Internet Engineering Task Force T. Lemon
Internet-Draft S. Cheshire
Intended status: Standards Track Apple Inc.
Expires: 26 October 2022 24 April 2022
Service Registration Protocol for DNS-Based Service Discovery
draft-ietf-dnssd-srp-13
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 802.11
(Wi-Fi) and 802.15.4 (IoT) networks. DNS-SD Service registration
uses public keys and SIG(0) to allow services to defend their
registrations against attack.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 26 October 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
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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extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Service Registration Protocol . . . . . . . . . . . . . . . . 5
2.1. Protocol Variants . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Full-featured Hosts . . . . . . . . . . . . . . . . . 5
2.1.2. Constrained Hosts . . . . . . . . . . . . . . . . . . 6
2.1.3. Why two variants? . . . . . . . . . . . . . . . . . . 6
2.2. Protocol Details . . . . . . . . . . . . . . . . . . . . 7
2.2.1. What to publish . . . . . . . . . . . . . . . . . . . 7
2.2.2. Where to publish it . . . . . . . . . . . . . . . . . 7
2.2.3. How to publish it . . . . . . . . . . . . . . . . . . 8
2.2.3.1. How DNS-SD Service Registration differs from
standard RFC2136 DNS Update . . . . . . . . . . . . 8
2.2.4. How to secure it . . . . . . . . . . . . . . . . . . 9
2.2.4.1. First-Come First-Served Naming . . . . . . . . . 9
2.2.5. Service Behavior . . . . . . . . . . . . . . . . . . 9
2.2.5.1. Public/Private key pair generation and storage . 9
2.2.5.2. Name Conflict Handling . . . . . . . . . . . . . 10
2.2.5.3. Record Lifetimes . . . . . . . . . . . . . . . . 10
2.2.5.4. Compression in SRV records . . . . . . . . . . . 11
2.2.5.5. Removing published services . . . . . . . . . . . 11
2.3. Validation and Processing of SRP Updates . . . . . . . . 12
2.3.1. Validation of Adds and Deletes . . . . . . . . . . . 12
2.3.1.1. Service Discovery Instruction . . . . . . . . . . 13
2.3.1.2. Service Description Instruction . . . . . . . . . 13
2.3.1.3. Host Description Instruction . . . . . . . . . . 14
2.3.2. Valid SRP Update Requirements . . . . . . . . . . . . 14
2.3.3. FCFS Name And Signature Validation . . . . . . . . . 15
2.3.4. Handling of Service Subtypes . . . . . . . . . . . . 16
2.3.5. SRP Update response . . . . . . . . . . . . . . . . . 16
2.3.6. Optional Behavior . . . . . . . . . . . . . . . . . . 16
3. TTL Consistency . . . . . . . . . . . . . . . . . . . . . . . 17
4. Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Cleaning up stale data . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5.1. Source Validation . . . . . . . . . . . . . . . . . . . . 19
5.2. SRP Server Authentication . . . . . . . . . . . . . . . . 20
5.3. Required Signature Algorithm . . . . . . . . . . . . . . 20
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
7. Delegation of 'service.arpa.' . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8.1. Registration and Delegation of 'service.arpa' as a
Special-Use Domain Name . . . . . . . . . . . . . . . . . 21
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8.2. 'dnssd-srp' Service Name . . . . . . . . . . . . . . . . 21
8.3. 'dnssd-srp-tls' Service Name . . . . . . . . . . . . . . 22
8.4. Anycast Address . . . . . . . . . . . . . . . . . . . . . 22
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
11. Normative References . . . . . . . . . . . . . . . . . . . . 24
12. Informative References . . . . . . . . . . . . . . . . . . . 26
Appendix A. Testing using standard RFC2136-compliant servers . . 27
Appendix B. How to allow services to update standard
RFC2136-compliant servers . . . . . . . . . . . . . . . . 28
Appendix C. Sample BIND9 configuration for
default.service.arpa. . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
DNS-Based Service Discovery [RFC6763] is a component of Zero
Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].
This document describes an enhancement to DNS-Based Service Discovery
[RFC6763] that allows services to register their services 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.
