Internet Engineering Task Force (IETF) T. Reddy.K
Request for Comments: 9606 Nokia
Category: Standards Track M. Boucadair
ISSN: 2070-1721 Orange
June 2024
DNS Resolver Information
Abstract
This document specifies a method for DNS resolvers to publish
information about themselves. DNS clients can use the resolver
information to identify the capabilities of DNS resolvers. How DNS
clients use such information is beyond the scope of this document.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9606.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Terminology
3. Retrieving Resolver Information
4. Format of the Resolver Information
5. Resolver Information Keys/Values
6. An Example
7. Security Considerations
8. IANA Considerations
8.1. RESINFO RR Type
8.2. DNS Resolver Information Keys Registration
8.3. Guidelines for the Designated Experts
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Historically, DNS clients communicated with recursive resolvers
without needing to know anything about the features supported by
these resolvers. However, more and more recursive resolvers expose
different features that may impact delivered DNS services (privacy
preservation, filtering, transparent behavior, etc.). DNS clients
can discover and authenticate encrypted DNS resolvers provided by a
local network, for example, using the Discovery of Network-designated
Resolvers (DNR) [RFC9463] and the Discovery of Designated Resolvers
(DDR) [RFC9462]. However, these DNS clients can't retrieve
information from the discovered recursive resolvers about their
capabilities to feed the resolver selection process. Instead of
depending on opportunistic approaches, DNS clients need a more
reliable mechanism to discover the features that are configured on
these resolvers.
This document fills that void by specifying a mechanism that allows
communication of DNS resolver information to DNS clients for use in
resolver selection decisions. For example, the resolver selection
procedure may use the retrieved resolver information to prioritize
privacy-preserving resolvers over those that don't enable QNAME
minimisation [RFC9156]. Another example is when a DNS client selects
a resolver based on its filtering capability. For instance, a DNS
client can choose a resolver that filters domains according to a
security policy using the Blocked (15) Extended DNS Error (EDE)
[RFC8914]. Alternatively, the client may have a policy not to select
a resolver that forges responses using the Forged Answer (4) EDE.
However, it is out of the scope of this document to define the
selection procedure and policies. Once a resolver is selected by a
DNS client, and unless explicitly mentioned, this document does not
interfere with that resolver's DNS operations.
Specifically, this document defines a new resource record (RR) type
for DNS clients to query the recursive resolvers. The initial
information that a resolver might want to expose is defined in
Section 5. That information is scoped to cover properties that are
used to infer privacy and transparency policies of a resolver. Other
information can be registered in the future per the guidance in
Section 8.2. The information is not intended for end-user
consumption.
2. Terminology
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.
This document makes use of the terms defined in [RFC8499]. The
following additional terms are used:
Encrypted DNS: Refers to a DNS scheme where DNS exchanges are
transported over an encrypted channel between a DNS client and
server (e.g., DNS over HTTPS (DoH) [RFC8484], DNS over TLS (DoT)
[RFC7858], or DNS over QUIC (DoQ) [RFC9250]).
Encrypted DNS resolver: Refers to a DNS resolver that supports any
encrypted DNS scheme.
Reputation: Defined as "the estimation in which an identifiable
actor is held, especially by the community or the Internet public
generally" per Section 1 of [RFC7070].
3. Retrieving Resolver Information
A DNS client that wants to retrieve the resolver information may use
the RR type "RESINFO" defined in this document. The content of the
RDATA in a response to a query for RESINFO RR QTYPE is defined in
Section 5. If the resolver understands the RESINFO RR type, the
RRset MUST have exactly one record. Invalid records MUST be silently
ignored by DNS clients. RESINFO is a property of the resolver and is
not subject to recursive resolution.
A DNS client can retrieve the resolver information using the RESINFO
RR type and the QNAME of the domain name that is used to authenticate
the DNS resolver (referred to as the Authentication Domain Name (ADN)
in DNR [RFC9463]).
If the Special-Use Domain Name "resolver.arpa", defined in [RFC9462],
is used to discover an encrypted DNS resolver, the client can
retrieve the resolver information using the RESINFO RR type and QNAME
of "resolver.arpa". In this case, a client has to contend with the
risk that a resolver does not support RESINFO. The resolver might
pass the query upstream, and then the client can receive a positive
RESINFO response from either a legitimate DNS resolver or an
attacker.
The DNS client MUST set the Recursion Desired (RD) bit of the query
to 0. The DNS client MUST discard the response if the AA flag in the
response is set to 0, indicating that the DNS resolver is not
authoritative for the response.
If a group of resolvers is sharing the same ADN and/or anycast
address, then these instances SHOULD expose a consistent RESINFO.
4. Format of the Resolver Information
The resolver information record uses the same format as DNS TXT
records. The format rules for TXT records are defined in the base
DNS specification (Section 3.3.14 of [RFC1035]) and are further
elaborated in the DNS-based Service Discovery (DNS-SD) specification
(Section 6.1 of [RFC6763]). The recommendations to limit the TXT
record size are discussed in Section 6.1 of [RFC6763].
