RR given in the Host Description).
* It contains zero or more "Add to an RRset" TXT RRs,
* If there is one "Add to an RRset" SRV update, there MUST be at * It contains zero or more "Add To An RRset" TXT RRs.
least one "Add to an RRset" TXT update.
* The target of the SRV RR Add, if present points to a hostname for * If there is one "Add To An RRset" SRV update, there MUST be at
which there is a Host Description Instruction in the SRP Update, least one "Add To An RRset" TXT update.
or
* If there is no "Add to an RRset" SRV RR, then either: * The target of the SRV RR Add, if present, points to a hostname for
- the name to which the "Delete all RRsets from a name" applies which there is a Host Description Instruction in the SRP Update;
or if there is no "Add To An RRset" SRV RR, then either:
- the name to which the "Delete All RRsets From A Name" applies
does not exist, or does not exist, or
- there is an existing KEY RR on that name, which 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 * No other resource records on the Service Instance Name are
modified. modified.
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
An instruction is a Host Description Instruction if, for the An instruction is a Host Description Instruction if, for the
appropriate hostname, it contains 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,
* exactly one "Add to an RRset" RR that adds a KEY RR that contains * 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
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
records. Host Description Instructions do not modify 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 a link-scope address or IPv4 autoconfiguration
address is provided by the SRP requestor, the SRP registrar could not address is provided by the SRP requestor, the SRP registrar could not
publish this in a DNS zone. However, in some situations, the publish this in a DNS zone. However, in some situations, the
registrar might make the records available through a mechanism such registrar might make the records available through a mechanism such
as an advertising proxy only on the specific link from which the SRP as an advertising proxy only on the specific link from which the SRP
update originated; in such a situation, locally-scoped records are update originated. In such a situation, locally scoped records are
still valid. 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 In addition, there MUST NOT be any Service Description Instruction to
which no Service Discovery Instruction points. A DNS Update that which no Service Discovery Instruction points. A DNS Update that
contains any additional adds or deletes that cannot be identified as contains any additional adds or deletes that cannot be identified as
Service Discovery, Service Description or Host Description Service Discovery, Service Description, or Host Description
Instructions is not an SRP Update. A DNS update that contains any Instructions is not an SRP Update. A DNS update that contains any
prerequisites is not an SRP Update. prerequisites is not an SRP Update.
An SRP Update MUST include an EDNS(0) Update Lease option An SRP Update MUST include an EDNS(0) Update Lease option (see
[I-D.ietf-dnssd-update-lease]. The LEASE time specified in the [RFC9664]). The LEASE time specified in the Update Lease option MUST
Update Lease option MUST be less than or equal to the KEY-LEASE time. be less than or equal to the KEY-LEASE time. A DNS update that does
A DNS update that does not include the Update Lease option, or that not include the Update Lease option, or that includes a KEY-LEASE
includes a KEY-LEASE value that is less than the LEASE value, is not value that is less than the LEASE value, is not an SRP update.
an SRP update.
When an SRP registrar receives a DNS Update that is not an SRP When an SRP registrar receives a DNS Update that is not an SRP
update, it MAY process the update as regular RFC2136 updates, update, it MAY process the update as regular updates described in
including access control checks and constraint checks, if supported. [RFC2136], including access control checks and constraint checks, if
Otherwise the SRP registrar MUST reject the DNS Update with the supported. Otherwise, the SRP registrar MUST reject the DNS Update
Refused RCODE. with the 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 Updates that look very much like SRP Updates nevertheless will
fail to validate. For example, a DNS update that contains an Add to fail to validate. For example, a DNS update that contains an "Add To
an RRset instruction for a Service Name and an Add to an RRset An RRset" instruction for a Service Name and an Add to an RRset
instruction for a Service Instance Name, where the PTR record added instruction for a Service Instance Name, where the PTR record added
to the Service Name does not reference the Service Instance Name, is to the Service Name does not reference the Service Instance Name, is
not a valid SRP Update message, but may be a valid RFC2136 update. not a valid SRP Update message but may be a valid update as described
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 a DNS Update message has been validated with these
conditions and is a valid SRP Update, the SRP registrar checks that conditions and is a valid SRP Update, the SRP registrar checks that
the name in the Host Description Instruction exists. If so, then the the name in the Host Description Instruction exists. If so, then the
registrar checks to see if the KEY record on that name is the same as registrar checks to see if the KEY record on that name is the same as
the KEY record in the Host Description Instruction. The registrar the KEY record in the Host Description Instruction. The registrar
performs the same check for the KEY records in any Service performs the same check for the KEY records in any Service
Description Instructions. For KEY records that were omitted from Description Instructions. For KEY records that were omitted from
Service Description Instructions, the KEY from the Host Description Service Description Instructions, the KEY from the Host Description
Instruction is used. If any existing KEY record corresponding to a Instruction is used. If any existing KEY record corresponding to a
KEY record in the SRP Update does not match the KEY record in the SRP KEY record in the SRP Update does not match the KEY record in the SRP
Update (whether provided or taken from the Host Description Update (whether provided or taken from the Host Description
Instruction), then the SRP registrar MUST reject the SRP Update with Instruction), then the SRP registrar MUST reject the SRP Update with
the YXDomain RCODE. the YXDomain RCODE.
Otherwise, the SRP registrar validates the SRP Update using SIG(0) Otherwise, the SRP registrar validates the SRP Update using SIG(0)
against the public key in the KEY record of the Host Description against the public key in the KEY record of the Host Description
Instruction. If the validation fails, the registrar MUST reject the Instruction. If the validation fails, the registrar MUST reject the
SRP Update with the Refused RCODE. Otherwise, the SRP Update is SRP Update with the Refused RCODE. Otherwise, the SRP Update is
considered valid and authentic, and is processed according to the considered valid and authentic and is processed according to the
method described in RFC2136. method described in [RFC2136].
