Internet DRAFT - draft-pauly-add-resolver-discovery
draft-pauly-add-resolver-discovery
Network Working Group T. Pauly
Internet-Draft E. Kinnear
Intended status: Standards Track Apple Inc.
Expires: 14 January 2021 C.A. Wood
Cloudflare
P. McManus
Fastly
T. Jensen
Microsoft
13 July 2020
Adaptive DNS Resolver Discovery
draft-pauly-add-resolver-discovery-01
Abstract
This document defines a method for dynamically discovering resolvers
that support encrypted transports, and introduces the concept of
designating a resolver to be used for a subset of client queries
based on domain. This method is intended to work both for locally-
hosted resolvers and resolvers accessible over the broader Internet.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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 14 January 2021.
Copyright Notice
Copyright (c) 2020 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 (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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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 Simplified BSD License text
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Designated Resolvers . . . . . . . . . . . . . . . . . . . . 3
3.1. Designating with Service Binding DNS Records . . . . . . 4
3.2. Additional Designation with PvD JSON . . . . . . . . . . 5
3.3. Confirmation of Designation with Zone Apex PvD . . . . . 6
3.4. Confirmation of Designation with TLS Certificates . . . . 8
4. Explicit Discovery of Local Resolvers . . . . . . . . . . . . 9
5. Discovery of DoH Capabilities for Direct Resolvers . . . . . 9
6. Server Deployment Considerations . . . . . . . . . . . . . . 10
6.1. Single Content Provider . . . . . . . . . . . . . . . . . 10
6.2. Multiple Content Providers . . . . . . . . . . . . . . . 10
6.3. Avoid Narrow Deployments . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9.1. DoH Template PvD Key . . . . . . . . . . . . . . . . . . 12
9.2. Trusted Names PvD Key . . . . . . . . . . . . . . . . . . 12
9.3. DoH URI Template DNS Service Parameter . . . . . . . . . 13
9.4. Special Use Domain Name "resolver.arpa" . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Rationale for using SVCB records . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
When clients need to resolve names into addresses in order to
establish networking connections, they traditionally use by default
the DNS resolver that is provisioned by the local network along with
their IP address [RFC2132] [RFC8106]. Alternatively, they can use a
resolver indicated by a tunneling service such as a VPN.
However, privacy-sensitive clients might prefer to use an encrypted
DNS service other than the one locally provisioned in order to
prevent interception, profiling, or modification by entities other
than the operator of the name service for the name being resolved.
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Protocols that can improve the transport security of a client when
using DNS or creating TLS connections include DNS-over-TLS (DoT)
[RFC7858], DNS-over-HTTPS (DoH) [RFC8484], and Encrypted TLS Client
Hellos [I-D.ietf-tls-esni].
This document defines a method for dynamically discovering resolvers
that support encrypted transports, and introduces the concept of
designating a resolver to be used for a subset of client queries
based on domain. This method is intended to work both for locally-
hosted resolvers and resolvers accessible over the broader Internet.
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:
Direct Resolver: A DNS resolver using any transport, encrypted or
unencrypted, that is provisioned directly by a local router or a
VPN.
Designated Resolver: A DNS resolver that is designated as a
responsible resolver for a given domain or zone. Designated
resolvers use encrypted transports.
Companion DoH Server: A DNS resolver that provides connectivity over
HTTPS (DoH) that is designated as equivalent to querying a
particular Direct Resolver.
3. Designated Resolvers
An encrypted DNS resolver, such as a DoH or DoT server, can be
designated for use in resolving names within one or more zones. This
means that clients can learn about an explicit mapping from a given
domain or zone to one or more Designated Resolvers, and use that
mapping to select the best resolver for a given query.
Designating a resolver MUST rely on agreement between the entity
managing a zone (the Domain Owner) and the entity operating the
resolver, such that clients can securely validate this designation.
These entities can be one and the same, or a Domain Owner can choose
to designate a third-party resolver to handle its traffic. Proof of
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this agreement asserts to clients that sending any query to the
designated resolver exposes no more information than sending that
query to the entity managing the corresponding zone.
