dnssd T. Lemon
Internet-Draft S. Cheshire
Intended status: Standards Track Apple Inc.
Expires: 22 August 2025 February 2025
Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-27
Abstract
The Service Registration Protocol (SRP) for DNS-based Service
Discovery (DNS-SD) uses the standard DNS Update mechanism to enable
DNS-SD using only unicast packets. This makes it possible to deploy
DNS-SD without multicast, which greatly improves scalability and
improves performance on networks where multicast service is not an
optimal choice, particularly IEEE 802.11 (Wi-Fi) and IEEE 802.15.4
networks. DNS-SD Service registration uses public keys and SIG(0) to
allow services to defend their registrations.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://dnssd-
wg.github.io/draft-ietf-dnssd-srp/draft-ietf-dnssd-srp.html. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-dnssd-srp/.
Discussion of this document takes place on the DNS-SD Working Group
mailing list (mailto:dnssd@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/dnssd/. Subscribe at
https://www.ietf.org/mailman/listinfo/dnssd/.
Source for this draft and an issue tracker can be found at
https://github.com/dnssd-wg/draft-ietf-dnssd-srp.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 August 2025.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology Used in This Document . . . . . . 7
3. Service Registration Protocol . . . . . . . . . . . . . . . . 7
3.1. Protocol Variants . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Full-Featured Hosts . . . . . . . . . . . . . . . . . 7
3.1.2. Constrained Hosts . . . . . . . . . . . . . . . . . . 8
3.1.3. Why two variants? . . . . . . . . . . . . . . . . . . 8
3.2. Protocol Details . . . . . . . . . . . . . . . . . . . . 9
3.2.1. What to Publish . . . . . . . . . . . . . . . . . . . 9
3.2.2. Where to Publish It . . . . . . . . . . . . . . . . . 10
3.2.3. How to Publish It . . . . . . . . . . . . . . . . . . 10
3.2.3.1. How the DNS-SD Service Registration Process Differs
from DNS Update . . . . . . . . . . . . . . . . . . 11
3.2.3.2. Retransmission Strategy . . . . . . . . . . . . . 12
3.2.3.3. Successive Updates . . . . . . . . . . . . . . . 12
3.2.4. How to Secure It . . . . . . . . . . . . . . . . . . 12
3.2.4.1. FCFS Naming . . . . . . . . . . . . . . . . . . . 13
3.2.5. SRP Requester Behavior . . . . . . . . . . . . . . . 13
3.2.5.1. Public/Private Key Pair Generation and Storage . 13
3.2.5.2. Name Conflict Handling . . . . . . . . . . . . . 14
3.2.5.3. Record Lifetimes . . . . . . . . . . . . . . . . 14
3.2.5.4. Compression in SRV Records . . . . . . . . . . . 15
3.2.5.5. Removing Published Services . . . . . . . . . . . 15
3.3. Validation and Processing of SRP Updates . . . . . . . . 17
3.3.1. Validation of DNS Update Add and Delete RRs . . . . . 17
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3.3.1.1. Service Discovery Instruction . . . . . . . . . . 18
3.3.1.2. Service Description Instruction . . . . . . . . . 18
3.3.1.3. Host Description Instruction . . . . . . . . . . 19
3.3.2. Valid SRP Update Requirements . . . . . . . . . . . . 20
3.3.3. FCFS Name and Signature Validation . . . . . . . . . 21
3.3.4. Handling of Service Subtypes . . . . . . . . . . . . 22
3.3.5. SRP Update Response . . . . . . . . . . . . . . . . . 22
3.3.6. Optional Behavior . . . . . . . . . . . . . . . . . . 22
4. TTL Consistency . . . . . . . . . . . . . . . . . . . . . . . 23
5. Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.1. Cleaning Up Stale Data . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 26
6.1. Source Validation . . . . . . . . . . . . . . . . . . . . 26
6.2. Other DNS Updates . . . . . . . . . . . . . . . . . . . . 27
6.3. Risks of Allowing Arbitrary Names to be Registered in SRP
Updates . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.4. Security of Local Service Discovery . . . . . . . . . . . 28
6.5. SRP Registrar Authentication . . . . . . . . . . . . . . 28
6.6. Required Signature Algorithm . . . . . . . . . . . . . . 28
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 29
8. Domain Name Reservation Considerations . . . . . . . . . . . 29
8.1. Users . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.2. Application Software . . . . . . . . . . . . . . . . . . 30
8.3. Name Resolution APIs and Libraries . . . . . . . . . . . 30
8.4. Recursive Resolvers . . . . . . . . . . . . . . . . . . . 30
8.5. Authoritative DNS Servers . . . . . . . . . . . . . . . . 31
8.6. DNS Server Operators . . . . . . . . . . . . . . . . . . 31
8.7. DNS Registries/Registrars . . . . . . . . . . . . . . . . 32
9. Delegation of "service.arpa." . . . . . . . . . . . . . . . . 32
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
10.1. Registration and Delegation of "service.arpa" as a
Special-Use Domain Name . . . . . . . . . . . . . . . . 32
10.2. Addition of "service.arpa" to the Locally-Served Zones
Registry . . . . . . . . . . . . . . . . . . . . . . . . 32
10.3. Subdomains of "service.arpa." . . . . . . . . . . . . . 32
10.4. Service Name Registrations . . . . . . . . . . . . . . . 33
10.4.1. 'dnssd-srp' Service Name . . . . . . . . . . . . . . 33
10.4.2. 'dnssd-srp-tls' Service Name . . . . . . . . . . . . 33
10.5. Anycast Address . . . . . . . . . . . . . . . . . . . . 34
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
11.1. Normative References . . . . . . . . . . . . . . . . . . 35
11.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. Using Standard Authoritative DNS Servers Compliant
with RFC 2136 to test SRP requesters . . . . . . . . . . 41
Appendix B. How to Allow SRP Requesters to Update Standard Servers
Compliant with RFC 2136 . . . . . . . . . . . . . . . . . 41
Appendix C. Sample BIND 9 Configuration for
"default.service.arpa." . . . . . . . . . . . . . . . . . 42
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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
DNS-SD [RFC6763] is a component of Zero Configuration Networking
[RFC6760] [ZC] [ROADMAP].
This document describes an enhancement to DNS-SD that allows servers
to register the services they offer using the DNS protocol over
unicast rather than using Multicast DNS (mDNS) [RFC6762]. There is
already a large installed base of DNS-SD clients that can discover
services using the DNS protocol (e.g., Android, Windows, Linux,
Apple).
This document is intended for three audiences: implementers of
software that provides services that should be advertised using
DNS-SD, implementers of authoritative DNS servers that will be used
in contexts where DNS-SD registration is needed, and administrators
of networks where DNS-SD service is required. The document is
expected to provide sufficient information to allow interoperable
implementation of the Service Registration Protocol.
DNS-SD allows servers to publish the information required to access
the services they provide. DNS-SD clients can then discover the set
of services of a particular type that are available. They can then
select a service from among those that are available and obtain the
information required to use it. Although DNS-SD using the DNS
protocol can be more efficient and versatile than using mDNS, it is
not common in practice because of the difficulties associated with
updating authoritative DNS services with service information.
The existing practice for updating DNS zones is either to enter new
data manually or to use DNS Update [RFC2136]. Unfortunately, DNS
Update requires either:
* that the authoritative DNS server automatically trust updates or
* that the DNS Update requester have some kind of shared secret or
public key that is known to the authoritative DNS server and can
be used to authenticate the update.
Furthermore, DNS Update can be a fairly chatty process, requiring
multiple roundtrips with different conditional predicates to complete
the update process.
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The Service Registration Protocol (SRP) adds a set of default
heuristics for processing DNS updates that eliminates the need for
conditional predicates. Instead, the SRP registrar (an authoritative
DNS server that supports SRP Updates) has a set of default predicates
that are applied to the update; and the update either succeeds
entirely or fails in a way that allows the requester to know what
went wrong and construct a new update.
SRP also adds a feature called "First Come, First Served Naming" (or
"FCFS Naming"), which allows the requester to:
* claim a name that is not yet in use, and
* using SIG(0) [RFC2931], authenticate both the initial claim (to
ensure it has not been modified in transit) and subsequent updates
(to ensure they come from the same entity that performed the
initial claim).
This prevents a new service instance from "stealing" a name that is
already in use: a second SRP requester attempting to claim an
existing name will not possess the SIG(0) key used by the first
requester to claim it. Because of this, its claim will be rejected.
This will force it to choose a new name.
It is important to understand that "authenticate" here just means
that we can tell that an update came from the same source as the
original registration. We have not established trust. This has
important implications for what we can and can't do with data the SRP
requester sends us. You will notice as you read this document that
we only support adding a very restricted set of records, and the
content of those records is further constrained.
The reason for this is precisely that we have not established trust.
