draft-ietf-add-ddr-02.txt		draft-ietf-add-ddr-03.txt
	ADD T. Pauly		ADD T. Pauly
	Internet-Draft E. Kinnear		Internet-Draft E. Kinnear
	Intended status: Standards Track Apple Inc.		Intended status: Standards Track Apple Inc.
	Expires: 9 January 2022 C.A. Wood		Expires: 4 April 2022 C.A. Wood
	Cloudflare		Cloudflare
	P. McManus		P. McManus
	Fastly		Fastly
	T. Jensen		T. Jensen
	Microsoft		Microsoft
	8 July 2021		1 October 2021
	Discovery of Designated Resolvers		Discovery of Designated Resolvers
	draft-ietf-add-ddr-02		draft-ietf-add-ddr-03
	Abstract		Abstract
	This document defines Discovery of Designated Resolvers (DDR), a		This document defines Discovery of Designated Resolvers (DDR), a
	mechanism for DNS clients to use DNS records to discover a resolver's		mechanism for DNS clients to use DNS records to discover a resolver's
	encrypted DNS configuration. This mechanism can be used to move from		encrypted DNS configuration. This mechanism can be used to move from
	unencrypted DNS to encrypted DNS when only the IP address of a		unencrypted DNS to encrypted DNS when only the IP address of a
	resolver is known. It can also be used to discover support for		resolver is known. This mechanism is designed to be limited to cases
	encrypted DNS protocols when the name of an encrypted resolver is		where unencrypted resolvers and their designated resolvers are
	known. This mechanism is designed to be limited to cases where		operated by the same entity or cooperating entities. It can also be
	unencrypted resolvers and their designated resolvers are operated by		used to discover support for encrypted DNS protocols when the name of
	the same entity or cooperating entities.		an encrypted resolver is known.
	Discussion Venues		Discussion Venues
	This note is to be removed before publishing as an RFC.		This note is to be removed before publishing as an RFC.
	Discussion of this document takes place on the Adaptive DNS Discovery		Discussion of this document takes place on the Adaptive DNS Discovery
	Working Group mailing list (add@ietf.org), which is archived at		Working Group mailing list (add@ietf.org), which is archived at
	https://mailarchive.ietf.org/arch/browse/add/.		https://mailarchive.ietf.org/arch/browse/add/.
	Source for this draft and an issue tracker can be found at		Source for this draft and an issue tracker can be found at
	skipping to change at page 2, line 10 ¶		skipping to change at page 2, line 10 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on 9 January 2022.		This Internet-Draft will expire on 4 April 2022.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 IETF Trust and the persons identified as the		Copyright (c) 2021 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents (https://trustee.ietf.org/		Provisions Relating to IETF Documents (https://trustee.ietf.org/
	license-info) in effect on the date of publication of this document.		license-info) in effect on the date of publication of this document.
