wdiff rfc9462v2.txt rfc9462.txt

Internet Engineering Task Force (IETF) T. Pauly
Request for Comments: 9462 E. Kinnear
Category: Standards Track Apple Inc.
ISSN: 2070-1721 C. A. Wood
Cloudflare
P. McManus
Fastly
T. Jensen
Microsoft
September
November 2023

Discovery of Designated Resolvers

Abstract

This document defines Discovery of Designated Resolvers (DDR), a set
of mechanisms for DNS clients to use DNS records to discover a
resolver's encrypted DNS configuration. An Encrypted DNS Resolver
discovered in this manner is referred to as a "Designated Resolver".
These mechanisms can be used to move from unencrypted DNS to
encrypted DNS when only the IP address of a resolver is known. These
mechanisms are designed to be limited to cases where Unencrypted DNS
Resolvers and their Designated Resolvers are operated by the same
entity or cooperating entities. It can also be used to discover
support for encrypted DNS protocols when the name of an Encrypted DNS
Resolver is known.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9462.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with 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.

Table of Contents

1. Introduction
1.1. Specification of Requirements
2. Terminology
3. DNS Service Binding Records
4. Discovery Using Resolver IP Addresses
4.1. Use of Designated Resolvers
4.1.1. Use of Designated Resolvers across Network Changes
4.2. Verified Discovery
4.3. Opportunistic Discovery
5. Discovery Using Resolver Names
6. Deployment Considerations
6.1. Caching Forwarders
6.2. Certificate Management
6.3. Server Name Handling
6.4. Handling Non-DDR Queries for resolver.arpa
6.5. Interaction with Network-Designated Resolvers
7. Security Considerations
8. IANA Considerations
8.1. Special-Use Domain Name "resolver.arpa"
8.2. Domain Name Reservation Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Rationale for Using a Special-Use Domain Name
Appendix B. Rationale for Using SVCB Records
Authors' Addresses

1. Introduction

When DNS clients wish to use encrypted DNS protocols such as DNS over
TLS (DoT) [RFC7858], DNS over QUIC (DoQ) [RFC9250], or DNS over HTTPS
(DoH) [RFC8484], they can require additional information beyond the
IP address of the DNS server, such as the resolver's hostname,
alternate IP addresses, non-standard ports, or URI Templates.
However, common configuration mechanisms only provide the resolver's
IP address during configuration. Such mechanisms include network
provisioning protocols like DHCP [RFC2132] [RFC8415] and IPv6 Router
Advertisement (RA) options [RFC8106], as well as manual
configuration.

This document defines two mechanisms for clients to discover
Designated Resolvers that support these encrypted protocols using DNS
server Service Binding (SVCB) records [RFC9460]:

1. When only an IP address of an Unencrypted DNS Resolver is known,
the client queries a Special-Use Domain Name (SUDN) [RFC6761] to
discover DNS SVCB records associated with one or more Encrypted
DNS Resolvers the Unencrypted DNS Resolver has designated for use
when support for DNS encryption is requested (Section 4).

2. When the hostname of an Encrypted DNS Resolver is known, the
client requests details by sending a query for a DNS SVCB record.
This can be used to discover alternate encrypted DNS protocols
supported by a known server, or to provide details if a resolver
name is provisioned by a network (Section 5).

Both of these approaches allow clients to confirm that a discovered
Encrypted DNS Resolver is designated by the originally provisioned
resolver. "Designated" in this context means that the resolvers are
operated by the same entity or cooperating entities; for example, the
resolvers are accessible on the same IP address, or there is a
certificate that contains the IP address for the original designating
resolver.

1.1. Specification of Requirements

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.

2. Terminology

This document defines the following terms:

DDR: Discovery of Designated Resolvers. "DDR" refers to the
mechanisms defined in this document.

Designated Resolver: A resolver, presumably an Encrypted DNS
Resolver, designated by another resolver for use in its own place.
This designation can be verified with TLS certificates.

Encrypted DNS Resolver: A DNS resolver using any encrypted DNS
transport. This includes current mechanisms such as DoH, DoT, and
DoQ, as well as future mechanisms.

Unencrypted DNS Resolver: A DNS resolver using a transport without
encryption, historically TCP or UDP port 53.