So, we can only publish information that we feel safe in publishing
even though we do not have any basis for trusting the requester. We
reason that mDNS [RFC6762] allows arbitrary hosts on a single IP link
to advertise services [RFC6763], relying on whatever service is
advertised to provide authentication as a part of its protocol rather
than in the service advertisement.
This is considered reasonably safe because it requires physical
presence on the network in order to advertise. An off-network mDNS
attack is simply not possible. Our goal with this specification is
to impose similar constraints. Therefore, you will see in
Section 3.3.1 that a very restricted set of records with a very
restricted set of relationships are allowed. You will also see in
Section 6.1 that we give advice on how to prevent off-network
attacks.
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This leads us to the disappointing observation that this protocol is
not a mechanism for adding arbitrary information to DNS zones. We
have not evaluated the security properties of adding, for example, an
SOA record, an MX record, or a CNAME record; therefore, these are
forbidden. Future updates to this specification might include
analyses for other records and extend the set of records and/or
record content that can be registered here. Or it might require
establishment of trust, and add an authorization model to the
authentication model we now have. But that is work for a future
document.
Finally, SRP adds the concept of a "lease" [RFC9664], analogous to
leases in DHCP [RFC2131][RFC8415]. The SRP registration itself has a
lease that may be on the order of two hours; if the requester does
not renew the lease before it has elapsed, the registration is
removed. The claim on the name can have a longer lease so that
another requester cannot claim the name, even though the registration
has expired.
The Service Registration Protocol for DNS-SD specified in this
document provides a reasonably secure mechanism for publishing this
information. Once published, these services can be readily
discovered by DNS-SD clients using standard DNS lookups.
Section 10 of the DNS-SD specification [RFC6763] briefly discusses
ways that servers can advertise the services they provide in the DNS
namespace. In the case of mDNS, it allows servers to advertise their
services on the local link, using names in the ".local" namespace,
which makes their services directly discoverable by peers attached to
that same local link.
DNS-SD [RFC6763] also allows clients to discover services using the
DNS protocol over traditional unicast [RFC1035]. This can be done by
having a system administrator manually configure service information
in the DNS; however, manually populating DNS authoritative server
databases is costly and potentially error-prone and requires a
knowledgeable network administrator. Consequently, although all
DNS-SD client implementations of which we are aware support DNS-SD
using DNS queries, in practice it is used much less frequently than
mDNS.
The Discovery Proxy [RFC8766] provides one way to automatically
populate the DNS namespace but is only appropriate on networks where
services are easily advertised using mDNS. The present document
describes a solution more suitable for networks where multicast is
inefficient, or where sleepy devices are common, by supporting use of
unicast for both the offering of services and the discovery of
services.
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2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Service Registration Protocol
Services that implement SRP use DNS Update [RFC2136] with SIG(0)
[RFC3007] to publish service information in the DNS. Two variants
exist: one for full-featured hosts and one for devices designed for
Constrained-Node Networks (CNNs) [RFC7228]. An SRP registrar is most
likely an authoritative DNS server, or is a source of data for one or
more authoritative DNS servers. There is no requirement that the
authoritative DNS server that is receiving SRP Updates be the same
authoritative DNS server that is answering queries that return
records that have been registered. For example, an SRP registrar
could be the "hidden primary" that is the source of data for a fleet
of secondary authoritative DNS servers.
3.1. Protocol Variants
3.1.1. Full-Featured Hosts
Full-featured hosts either are configured manually with a
registration domain or discover the default registration domain
automatically using the Domain Enumeration process described in
Section 11 of the DNS-SD specification [RFC6763]. If this process
does not produce a default registration domain, the SRP registrar is
not discoverable on the local network using this mechanism. Other
discovery mechanisms are possible, but they are out of scope for this
document.
Configuration of the registration domain can be done either:
* by querying the list of available registration domains
("r._dns-sd._udp") and allowing the user to select one from the UI
or
* by any other means appropriate to the particular use case being
addressed.
Full-featured devices construct the names of the SRV, TXT, and PTR
records describing their service 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
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queries as described in Section 6.1 of the DNS Push Notification
specification [RFC8765]. Having discovered the enclosing DNS zone,
they query for the "_dnssd-srp._tcp.<zone>" SRV record to discover
the SRP registrar to which they can send SRP Updates. Hosts that
support SRP Updates using TLS use the "_dnssd-srp-tls._tcp.<zone>"
SRV record instead.
Examples of full-featured hosts include devices such as home
computers, laptops, powered peripherals with network connections
(such as printers and home routers), and even battery-operated
devices such as mobile phones that have long battery lives.
3.1.2. Constrained Hosts
For devices designed for CNNs [RFC7228], some simplifications are
available. Instead of being configured with (or discovering) the
service registration domain, the special-use domain name [RFC6761]
"default.service.arpa" is used. The details of how SRP registrars
are discovered will be specific to the constrained network;
therefore, we do not suggest a specific mechanism here.
SRP requesters on CNNs are expected to receive, from the network, a
list of SRP registrars with which to register. It is the
responsibility of a CNN supporting SRP to provide one or more
registrar addresses. It is the responsibility of the registrar
supporting a CNN to handle the updates appropriately. In some
network environments, updates may be accepted directly into a local
"default.service.arpa" zone, which has only local visibility. In
other network environments, updates for names ending in
"default.service.arpa" may be rewritten by the registrar to names
with broader visibility [RFC8766].
3.1.3. Why two variants?
The reason for these different variants is that low-power devices
that typically use CNNs may have very limited battery capacity. The
series of DNS lookups required to discover an SRP registrar and then
communicate with it will increase the energy required to advertise a
service; for low-power devices, the additional flexibility this
provides does not justify the additional use of energy. It is also
fairly typical of such networks that some network service information
is obtained as part of the process of joining the network; thus, this
can be relied upon to provide nodes with the information they need.
Networks that are not CNNs can have more complicated topologies at
the IP layer. Nodes connected to such networks can be assumed to be
able to do DNS-SD service registration domain discovery. Such
networks are generally able to provide registration domain discovery
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and routing. This creates the possibility of off-network spoofing,
where a device from a foreign network registers a service on the
local network in order to attack devices on the local network. To
prevent such spoofing, TCP is required for such networks.
3.2. Protocol Details
We will discuss several parts to this process:
* how to know what to publish (Section 3.2.1),
* how to know where to publish it (under what name) (Section 3.2.2),
* how to publish it (Section 3.2.3),
* how to secure its publication (Section 3.2.4), and
* how to maintain the information once published (Section 5).
3.2.1. What to Publish
SRP Updates are sent by SRP requesters to SRP registrars. Three
types of instructions appear in an SRP Update: Service Discovery
instructions, Service Description instructions, and Host Description
instructions. These instructions are made up of DNS Update Resource
Records (RRs) that are either adds or deletes. The types of records
that are added, updated, and removed in each of these instructions,
as well as the constraints that apply to them, are described in
Section 3.3. An SRP Update is a DNS Update message [RFC2136] that is
constructed so as to meet the constraints described in that section.
The following is a brief overview of what is included in a typical
SRP Update:
* Service Discovery PTR RR(s) for service(s), which map from a
generic service type (or subtype(s)) to a specific Service
Instance Name [RFC6763].
* For each 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. Currently 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 always exactly one hostname in a single SRP Update. A
DNS Update containing more than one hostname is not an SRP Update.
The hostname has one or more address RRs (AAAA or A) and a KEY RR
(used for FCFS Naming). Depending on the use case, an SRP
requester may be required to suppress some addresses that would
not be usable by hosts discovering the service through the SRP
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registrar. The exact address record suppression behavior required
may vary for different types of SRP requesters. Guidance for
suppressing unusable records can be found in Section 5.5.2 of the
Discovery Proxy specification [RFC8766].
The DNS-Based Service Discovery specification [RFC6763] describes the
details of what each of these RR types mean, with the exception of
the KEY RR, which was defined in the specification for how to store
Diffie-Hellman Keys in the DNS [RFC2539]. These specifications
should be considered the definitive sources for information about
what to publish; the reason for summarizing this here is to provide
the reader with enough information about what will be published that
the service registration process can be understood at a high level
without first learning the full details of DNS-SD. Also, the
"Service Instance Name" is an important aspect of FCFS Naming, which
we describe later on in this document.
3.2.2. Where to Publish It
Multicast DNS (mDNS) uses a single namespace, ".local". Subdomains
of ".local" are specific to the local link on which they are
advertised. This convenience is not available for DNS-SD using the
DNS protocol: services must exist in some specific DNS namespace that
is chosen either by the network operator or automatically.
As described above, full-featured devices are responsible for knowing
the domain in which to register their services. Such devices MAY
optionally support configuration of a registration domain by the
operator of the device. However, such devices MUST support
registration domain discovery as described in Section 11 of the
DNS-SD specification [RFC6763].
Devices made for CNNs register in the special-use domain name
[RFC6761] "default.service.arpa" and let the SRP registrar handle
rewriting that to a different domain if necessary, using the same
techniques as Discovery Proxies [RFC8766].