	Please review these documents carefully, as they describe your rights		Please review these documents carefully, as they describe your rights
	skipping to change at page 2, line 33 ¶		skipping to change at page 2, line 33 ¶
	as described in Section 4.e of the Trust Legal Provisions and are		as described in Section 4.e of the Trust Legal Provisions and are
	provided without warranty as described in the Simplified BSD License.		provided without warranty as described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Specification of Requirements . . . . . . . . . . . . . . 3		1.1. Specification of Requirements . . . . . . . . . . . . . . 3
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3. DNS Service Binding Records . . . . . . . . . . . . . . . . . 4		3. DNS Service Binding Records . . . . . . . . . . . . . . . . . 4
	4. Discovery Using Resolver IP Addresses . . . . . . . . . . . . 5		4. Discovery Using Resolver IP Addresses . . . . . . . . . . . . 5
	4.1. Authenticated Discovery . . . . . . . . . . . . . . . . . 6		4.1. Use of Designated Resolvers . . . . . . . . . . . . . . . 6
	4.2. Opportunistic Discovery . . . . . . . . . . . . . . . . . 6		4.2. Authenticated Discovery . . . . . . . . . . . . . . . . . 6
			4.3. Opportunistic Discovery . . . . . . . . . . . . . . . . . 7
	5. Discovery Using Resolver Names . . . . . . . . . . . . . . . 7		5. Discovery Using Resolver Names . . . . . . . . . . . . . . . 7
	6. Deployment Considerations . . . . . . . . . . . . . . . . . . 7		6. Deployment Considerations . . . . . . . . . . . . . . . . . . 8
	6.1. Caching Forwarders . . . . . . . . . . . . . . . . . . . 8		6.1. Caching Forwarders . . . . . . . . . . . . . . . . . . . 8
	6.2. Certificate Management . . . . . . . . . . . . . . . . . 8		6.2. Certificate Management . . . . . . . . . . . . . . . . . 9
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 8		7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
	8.1. Special Use Domain Name "resolver.arpa" . . . . . . . . . 9		8.1. Special Use Domain Name "resolver.arpa" . . . . . . . . . 10
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
	9.1. Normative References . . . . . . . . . . . . . . . . . . 9		9.1. Normative References . . . . . . . . . . . . . . . . . . 10
	9.2. Informative References . . . . . . . . . . . . . . . . . 10		9.2. Informative References . . . . . . . . . . . . . . . . . 11
	Appendix A. Rationale for using SVCB records . . . . . . . . . . 11		Appendix A. Rationale for using SVCB records . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
	1. Introduction		1. Introduction
	When DNS clients wish to use encrypted DNS protocols such as DNS-		When DNS clients wish to use encrypted DNS protocols such as DNS-
	over-TLS (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [RFC8484], they		over-TLS (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [RFC8484], they
	require additional information beyond the IP address of the DNS		require additional information beyond the IP address of the DNS
	server, such as the resolver's hostname, non-standard ports, or URL		server, such as the resolver's hostname, non-standard ports, or URL
	paths. However, common configuration mechanisms only provide the		paths. However, common configuration mechanisms only provide the
	resolver's IP address during configuration. Such mechanisms include		resolver's IP address during configuration. Such mechanisms include
	network provisioning protocols like DHCP [RFC2132] and IPv6 Router		network provisioning protocols like DHCP [RFC2132] and IPv6 Router
	skipping to change at page 4, line 25 ¶		skipping to change at page 4, line 25 ¶
	3. DNS Service Binding Records		3. DNS Service Binding Records
	DNS resolvers can advertise one or more Designated Resolvers that may		DNS resolvers can advertise one or more Designated Resolvers that may
	offer support over encrypted channels and are controlled by the same		offer support over encrypted channels and are controlled by the same
	entity.		entity.
	When a client discovers Designated Resolvers, it learns information		When a client discovers Designated Resolvers, it learns information
	such as the supported protocols, ports, and server name to use in		such as the supported protocols, ports, and server name to use in
	certificate validation. This information is provided in Service		certificate validation. This information is provided in Service
	Binding (SVCB) records for DNS Servers, defined by		Binding (SVCB) records for DNS Servers. The formatting of these
	[I-D.schwartz-svcb-dns].		records, including the DNS-unique parameters such as "dohpath", are
			defined by [I-D.ietf-add-svcb-dns].
	The following is an example of an SVCB record describing a DoH server		The following is an example of an SVCB record describing a DoH server
	discovered by querying for "_dns.example.net":		discovered by querying for _dns.example.net:
	_dns.example.net 7200 IN SVCB 1 . (		_dns.example.net 7200 IN SVCB 1 . (
	alpn=h2 dohpath=/dns-query{?dns} )		alpn=h2 dohpath=/dns-query{?dns} )
	The following is an example of an SVCB record describing a DoT server		The following is an example of an SVCB record describing a DoT server
	discovered by querying for "_dns.example.net":		discovered by querying for _dns.example.net:
	_dns.example.net 7200 IN SVCB 1 dot.example.net (		_dns.example.net 7200 IN SVCB 1 dot.example.net (
	alpn=dot port=8530 )		alpn=dot port=8530 )
	If multiple Designated Resolvers are available, using one or more		If multiple Designated Resolvers are available, using one or more
	encrypted DNS protocols, the resolver deployment can indicate a		encrypted DNS protocols, the resolver deployment can indicate a
	preference using the priority fields in each SVCB record		preference using the priority fields in each SVCB record
	[I-D.ietf-dnsop-svcb-https].		[I-D.ietf-dnsop-svcb-https].