3. DNS Service Binding Records

DNS resolvers can advertise one or more Designated Resolvers that may
offer support over encrypted channels and are controlled by the same
entity.

When a client discovers Designated Resolvers, it learns information
such as the supported protocols and ports. This information is
provided in ServiceMode SVCB records for DNS servers, although
AliasMode SVCB records can be used to direct clients to the needed
ServiceMode SVCB record per [RFC9460]. The formatting of these
records, including the DNS-unique parameters such as "dohpath", are
defined by [RFC9461].

The following is an example of an a SVCB record describing a DoH server
discovered by querying for _dns.example.net:

_dns.example.net. 7200 IN SVCB 1 example.net. (
alpn=h2 dohpath=/dns-query{?dns} )

The following is an example of an a SVCB record describing a DoT server
discovered by querying for _dns.example.net:

_dns.example.net. 7200 IN SVCB 1 dot.example.net (
alpn=dot port=8530 )

The following is an example of an a SVCB record describing a DoQ server
discovered by querying for _dns.example.net:

_dns.example.net. 7200 IN SVCB 1 doq.example.net (
alpn=doq port=8530 )

If multiple Designated Resolvers are available, using one or more
encrypted DNS protocols, the resolver deployment can indicate a
preference using the priority fields in each SVCB record [RFC9460].

If the client encounters a mandatory parameter in an a SVCB record it
does not understand, it MUST NOT use that record to discover a
Designated Resolver, in accordance with Section 8 of [RFC9460]. The
client can still use other records in the same response if the client
can understand all of their mandatory parameters. This allows future
encrypted deployments to simultaneously support protocols even if a
given client is not aware of all those protocols. For example, if
the Unencrypted DNS Resolver returns three SVCB records -- one for
DoH, one for DoT, and one for a yet-to-exist protocol -- a client
that only supports DoH and DoT should be able to use those records
while safely ignoring the third record.

To avoid name lookup deadlock, clients that use Designated Resolvers
need to ensure that a specific Encrypted DNS Resolver is not used for
any queries that are needed to resolve the name of the resolver
itself or to perform certificate revocation checks for the resolver,
as described in Section 10 of [RFC8484]. Designated Resolvers need
to ensure that this deadlock is avoidable, as also described in
Section 10 of [RFC8484].

This document focuses on discovering DoH, DoT, and DoQ Designated
Resolvers. Other protocols can also use the format defined by
[RFC9461]. However, if any such protocol does not involve some form
of certificate validation, new validation mechanisms will need to be
defined to support validating designation as defined in Section 4.2.

4. Discovery Using Resolver IP Addresses

When a DNS client is configured with an Unencrypted DNS Resolver IP
address, it SHOULD query the resolver for SVCB records of a service
with a scheme of "dns" and an authority of "resolver.arpa" before
making other queries. This allows the client to switch to using
encrypted DNS for all other queries, if possible. Specifically, the
client issues a query for _dns.resolver.arpa. with the SVCB resource
record type (64) [RFC9460].

Responses to the SVCB query for the "resolver.arpa" SUDN describe
Designated Resolvers. To ensure that different Designated Resolver
configurations can be correctly distinguished and associated with A
and AAAA records for the resolver, ServiceMode SVCB responses to
these queries MUST NOT use the "." or "resolver.arpa" value for the
TargetName. Similarly, clients MUST NOT perform A or AAAA queries
for "resolver.arpa".

The following is an example of an a SVCB record describing a DoH server
discovered by querying for _dns.resolver.arpa.:

_dns.resolver.arpa. 7200 IN SVCB 1 doh.example.net (
alpn=h2 dohpath=/dns-query{?dns} )

The following is an example of an a SVCB record describing a DoT server
discovered by querying for _dns.resolver.arpa.:

_dns.resolver.arpa. 7200 IN SVCB 1 dot.example.net (
alpn=dot port=8530 )

The following is an example of an a SVCB record describing a DoQ server
discovered by querying for _dns.resolver.arpa.:

_dns.resolver.arpa. 7200 IN SVCB 1 doq.example.net (
alpn=doq port=8530 )

If the recursive resolver that receives this query has one or more
Designated Resolvers, it will return the corresponding SVCB records.
When responding to these special queries for "resolver.arpa", the
recursive resolver SHOULD include the A and AAAA records for the name
of the Designated Resolver in the Additional Answers section. This
will save the DNS client an additional round trip to retrieve the
address of the Designated Resolver; see Section 5 of [RFC9460].