KEY record updates omitted from Service Description Instruction are KEY record updates omitted from Service Description Instruction are
processed as if they had been explicitly present: every Service processed as if they had been explicitly present. After the SRP
Description that is updated MUST, after the SRP Update has been Update has been applied, every Service Description that is updated
applied, have a KEY RR, and it must be the same KEY RR that is MUST have a KEY RR: and it must be the same KEY RR that is present in
present in the Host Description to which the Service Description the Host Description to which the Service Description refers.
refers.
[RFC3445] states that the flags field in the KEY RR MUST be zero [RFC3445] states that the flags field in the KEY RR MUST be zero
except for bit 7, which can be one in the case of a zone key. except for bit 7, which can be one in the case of a zone key.
However, the SRP registrar MUST NOT validate the flags field. However, the SRP registrar MUST NOT validate the flags field.
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
skipping to change at page 20, line 5 skipping to change at line 914
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 Similarly, there is no mechanism for deleting subtypes. A delete of
a service deletes all of its subtypes. To delete an individual a service deletes all of its subtypes. To delete an individual
subtype, an SRP Update must be constructed that contains the service subtype, an SRP Update must be constructed that contains the service
type and all subtypes for that service except for the one to be type and all subtypes for that service except for the one to be
deleted. 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: all update and can be either NoError, ServFail, Refused, or YXDomain.
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
[RFC2136]. [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
[RFC2136] specifies that the server is permitted to respond either [RFC2136] specifies that the server is permitted to respond either
with no RRs or to copy the RRs sent by the client into the response. with no RRs or to copy the RRs sent by the client into the response.
The SRP Requestor MUST NOT attempt to validate any RRs that are The SRP requestor MUST NOT attempt to validate any RRs that are
included in the response. It is possible that a future SRP extension included in the response. It is possible that a future SRP extension
may include per-RR indications as to why the update failed, but at may include per-RR indications as to why the update failed, but at
present this is not specified, so if a client were to attempt to the time of writing this is not specified. So, if a client were to
validate the RRs in the response, it might reject such a response, attempt to validate the RRs in the response, it might reject such a
since it would contain RRs, but probably not a set of RRs identical response since it would contain RRs but probably not a set of RRs
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 (Section 3.5 of The SRP registrar MAY add a Reverse Mapping (see Section 3.5 of
[RFC1035], Section 2.5 of [RFC3596]) that corresponds to the Host [RFC1035] and Section 2.5 of [RFC3596]) that corresponds to the Host
Description. This is not required because the Reverse Mapping serves Description. This is not required because the reverse mapping serves
no protocol function, but it may be useful for debugging, e.g. in no protocol function, but it may be useful for debugging, e.g., in
annotating network packet traces or logs. In order for the registrar annotating network packet traces or logs. 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 requestor 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 pre-registered registration of keys and only accept updates from preregistered keys.
keys. In this case, an update for a key that has not been registered In this case, an update for a key that has not been registered SHOULD
SHOULD be rejected with the Refused RCODE. be rejected with the Refused RCODE.
There are at least two benefits to doing this rather than simply There are at least two benefits to doing this rather than simply
using normal SIG(0) DNS updates. First, the same registration using normal SIG(0) DNS updates:
protocol can be used in both cases, so both use cases can be
addressed by the same SRP requestor implementation. Second, the 1. The same registration protocol can be used in both cases, so both
registration protocol includes maintenance functionality not present use cases can be addressed by the same SRP requestor
with normal DNS updates. implementation.
2. The registration protocol includes maintenance functionality not
present with normal DNS updates.
Note that the semantics of using SRP in this way are different than Note that the semantics of using SRP in this way are different than
for typical RFC2136 implementations: the KEY used to sign the SRP for typical implementations described in [RFC2136]. The KEY used to
Update only allows the SRP requestor to update records that refer to sign the SRP Update only allows the SRP requestor to update records
its Host Description. RFC2136 implementations do not normally that refer to its Host Description. Implementations specific to
provide a way to enforce a constraint of this type. [RFC2136] do not normally provide a way to enforce a constraint of
this type.
The SRP registrar could also have a dictionary of names or name 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 All RRs within an RRset are required to have the same TTL (see
(Clarifications to the DNS Specification [RFC2181], Section 5.2). In Section 5.2 of [RFC2181]). In order to avoid inconsistencies, SRP
order to avoid inconsistencies, SRP places restrictions on TTLs sent places restrictions on TTLs sent by requestors and requires that SRP
by requestors and requires that SRP registrars enforce consistency. registrars enforce consistency.
Requestors sending SRP Updates MUST use consistent TTLs in all RRs Requestors sending SRP Updates MUST use consistent TTLs in all RRs
within the SRP Update. within the 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 the SRP
Update are the same. If they are not, the SRP update MUST be Update are the same. If they are not, the SRP update MUST be
rejected with a Refused RCODE. 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 requestor'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 server, and also increases network load, which may
be an issue for constrained networks. Longer TTLs will increase the be an issue for constrained networks. Longer TTLs will increase the
likelihood that data in caches will be stale. TTL minimums and likelihood that data in caches will be stale. TTL minimums and
maximums SHOULD be configurable by the operator of the SRP registrar. maximums SHOULD be configurable by the operator of the SRP registrar.
5. Maintenance 5. Maintenance
5.1. Cleaning up stale data
Because the DNS-SD registration protocol is automatic, and not 5.1. Cleaning Up Stale Data
managed by humans, some additional bookkeeping is required. When an
update is constructed by the SRP requestor, it MUST include an
EDNS(0) Update Lease Option [I-D.ietf-dnssd-update-lease]. The
Update Lease Option contains two lease times: the Lease Time and the
KEY Lease Time.
These leases are promises, similar to DHCP leases [RFC2131], from the Because the DNS-SD registration protocol is automatic and not managed
SRP requestor that it will send a new update for the service by humans, some additional bookkeeping is required. When an update
is constructed by the SRP requestor, it MUST include an EDNS(0)
Update Lease Option (see [RFC9664]). The Update Lease Option
contains two lease times: the Lease Time and the KEY Lease Time.
