Internet DRAFT - draft-ietf-dnssd-srp
draft-ietf-dnssd-srp
Internet Engineering Task Force T. Lemon
Internet-Draft S. Cheshire
Intended status: Standards Track Apple Inc.
Expires: 5 September 2024 4 March 2024
Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-25
Abstract
The Service Registration Protocol for DNS-Based Service Discovery
uses the standard DNS Update mechanism to enable DNS-Based Service
Discovery using only unicast packets. This makes it possible to
deploy DNS Service Discovery without multicast, which greatly
improves scalability and improves performance on networks where
multicast service is not an optimal choice, particularly IEEE 802.11
(Wi-Fi) and IEEE 802.15.4 networks. DNS-SD Service registration uses
public keys and SIG(0) to allow services to defend their
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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This Internet-Draft will expire on 5 September 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology Used in This Document . . . . . . 6
3. Service Registration Protocol . . . . . . . . . . . . . . . . 6
3.1. Protocol Variants . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Full-featured Hosts . . . . . . . . . . . . . . . . . 7
3.1.2. Constrained Hosts . . . . . . . . . . . . . . . . . . 7
3.1.3. Why two variants? . . . . . . . . . . . . . . . . . . 8
3.2. Protocol Details . . . . . . . . . . . . . . . . . . . . 8
3.2.1. What to publish . . . . . . . . . . . . . . . . . . . 8
3.2.2. Where to publish it . . . . . . . . . . . . . . . . . 9
3.2.3. How to publish it . . . . . . . . . . . . . . . . . . 10
3.2.3.1. How the DNS-SD Service Registration process differs
from DNS Update as specified in RFC2136 . . . . . . 10
3.2.3.2. Retransmission Strategy . . . . . . . . . . . . . 11
3.2.3.3. Successive Updates . . . . . . . . . . . . . . . 11
3.2.4. How to secure it . . . . . . . . . . . . . . . . . . 11
3.2.4.1. First-Come First-Served Naming . . . . . . . . . 11
3.2.5. SRP Requestor Behavior . . . . . . . . . . . . . . . 12
3.2.5.1. Public/Private key pair generation and storage . 12
3.2.5.2. Name Conflict Handling . . . . . . . . . . . . . 13
3.2.5.3. Record Lifetimes . . . . . . . . . . . . . . . . 13
3.2.5.4. Compression in SRV records . . . . . . . . . . . 13
3.2.5.5. Removing published services . . . . . . . . . . . 14
3.3. Validation and Processing of SRP Updates . . . . . . . . 15
3.3.1. Validation of DNS Update Add and Delete RRs . . . . . 15
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3.3.1.1. Service Discovery Instruction . . . . . . . . . . 16
3.3.1.2. Service Description Instruction . . . . . . . . . 17
3.3.1.3. Host Description Instruction . . . . . . . . . . 17
3.3.2. Valid SRP Update Requirements . . . . . . . . . . . . 18
3.3.3. FCFS Name And Signature Validation . . . . . . . . . 18
3.3.4. Handling of Service Subtypes . . . . . . . . . . . . 19
3.3.5. SRP Update response . . . . . . . . . . . . . . . . . 20
3.3.6. Optional Behavior . . . . . . . . . . . . . . . . . . 20
4. TTL Consistency . . . . . . . . . . . . . . . . . . . . . . . 21
5. Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Cleaning up stale data . . . . . . . . . . . . . . . . . 22
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6.1. Source Validation . . . . . . . . . . . . . . . . . . . . 24
6.2. Other DNS updates . . . . . . . . . . . . . . . . . . . . 24
6.3. Risks of allowing arbitrary names to be registered in SRP
updates . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.4. Security of local service discovery . . . . . . . . . . . 25
6.5. SRP Registrar Authentication . . . . . . . . . . . . . . 26
6.6. Required Signature Algorithm . . . . . . . . . . . . . . 26
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 26
8. Domain Name Reservation Considerations . . . . . . . . . . . 27
8.1. Users . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.2. Application Software . . . . . . . . . . . . . . . . . . 27
8.3. Name Resolution APIs and Libraries . . . . . . . . . . . 27
8.4. Caching DNS Servers . . . . . . . . . . . . . . . . . . . 28
8.5. Authoritative DNS Servers . . . . . . . . . . . . . . . . 29
8.6. DNS Server Operators . . . . . . . . . . . . . . . . . . 29
8.7. DNS Registries/Registrars . . . . . . . . . . . . . . . . 29
9. Delegation of 'service.arpa.' . . . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10.1. Registration and Delegation of 'service.arpa' as a
Special-Use Domain Name . . . . . . . . . . . . . . . . 30
10.2. Subdomains of 'service.arpa.' . . . . . . . . . . . . . 30
10.3. Service Name registrations . . . . . . . . . . . . . . . 30
10.4. 'dnssd-srp' Service Name . . . . . . . . . . . . . . . . 31
10.5. 'dnssd-srp-tls' Service Name . . . . . . . . . . . . . . 31
10.6. Anycast Address . . . . . . . . . . . . . . . . . . . . 32
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 32
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
13. Normative References . . . . . . . . . . . . . . . . . . . . 33
14. Informative References . . . . . . . . . . . . . . . . . . . 36
Appendix A. Testing using standard RFC2136-compliant DNS
servers . . . . . . . . . . . . . . . . . . . . . . . . . 38
Appendix B. How to allow SRP requestors to update standard
RFC2136-compliant servers . . . . . . . . . . . . . . . . 39
Appendix C. Sample BIND9 configuration for
default.service.arpa. . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
DNS-Based Service Discovery [RFC6763] is a component of Zero
Configuration Networking [RFC6760] [ZC] [ROADMAP].
This document describes an enhancement to DNS-Based Service Discovery
[RFC6763] (DNS-SD) that allows servers to register the services they
offer using the DNS protocol rather than using Multicast DNS
[RFC6762] (mDNS). There is already a large installed base of DNS-SD
clients that can discover services using the DNS protocol (e.g.
Android, Windows, Linux, Apple).
This document is intended for three audiences: implementors of
software that provides services that should be advertised using
DNS-SD, implementors of DNS servers that will be used in contexts
where DNS-SD registration is needed, and administrators of networks
where DNS-SD service is required. The document is expected to
provide sufficient information to allow interoperable implementation
of the registration protocol.
DNS-Based Service Discovery (DNS-SD) allows services to advertise the
fact that they provide service, and to provide the information
required to access that service. DNS-SD clients can then discover
the set of services of a particular type that are available. They
can then select a service from among those that are available and
obtain the information required to use it. Although DNS Service
Discovery (DNS-SD) using the DNS protocol (as opposed to mDNS) can be
more efficient and versatile, it is not common in practice, because
of the difficulties associated with updating authoritative DNS
services with service information.
Existing practice for updating DNS zones is to either manually enter
new data, or else use DNS Update [RFC2136]. Unfortunately DNS Update
requires either that the authoritative DNS server automatically trust
updates, or else that the DNS Update requestor have some kind of
shared secret or public key that is known to the DNS server and can
be used to authenticate the update. Furthermore, DNS Update can be a
fairly chatty process, requiring multiple round trips with different
conditional predicates to complete the update process.
The Service Registration Protocol (SRP) adds a set of default
heuristics for processing DNS updates that eliminates the need for
DNS update conditional predicates: instead, the SRP registrar (a DNS
server that supports SRP updates) has a set of default predicates
that are applied to the update, and the update either succeeds
entirely, or fails in a way that allows the requestor to know what
went wrong and construct a new update.
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SRP also adds a feature called First-Come, First-Served (FCFS)
Naming, which allows the requestor to claim a name that is not yet in
use, and, using SIG(0) [RFC2931], to authenticate both the initial
claim and subsequent updates. This prevents name conflicts, since a
second SRP requestor attempting to claim the same name will not
possess the SIG(0) key used by the first requestor to claim it, and
so its claim will be rejected and the second requestor will have to
choose a new name.