Designated Resolvers SHOULD be accessible using the IP address
families that are supported by their associated Unencrypted DNS
Resolvers. If an Unencrypted DNS Resolver is accessible using an
IPv4 address, it ought to provide an A record for an IPv4 address of
the Designated Resolver; similarly, if it is accessible using an IPv6
address, it ought to provide a AAAA record for an IPv6 address of the
Designated Resolver. The Designated Resolver MAY support more
address families than the Unencrypted DNS Resolver, but it SHOULD NOT
support fewer. If this is not done, clients that only have
connectivity over one address family might not be able to access the
Designated Resolver.

If the recursive resolver that receives this query has no Designated
Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa"
zone, to provide a consistent and accurate signal to clients that it
does not have a Designated Resolver.

4.1. Use of Designated Resolvers

When a client discovers Designated Resolvers from an Unencrypted DNS
Resolver IP address, it can choose to use these Designated Resolvers
either (1) automatically or (2) based on some other policy,
heuristic, or user choice.

This document defines two preferred methods for automatically using
Designated Resolvers:

* Verified Discovery (Section 4.2), for when a TLS certificate can
be used to validate the resolver's identity.

* Opportunistic Discovery (Section 4.3), for when a resolver's IP
address is a private or local address.

A client MAY additionally use a discovered Designated Resolver
without either of these methods, based on implementation-specific
policy or user input. Details of such policy are out of scope for
this document. Clients MUST NOT automatically use a Designated
Resolver without some sort of validation, such as the two methods
defined in this document or a future mechanism. Use without
validation can allow an attacker to direct traffic to an Encrypted
DNS Resolver that is unrelated to the original Unencrypted DNS
Resolver, as described in Section 7.

A client MUST NOT reuse a designation discovered using the IP address
of one Unencrypted DNS Resolver in place of any other Unencrypted DNS
Resolver. Instead, the client needs to repeat the discovery process
to discover the Designated Resolver of the other Unencrypted DNS
Resolver. In other words, designations are per-resolver and MUST NOT
be used to configure the client's universal DNS behavior. This
ensures in all cases that queries are being sent to a party
designated by the resolver originally being used.

4.1.1. Use of Designated Resolvers across Network Changes

If a client is configured with the same Unencrypted DNS Resolver IP
address on multiple different networks, a Designated Resolver that
has been discovered on one network SHOULD NOT be reused on any of the
other networks without repeating the discovery process for each
network, since the same IP address may be used for different servers
on the different networks.

4.2. Verified Discovery

Verified Discovery is a mechanism that allows the automatic use of a
Designated Resolver that supports DNS encryption that performs a TLS
handshake.

In order to be considered a verified Designated Resolver, the TLS
certificate presented by the Designated Resolver needs to pass the
following checks made by the client:

1. The client MUST verify the chain of certificates up to a trust
anchor as described in Section 6 of [RFC5280]. The client SHOULD
use the default system or application trust anchors, unless
otherwise configured.

2. The client MUST verify that the certificate contains the IP
address of the designating Unencrypted DNS Resolver in an
iPAddress entry of the subjectAltName extension as described in
Section 4.2.1.6 of [RFC5280].

If these checks pass, the client SHOULD use the discovered Designated
Resolver for any cases in which it would have otherwise used the
Unencrypted DNS Resolver, so as to prefer encrypted DNS whenever
possible.

If these checks fail, the client MUST NOT automatically use the
discovered Designated Resolver if this designation was only
discovered via a _dns.resolver.arpa. query (if the designation was
advertised directly by the network as described in Section 6.5, the
server can still be used). Additionally, the client SHOULD suppress
any further queries for Designated Resolvers using this Unencrypted
DNS Resolver for the length of time indicated by the SVCB record's
Time to Live (TTL) in order to avoid excessive queries that will lead
to further failed validations. The client MAY issue new queries if
the SVCB record's TTL is excessively long (as determined by client
policy) to minimize the length of time an intermittent attacker can
prevent the use of encrypted DNS.