3.2.3. How to Publish It
It is possible to send a DNS Update message that does several things
at once: for example, it's possible in a single transaction to add or
update a single Host Description while also adding or updating the
RRs comprising the Service Description(s) for one or more service
instance(s) available on that host, and adding or updating the RRs
comprising the Service Discovery instruction(s) for those service
instance(s).
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An SRP Update takes advantage of this: it is implemented as a single
DNS Update message that contains a service's Service Discovery
records, Service Description records, and Host Description records.
Updates done according to this specification are somewhat different
from normal DNS Updates [RFC2136] where the update process could
involve many update attempts. You might first attempt to add a name
if it doesn't exist; if that fails, then in a second message you
might update the name if it does exist but matches certain
preconditions. Because the Service Registration Protocol described
in this document uses a single transaction, some of this adaptability
is lost.
In order to allow updates to happen in a single transaction, SRP
Updates do not include update prerequisites. The requirements
specified in Section 3.3 are implicit in the processing of SRP
Updates; thus, there is no need for the SRP requester to put in any
explicit prerequisites.
3.2.3.1. How the DNS-SD Service Registration Process Differs from DNS
Update
DNS-SD Service Registration uses DNS Update [RFC2136], with some
additions:
* It implements FCFS Naming, protected using SIG(0) [RFC2931].
* It enforces policy about what updates are allowed.
* It optionally performs rewriting of "default.service.arpa" to some
other domain.
* It optionally performs automatic population of the address-to-name
reverse mapping domains.
* An SRP registrar is not required to implement general DNS Update
prerequisite processing.
* CNN SRP requesters are allowed to send updates to the generic
domain "default.service.arpa.".
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3.2.3.2. Retransmission Strategy
The DNS protocol, including DNS updates, can operate over UDP or TCP.
When using UDP, reliable transmission must be guaranteed by
retransmitting if a DNS UDP message is not acknowledged in a
reasonable interval. Section 4.2.1 of the DNS specification
[RFC1035] provides some guidance on this topic, as does Section 1 of
the IETF document describing common DNS implementation errors
[RFC1536]. Section 3.1.3 of the UDP Usage Guidelines document
[RFC8085] also provides useful guidance that is particularly relevant
to DNS.
3.2.3.3. Successive Updates
SRP does not require that every update contain the same information.
When an SRP requester needs to send more than one SRP Update to the
SRP registrar, it SHOULD combine these into a single SRP Update, when
possible, subject to DNS message size limits and link-specific size
limits (e.g. an 802.15.4 network will perform poorly when asked to
deliver a packet larger than about 500 bytes). If the updates do not
fit into a single SRP Update then the SRP requester MUST send
subsequent SRP Update sequentially: until an earlier SRP Update has
been acknowledged, the requester MUST NOT send any subsequent SRP
Update. If a configuration change occurs while an outstanding SRP
Update is in flight, the SRP registrar MUST defer sending a new SRP
Update for that change until the previous SRP Update has completed.
3.2.4. How to Secure It
DNS Update messages can be secured using secret key transaction
signatures (TSIG) [RFC8945]. This approach uses a secret key shared
between the DNS Update requester (which issues the update) and the
authoritative DNS server (which authenticates it). This model does
not work for automatic service registration.
The goal of securing the DNS-SD Registration Protocol is to provide
the best possible security given the constraint that service
registration has to be automatic. It is possible to layer more
operational security on top of what we describe here, but FCFS Naming
is already an improvement over the security of mDNS.
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3.2.4.1. FCFS Naming
FCFS Naming provides a limited degree of security. A server that
registers its service using SRP is given ownership of a name for an
extended period of time based on a lease specific to the key used to
authenticate the SRP Update, which may be longer than the lease
associated with the registered RRs. As long as the registrar
remembers the name and does not reveal the key used to register RRs
on that name, no other SRP requester can add or update the
information associated with that name. If the SRP requester fails to
renew its service registration before the KEY lease expires
(Section 4 of the DNS Update Lease specification [RFC9664]) its name
is no longer protected. FCFS Naming is used to protect both the
Service Description and the Host Description.
3.2.5. SRP Requester Behavior
3.2.5.1. Public/Private Key Pair Generation and Storage
The requester generates a public/private key pair (Section 6.6).
This key pair MUST be stored in stable storage; if there is no
writable stable storage on the SRP requester, the SRP requester MUST
be preconfigured with a public/private key pair in read-only storage.
This key pair MUST be unique to the device. A device with rewritable
storage SHOULD retain this key indefinitely. When the device changes
ownership, it may be appropriate for the former owner to erase the
old key pair, which would then require the new owner to install a new
one. Therefore, the SRP requester on the device SHOULD provide a
mechanism to erase the key (for example, as the result of a "factory
reset") and to generate a new key.
Note that when a new key is generated, this will prevent the device
from registering with the name associated with the old key in the
same domain where it had previously registered. So implicit in the
generation of a new key is the generation of a new name; this can
either be done proactively when regenerating a key, or can be done
when the SRP update produces a name conflict.
The policy described here for managing keys assumes that the keys are
only used for SRP. If a key that is used for SRP is also used for
other purposes, the policy described here is likely to be
insufficient. The policy stated here is NOT RECOMMENDED in such a
situation: a policy appropriate to the full set of uses for the key
must be chosen. Specifying such a policy is out of scope for this
document.
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When sending DNS updates, the requester includes a KEY record
containing the public portion of the key in each Host Description
Instruction and each Service Description Instruction. Each KEY
record MUST contain the same public key. The update is signed using
SIG(0), using the private key that corresponds to the public key in
the KEY record. The lifetimes of the records in the update are set
using the EDNS(0) Update Lease option [RFC9664].
The format of the KEY resource record in the SRP Update is defined in
the IETF specification for DNSSEC Resource Records [RFC4034].
Because the KEY RR used in SIG(0) is not a zone-signing key, the
flags field in the KEY RR MUST be all zeroes.
The KEY record in Service Description updates MAY be omitted for
brevity; if it is omitted, the SRP registrar MUST behave as if the
same KEY record that is given for the Host Description is also given
for each Service Description for which no KEY record is provided.
Omitted KEY records are not used when computing the SIG(0) signature.
3.2.5.2. Name Conflict Handling
"Add" operations for both Host Description RRs and Service
Description RRs can have names that result in name conflicts.
Service Discovery record "Add" operations cannot have name conflicts.
If any Host Description or Service Description record is found by the
SRP registrar to have a conflict with an existing name, the registrar
will respond to the SRP Update with a YXDomain RCODE [RFC2136]. In
this case, the SRP requester MUST choose a new name or give up.
There is no specific requirement for how the SRP requester should
choose a new name. Typically, however, the requester will append a
number to the preferred name. This number could be sequentially
increasing or could be chosen randomly. One existing implementation
attempts several sequential numbers before choosing randomly. For
instance, it might try host.default.service.arpa, then
host-1.default.service.arpa, then host-2.default.service.arpa, then
host-31773.default.service.arpa.
3.2.5.3. Record Lifetimes
The lifetime of the DNS-SD PTR, SRV, A, AAAA, and TXT records
[RFC6763] uses the LEASE field of the Update Lease option and is
typically set to two hours. Thus, if a device is disconnected from
the network, it does not continue to appear for too long in the user
interfaces of devices looking for instances of that service type.
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The lifetime of the KEY records is set using the KEY-LEASE field of
the Update Lease Option and SHOULD be set to a much longer time,
typically 14 days. The result being that even though a device may be
temporarily unplugged -- disappearing from the network for a few days
-- it makes a claim on its name that lasts much longer.
Therefore, even if a device is unplugged from the network for a few
days, and its services are not available for that time, no other
device can come along and claim its name the moment it disappears
from the network. In the event that a device is unplugged from the
network and permanently discarded, then its name is eventually
cleaned up and made available for reuse.
3.2.5.4. Compression in SRV Records
Although the original SRV specification [RFC2782] requires that the
target hostname in the rdata of an SRV record not be compressed in
DNS queries and responses, an SRP requester MAY compress the target
in the SRV record, since an SRP Update is neither a DNS query nor a
DNS response. The motivation for _not_ compressing is not stated in
the SRV specification, but is assumed to be because a recursive
resolver (caching server) that does not understand the format of the
SRV record might store it as binary data without decoding a
compression pointer embedded with the target hostname field, and thus
return nonsensical rdata in response to a query. This concern does
not apply in the case of SRP. An SRP registrar needs to understand
SRV records in order to validate the SRP Update. Compression of the
target can save space in the SRP Update, so we want SRP requesters to
be able to assume that the registrar will handle this. Therefore,
SRP registrars MUST support compression of SRV RR targets.
Note that this document does not update the SRV specification
[RFC2782]: authoritative DNS servers still MUST NOT compress SRV
record targets. The requirement to accept compressed SRV records in
updates only applies to SRP registrars, and SRP registrars that are
also authoritative DNS servers still MUST NOT compress SRV record
targets in DNS responses. We note also that Multicast DNS [RFC6762]
similarly compresses SRV records in mDNS messages.