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 intended 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-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 client have some kind of shared
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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 SRP protocol adds a set of default heuristics for processing DNS
updates that eliminates the need for DNS update conditional
predicates: instead, the SRP server 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 registering service to
know what went wrong and construct a new update.
SRP also adds a feature called First-Come, First-Served Naming, which
allows the registering service 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 service attempting to claim the same name will not possess
the SIG(0) key used by the first service to claim it, and so its
claim will be rejected and the second service will have to choose a
new name.
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
registering service 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 service cannot claim the name, even though the
registration has expired.
The Service Registration Protocol for DNS-SD (SRP), described 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 services can publish
their information in the DNS namespace. In the case of mDNS, it
allows services 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.
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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 knowledgable 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. 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 server 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
SRP requests be the same server that is answering queries that return
records that have been registered.
2.1. Protocol Variants
2.1.1. Full-featured Hosts
Full-featured hosts are either configured manually with a
registration domain, or use the "dr._dns-sd._udp.<domain>" query
([RFC6763], Section 11) to learn the default registration domain from
the network. RFC6763 says to discover the registration domain using
either ".local" or a network-supplied domain name for <domain>.
Services using SRP MUST use the domain name received through the
DHCPv4 Domain Name option ([RFC2132], Section 3.17), if available, or
the Neighbor Discovery DNS Search List option [RFC8106]. If the DNS
Search List option contains more than one domain name, it MUST NOT be
used. If neither option is available, the Service Registration
protocol is not available on the local network.
Manual configuration of the registration domain can be done either by
querying the list of available registration zones ("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
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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 [RFC8765].
Having discovered the enclosing DNS zone, they query for the
"_dnssd-srp._tcp.<zone>" SRV record to discover the server to which
they should send DNS updates. Hosts that support SRP Updates using
TLS use the "_dnssd-srp-tls._tcp.<zone>" SRV record instead.
2.1.2. 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 (proposed) special-
use domain name (see [RFC6761]) "default.service.arpa" is used. The
details of how SRP server(s) are discovered will be specific to the
constrained network, and therefore we do not suggest a specific
mechanism here.
SRP clients on constrained networks are expected to receive from the
network a list of SRP servers with which to register. It is the
responsibility of a Constrained-Node Network supporting SRP to
provide one or more SRP server addresses. It is the responsibility
of the SRP server 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 internally to
names with broader visibility.
2.1.3. Why two variants?
The reason for these different assumptions is that low-power devices
that typically use Constrained-Node Networks may have very limited
battery power. The series of DNS lookups required to discover an SRP
server and then communicate with it will increase the power required
to advertise a service; for low-power devices, the additional
flexibility this provides does not justify the additional use of
power. 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 Internet layer. Nodes connected to such networks
can be assumed to be able to do DNSSD service registration domain
discovery. Such networks are generally able to provide registration
domain discovery and routing. By requiring the use of TCP, the
possibility of off-network spoofing is eliminated.
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2.2. 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, how to secure its publication, and how to maintain the
information once published.
2.2.1. What to publish
We refer to the DNS Update message sent by services using SRP as an
SRP Update. Three types of updates appear in an SRP update: Service
Discovery records, Service Description records, and Host Description
records.
* Service Discovery records are one or more PTR RRs, mapping from
the generic service type (or subtype) to the specific Service
Instance Name.
* Service Description records are exactly one SRV RR, exactly one
KEY RR, and one or more TXT RRs, all with the same name, the
Service Instance Name ([RFC6763], Section 4.1). In principle
Service Description records can include other record types, with
the same Service Instance Name, though in practice they rarely do.
The Service Instance Name MUST be referenced by one or more
Service Discovery PTR records, unless it is a placeholder service
registration for an intentionally non-discoverable service name.
* The Host Description records for a service are a KEY RR, used to
claim exclusive ownership of the service registration, and one or
more RRs of type A or AAAA, giving the IPv4 or IPv6 address(es) of
the host where the service resides.
[RFC6763] describes the details of what each of these types of
updates contains, 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
first-come, first-serve naming, which we describe later on in this
document.