It is important to understand that "authenticate" here just means
that we can tell that an update came from the same source as the
original registration. We have not established trust. This has
important implications for what we can and can't do with data the
client sends us. You will notice as you read this document that we
only support adding a very restricted set of records, and the content
of those records is further constrained.
The reason for this is precisely that we have not established trust.
So we can only publish information that we feel safe in publishing
even though we do not have any basis for trusting the requestor. We
reason that mDNS [RFC6762] allows arbitrary hosts on a single IP link
to advertise services [RFC6763], relying on whatever service is
advertised to provide authentication as a part of its protocol rather
than in the service advertisement.
This is considered reasonably safe because it requires physical
presence on the network in order to advertise. An off-network mDNS
attack is simply not possible. Our goal with this specification is
to impose similar constraints. Because of this you will see in
Section 3.3.1 that a very restricted set of records with a very
restricted set of relationships are allowed. You will also see in
Section 6.1 that we give advice on how to prevent off-network
attacks.
This leads us to the disappointing observation that this protocol is
not a mechanism for adding arbitrary information to DNS zones. We
have not evaluated the security properties of adding, for example, an
SOA record, an MX record, or a CNAME record, and so these are
forbidden. A future protocol specification might include analyses
for other records, and extend the set of records that can be
registered here. Or it might require establishment of trust, and add
an authorization model to the authentication model we now have. But
this is work for a future document.
Finally, SRP adds the concept of a 'lease,' similar to leases in
Dynamic Host Configuration Protocol [RFC8415]. The SRP registration
itself has a lease which may be on the order of an hour; if the
requestor does not renew the lease before it has elapsed, the
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registration is removed. The claim on the name can have a longer
lease, so that another requestor cannot claim the name, even though
the registration has expired.
The Service Registration Protocol for DNS-SD (SRP), specified in this
document, provides a reasonably secure mechanism for publishing this
information. Once published, these services can be readily
discovered by DNS-SD clients using standard DNS lookups.
The DNS-SD specification ([RFC6763], Section 10, “Populating the DNS
with Information”), briefly discusses ways that servers can publish
their information in the DNS namespace. In the case of mDNS, it
allows servers to publish their information on the local link, using
names in the ".local" namespace, which makes their services directly
discoverable by peers attached to that same local link.
RFC6763 also allows clients to discover services using the DNS
protocol [RFC1035]. This can be done by having a system
administrator manually configure service information in the DNS, but
manually populating DNS authoritative server databases is costly and
potentially error-prone, and requires a knowledgeable network
administrator. Consequently, although all DNS-SD client
implementations of which we are aware support DNS-SD using DNS
queries, in practice it is used much less frequently than mDNS.
The Discovery Proxy [RFC8766] provides one way to automatically
populate the DNS namespace, but is only appropriate on networks where
services are easily advertised using mDNS. This document describes a
solution more suitable for networks where multicast is inefficient,
or where sleepy devices are common, by supporting both offering of
services, and discovery of services, using unicast.
2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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] [RFC3007] to
publish service information in the DNS. Two variants exist, one for
full-featured hosts, and one for devices designed for "Constrained-
Node Networks" [RFC7228]. An SRP registrar is most likely an
authoritative DNS server, or else is updating an authoritative DNS
server. There is no requirement that the server that is receiving
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SRP updates be the same server that is answering queries that return
records that have been registered.
3.1. Protocol Variants
3.1.1. Full-featured Hosts
Full-featured hosts either are configured manually with a
registration domain, or discover the default registration domain as
described in Section 11 of [RFC6763]. If this process does not
produce a default registration domain, the Service Registration
protocol is not discoverable on the local network using this
mechanism. Other discovery mechanisms are possible, but are out of
scope for this document.
Manual configuration of the registration domain can be done either by
querying the list of available registration domains
("r._dns-sd._udp") and allowing the user to select one from the UI,
or by any other means appropriate to the particular use case being
addressed. Full-featured devices construct the names of the SRV,
TXT, and PTR records describing their service(s) as subdomains of the
chosen service registration domain. For these names they then
discover the zone apex of the closest enclosing DNS zone using SOA
queries Section 6.1 of [RFC8765]. Having discovered the enclosing
DNS zone, they query for the "_dnssd-srp._tcp.<zone>" SRV record to
discover the server to which they can send SRP updates. Hosts that
support SRP Updates using TLS use the "_dnssd-srp-tls._tcp.<zone>"
SRV record instead.
Examples of full-featured hosts include devices such as home
computers, laptops, powered peripherals with network connections such
as printers, home routers, and even battery-operated devices such as
mobile phones that have long battery lives.
3.1.2. Constrained Hosts
For devices designed for Constrained-Node Networks [RFC7228] some
simplifications are available. Instead of being configured with (or
discovering) the service registration domain, the special-use domain
name (see [RFC6761]) "default.service.arpa" is used. The details of
how SRP registrar(s) are discovered will be specific to the
constrained network, and therefore we do not suggest a specific
mechanism here.
SRP requestors on constrained networks are expected to receive from
the network a list of SRP registrars with which to register. It is
the responsibility of a Constrained-Node Network supporting SRP to
provide one or more registrar addresses. It is the responsibility of
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the registrar supporting a Constrained-Node Network to handle the
updates appropriately. In some network environments, updates may be
accepted directly into a local "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.
3.1.3. Why two variants?
The reason for these different variants is that low-power devices
that typically use Constrained-Node Networks may have very limited
battery storage. The series of DNS lookups required to discover an
SRP registrar and then communicate with it will increase the energy
required to advertise a service; for low-power devices, the
additional flexibility this provides does not justify the additional
use of energy. It is also fairly typical of such networks that some
network service information is obtained as part of the process of
joining the network, and so this can be relied upon to provide nodes
with the information they need.
Networks that are not constrained networks can have more complicated
topologies at the IP layer. Nodes connected to such networks can be
assumed to be able to do DNS-SD service registration domain
discovery. Such networks are generally able to provide registration
domain discovery and routing. This creates the possibility of off-
network spoofing, where a device from a foreign network registers a
service on the local network in order to attack devices on the local
network. To prevent such spoofing, TCP is required for such
networks.
3.2. Protocol Details
We will discuss several parts to this process: how to know what to
publish, how to know where to publish it (under what name), how to
publish it, and how to secure its publication. In Section 5, we
specify how to maintain the information once published.
3.2.1. What to publish
SRP Updates are sent by SRP requestors to SRP registrars. Three
types of instructions appear in an SRP update: Service Discovery
instructions, Service Description instructions, and Host Description
instructions. These instructions are made up of DNS Update RRs that
are either adds or deletes. The types of records that are added,
updated and removed in each of these instructions, as well as the
constraints that apply to them, are described in Section 3.3. An SRP
Update is a DNS Update message that is constructed so as to meet the
constraints described in that section. The following is a brief
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overview of what is included in a typical SRP Update:
* PTR Resource Record (RR) for services, which map from a generic
service type (or subtype) name to a specific Service Instance
Name.
* For any Service Instance Name ([RFC6763], Section 4.1), an SRV RR,
one or more TXT RRs, and a KEY RR. Although in principle DNS-SD
Service Description records can include other record types with
the same Service Instance Name, in practice they rarely do. SRP
does not permit other record types. The KEY RR is used to support
FCFS naming, and has no specific meaning for DNS-SD lookups. SRV
records for all services described in an SRP update point to the
same hostname.
* There is never more than one hostname in a single SRP update. The
hostname has one or more address RRs (AAAA or A) and a KEY RR
(used for FCFS naming). Depending on the use case, an SRP
requestor may be required to suppress some addresses that would
not be usable by hosts discovering the service through the SRP
registrar. The exact address record suppression behavior required
may vary for different types of SRP requestors. An example of
such advice can be found in Section 5.5.2 of [RFC8766].
