



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.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

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 [RFC9499].  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 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 [RFC9499]) 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, 27 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>.

   [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>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [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 Éric Vyncke for the AD review.

   Thanks to Gunter Van de Velde, 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