In addition, we note that an implementer of an SRP requester might
update existing code that creates SRV records or compresses DNS
messages so that it compresses the target of an SRV record. Care
must be taken if such code is used both in requesters and in
authoritative DNS servers that the code only compresses in the case
where a requester is generating an SRP Update.
3.2.5.5. Removing Published Services
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3.2.5.5.1. Removing All Published Services
To remove all the services registered to a particular hostname, the
SRP requester transmits an SRP Update for that hostname with an
Update Lease option that has a LEASE value of zero. The SRP Update
MUST contain exactly one Host Description Instruction, containing
exactly one "Delete All RRsets From A Name" instruction for the
hostname, and no "Add to an RRSet" instructions for that hostname.
If the registration is to be permanently removed, KEY-LEASE SHOULD
also be zero. Otherwise, it SHOULD be set to the same value it had
previously; this holds the name in reserve for when the SRP requester
is once again able to provide the service.
This method of removing services is intended for the case where the
requester is going offline and does not want any of its services to
continue being advertised.
To support this, when removing a hostname an SRP registrar MUST
remove all service instances pointing to that hostname, and all
Service Discovery PTR records pointing to those service instances,
even if the SRP requester doesn't list them explicitly. If the KEY
lease time is nonzero, the SRP registrar MUST NOT delete the KEY
records for these SRP requesters.
3.2.5.5.2. Removing Some Published Services
In some use cases, a requester may need to remove a specific service
without removing its other services. For example, a device may shut
down its remote screen access (_rfb._tcp) service while retaining its
command-line login (_ssh._tcp) service. This can be accomplished in
one of two ways:
1. To simply remove a specific service, the requester sends a valid
SRP Update with a Service Description Instruction
(Section 3.3.1.2) containing a single "Delete All RRsets From A
Name" update to the Service Instance Name. The SRP Update SHOULD
include Service Discovery Instructions (Section 3.3.1.1)
consisting of "Delete An RR From An RRset" updates [RFC2136] that
delete any Service Discovery PTR records whose target is the
Service Instance Name. However, even in the absence of such
Service Discovery Instructions, the SRP registrar MUST delete any
Service Discovery PTR records that point to the deleted Service
Instance Name.
2. When deleting one service instance while simultaneously creating
a new service instance with a different service instance name, an
alternative is to perform both operations using a single SRP
Update. In this case, the old service is deleted as in the first
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alternative. The new service is added, just as it would be in an
update that wasn't deleting the old service. Because both the
removal of the old service and the add of the new service consist
of a valid Service Discovery Instruction and a valid Service
Description Instruction, the update as a whole is a valid SRP
Update and will result in the old service being removed and the
new one added; or, to put it differently, the SRP Update will
result in the old service being replaced by the new service.
It is perhaps worth noting that, if a service is being updated
without the Service Instance Name changing (for example, when only
the target port in the SRV record is being updated), that SRP Update
will look very much like the second alternative above. The PTR
record in the Service Discovery Instruction will be the same for both
the "Delete An RR From An RRset" update and the "Add To An RRset"
update [RFC2136]. Since the removal of the old service and the
addition of the new service are both valid SRP Update operations, the
combined operation is a valid SRP Update operation. The SRP
registrar does not need to include code to recognize this special
case, and does not need to take any special actions to handle it
correctly.
Whichever of these two alternatives is used, the hostname lease will
be updated with the lease time provided in the SRP update. In
neither of these cases is it permissible to delete the hostname. All
services must point to a hostname. If a hostname is to be deleted,
this must be done using the method described in Section 3.2.5.5.1,
which deletes the hostname and all services that have that hostname
as their target.
3.3. Validation and Processing of SRP Updates
3.3.1. Validation of DNS Update Add and Delete RRs
The SRP registrar first validates that the DNS Update message is a
syntactically and semantically valid DNS Update message according to
the usual DNS Update rules [RFC2136].
SRP Updates consist of a set of _instructions_ that together add or
remove one or more services. Each SRP Update consists of one or more
delete update(s), or one or more add update(s), or some combination
of both delete updates and add updates.
The SRP registrar checks each instruction in the SRP Update to see
that it is either a Service Discovery Instruction, a Service
Description Instruction, or a Host Description Instruction. Order
matters in DNS updates. Specifically, deletes must precede adds for
records that the deletes would affect; otherwise, the add will have
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no effect. This is the only ordering constraint: aside from this
constraint, updates may appear in whatever order is convenient when
constructing the update.
Because the SRP Update is a DNS update, it MUST contain a single
entry in the Zone Section (what would be the Question Section in a
traditional DNS message) that indicates the zone to be updated.
Every delete and update in an SRP Update MUST be within the zone that
is specified for the SRP Update.
3.3.1.1. Service Discovery Instruction
An instruction is a Service Discovery Instruction if it:
* consists of exactly one "Add To An RRSet" or exactly one "Delete
An RR From An RRSet" RR update (Section 2.5 of the DNS Update
specification [RFC2136]),
* which updates a PTR RR,
* the target of which is a Service Instance Name
* for which name a Service Description Instruction is present in the
SRP Update, and:
- if the Service Discovery Instruction is an "Add To An RRSet"
instruction, that Service Description Instruction contains a
"Delete All RRsets From A Name" instruction for that Service
Instance Name followed by "Add To An RRset" instructions for
the SRV and TXT records describing that service; or
- if the Service Discovery Instruction is a "Delete An RR From An
RRSet" instruction, that Service Description Instruction
contains a "Delete All RRsets From A Name" instruction for that
Service Instance Name with no following "Add To An RRset"
instructions for the SRV and TXT records describing that
service.
Note that there can be more than one Service Discovery Instruction
for the same service name (the owner name of the Service Discovery
PTR record) if the SRP requester is advertising more than one
instance of the same service type or is changing the target of a PTR
RR. When subtypes are being used (Section 7.1 of the DNS-SD
specification [RFC6763]) each subtype is a separate Service Discovery
Instruction. For each such PTR RR add or delete, the above
constraints must be met.
3.3.1.2. Service Description Instruction
An instruction is a Service Description Instruction if, for the given
Service Instance Name, all of the following are true:
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* It contains exactly one "Delete All RRsets From A Name" update for
the Service Instance Name (Section 2.5.3 of the DNS Update
specification [RFC2136]),
* It contains zero or one "Add To An RRset" KEY RRs that, if
present, contains the public key corresponding to the private key
that was used to sign the message (if present, the KEY RR MUST
match the KEY RR given in the Host Description),
* It contains zero or one "Add To An RRset" SRV RR,
* If an "Add To An RRSet" update for an SRV RR is present, there
MUST be at least one "Add To An RRset" update for the
corresponding TXT RR, and the target of the SRV RR MUST be the
hostname given in the Host Description Instruction in the SRP
Update, or
* If there is no "Add To An RRset" update for an SRV RR, then there
MUST be no "Add To An RRset" updates for the corresponding TXT RR,
and either:
- the name to which the "Delete All RRsets From A Name" applies
does not exist, or
- there is an existing KEY RR on that name that matches the key
with which the SRP Update was signed.
Service Description Instructions do not modify any other resource
records.
An SRP registrar MUST correctly handle compressed names in the SRV
target.
3.3.1.3. Host Description Instruction
Every SRP Update alway contains exactly one Host Description
Instruction.
An instruction is a Host Description Instruction if, for the
appropriate hostname, it contains the following:
* exactly one "Delete All RRsets From A Name" RR
* exactly one "Add To An RRset" RR that adds a KEY RR that contains
the public key corresponding to the private key that was used to
sign the message
* zero "Add To An RRset" operations (in the case of deleting a
registration) or one or more "Add To An RRset" RRs of type A and/
or AAAA (in the case of creating or updating a registration)
Host Description Instructions do not modify any other resource
records.
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A and/or AAAA records that are not of sufficient scope to be validly
published in a DNS zone MAY be ignored by the SRP registrar, which
could result in a Host Description effectively containing zero
reachable addresses even when it contains one or more addresses.
For example, if an IPv4 link-local address [RFC3927] or an IPv6 link-
local address [RFC4862] is provided by the SRP requester, the SRP
registrar could elect not to publish this in a DNS zone. However, in
some situations, the registrar might make the records available
through a mechanism such as an advertising proxy only on the specific
link from which the SRP Update originated. In such a situation,
locally scoped records are still valid.
3.3.2. Valid SRP Update Requirements
An SRP Update MUST contain exactly one Host Description Instruction.
Multiple Service Discovery updates and Service Description updates
may be combined into a single single SRP Update along with a single
Host Description update, as described in Section 3.2.3. A DNS Update
message that contains any additional adds or deletes that cannot be
identified as Service Discovery, Service Description, or Host
Description Instructions is not an SRP Update. A DNS update that
contains any prerequisites is not an SRP Update.
An SRP Update MUST include an EDNS(0) Update Lease option [RFC9664].