[RFC6763] describes the details of what each of these types of RR
mean, with the exception of the KEY RR, which is defined in
[RFC2539]. These RFCs should be considered the definitive source for
information about what to publish; the reason for summarizing this
here is to provide the reader with enough information about what will
be published that the service registration process can be understood
at a high level without first learning the full details of DNS-SD.
Also, the "Service Instance Name" is an important aspect of FCFS
naming, which we describe later on in this document.
3.2.2. Where to publish it
Multicast DNS uses a single namespace, ".local", which is valid on
the local link. This convenience is not available for DNS-SD using
the DNS protocol: services must exist in some specific DNS namespace
that is chosen either by the network operator, or automatically.
As described above, full-featured devices are responsible for knowing
the domain in which to register their services. Such devices MAY
optionally support configuration of a registration domain by the
operator of the device. However, such devices MUST support
registration domain discovery as described in Section 11 of
[RFC6763], "Discovery of Browsing and Registration Domains".
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Devices made for Constrained-Node Networks register in the special
use domain name [RFC6761] "default.service.arpa", and let the SRP
registrar handle rewriting that to a different domain if necessary.
3.2.3. How to publish it
It is possible to issue a DNS Update that does several things at
once; this means that it's possible to do all the work of adding a
PTR resource record to the PTR RRset on the Service Name, and
creating or updating the Service Instance Name and Host Description,
in a single transaction.
An SRP Update takes advantage of this: it is implemented as a single
DNS Update message that contains a service's Service Discovery
records, Service Description records, and Host Description records.
Updates done according to this specification are somewhat different
than regular DNS Updates as defined in [RFC2136]. The [RFC2136]
update process can involve many update attempts: you might first
attempt to add a name if it doesn't exist; if that fails, then in a
second message you might update the name if it does exist but matches
certain preconditions. Because the registration protocol uses a
single transaction, some of this adaptability is lost.
In order to allow updates to happen in a single transaction, SRP
Updates do not include update prerequisites. The requirements
specified in Section 3.3 are implicit in the processing of SRP
Updates, and so there is no need for the SRP requestor to put in any
explicit prerequisites.
3.2.3.1. How the DNS-SD Service Registration process differs from DNS
Update as specified in RFC2136
DNS-SD Service Registration is based on standard RFC2136 DNS Update,
with some differences:
* It implements first-come first-served name allocation, protected
using SIG(0) [RFC2931].
* It enforces policy about what updates are allowed.
* It optionally performs rewriting of "default.service.arpa" to some
other domain.
* It optionally performs automatic population of the address-to-name
reverse mapping domains.
* An SRP registrar is not required to implement general DNS Update
prerequisite processing.
* Constrained-Node SRP requestors 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 [RFC1035] provides some
guidance on this topic, as does Section 1 of [RFC1536].
Section 3.1.3 of [RFC8085] also provides useful guidance that is
particularly relevant to DNS.
3.2.3.3. Successive Updates
Service Registration Protocol does not require that every update
contain the same information. When an SRP requestor needs to send
more than one SRP update to the SRP registrar, it MUST send these
sequentially: until an earlier update has been successfully
acknowledged, the requestor MUST NOT begin sending a subsequent
update.
3.2.4. How to secure it
DNS update as described in [RFC2136] is secured using Secret Key
Transaction Signatures, [RFC8945], which uses a secret key shared
between the DNS Update requestor (which issues the update) and the
server (which authenticates it). This model does not work for
automatic service registration.
The goal of securing the DNS-SD Registration Protocol is to provide
the best possible security given the constraint that service
registration has to be automatic. It is possible to layer more
operational security on top of what we describe here, but FCFS naming
is already an improvement over the security of mDNS.
3.2.4.1. First-Come First-Served Naming
First-Come First-Serve naming provides a limited degree of security:
a server that registers its service using DNS-SD Registration
protocol is given ownership of a name for an extended period of time
based on a lease specific to the key used to authenticate the DNS
Update, which may be longer than the lease associated with the
registered records. As long as the registration service remembers
the name and the key used to register that name, no other server can
add or update the information associated with that. If the server
fails to renew its service registration before the KEY lease
(Section 4 of [I-D.ietf-dnssd-update-lease]) expires, its name is no
longer protected. FCFS naming is used to protect both the Service
Description and the Host Description.
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3.2.5. SRP Requestor Behavior
3.2.5.1. Public/Private key pair generation and storage
The requestor generates a public/private key pair (See Section 6.6).
This key pair MUST be stored in stable storage; if there is no
writable stable storage on the SRP requestor, the SRP requestor MUST
be pre-configured with a public/private key pair in read-only storage
that can be used. This key pair MUST be unique to the device. A
device with rewritable storage SHOULD retain this key indefinitely.
When the device changes ownership, it may be appropriate for the
former owner to erase the old key pair, which would then require the
new owner to install a new one. Therefore, the SRP requestor on the
device SHOULD provide a mechanism to erase the key, for example as
the result of a "factory reset," and to generate a new key.
The policy described here for managing keys assumes that the keys are
only used for SRP. If a key that is used for SRP is also used for
other purposes, the policy described here is likely to be
insufficient. The policy stated here is NOT RECOMMENDED in such a
situation: a policy appropriate to the full set of uses for the key
must be chosen. Specifying such a policy is out of scope for this
document.
When sending DNS updates, the requestor includes a KEY record
containing the public portion of the key in each Host Description
Instruction and each Service Description Instruction. Each KEY
record MUST contain the same public key. The update is signed using
SIG(0), using the private key that corresponds to the public key in
the KEY record. The lifetimes of the records in the update is set
using the EDNS(0) Update Lease option [I-D.ietf-dnssd-update-lease].
The format of the KEY resource record in the SRP Update is defined in
[RFC3445]. Because the KEY RR used in TSIG is not a zone-signing
key, the flags field in the KEY RR MUST be all zeroes.
The KEY record in Service Description updates MAY be omitted for
brevity; if it is omitted, the SRP registrar MUST behave as if the
same KEY record that is given for the Host Description is also given
for each Service Description for which no KEY record is provided.
Omitted KEY records are not used when computing the SIG(0) signature.
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3.2.5.2. Name Conflict Handling
Both Host Description RR adds and Service Description RR adds can
have names that result in name conflicts. Service Discovery record
adds cannot have name conflicts. If any Host Description or Service
Description record is found by the SRP registrar to have a conflict
with an existing name, the registrar will respond to the SRP Update
with a YXDomain RCODE (Section 2.2 of [RFC2136]). In this case, the
requestor MUST choose a new name or give up.
There is no specific requirement for how this is done; typically,
however, the requestor will append a number to the preferred name.
This number could be sequentially increasing, or could be chosen
randomly. One existing implementation attempts several sequential
numbers before choosing randomly. So for instance, it might try
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. This means that if a device is
disconnected from the network, it does not appear in the user
interfaces of devices looking for services of that type for too long.
The lifetime of the KEY records is set using the KEY-LEASE field of
the Update Lease Option, and SHOULD be set to a much longer time,
typically 14 days. The result of this is that even though a device
may be temporarily unplugged, disappearing from the network for a few
days, it makes a claim on its name that lasts much longer.
This means that even if a device is unplugged from the network for a
few days, and its services are not available for that time, no other
device can come along and claim its name the moment it disappears
from the network. In the event that a device is unplugged from the
network and permanently discarded, then its name is eventually
cleaned up and made available for re-use.
3.2.5.4. Compression in SRV records
Although [RFC2782] requires that the target name in the SRV record
not be compressed, an SRP requestor MAY compress the target in the
SRV record. The motivation for _not_ compressing in [RFC2782] is not
stated, but is assumed to be because a caching resolver that does not
understand the format of the SRV record might store it as binary data
and thus return an invalid pointer in response to a query. This does
not apply in the case of SRP: an SRP registrar needs to understand
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SRV records in order to validate the SRP Update. Compression of the
target can save space in the SRP Update, so we want clients to be
able to assume that the registrar will handle this. Therefore, SRP
registrars MUST support compression of SRV RR targets.