As an example with only one entity, a company that runs many sites
within "enterprise.example.com" can provide its own DoH resolver,
"doh.enterprise.example.com", and designate only that resolver for
all names that fall within "enterprise.example.com". This means that
no other resolver would be designated for those names, and clients
would only resolve names with the same entity that would service TLS
connections.
As an example with several entities, the organization that operates
sites within "example.org" may work with two different Content
Delivery Networks (CDNs) to serve its sites. It might designate
names under "example.com" to two different entities, "doh.cdn-a.net"
and "doh.cdn-b.net". These are CDNs that have an existing
relationship with the organization that runs "example.org", and have
agreements with that organization about how data with information on
names and users is handled.
There are several methods that can be used to discover and validate a
resolver designation:
* Discovery using SVCB DNS records (Section 3.1), and validation
using DNSSEC
* Discovery using information in a provisioning domain (PvD) file
from the Designated DoH Resolver (Section 3.2)
* Validation using a file hosted on a well-known HTTPS URI based on
a zone apex (Section 3.3)
* Validation using TLS certificates to confirm of domain name
ownership (Section 3.4)
Note that clients MUST NOT accept designations for effective top-
level domains (eTLDs), such as ".com".
3.1. Designating with Service Binding DNS Records
The primary source for discovering Designated DoH Server
configurations is from properties stored in a SVCB DNS resource
record, or a SVCB-conformant resource record type, like HTTPS
[I-D.ietf-dnsop-svcb-https]. This record provides the URI Template
of a DoH server that is designated for a specific domain. A specific
domain may have more than one such record.
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The rationale for using SVCB records for recolver discovery is
discussed in Appendix A.
In order to designate a DoH server for a domain, a SVCB record can
contain the "dohuri" (Section 9). The value stored in the parameter
is a URI, which is the DoH URI template [RFC8484].
The following example shows a record containing a DoH URI, as
returned by a query for the HTTPS variant of the SVCB record type on
"foo.example.com".
foo.example.com. 7200 IN HTTPS 1 . (
dohuri=https://doh.example.net/dns-query )
If this record is DNSSEC-signed [RFC4033], clients can immediately
create a mapping that indicates the server (doh.example.net) as a
Designated Resolver for the name in the SVCB record
(foo.example.com).
Once a record that designated a DoH server has expired, the client
SHOULD issue another SVCB/HTTPS query whenever issuing queries within
the designated domain. This query SHOULD still be performed using
the designated DoH server. If the response designates a different
DoH server, the client should verify and use the new designation.
If this record is not DNSSEC-signed, clients MUST perform other
validation to determine that the zone designation is permitted, as
described in Section 3.3.
3.2. Additional Designation with PvD JSON
A provisioning domain (PvD) defines a coherent set of information
that can be used to access a network and resolve names. Section 4.3
of [I-D.ietf-intarea-provisioning-domains] defines a JSON dictionary
format that can be fetched over HTTPS at the well-known URI "/.well-
known/pvd".
Designated Resolvers that support DoH SHOULD provide a PvD JSON
dictionary available at the well-known PvD URI with the path of the
DoH server's URI template appended.
For example, the PvD JSON for the DoH server
"https://doh.example.net/dns-query" would be available at
"https://doh.example.net/.well-known/pvd/dns-query".
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The key "dohTemplate" is defined within the JSON dictionary
(Section 9) to point back to the DoH URI Template itself. This is
used for confirming the DoH server when the PvD is discovered locally
or during zone apex confirmation (Section 3.3).
Names that are listed in the "dnsZones" key in the JSON dictionary
indicate a set of zones that designate the resolver. These are the
zones that are available to resolve through the associated DoH
server. Note that this list does not need to be exhaustive, but is
the set of common zones managed by the resolver that all clients
should be aware of. Before using DNS results for these names,
clients MUST validate the designation either with a DNSSEC-signed
SVCB record (Section 3.1), or the confirmation methods described in
Section 3.3 and Section 3.4. DNS queries for validating records
SHOULD be sent to the DoH resolver. In order to optimize the
validation of these domains, servers MAY use HTTP Server Push to
deliver the signed SVCB answers prior to requests being made.