Similar to DHCP leases (see [RFC2131]), these leases are promises
from the SRP requestor that it will send a new update for the service
registration before the lease time expires. The Lease time is chosen 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 time after the update during which the registered
records other than the KEY record can be assumed to be valid. The records other than the KEY record can be assumed to be valid. The
KEY lease time represents the time after the update during which the KEY lease time represents the time after the update during which the
KEY record can be assumed to be valid. KEY record can be assumed to be valid.
The reasoning behind the different lease times is discussed in the The reasoning behind the different lease times is discussed in
section on FCFS naming (Section 3.2.4.1). SRP registrars may be Section 3.2.4.1. SRP registrars may be configured with limits for
configured with limits for these values. A default limit of two these values. At the time of writing, a default limit of two hours
hours for the Lease and 14 days for the SIG(0) KEY are currently for the Lease and 14 days for the SIG(0) KEY are thought to be good
thought to be good choices. Constrained devices with limited battery choices. Constrained devices with limited battery that wake
that wake infrequently are likely to request longer leases; infrequently are likely to request longer leases; registrars that
registrars that support such devices may need to set higher limits. support such devices may need to set higher limits. SRP requestors
SRP requestors that are going to continue to use names on which they that are going to continue to use names on which they hold leases
hold leases SHOULD update well before the lease ends, in case the SHOULD update well before the lease ends in case the registrar is
registrar is unavailable or under heavy load. unavailable or under heavy load.
The lease time applies specifically to the host. All service The lease time applies specifically to the host. 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 host. When the lease on a host expires, the host and all
services that reference it MUST be removed at the same time—it is services that reference it MUST be removed at the same time: it is
never valid for a service instance to remain when the host it never valid for a service instance to remain when the host it
references has been removed. If the KEY record for the host is to references has been removed. If the KEY record for the host is to
remain, the KEY record for any services that reference it MUST also remain, the KEY record for any services that reference it MUST also
remain. However, the service PTR record MUST be removed, since it remain. However, the service PTR record MUST be removed since it has
has no key associated with it, and since it is never valid to have a no key associated with it and since it is never valid to have a
service PTR record for which there is no service instance on the service PTR record for which there is no service instance on the
target of the PTR record. 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 for doing this is that a requestor may re-register a host The reason being that a requestor may re-register a host with a
with a different set of services, and not remember that some different set of services and not remember that some different
different service instance had previously been registered. In this service instance had previously been registered. In this case, when
case, when that service instance lease expires, the SRP registrar that service instance lease expires, the SRP registrar MUST remove
MUST remove the service instance (although the KEY record for the the service instance (although the KEY record for the service
service instance SHOULD be retained until the KEY lease on that instance SHOULD be retained until the KEY lease on that service
service expires). This is beneficial because otherwise if the SRP expires). This is beneficial because, otherwise, if the SRP
requestor continues to renew the host, but never mentions the stale requestor continues to renew the host but never mentions the stale
service again, the stale service will continue to be advertised. service again, the stale service will continue to be advertised.
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 if the lease time proposed by the requestor has been
shortened or lengthened by the registrar. The requestor MUST check shortened or lengthened by the registrar. The requestor MUST check
for the EDNS(0) Update Lease option in the response and MUST use the for the EDNS(0) Update Lease option in the response and MUST use the
lease times from that option in place of the options that it sent to lease times from that option in place of the options that it sent to
the registrar when deciding when to renew its registration. The the registrar when deciding when to renew its registration. The
times may be shorter or longer than those specified in the SRP times may be shorter or longer than those specified in the SRP
Update; the SRP requestor must honor them in either case. Update: the SRP requestor must honor them in either case.
SRP requestors SHOULD assume that each lease ends N seconds after the SRP requestors 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 lease duration). SRP
Registrars SHOULD assume that each lease ends N seconds after the 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 requestor sent
it, this avoids the possibility of misunderstandings. it, this avoids the possibility of misunderstandings.
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 SHOULD differentiate between SRP Updates and other updates, and but they SHOULD differentiate between SRP Updates and other updates
MUST reject updates that would otherwise be SRP Updates if they do and MUST reject updates that would otherwise be SRP Updates if they
not include leases. do not include leases.
Lease times have a completely different function than TTLs. On an Lease times have a completely different function than TTLs. On an
authoritative DNS server, the TTL on a resource record is a constant: authoritative DNS server, the TTL on a resource record is a constant.
whenever that RR is served in a DNS response, the TTL value sent in Whenever that RR is served in a DNS response, the TTL value sent in
the answer is the same. The lease time is never sent as a TTL; its the answer is the same. The lease time is never sent as a TTL; its
sole purpose is to determine when the authoritative DNS server will sole purpose is to determine when the authoritative DNS server will
delete stale records. It is not an error to send a DNS response with delete stale records. It is not an error to send a DNS response with
a TTL of 'n' when the remaining time on the lease is less than 'n'. a TTL of 'n' when the remaining time on the lease is less than 'n'.