Note that this does not update [RFC2782]: DNS servers still MUST NOT
compress SRV record targets. The requirement to accept compressed
SRV records in updates only applies to SRP registrars, and SRP
registrars that are also DNS servers still MUST NOT compress SRV
record targets in DNS responses. We note also that [RFC6762]
recomments that SRV records be compressed in mDNS messages, so
[RFC2782] does not apply to mDNS messages.
In addition, we note that an implementor of an SRP requestor might
update existing code that creates SRV records or compresses DNS
messages so that it compresses the target of an SRV record. Care
must be taken if such code is used both in requestors and in DNS
servers that the code only compresses in the case where a requestor
is generating an SRP update.
3.2.5.5. Removing published services
3.2.5.5.1. Removing all published services
To remove all the services registered to a particular host, the SRP
requestor transmits an SRP update for that host with an Update Lease
option that has a LEASE value of zero. If the registration is to be
permanently removed, KEY-LEASE SHOULD also be zero. Otherwise, it
SHOULD be set to the same value it had previously; this holds the
name in reserve for when the SRP requestor is once again able to
provide the service.
SRP requestors are normally expected to remove all service instances
when removing a host. However, in some cases an SRP requestor may
not have retained sufficient state to know that some service instance
is pointing to a host that it is removing. This method of removing
services is intended for the case where the requestor is going
offline and does not want its services advertised. Therefore, it is
sufficient for the requestor to send the Host Description Instruction
(Section 3.3.1.3).
To support this, when removing services based on the lease time being
zero, an SRP registrar MUST remove all service instances pointing to
a host when a host is removed, even if the SRP requestor doesn't list
them explicitly. If the KEY lease time is nonzero, the SRP registrar
MUST NOT delete the KEY records for these SRP requestors.
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3.2.5.5.2. Removing some published services
In some use cases a requestor may need to remove some specific
service, without removing its other services. This can be
accomplished in one of two ways. To simply remove a specific
service, the requestor sends a valid SRP Update where the Service
Discovery Instruction (Section 3.3.1.1) contains a single Delete an
RR from an RRset ([RFC2136], Section 2.5.4) update that deletes the
PTR record whose target is the service instance name. The Service
Description Instruction (Section 3.3.1.2) in this case contains a
single Delete all RRsets from a Name ([RFC2136], Section 2.5.3)
update to the service instance name.
The second alternative is used when some service is being replaced by
a different service with a different service instance name. In this
case, the old service is deleted as in the first alternative. The
new service is added, just as it would be in an update that wasn't
deleting the old service. Because both the removal of the old
service and the add of the new service consist of a valid Service
Discovery Instruction and a valid Service Description Instruction,
the update as a whole is a valid SRP Update, and will result in the
old service being removed and the new one added, or, to put it
differently, in the old service being replaced by the new service.
It is perhaps worth noting that if a service is being updated without
the service instance name changing, that will look very much like the
second alternative above. The difference is that because the target
for the PTR record in the Service Discovery Instruction is the same
for both the Delete An RR From An RRset update and the Add To An
RRSet update, there is no way to tell whether they were intended to
be one or two Instructions. The same would be true of the Service
Description Instruction.
Whichever of these two alternatives is used, the host lease will be
updated with the lease time provided in the SRP update. In neither
of these cases is it permissible to delete the host. All services
must point to a host. If a host is to be deleted, this must be done
using the method described in Section 3.2.5.5.1, which deletes the
host and all services that have that host as their target.
3.3. Validation and Processing of SRP Updates
3.3.1. Validation of DNS Update Add and Delete RRs
The SRP registrar first validates that the DNS Update is a
syntactically and semantically valid DNS Update according to the
rules specified in [RFC2136].
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SRP Updates consist of a set of _instructions_ that together add or
remove one or more services. Each instruction consists of some
combination of delete updates and add updates. When an instruction
contains a delete and an add, the delete MUST precede the add.
The SRP registrar checks each instruction in the SRP Update to see
that it is either a Service Discovery Instruction, a Service
Description Instruction, or a Host Description Instruction. Order
matters in DNS updates. Specifically, deletes must precede adds for
records that the deletes would affect; otherwise the add will have no
effect. This is the only ordering constraint; aside from this
constraint, updates may appear in whatever order is convenient when
constructing the update.
Because the SRP Update is a DNS update, it MUST contain a single
question that indicates the zone to be updated. Every delete and
update in an SRP Update MUST be within the zone that is specified for
the SRP Update.
3.3.1.1. Service Discovery Instruction
An instruction is a Service Discovery Instruction if it contains
* exactly one "Add to an RRSet" ([RFC2136], Section 2.5.1) or
exactly one "Delete an RR from an RRSet" ([RFC2136],
Section 2.5.4) RR update,
* which updates a PTR RR,
* the target of which is a Service Instance Name
* for which name a Service Description Instruction is present in the
SRP Update, and:
- if the RR Update is an "Add to an RRSet" instruction, that
Service Description Instruction contains an "Add to an RRset"
RR update for the SRV RR describing that service and no other
"Delete from an RRset" instructions for that Service Instance
Name; or
- if the RR Update is a "Delete an RR from an RRSet" instruction,
that Service Description Instruction contains a "Delete from an
RRset" RR update and no other "Add to an RRset" instructions
for that Service Instance Name.
* and contains no other add or delete RR updates for the same name
as the PTR RR Update.
Note that there can be more than one Service Discovery Instruction
for the same name if the SRP requestor is advertising more than one
service of the same type, or is changing the target of a PTR RR.
This is also true for SRP subtypes (Section 7.1 of [RFC6763]). For
each such PTR RR add or delete, the above constraints must be met.
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3.3.1.2. Service Description Instruction
An instruction is a Service Description Instruction if, for the
appropriate Service Instance Name, the following are true:
* It contains exactly one "Delete all RRsets from a name" update for
the service instance name ([RFC2136], Section 2.5.3),
* It contains zero or one "Add to an RRset" SRV RR,
* It contains zero or one "Add to an RRset" KEY RR that, if present,
contains the public key corresponding to the private key that was
used to sign the message (if present, the KEY MUST match the KEY
RR given in the Host Description),
* It contains zero or more "Add to an RRset" TXT RRs,
* If there is one "Add to an RRset" SRV update, there MUST be at
least one "Add to an RRset" TXT update.
* The target of the SRV RR Add, if present points to a hostname for
which there is a Host Description Instruction in the SRP Update,
or
* If there is no "Add to an RRset" SRV RR, then either:
- the name to which the "Delete all RRsets from a name" applies
does not exist, or
- there is an existing KEY RR on that name, which matches the key
with which the SRP Update was signed.
* No other resource records on the Service Instance Name are
modified.
An SRP registrar MUST correctly handle compressed names in the SRV
target.
3.3.1.3. Host Description Instruction
An instruction is a Host Description Instruction if, for the
appropriate hostname, it contains
* exactly one "Delete all RRsets from a name" RR,
* one or more "Add to an RRset" RRs of type A and/or AAAA,
* exactly one "Add to an RRset" RR that adds a KEY RR that contains
the public key corresponding to the private key that was used to
sign the message,
* Host Description Instructions do not modify any other resource
records.
A and/or AAAA records that are not of sufficient scope to be validly
published in a DNS zone MAY be ignored by the SRP registrar, which
could result in a host description effectively containing zero
reachable addresses even when it contains one or more addresses.
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For example, if a link-scope address or IPv4 autoconfiguration
address is provided by the SRP requestor, the SRP registrar could not
publish this in a DNS zone. However, in some situations, the
registrar might make the records available through a mechanism such
as an advertising proxy only on the specific link from which the SRP
update originated; in such a situation, locally-scoped records are
still valid.