	To avoid name lookup deadlock, Designated Resolvers SHOULD follow the		To avoid name lookup deadlock, Designated Resolvers SHOULD follow the
	guidance in Section 10 of [RFC8484] regarding the avoidance of DNS-		guidance in Section 10 of [RFC8484] regarding the avoidance of DNS-
	based references that block the completion of the TLS handshake.		based references that block the completion of the TLS handshake.
	This document focuses on discovering DoH and DoT Designated		This document focuses on discovering DoH and DoT Designated
	Resolvers. Other protocols can also use the format defined by		Resolvers. Other protocols can also use the format defined by
	[I-D.schwartz-svcb-dns]. However, if any protocol does not involve		[I-D.ietf-add-svcb-dns]. However, if any protocol does not involve
	some form of certificate validation, new validation mechanisms will		some form of certificate validation, new validation mechanisms will
	need to be defined to support validating designation as defined in		need to be defined to support validating designation as defined in
	Section 4.1.		Section 4.2.
	4. Discovery Using Resolver IP Addresses		4. Discovery Using Resolver IP Addresses
	When a DNS client is configured with an Unencrypted Resolver IP		When a DNS client is configured with an Unencrypted Resolver IP
	address, it SHOULD query the resolver for SVCB records for		address, it SHOULD query the resolver for SVCB records for
	"dns://resolver.arpa" before making other queries. Specifically, the		"dns://resolver.arpa" before making other queries. Specifically, the
	client issues a query for "_dns.resolver.arpa" with the SVCB resource		client issues a query for _dns.resolver.arpa with the SVCB resource
	record type (64) [I-D.ietf-dnsop-svcb-https].		record type (64) [I-D.ietf-dnsop-svcb-https].
	Because this query is for an SUDN, which no entity can claim		Because this query is for an SUDN, which no entity can claim
	ownership over, the SVCB response MUST NOT use the "." value for the		ownership over, the SVCB response MUST NOT use the "." value for the
	SvcDomainName. Instead, the domain name used for DoT or used to		TargetName. Instead, the domain name used for DoT or used to
	construct the DoH template MUST be provided.		construct the DoH template MUST be provided.
	The following is an example of an SVCB record describing a DoH server		The following is an example of an SVCB record describing a DoH server
	discovered by querying for "_dns.resolver.arpa":		discovered by querying for _dns.resolver.arpa:
	_dns.resolver.arpa 7200 IN SVCB 1 doh.example.net (		_dns.resolver.arpa 7200 IN SVCB 1 doh.example.net (
	alpn=h2 dohpath=/dns-query{?dns} )		alpn=h2 dohpath=/dns-query{?dns} )
	The following is an example of an SVCB record describing a DoT server		The following is an example of an SVCB record describing a DoT server
	discovered by querying for "_dns.resolver.arpa":		discovered by querying for _dns.resolver.arpa:
	_dns.resolver.arpa 7200 IN SVCB 1 dot.example.net (		_dns.resolver.arpa 7200 IN SVCB 1 dot.example.net (
	alpn=dot port=8530 )		alpn=dot port=8530 )
	If the recursive resolver that receives this query has one or more		If the recursive resolver that receives this query has one or more
	Designated Resolvers, it will return the corresponding SVCB records.		Designated Resolvers, it will return the corresponding SVCB records.
	When responding to these special queries for "dns://resolver.arpa",		When responding to these special queries for "dns://resolver.arpa",
	the recursive resolver SHOULD include the A and AAAA records for the		the recursive resolver SHOULD include the A and AAAA records for the
	name of the Designated Resolver in the Additional Answers section.		name of the Designated Resolver in the Additional Answers section.