The LEASE time specified in the Update Lease option MUST be less than
or equal to the KEY-LEASE time. A DNS update that does not include
the Update Lease option, or that includes a KEY-LEASE value that is
less than the LEASE value, is not an SRP Update.
When an SRP registrar receives a DNS Update message that is not an
SRP update, it MAY process the update as normal DNS Update [RFC2136],
including access control checks and constraint checks, if supported.
Otherwise, the SRP registrar MUST reject the DNS Update with the
Refused RCODE.
If the definitions of each of these instructions are followed
carefully and the update requirements are validated correctly, many
DNS Update messages that look very much like SRP Updates nevertheless
will fail to validate. For example, a DNS update that contains an
"Add To An RRset" instruction for a Service Name, and an "Add to an
RRset" instruction for a Service Instance Name, where the PTR record
added to the Service Name does not reference the Service Instance
Name, is not a valid SRP Update, but may be a valid DNS Update.
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3.3.3. FCFS Name and Signature Validation
Assuming that the SRP registrar has confirmed that a DNS Update
message is a valid SRP Update (Section 3.3.2), it then checks that
the name in the Host Description Instruction exists in the zone being
updated. If so, then the registrar checks to see if the KEY record
on that name is the same as the KEY record in the Host Description
Instruction. The registrar performs the same check for the KEY
records in any Service Description Instructions. For KEY records
that were omitted from Service Description Instructions, the KEY from
the Host Description Instruction is used. If any existing KEY record
corresponding to a KEY record in the SRP Update does not match the
KEY record in the SRP Update (whether provided or taken from the Host
Description Instruction), then the SRP registrar MUST reject the SRP
Update with the YXDomain RCODE. This informs the SRP requester that
it should select a different name and try again.
If the SRP Update is not in conflict with existing data in the zone
being updated, the SRP registrar validates the SRP Update using
SIG(0) against the public key in the KEY record of the Host
Description Instruction. If the validation fails, the SRP Update is
malformed and the registrar MUST reject the SRP Update with the
Refused RCODE. Otherwise, the SRP Update is considered valid and
authentic and is processed as for a normal DNS Update [RFC2136].
KEY record updates omitted from Service Description Instruction(s)
are processed as if they had been explicitly present. After the SRP
Update has been applied, every Service Description that is updated
MUST have a KEY RR, which MUST have the same valua as the KEY RR that
is present in the Host Description to which the Service Description
refers.
The IETF specification for DNSSEC Resource Records [RFC4034] states
that the flags field in the KEY RR MUST be zero except for bit 7,
which can be one in the case of a zone key. SRP requesters
implementing this version of the SRP specification MUST set the flags
field in the KEY RR to all zeroes. SRP registrars implementing this
version of the SRP specification MUST accept and store the flags
field in the KEY RR as received, without checking or modifying its
value.
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3.3.4. Handling of Service Subtypes
SRP registrars MUST treat the update instructions for a service type
and all its subtypes as atomic. That is, when a service and its
subtypes are being updated, whatever information appears in the SRP
Update is the entirety of information about that service and its
subtypes. If any subtype appeared in a previous update but does not
appear in the current update, then the SRP registrar MUST remove that
subtype.
There is intentionally no mechanism for deleting a single subtype
individually. A delete of a service deletes all of its subtypes. To
delete a single subtype individually, an SRP Update must be
constructed that contains the service type and all subtypes for that
service except for the subtype(s) to be deleted.
3.3.5. SRP Update Response
The status that is returned depends on the result of processing the
update and can be either NoError, ServFail, Refused, or YXDomain.
All other possible outcomes will already have been accounted for when
applying the constraints that qualify the update as an SRP Update.
The meanings of these responses are explained in Section 2.2 of the
DNS Update specification [RFC2136].
In the case of a response other than NoError, Section 3.8 of the DNS
Update specification [RFC2136] states that the authoritative DNS
server is permitted to respond either with no RRs or to copy the RRs
sent by the DNS Update client into the response. The SRP requester
MUST NOT attempt to validate any RRs that are included in the
response. It is possible that a future SRP extension may include
per-RR indications as to why the update failed, but at the time of
writing this is not specified. So, if an SRP requester were to
attempt to validate the RRs in the response, it might reject such a
response since it would contain RRs but probably not a set of RRs
identical to what was sent in the SRP Update.
3.3.6. Optional Behavior
The SRP registrar MAY add a Reverse Mapping PTR record (described for
IPv4 in Section 3.5 of the DNS specification [RFC1035] and for IPv6
in Section 2.5 of the later document updating DNS for IPv6 [RFC3596])
that corresponds to the Host Description. This is optional because
the reverse mapping PTR record serves no essential protocol function,
but it may be useful for debugging, for example in annotating network
packet traces or logs. In order for the registrar to do a reverse
mapping update, it must be authoritative for the zone that would need
to be updated or have credentials to do the update. The SRP
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requester MAY also do a reverse mapping update if it has credentials
to do so.
The SRP registrar MAY apply additional criteria when accepting
updates. In some networks, it may be possible to do out-of-band
registration of keys and only accept updates from preregistered keys.
In this case, an update for a key that has not been registered SHOULD
be rejected with the Refused RCODE. When use of managed keys is
desired, there are at least two benefits to doing this in conjunction
with SRP rather than simply performing traditional DNS Updates using
SIG(0) keys:
1. The same over-the-air registration protocol is used in both
cases, so both use cases can be addressed by the same SRP
requester implementation.
2. The Service Registration Protocol includes maintenance
functionality not present with normal DNS updates.
Note that the semantics of using SRP in this way are different from
the semantics of typical implementations of DNS Update. The KEY used
to sign the SRP Update only allows the SRP requester to update
records that refer to its Host Description. Implementations of
traditional DNS Update [RFC2136] do not normally provide a way to
enforce a constraint of this type.
The SRP registrar could also have a dictionary of names or name
patterns that are not permitted. If such a list is used, updates for
Service Instance Names that match entries in the dictionary are
rejected with a Refused RCODE.
4. TTL Consistency
All RRs within an RRset are required to have the same TTL (required
by Section 5.2 of the DNS Clarifications document [RFC2181]). In
order to avoid inconsistencies, SRP places restrictions on TTLs sent
by requesters and requires that SRP registrars enforce consistency.
Requesters sending SRP Updates MUST use consistent TTLs in all RRs
within each RRset contained within an SRP Update.
SRP registrars MUST check that the TTLs for all RRs within each RRset
contained within an SRP Update are the same. If they are not, the
SRP update MUST be rejected with a Refused RCODE.
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Additionally, when adding RRs to an RRset (for example, when
processing Service Discovery records), the SRP registrar MUST use the
same TTL on all RRs in the RRset. How this consistency is enforced
is up to the implementation.
TTLs sent in SRP Updates are advisory: they indicate the SRP
requester's guess as to what a good TTL would be. SRP registrars may
override these TTLs. SRP registrars SHOULD ensure that TTLs are
reasonable: neither too long nor too short. The TTL SHOULD NOT ever
be longer than the lease time (Section 5.1). Shorter TTLs will
result in more frequent data refreshes; this increases latency on the
DNS-SD client side, increases load on any caching resolvers and on
the authoritative DNS server, and also increases network load, which
may be an issue for CNNs. Longer TTLs will increase the likelihood
that data in caches will be stale. TTL minimums and maximums SHOULD
be configurable by the operator of the SRP registrar.
5. Maintenance
5.1. Cleaning Up Stale Data
Because the DNS-SD Service Registration Protocol is automatic and not
managed by humans, some additional bookkeeping is required. When an
update is constructed by the SRP requester, it MUST include an
EDNS(0) Update Lease Option [RFC9664]. The Update Lease Option
contains two lease times: the Lease Time and the KEY Lease Time.
Similar to DHCP leases [RFC2131], these leases are promises from the
SRP requester that it will send a new update for the service
registration before the lease time expires. The Lease time is chosen
to represent the duration after the update during which the
registered records other than the KEY record can be assumed to be
valid. The KEY lease time represents the duration after the update
during which the KEY record can be assumed to be valid. The
reasoning behind the different lease times is discussed in
Section 3.2.4.1 and Section 3.2.5.3.
SRP registrars may be configured with limits for these values. At
the time of writing, a default limit of two hours for the Lease and
14 days for the SIG(0) KEY are thought to be good choices. Devices
with limited battery that wake infrequently are likely to request
longer leases; registrars that support such devices may need to set
higher limits. SRP requesters that are going to continue to use
names on which they hold leases SHOULD refresh them well before the
lease ends in case the registrar is temporarily unavailable or under
heavy load.
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The lease time applies specifically to the hostname. All service
instances, and all service entries for such service instances, depend
on the hostname. When the lease on a hostname expires, the hostname
and all services that reference it MUST be removed at the same time:
it is never valid for a service instance to remain when the hostname
it references has been removed. If the KEY record for the hostname
is to remain, the KEY record for any services that reference it MUST
also remain. However, the Service Discovery PTR record MUST be
removed since it has no key associated with it and since it is never
valid to have a Service Discovery PTR record for which there is no
service instance on the target of the PTR record.