3.3.2. Valid SRP Update Requirements
An SRP Update MUST contain exactly one Host Description Instruction.
In addition, there MUST NOT be any Service Description Instruction to
which no Service Discovery Instruction points. A DNS Update that
contains any additional adds or deletes that cannot be identified as
Service Discovery, Service Description or Host Description
Instructions is not an SRP Update. A DNS update that contains any
prerequisites is not an SRP Update.
An SRP Update MUST include an EDNS(0) Update Lease option
[I-D.ietf-dnssd-update-lease]. The LEASE time specified in the
Update Lease option MUST be less than or equal to the KEY-LEASE time.
A DNS update that does not include the Update Lease option, or that
includes a KEY-LEASE value that is less than the LEASE value, is not
an SRP update.
When an SRP registrar receives a DNS Update that is not an SRP
update, it MAY process the update as regular RFC2136 updates,
including access control checks and constraint checks, if supported.
Otherwise the SRP registrar MUST reject the DNS Update with the
Refused RCODE.
If the definitions of each of these instructions are followed
carefully and the update requirements are validated correctly, many
DNS Updates that look very much like SRP Updates nevertheless will
fail to validate. For example, a DNS update that contains an Add to
an RRset instruction for a Service Name and an Add to an RRset
instruction for a Service Instance Name, where the PTR record added
to the Service Name does not reference the Service Instance Name, is
not a valid SRP Update message, but may be a valid RFC2136 update.
3.3.3. FCFS Name And Signature Validation
Assuming that a DNS Update message has been validated with these
conditions and is a valid SRP Update, the SRP registrar checks that
the name in the Host Description Instruction exists. If so, then the
registrar checks to see if the KEY record on that name is the same as
the KEY record in the Host Description Instruction. The registrar
performs the same check for the KEY records in any Service
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Description Instructions. For KEY records that were omitted from
Service Description Instructions, the KEY from the Host Description
Instruction is used. If any existing KEY record corresponding to a
KEY record in the SRP Update does not match the KEY record in the SRP
Update (whether provided or taken from the Host Description
Instruction), then the SRP registrar MUST reject the SRP Update with
the YXDomain RCODE.
Otherwise, the SRP registrar validates the SRP Update using SIG(0)
against the public key in the KEY record of the Host Description
Instruction. If the validation fails, the registrar MUST reject the
SRP Update with the Refused RCODE. Otherwise, the SRP Update is
considered valid and authentic, and is processed according to the
method described in RFC2136.
KEY record updates omitted from Service Description Instruction are
processed as if they had been explicitly present: every Service
Description that is updated MUST, after the SRP Update has been
applied, have a KEY RR, and it must be the same KEY RR that is
present in the Host Description to which the Service Description
refers.
[RFC3445] states that the flags field in the KEY RR MUST be zero
except for bit 7, which can be one in the case of a zone key.
However, the SRP registrar MUST NOT validate the flags field.
3.3.4. Handling of Service Subtypes
SRP registrars MUST treat the update instructions for a service type
and all its subtypes as atomic. That is, when a service and its
subtypes are being updated, whatever information appears in the SRP
Update is the entirety of information about that service and its
subtypes. If any subtype appeared in a previous update but does not
appear in the current update, then the SRP registrar MUST remove that
subtype.
Similarly, there is no mechanism for deleting subtypes. A delete of
a service deletes all of its subtypes. To delete an individual
subtype, an SRP Update must be constructed that contains the service
type and all subtypes for that service except for the one to be
deleted.
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3.3.5. SRP Update response
The status that is returned depends on the result of processing the
update, and can be either NoError, ServFail, Refused or YXDomain: all
other possible outcomes will already have been accounted for when
applying the constraints that qualify the update as an SRP Update.
The meanings of these responses are explained in Section 2.2 of
[RFC2136].
In the case of a response other than NoError, Section 3.8 of
[RFC2136] specifies that the server is permitted to respond either
with no RRs or to copy the RRs sent by the client into the response.
The SRP Requestor MUST NOT attempt to validate any RRs that are
included in the response. It is possible that a future SRP extension
may include per-RR indications as to why the update failed, but at
present this is not specified, so if a client were to attempt to
validate the RRs in the response, it might reject such a response,
since it would contain RRs, but probably not a set of RRs identical
to what was sent in the SRP Update.
3.3.6. Optional Behavior
The SRP registrar MAY add a Reverse Mapping (Section 3.5 of
[RFC1035], Section 2.5 of [RFC3596]) that corresponds to the Host
Description. This is not required because the Reverse Mapping serves
no protocol function, but it may be useful for debugging, e.g. in
annotating network packet traces or logs. In order for the registrar
to do a reverse mapping update, it must be authoritative for the zone
that would need to be updated, or have credentials to do the update.
The SRP requestor MAY also do a reverse mapping update if it has
credentials to do so.
The SRP registrar MAY apply additional criteria when accepting
updates. In some networks, it may be possible to do out-of-band
registration of keys, and only accept updates from pre-registered
keys. In this case, an update for a key that has not been registered
SHOULD be rejected with the Refused RCODE.
There are at least two benefits to doing this rather than simply
using normal SIG(0) DNS updates. First, the same registration
protocol can be used in both cases, so both use cases can be
addressed by the same SRP requestor implementation. Second, the
registration protocol includes maintenance functionality not present
with normal DNS updates.
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Note that the semantics of using SRP in this way are different than
for typical RFC2136 implementations: the KEY used to sign the SRP
Update only allows the SRP requestor to update records that refer to
its Host Description. RFC2136 implementations do not normally
provide a way to enforce a constraint of this type.
The SRP registrar could also have a dictionary of names or name
patterns that are not permitted. If such a list is used, updates for
Service Instance Names that match entries in the dictionary are
rejected with a Refused RCODE.
4. TTL Consistency
All RRs within an RRset are required to have the same TTL
(Clarifications to the DNS Specification [RFC2181], Section 5.2). In
order to avoid inconsistencies, SRP places restrictions on TTLs sent
by requestors and requires that SRP registrars enforce consistency.
Requestors sending SRP Updates MUST use consistent TTLs in all RRs
within the SRP Update.
SRP registrars MUST check that the TTLs for all RRs within the SRP
Update are the same. If they are not, the SRP update MUST be
rejected with a Refused RCODE.
Additionally, when adding RRs to an RRset, for example when
processing Service Discovery records, the SRP registrar MUST use the
same TTL on all RRs in the RRset. How this consistency is enforced
is up to the implementation.
TTLs sent in SRP Updates are advisory: they indicate the SRP
requestor's guess as to what a good TTL would be. SRP registrars may
override these TTLs. SRP registrars SHOULD ensure that TTLs are
reasonable: neither too long nor too short. The TTL SHOULD NOT ever
be longer than the lease time (Section 5.1). Shorter TTLs will
result in more frequent data refreshes; this increases latency on the
DNS-SD client side, increases load on any caching resolvers and on
the authoritative server, and also increases network load, which may
be an issue for constrained networks. Longer TTLs will increase the
likelihood that data in caches will be stale. TTL minimums and
maximums SHOULD be configurable by the operator of the SRP registrar.
5. Maintenance
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5.1. Cleaning up stale data
Because the DNS-SD registration protocol is automatic, and not
managed by humans, some additional bookkeeping is required. When an
update is constructed by the SRP requestor, it MUST include an
EDNS(0) Update Lease Option [I-D.ietf-dnssd-update-lease]. The
Update Lease Option contains two lease times: the Lease Time and the
KEY Lease Time.
These leases are promises, similar to DHCP leases [RFC2131], from the
SRP requestor that it will send a new update for the service
registration before the lease time expires. The Lease time is chosen
to represent the time after the update during which the registered
records other than the KEY record can be assumed to be valid. The
KEY lease time represents the time after the update during which the
KEY record can be assumed to be valid.