	This will allow the DNS client to make queries over an encrypted		This will allow the DNS client to make queries over an encrypted
	skipping to change at page 6, line 5 ¶		skipping to change at page 6, line 5 ¶
	[I-D.ietf-dnsop-svcb-https]. If no A/AAAA records or SVCB IP address		[I-D.ietf-dnsop-svcb-https]. If no A/AAAA records or SVCB IP address
	hints are included, clients will be forced to delay use of the		hints are included, clients will be forced to delay use of the
	Encrypted Resolver until an additional DNS lookup for the A and AAAA		Encrypted Resolver until an additional DNS lookup for the A and AAAA
	records can be made to the Unencrypted Resolver (or some other		records can be made to the Unencrypted Resolver (or some other
	resolver the DNS client has been configured to use).		resolver the DNS client has been configured to use).
	If the recursive resolver that receives this query has no Designated		If the recursive resolver that receives this query has no Designated
	Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa"		Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa"
	SUDN.		SUDN.
	4.1. Authenticated Discovery		4.1. Use of Designated Resolvers
			When a client discovers Designated Resolvers from an Unencrypted
			Resolver IP address, it can choose to use these Designated Resolvers
			either automatically, or based on some other policy, heuristic, or
			user choice.
			This document defines two preferred methods to automatically use
			Designated Resolvers:
			* Authenticated 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 is
			accessed using a non-public IP 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 of
			this document. Clients SHOULD NOT automatically use a Designated
			Resolver without some sort of validation, such as the two methods
			defined in this document or a future mechanism.
			4.2. Authenticated Discovery
			Authenticated Discovery is a mechanism that allows automatic use of a
			Designated Resolver that supports DNS encryption that performs a TLS
			handshake.
	In order to be considered an authenticated Designated Resolver, the		In order to be considered an authenticated Designated Resolver, the
	TLS certificate presented by the Encrypted Resolver MUST contain both		TLS certificate presented by the Designated Resolver MUST contain
	the domain name (from the SVCB answer) and the IP address of the		both the domain name (from the SVCB answer) and the IP address of the
	designating Unencrypted Resolver within the SubjectAlternativeName		designating Unencrypted Resolver within the SubjectAlternativeName
	certificate field. The client MUST check the SubjectAlternativeName		certificate field. The client MUST check the SubjectAlternativeName
	field for both the Unencrypted Resolver's IP address and the		field for both the Unencrypted Resolver's IP address and the
	advertised name of the Designated Resolver. If the certificate can		advertised name of the Designated Resolver. If the certificate can
	be validated, the client SHOULD use the discovered Designated		be validated, the client SHOULD use the discovered Designated
	Resolver for any cases in which it would have otherwise used the		Resolver for any cases in which it would have otherwise used the
	Unencrypted Resolver. If the Designated Resolver has a different IP		Unencrypted Resolver. If the Designated Resolver has a different IP
	address than the Unencrypted Resolver and the TLS certificate does		address than the Unencrypted Resolver and the TLS certificate does
	not cover the Unencrypted Resolver address, the client MUST NOT use		not cover the Unencrypted Resolver address, the client MUST NOT
	the discovered Encrypted Resolver. Additionally, the client SHOULD		automatically use the discovered Designated Resolver. Additionally,
	suppress any further queries for Designated Resolvers using this		the client SHOULD suppress any further queries for Designated
	Unencrypted Resolver for the length of time indicated by the SVCB		Resolvers using this Unencrypted Resolver for the length of time
	record's Time to Live (TTL).		indicated by the SVCB record's Time to Live (TTL).
	If the Designated Resolver and the Unencrypted Resolver share an IP		If the Designated Resolver and the Unencrypted Resolver share an IP
	address, clients MAY choose to opportunistically use the Encrypted		address, clients MAY choose to opportunistically use the Designated
	Resolver even without this certificate check (Section 4.2).		Resolver even without this certificate check (Section 4.3).
