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Versions: 00 01 03

add                                                           D. Migault
Internet-Draft                                                  Ericsson
Intended status: Informational                             July 28, 2020
Expires: January 29, 2021


            DNS Resolving service Discovery Protocol (DRDP)
                         draft-mglt-add-rdp-03

Abstract

   This document describes the DNS Resolver Discovery Protocol (DRDP)
   that enables a DNS client to discover various available local and
   global resolving service.  The discovery is primarily initiated by a
   DNS client, but a resolving service may also inform the DNS client
   other resolving services are available and eventually preferred.

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 https://datatracker.ietf.org/drafts/current/.

   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 January 29, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/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 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.



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Table of Contents

   1.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Pointer to a list of Resolving Domains  . . . . . . . . . . .   5
   6.  Discovery of Resolving Services . . . . . . . . . . . . . . .   6
   7.  TTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  SvcParamKey . . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Resolver advertising other service sub type . . . . . . . . .   8
   10. Migration to service sub types  . . . . . . . . . . . . . . .   8
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   8
     11.1.  Use of protected channel is RECOMMENDED  . . . . . . . .   8
     11.2.  DNSSEC is RECOMMENDED  . . . . . . . . . . . . . . . . .   9
     11.3.  TLSA is RECOMMENDED  . . . . . . . . . . . . . . . . . .  10
   12. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   15. Appendices  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     15.1.  DRDP Requirements  . . . . . . . . . . . . . . . . . . .  12
     15.2.  Discovery of specific service instance . . . . . . . . .  13
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     16.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Requirements Notation

   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.

2.  Introduction

   A DNS client can proceed to DNS resolution using various resolving
   services.  These services can be local or global and can use a wide
   range of DNS transport protocols such as, for example, standard DNS
   [RFC1035], DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484].  The
   local scope of these services may take various forms.  For example,
   it could be associated to a network perspective ( restricted to the
   network the DNS client is connected to ) or to an application
   perspective ( restricted to some domain names ).

   The purpose of the DNS Resolving service Discovery Protocol (DRDP) is
   to discover resolving services available to the DNS client.  These



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   available resolving services to a given DNS client may highly depend
   on its location or browsing activity.  The number of resolving
   services available to the DNS client is expected to remain quite
   consequent and evolve over time.  Similarly, characteristics
   associated to these resolving services may also evolve over time.  As
   a result, the DNS client is unlikely willing to synchronize such a
   huge data base of resolving services.  DRDP proposes an alternative
   that consists in adaptively discovering the available resolving
   services based on the DNS client context.

   DRDP adopts a hierarchical approach where the DNS client (or DRDP
   client) discovers the resolving services from resolving domains (RD)
   or a pointer to a list of resolving domains (Pointer).

   The document does not describe how the DNS client is provisioned with
   RD or RD_list.  The DNS client may obtain the contextual resolving
   domains via various way, including a configuration, via DHCP Options
   [I-D.btw-add-home] or derived from specific procedures [I-D.mglt-add-
   drdp-isp].  The DNS client is expected to discover resolving services
   from all RD or RD_list before proceeding to a selection process.  The
   selection process of the resolving service is out of scope of this
   document.

3.  Terminology

   DNS client  the client that sends DNS queries fro resolution.  In
      this document the DNS client designates also the end entity that
      is collecting information about the available Resolving Services
      and then proceed to the selection of a subset them.  The selection
      is processed according to the DNS client's policy.

   Resolving Service  designates a service that receives DNS queries
      from a DNS client and resolves them.  A Resolving Service is
      implemented by one or multiple resolvers.

   Resolver: A resolver designates the software or hardware handling the
   DNS exchange.  See [RFC7719] for more details.

   DNS transport  designates the necessary parameters a DNS client needs
      to establish a session with a Resolving Service.

   Resolving Domain  a DNS domain that hosts one or multiple resolving
      services.








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4.  Overview

   DRDP is a DNS based protocol that addresses the requirements listed
   in Section 15.1.  The figure below represents provides a high level
   description of DRDP.

   1.  The DRDP client considers the source of resolving domains (RD).
       This document defines two ways to collect RDs: RD are directly
       provisioned - in our case rd.org) , or RD are retrieved from a
       Pointer - in our case rd_pointer.org.  In the later case RD are
       collected from a DNS request of type PTR.  In the case of Pointer
       being rd_pointer.org, the QNAME of the PTR request would be
       b._dns.rd_pointer.org.