SRP registrars MUST also track a lease time per service instance.
The reason being that a requester may re-register a hostname with a
different set of services and not remember that some different
service instance had previously been registered. In this case, when
that service instance lease expires, the SRP registrar MUST remove
the service instance, and any associated Service Discovery PTR
records pointing to that service instance, (although the KEY record
for the service instance SHOULD be retained until the KEY lease on
that service expires). This is beneficial because it avoids stale
services continuing to be advertised after the SRP requester has
forgotten about them.
The SRP registrar MUST include an EDNS(0) Update Lease option in the
response. The requester MUST check for the EDNS(0) Update Lease
option in the response, and when deciding when to renew its
registration the requester MUST use the lease times from that
received option in place of the lease times that it originally
requested from the registrar. The times may be shorter or longer
than those specified in the SRP Update. The SRP requester must honor
them in either case.
SRP requesters SHOULD assume that each lease ends N seconds after the
update was first transmitted (where N is the granted lease duration).
SRP registrars SHOULD assume that each lease ends N seconds after the
update that was successfully processed was received. Because the
registrar will always receive the update after the SRP requester sent
it, this avoids the possibility of a race condition where the SRP
registrar prematurely removes a service when the SRP requester thinks
the lease has not yet expired. In addition, the SRP requester MUST
begin attempting to renew its lease in advance of the expected
expiration time, as required by the DNS Update Lease specification
[RFC9664], to accomodate the situation where the clocks on the SRP
requester and the SRP registrar to not run at precisely the same
rate.
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SRP registrars MUST reject updates that do not include an EDNS(0)
Update Lease option. DNS authoritative servers that allow both SRP
and non-SRP DNS updates MAY accept updates that don't include leases,
but they SHOULD differentiate between SRP Updates and other updates
and MUST reject updates that would otherwise be SRP Updates if they
do not include leases.
The function of Lease times and the function of TTLs are completely
different. On an authoritative DNS server, the TTL on a resource
record is a constant. Whenever that RR is served in a DNS response,
the TTL value sent in the answer is the same. The lease time is
never sent as a TTL; its sole purpose is to determine when the
authoritative DNS server will delete stale records. It is not an
error to send a DNS response with a TTL of M when the remaining time
on the lease is less than M.
6. Security Considerations
6.1. Source Validation
SRP Updates have no authorization semantics other than "First Come,
First Served" (FCFS). Thus, if an attacker from outside the
administrative domain of the SRP registrar knows the registrar's IP
address, it can, in principle, send updates to the registrar that
will be processed successfully. Therefore, SRP registrars SHOULD be
configured to reject updates from source addresses outside of the
administrative domain of the registrar.
For TCP updates, the initial SYN-SYN+ACK handshake prevents updates
being forged by an off-path attacker. In order to ensure that this
handshake happens, SRP registrars relying on three-way-handshake
validation MUST NOT accept TCP Fast Open payloads [RFC7413]. If the
network infrastructure allows it, an SRP registrar MAY accept TCP
Fast Open payloads if all such packets are validated along the path,
and the network is able to reject this type of spoofing at all
ingress points.
For UDP updates from CNN devices, spoofing would have to be prevented
with appropriate source address filtering on routers [RFC2827]. This
would ordinarily be accomplished by measures such as those described
in Section 4.5 of the IPv6 CE Router Requirements document [RFC7084].
For example, a stub router [SNAC-SIMPLE] for a CNN might only accept
UDP updates from source addresses known to be on-link on that stub
network and might further validate that the UDP update was actually
received on the stub network interface and not the interface
connected to the adjacent infrastructure link.
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6.2. Other DNS Updates
Note that these rules only apply to the validation of SRP Updates.
An authoritative DNS server that accepts updates from SRP requesters
may also accept other DNS Update messages, and those DNS Update
messages may be validated using different rules. However, in the
case of an authoritative DNS server that accepts SRP updates, the
intersection of the SRP Update rules and whatever other update rules
are present must be considered very carefully.
For example, a normal authenticated DNS update to any RR that was
added using SRP, but is authenticated using a different key, could be
used to override a promise made by the SRP registrar to an SRP
requester by replacing all or part of the service registration
information with information provided by an authenticated DNS update
requester. An implementation that allows both kinds of updates
SHOULD NOT allow DNS Update requesters that are using different
authentication and authorization credentials to update records added
by SRP requesters.
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 also used for
non-DNS-SD records. For example, imagine that you set up SRP service
for example.com. SRP requesters can now register names like "www" or
"mail" or "smtp" in this domain. In addition, SRP Updates using FCFS
Naming can insert names that are obscene or offensive into the zone.
There is no simple solution to these problems. However, we have two
recommendations to address this problem:
* Do not provide SRP service in organization-level zones. Use
subdomains of the organizational domain for DNS-SD. This does not
prevent registering names as mentioned above but does ensure that
genuinely important names are not accidentally claimed by SRP
requesters. So, for example, the zone "dnssd.example.com" could
be used instead of "example.com" for SRP Updates. Because of the
way that DNS-browsing domains are discovered, there is no need for
the DNS-SD discovery zone that is updated by SRP to have a user-
friendly or important-sounding name.
* Configure a dictionary of names that are prohibited. Dictionaries
of common obscene and offensive names are no doubt available and
can be augmented with a list of typical "special" names like
"www", "mail", "smtp", and so on. Lists of names are generally
available or can be constructed manually. Names rejected due to
this should return a Refused RCODE, indicating to the SRP
requester that it should not append or increment a number at the
end of the name and try again in an infinite loop. If a name is
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considered unacceptable because it is obscene or offensive, adding
a number on the end is unlikely to make the name become
acceptable.
6.4. Security of Local Service Discovery
Local links can be protected by managed services such as RA Guard
[RFC6105], but multicast services like DHCP [RFC2131] DHCPv6
[RFC8415], and IPv6 Neighbor Discovery [RFC4861] are, in most cases,
not authenticated and can't be controlled on unmanaged networks, such
as home networks and small office networks where no network
management staff are present. In such situations, the SRP service
has comparatively fewer potential security exposures and, hence, is
not the weak link. This is discussed in more detail in
Section 3.2.4.
The fundamental protection for networks of this type is the user's
choice of what devices to add to the network. Work is being done in
other working groups and standards bodies to improve the state of the
art for network on-boarding and device isolation (e.g., Manufacturer
Usage Descriptions [RFC8520] provide a means for constraining what
behaviors are allowed for a device in an automatic way), but such
work is out of scope for this document.
6.5. SRP Registrar Authentication
This specification does not provide a mechanism for validating
responses from SRP registrars to SRP requesters. In principle, a KEY
RR could be used by a non-CNN SRP requester to validate responses
from the registrar, but this is not required, nor do we specify a
mechanism for determining which key to use.
In addition, for DNS-over-TLS connections, out-of-band key pinning as
described in Section 4.2 of the DNS-over-TLS specification [RFC7858]
could be used for authentication of the SRP registrar, e.g., to
prevent man-in-the-middle attacks. However, the use of such keys is
impractical for an unmanaged service registration protocol; hence, it
is out of scope for this document.
6.6. Required Signature Algorithm
For validation, SRP registrars MUST implement the ECDSAP256SHA256
signature algorithm. SRP registrars SHOULD implement the algorithms
that are listed in Section 3.1 of the DNSSEC Cryptographic Algorithms
specification [RFC8624], in the validation column of the table, that
are numbered 13 or higher, and that have a "MUST", "RECOMMENDED", or
"MAY" designation in the validation column of the table. SRP
requesters MUST NOT assume that any algorithm numbered lower than 13
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is available for use in validating SIG(0) signatures.
7. Privacy Considerations
Because DNS-SD SRP Updates can be sent off-link, the privacy
implications of SRP are different from those for mDNS responses. SRP
Requester implementations that are using TCP SHOULD also use DNS-
over-TLS [RFC7858] if available. SRP registrar implementations MUST
offer TLS support. Because there is no mechanism for sharing keys,
validation of DNS-over-TLS keys is not possible; DNS-over-TLS is used
only for Opportunistic Privacy, as documented in Section 4.1 of the
DNS-over-TLS specification [RFC7858].
SRP requesters that are able to use TLS SHOULD NOT fall back to TCP.
Since all SRP registrars are required to support TLS, whether to use
TLS is entirely the decision of the SRP requester.
Public keys can be used as identifiers to track hosts. SRP
registrars MAY elect not to return KEY records for queries for SRP
registrations. To avoid DNSSEC validation failures, an SRP registrar
that signs the zone for DNSSEC but refuses to return a KEY record
MUST NOT store the KEY record in the zone itself. Because the KEY
record isn't in the zone, the nonexistence of the KEY record can be
validated. If the zone is not signed, the authoritative DNS server
MAY instead return a negative non-error response (either NXDOMAIN or
no data).