The reasoning behind the different lease times is discussed in the
section on FCFS naming (Section 3.2.4.1). SRP registrars may be
configured with limits for these values. A default limit of two
hours for the Lease and 14 days for the SIG(0) KEY are currently
thought to be good choices. Constrained devices with limited battery
that wake infrequently are likely to request longer leases;
registrars that support such devices may need to set higher limits.
SRP requestors that are going to continue to use names on which they
hold leases SHOULD update well before the lease ends, in case the
registrar is unavailable or under heavy load.
The lease time applies specifically to the host. All service
instances, and all service entries for such service instances, depend
on the host. When the lease on a host expires, the host and all
services that reference it MUST be removed at the same time—it is
never valid for a service instance to remain when the host it
references has been removed. If the KEY record for the host is to
remain, the KEY record for any services that reference it MUST also
remain. However, the service PTR record MUST be removed, since it
has no key associated with it, and since it is never valid to have a
service PTR record for which there is no service instance on the
target of the PTR record.
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SRP registrars MUST also track a lease time per service instance.
The reason for doing this is that a requestor may re-register a host
with a different set of services, and not remember that some
different service instance had previously been registered. In this
case, when that service instance lease expires, the SRP registrar
MUST remove the service instance (although the KEY record for the
service instance SHOULD be retained until the KEY lease on that
service expires). This is beneficial because otherwise if the SRP
requestor continues to renew the host, but never mentions the stale
service again, the stale service will continue to be advertised.
The SRP registrar MUST include an EDNS(0) Update Lease option in the
response if the lease time proposed by the requestor has been
shortened or lengthened by the registrar. The requestor MUST check
for the EDNS(0) Update Lease option in the response and MUST use the
lease times from that option in place of the options that it sent to
the registrar when deciding when to renew its registration. The
times may be shorter or longer than those specified in the SRP
Update; the SRP requestor must honor them in either case.
SRP requestors SHOULD assume that each lease ends N seconds after the
update was first transmitted, where N is the lease duration. SRP
Registrars SHOULD assume that each lease ends N seconds after the
update that was successfully processed was received. Because the
registrar will always receive the update after the SRP requestor sent
it, this avoids the possibility of misunderstandings.
SRP registrars MUST reject updates that do not include an EDNS(0)
Update Lease option. DNS authoritative servers that allow both SRP
and non-SRP DNS updates MAY accept updates that don't include leases,
but SHOULD differentiate between SRP Updates and other updates, and
MUST reject updates that would otherwise be SRP Updates if they do
not include leases.
Lease times have a completely different function than TTLs. On an
authoritative DNS server, the TTL on a resource record is a constant:
whenever that RR is served in a DNS response, the TTL value sent in
the answer is the same. The lease time is never sent as a TTL; its
sole purpose is to determine when the authoritative DNS server will
delete stale records. It is not an error to send a DNS response with
a TTL of 'n' when the remaining time on the lease is less than 'n'.
6. Security Considerations
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6.1. Source Validation
SRP Updates have no authorization semantics other than FCFS. This
means that if an attacker from outside of the administrative domain
of the SRP registrar knows the registrar's IP address, it can in
principle send updates to the registrar that will be processed
successfully. SRP Registrars SHOULD therefore be configured to
reject updates from source addresses outside of the administrative
domain of the registrar.
For TCP updates, the initial SYN-SYN+ACK handshake prevents updates
being forged by an off-network attacker. In order to ensure that
this handshake happens, SRP registrars relying on three-way-handshake
validation MUST NOT accept TCP Fast Open [RFC7413] payloads. If the
network infrastructure allows it, an SRP registrar MAY accept TCP
Fast Open payloads if all such packets are validated along the path,
and the network is able to reject this type of spoofing at all
ingress points.
For UDP updates from constrained devices, spoofing would have to be
prevented with appropriate source address filtration on routers
[RFC2827]. This would ordinarily be accomplished by measures such as
are described in Section 4.5 of [RFC7084]. For example, a stub
router [I-D.ietf-snac-simple] for a constrained network might only
accept UDP updates from source addresses known to be on-link on that
stub network, and might further validate that the UDP update was
actually received on the stub network interface and not the interface
connected to the adjacent infrastructure link.
6.2. Other DNS updates
Note that these rules only apply to the validation of SRP Updates. A
server that accepts updates from SRP requestors may also accept other
DNS updates, and those DNS updates may be validated using different
rules. However, in the case of a DNS server that accepts SRP
updates, the intersection of the SRP Update rules and whatever other
update rules are present must be considered very carefully.
For example, a normal, authenticated DNS update to any RR that was
added using SRP, but that is authenticated using a different key,
could be used to override a promise made by the SRP registrar to an
SRP requestor, by replacing all or part of the service registration
information with information provided by an authenticated DNS update
requestor. An implementation that allows both kinds of updates
SHOULD NOT allow DNS Update requestors that are using different
authentication and authorization credentials to update records added
by SRP requestors.
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6.3. Risks of allowing arbitrary names to be registered in SRP updates
It is possible to set up SRP updates for a zone that is used for non-
DNSSD services. For example, imagine that you set up SRP service for
example.com. SRP hosts can now register names like "www" or "mail"
or "smtp" in this domain. In addition, SRP updates using FCFS naming
can insert names that are obscene or offensive into the zone. There
is no simple solution to these problems. We have two recommendations
to address this problem, however:
* Do not provide SRP service in organization-level zones. Use
subdomains of the organizational domain for DNS service discovery.
This does not prevent registering names as mentioned above, but
does ensure that genuinely important names are not accidentally
reserved for SRP clients. So for example, the zone
"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 DNSSD discovery zone that is
updated by SRP to have a user-friendly or important-sounding name.
* Configure a dictionary of names that are prohibited. Dictionaries
of common obscene and offensive names are no doubt available, and
can be augmented with a list of typical "special" names like
"www", "mail", "smtp" and so on. Lists of names are generally
available, or can be constructed manually.
6.4. Security of local service discovery
Local links can be protected by managed services such as RA Guard
[RFC6105], but multicast services like DHCP [RFC2131], DHCPv6
[RFC8415] and IPv6 Neighbor Discovery [RFC4861] are in most cases not
authenticated and can't be controlled on unmanaged networks, such as
home networks and small-office networks where no network management
staff are present. In such situations, the SRP service has
comparatively fewer potential security exposures and hence is not the
weak link. This is discussed in more detail in Section 3.2.4.
The fundamental protection for networks of this type is the user's
choice of what devices to add to the network. Work is being done in
other working groups and standards bodies to improve the state of the
art for network on-boarding and device isolation (e.g., [RFC8520]
provides a means for constraining what behaviors are allowed for a
device in an automatic way), but such work is out of scope for this
document.
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6.5. SRP Registrar Authentication
This specification does not provide a mechanism for validating
responses from SRP Registrars to SRP requestors. In principle, a KEY
RR could be used by a non-constrained SRP requestor to validate
responses from the registrar, but this is not required, nor do we
specify a mechanism for determining which key to use.
In addition, for DNS-over-TLS connections, out-of-band key pinning as
described in [RFC7858], Section 4.2 could be used for authentication
of the SRP registrar, e.g. to prevent man-in-the-middle attacks.
However the use of such keys is impractical for an unmanaged service
registration protocol, and hence is out of scope for this document.
6.6. Required Signature Algorithm
For validation, SRP registrars MUST implement the ECDSAP256SHA256
signature algorithm. SRP registrars SHOULD implement the algorithms
specified in [RFC8624], Section 3.1, in the validation column of the
table, that are numbered 13 or higher and have a "MUST",
"RECOMMENDED", or "MAY" designation in the validation column of the
table. SRP requestors MUST NOT assume that any algorithm numbered
lower than 13 is available for use in validating SIG(0) signatures.
7. Privacy Considerations
Because DNS-SD SRP Updates can be sent off-link, the privacy
implications of SRP are different than for multicast DNS responses.