6. Security Considerations 6. Security Considerations
6.1. Source Validation 6.1. Source Validation
SRP Updates have no authorization semantics other than FCFS. This SRP Updates have no authorization semantics other than FCFS. Thus,
means that if an attacker from outside of the administrative domain if an attacker from outside the administrative domain of the SRP
of the SRP registrar knows the registrar's IP address, it can in registrar knows the registrar's IP address, it can, in principle,
principle send updates to the registrar that will be processed send updates to the registrar that will be processed successfully.
successfully. SRP Registrars SHOULD therefore be configured to Therefore, SRP registrars SHOULD be configured to reject updates from
reject updates from source addresses outside of the administrative source addresses outside of the administrative domain of the
domain of the registrar. 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-network attacker. In order to ensure that
this handshake happens, SRP registrars relying on three-way-handshake this handshake happens, SRP registrars relying on three-way-handshake
validation MUST NOT accept TCP Fast Open [RFC7413] payloads. If the validation MUST NOT accept TCP Fast Open payloads (see [RFC7413]).
network infrastructure allows it, an SRP registrar MAY accept TCP If the network infrastructure allows it, an SRP registrar MAY accept
Fast Open payloads if all such packets are validated along the path, TCP Fast Open payloads if all such packets are validated along the
and the network is able to reject this type of spoofing at all path, 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 constrained devices, spoofing would have to be
prevented with appropriate source address filtration on routers prevented with appropriate source address filtration on routers
[RFC2827]. This would ordinarily be accomplished by measures such as ([RFC2827]). This would ordinarily be accomplished by measures such
are described in Section 4.5 of [RFC7084]. For example, a stub as those described in (Section 4.5 of [RFC7084]). For example, a
router [I-D.ietf-snac-simple] for a constrained network might only stub router ([SNAC-SIMPLE]) for a constrained network might only
accept UDP updates from source addresses known to be on-link on that accept UDP updates from source addresses known to be on-link on that
stub network, and might further validate that the UDP update was stub network and might further validate that the UDP update was
actually received on the stub network interface and not the interface actually 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. A
server that accepts updates from SRP requestors may also accept other server that accepts updates from SRP requestors may also accept other
DNS updates, and those DNS updates may be validated using different DNS updates, and those DNS updates may be validated using different
rules. However, in the case of a DNS server that accepts SRP rules. However, in the case of a DNS server that accepts SRP
updates, the intersection of the SRP Update rules and whatever other updates, the intersection of the SRP Update rules and whatever other
update rules are present must be considered very carefully. 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 that is authenticated using a different key,
could be used to override a promise made by the SRP registrar to an could be used to override a promise made by the SRP registrar to an
SRP requestor, by replacing all or part of the service registration SRP requestor 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 requestor. An implementation that allows both kinds of updates
SHOULD NOT allow DNS Update requestors that are using different SHOULD NOT allow DNS Update requestors 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 requestors.
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 used for non-
DNSSD services. For example, imagine that you set up SRP service for DNSSD services. For example, imagine that you set up SRP service for
example.com. SRP hosts can now register names like "www" or "mail" example.com. SRP hosts can now register names like "www" or "mail"
or "smtp" in this domain. In addition, SRP updates using FCFS naming or "smtp" in this domain. In addition, SRP updates using FCFS naming
can insert names that are obscene or offensive into the zone. There can insert names that are obscene or offensive into the zone. There
is no simple solution to these problems. We have two recommendations is no simple solution to these problems. However, we have two
to address this problem, however: 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 service discovery. subdomains of the organizational domain for DNS-SD. This does not
This does not prevent registering names as mentioned above, but prevent registering names as mentioned above but does ensure that
does ensure that genuinely important names are not accidentally genuinely important names are not accidentally reserved for SRP
reserved for SRP clients. So for example, the zone clients. So, for example, the zone "dnssd.example.com" could be
"dnssd.example.com" could be used instead of "example.com" for SRP used instead of "example.com" for SRP updates. Because of the way
updates. Because of the way that DNS browsing domains are that DNS-browsing domains are discovered, there is no need for the
discovered, there is no need for the DNSSD discovery zone that is DNSSD discovery zone that is updated by SRP to have a user-
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.
6.4. Security of local service discovery 6.4. Security of Local Service Discovery
Local links can be protected by managed services such as RA Guard Local links can be protected by managed services such as Router
[RFC6105], but multicast services like DHCP [RFC2131], DHCPv6 Advertisement Guard (see [RFC6105]), but multicast services like
[RFC8415] and IPv6 Neighbor Discovery [RFC4861] are in most cases not DHCP, DHCPv6, and IPv6 Neighbor Discovery (see [RFC2131], [RFC8415],
authenticated and can't be controlled on unmanaged networks, such as and [RFC4861], respectively) are, in most cases, not authenticated
home networks and small-office networks where no network management and can't be controlled on unmanaged networks, such as home networks
staff are present. In such situations, the SRP service has and small office networks where no network management staff are
comparatively fewer potential security exposures and hence is not the present. In such situations, the SRP service has comparatively fewer
weak link. This is discussed in more detail in Section 3.2.4. potential security exposures and, hence, is not the weak link. This
is discussed in more detail in Section 3.2.4.
The fundamental protection for networks of this type is the user's 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., [RFC8520]
provides a means for constraining what behaviors are allowed for a provides a means for constraining what behaviors are allowed for a
device in an automatic way), but such work is out of scope for this device in an automatic way), but such work is out of scope for this
document. 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 requestors. In principle, a KEY
RR could be used by a non-constrained SRP requestor to validate RR could be used by a non-constrained SRP requestor to validate
responses from the registrar, but this is not required, nor do we responses from the registrar, but this is not required, nor do we
specify a mechanism for determining which key to use. specify a 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 [RFC7858], Section 4.2 could be used for authentication described in Section 4.2 of [RFC7858] could be used for
of the SRP registrar, e.g. to prevent man-in-the-middle attacks. authentication of the SRP registrar, e.g., to prevent man-in-the-
However the use of such keys is impractical for an unmanaged service middle attacks. However, the use of such keys is impractical for an
registration protocol, and hence is out of scope for this document. unmanaged service registration protocol; hence, it 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
specified in [RFC8624], Section 3.1, in the validation column of the that are specified in Section 3.1 of [RFC8624], in the validation
table, that are numbered 13 or higher and have a "MUST", column of the table, that are numbered 13 or higher, and that have a
"RECOMMENDED", or "MAY" designation in the validation column of the "MUST", "RECOMMENDED", or "MAY" designation in the validation column
table. SRP requestors MUST NOT assume that any algorithm numbered of the table. SRP requestors MUST NOT assume that any algorithm
lower than 13 is available for use in validating SIG(0) signatures. numbered lower than 13 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 multicast DNS responses. implications of SRP are different than for mDNS responses. Host
Host implementations that are using TCP SHOULD also use TLS if implementations that are using TCP SHOULD also use TLS if available.
available. SRP Registrar implementations MUST offer TLS support. SRP registrar implementations MUST offer TLS support. The use of TLS
The use of TLS with DNS is described in [RFC7858]. Because there is with DNS is described in [RFC7858]. Because there is no mechanism
no mechanism for sharing keys, validation of DNS-over-TLS keys is not for sharing keys, validation of DNS-over-TLS keys is not possible;
possible; DNS-over-TLS is used only as described in [RFC7858], DNS-over-TLS is used only as described in Section 4.1 of [RFC7858].