If the Designated Resolver and the Unencrypted DNS Resolver share an
IP address, clients MAY choose to opportunistically use the
Designated Resolver even without this certificate check
(Section 4.3). If the IP address is not shared, opportunistic use
allows for attackers to redirect queries to an unrelated Encrypted
DNS Resolver, as described in Section 7.

Connections to a Designated Resolver can use a different IP address
than the IP address of the Unencrypted DNS Resolver -- for example,
if the process of resolving the SVCB service yields additional
addresses. Even when a different IP address is used for the
connection, the TLS certificate checks described in this section
still apply for the original IP address of the Unencrypted DNS
Resolver.

4.3. Opportunistic Discovery

There are situations where Verified Discovery of encrypted DNS
configuration over unencrypted DNS is not possible. For example, the
identities of Unencrypted DNS Resolvers on private IP addresses
[RFC1918], Unique Local Addresses (ULAs) [RFC4193], and Link-Local
addresses [RFC3927] [RFC4291] cannot be safely confirmed using TLS
certificates under most conditions.

An opportunistic privacy profile is defined for DoT in Section 4.1 of
[RFC7858] as a mode in which clients do not validate the name of the
resolver presented in the certificate. This opportunistic privacy
profile similarly applies to DoQ [RFC9250]. For this profile,
Section 4.1 of [RFC7858] explains that clients might or might not
validate the resolver; however, even if clients choose to perform
some certificate validation checks, they will not be able to validate
the names presented in the SubjectAltName (SAN) field of the
certificate for private and local IP addresses.

A client MAY use information from the SVCB record for
_dns.resolver.arpa. with this opportunistic privacy profile as long
as the IP address of the Encrypted DNS Resolver does not differ from
the IP address of the Unencrypted DNS Resolver. Clients SHOULD use
this mode only for resolvers using private or local IP addresses,
since resolvers that use other addresses are able to provision TLS
certificates for their addresses.

5. Discovery Using Resolver Names

A DNS client that already knows the name of an Encrypted DNS Resolver
can use DDR to discover details about all supported encrypted DNS
protocols. This situation can arise if a client has been configured
to use a given Encrypted DNS Resolver, or if a network provisioning
protocol (such as DHCP or IPv6 RAs) provides a name for an Encrypted
DNS Resolver alongside the resolver IP address, such as by using
Discovery of Network-designated Resolvers (DNR) [RFC9463].

For these cases, the client simply sends a DNS SVCB query using the
known name of the resolver. This query can be issued to the named
Encrypted DNS Resolver itself or to any other resolver. Unlike the
case of bootstrapping from an Unencrypted DNS Resolver (Section 4),
these records SHOULD be available in the public DNS if the same
domain name's A or AAAA records are available in the public DNS to
allow using any resolver to discover another resolver's Designated
Resolvers. When the name can only be resolved in private namespaces,
these records SHOULD be available to the same audience as the A and
AAAA records.

For example, if the client already knows about a DoT server
resolver.example.com, it can issue an a SVCB query for
_dns.resolver.example.com to discover if there are other encrypted
DNS protocols available. In the following example, the SVCB answers
indicate that resolver.example.com supports both DoH and DoT and that
the DoH server indicates a higher priority than the DoT server.

_dns.resolver.example.com. 7200 IN SVCB 1 resolver.example.com. (
alpn=h2 dohpath=/dns-query{?dns} )
_dns.resolver.example.com. 7200 IN SVCB 2 resolver.example.com. (
alpn=dot )

Clients MUST validate that for any Encrypted DNS Resolver discovered
using a known resolver name, the TLS certificate of the resolver
contains the known name in a subjectAltName extension. In the
example above, this means that both servers need to have certificates
that cover the name resolver.example.com. Often, the various
supported encrypted DNS protocols will be specified such that the
SVCB TargetName matches the known name, as is true in the example
above. However, even when the TargetName is different (for example,
if the DoH server had a TargetName of doh.example.com), the clients
still check for the original known resolver name in the certificate.

Note that this resolver validation is not related to the DNS resolver
that provided the SVCB answer.

