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DNS Resolver Information

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Tirumaleswar Reddy.K , Mohamed Boucadair
Last updated 2023-10-04 (Latest revision 2023-09-20)
Replaces draft-reddy-add-resolver-info
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state In WG Last Call
Document shepherd Tommy Jensen
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ADD                                                             T. Reddy
Internet-Draft                                                     Nokia
Intended status: Standards Track                            M. Boucadair
Expires: 6 April 2024                                             Orange
                                                          4 October 2023

                        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 the

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 6 April 2024.

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Copyright Notice

   Copyright (c) 2023 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.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  RESINFO RR Type . . . . . . . . . . . . . . . . . . . . .   6
     7.2.  DNS Resolver Information Key Registration . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Historically, DNS stub resolvers communicated with upstream resolvers
   without needing to know anything about the features supported by
   these recursive resolvers.  As more and more recursive resolvers
   expose different features that may impact delivered DNS services,
   means to help stub resolvers to identify the capabilities of
   resolvers are valuable.  Typically, stub resolvers can discover and
   authenticate encrypted DNS resolvers provided by a local network, for
   example, using the Discovery of Network-designated Resolvers (DNR)
   [I-D.ietf-add-dnr] and the Discovery of Designated Resolvers (DDR)
   [I-D.ietf-add-ddr].  However, these stub resolvers need a mechanism
   to retrieve information from the discovered recursive resolvers about
   their capabilities.

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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 stub resolvers 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.

3.  Retrieving Resolver Information

   A stub resolver 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 RESINFO RR type query is
   defined in Section 5.  If the resolver understands the RESINFO RR
   type, the RRSet in the Answer section MUST have exactly one record.

   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 [I-D.ietf-add-dnr]).

   If the Special-Use Domain Name "", defined in
   [I-D.ietf-add-ddr], is used to discover an encrypted DNS resolver,
   the client can retrieve the resolver information using the RESINFO RR
   type and QNAME of "".  If a resolver supports DDR but
   does not support RESINFO, the client can receive a positive RESINFO

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   response from a legitimate upstream DNS resolver or an attacker.
   While DNSSEC can be considered as a candidate mechanism to protect
   against the attack, it is important to note that DNSSEC protection is
   not possible with "" due to the lack of uniqueness.  To
   prevent such an attack, resolvers supporting DDR MUST convey the
   "sig" attribute as defined in Section 5.  DNS clients using DDR for
   encrypted resolver discovery querying for a RESINFO RR MUST validate
   the signature in the "sig" attribute for data origin authentication
   using the public key in the certificate, including ensuring that the
   certificate contains the IP address that DDR uses for validation as
   per Section 4.2 of [I-D.ietf-add-ddr].  If the signature validation
   fails, the DNS client MUST reject the RESINFO RR.

4.  Format of the Resolver Information

   The resolver information uses the same format as DNS TXT records.
   The motivation for using the same format as TXT records is to convey
   a small amount of useful information about a DNS resolver.  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 7.2) 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.

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   exterr:  If the DNS resolver supports extended DNS errors (EDE)
      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, how to report a problem) for
      troubleshooting purposes.

      The 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 possibily
      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.  For example, a DoT server may not
      want to host an HTTPS server.

   sig:  It contains the signature of the RESINFO RR excluding the "sig"
      attribute.  The signature is generated by the encrypted DNS
      resolver using all of the attributes in the RESINFO RR except the
      "sig" attribute.  The signature algorithm MUST be compatible with
      the public key in the DNS resolver's end-entity certificate.  As a
      reminder, the server's end-entity certificate's public key will be
      compatible with the selected authentication algorithm from the
      client's "signature_algorithms" TLS extension (Section of

      This is an optional attribute.

   New keys can be defined as per the procedure defined in Section 7.2.

   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

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6.  Security Considerations

   To prevent DNS response forgery attacks, DNS clients using DNR MUST
   employ one of the following measures:

   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.

   In order to prevent DNS response forgery attacks, DNS clients using
   DDR MUST perform the validation explained in Section 3 for data
   origin authentication of RESINFO RR.

7.  IANA Considerations

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

7.1.  RESINFO RR Type

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

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

7.2.  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

   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.

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   Value Type:  The type of the value to be used in the key.

   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   |  Value   | Description                | Specification |
   |          |   Type   |                            |               |
   | qnamemin |   None   | The presence of the key    |    RFCXXXX    |
   |          |          | name indicates that        |               |
   |          |          | 'qnameminimization' is     |               |
   |          |          | enabled                    |               |
   |  exterr  |  16-bit  | Lists the set of supported |    RFCXXXX    |
   |          | unsigned | extended DNS errors.  It   |               |
   |          | integer  | must be a value in the     |               |
   |          |          | "Extended DNS Error Codes" |               |
   |          |          | registry.                  |               |
   | infourl  |  string  | Provides an URL that       |    RFCXXXX    |
   |          |          | points to an unstructured  |               |
   |          |          | resolver information that  |               |
   |          |          | is used for                |               |
   |          |          | troubleshooting            |               |
   |   sig    |  binary  | Includes a signature of    |    RFCXXXX    |
   |          |          | RESINFO RR for data origin |               |
   |          |          | authentication             |               |

                    Table 1: Initial RESINFO Registry

8.  References

8.1.  Normative References

              Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", Work in
              Progress, Internet-Draft, draft-ietf-add-ddr-10, 5 August
              2022, <

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              Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for the
              Discovery of Network-designated Resolvers (DNR)", Work in
              Progress, Internet-Draft, draft-ietf-add-dnr-16, 27 April
              2023, <

   [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,

   [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, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [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,

8.2.  Informative References

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              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",

   [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, Tim Wicinski, and
   Steffen Nurpmeso for the discussion on the RR formatting rules.

   Thanks to Tommy Jensen for the Shepherd review.

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   Thanks to Johan Stenstam for the dns-dir review.

Authors' Addresses

   Tirumaleswar Reddy

   Mohamed Boucadair
   35000 Rennes

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