8. Domain Name Reservation Considerations
This section specifies considerations for systems involved in domain
name resolution when resolving queries for names ending with
".service.arpa.". Each item in this section addresses some aspect of
the DNS or the process of resolving domain names that would be
affected by this special-use allocation. Detailed explanations of
these items can be found in Section 5 of the Special-Use Domain Names
specification [RFC6761].
8.1. Users
The current proposed use for "service.arpa" does not require special
knowledge on the part of the user. While the "default.service.arpa."
subdomain is used as a generic name for registration, users are not
expected to see this name in user interfaces. In the event that it
does show up in a user interface, it is just a domain name and
requires no special treatment by the user.
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8.2. Application Software
Application software does not need to handle subdomains of
"service.arpa" specially. "service.arpa" SHOULD NOT be treated as
more trustworthy than any other insecure DNS domain, simply because
it is locally served (or for any other reason). It is not possible
to register a PKI certificate for a subdomain of "service.arpa."
because it is a locally served domain name. So, no such subdomain
can be considered to be uniquely identifying a particular host, as
would be required for such a PKI certificate to be issued. If a
subdomain of "service.arpa." is returned by an API or entered in an
input field of an application, PKI authentication of the endpoint
being identified by the name will not be possible. Alternative
methods and practices for authenticating such endpoints are out of
scope for this document.
8.3. Name Resolution APIs and Libraries
Name resolution APIs and libraries MUST NOT recognize names that end
in "service.arpa." as special and MUST NOT treat them as having
special significance, except that it may be necessary that such APIs
not bypass the locally discovered recursive resolvers.
One or more IP addresses for recursive resolvers will usually be
supplied to the SRP requester through router advertisements or DHCP.
For an administrative domain that uses subdomains of "service.arpa.",
the recursive resolvers provided by that domain will be able to
answer queries for subdomains of "service.arpa.". Other (non-local)
resolvers will not, or they will provide answers that are not correct
within that administrative domain.
A host that is configured to use a resolver other than one that has
been provided by the local network may not be able to resolve or may
receive incorrect results for subdomains of "service.arpa.". In
order to avoid this, hosts SHOULD use the resolvers that are locally
provided for resolving "service.arpa." names, even when they are
configured to use other resolvers for other names.
8.4. Recursive Resolvers
There are two considerations for recursive resolvers (also known as
"caching DNS servers" or "recursive DNS servers") that follow this
specification:
1. For correctness, recursive resolvers at sites using
'service.arpa.' must, in practice, transparently support DNSSEC
queries: queries for DNSSEC records and queries with the DNSSEC
OK (DO) bit set (Section 3.2.1 of the DNSSEC specification
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[RFC4035]). DNSSEC validation [RFC9364] is a best current
practice: although validation is not required, a caching
recursive resolver that does not validate answers that can be
validated may cache invalid data. In turn, this would prevent
validating stub resolvers from successfully validating answers.
Hence, as a practical matter, recursive resolvers at sites using
"service.arpa" should do DNSSEC validation.
2. Unless configured otherwise, recursive resolvers and DNS proxies
MUST behave following the rules prescribed for Iterative
Resolvers in Section 3 of the IETF Locally Served DNS Zones
document [RFC6303]. That is, queries for "service.arpa." and
subdomains of "service.arpa." MUST NOT be forwarded, with one
important exception: a query for a DS record with the DO bit set
MUST return the correct answer for that question, including
correct information in the authority section that proves that the
record is nonexistent.
So, for example, a query for the NS record for "service.arpa."
MUST NOT result in that query being forwarded to an upstream
cache nor to the authoritative DNS server for ".arpa.". However,
to provide accurate authority information, a query for the DS
record MUST result in forwarding whatever queries are necessary.
Typically, this will just be a query for the DS record since the
necessary authority information will be included in the authority
section of the response if the DO bit is set.
8.5. Authoritative DNS Servers
No special processing of "service.arpa." is required for
authoritative DNS server implementations. It is possible that an
authoritative DNS server might attempt to check the authoritative DNS
servers for "service.arpa." for a delegation beneath that name before
answering authoritatively for such a delegated name. In such a case,
because the name always has only local significance, there will be no
such delegation in the "service.arpa." zone; therefore, the
authoritative DNS server would refuse to answer authoritatively for
such a zone. An authoritative DNS server that implements this sort
of check MUST be configurable so that either it does not do this
check for the "service.arpa." domain or it ignores the results of the
check.
8.6. DNS Server Operators
DNS server operators MAY configure an authoritative DNS server for
"service.arpa." for use with SRP. The operator for the DNS servers
that are authoritative for "service.arpa." in the global DNS will
configure any such DNS servers as described in Section 9.
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8.7. DNS Registries/Registrars
"service.arpa." is a subdomain of the "arpa" top-level domain, which
is operated by IANA under the authority of the Internet Architecture
Board (IAB) [RFC3172]. There are no other DNS registrars for
".arpa".
9. Delegation of "service.arpa."
The owner of the 'arpa.' zone, at the time of writing the IAB
[IAB-ARPA], has added a delegation of 'service.arpa.' in the '.arpa.'
zone [RFC3172], following the guidance provided in Section 7 of the
'home.arpa.' specification [RFC8375].
10. IANA Considerations
10.1. Registration and Delegation of "service.arpa" as a Special-Use
Domain Name
IANA has recorded the domain name "service.arpa." in the "Special-Use
Domain Names" registry [SUDN]. IANA has implemented the delegation
requested in Section 9.
10.2. Addition of "service.arpa" to the Locally-Served Zones Registry
IANA has also added a new entry to the "Transport-Independent
Locally-Served Zones Registry" registry of the "Locally-Served DNS
Zones" group [LSDZ]. The entry is for the domain "SERVICE.ARPA" with
the description "DNS-SD Service Registration Protocol Special-Use
Domain" and lists this document as the reference.
10.3. Subdomains of "service.arpa."
This document only makes use of the "default.service.arpa" subdomain
of "service.arpa." Other subdomains are reserved for future use by
DNS-SD or related work. IANA has created the "service.arpa
Subdomain" registry [SUB]. The IETF has change control for this
registry. New entries may be added either as a result of Standards
Action or with IESG Approval, provided that a specification exists
[RFC8126].
IANA has grouped the "service.arpa Subdomain" registry with the
"Locally-Served DNS Zones" group. The registry is a table with three
columns: the subdomain name (expressed as a fully qualified domain
name), a brief description of how it is used, and a reference to the
document that describes its use in detail.
This initial contents of this registry are as follows:
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+=======================+=================+===========+
| Subdomain Name | Description | Reference |
+=======================+=================+===========+
| default.service.arpa. | Default domain | RFC 9665 |
| | for SRP Updates | |
+-----------------------+-----------------+-----------+
Table 1
10.4. Service Name Registrations
IANA has added two new entries to the "Service Name and Transport
Protocol Port Number Registry" [PORT]. The following subsections
contain tables with the fields required by Section 8.1.1 of IANA's
Procedures for Service Name allocation [RFC6335].
10.4.1. 'dnssd-srp' Service Name
+====================+=============================+
| Field Name | Value |
+====================+=============================+
| Service Name | dnssd-srp |
+--------------------+-----------------------------+
| Transport Protocol | tcp |
+--------------------+-----------------------------+
| Assignee | IESG <iesg@ietf.org> |
+--------------------+-----------------------------+
| Contact | IETF Chair <chair@ietf.org> |
+--------------------+-----------------------------+
| Description | DNS-SD Service Discovery |
+--------------------+-----------------------------+
| Reference | RFC 9665 |
+--------------------+-----------------------------+
| Port Number | None |
+--------------------+-----------------------------+
| Service Code | None |
+--------------------+-----------------------------+
Table 2
10.4.2. 'dnssd-srp-tls' Service Name
+====================+================================+
| Field Name | Value |
+====================+================================+
| Service Name | dnssd-srp-tls |
+--------------------+--------------------------------+
| Transport Protocol | tcp |
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+--------------------+--------------------------------+
| Assignee | IESG <iesg@ietf.org> |
+--------------------+--------------------------------+
| Contact | IETF Chair <chair@ietf.org> |
+--------------------+--------------------------------+
| Description | DNS-SD Service Discovery (TLS) |
+--------------------+--------------------------------+
| Reference | RFC 9665 |
+--------------------+--------------------------------+
| Port Number | None |
+--------------------+--------------------------------+
| Service Code | None |
+--------------------+--------------------------------+
Table 3
10.5. Anycast Address
IANA has allocated an IPv6 anycast address from the "IANA IPv6
Special-Purpose Address Registry" [IPv6], similar to the Port Control
Protocol [RFC6887] anycast address [RFC7723]. The purpose of this
allocation is to provide a fixed anycast address that can be commonly
used as a destination for SRP Updates when no SRP registrar is
explicitly configured. The initial values for the registry are as
follows:
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+======================+=============================+
| Attribute | Value |
+======================+=============================+
| Address Block | 2001:1::3/128 |
+----------------------+-----------------------------+
| Name | DNS-SD Service Registration |
| | Protocol Anycast Address |
+----------------------+-----------------------------+
| RFC | RFC 9665 |
+----------------------+-----------------------------+
| Allocation Date | 2024-04 |
+----------------------+-----------------------------+
| Termination Date | N/A |
+----------------------+-----------------------------+
| Source | True |
+----------------------+-----------------------------+
| Destination | True |
+----------------------+-----------------------------+
| Forwardable | True |
+----------------------+-----------------------------+
| Globally Reachable | True |
+----------------------+-----------------------------+
| Reserved-by-Protocol | False |
+----------------------+-----------------------------+
Table 4
11. References
11.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
<https://www.rfc-editor.org/info/rfc1536>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC2539] Eastlake 3rd, D., "Storage of Diffie-Hellman Keys in the
Domain Name System (DNS)", RFC 2539, DOI 10.17487/RFC2539,
March 1999, <https://www.rfc-editor.org/info/rfc2539>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
2000, <https://www.rfc-editor.org/info/rfc2931>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
RFC 6303, DOI 10.17487/RFC6303, July 2011,
<https://www.rfc-editor.org/info/rfc6303>.