The "expires" key indicates a time after which the content of the PvD
file is no longer valid. Clients SHOULD re-fetch PvD information if
the expiration time has passed before using any designations that
were based on the PvD content.
{
"identifier": "doh.example.net.",
"dohTemplate": "https://doh.example.net/dns-query",
"dnsZones": ["example.com"],
"expires": "2020-08-23T06:00:00Z"
}
3.3. Confirmation of Designation with Zone Apex PvD
Designated DoH Resolvers that provide the PvD JSON described in
Section 3.2 can also provide information to validate of zone's
designation without DNSSEC. In order to confirm the designation, the
client requests a well-known HTTPS URI based on a zone apex name, and
checks a PvD file to ensure that it matches the DoH resolver. This
ensures that a DoH resolver cannot claim a designation for a given
zone without cooperation from the entity that owns the certificate
for the apex of that zone.
In order to enumerate the zone apex names that confirm designation in
this manner, the DoH resolver's PvD JSON dictionary can contain an
array of strings, with the key "trustedNames". Clients can validate
the resolver designation by checking a resource hosted by a name
indicated in "trustedNames". The client first issues an HTTP GET
request by appending "/.well-known/pvd" to the trusted name, using
the "https" scheme. In order to validate the designation, the PvD
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JSON MUST contain a "dohTemplate" key pointing to the correct DoH
resolver. The client's query for the IP addresses of the trusted
name MAY use the DoH resolver prior to fully validating the
designation, since the validation uses HTTPS to authenticate the
designation.
Note that the names listed in "trustedNames" are only useful for
confirming a designation that was indicated either by a non-DNSSEC-
signed SVCB designation (Section 3.1), or an additional designation
provided by the DoH resolver's PvD (Section 3.2). A trusted name
MUST be an exact match of a designating name, or else a parent of a
designating name.
If a name has more specific sub-domains that should not be allowed to
designate a given DoH resolver, this method of confirmation MUST NOT
be used.
As an example of this process, the JSON dictionary for the DoH server
"https://doh.example.net/dns-query", which is retrieved from
"https://doh.example.net/.well-known/pvd/dns-query", could contain
the following contents:
{
"identifier": "doh.example.net.",
"dohTemplate": "https://doh.example.net/dns-query",
"dnsZones": ["example.com"],
"trustedNames": ["example.com"],
"expires": "2020-08-23T06:00:00Z"
}
This indicates that "example.com" should be treated as a designated
domain, and that it can be validated by checking with the
"example.com" server rather than using DNSSEC.
In this example, the well-known URI used for validation is
"https://example.com/.well-known/pvd". In order to trust the
designation, this request must return valid JSON with the
"dohTemplate" key matching the original DoH resolver. For example,
this dictionary could contain the following contents:
{
"identifier": "example.com.",
"dohTemplate": "https://doh.example.net/dns-query",
"expires": "2020-08-23T06:00:00Z"
}
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A client MUST NOT trust a designation if the JSON content is not
present, does not contain a "dohTemplate" key, or the value in the
"dohTemplate" key does not match. The following result would not be
acceptable for the example above:
{
"identifier": "example.com.",
"dohTemplate": "https://not-the-doh-youre-looking-for.example.net/dns-query"
"expires": "2020-08-23T06:00:00Z"
}
Note that the domains listed in "trustedNames" may be broader than
the zones that designate the resolver. In the following example,
names under "foo.example.com" and "bar.example.com" designate the DoH
server "https://doh.example.net/dns-query", and use the PvD JSON from
"example.com" to validate the designation. However, the client would
not designate the DoH server for all names under "example.com".
{
"identifier": "doh.example.net.",
"dohTemplate": "https://doh.example.net/dns-query",
"dnsZones": ["foo.example.com", "bar.example.com"],
"trustedNames": ["example.com"],
"expires": "2020-08-23T06:00:00Z"
}
3.4. Confirmation of Designation with TLS Certificates
A DoH server designation can also be validated by checking the
SubjectAlternativeName field in the DoH server's own TLS certificate.
When a client wants to confirm the validity of the designation in
this situation, it can check the TLS certificate of the DoH server
for the name of the domain which triggered the original designation
query.
