Network Working Group                                            P. Sood
Internet-Draft                                                    Google
Intended status: Standards Track                              P. Hoffman
Expires: December 29, 2020                                         ICANN
                                                           June 27, 2020

               DNS Resolver Information Self-publication


   This document describes methods for DNS resolvers to self-publish
   information about themselves.  The information is returned as a JSON
   object.  The names in this object are defined in an IANA registry
   that allows for light-weight registration.  Applications and
   operating systems can use the methods defined here to get the
   information from resolvers in order to make choices about how to send
   future queries to those resolvers.

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
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   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 December 29, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Retrieving Resolver Information by DNS  . . . . . . . . . . .   3
   3.  Contents of the Returned I-JSON Object  . . . . . . . . . . .   4
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  RESINFO RRtype  . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Registry for DNS Resolver Information . . . . . . . . . .   5
     4.3. Special-Use Domain Name  . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Ideas From Earlier Work that was Abandoned . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Historically, DNS stub resolvers typically communicated with the
   recursive resolvers in their configuration without needing to know
   anything about the features of the recursive resolvers.  More
   recently, recursive resolvers have different features that may cause
   stub resolvers to make choices about which configured resolver from
   its configuration to use, and also how to communicate with the
   recursive resolver (such as over different transports).  Thus stub
   resolvers need a way to get information from recursive resolvers
   about features that might affect the communication.

   This document specifies a method for stub resolvers to ask recursive
   resolvers for such information.  In short, a new RRtype is defined
   for stub resolvers to query using the DNS to a special-use domain

   The response from this method is a JSON object.  The JSON object MUST
   use the I-JSON message format defined in [RFC7493].  Note that
   [RFC7493] was based on RFC 7159, but RFC 7159 was replaced by
   [RFC8259].  Requiring the use of I-JSON instead of more general JSON
   format greatly increases the likelihood of interoperability.

   The information that a resolver might want to give to a recursive
   resolver is not defined in this document; instead other documents

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   will follow that will specify that information and the format that it
   comes in.

   In nearly every common scenario today, a DNS stub resolver gets the
   IP addresses of the recursive resolvers that it will use in an
   insecure fashion, such as from DHCP.  Because these addresses were
   obtained insecurely, the protocol specified here does not try to use
   authenticated communication.  If, in the future, more stub resolvers
   get the addresses of their recursive resolvers in a secure fashion,
   this protocol can be enhanced to include authenticated ways of
   getting information from the resolver.

1.1.  Definitions

   In the rest of this document, the term "resolver" without
   qualification means "recursive resolver" as defined in [RFC8499].
   Also, the term "stub" is used to mean "stub resolver".

   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.

2.  Retrieving Resolver Information by DNS

   A stub that wants to use the DNS to get information about a resolver
   can use the DNS query defined here.  The query a stub resolver uses
   is  The RRtype "RESINFO" is defined in
   this document, and the IANA assignment is given in Section 4.1.  The
   contents of the Rdata in the response to this query is defined in
   Section 3.  If the resolver understands the RESINFO RRtype, the RRset
   in the Answer section MUST have exactly one record.

   The name is defined in this document, and the IANA
   assignment is given in Section 4.3.  As described in Section 4.3, the
   zone is not actually delegated and never will be.
   The resolver that receives this query acts as if it is delegated, and
   responds with its own RESINFO data in the Answer section.

   A resolver that receives a query with the RRtype of RESINFO with a
   QNAME of acts as if it is delegated, and responds
   with its own RESINFO data in the Answer section.

   A resolver MAY be configured to respond to queries for the RESINFO
   RRtype on names other than  For example, a
   resolver might be known to some of its clients by both an IP address
   and a few domain names, and be configured to be authoritative for

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   those names.  For all names other than or names
   that the resolver is configured to be authoritative for, a query for
   the RESINFO RRtype is meaningless and MUST result in a NODATA or
   NXDOMAIN response.

3.  Contents of the Returned I-JSON Object

   The JSON object returned by a DNS query or an HTTPS query MAY contain
   any name/value pairs.

   All names in the returned object MUST either be defined in the IANA
   registry or, if for local use only, begin with the substring "temp-".
   The IANA registry (Section 4.2) will never register names that begin
   with "temp-".