Host implementations that are using TCP SHOULD also use TLS if
available. SRP Registrar implementations MUST offer TLS support.
The use of TLS with DNS is described in [RFC7858]. Because there is
no mechanism for sharing keys, validation of DNS-over-TLS keys is not
possible; DNS-over-TLS is used only as described in [RFC7858],
Section 4.1
Hosts that implement TLS support SHOULD NOT fall back to TCP; since
SRP registrars are required to support TLS, it is entirely up to the
host implementation whether to use it.
Public keys can be used as identifiers to track hosts. SRP
registrars MAY elect not to return KEY records for queries for SRP
registrations. To avoid DNSSEC validation failures, an SRP registrar
that signs the zone for DNSSEC but refuses to return a KEY record
MUST NOT store the KEY record in the zone itself. Because the KEY
record isn't in the zone, the nonexistance of the KEY record can be
validated. If the zone is not signed, the server MAY instead return
a negative non-error response (either NXDOMAIN or no data).
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8. Domain Name Reservation Considerations
This section specifies considerations for systems involved in domain
name resolution when resolving queries for names ending with
'.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 [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. Users are not expected to
see this name in user interfaces, although it's certainly possible
that they might. If they do, they are not expected to treat it
specially.
8.2. Application Software
Application software does not need to handle subdomains of
'service.arpa' 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 as uniquely identifying a particular host, as would be
required for such a PKI cert 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 configured recursive resolvers.
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One or more IP addresses for recursive DNS servers will usually be
supplied to the client through router advertisements or DHCP. For an
administrative domain that uses subdomains of '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 be unable to resolve, or may
receive incorrect results for, subdomains of 'service.arpa.'. In
order to avoid this, it is permissible that hosts use the resolvers
that are locally provided for resolving 'service.arpa.', even when
they are configured to use other resolvers.
8.4. Caching DNS Servers
There are three considerations for caching DNS servers that follow
this specification:
1. For correctness, recursive resolvers at sites using
'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 [RFC4035]). DNSSEC validation
is a Best Current Practice [RFC9364]: although validation is not
required, a caching recursive resolver that does not validate
answers that can be validated may cache invalid data. This, in
turn, would prevent validating stub resolvers from successfully
validating answers. Hence, as a practical matter, recursive
resolvers at sites using 'service.arpa' should do DNSSEC
validation.
2. Unless configured otherwise, recursive resolvers and DNS proxies
MUST behave as described in Locally Served Zones, Section 3 of
[RFC6303]. That is, queries for '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.
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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,
as necessary to provide accurate authority information, a query
for the DS record MUST result in forwarding whatever queries are
necessary; typically, this will just be a query for the DS
record, since the necessary authority information will be
included in the authority section of the response if the DO bit
is set.
8.5. Authoritative DNS Servers
No special processing of 'service.arpa.' is required for
authoritative DNS server implementations. It is possible that an
authoritative DNS server might attempt to check the authoritative
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, and so the server would
refuse to answer authoritatively for such a zone. A server that
implements this sort of check MUST be configurable so that either it
does not do this check for the 'service.arpa.' domain or it ignores
the results of the check.
8.6. DNS Server Operators
DNS server operators MAY configure an authoritative server for
'service.arpa.' for use with SRP. The operator for the DNS servers
authoritative for 'service.arpa.' in the global DNS will configure
any such servers as described in Section 9.
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 according to the rules established in [RFC3172]. There are no
other DNS registrars for '.arpa'.
9. Delegation of 'service.arpa.'
In order to be fully functional, the owner of the 'arpa.' zone must
add a delegation of 'service.arpa.' in the '.arpa.' zone [RFC3172].
This delegation is to be set up as was done for 'home.arpa', as a
result of the specification in Section 7 of [RFC8375]. This is
currently the responsibility of the IAB [IAB-ARPA]
10. IANA Considerations
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10.1. Registration and Delegation of 'service.arpa' as a Special-Use
Domain Name
IANA is requested to record the domain name 'service.arpa.' in the
Special-Use Domain Names registry [SUDN]. IANA is requested, with
the approval of IAB, to implement the delegation requested in
Section 9.
IANA is further requested to add a new entry to the "Transport-
Independent Locally-Served Zones" subregistry of the "Locally-Served
DNS Zones" registry [LSDZ]. The entry will be for the domain
'service.arpa.' with the description "DNS-SD Service Registration
Protocol Special-Use Domain", listing this document as the reference.
10.2. 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. The IANA is requested to create a registry,
the "service.arpa Subdomain" registry. The IETF shall have change
control for this registry. New entries may be added either as a
result of Standards Action Section 4.9 of [RFC8126] or with IESG
approval Section 4.10 of [RFC8126], provided that a specification
exists Section 4.6 of [RFC8126].
The IANA shall group the "service.arpa Subdomain" registry with the
"Locally-Served DNS Zones" registry. The registry shall be a table
with three columns: the subdomain name (expressed as a fully-
qualified domain name), a brief description of how it is used, and a
reference to the document that describes its use in detail.
This registry shall begin as the following table:
+=======================+=================+===========+
| Subdomain Name | Description | reference |
+=======================+=================+===========+
| default.service.arpa. | Default domain | [THIS |
| | for SRP updates | DOCUMENT] |
+-----------------------+-----------------+-----------+
Table 1
10.3. Service Name registrations
IANA is requested to add two new entries to the Service Names and
Port Numbers registry. The following sections contain tables with
the fields required by Section 8.1.1 of [RFC6335].
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10.4. 'dnssd-srp' Service Name
+--------------------+-----------------------------+
| Field Name | Value |
+--------------------+-----------------------------+
| Service Name | dnssd-srp |
+--------------------+-----------------------------+
| Transport Protocol | TCP |
+--------------------+-----------------------------+
| Assignee | IESG <iesg@ietf.org> |
+--------------------+-----------------------------+
| Contact | IETF Chair <chair@ietf.org> |
+--------------------+-----------------------------+
| Description | DNS-SD Service Registration |
+--------------------+-----------------------------+
| Reference | this document |
+--------------------+-----------------------------+
| Port Number | None |
+--------------------+-----------------------------+
| Service Code | None |
+--------------------+-----------------------------+
Table 2
10.5. 'dnssd-srp-tls' Service Name
+--------------------+-----------------------------------+
| Field Name | Value |
+--------------------+-----------------------------------+
| Service Name | dnssd-srp-tls |
+--------------------+-----------------------------------+
| Transport Protocol | TCP |
+--------------------+-----------------------------------+
| Assignee | IESG |
+--------------------+-----------------------------------+
| Contact | IETF Chair |
+--------------------+-----------------------------------+
| Description | DNS-SD Service Registration (TLS) |
+--------------------+-----------------------------------+
| Reference | this document |
+--------------------+-----------------------------------+
| Port Number | None |
+--------------------+-----------------------------------+
| Service Code | None |
+--------------------+-----------------------------------+
Table 3
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10.6. Anycast Address
IANA is requested to allocate an IPv6 Anycast address from the IPv6
Special-Purpose Address Registry, similar to the Port Control
Protocol anycast address, 2001:1::1. The value TBD is to be replaced
with the actual allocation in the table that follows. The purpose of
this allocation is to provide a fixed anycast address that can be
commonly used as a destination for SRP updates when no SRP registrar
is explicitly configured. The values for the registry are:
+----------------------+-----------------------------+
| Attribute | value |
+----------------------+-----------------------------+
| Address Block | 2001:1::TBD/128 |
+----------------------+-----------------------------+
| Name | DNS-SD Service Registration |
| | Protocol Anycast Address |
+----------------------+-----------------------------+
| RFC | [this document] |
+----------------------+-----------------------------+
| Allocation Date | [date of allocation] |
+----------------------+-----------------------------+
| Termination Date | N/A |
+----------------------+-----------------------------+
| Source | True |
+----------------------+-----------------------------+
| Destination | True |
+----------------------+-----------------------------+
| Forwardable | True |
+----------------------+-----------------------------+
| Global | True |
+----------------------+-----------------------------+
| Reserved-by-protocol | False |
+----------------------+-----------------------------+
Table 4
11. Implementation Status
[Note to the RFC Editor: please remove this section prior to
publication.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 7942.