As another example, being able to discover a Designated Resolver for
a known Encrypted DNS Resolver is useful when a client has a DoT
configuration for foo.resolver.example.com but is on a network that
blocks DoT traffic. The client can still send a query to any other
accessible resolver (either the local network resolver or an
accessible DoH server) to discover if there is a designated DoH
server for foo.resolver.example.com.

6. Deployment Considerations

Resolver deployments that support DDR are advised to consider the
following points.

6.1. Caching Forwarders

A DNS forwarder SHOULD NOT forward queries for "resolver.arpa" (or
any subdomains) upstream. This prevents a client from receiving an a
SVCB record that will fail to authenticate because the forwarder's IP
address is not in the SubjectAltName (SAN) field of the upstream
resolver's Designated Resolver's TLS certificate. A DNS forwarder
that already acts as a completely transparent forwarder MAY choose to
forward these queries when the operator expects that this does not
apply, because the operator either knows that the upstream resolver
does have the forwarder's IP address in its TLS certificate's SAN
field or expects clients to validate the connection via some future
mechanism.

Operators who choose to forward queries for "resolver.arpa" upstream
should note that client behavior is never guaranteed and that the use
of DDR by a resolver does not communicate a requirement for clients
to use the SVCB record when it cannot be verified.

6.2. Certificate Management

Resolver owners that support Verified Discovery will need to list
valid referring IP addresses in their TLS certificates. This may
pose challenges for resolvers with a large number of referring IP
addresses.

6.3. Server Name Handling

Clients MUST NOT use "resolver.arpa" as the server name in either
(1) the TLS Server Name Indication (SNI) [RFC8446] for DoT, DoQ, or
DoH connections or (2) the URI host for DoH requests.

When performing discovery using resolver IP addresses, clients MUST
use the original IP address of the Unencrypted DNS Resolver as the
URI host for DoH requests.

Note that since IP addresses are not supported by default in the TLS
SNI, resolvers that support discovery using IP addresses will need to
be configured to present the appropriate TLS certificate when no SNI
is present for DoT, DoQ, and DoH.

6.4. Handling Non-DDR Queries for resolver.arpa

DNS resolvers that support DDR by responding to queries for
_dns.resolver.arpa. MUST treat resolver.arpa as a locally served zone
per [RFC6303]. In practice, this means that resolvers SHOULD respond
to queries of any type other than SVCB for _dns.resolver.arpa. with
NODATA and queries of any type for any domain name under
resolver.arpa with NODATA.

6.5. Interaction with Network-Designated Resolvers

DNR [RFC9463] allows a network to provide designation of resolvers
directly through DHCP [RFC2132] [RFC8415] and through IPv6 RA options
[RFC8106]. When such indications are present, clients can suppress
queries for "resolver.arpa" to the unencrypted DNS server indicated
by the network over DHCP or RAs, and the DNR indications SHOULD take
precedence over those discovered using "resolver.arpa" for the same
resolver if there is a conflict, since DNR is considered a more
reliable source.

The Designated Resolver information in DNR might not contain a full
set of SvcParams needed to connect to an Encrypted DNS Resolver. In
such a case, the client can use an a SVCB query using a resolver name,
as described in Section 5, to the Authentication Domain Name (ADN).

7. Security Considerations

Since clients can receive DNS SVCB answers over unencrypted DNS, on-
path attackers can prevent successful discovery by dropping SVCB
queries or answers and thus can prevent clients from switching to
using encrypted DNS. Clients should be aware that it might not be
possible to distinguish between resolvers that do not have any
Designated Resolver and such an active attack. To limit the impact
of discovery queries being dropped either maliciously or
unintentionally, clients can re-send their SVCB queries periodically.

Section 8.2 of [RFC9461] describes another type of downgrade attack
where an attacker can block connections to the encrypted DNS server.
For DDR, clients need to validate a Designated Resolver using a
connection to the server before trusting it, so attackers that can
block these connections can prevent clients from switching to using
encrypted DNS.

Encrypted DNS Resolvers that allow discovery using DNS SVCB answers
over unencrypted DNS MUST NOT provide differentiated behavior based
solely on metadata in the SVCB record, such as the HTTP path or
alternate port number, which are parameters that an attacker could
modify. For example, if a DoH resolver provides a filtering service
for one URI path and a non-filtered service for another URI path, an
attacker could select which of these services is used by modifying
the "dohpath" parameter. These attacks can be mitigated by providing
separate resolver IP addresses or hostnames.