   All names MUST consist only of lower-case ASCII characters, digits,
   and hyphens (that is, Unicode characters U+0061 through 007A, U+0030
   through U+0039, and U+002D), and MUST be 63 characters or shorter.
   As defined in Section 4.2, the IANA registry will not register names
   that begin with "temp-", so these names can be used freely by any

   Note that the message returned by the resolver MUST be in I-JSON
   format.  I-JSON requires that the message MUST be encoded in UTF8.

3.1.  Example

   The I-JSON object that a resolver returns might look like the

      "temp-field2": 42

   As specified in [RFC7493], the I-JSON object is encoded as UTF8.
   [RFC7493] explicitly allows the returned objects to be in any order.

4.  IANA Considerations

4.1.  RESINFO RRtype

   This document defines a new DNS RR type, RESINFO, whose value TBD
   will be allocated by IANA from the "Resource Record (RR) TYPEs" sub-
   registry of the "Domain Name System (DNS) Parameters" registry:

   Type: RESINFO

   Value: TBD

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   Meaning: Information self-published by a resolver as an I-JSON (RFC
   7493) object

   Reference: This document

4.2.  Registry for DNS Resolver Information

   IANA will create a new registry titled "DNS Resolver Information"
   that will contain definitions of the names that can be used with the
   protocols defined in this document.  The registration procedure is by
   Expert Review and Specification Required, as defined in [RFC8126].

   The specification that is required for registration can be either an
   Internet-Draft or an RFC.  The reviewer for this registry is
   instructed to generally be liberal in what they accept into the
   registry: as long as the specification that comes with the
   registration request is reasonably understandable, the registration
   should be accepted.

   The registry has the following fields for each element:

   Name: The name to be used in the JSON object.  This name MUST NOT
   begin with "temp-".  This name MUST conform to the definition of
   "string" in I-JSON [RFC7493] message format.

   Value type: The type of data to be used in the JSON object.

   Specification: The name of the specification for the registered

4.3. Special-Use Domain Name

   IANA will record the domain name "" in the
   "Special-Use Domain Names" registry [SUDN].  IANA MUST NOT delegate in the .arpa zone.

5.  Security Considerations

   Unless a DNS request for as described
   in Section 2 is sent over DNS-over-TLS (DoT) [RFC7858] or DNS-over-
   HTTPS (DoH) [RFC8484], the response is susceptible to forgery.  Given
   that one of the first expected uses for the protocol in this document
   is to find out whether DoT or DoH is available for the resolver, it
   is thus expected that most if not all such DNS requests will be sent
   without any chance of authentication.  Stubs and resolvers SHOULD use
   normal DNS methods for avoiding forgery such as query ID
   randomization and source port randomization.

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6.  References

6.1.  Normative References

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

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,

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

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

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

   [SUDN]     "Special-Use Domain Names", n.d.,

6.2.  Informative References

   [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, <>.

   [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,

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

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Appendix A.  Ideas From Earlier Work that was Abandoned

   This document is based on work done earlier in the DNSOP working
   group, and personal drafts before that.

   In that earlier work, "<reverse-ip>.{in-addr,ip6}.arpa" was proposed
   as the domain name to allow for the possibility of DNSSEC-signed
   responses.  However, it was pointed out that people often do not
   control their reverse IP names and thus their ISP (or their ISP's
   ISP) could spoof responses and make them look legitimate by signing
   with DNSSEC.

   In an earlier version of this draft, a second way to get the resolver
   information was specified: using a query to a well-known URI over
   HTTPS, possibly with authentication.  Many participants in the ADD
   Working Group in early 2020 disagreed with specifying this transport
   because the IP address being used was most likely obtained by the
   stub resolver in an insecure fashion, so using an authenticated
   method could lead to inappropriate assumptions about the security of
   the answer.


   The idea of various types of servers publishing information about
   themselves has been around for decades.  However this idea has not
   been used in the DNS.  This document aims to fix this omission.

   Roy Arends contributed many ideas to an earlier version of this draft
   before it was moved to the ADD working group.

Authors' Addresses

   Puneet Sood


   Paul Hoffman


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