   2.  The DRDP client collects the resolving services and associated
       parameters under the umbrella of each RD.  The resolving services
       offered by the RD are collected via a DNS request of type SVCB
       and the associated parameters are provided through SvcParameters.
       In the case of the RD being rd.org, the QNAME of the SVCB request
       would be _dns.rd.org.

                        +---------------------------------------------+
      +---------------->| source of Resolving Domains                 |
      |                 +---------------------------------------------+
   1. collect           | * resolving domains (rd.org)                |
   Resolving Domains    | * list of resolving domains (rd_pointer.net)|
      |                 | * other means: configuration, DHCP,         |
      |                 |                Website co-hosting, derived  |
      |                 +---------------------------------------------+
      |
   +-------------+      +---------------------------------------------+
   | DRDP client |----->|  Resolving Domain                           |
   +-------------+      +---------------------------------------------+
   2. collect           | rd.1.com,   ...,  rd.i.com,  ...,  rd.n.com |
   Resolving Services   +---------------------|------------------|----+
   and associated       |                     |                  |
   parameters           +---------------------|------------------|----+
                        | Resolving Services  |                  |    |
                        +---------------------v------------------v----+
                        | +-------------------+      +--------------+ |
   3. Proceed           | | doh.resolver.net  |      | doh.isp.com  | |
   Selection            | +-------------------+ ...  +--------------+ |
                        | | dot.resolver.net  |      | do53.isp.com | |
                        | +-------------------+      +--------------+ |
                        | | do53.resolver.net |                       |
                        | +-------------------+                       |
                        +---------------------------------------------+




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5.  Pointer to a list of Resolving Domains

   A Pointer is a FQDN that points to a list of FQDN that designates RD.
   If Pointer is represented by rd_pointer.net, the associated RDs are
   retrieved by the DNS query of type PTR for b._dns.rd_pointer.org.

   The zone file below is inspired from DNS-SD where b indicates a
   browsing domain, _dns indicates the DNS resolving service,
   rd_pointer.org the Pointer and rd.1.com, ... rd.i.com the associated
   RDs.  Note that they do not necessarily need to share a TLD.  The
   order of the resolving domains is irrelevant, and the zone
   administrator SHOULD regularly reorder them.  The RRsets MUST be
   signed with DNSSEC.

   b._dns.rd_pointer.net  PTR rd.1.com
   [...]
   b._dns.rd_pointer.net  PTR rd.n.com

   Using the DNS provides the advantage to retrieve the resolving domain
   without requiring other libraries than DNS as well as benefit from
   the DNS caching infrastructure including the use of the TTL.

   An EDNS buffer size of 1232 bytes will avoid fragmentation on nearly
   all current networks.  This is based on an MTU of 1280, which is
   required by the IPv6 specification, minus 48 bytes for the IPv6 and
   UDP headers.  This document RECOMMENDS that the number of RDs
   associated to a Pointer do not generate fragmentation of the DNS UDP
   packet.  It is believed to address most common needs or expectation
   from a vast majority of stub DNS client.

   When the number of RD exceeds this limit, the DNS client may carry
   this over TCP which is likely to be supported by DNS client wiling to
   upgrade to DoH or DoT resolving services.  However, the transfer of
   large number of RDs is considered as an application specificity that
   would benefit benefit from the compression of the transferred data
   provided by ftp or http.  In such case, these application may define
   there own specific mechanism to provision the RDs.

   As of July 27 2020, 1232 bytes correspond to the 94 first most
   popular FQDN listed by [moz.com].  The current size of such lists
   [curl][dnsprivacy.org] have less than 50 resolving domains.  Other
   lists such as [public-dns.info] have as much as 11.000 entries, but
   such lists seems to contain open resolvers which is out side of the
   scope of a selection process.
   Web browser (Google Chrome) also have lists over 10.000 entries, but
   in case a significant number of entries seems to be IP addresses that
   have a very limited network scope ( e.g. limited to the ISP ).  The
   length of the list in scope to the DNS client is in fact significant



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   smaller in term of IP addresses and even smaller if resolving domain
   are able to represent multiple IP addresses.  Overall, the size of
   such lists are currently due to the absence of discovery protocols.

6.  Discovery of Resolving Services

   The discovery of resolving services is performed by the RDP client
   with all the available RDs.  Given a RD rd.org, a DRDP client sends a
   DNS request of type SVCB for _dns.rd.org.