While the IP address of the Unencrypted DNS Resolver is often
provisioned over insecure mechanisms, it can also be provisioned
securely, such as via manual configuration, on a VPN, or on a network
with protections like RA-Guard [RFC6105]. An attacker might try to
direct encrypted DNS traffic to itself by causing the client to think
that a discovered Designated Resolver uses a different IP address
from the Unencrypted DNS Resolver. Such a Designated Resolver might
have a valid certificate but might be operated by an attacker that is
trying to observe or modify user queries without the knowledge of the
client or network.

If the IP address of a Designated Resolver differs from that of an
Unencrypted DNS Resolver, clients applying Verified Discovery
(Section 4.2) MUST validate that the IP address of the Unencrypted
DNS Resolver is covered by the SubjectAltName (SAN) of the Designated
Resolver's TLS certificate. If that validation fails, the client
MUST NOT automatically use the discovered Designated Resolver.

Clients using Opportunistic Discovery (Section 4.3) MUST be limited
to cases where the Unencrypted DNS Resolver and Designated Resolver
have the same IP address, which SHOULD be a private or local IP
address. Clients that do not follow Opportunistic Discovery
(Section 4.3) and instead try to connect without first checking for a
designation run the possible risk of being intercepted by an attacker
hosting an Encrypted DNS Resolver on an IP address of an Unencrypted
DNS Resolver where the attacker has failed to gain control of the
Unencrypted DNS Resolver.

The constraints on the use of Designated Resolvers specified here
apply specifically to the automatic discovery mechanisms defined in
this document, which are referred to as Verified Discovery and
Opportunistic Discovery. Clients MAY use some other mechanism to
verify and use Designated Resolvers discovered using the DNS SVCB
record. However, the use of such an alternate mechanism needs to
take into account the attack scenarios detailed here.

8. IANA Considerations

8.1. Special-Use Domain Name "resolver.arpa"

IANA has registered "resolver.arpa" in the "Special-Use Domain Names"
registry established by [RFC6761].

IANA has added an entry in the "Transport-Independent Locally-Served
DNS Zone Registry" for 'resolver.arpa.' with the description "DNS
Resolver Special-Use Domain" and listed this document as the
reference.

8.2. Domain Name Reservation Considerations

In accordance with Section 5 of [RFC6761], the answers to the
following questions are provided relative to this document:

1. Are human users expected to recognize these names as special and
use them differently? In what way?

No. This name is used automatically by DNS stub resolvers
running on client devices on behalf of users, and users will
never see this name directly.

2. Are writers of application software expected to make their
software recognize these names as special and treat them
differently? In what way?

No. There is no use case where a non-DNS application (covered by
the next question) would need to use this name.

3. Are writers of name resolution APIs and libraries expected to
make their software recognize these names as special and treat
them differently? If so, how?

Yes. DNS client implementors are expected to use this name when
querying for a resolver's properties instead of records for the
name itself. DNS servers are expected to respond to queries for
this name with their own properties instead of checking the
matching zone as it would for normal domain names.

4. Are developers of caching domain name servers expected to make
their implementations recognize these names as special and treat
them differently? If so, how?

Yes. Caching domain name servers should not forward queries for
this name, to avoid causing validation failures due to IP address
mismatch.

5. Are developers of authoritative domain name servers expected to
make their implementations recognize these names as special and
treat them differently? If so, how?

No. DDR is designed for use by recursive resolvers.
Theoretically, an authoritative server could choose to support
this name if it wants to advertise support for encrypted DNS
protocols over plaintext DNS, but that scenario is covered by
other work in the IETF DNSOP Working Group.

6. Does this reserved Special-Use Domain Name have any potential
impact on DNS server operators? If they try to configure their
authoritative DNS server as authoritative for this reserved name,
will compliant name server software reject it as invalid? Do DNS
server operators need to know about that and understand why?
Even if the name server software doesn't prevent them from using
this reserved name, are there other ways that it may not work as
expected, of which the DNS server operator should be aware?

This name is locally served, and any resolver that supports this
name should never forward the query. DNS server operators should
be aware that records for this name will be used by clients to
modify the way they connect to their resolvers.