The following example shows an HTTPS variant of the SVCB record type
for "foo.example.com". If this record was received without DNSSEC,
the client can confirm its validity by establishing a connection to
"doh.example.net" and verifying the TLS certificate contains an exact
match for the "foo.example.com" name. If the queried domain is not
present in the TLS certificate of the designated DoH server, the
client may confirm the validity by an alternate method such as zone
apex confirmation (Section 3.3) but MUST NOT use the record until
otherwise validated.
foo.example.com. 7200 IN HTTPS 1 . (
dohuri=https://doh.example.net/dns-query )
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4. Explicit Discovery of Local Resolvers
If the local network provides configuration with an Explicit
Provisioning Domain (PvD), as defined by
[I-D.ietf-intarea-provisioning-domains], clients can learn about
domains for which the local network's resolver is authoritative. The
keys for DoH resolvers described in Section 3.2 also allow this local
PvD to be used for resolver discovery.
If an RA provided by the router on the network defines an Explicit
PvD that has additional information, and this additional information
JSON dictionary contains the key "dohTemplate", then the client
SHOULD add this DoH server to its list of known DoH configurations.
The domains that the DoH server claims authority for are listed in
the "dnsZones" key.
Local deployments that want to designate a resolver for a private
name that is not easily signed with DNSSEC MUST provide an alternate
method of validating a designation, such as described in Section 3.3
or Section 3.4.
5. Discovery of DoH Capabilities for Direct Resolvers
Direct Resolvers can advertise a Companion DoH server that offers
equivalent services and is controlled by the same entity. To do
this, a DNS server returns an SVCB record for "dns://resolver.arpa"
with "ipv4hint" and/or "ipv6hint" set to a valid IP address and the
"dohuri" key set to a valid DoH URI template as with the Designated
DoH Server SVCB record. The TLS certificate used with the DoH URI
MUST have the IP addresses for each of its DNS endpoints, classic or
DoH, within the SubjectAlternativeName field to allow the client to
verify ownership.
Once a client is configured to query a Direct Resolver, it SHOULD
query the resolver for SVCB records for "dns://resolver.arpa" before
making other queries. This will help the client avoid leaking
queries that could go over DoH once the Companion DoH Server is
discovered. If an SVCB record is returned, its "dohip" field
designates an IP address the client can send DoH queries to in lieu
of sending classic DNS queries to the Direct Resolver. The "dohuri"
field contains the DoH URI similarly to the SVCB record for a
Designated DoH Server.
To validate the Companion DoH Server and the resolver that advertised
it are related, the client MUST check the SubjectAlternativeName
field of the Companion DoH Server's TLS certificate for the original
resolver's IP address and the advertised IP address for the Companion
DoH server. If both are present, the discovered Companion DoH Server
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MUST be used whenever the original Direct Resolver would be used.
Otherwise, the client SHOULD suppress queries for Companion DoH
Servers against this resolver for the TTL of the negative or invalid
response and continue to use the original Direct Resolver.
The following example shows a record containing a Companion DoH URI,
as returned by a query for an SVCB record for "dns://resolver.arpa":
_dns.resolver.arpa 7200 IN SVCB 1 doh.example.net (
ipv4hint=x.y.z.w
dohuri=https://doh.example.net/dns-query )
A DNS resolver MAY return more than one SVCB record of this form to
advertise multiple Companion DoH Servers that are valid as a
replacement for itself. Any or all of these servers may have the
same IP address as the DNS resolver itself. In this case, clients
will only have one IP address to check for when verifying ownership
of the Companion DoH server.
6. Server Deployment Considerations
When servers designate DoH servers for their names, the specific
deployment model can impact the effective privacy and performance
characteristics.
6.1. Single Content Provider
If a name always resolves to server IP addresses that are hosted by a
single content provider, the name ought to designate a single DoH
server. This DoH server will be most optimal when it is designated
by many or all names that are hosted by the same content provider.
This ensures that clients can increase connection reuse to reduce
latency in connection setup.
A DoH server that corresponds to the content provider that hosts
content has an opportunity to tune the responses provided to a client
based on the location inferred by the client IP address.