Section 4.1
Hosts that implement TLS support SHOULD NOT fall back to TCP; since Hosts that implement TLS support SHOULD NOT fall back to TCP. Since
SRP registrars are required to support TLS, it is entirely up to the SRP registrars are required to support TLS, it is entirely up to the
host implementation whether to use it. host implementation whether to use it.
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 nonexistance 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 server MAY instead return
a negative non-error response (either NXDOMAIN or no data). a negative non-error 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 [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. Users are not expected to
see this name in user interfaces, although it's certainly possible see this name in user interfaces, although it's certainly possible
that they might. If they do, they are not expected to treat it that they might. If they do, they are not expected to treat it
specially. 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 can because it is a locally served domain name. So, no such subdomain
be considered as uniquely identifying a particular host, as would be can be considered to be uniquely identifying a particular host, as
required for such a PKI cert to be issued. If a subdomain of would be required for such a PKI certificate to be issued. If a
'service.arpa.' is returned by an API or entered in an input field of subdomain of "service.arpa." is returned by an API or entered in an
an application, PKI authentication of the endpoint being identified input field of an application, PKI authentication of the endpoint
by the name will not be possible. Alternative methods and practices being identified by the name will not be possible. Alternative
for authenticating such endpoints are out of scope for this document. methods and practices for authenticating such endpoints are out of
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 configured recursive resolvers.
One or more IP addresses for recursive DNS servers will usually be One or more IP addresses for recursive DNS servers will usually be
supplied to the client through router advertisements or DHCP. For an supplied to the client through router advertisements or DHCP. For an
administrative domain that uses subdomains of 'service.arpa.', the administrative domain that uses subdomains of "service.arpa.", the
recursive resolvers provided by that domain will be able to answer recursive resolvers provided by that domain will be able to answer
queries for subdomains of 'service.arpa.'; other (non-local) 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 be unable 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, it is permissible that hosts use the resolvers
that are locally provided for resolving 'service.arpa.', even when that are locally provided for resolving "service.arpa.", even when
they are configured to use other resolvers. they are configured to use other resolvers.
8.4. Caching DNS Servers 8.4. Caching DNS Servers
There are three considerations for caching DNS servers that follow There are three considerations for caching DNS servers that follow
this specification: 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 [RFC4035]). DNSSEC validation
is a Best Current Practice [RFC9364]: although validation is not is a Best Current Practice ([RFC9364]): although validation is
required, a caching recursive resolver that does not validate not required, a caching recursive resolver that does not validate
answers that can be validated may cache invalid data. This, in answers that can be validated may cache invalid data. In turn,
turn, would prevent validating stub resolvers from successfully this would prevent validating stub resolvers from successfully
validating answers. Hence, as a practical matter, recursive validating answers. Hence, as a practical matter, recursive
resolvers at sites using 'service.arpa' should do DNSSEC resolvers at sites using "service.arpa" should do DNSSEC
validation. 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 as described in Locally Served Zones (Section 3 of
[RFC6303]. That is, queries for 'service.arpa.' and subdomains [RFC6303]). That is, queries for "service.arpa." and subdomains
of 'service.arpa.' MUST NOT be forwarded, with one important of "service.arpa." MUST NOT be forwarded, with one important
exception: a query for a DS record with the DO bit set MUST exception: a query for a DS record with the DO bit set MUST
return the correct answer for that question, including correct return the correct answer for that question, including correct
information in the authority section that proves that the record information in the authority section that proves that the record
is nonexistent. 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 as necessary to provide accurate authority information, a query
for the DS record MUST result in forwarding whatever queries are for the DS record MUST result in forwarding whatever queries are
necessary; typically, this will just be a query for the DS necessary. Typically, this will just be a query for the DS
record, since the necessary authority information will be record since the necessary authority information will be included
included in the authority section of the response if the DO bit in the authority section of the response if the DO bit is set.
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
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, and so the server would such delegation in the "service.arpa." zone; therefore, the server
refuse to answer authoritatively for such a zone. A server that would refuse to answer authoritatively for such a zone. A server
implements this sort of check MUST be configurable so that either it that implements this sort of check MUST be configurable so that
does not do this check for the 'service.arpa.' domain or it ignores either it does not do this check for the "service.arpa." domain or it
the results of the check. 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 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 authoritative for "service.arpa." in the global DNS will configure
any such servers as described in Section 9. any such 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 according to the rules established in [RFC3172]. There are no Board (IAB) according to the rules established in [RFC3172]. There
other DNS registrars for '.arpa'. are no other DNS registrars for ".arpa".
9. Delegation of 'service.arpa.' 9. Delegation of "service.arpa."
In order to be fully functional, the owner of the 'arpa.' zone must In order to be fully functional, the owner of the "arpa." zone must
add a delegation of 'service.arpa.' in the '.arpa.' zone [RFC3172]. add a delegation of "service.arpa." in the ".arpa." zone (see
This delegation is to be set up as was done for 'home.arpa', as a [RFC3172]). This delegation is to be set up as was done for
result of the specification in Section 7 of [RFC8375]. This is "home.arpa", as a result of the specification in Section 7 of
currently the responsibility of the IAB [IAB-ARPA] [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 is requested to record the domain name 'service.arpa.' in the IANA has recorded the domain name "service.arpa." in the "Special-Use
Special-Use Domain Names registry [SUDN]. IANA is requested, with Domain Names" registry (see [SUDN]). IANA has implemented the
the approval of IAB, to implement the delegation requested in delegation requested in Section 9.