7. How should DNS Registries/Registrars treat requests to register
this reserved domain name? Should such requests be denied?
Should such requests be allowed, but only to a specially
designated entity?

IANA holds the registration for this name. Non-IANA requests to
register this name should always be denied by DNS Registries/
Registrars.

9. References

9.1. Normative References

[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<https://www.rfc-editor.org/info/rfc3927>.

[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
RFC 6303, DOI 10.17487/RFC6303, July 2011,
<https://www.rfc-editor.org/info/rfc6303>.

[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>.

[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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.

[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/info/rfc9250>.

[RFC9460] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (DNS SVCB (SVCB and HTTPS
Resource Records (RRs))", Records)", RFC 9460, DOI 10.17487/RFC9460, September
November 2023, <https://www.rfc-editor.org/info/rfc9460>.

[RFC9461] Schwartz, B., "Service Binding Mapping for DNS Servers",
RFC 9461, DOI 10.17487/RFC9461, September November 2023,
<https://www.rfc-editor.org/info/rfc9461>.

[RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
and T. Jensen, "DHCP and Router Advertisement Options for
the Discovery of Network-designated Resolvers (DNR)",
RFC 9463, DOI 10.17487/RFC9463, September November 2023,
<https://www.rfc-editor.org/info/rfc9463>.

9.2. Informative References

[DoH-HINTS]
Schinazi, D., Sullivan, N., and J. Kipp, "DoH Preference
Hints for HTTP", Work in Progress, Internet-Draft, draft-
schinazi-httpbis-doh-preference-hints-02, 13 July 2020,
<https://datatracker.ietf.org/doc/html/draft-schinazi-
httpbis-doh-preference-hints-02>.

[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-16, 6 April
draft-ietf-tls-esni-17, 9 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-16>.
esni-17>.

[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>.

[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
DOI 10.17487/RFC6105, February 2011,
<https://www.rfc-editor.org/info/rfc6105>.

[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>.

[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>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name
'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August
2020, <https://www.rfc-editor.org/info/rfc8880>.

Appendix A. Rationale for Using a Special-Use Domain Name

The "resolver.arpa" SUDN is similar to "ipv4only.arpa" in that the
querying client is not interested in an answer from the authoritative
"arpa" name servers. The intent of the SUDN is to allow clients to
communicate with the Unencrypted DNS Resolver much like
"ipv4only.arpa" allows for client-to-middlebox communication. For
more context, see [RFC8880] for the rationale behind "ipv4only.arpa".

Appendix B. Rationale for Using SVCB Records

These mechanisms use SVCB/HTTPS resource records [RFC9460] to
communicate that a given domain designates a particular Designated
Resolver for clients to use in place of an Unencrypted DNS Resolver
(using a SUDN) or another Encrypted DNS Resolver (using its domain
name).

There are various other proposals for how to provide similar
functionality. There are several reasons that these mechanisms have
chosen SVCB records:

* Discovering Encrypted DNS Resolvers using DNS records keeps client
logic for DNS self-contained and allows a DNS resolver operator to
define which resolver names and IP addresses are related to one
another.

* Using DNS records also does not rely on bootstrapping with higher-
level application operations (such as those discussed in
[DoH-HINTS]).

* SVCB records are extensible and allow the definition of parameter
keys, making them a superior mechanism for extensibility as
compared to approaches such as overloading TXT records. The same
keys can be used for discovering Designated Resolvers of different
transport types as well as those advertised by Unencrypted DNS
Resolvers or another Encrypted DNS Resolver.

* Clients and servers that are interested in privacy of names will
already need to support SVCB records in order to use the TLS
Encrypted ClientHello [ECH]. Without encrypting names in TLS, the
value of encrypting DNS is reduced, so pairing the solutions
provides the greatest benefit.

Authors' Addresses

Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: tpauly@apple.com

Eric Kinnear
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: ekinnear@apple.com

Christopher A. Wood
Cloudflare
101 Townsend St
San Francisco, California 94107
United States of America
Email: caw@heapingbits.net

Patrick McManus
Fastly
Email: mcmanus@ducksong.com

Tommy Jensen
Microsoft
Email: tojens@microsoft.com