6.2. Multiple Content Providers
Some hostnames may resolve to server IP addresses that are hosted by
multiple content providers. In such scenarios, the deployment may
want to be able to control the percentage of traffic that flows to
each content provider.
In these scenarios, there can either be:
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* multiple designated DoH servers that are advertised via SVCB DNS
Records; or,
* a single designated DoH server that can be referenced by one or
more SVCB DNS Records, operated by a party that is aware of both
content providers and can manage splitting the traffic.
If a server deployment wants to easily control the split of traffic
between different content providers, it ought to use the latter model
of using a single designated DoH server that can better control which
IP addresses are provided to clients. Otherwise, if a client is
aware of multiple DoH servers, it might use a single resolver
exclusively, which may lead to inconsistent behavior between clients
that choose different resolvers.
6.3. Avoid Narrow Deployments
Using designated DoH servers can improve the privacy of name
resolution whenever a DoH server is designated by many different
names within one or more domains. This limits the amount of
information leaked to an attacker observing traffic between a client
and a DoH server: the attacker only learns that the client might be
resolving one of the many names for which the server is designated.
However, if a deployment designates a given DoH server for only one
name, or a very small set of names, then it becomes easier for an
attacker to infer that a specific name is being accessed by a client.
For this reason, deployments are encouraged to avoid deploying a DoH
server that is only designated by a small number of names. Clients
can also choose to only allow DoH servers that are associated with
many names.
Beyond the benefits to privacy, having a larger number of names
designate a given DoH server improves the opportunity for DoH
connection reuse, which can improve the performance of name
resolutions.
7. Security Considerations
In order to avoid interception and modification of the information
sent between clients and Designated Resolvers, all exchanges between
clients and servers are performed over encrypted connections, e.g.,
TLS.
Malicious adversaries may block client connections to a Designated
Resolver as a Denial-of-Service (DoS) measure. Clients which cannot
connect these resolvers may be forced to, if local policy allows,
fall back to unencrypted DNS if this occurs.
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8. Privacy Considerations
Clients must be careful in determining to which DoH servers they send
queries directly. A malicious resolver that can direct queries to
itself can track or profile client activity. In order to avoid the
possibility of a spoofed SVCB record designating a malicious DoH
server for a name, clients MUST ensure that such records validate
using DNSSEC (Section 3.1), using zone apex confirmation
(Section 3.3), or using domain names in TLS certificates
(Section 3.4).
Even servers that are validly designated can risk leaking or logging
information about client lookups. Such risk can be mitigated by
further restricting the list of resolvers that are allowed for direct
use based on client policy.
An adversary able to see traffic on each path segment of a DoH query
(e.g., from client to a Designated Resolver, and the Designated
Resolver to an authoritative DNS server) can link queries to specific
clients with high probability. Failure to observe traffic on any one
of these path segments makes this linkability increasingly difficult.
For example, if an adversary can only observe traffic between a
client and proxy and egress traffic from a target, then it may be
difficult identify a specific client's query among the recursive
queries generated by the target.
9. IANA Considerations
9.1. DoH Template PvD Key
This document adds a key to the "Additional Information PvD Keys"
registry [I-D.ietf-intarea-provisioning-domains].
+============+=============+======+=================================+
| JSON key | Description | Type | Example |
+============+=============+======+=================================+
|dohTemplate | DoH URI |String| "https://dnsserver.example.net/ |
| | Template | | dns-query{?dns}" |
| | [RFC8484] | | |
+------------+-------------+------+---------------------------------+
Table 1
9.2. Trusted Names PvD Key
This document adds a key to the "Additional Information PvD Keys"
registry [I-D.ietf-intarea-provisioning-domains].
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+==============+=======================+=========+===============+
| JSON key | Description | Type | Example |
+==============+=======================+=========+===============+
| trustedNames | Names of servers that | Array | [ |
| | can validate resolver | of | "example.com" |
| | designation. | Strings | ] |
+--------------+-----------------------+---------+---------------+
Table 2
9.3. DoH URI Template DNS Service Parameter
This document adds a parameter to the "Service Binding (SVCB)
Parameter" registry. The allocation request is 32768, taken from the
to the First Come First Served range.