Section 9.
IANA is further requested to add a new entry to the "Transport- IANA has also added a new entry to the "Transport-Independent
Independent Locally-Served Zones" subregistry of the "Locally-Served Locally-Served Zones Registry" registry of the "Locally-Served DNS
DNS Zones" registry [LSDZ]. The entry will be for the domain Zones" group (see [LSDZ]). The entry is for the domain
'service.arpa.' with the description "DNS-SD Service Registration "SERVICE.ARPA" with the description "DNS-SD Service Registration
Protocol Special-Use Domain", listing this document as the reference. Protocol Special-Use Domain" and lists this document as the
reference.
10.2. Subdomains of 'service.arpa.' 10.2. 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. The IANA is requested to create a registry, DNS-SD or related work. IANA has created the "service.arpa
the "service.arpa Subdomain" registry. The IETF shall have change Subdomain" registry (see [SUB]). The IETF has change control for
control for this registry. New entries may be added either as a this registry. New entries may be added either as a result of
result of Standards Action Section 4.9 of [RFC8126] or with IESG Standards Action (Section 4.9 of [RFC8126]) or with IESG Approval
approval Section 4.10 of [RFC8126], provided that a specification (Section 4.10 of [RFC8126]), provided that a specification exists
exists Section 4.6 of [RFC8126]. (Section 4.6 of [RFC8126]).
The IANA shall group the "service.arpa Subdomain" registry with the IANA has grouped the "service.arpa Subdomain" registry with the
"Locally-Served DNS Zones" registry. The registry shall be a table "Locally-Served DNS Zones" group. The registry is a table with three
with three columns: the subdomain name (expressed as a fully- columns: the subdomain name (expressed as a fully qualified domain
qualified domain name), a brief description of how it is used, and a name), a brief description of how it is used, and a reference to the
reference to the document that describes its use in detail. document that describes its use in detail.
This registry shall begin as the following table: This initial contents of this registry are as follows:
+=======================+=================+===========+ +=======================+=================+===========+
| Subdomain Name | Description | reference | | Subdomain Name | Description | Reference |
+=======================+=================+===========+ +=======================+=================+===========+
| default.service.arpa. | Default domain | [THIS | | default.service.arpa. | Default domain | RFC 9665 |
| | for SRP updates | DOCUMENT] | | | for SRP updates | |
+-----------------------+-----------------+-----------+ +-----------------------+-----------------+-----------+
Table 1 Table 1
10.3. Service Name registrations 10.3. Service Name Registrations
IANA is requested to add two new entries to the Service Names and IANA has added two new entries to the "Service Name and Transport
Port Numbers registry. The following sections contain tables with Protocol Port Number Registry" (see [PORT]). The following
the fields required by Section 8.1.1 of [RFC6335]. subsections contain tables with the fields required by Section 8.1.1
of [RFC6335].
10.4. 'dnssd-srp' Service Name 10.3.1. '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> |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Contact | IETF Chair <chair@ietf.org> | | Contact | IETF Chair <chair@ietf.org> |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Description | DNS-SD Service Registration | | Description | DNS-SD Service Discovery |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Reference | this document | | Reference | RFC 9665 |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Port Number | None | | Port Number | None |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
| Service Code | None | | Service Code | None |
+--------------------+-----------------------------+ +--------------------+-----------------------------+
Table 2 Table 2
10.5. 'dnssd-srp-tls' Service Name 10.3.2. '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 | | Assignee | IESG <iesg@ietf.org> |
+--------------------+-----------------------------------+ +--------------------+--------------------------------+
| Contact | IETF Chair | | Contact | IETF Chair<chair@ietf.org> |
+--------------------+-----------------------------------+ +--------------------+--------------------------------+
| Description | DNS-SD Service Registration (TLS) | | Description | DNS-SD Service Discovery (TLS) |
+--------------------+-----------------------------------+ +--------------------+--------------------------------+
| Reference | this document | | Reference | RFC 9665 |
+--------------------+-----------------------------------+ +--------------------+--------------------------------+
| Port Number | None | | Port Number | None |
+--------------------+-----------------------------------+ +--------------------+--------------------------------+
| Service Code | None | | Service Code | None |
+--------------------+-----------------------------------+ +--------------------+--------------------------------+
Table 3 Table 3
10.6. Anycast Address 10.4. Anycast Address
IANA is requested to allocate an IPv6 Anycast address from the IPv6 IANA has allocated an IPv6 Anycast address from the "IANA IPv6
Special-Purpose Address Registry, similar to the Port Control Special-Purpose Address Registry" (see [IPv6]), similar to the Port
Protocol anycast address, 2001:1::1. The value TBD is to be replaced Control Protocol anycast address: 2001:1::1. The purpose of this
with the actual allocation in the table that follows. The purpose of allocation is to provide a fixed anycast address that can be commonly
this allocation is to provide a fixed anycast address that can be used as a destination for SRP updates when no SRP registrar is
commonly used as a destination for SRP updates when no SRP registrar explicitly configured. The initial values for the registry are as
is explicitly configured. The values for the registry are: follows:
+----------------------+-----------------------------+ +======================+=============================+
| Attribute | value | | Attribute | Value |
+----------------------+-----------------------------+ +======================+=============================+
| Address Block | 2001:1::TBD/128 | | Address Block | 2001:1::3/128 |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Name | DNS-SD Service Registration | | Name | DNS-SD Service Registration |
| | Protocol Anycast Address | | | Protocol Anycast Address |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| RFC | [this document] | | RFC | RFC 9665 |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Allocation Date | [date of allocation] | | Allocation Date | 2024-04 |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Termination Date | N/A | | Termination Date | N/A |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Source | True | | Source | True |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Destination | True | | Destination | True |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Forwardable | True | | Forwardable | True |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Global | True | | Globally Reachable | True |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
| Reserved-by-protocol | False | | Reserved-by-Protocol | False |
+----------------------+-----------------------------+ +----------------------+-----------------------------+
Table 4 Table 4
11. Implementation Status 11. References
[Note to the RFC Editor: please remove this section prior to
publication.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 7942.