2.2.2. 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 unicast
namespace.
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As described above, full-featured devices are responsible for knowing
in what domain they should register their services. Devices made for
Constrained-Node Networks register in the (proposed) special use
domain name [RFC6761] "default.service.arpa", and let the SRP server
handle rewriting that to a different domain if necessary.
2.2.3. 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 2.3 are implicit in the processing of SRP
Updates, and so there is no need for the service sending the SRP
Update to put in any explicit prerequisites.
2.2.3.1. How DNS-SD Service Registration differs from standard RFC2136
DNS Update
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 server is not required to implement general DNS Update
prerequisite processing.
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* Constrained-Node SRP clients are allowed to send updates to the
generic domain "default.service.arpa"
2.2.4. How to secure it
Traditional DNS update is secured using the TSIG protocol, which uses
a secret key shared between the DNS Update client (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 what we
describe here is an improvement over the security of mDNS. The goal
is not to provide the level of security of a network managed by a
skilled operator.
2.2.4.1. First-Come First-Served Naming
First-Come First-Serve naming provides a limited degree of security:
a service that registers its service using DNS-SD Registration
protocol is given ownership of a name for an extended period of time
based on the key used to authenticate the DNS Update. As long as the
registration service remembers the name and the key used to register
that name, no other service can add or update the information
associated with that. FCFS naming is used to protect both the
Service Description and the Host Description.
2.2.5. Service Behavior
2.2.5.1. Public/Private key pair generation and storage
The service generates a public/private key pair. This key pair MUST
be stored in stable storage; if there is no writable stable storage
on the SRP client, the SRP client 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 to erase the old key and install a
new one. Therefore, the SRP client on the device SHOULD provide a
mechanism to overwrite the key, for example as the result of a
"factory reset."
When sending DNS updates, the service 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
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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.sekar-dns-ul].
The KEY record in Service Description updates MAY be omitted for
brevity; if it is omitted, the SRP server 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.
2.2.5.2. Name Conflict Handling
Both Host Description records and Service Description Records can
have names that result in name conflicts. Service Discovery records
cannot have name conflicts. If any Host Description or Service
Description record is found by the server to have a conflict with an
existing name, the server will respond to the SRP Update with a
YXDOMAIN rcode. In this case, the Service MUST either abandon the
service registration attempt, or else choose a new name.
There is no specific requirement for how this is done; typically,
however, the service 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.service.arpa, then host-1.service.arpa, then host-
2.service.arpa, then host-31773.service.arpa.
2.2.5.3. 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.
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2.2.5.4. Compression in SRV records
Although [RFC2782] requires that the target name in the SRV record
not be compressed, an SRP client SHOULD 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 server needs to understand SRV
records in order to validate the SRP Update. Compression of the
target potentially saves substantial space in the SRP Update.
2.2.5.5. Removing published services
2.2.5.5.1. Removing all published services
To remove all the services registered to a particular host, the SRP
client retransmits its most recent update 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 have the same value it had previously; this holds the name in
reserve for when the SRP client is once again able to provide the
service.
SRP clients are normally expected to remove all service instances
when removing a host. However, in some cases a SRP client 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 client is going offline
and does not want its services advertised. Therefore, it is
sufficient for the client to send the Host Description Instruction
(Section 2.3.1.3).
To support this, when removing services based on the lease time being
zero, an SRP server MUST remove all service instances pointing to a
host when a host is removed, even if the SRP client doesn't list them
explicitly. If the key lease time is nonzero, the SRP server MUST
NOT delete the KEY records for these SRP clients.
2.2.5.5.2. Removing some published services
In some use cases a client 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 client sends a
valid SRP Update where the Service Discovery Instruction
(Section 2.3.1.1) 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
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Instruction (Section 2.3.1.2) 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, these will be seen as a single Service Description
Instruction, not as 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 2.2.5.5.1, which deletes the
host and all services that have that host as their target.
2.3. Validation and Processing of SRP Updates
2.3.1. Validation of Adds and Deletes
The SRP server first validates that the DNS Update is a syntactically
and semantically valid DNS Update according to the rules specified in
RFC2136.
