ADD T. Pauly
Internet-Draft E. Kinnear
Intended status: Standards Track Apple Inc.
Expires: 9 January 2022 C.A. Wood
Cloudflare
P. McManus
Fastly
T. Jensen
Microsoft
8 July 2021
Discovery of Designated Resolvers
draft-ietf-add-ddr-02
Abstract
This document defines Discovery of Designated Resolvers (DDR), a
mechanism for DNS clients to use DNS records to discover a resolver's
encrypted DNS configuration. This mechanism can be used to move from
unencrypted DNS to encrypted DNS when only the IP address of a
resolver is known. It can also be used to discover support for
encrypted DNS protocols when the name of an encrypted resolver is
known. This mechanism is designed to be limited to cases where
unencrypted resolvers and their designated resolvers are operated by
the same entity or cooperating entities.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Adaptive DNS Discovery
Working Group mailing list (add@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/add/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-add/draft-ietf-add-ddr.
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/.
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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 9 January 2022.
Copyright Notice
Copyright (c) 2021 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/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DNS Service Binding Records . . . . . . . . . . . . . . . . . 4
4. Discovery Using Resolver IP Addresses . . . . . . . . . . . . 5
4.1. Authenticated Discovery . . . . . . . . . . . . . . . . . 6
4.2. Opportunistic Discovery . . . . . . . . . . . . . . . . . 6
5. Discovery Using Resolver Names . . . . . . . . . . . . . . . 7
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
6.1. Caching Forwarders . . . . . . . . . . . . . . . . . . . 8
6.2. Certificate Management . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. Special Use Domain Name "resolver.arpa" . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Rationale for using SVCB records . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
When DNS clients wish to use encrypted DNS protocols such as DNS-
over-TLS (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [RFC8484], they
require additional information beyond the IP address of the DNS
server, such as the resolver's hostname, non-standard ports, or URL
paths. However, common configuration mechanisms only provide the
resolver's IP address during configuration. Such mechanisms include
network provisioning protocols like DHCP [RFC2132] and IPv6 Router
Advertisement (RA) options [RFC8106], as well as manual
configuration.
This document defines two mechanisms for clients to discover
designated resolvers using DNS server Service Binding (SVCB,
[I-D.ietf-dnsop-svcb-https]) records:
1. When only an IP address of an Unencrypted Resolver is known, the
client queries a special use domain name to discover DNS SVCB
records associated with one or more Encrypted Resolvers the
Unencrypted Resolver has designated for use when support for DNS
encryption is requested (Section 4).
2. When the hostname of an Encrypted 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 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 claims ownership over both resolvers.
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. Refers to the mechanisms
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defined in this document.
Designated Resolver: A resolver, presumably an Encrypted Resolver,
designated by another resolver for use in its own place. This
designation can be authenticated with TLS certificates.
Encrypted Resolver: A DNS resolver using any encrypted DNS
transport. This includes current mechanisms such as DoH and DoT
as well as future mechanisms.
Unencrypted Resolver: A DNS resolver using 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, ports, and server name to use in
certificate validation. This information is provided in Service
Binding (SVCB) records for DNS Servers, defined by
[I-D.schwartz-svcb-dns].
The following is an example of an SVCB record describing a DoH server
discovered by querying for "_dns.example.net":
_dns.example.net 7200 IN SVCB 1 . (
alpn=h2 dohpath=/dns-query{?dns} )
The following is an example of an 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 )
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
[I-D.ietf-dnsop-svcb-https].
To avoid name lookup deadlock, Designated Resolvers SHOULD follow the
guidance in Section 10 of [RFC8484] regarding the avoidance of DNS-
based references that block the completion of the TLS handshake.
This document focuses on discovering DoH and DoT Designated
Resolvers. Other protocols can also use the format defined by
[I-D.schwartz-svcb-dns]. However, if any protocol does not involve
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some form of certificate validation, new validation mechanisms will
need to be defined to support validating designation as defined in
Section 4.1.
4. Discovery Using Resolver IP Addresses
When a DNS client is configured with an Unencrypted Resolver IP
address, it SHOULD query the resolver for SVCB records for
"dns://resolver.arpa" before making other queries. Specifically, the
client issues a query for "_dns.resolver.arpa" with the SVCB resource
record type (64) [I-D.ietf-dnsop-svcb-https].
Because this query is for an SUDN, which no entity can claim
ownership over, the SVCB response MUST NOT use the "." value for the
SvcDomainName. Instead, the domain name used for DoT or used to
construct the DoH template MUST be provided.
The following is an example of an 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 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 )
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 "dns://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 allow the DNS client to make queries over an encrypted
connection without waiting to resolve the Encrypted Resolver name per
[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
Encrypted Resolver until an additional DNS lookup for the A and AAAA
records can be made to the Unencrypted Resolver (or some other
resolver the DNS client has been configured to use).
If the recursive resolver that receives this query has no Designated
Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa"
SUDN.
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4.1. Authenticated Discovery
In order to be considered an authenticated Designated Resolver, the
TLS certificate presented by the Encrypted Resolver MUST contain both
the domain name (from the SVCB answer) and the IP address of the
designating Unencrypted Resolver within the SubjectAlternativeName
certificate field. The client MUST check the SubjectAlternativeName
field for both the Unencrypted Resolver's IP address and the
advertised name of the Designated Resolver. If the certificate can
be validated, the client SHOULD use the discovered Designated
Resolver for any cases in which it would have otherwise used the
Unencrypted Resolver. If the Designated Resolver has a different IP
address than the Unencrypted Resolver and the TLS certificate does
not cover the Unencrypted Resolver address, the client MUST NOT use
the discovered Encrypted Resolver. Additionally, the client SHOULD
suppress any further queries for Designated Resolvers using this
Unencrypted Resolver for the length of time indicated by the SVCB
record's Time to Live (TTL).