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
There are two known independent implementations of SRP requestors:
* SRP Client for OpenThread:
https://github.com/openthread/openthread/pull/6038
* mDNSResponder open source project: https://github.com/Abhayakara/
mdnsresponder
There are two related implementations of an SRP registrar. One acts
as a DNS Update proxy, taking an SRP Update and applying it to the
specified DNS zone using DNS update. The other acts as an
Advertising Proxy [AP]. Both are included in the mDNSResponder open
source project mentioned above.
12. Acknowledgments
Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
Dong and Abtin Keshavarzian for their thorough technical reviews.
Thanks to Kangping and Abtin as well for testing the document by
doing an independent implementation. Thanks to Tamara Kemper for
doing a nice developmental edit, Tim Wattenberg for doing an SRP
requestor proof-of-concept implementation at the Montreal Hackathon
at IETF 102, and Tom Pusateri for reviewing during the hackathon and
afterwards. Thanks to Esko for a really thorough second last call
review. Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
Dong, Martin Turon, and Michael Cowan for their detailed second last
call reviews. Thanks to Patrik Fältström, Dhruv Dhody, David Dong,
Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective
directorate reviews. Thanks to Paul Wouters for a _really_ detailed
IESG review! Thanks also to the other IESG members who provided
comments or simply took the time to review the document.
13. Normative References
[I-D.ietf-dnssd-update-lease] 11.1. Normative References
Cheshire, S. and T. Lemon, "An EDNS(0) option to negotiate
Leases on DNS Updates", Work in Progress, Internet-Draft,
draft-ietf-dnssd-update-lease-08, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
update-lease-08>.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>. November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. [RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993, Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
<https://www.rfc-editor.org/info/rfc1536>. <https://www.rfc-editor.org/info/rfc1536>.
skipping to change at page 36, line 13 skipping to change at line 1602
<https://www.rfc-editor.org/info/rfc8624>. <https://www.rfc-editor.org/info/rfc8624>.
[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>.
14. Informative References [RFC9664] Cheshire, S. and T. Lemon, "An EDNS(0) Option to Negotiate
Leases on DNS Updates", RFC 9664, DOI 10.17487/RFC9664,
October 2024, <https://www.rfc-editor.org/info/rfc9664>.
11.2. Informative References
[IAB-ARPA] "Internet Architecture Board statement on the registration
of special use names in the ARPA domain", March 2017,
<https://www.iab.org/documents/correspondence-reports-
documents/2017-2/iab-statement-on-the-registration-of-
special-use-names-in-the-arpa-domain/>.
[IPv6] IANA, "IANA IPv6 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv6-special-
registry>.
[LSDZ] IANA, "Locally-Served DNS Zones",
<https://www.iana.org/assignments/locally-served-dns-
zones>.
[PORT] IANA, "Service Name and Transport Protocol Port Number
Registry", <https://www.iana.org/assignments/service-
names-port-numbers>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", [RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997, RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>. <https://www.rfc-editor.org/info/rfc2131>.
[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>.
skipping to change at page 38, line 5 skipping to change at line 1709
Gudmundsson, O., and B. Wellington, "Secret Key Gudmundsson, O., and B. Wellington, "Secret Key
Transaction Authentication for DNS (TSIG)", STD 93, Transaction Authentication for DNS (TSIG)", STD 93,
RFC 8945, DOI 10.17487/RFC8945, November 2020, RFC 8945, DOI 10.17487/RFC8945, November 2020,
<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>.
[AP] Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD [SNAC-SIMPLE]
Service Registration Protocol", Work in Progress,
Internet-Draft, draft-ietf-dnssd-advertising-proxy-03, 28
July 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-dnssd-advertising-proxy-03>.
[I-D.ietf-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-03, 30 January Internet-Draft, draft-ietf-snac-simple-05, 8 July 2024,
2024, <https://datatracker.ietf.org/doc/html/draft-ietf- <https://datatracker.ietf.org/doc/html/draft-ietf-snac-
snac-simple-03>. simple-05>.
[SUDN] "Special-Use Domain Names Registry", July 2012,
<https://www.iana.org/assignments/special-use-domain-
names/special-use-domain-names.xhtml>.
[LSDZ] "Locally-Served DNS Zones Registry", July 2011, [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.xhtml>. zones/locally-served-dns-zones>.
[IAB-ARPA] "Internet Architecture Board statement on the registration [SUDN] IANA, "Special-Use Domain Names",
of special use names in the ARPA domain", March 2017, <https://www.iana.org/assignments/special-use-domain-
<https://www.iab.org/documents/correspondence-reports- names>.
documents/2017-2/iab-statement-on-the-registration-of-
special-use-names-in-the-arpa-domain/>.
[ZC] Cheshire, S. and D.H. Steinberg, "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 0-596-10100-7, December 2005. ISBN 9780596101008, December 2005.