	If resolving the name of an Encrypted Resolver from an SVCB record		If resolving the name of a Designated Resolver from an SVCB record
	yields an IP address that was not presented in the Additional Answers		yields an IP address that was not presented in the Additional Answers
	section or ipv4hint or ipv6hint fields of the original SVCB query,		section or ipv4hint or ipv6hint fields of the original SVCB query,
	the connection made to that IP address MUST pass the same TLS		the connection made to that IP address MUST pass the same TLS
	certificate checks before being allowed to replace a previously known		certificate checks before being allowed to replace a previously known
	and validated IP address for the same Encrypted Resolver name.		and validated IP address for the same Designated Resolver name.
	4.2. Opportunistic Discovery		4.3. Opportunistic Discovery
	There are situations where authenticated discovery of encrypted DNS		There are situations where authenticated discovery of encrypted DNS
	configuration over unencrypted DNS is not possible. This includes		configuration over unencrypted DNS is not possible. This includes
	Unencrypted Resolvers on non-public IP addresses such as those		Unencrypted Resolvers on non-public IP addresses such as those
	defined in [RFC1918] whose identity cannot be confirmed using TLS		defined in [RFC1918] whose identity cannot be confirmed using TLS
	certificates.		certificates.
	Opportunistic Privacy is defined for DoT in Section 4.1 of [RFC7858]		Opportunistic Privacy is defined for DoT in Section 4.1 of [RFC7858]
	as a mode in which clients do not validate the name of the resolver		as a mode in which clients do not validate the name of the resolver
	presented in the certificate. A client MAY use information from the		presented in the certificate. A client MAY use information from the
	skipping to change at page 7, line 21 ¶		skipping to change at page 8, line 6 ¶
	protocol (such as DHCP or IPv6 Router Advertisements) provides a name		protocol (such as DHCP or IPv6 Router Advertisements) provides a name
	for an Encrypted Resolver alongside the resolver IP address.		for an Encrypted Resolver alongside the resolver IP address.
	For these cases, the client simply sends a DNS SVCB query using the		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		known name of the resolver. This query can be issued to the named
	Encrypted Resolver itself or to any other resolver. Unlike the case		Encrypted Resolver itself or to any other resolver. Unlike the case
	of bootstrapping from an Unencrypted Resolver (Section 4), these		of bootstrapping from an Unencrypted Resolver (Section 4), these
	records SHOULD be available in the public DNS.		records SHOULD be available in the public DNS.
	For example, if the client already knows about a DoT server		For example, if the client already knows about a DoT server
	"resolver.example.com", it can issue an SVCB query for		resolver.example.com, it can issue an SVCB query for
	"_dns.resolver.example.com" to discover if there are other encrypted		_dns.resolver.example.com to discover if there are other encrypted
	DNS protocols available. In the following example, the SVCB answers		DNS protocols available. In the following example, the SVCB answers
	indicate that "resolver.example.com" supports both DoH and DoT, and		indicate that resolver.example.com supports both DoH and DoT, and
	that the DoH server indicates a higher priority than the DoT server.		that the DoH server indicates a higher priority than the DoT server.
	_dns.resolver.example.com 7200 IN SVCB 1 . (		_dns.resolver.example.com 7200 IN SVCB 1 . (
	alpn=h2 dohpath=/dns-query{?dns} )		alpn=h2 dohpath=/dns-query{?dns} )
	_dns.resolver.example.com 7200 IN SVCB 2 . (		_dns.resolver.example.com 7200 IN SVCB 2 . (
	alpn=dot )		alpn=dot )
	Often, the various supported encrypted DNS protocols will be		Often, the various supported encrypted DNS protocols will be
	accessible using the same hostname. In the example above, both DoH		accessible using the same hostname. In the example above, both DoH
	and DoT use the name "resolver.example.com" for their TLS		and DoT use the name resolver.example.com for their TLS certificates.
	certificates. If a deployment uses a different hostname for one		If a deployment uses a different hostname for one protocol, but still
	protocol, but still wants clients to treat both DNS servers as		wants clients to treat both DNS servers as designated, the TLS
	designated, the TLS certificates MUST include both names in the		certificates MUST include both names in the SubjectAlternativeName
	SubjectAlternativeName fields. Note that this name verification is		fields. Note that this name verification is not related to the DNS
	not related to the DNS resolver that provided the SVCB answer.		resolver that provided the SVCB answer.