Similar to DNS-SD, the RESINFO RR type uses "key/value" pairs to
convey the resolver information. Each key/value pair is encoded
using the format rules defined in Section 6.3 of [RFC6763]. Using
standardized key/value syntax within the RESINFO RR type makes it
easier for future keys to be defined. If a DNS client sees unknown
keys in a RESINFO RR type, it MUST silently ignore them. The same
rules for the keys, as defined in Section 6.4 of [RFC6763], MUST be
followed for RESINFO.
Resolver information keys MUST either be defined in the IANA registry
(Section 8.2) or begin with the substring "temp-" for names defined
for local use only.
5. Resolver Information Keys/Values
The following resolver information keys are defined:
qnamemin: The presence of this key indicates that the DNS resolver
supports QNAME minimisation [RFC9156] to improve DNS privacy.
Note that, per the rules for the keys defined in Section 6.4 of
[RFC6763], if there is no '=' in a key, then it is a boolean
attribute, simply identified as being present, with no value.
The presence of this key indicates that the DNS resolver is
configured to minimise the amount of privacy-sensitive data sent
to an authoritative name server.
This is an optional attribute.
exterr: If the DNS resolver supports an the EDE option defined in
[RFC8914] to return additional information about the cause of DNS
errors, the value of this key lists the possible EDE codes that
can be returned by this DNS resolver. A value can be an
individual EDE or a range of EDEs. Range values MUST be
identified by "-". When multiple non-contiguous values are
present, these values MUST be comma-separated.
Returned EDEs (e.g., Blocked (15), Censored (16), and Filtered
(17)) indicate whether the DNS resolver is configured to reveal
the reason why a query was filtered/blocked when such an event
happens. If the resolver's capabilities are updated to include
new similar error codes, the resolver can terminate the TLS
session, prompting the client to initiate a new TLS connection and
retrieve the resolver information again. This allows the client
to become aware of the resolver's updated capabilities.
Alternatively, if the client receives an EDE for a DNS request,
but that EDE was not listed in the "exterr", the client can query
the resolver again to learn about the updated resolver's
capabilities to return new error codes. If a mismatch still
exists, the client can identify that the resolver information is
inaccurate and discard it.
This is an optional attribute.
infourl: A URL that points to the generic unstructured resolver
information (e.g., DoH APIs supported, possible HTTP status codes
returned by the DoH server, or how to report a problem) for
troubleshooting purposes. The server that exposes such
information is called "resolver information server".
The resolver information server MUST support only the content-type
"text/html" for the resolver information. The DNS client MUST
reject the URL as invalid if the scheme is not "https". Invalid
URLs MUST be ignored. The URL MUST be treated only as diagnostic
information for IT staff. It is not intended for end-user
consumption as the URL can possibly provide misleading
information.
This key can be used by IT staff to retrieve other useful
information about the resolver and also the procedure to report
problems (e.g., invalid filtering).
This is an optional attribute.
New keys can be defined as per the procedure defined in Section 8.2.
6. An Example
Figure 1 shows an example of a published resolver information record.
resolver.example.net. 7200 IN RESINFO qnamemin exterr=15-17
infourl=https://resolver.example.com/guide
Figure 1: An Example of a Resolver Information Record
As mentioned in Section 3, a DNS client that discovers the ADN
"resolver.example.net" of its resolver using DNR will issue a query
for RESINFO RR QTYPE for that ADN and will learn that:
* the resolver enables QNAME minimisation,
* the resolver can return Blocked (15), Censored (16), and Filtered
(17) EDEs, and
* more information can be retrieved from
"https://resolver.example.com/guide".
7. Security Considerations
DNS clients communicating with discovered DNS resolvers MUST use one
of the following measures to prevent DNS response forgery attacks:
1. Establish an authenticated secure connection to the DNS resolver.
2. Implement local DNSSEC validation (Section 10 of [RFC8499]) to
verify the authenticity of the resolver information.
It is important to note that, of these two measures, only the first
one can apply to queries for "resolver.arpa".
An encrypted resolver may return incorrect information in RESINFO.
If the client cannot validate the attributes received from the
resolver, which will be used for resolver selection or displayed to
the end user, the client should process those attributes only if the
encrypted resolver has sufficient reputation according to local
policy (e.g., user configuration, administrative configuration, or a
built-in list of reputable resolvers). This approach limits the
ability of a malicious encrypted resolver to cause harm with false
claims.
8. IANA Considerations
8.1. RESINFO RR Type
IANA has updated the "Resource Record (RR) TYPEs" registry under the
"Domain Name System (DNS) Parameters" registry group [RRTYPE] as
follows:
Type: RESINFO
Value: 261
Meaning: Resolver Information as Key/Value Pairs
Reference: RFC 9606
8.2. DNS Resolver Information Keys Registration
IANA has created a new registry called "DNS Resolver Information
Keys" under the "Domain Name System (DNS) Parameters" registry group
[IANA-DNS]. This new registry contains definitions of the keys that
can be used to provide the resolver information.