Appendix A. Testing using standard RFC2136-compliant DNS servers Appendix A. Testing Using Standard DNS Servers Compliant with RFC 2136
It may be useful to set up an authoritative DNS server for testing It may be useful to set up an authoritative DNS server for testing
that does not implement SRP. This can be done by configuring the that does not implement SRP. This can be done by configuring the
server to listen on the anycast address, or advertising it in the server to listen on the anycast address or by advertising it in the
_dnssd-srp._tcp.<zone> SRV and _dnssd-srp-tls._tcp.<zone> record. It _dnssd-srp._tcp.<zone> SRV and _dnssd-srp-tls._tcp.<zone> record. It
must be configured to be authoritative for "default.service.arpa", must be configured to be authoritative for "default.service.arpa" and
and to accept updates from hosts on local networks for names under to accept updates from hosts on local networks for names under
"default.service.arpa" without authentication, since such servers "default.service.arpa" without authentication since such servers will
will not have support for FCFS authentication (Section 3.2.4.1). 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 requestors that send
SRP updates. However, no prerequisites will be applied, and this SRP updates. However, no prerequisites will be applied; this means
means that the test server will accept internally inconsistent SRP that the test server will accept internally inconsistent SRP Updates
Updates, and will not stop two SRP Updates, sent by different and will not stop two SRP Updates sent by different services that
services, that claim the same name(s), from overwriting each other. 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 requestor can be done by manually installing
the requestor's public key on the DNS server that will be receiving the requestor's public key on the DNS server that will be receiving
the updates. The key can then be used to authenticate the SRP the updates. The key can then be used to authenticate the SRP update
update, and can be used as a requirement for the update. An example and can be used as a requirement for the update. An example
configuration for testing SRP using BIND 9 is given in Appendix C. configuration for testing SRP using BIND 9 is given in Appendix C.
Appendix B. How to allow SRP requestors to update standard Appendix B. How to Allow SRP Requestors to Update Standard Servers
RFC2136-compliant servers Compliant with RFC 2136
Ordinarily SRP Updates will fail when sent to an RFC 2136-compliant Ordinarily, SRP Updates will fail when sent to a server compliant
server that does not implement SRP because the zone being updated is with [RFC2136] that does not implement SRP because the zone being
"default.service.arpa", and no DNS server that is not an SRP updated is "default.service.arpa" and because no DNS server that is
registrar would normally be configured to be authoritative for not an SRP registrar would normally be configured to be authoritative
"default.service.arpa". Therefore, a requestor that sends an SRP for "default.service.arpa". Therefore, a requestor that sends an SRP
Update can tell that the receiving server does not support SRP, but Update can tell that the receiving server does not support SRP but
does support RFC2136, because the RCODE will either be NotZone, does support [RFC2136] because the RCODE will either be NotZone,
NotAuth or Refused, or because there is no response to the update NotAuth, or Refused or because there is no response to the update
request (when using the anycast address) request (when using the anycast address).
In this case a requestor MAY attempt to register itself using regular In this case, a requestor MAY attempt to register itself using
RFC2136 DNS updates. To do so, it must discover the default regular DNS updates described in [RFC2136]. To do so, it must
registration zone and the DNS server designated to receive updates discover the default registration zone and the DNS server designated
for that zone, as described earlier, using the _dns-update._udp SRV to receive updates for that zone, as described earlier, using the
record. It can then send the update to the port and host pointed to _dns-update._udp SRV record. It can then send the update to the port
by the SRV record, and is expected to use appropriate prerequisites and host pointed to by the SRV record, and it is expected to use
to avoid overwriting competing records. Such updates are out of appropriate prerequisites to avoid overwriting competing records.
scope for SRP, and a requestor that implements SRP MUST first attempt Such updates are out of scope for SRP, and a requestor that
to use SRP to register itself, and only attempt to use RFC2136 implements SRP MUST first attempt to use SRP to register itself and
backwards compatibility if that fails. Although the owner name for only attempt to use backwards capability with [RFC2136] if that
the SRV record specifies the UDP protocol for updates, it is also fails. Although the owner name for the SRV record specifies UDP for
possible to use TCP, and TCP SHOULD be required to prevent spoofing. updates, it is also possible to use TCP, and TCP SHOULD be required
to prevent spoofing.
Appendix C. Sample BIND9 configuration for default.service.arpa. Appendix C. Sample BIND9 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 . $ORIGIN .
skipping to change at page 40, line 39 skipping to change at line 1824
$TTL 300 ; 5 minutes $TTL 300 ; 5 minutes
ns3 AAAA 2001:db8:0:1::1 ns3 AAAA 2001:db8:0:1::1
$TTL 3600 ; 1 hour $TTL 3600 ; 1 hour
demo AAAA 2001:db8:0:2::1 demo AAAA 2001:db8:0:2::1
KEY 0 3 13 ( KEY 0 3 13 (
qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg== 9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
); alg = ECDSAP256SHA256 ; key id = 15008 ); alg = ECDSAP256SHA256 ; key id = 15008
AAAA ::1 AAAA ::1
Figure 2: Example Zone file Figure 2: Example Zone File
Acknowledgments
Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
Dong, and Abtin Keshavarzian for their thorough technical reviews.
Thanks to Kangping and Abtin as well for testing the document by
doing an independent implementation. Thanks to Tamara Kemper for
doing a nice developmental edit, Tim Wattenberg for doing an SRP
requestor proof-of-concept implementation at the Montreal Hackathon
at IETF 102, and Tom Pusateri for reviewing during the hackathon and
afterwards. Thanks to Esko for a really thorough second Last Call
review. Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
Dong, Martin Turon, and Michael Cowan for their detailed second last
call reviews. Thanks to Patrik Fältström, Dhruv Dhody, David Dong,
Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective
directorate reviews. Thanks to Paul Wouters for a _really_ detailed
IESG review! Thanks also to the other IESG members who provided
comments or simply took the time to review the document.
Authors' Addresses Authors' Addresses
Ted Lemon Ted Lemon
Apple Inc. Apple Inc.
One Apple Park Way One Apple Park Way
Cupertino, California 95014 Cupertino, CA 95014
United States of America United States of America
Email: mellon@fugue.com Email: mellon@fugue.com
Stuart Cheshire Stuart Cheshire
Apple Inc. Apple Inc.
One Apple Park Way One Apple Park Way
Cupertino, California 95014 Cupertino, CA 95014
United States of America United States of America
Phone: +1 408 974 3207 Phone: +1 408 974 3207
Email: cheshire@apple.com Email: cheshire@apple.com
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