If present, this parameters indicates the URI template of a DoH
server that is designated for use with the name being resolved. This
is a string encoded as UTF-8 characters.
Name: dohuri
SvcParamKey: 32768
Meaning: URI template for a designated DoH server
Reference: This document.
9.4. Special Use Domain Name "resolver.arpa"
This document calls for the creation of the "resolver.arpa" SUDN.
This will allow resolvers to respond to queries directed at
themselves rather than a specific domain name. While this document
uses "resolver.arpa" to return SVCB records indicating DoH
capability, the name is generic enough to allow future reuse for
other purposes where the resolver wishes to provide information about
itself to the client.
10. Acknowledgments
Thanks to Erik Nygren, Lorenzo Colitti, Mikael Abrahamsson, Ben
Schwartz, Ask Hansen, Leif Hedstrom, Tim McCoy, Stuart Cheshire,
Miguel Vega, Joey Deng, Ted Lemon, and Elliot Briggs for their
feedback and input on this document.
11. References
11.1. Normative References
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[I-D.ietf-dnsop-svcb-https]
Schwartz, B., Bishop, M., and E. Nygren, "Service binding
and parameter specification via the DNS (DNS SVCB and
HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
dnsop-svcb-https-00, 12 June 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-dnsop-svcb-https-00.txt>.
[I-D.ietf-intarea-provisioning-domains]
Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W.
Shao, "Discovering Provisioning Domain Names and Data",
Work in Progress, Internet-Draft, draft-ietf-intarea-
provisioning-domains-11, 31 January 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-intarea-
provisioning-domains-11.txt>.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-07, 1 June 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-tls-esni-07.txt>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[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>.
[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>.
11.2. Informative References
[I-D.schinazi-httpbis-doh-preference-hints]
Schinazi, D., Sullivan, N., and J. Kipp, "DoH Preference
Hints for HTTP", Work in Progress, Internet-Draft, draft-
schinazi-httpbis-doh-preference-hints-01, 8 January 2020,
<http://www.ietf.org/internet-drafts/draft-schinazi-
httpbis-doh-preference-hints-01.txt>.
[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>.
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[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>.
[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
Ed., "Design Choices When Expanding the DNS", RFC 5507,
DOI 10.17487/RFC5507, April 2009,
<https://www.rfc-editor.org/info/rfc5507>.
[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>.
[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>.
Appendix A. Rationale for using SVCB records
This mechanism uses SVCB/HTTPS resource records
[I-D.ietf-dnsop-svcb-https] to communicate that a given domain
designates a particular DoH resolver for clients to use for
subsequent queries to within the domain.
There are various other proposals for how to provide similar
functionality. There are several reasons that this mechanism has
chosen SVCB records:
* Discovering encrypted resolver using DNS records keeps client
logic for DNS self-contained, and allows an operator of a DNS zone
to define exactly which names should use a given DoH server.
* Using DNS records also doesn't rely on bootstrapping with higher-
level application operations (such as
[I-D.schinazi-httpbis-doh-preference-hints]).
* SVCB records are extensible and allow definition of parameter
keys. This makes them a superior mechanism for extensibility, as
compared to approaches such as overloading TXT records. The same
keys can be used both for upgrading direct resolvers to DoH
through an explicit query (Section 5) and for discovering
designated resolvers when issuing standard HTTPS queries
(Section 3.1).
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* Clients and servers that are interested in privacy of names will
already need to support SVCB records in order to use Encrypted TLS
Client Hello [I-D.ietf-tls-esni]. Without encrypting names in
TLS, the value of encrypting DNS is reduced, so pairing the
solutions provides the largest benefit.
* Clients that support SVCB will generally send out three queries
when accessing web content on a dual-stack network: A, AAAA, and
HTTPS queries. Discovering a resolver designation for a zone as
part of one of these queries, without having to add yet another
query, minimizes the total number of queries clients send. While
[RFC5507] recommends adding new RRTypes for new functionality,
SVCB provides an extension mechanism that simplifies client
behavior.
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,
United States of America
Email: caw@heapingbits.net
Patrick McManus
Fastly
Email: mcmanus@ducksong.com
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Tommy Jensen
Microsoft
Email: tojens@microsoft.com
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