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 server 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
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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.
2.3.1.1. Service Discovery Instruction
An instruction is a Service Discovery Instruction if it contains
* exactly one "Add to an RRSet" or exactly one "Delete an RR from an
RRSet" ([RFC2136], Section 2.5.1) 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
* if the Service Discovery Instruction is an "Add to an RRSet"
instruction, the Service Description Instruction does not match if
it does not contain an "Add to an RRset" update for the SRV RR
describing that service.
* if the Service Discovery Instruction is a "Delete an RR from an
RRSet" update, the Service Description Instruction does not match
if it contains an "Add to an RRset" update.
* Service Discovery Instructions do not contain any other add or
delete updates.
2.3.1.2. Service Description Instruction
An instruction is a Service Description Instruction if, for the
appropriate Service Instance Name, it contains
* exactly one "Delete all RRsets from a name" update for the service
instance name ([RFC2136], Section 2.5.3),
* zero or one "Add to an RRset" SRV RR,
* 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),
* 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
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* 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.
* Service Descriptions Instructions do not modify any other resource
records.
An SRP server MUST correctly handle compressed names in the SRV
target.
2.3.1.3. 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,
* A and/or AAAA records must be of sufficient scope to be reachable
by all hosts that might query the DNS. If a link-scope address or
IPv4 autoconfiguration address is provided by the SRP client, the
SRP server MUST treat this as if no address records were received;
that is, the Host Description is not valid.
* 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,
* there is a Service Instance Name Instruction in the SRP Update for
which the SRV RR that is added points to the hostname being
updated by this update.
* Host Description Instructions do not modify any other resource
records.
2.3.2. Valid SRP Update Requirements
An SRP Update MUST include zero or more Service Discovery
Instructions. For each Service Discovery Instruction, there MUST be
at least one Service Description Instruction. Note that in the case
of SRP subtypes (Section 7.1 of [RFC6763]), it's quite possible that
two Service Discovery Instructions might reference the same Service
Description Instruction. For each Service Description Instruction
there MUST be at least one Service Discovery Instruction with its
service instance name as the target of its PTR record. There MUST be
exactly one Host Description Instruction. Every Service Description
Instruction must have that Host Description Instruction as the target
of its SRV record. A DNS Update that does not meet these constraints
is not an SRP Update.
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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. Such messages
should either be processed as regular RFC2136 updates, including
access control checks and constraint checks, if supported, or else
rejected with RCODE=REFUSED.
In addition, in order for an update to be a valid SRP Update, the
target of every Service Discovery Instruction MUST be a Service
Description Instruction that is present in the SRP Update. There
MUST NOT be any Service Description Instruction to which no Service
Discovery Instruction points. The target of the SRV record in every
Service Description Instruction MUST be the single Host Description
Instruction.
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.
2.3.3. FCFS Name And Signature Validation
Assuming that a DNS Update message has been validated with these
conditions and is a valid SRP Update, the server checks that the name
in the Host Description Instruction exists. If so, then the server
checks to see if the KEY record on that name is the same as the KEY
record in the Host Description Instruction. The server 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 server MUST reject the SRP Update with the
YXDOMAIN RCODE.
Otherwise, the server 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 server 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.
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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.
2.3.4. Handling of Service Subtypes
SRP servers 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 DNS server 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.
2.3.5. SRP Update response
The status that is returned depends on the result of processing the
update, and can be either SUCCESS or SERVFAIL: all other possible
outcomes should already have been accounted for when applying the
constraints that qualify the update as an SRP Update.
2.3.6. Optional Behavior
The server MAY add a Reverse Mapping 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 server to
add a reverse mapping update, it must be authoritative for the zone
or have credentials to do the update. The SRP client MAY also do a
reverse mapping update if it has credentials to do so.
The server 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.
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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 service implementation. Second, the
registration protocol includes maintenance functionality not present
with normal DNS updates.
Note that the semantics of using SRP in this way are different than
for typical RFC2136 implementations: the KEY used to sign the SRP
Update only allows the SRP client 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 server may 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
YXDOMAIN.