The registration procedure is Specification Required (Section 4.6 of
[RFC8126]). Designated experts should carefully consider the
security implications of allowing a resolver to include new keys in
this registry. Additional considerations are provided in
Section 8.3.
The structure of the registry is as follows:
Name: The key name. The name MUST conform to the definition in
Section 4 of this document. The IANA registry MUST NOT register
names that begin with "temp-" so that these names can be used
freely by any implementer.
Description: A description of the registered key.
Reference: The reference specification for the registered element.
The initial contents of this registry are provided in Table 1.
+==========+=====================================+===========+
| Name | Description | Reference |
+==========+=====================================+===========+
| qnamemin | The presence of the key name | RFC 9606 |
| | indicates that QNAME minimisation | |
| | is enabled. | |
+----------+-------------------------------------+-----------+
| exterr | Lists the set of enabled extended | RFC 9606 |
| | DNS errors. It must be an INFO- | |
| | CODE decimal value in the "Extended | |
| | DNS Error Codes" registry | |
| | <https://www.iana.org/assignments/ | |
| | dns-parameters/>. | |
+----------+-------------------------------------+-----------+
| infourl | Provides a URL that points to | RFC 9606 |
| | unstructured resolver information | |
| | that is used for troubleshooting. | |
+----------+-------------------------------------+-----------+
Table 1: Initial Contents of the DNS Resolver Information
Keys Registry
8.3. Guidelines for the Designated Experts
It is suggested that multiple designated experts be appointed for
registry change requests.
Criteria that should be applied by the designated experts include
determining whether the proposed registration duplicates existing
entries and whether the registration description is clear and fits
the purpose of this registry.
Registration requests are evaluated within a two-week review period
on the advice of one or more designated experts. Within the review
period, the designated experts will either approve or deny the
registration request, communicating this decision to IANA. Denials
should include an explanation and, if applicable, suggestions as to
how to make the request successful.
9. References
9.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", RFC 8914,
DOI 10.17487/RFC8914, October 2020,
<https://www.rfc-editor.org/info/rfc8914>.
[RFC9156] Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
Name Minimisation to Improve Privacy", RFC 9156,
DOI 10.17487/RFC9156, November 2021,
<https://www.rfc-editor.org/info/rfc9156>.
[RFC9462] Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", RFC 9462,
DOI 10.17487/RFC9462, November 2023,
<https://www.rfc-editor.org/info/rfc9462>.
[RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
and T. Jensen, "DHCP and Router Advertisement Options for
the Discovery of Network-designated Resolvers (DNR)",
RFC 9463, DOI 10.17487/RFC9463, November 2023,
<https://www.rfc-editor.org/info/rfc9463>.
9.2. Informative References
[IANA-DNS] IANA, "Domain Name System (DNS) Parameters",
<https://www.iana.org/assignments/dns-parameters/>.
[RESINFO] Sood, P. and P. Hoffman, "DNS Resolver Information Self-
publication", Work in Progress, Internet-Draft, draft-pp-
add-resinfo-02, 30 June 2020,
<https://datatracker.ietf.org/doc/html/draft-pp-add-
resinfo-02>.
[RFC7070] Borenstein, N. and M. Kucherawy, "An Architecture for
Reputation Reporting", RFC 7070, DOI 10.17487/RFC7070,
November 2013, <https://www.rfc-editor.org/info/rfc7070>.
[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 8499, DOI 10.17487/RFC8499, January
2019, <https://www.rfc-editor.org/info/rfc8499>.
[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/info/rfc9250>.
[RRTYPE] IANA, "Resource Record (RR) TYPEs",
<https://www.iana.org/assignments/dns-parameters/>.
Acknowledgments
This specification leverages the work that has been documented in
[RESINFO].
Thanks to Tommy Jensen, Vittorio Bertola, Vinny Parla, Chris Box, Ben
Schwartz, Tony Finch, Daniel Kahn Gillmor, Eric Rescorla, Shashank
Jain, Florian Obser, Richard Baldry, and Martin Thomson for the
discussion and comments.
Thanks to Mark Andrews, Joe Abley, Paul Wouters, and Tim Wicinski for
the discussion on RR formatting rules.
Special thanks to Tommy Jensen for the careful and thoughtful
Shepherd review.
Thanks to Johan Stenstam and Jim Reid for the dns-dir reviews, Ray
Bellis for the RRTYPE allocation review, Arnt Gulbrandsen for the ART
review, and Mallory Knodel for the gen-art review.
Thanks to Eric Éric Vyncke for the AD review.
Thanks to Gunter Van de Velde, Érik Erik Kline, Paul Wouters, Orie Steele,
Warren Kumari, Roman Danyliw, and Murray Kucherawy for the IESG
review.
Authors' Addresses
Tirumaleswar Reddy.K
Nokia
India
Email: kondtir@gmail.com
Mohamed Boucadair
Orange
35000 Rennes
France
Email: mohamed.boucadair@orange.com