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
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here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
There are two known independent implementations of SRP requestors:
* SRP Client for OpenThread:
https://github.com/openthread/openthread/pull/6038
* mDNSResponder open source project: https://github.com/Abhayakara/
mdnsresponder
There are two related implementations of an SRP registrar. One acts
as a DNS Update proxy, taking an SRP Update and applying it to the
specified DNS zone using DNS update. The other acts as an
Advertising Proxy [AP]. Both are included in the mDNSResponder open
source project mentioned above.
12. Acknowledgments
Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping
Dong and Abtin Keshavarzian for their thorough technical reviews.
Thanks to Kangping and Abtin as well for testing the document by
doing an independent implementation. Thanks to Tamara Kemper for
doing a nice developmental edit, Tim Wattenberg for doing an SRP
requestor proof-of-concept implementation at the Montreal Hackathon
at IETF 102, and Tom Pusateri for reviewing during the hackathon and
afterwards. Thanks to Esko for a really thorough second last call
review. Thanks also to Nathan Dyck, Gabriel Montenegro, Kangping
Dong, Martin Turon, and Michael Cowan for their detailed second last
call reviews. Thanks to Patrik Fältström, Dhruv Dhody, David Dong,
Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective
directorate reviews. Thanks to Paul Wouters for a _really_ detailed
IESG review! Thanks also to the other IESG members who provided
comments or simply took the time to review the document.
13. Normative References
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[I-D.ietf-dnssd-update-lease]
Cheshire, S. and T. Lemon, "An EDNS(0) option to negotiate
Leases on DNS Updates", Work in Progress, Internet-Draft,
draft-ietf-dnssd-update-lease-08, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
update-lease-08>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
<https://www.rfc-editor.org/info/rfc1536>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC2539] Eastlake 3rd, D., "Storage of Diffie-Hellman Keys in the
Domain Name System (DNS)", RFC 2539, DOI 10.17487/RFC2539,
March 1999, <https://www.rfc-editor.org/info/rfc2539>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
2000, <https://www.rfc-editor.org/info/rfc2931>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.
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[RFC3445] Massey, D. and S. Rose, "Limiting the Scope of the KEY
Resource Record (RR)", RFC 3445, DOI 10.17487/RFC3445,
December 2002, <https://www.rfc-editor.org/info/rfc3445>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
RFC 6303, DOI 10.17487/RFC6303, July 2011,
<https://www.rfc-editor.org/info/rfc6303>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8375] Pfister, P. and T. Lemon, "Special-Use Domain
'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
<https://www.rfc-editor.org/info/rfc8375>.
[RFC8624] Wouters, P. and O. Sury, "Algorithm Implementation
Requirements and Usage Guidance for DNSSEC", RFC 8624,
DOI 10.17487/RFC8624, June 2019,
<https://www.rfc-editor.org/info/rfc8624>.
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[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/info/rfc8765>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/info/rfc9364>.
14. Informative References
[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>.
[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>.
[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>.
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[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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>.
[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>.
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[AP] Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
Service Registration Protocol", Work in Progress,
Internet-Draft, draft-ietf-dnssd-advertising-proxy-03, 28
July 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-dnssd-advertising-proxy-03>.
[I-D.ietf-snac-simple]
Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-03, 30 January
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
snac-simple-03>.
[SUDN] "Special-Use Domain Names Registry", July 2012,
<https://www.iana.org/assignments/special-use-domain-
names/special-use-domain-names.xhtml>.
[LSDZ] "Locally-Served DNS Zones Registry", July 2011,
<https://www.iana.org/assignments/locally-served-dns-
zones/locally-served-dns-zones.xhtml>.
[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/>.
[ZC] Cheshire, S. and D.H. Steinberg, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc. ,
ISBN 0-596-10100-7, December 2005.
Appendix A. Testing using standard RFC2136-compliant DNS servers
It may be useful to set up an authoritative DNS server for testing
that does not implement SRP. This can be done by configuring the
server to listen on the anycast address, or advertising it in the
_dnssd-srp._tcp.<zone> SRV and _dnssd-srp-tls._tcp.<zone> record. It
must be configured to be authoritative for "default.service.arpa",
and to accept updates from hosts on local networks for names under
"default.service.arpa" without authentication, since such servers
will not have support for FCFS authentication (Section 3.2.4.1).
An authoritative DNS server configured in this way will be able to
successfully accept and process SRP Updates from requestors that send
SRP updates. However, no prerequisites will be applied, and this
means that the test server will accept internally inconsistent SRP
Updates, and will not stop two SRP Updates, sent by different
services, that claim the same name(s), from overwriting each other.
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Since SRP Updates are signed with keys, validation of the SIG(0)
algorithm used by the requestor can be done by manually installing
the requestor's public key on the DNS server that will be receiving
the updates. The key can then be used to authenticate the SRP
update, and can be used as a requirement for the update. An example
configuration for testing SRP using BIND 9 is given in Appendix C.
Appendix B. How to allow SRP requestors to update standard
RFC2136-compliant servers
Ordinarily SRP Updates will fail when sent to an RFC 2136-compliant
server that does not implement SRP because the zone being updated is
"default.service.arpa", and no DNS server that is not an SRP
registrar would normally be configured to be authoritative for
"default.service.arpa". Therefore, a requestor that sends an SRP
Update can tell that the receiving server does not support SRP, but
does support RFC2136, because the RCODE will either be NotZone,
NotAuth or Refused, or because there is no response to the update
request (when using the anycast address)
In this case a requestor MAY attempt to register itself using regular
RFC2136 DNS updates. To do so, it must discover the default
registration zone and the DNS server designated to receive updates
for that zone, as described earlier, using the _dns-update._udp SRV
record. It can then send the update to the port and host pointed to
by the SRV record, and is expected to use appropriate prerequisites
to avoid overwriting competing records. Such updates are out of
scope for SRP, and a requestor that implements SRP MUST first attempt
to use SRP to register itself, and only attempt to use RFC2136
backwards compatibility if that fails. Although the owner name for
the SRV record specifies the UDP protocol for updates, it is also
possible to use TCP, and TCP SHOULD be required to prevent spoofing.
Appendix C. Sample BIND9 configuration for 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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$ORIGIN .
$TTL 57600 ; 16 hours
default.service.arpa IN SOA ns3.default.service.arpa.
postmaster.default.service.arpa. (
2951053287 ; serial
3600 ; refresh (1 hour)
1800 ; retry (30 minutes)
604800 ; expire (1 week)
3600 ; minimum (1 hour)
)
NS ns3.default.service.arpa.
SRV 0 0 53 ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 3600 ; 1 hour
_ipps._tcp PTR demo._ipps._tcp
$ORIGIN _ipps._tcp.default.service.arpa.
demo TXT "0"
SRV 0 0 9992 demo.default.service.arpa.
$ORIGIN _udp.default.service.arpa.
$TTL 3600 ; 1 hour
_dns-update PTR ns3.default.service.arpa.
$ORIGIN _tcp.default.service.arpa.
_dnssd-srp PTR ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 300 ; 5 minutes
ns3 AAAA 2001:db8:0:1::1
$TTL 3600 ; 1 hour
demo AAAA 2001:db8:0:2::1
KEY 0 3 13 (
qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
); alg = ECDSAP256SHA256 ; key id = 15008
AAAA ::1
Figure 2: Example Zone file
Authors' Addresses
Ted Lemon
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: mellon@fugue.com
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Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 408 974 3207
Email: cheshire@apple.com
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