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.¶
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/.¶
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 5 September 2024.¶
Copyright (c) 2024 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
Provisions Relating to IETF Documents
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respect to this document. Code Components extracted from this
document must include Revised BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Revised BSD License.¶
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.¶
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 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.¶
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.¶
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 SRP updates be the same server that is answering
queries that return records that have been registered.¶
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.¶
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 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.¶
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.¶
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.¶
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 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.¶
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".¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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 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.¶
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.¶
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.¶
The SRP registrar 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 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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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 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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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'.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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).¶
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].¶
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.¶
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.¶
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.¶
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.¶
There are three considerations for caching DNS servers that
follow this specification:¶
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.¶
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.¶
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.¶
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.¶
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.¶
'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'.¶
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]¶
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.¶
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:¶
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].¶
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:¶
[Note to the RFC Editor: please remove this section prior to publication.]¶
This section records the status of known implementations of the protocol defined by this specification at the time of
posting of this Internet-Draft, and is based on a proposal described in RFC 7942. The description of implementations in
this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the
listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent
to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be
construed to be, a catalog of available implementations or their features. Readers are advised to note that other
implementations may exist.¶
According to RFC 7942, "this will allow reviewers and working groups to assign due consideration to documents that have the
benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented
protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
There are two known independent implementations of SRP requestors:¶
SRP Client for OpenThread: https://github.com/openthread/openthread/pull/6038¶
mDNSResponder open source project: https://github.com/Abhayakara/mdnsresponder¶
There are two related implementations of an SRP registrar. One acts as a DNS Update proxy, taking an SRP Update and applying it
to the specified DNS zone using DNS update. The other acts as an Advertising Proxy
[AP]. Both are included in the mDNSResponder open source project mentioned above.¶
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.¶
Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. Miller, "Common DNS Implementation Errors and Suggested Fixes", RFC 1536, DOI 10.17487/RFC1536, , <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, , <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, , <https://www.rfc-editor.org/info/rfc2136>.
Eastlake 3rd, D., "Storage of Diffie-Hellman Keys in the Domain Name System (DNS)", RFC 2539, DOI 10.17487/RFC2539, , <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, , <https://www.rfc-editor.org/info/rfc2782>.
Huston, G., Ed., "Management Guidelines & Operational Requirements for the Address and Routing Parameter Area Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, , <https://www.rfc-editor.org/info/rfc3172>.
Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", STD 88, RFC 3596, DOI 10.17487/RFC3596, , <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, , <https://www.rfc-editor.org/info/rfc4035>.
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, , <https://www.rfc-editor.org/info/rfc7858>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
Wouters, P. and O. Sury, "Algorithm Implementation Requirements and Usage Guidance for DNSSEC", RFC 8624, DOI 10.17487/RFC8624, , <https://www.rfc-editor.org/info/rfc8624>.
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, , <https://www.rfc-editor.org/info/rfc2827>.
Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc6760>.
Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic Requirements for IPv6 Customer Edge Routers", RFC 7084, DOI 10.17487/RFC7084, , <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, , <https://www.rfc-editor.org/info/rfc7228>.
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, , <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, , <https://www.rfc-editor.org/info/rfc8520>.
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, , <https://www.rfc-editor.org/info/rfc8945>.
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.¶
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.¶
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.¶