3. 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 services and requires that SRP servers enforce consistency.
Services sending SRP Updates MUST use consistent TTLs in all RRs
within the SRP Update.
SRP servers 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 server 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 client's
guess as to what a good TTL would be. SRP servers may override these
TTLs. SRP servers SHOULD ensure that TTLs are reasonable: neither
too long nor too short. The TTL should never be longer than the
lease time (Section 4.1). 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 server.
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4. Maintenance
4.1. 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 client, it MUST include an EDNS(0)
Update Lease Option [I-D.sekar-dns-ul]. 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 client 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 should be assumed to be valid. The
Key Lease time represents the time after the update during which the
KEY record should be assumed to be valid.
The reasoning behind the different lease times is discussed in the
section on first-come, first-served naming (Section 2.2.4.1). SRP
servers 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; servers that support such devices may need to set higher
limits. SRP clients that are going to continue to use names on which
they hold leases should update well before the lease ends, in case
the registration service 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.
SRP Servers SHOULD also track a lease time per service instance. The
reason for doing this is that a client 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 server SHOULD remove the
service instance (although the KEY record for the service instance
SHOULD be retained until the key lease on that service expires).
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This is beneficial because if the SRP client continues to renew the
host, but never mentions the stale service again, the stale service
will continue to be advertised.
The SRP server MUST include an EDNS(0) Update Lease option in the
response if the lease time proposed by the service has been shortened
or lengthened. The service 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 server when deciding when
to update its registration. The times may be shorter or longer than
those specified in the SRP Update; the SRP client must honor them in
either case.
SRP clients should assume that each lease ends N seconds after the
update was first transmitted, where N is the lease duration. Servers
should assume that each lease ends N seconds after the update that
was successfully processed was received. Because the server will
always receive the update after the SRP client sent it, this avoids
the possibility of misunderstandings.
SRP servers MUST reject updates that do not include an EDNS(0) Update
Lease option. Dual-use servers 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'.
5. Security Considerations
5.1. Source Validation
SRP Updates have no authorization semantics other than first-come,
first-served. This means that if an attacker from outside of the
administrative domain of the server knows the server's IP address, it
can in principle send updates to the server that will be processed
successfully. Servers should therefore be configured to reject
updates from source addresses outside of the administrative domain of
the server.
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For updates sent to an anycast IP address of an SRP server, this
validation must be enforced by every router on the path from the
Constrained-Device Network to the unconstrained portion of the
network. 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 servers relying on three-way-
handshake validation MUST NOT accept TCP Fast Open payloads. If the
network infrastructure allows it, an SRP server 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.
Note that these rules only apply to the validation of SRP Updates. A
server that accepts updates from SRP clients may also accept other
DNS updates, and those DNS updates may be validated using different
rules. However, in the case of a DNS service 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 Server to an SRP
client, by replacing all or part of the service registration
information with information provided by an authenticated DNS update
client. An implementation that allows both kinds of updates should
not allow DNS Update clients that are using different authentication
and authorization credentials to to update records added by SRP
clients.
5.2. SRP Server Authentication
This specification does not provide a mechanism for validating
responses from DNS servers to SRP clients. In the case of
Constrained Network/Constrained Node clients, such validation isn't
practical because there's no way to establish trust. In principle, a
KEY RR could be used by a non-constrained SRP client to validate
responses from the server, but this is not required, nor do we
specify a mechanism for determining which key to use.
5.3. Required Signature Algorithm
For validation, SRP servers MUST implement the ECDSAP256SHA256
signature algorithm. SRP servers 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 clients MUST NOT assume that any algorithm numbered lower
than 13 is available for use in validating SIG(0) signatures.
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6. Privacy Considerations
Because DNSSD 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. Server implementations MUST offer TLS support. The use
of TLS with DNS is described in [RFC7858] and [RFC8310].
Hosts that implement TLS support SHOULD NOT fall back to TCP; since
servers 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 servers
MAY elect not to return KEY records for queries for SRP
registrations.
7. 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 should be set up as was done for 'home.arpa', as a
result of the specification in Section 7 of [RFC8375].