	For example, being able to discover a Designated Resolver for a known		For example, being able to discover a Designated Resolver for a known
	Encrypted Resolver is useful when a client has a DoT configuration		Encrypted Resolver is useful when a client has a DoT configuration
	for "foo.resolver.example.com" but is on a network that blocks DoT		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		traffic. The client can still send a query to any other accessible
	resolver (either the local network resolver or an accessible DoH		resolver (either the local network resolver or an accessible DoH
	server) to discover if there is a designated DoH server for		server) to discover if there is a designated DoH server for
	"foo.resolver.example.com".		foo.resolver.example.com.
	6. Deployment Considerations		6. Deployment Considerations
	Resolver deployments that support DDR are advised to consider the		Resolver deployments that support DDR are advised to consider the
	following points.		following points.
	6.1. Caching Forwarders		6.1. Caching Forwarders
	A DNS forwarder SHOULD NOT forward queries for "resolver.arpa"		A DNS forwarder SHOULD NOT forward queries for "resolver.arpa"
	upstream. This prevents a client from receiving an SVCB record that		upstream. This prevents a client from receiving an SVCB record that
	skipping to change at page 8, line 32 ¶		skipping to change at page 9, line 19 ¶
	6.2. Certificate Management		6.2. Certificate Management
	Resolver owners that support authenticated discovery will need to		Resolver owners that support authenticated discovery will need to
	list valid referring IP addresses in their TLS certificates. This		list valid referring IP addresses in their TLS certificates. This
	may pose challenges for resolvers with a large number of referring IP		may pose challenges for resolvers with a large number of referring IP
	addresses.		addresses.
	7. Security Considerations		7. Security Considerations
	Since client can receive DNS SVCB answers over unencrypted DNS, on-		Since clients can receive DNS SVCB answers over unencrypted DNS, on-
	path attackers can prevent successful discovery by dropping SVCB		path attackers can prevent successful discovery by dropping SVCB
	packets. Clients should be aware that it might not be possible to		packets. Clients should be aware that it might not be possible to
	distinguish between resolvers that do not have any Designated		distinguish between resolvers that do not have any Designated
	Resolver and such an active attack.		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.
	While the IP address of the Unencrypted Resolver is often provisioned		While the IP address of the Unencrypted Resolver is often provisioned
	over insecure mechanisms, it can also be provisioned securely, such		over insecure mechanisms, it can also be provisioned securely, such
	as via manual configuration, a VPN, or on a network with protections		as via manual configuration, a VPN, or on a network with protections
	like RA guard [RFC6105]. An attacker might try to direct Encrypted		like RA guard [RFC6105]. An attacker might try to direct Encrypted
	DNS traffic to itself by causing the client to think that a		DNS traffic to itself by causing the client to think that a
	discovered Designated Resolver uses a different IP address from the		discovered Designated Resolver uses a different IP address from the
	Unencrypted Resolver. Such an Encrypted Resolver might have a valid		Unencrypted Resolver. Such a Designated Resolver might have a valid
	certificate, but be operated by an attacker that is trying to observe		certificate, but be operated by an attacker that is trying to observe
	or modify user queries without the knowledge of the client or		or modify user queries without the knowledge of the client or
	network.		network.
	If the IP address of a Designated Resolver differs from that of an		If the IP address of a Designated Resolver differs from that of an
	Unencrypted Resolver, clients MUST validate that the IP address of		Unencrypted Resolver, clients applying Authenicated Discovery
	the Unencrypted Resolver is covered by the SubjectAlternativeName of		(Section 4.2) MUST validate that the IP address of the Unencrypted
	the Encrypted Resolver's TLS certificate (Section 4.1).		Resolver is covered by the SubjectAlternativeName of the Designated
			Resolver's TLS certificate.