If the Designated Resolver and the Unencrypted Resolver share an IP
address, clients MAY choose to opportunistically use the Encrypted
Resolver even without this certificate check (Section 4.2).
If resolving the name of an Encrypted Resolver from an SVCB record
yields an IP address that was not presented in the Additional Answers
section or ipv4hint or ipv6hint fields of the original SVCB query,
the connection made to that IP address MUST pass the same TLS
certificate checks before being allowed to replace a previously known
and validated IP address for the same Encrypted Resolver name.
4.2. Opportunistic Discovery
There are situations where authenticated discovery of encrypted DNS
configuration over unencrypted DNS is not possible. This includes
Unencrypted Resolvers on non-public IP addresses such as those
defined in [RFC1918] whose identity cannot be confirmed using TLS
certificates.
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
presented in the certificate. A client MAY use information from the
SVCB record for "dns://resolver.arpa" with this "opportunistic"
approach (not validating the names presented in the
SubjectAlternativeName field of the certificate) as long as the IP
address of the Encrypted Resolver does not differ from the IP address
of the Unencrypted Resolver. This approach can be used for any
encrypted DNS protocol that uses TLS.
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5. Discovery Using Resolver Names
A DNS client that already knows the name of an Encrypted 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 Resolver, or if a network provisioning
protocol (such as DHCP or IPv6 Router Advertisements) provides a name
for an Encrypted Resolver alongside the resolver IP address.
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 Resolver itself or to any other resolver. Unlike the case
of bootstrapping from an Unencrypted Resolver (Section 4), these
records SHOULD be available in the public DNS.
For example, if the client already knows about a DoT server
"resolver.example.com", it can issue an 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 . (
alpn=h2 dohpath=/dns-query{?dns} )
_dns.resolver.example.com 7200 IN SVCB 2 . (
alpn=dot )
Often, the various supported encrypted DNS protocols will be
accessible using the same hostname. In the example above, both DoH
and DoT use the name "resolver.example.com" for their TLS
certificates. If a deployment uses a different hostname for one
protocol, but still wants clients to treat both DNS servers as
designated, the TLS certificates MUST include both names in the
SubjectAlternativeName fields. Note that this name verification is
not related to the DNS resolver that provided the SVCB answer.
For example, being able to discover a Designated Resolver for a known
Encrypted 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.
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6.1. Caching Forwarders
A DNS forwarder SHOULD NOT forward queries for "resolver.arpa"
upstream. This prevents a client from receiving an SVCB record that
will fail to authenticate because the forwarder's IP address is not
in the upstream resolver's Designated Resolver's TLS certificate SAN
field. A DNS forwarder which already acts as a completely blind
forwarder MAY choose to forward these queries when the operator
expects that this does not apply, either because the operator knows
the upstream resolver does have the forwarder's IP address in its TLS
certificate's SAN field or that the operator expects clients of the
unencrypted resolver to use the SVCB information opportunistically.
Operators who choose to forward queries for "resolver.arpa" upstream
should note that client behavior is never guaranteed and use of DDR
by a resolver does not communicate a requirement for clients to use
the SVCB record when it cannot be authenticated.
6.2. Certificate Management
Resolver owners that support authenticated 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.
7. Security Considerations
Since client can receive DNS SVCB answers over unencrypted DNS, on-
path attackers can prevent successful discovery by dropping SVCB
packets. 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.
While the IP address of the Unencrypted Resolver is often provisioned
over insecure mechanisms, it can also be provisioned securely, such
as via manual configuration, 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 Resolver. Such an Encrypted Resolver might have a valid
certificate, but 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 Resolver, clients MUST validate that the IP address of
the Unencrypted Resolver is covered by the SubjectAlternativeName of
the Encrypted Resolver's TLS certificate (Section 4.1).
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Opportunistic use of Encrypted Resolvers MUST be limited to cases
where the Unencrypted Resolver and Designated Resolver have the same
IP address (Section 4.2).
8. IANA Considerations
8.1. 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 designated
encrypted 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.
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 Resolver much like "ipv4only.arpa"
allows for client-to-middlebox communication. For more context, see
the rationale behind "ipv4only.arpa" in [RFC8880].
9. References
9.1. Normative References
[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-06, 16 June 2021,
<https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
https-06.txt>.
[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>.
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[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>.
[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>.
9.2. 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-02, 13 July 2020,
<https://www.ietf.org/archive/id/draft-schinazi-httpbis-
doh-preference-hints-02.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>.
[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>.
[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>.
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[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>.
[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 SVCB records
This mechanism uses SVCB/HTTPS resource records
[I-D.ietf-dnsop-svcb-https] to communicate that a given domain
designates a particular Designated Resolver for clients to use in
place of an Unencrypted Resolver (using a SUDN) or another Encrypted
Resolver (using its domain name).
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 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
[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 for discovering Designated Resolvers of different
transport types as well as those advertised by Unencrypted
Resolvers or another Encrypted Resolver.
* 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.
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* 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 Designated Resolver 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
Tommy Jensen
Microsoft
Email: tojens@microsoft.com
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