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[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8375] Pfister, P. and T. Lemon, "Special-Use Domain
'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
<https://www.rfc-editor.org/info/rfc8375>.
[RFC8624] Wouters, P. and O. Sury, "Algorithm Implementation
Requirements and Usage Guidance for DNSSEC", RFC 8624,
DOI 10.17487/RFC8624, June 2019,
<https://www.rfc-editor.org/info/rfc8624>.
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/info/rfc8765>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/info/rfc9364>.
[RFC9664] Cheshire, S. and T. Lemon, "An EDNS(0) Option to Negotiate
Leases on DNS Updates", RFC 9664, DOI 10.17487/RFC9664,
October 2024, <https://www.rfc-editor.org/info/rfc9664>.
11.2. Informative References
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[IAB-ARPA] "Internet Architecture Board statement on the registration
of special use names in the ARPA domain", March 2017,
<https://www.iab.org/documents/correspondence-reports-
documents/2017-2/iab-statement-on-the-registration-of-
special-use-names-in-the-arpa-domain/>.
[IPv6] IANA, "IANA IPv6 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv6-special-
registry>.
[LSDZ] IANA, "Locally-Served DNS Zones",
<https://www.iana.org/assignments/locally-served-dns-
zones>.
[PORT] IANA, "Service Name and Transport Protocol Port Number
Registry", <https://www.iana.org/assignments/service-
names-port-numbers>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
<https://www.rfc-editor.org/info/rfc3007>.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<https://www.rfc-editor.org/info/rfc3927>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
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[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
DOI 10.17487/RFC6105, February 2011,
<https://www.rfc-editor.org/info/rfc6105>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6760] Cheshire, S. and M. Krochmal, "Requirements for a Protocol
to Replace the AppleTalk Name Binding Protocol (NBP)",
RFC 6760, DOI 10.17487/RFC6760, February 2013,
<https://www.rfc-editor.org/info/rfc6760>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC7723] Kiesel, S. and R. Penno, "Port Control Protocol (PCP)
Anycast Addresses", RFC 7723, DOI 10.17487/RFC7723,
January 2016, <https://www.rfc-editor.org/info/rfc7723>.
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[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>.
[RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
2020, <https://www.rfc-editor.org/info/rfc8766>.
[RFC8945] Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
Gudmundsson, O., and B. Wellington, "Secret Key
Transaction Authentication for DNS (TSIG)", STD 93,
RFC 8945, DOI 10.17487/RFC8945, November 2020,
<https://www.rfc-editor.org/info/rfc8945>.
[ROADMAP] Cheshire, S., "Service Discovery Road Map", Work in
Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
23 October 2018, <https://datatracker.ietf.org/doc/html/
draft-cheshire-dnssd-roadmap-03>.
[SNAC-SIMPLE]
Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-06, 4 November
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
snac-simple-06>.
[SUB] IANA, "service.arpa Subdomain",
<https://www.iana.org/assignments/locally-served-dns-
zones/locally-served-dns-zones>.
[SUDN] IANA, "Special-Use Domain Names",
<https://www.iana.org/assignments/special-use-domain-
names>.
[ZC] Steinberg, D.H. and S. Cheshire, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc.,
ISBN 9780596101008, December 2005.
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Appendix A. Using Standard Authoritative DNS Servers Compliant with RFC
2136 to test SRP requesters
For testing it may be useful to set up an authoritative DNS server
that does not implement SRP. This can be done by configuring the
authoritative DNS server to listen on the anycast address or by
advertising it in the "_dnssd-srp._tcp.<zone>" and
"_dnssd-srp-tls._tcp.<zone>" SRV records. It must be configured to
be authoritative for "default.service.arpa" and to accept updates
from hosts on local networks for names under "default.service.arpa"
without authentication since such authoritative DNS servers will not
have support for FCFS authentication (Section 3.2.4.1).
An authoritative DNS server configured in this way will be able to
successfully accept and process SRP Updates from requesters that send
SRP updates. However, no prerequisites will be applied; this means
that the test authoritative DNS server will accept internally
inconsistent SRP Updates and will not stop two SRP Updates sent by
different services that claim the same name or names from overwriting
each other.
Since SRP Updates are signed with keys, validation of the SIG(0)
algorithm used by the requester can be done by manually installing
the requester's public key on the authoritative DNS server that will
be receiving the updates. The key can then be used to authenticate
the SRP Update and can be used as a requirement for the update. An
example configuration for testing SRP using BIND 9 is given in
Appendix C.
Appendix B. How to Allow SRP Requesters to Update Standard Servers
Compliant with RFC 2136
Ordinarily, CNN SRP Updates sent to an authoritative DNS server that
implements standard DNS Update [RFC2136] but not SRP will fail
because the zone being updated is "default.service.arpa" and because
no authoritative DNS server that is not an SRP registrar would
normally be configured to be authoritative for
"default.service.arpa". Therefore, a requester that sends an SRP
Update can tell that the receiving authoritative DNS server does not
support SRP but does support standard DNS Update [RFC2136] because
the RCODE will either be NotZone, NotAuth, or Refused or because
there is no response to the update request (when using the anycast
address).
In this case, a requester MAY attempt to register itself using normal
DNS updates [RFC2136]. To do so, it must discover the default
registration zone and the authoritative DNS server designated to
receive updates for that zone, as described earlier, using the
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_dns-update._udp SRV record. It can then send the update to the port
and host pointed to by the SRV record, and it is expected to use
appropriate prerequisites to avoid overwriting competing records.
Such updates are out of scope for SRP, and a requester that
implements SRP MUST first attempt to use SRP to register itself and
only attempt to use backwards capability with normal DNS Update
[RFC2136] if that fails. Although the owner name of the SRV record
for DNS Update (_dns-update._udp) specifies UDP, it is also possible
to use TCP, and TCP SHOULD be required to prevent spoofing.
Appendix C. Sample BIND 9 Configuration for "default.service.arpa."
zone "default.service.arpa." {
type primary;
file "/etc/bind/primary/service.db";
allow-update { key demo.default.service.arpa.; };
};
Figure 1: Zone Configuration in named.conf
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$TTL 57600 ; 16 hours
@ IN SOA ns postmaster (
2951053287 ; serial
3600 ; refresh (1 hour)
1800 ; retry (30 minutes)
604800 ; expire (1 week)
3600 ; minimum (1 hour)
)
NS ns
ns AAAA 2001:db8:0:2::1
$TTL 3600 ; 1 hour
; Autoconguration bootstrap records
_dnssd-srp._tcp SRV 0 0 53 ns
_dnssd-srp-tls._tcp SRV 0 0 853 ns
; Service Discovery Instruction
_ipps._tcp PTR demo._ipps._tcp
; Service Description Instruction
demo._ipps._tcp SRV 0 0 631 demohost
TXT ""
; Host Description Instruction
demohost AAAA 2001:db8:0:2::2
KEY 0 3 13 (
qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
); alg = ECDSAP256SHA256 ; key id = 15008
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
requester proof-of-concept implementation at the Montreal Hackathon
at IETF 102, and Tom Pusateri for reviewing during the hackathon and
afterwards. Thanks to Esko for a really thorough second Last Call
review. Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
Dong, Martin Turon, and Michael Cowan for their detailed second last
call reviews. Thanks to Patrik Fältström, Dhruv Dhody, David Dong,
Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective
directorate reviews. Thanks to Paul Wouters for a _really_ detailed
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IESG review! Thanks also to the other IESG members who provided
comments or simply took the time to review the document.
Authors' Addresses
Ted Lemon
Apple Inc.
One Apple Park Way
Cupertino, CA 95014
United States of America
Email: mellon@fugue.com
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, CA 95014
United States of America
Phone: +1 408 974 3207
Email: cheshire@apple.com
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