	Opportunistic use of Encrypted Resolvers MUST be limited to cases		Clients using Opportunistic Discovery (Section 4.3) MUST be limited
	where the Unencrypted Resolver and Designated Resolver have the same		to cases where the Unencrypted Resolver and Designated Resolver have
	IP address (Section 4.2).		the same IP address.
			The constraints on validation of Designated Resolvers specified here
			apply specifically to the automatic discovery mechanisms defined in
			this document, which are referred to as Authenticated Discovery and
			Opportunistic Discovery. Clients MAY use some other mechanism to
			validate and use Designated Resolvers discovered using the DNS SVCB
			record. However, use of such an alternate mechanism needs to take
			into account the attack scenarios detailed here.
	8. IANA Considerations		8. IANA Considerations
	8.1. Special Use Domain Name "resolver.arpa"		8.1. Special Use Domain Name "resolver.arpa"
	This document calls for the creation of the "resolver.arpa" SUDN.		This document calls for the creation of the "resolver.arpa" SUDN.
	This will allow resolvers to respond to queries directed at		This will allow resolvers to respond to queries directed at
	themselves rather than a specific domain name. While this document		themselves rather than a specific domain name. While this document
	uses "resolver.arpa" to return SVCB records indicating designated		uses "resolver.arpa" to return SVCB records indicating designated
	encrypted capability, the name is generic enough to allow future		encrypted capability, the name is generic enough to allow future
	skipping to change at page 9, line 32 ¶		skipping to change at page 10, line 36 ¶
	querying client is not interested in an answer from the authoritative		querying client is not interested in an answer from the authoritative
	"arpa" name servers. The intent of the SUDN is to allow clients to		"arpa" name servers. The intent of the SUDN is to allow clients to
	communicate with the Unencrypted Resolver much like "ipv4only.arpa"		communicate with the Unencrypted Resolver much like "ipv4only.arpa"
	allows for client-to-middlebox communication. For more context, see		allows for client-to-middlebox communication. For more context, see
	the rationale behind "ipv4only.arpa" in [RFC8880].		the rationale behind "ipv4only.arpa" in [RFC8880].
	9. References		9. References
	9.1. Normative References		9.1. Normative References
			[I-D.ietf-add-svcb-dns]
			Schwartz, B., "Service Binding Mapping for DNS Servers",
			Work in Progress, Internet-Draft, draft-ietf-add-svcb-dns-
			00, 1 October 2021,
			<https://datatracker.ietf.org/doc/html/draft-ietf-add-
			svcb-dns-00>.
	[I-D.ietf-dnsop-svcb-https]		[I-D.ietf-dnsop-svcb-https]
	Schwartz, B., Bishop, M., and E. Nygren, "Service binding		Schwartz, B., Bishop, M., and E. Nygren, "Service binding
	and parameter specification via the DNS (DNS SVCB and		and parameter specification via the DNS (DNS SVCB and
	HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-		HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
	dnsop-svcb-https-06, 16 June 2021,		dnsop-svcb-https-07, 5 August 2021,
	<https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-		<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
	https-06.txt>.		svcb-https-07>.
	[I-D.ietf-tls-esni]
	Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
	Encrypted Client Hello", Work in Progress, Internet-Draft,
	draft-ietf-tls-esni-12, 7 July 2021,
	<https://www.ietf.org/archive/id/draft-ietf-tls-esni-
	12.txt>.
	[I-D.schwartz-svcb-dns]
	Schwartz, B., "Service Binding Mapping for DNS Servers",
	Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
	03, 19 April 2021, <https://www.ietf.org/archive/id/draft-
	schwartz-svcb-dns-03.txt>.