8. IANA Considerations
8.1. 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 7.
IANA is further requested to add a new entry to the "Transport-
Independent Locally-Served Zones" subregistry of the the "Locally-
Served DNS Zones" registry [LSDZ]. The entry will be for the domain
'service.arpa.' with the description "DNS-SD Registration Protocol
Special-Use Domain", listing this document as the reference.
8.2. 'dnssd-srp' Service Name
IANA is also requested to add a new entry to the Service Names and
Port Numbers registry for dnssd-srp with a transport type of tcp. No
port number is to be assigned. The reference should be to this
document, and the Assignee and Contact information should reference
the authors of this document. The Description should be as follows:
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Availability of DNS Service Discovery Service Registration Protocol
Service for a given domain is advertised using the
"_dnssd-srp._tcp.<domain>" SRV record gives the target host and port
where DNSSD Service Registration Service is provided for the named
domain.
8.3. 'dnssd-srp-tls' Service Name
IANA is also requested to add a new entry to the Service Names and
Port Numbers registry for dnssd-srp with a transport type of tcp. No
port number is to be assigned. The reference should be to this
document, and the Assignee and Contact information should reference
the authors of this document. The Description should be as follows:
Availability of DNS Service Discovery Service Registration Protocol
Service for a given domain over TLS is advertised using the
"_dnssd-srp-tls._tcp.<domain>." SRV record gives the target host and
port where DNSSD Service Registration Service is provided for the
named domain.
8.4. 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 should be
replaced with the actual allocation in the table that follows. The
values for the registry are:
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+----------------------+-----------------------------+
| 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 1
9. Implementation Status
[Note to the RFC Editor: please remove this section prior to
publication.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 7942.
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
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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 clients:
* 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 server. 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 [I-D.sctl-advertising-proxy]. Both are included in
the mDNSResponder open source project mentioned above.
10. 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 a SRP
client proof-of-concept implementation at the Montreal Hackathon at
IETF 102, and Tom Pusateri for reviewing during the hackathon and
afterwards.
11. Normative References
[I-D.sekar-dns-ul]
Cheshire, S. and T. Lemon, "An EDNS0 option to negotiate
Leases on DNS Updates", Work in Progress, Internet-Draft,
draft-sekar-dns-ul-03, 27 July 2021,
<https://datatracker.ietf.org/doc/html/draft-sekar-dns-ul-
03>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[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>.
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[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>.
[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>.
[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>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC8106, March 2017,
<https://www.rfc-editor.org/info/rfc8106>.
[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>.
[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>.
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12. Informative 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>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[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>.
[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>.
[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>.
[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>.
[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>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
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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>.
[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>.
[I-D.cheshire-dnssd-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>.
[I-D.cheshire-edns0-owner-option]
Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", Work
in Progress, Internet-Draft, draft-cheshire-edns0-owner-
option-01, 3 July 2017,
<https://datatracker.ietf.org/doc/html/draft-cheshire-
edns0-owner-option-01>.
[I-D.sctl-advertising-proxy]
Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
Service Registration Protocol", Work in Progress,
Internet-Draft, draft-sctl-advertising-proxy-02, 12 July
2021, <https://datatracker.ietf.org/doc/html/draft-sctl-
advertising-proxy-02>.
[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 servers
It may be useful to set up a 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 2.2.4.1).
A server configured in this way will be able to successfully accept
and process SRP Updates from services that send SRP updates.
However, no prerequisites will be applied, and this means that the
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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.
Since SRP Updates are signed with keys, validation of the SIG(0)
algorithm used by the client can be done by manually installing the
client public key on the DNS server that will be receiving the
updates. The key can then be used to authenticate the client, 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 services 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 server
should normally be configured to be authoritative for
"default.service.arpa". Therefore, a service 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 service 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 make the update using the port and host pointed
to by the SRV record, and should use appropriate prerequisites to
avoid overwriting competing records. Such updates are out of scope
for SRP, and a service that implements SRP MUST first attempt to use
SRP to register itself, and should 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 master;
file "/etc/bind/master/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 513 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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