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DNS Resolver Information

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9606.
Authors Tirumaleswar Reddy.K , Mohamed Boucadair
Last updated 2024-04-04 (Latest revision 2024-03-20)
Replaces draft-reddy-add-resolver-info
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tommy Jensen
Shepherd write-up Show Last changed 2024-02-14
IESG IESG state Became RFC 9606 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Has enough positions to pass.
Responsible AD Éric Vyncke
Send notices to
IANA IANA review state IANA OK - Actions Needed
ADD                                                             T. Reddy
Internet-Draft                                                     Nokia
Intended status: Standards Track                            M. Boucadair
Expires: 22 September 2024                                        Orange
                                                           21 March 2024

                        DNS Resolver Information


   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 such
   an information is then used by DNS clients is out of the scope of
   this document.

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 (, which is archived at

   Source for this draft and an issue tracker can be found at

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

   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 22 September 2024.

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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
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Retrieving Resolver Information . . . . . . . . . . . . . . .   3
   4.  Format of the Resolver Information  . . . . . . . . . . . . .   4
   5.  Resolver Information Keys/Values  . . . . . . . . . . . . . .   4
   6.  An Example  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  RESINFO RR Type . . . . . . . . . . . . . . . . . . . . .   6
     8.2.  DNS Resolver Information Key Registration . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Historically, DNS clients communicated with recursive resolvers
   without needing to know anything about the features supported by
   these resolvers.  However, recent developments (e.g., Extended Error
   Reporting [RFC8914] or encrypted DNS) imply that earlier assumption
   no longer generally applies.  Typically, DNS clients can discover and
   authenticate encrypted DNS resolvers provided by a local network
   (e.g., 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.  Instead of
   depending on opportunistic approaches, DNS clients need a more
   reliable mechanism to discover the features that are supported by

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   This document fills that void by specifying a method for stub
   resolvers to retrieve such information.  To that aim, a new resource
   record (RR) type is defined for DNS clients to query the recursive
   resolvers.  The information that a resolver might want to expose is
   defined in Section 5.

   Retrieved information can be used to feed the server selection
   procedure.  However, that selection procedure is out of the scope of
   this document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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:  "The estimation in which an identifiable actor is held,
      especially by the community or the Internet public generally"
      (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.  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]).

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   If the Special-Use Domain Name "", 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 "".  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 either from 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 encrypted DNS
   resolver is not authoritative for the response.

4.  Format of the Resolver Information

   The resolver information record uses the same format as DNS TXT
   records.  As a reminder, the format rules for TXT records are defined
   in the base DNS specification (Section 3.3.14 of [RFC1035]) and
   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 those defined in Section 6.4 of [RFC6763] MUST
   be followed for RESINFO.

   Keys MUST either be defined in the IANA registry (Section 8.2) or
   begin with the substring "temp-" for names defined for local use

5.  Resolver Information Keys/Values

   The following resolver information keys are defined:

   qnamemin:  If the DNS resolver supports QNAME minimisation [RFC9156]
      to improve DNS privacy, the key is present.  Note that, as 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.

      This is an optional attribute.

   exterr:  If the DNS resolver supports extended DNS errors (EDE)

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      option [RFC8914] to return additional information about the cause
      of DNS errors, the value of this key lists the possible extended
      DNS error codes that can be returned by this DNS resolver.  When
      multiple values are present, these values MUST be comma-separated.

      This is an optional attribute.

   infourl:  An 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 the content-type
      'text/html'.  The DNS client MUST reject the URL if the scheme is
      not "https".  The URL SHOULD 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.  A DNS client MAY choose to display the URL to the
      end user, if and only if the encrypted resolver has sufficient
      reputation, according to some local policy (e.g., user
      configuration, administrative configuration, or a built-in list of
      respectable resolvers).

      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. 7200 IN RESINFO qnamemin exterr=15,16,17

            Figure 1: An Example of Resolver Information Record

   As mentioned in Section 3, a DNS client that discovers the ADN
   "" of its resolver using DNR will issue a query
   for RESINFO RR QTYPE for that ADN and will learn that the resolver

   *  QNAME minimisation,

   *  Blocked (15), Censored (16), and Filtered (17) EDEs, and

   *  that more information can be retrieved from

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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 ''.

   An encrypted resolver may return incorrect information in RESINFO.
   If the client cannot validate the attributes received from the
   resolver, that 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

8.  IANA Considerations

      Note to the RFC Editor: Please update "RFCXXXX" occurrences with
      the RFC number to be assigned to this document.

8.1.  RESINFO RR Type

   This document requests IANA to update this entry from the "Resource
   Record (RR) TYPEs" registry of the "Domain Name System (DNS)
   Parameters" registry group available at [RRTYPE]:

   Type: RESINFO
   Value: 261
   Meaning: Resolver Information as Key/Value Pairs
   Reference: RFCXXXX

8.2.  DNS Resolver Information Key Registration

   This document requests IANA to create a new registry entitled "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

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   The registration procedure is Specification Required (Section 4.6 of

   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 these names can be used freely
      by any implementer.

   Description:  A description of the registered key.

   Specification:  The reference specification for the registered

   The initial content of this registry is provided in Table 1.

    |   Name   | Description                         | Specification |
    | qnamemin | The presence of the key name        |    RFCXXXX    |
    |          | indicates that QNAME minimization   |               |
    |          | is enabled                          |               |
    |  exterr  | Lists the set of supported extended |    RFCXXXX    |
    |          | DNS errors.  It must be an INFO-    |               |
    |          | CODE decimal value in the "Extended |               |
    |          | DNS Error Codes" registry.          |               |
    | infourl  | Provides an URL that points to an   |    RFCXXXX    |
    |          | unstructured resolver information   |               |
    |          | that is used for troubleshooting    |               |

                    Table 1: Initial RESINFO Registry

9.  References

9.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,

   [RFC9156]  Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
              Name Minimisation to Improve Privacy", RFC 9156,
              DOI 10.17487/RFC9156, November 2021,

   [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,

   [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,

9.2.  Informative References

              Sood, P. and P. E. Hoffman, "DNS Resolver Information
              Self-publication", Work in Progress, Internet-Draft,
              draft-pp-add-resinfo-02, 30 June 2020,

   [IANA-DNS] IANA, "Domain Name System (DNS) Parameters",

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   [RFC7070]  Borenstein, N. and M. Kucherawy, "An Architecture for
              Reputation Reporting", RFC 7070, DOI 10.17487/RFC7070,
              November 2013, <>.

   [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, <>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,

   [RRTYPE]   IANA, "Resource Record (RR) TYPEs",


   This specification leverages the work that has been documented in

   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 the 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 Vyncke for the AD review.

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Authors' Addresses

   Tirumaleswar Reddy

   Mohamed Boucadair
   35000 Rennes

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