	[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.		[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
	J., and E. Lear, "Address Allocation for Private		J., and E. Lear, "Address Allocation for Private
	Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,		Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
	February 1996, <https://www.rfc-editor.org/info/rfc1918>.		February 1996, <https://www.rfc-editor.org/rfc/rfc1918>.
	[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,		[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
	and P. Hoffman, "Specification for DNS over Transport		and P. Hoffman, "Specification for DNS over Transport
	Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May		Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
	2016, <https://www.rfc-editor.org/info/rfc7858>.		2016, <https://www.rfc-editor.org/rfc/rfc7858>.
	[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS		[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
	(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,		(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
	<https://www.rfc-editor.org/info/rfc8484>.		<https://www.rfc-editor.org/rfc/rfc8484>.
	9.2. Informative References		9.2. Informative References
			[I-D.ietf-tls-esni]
			Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
			Encrypted Client Hello", Work in Progress, Internet-Draft,
			draft-ietf-tls-esni-13, 12 August 2021,
			<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
			esni-13>.
	[I-D.schinazi-httpbis-doh-preference-hints]		[I-D.schinazi-httpbis-doh-preference-hints]
	Schinazi, D., Sullivan, N., and J. Kipp, "DoH Preference		Schinazi, D., Sullivan, N., and J. Kipp, "DoH Preference
	Hints for HTTP", Work in Progress, Internet-Draft, draft-		Hints for HTTP", Work in Progress, Internet-Draft, draft-
	schinazi-httpbis-doh-preference-hints-02, 13 July 2020,		schinazi-httpbis-doh-preference-hints-02, 13 July 2020,
	<https://www.ietf.org/archive/id/draft-schinazi-httpbis-		<https://datatracker.ietf.org/doc/html/draft-schinazi-
	doh-preference-hints-02.txt>.		httpbis-doh-preference-hints-02>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/rfc/rfc2119>.
	[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor		[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
	Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,		Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
	<https://www.rfc-editor.org/info/rfc2132>.		<https://www.rfc-editor.org/rfc/rfc2132>.
	[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,		[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
	Ed., "Design Choices When Expanding the DNS", RFC 5507,		Ed., "Design Choices When Expanding the DNS", RFC 5507,
	DOI 10.17487/RFC5507, April 2009,		DOI 10.17487/RFC5507, April 2009,
	<https://www.rfc-editor.org/info/rfc5507>.		<https://www.rfc-editor.org/rfc/rfc5507>.
	[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.		[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
	Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,		Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
	DOI 10.17487/RFC6105, February 2011,		DOI 10.17487/RFC6105, February 2011,
	<https://www.rfc-editor.org/info/rfc6105>.		<https://www.rfc-editor.org/rfc/rfc6105>.
	[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,		[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
	"IPv6 Router Advertisement Options for DNS Configuration",		"IPv6 Router Advertisement Options for DNS Configuration",
	RFC 8106, DOI 10.17487/RFC8106, March 2017,		RFC 8106, DOI 10.17487/RFC8106, March 2017,
	<https://www.rfc-editor.org/info/rfc8106>.		<https://www.rfc-editor.org/rfc/rfc8106>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
	[RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name		[RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name
	'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August		'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August
	2020, <https://www.rfc-editor.org/info/rfc8880>.		2020, <https://www.rfc-editor.org/rfc/rfc8880>.
	Appendix A. Rationale for using SVCB records		Appendix A. Rationale for using SVCB records
	This mechanism uses SVCB/HTTPS resource records		This mechanism uses SVCB/HTTPS resource records
	[I-D.ietf-dnsop-svcb-https] to communicate that a given domain		[I-D.ietf-dnsop-svcb-https] to communicate that a given domain
	designates a particular Designated Resolver for clients to use in		designates a particular Designated Resolver for clients to use in
	place of an Unencrypted Resolver (using a SUDN) or another Encrypted		place of an Unencrypted Resolver (using a SUDN) or another Encrypted
	Resolver (using its domain name).		Resolver (using its domain name).
	There are various other proposals for how to provide similar		There are various other proposals for how to provide similar
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