   The example below presents the use of an AliasForm followed by a
   ServiceForm which allows an indirection.  The Alias form is not
   madatory and instead only ServiceForm associated to _dns.rd.org could
   have been used instead.

   The SvcFieldPriority indicates the preference of the RD.  It
   typically enables an operator to indicate that an encrypted DNS is
   preferred.

   The SvcParamKey alpn MUST be present when TLS is used as its presence
   and value indicates the DNS transport.  The absence of the alpn
   SvcParamKey indicates Do53, alpn set to dot indicates DoT is served
   while h* indicates DoH is served.  Note that the port value (53, 853,
   443) is not used to determine the DNS transport as non standard port
   MAY be used.  The example below uses an non standard port 5353 for
   illustrative purpose.

   Other SvcParam are detailed in Section 8 and are optional.  A
   SvcParam not understood by the DNS client MUST be ignored.

   The RRsets MUST be protected with DNSSEC and when alpn is provided a
   TLSA RRset SHOULD be present.  These RRsets have been omitted for
   clarity.

## Discovery of all service instances
_dns.rd.org. 7200 IN SVCB 0 svc.example.com.
svc.example.com.    7200 IN SVCB 12 ( svc0.example.net.
                                      port="5353" user-display="Legacy Resolver" )
svc.example.com.    7200 IN SVCB 1 ( svc1.example.net.  alpn="dot"
                                      port="5353" esniconfig="..."
                                      user-display="Preferred Example's Choice" )
svc.example.com.    7200 IN SVCB 3 ( svc2.example.net. alpn="h2"
                                       port="5353" esniconfig="..." user-display= )
svc.example.com.    7200 IN SVCB 2 ( svc3.example.net. alpn="h3"
                                       port="5353" esniconfig="..." user-display= )






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   Note that Section 15.2 provides another variant to perform RDP.  Such
   variant is left for further discussion and address the need to be
   able to narrow down the discovery to a subset of resolving services
   such as DoH-only or DoT-only services.

   Some notes:

   1.  _domain uses SVCB but does not have TLS.  While SVCB has been
       created essentially for TLS based service, this does not appear
       to be mandatory.

   2.  Should we have some constraints regarding the SvcDomainName and
       QNAME ?

   3.  do we need the service subsets

7.  TTL

   The DNS client SHOULD perform DRDP at regular intervals as indicated
   by its policy.

   The selection process MAY remove resolving services with short TTL
   lower than a day as it indicates some source of instalbility.  Given
   a subset of selected resolving services, the DNS client may perform
   DRDP 1 hour before an SVB RRset expires.

8.  SvcParamKey

   This section defines a set of SvcParamKey that MAY be use to carry
   the necessary informations for the selection process.

   alpn :

   esniconfig :

   port :

   user-display  indicates a strings in UTF-8 that is expected to be
      representative to a potential end user.  Though there is no
      restriction in the scope of that string.  The string is likely to
      represent the service within the resolving domain.

   uri_template  designates the URI template for DoH.  This key MUST NOT
      be present on non DoH services and MUST be ignored by the DNS
      client on non DoH resolving Services.

   auth_domain  indicates the list of authoritative domain name the
      resolving service has strong relation with.  It is expected that a



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      resolving service may prefer to perform DNS resolution over these
      domains to that specific resolving service as to preserve its
      privacy.  This information MUST be verified and validated.

   scope_domain  indicates the limitation of resolved domains.  When
      present DNS request sent to the resolution service MUST belong to
      that domain.

   filtering  indicates the presence of a filtering service

   ip_subnet  indicates a subnetwork restriction.  This is mostly useful
      for resolving services that are not globally.

   dnssec  indicates whether dnssec is enabled or not.

9.  Resolver advertising other service sub type

   A resolving service receiving a DNS request over a service sub type
   MAY be willing to advertise the DNS client that other sub service
   type are available.  This is especially useful, when, for example, a
   resolver wants that the DNS resolver switches to other service sub
   types that are more secure.

   In order to do so the resolver MAY provide in the additional data
   field the _dns SRVCB of ServiceForm.

10.  Migration to service sub types

   The principle of the discovery mechanism is that the resolver
   indicates the available service sub types and let the DNS client
   chose which sub type it prefers.  On the other hand, the resolver MAY
   also indicate a preference using the priority and weight fields.
   Redirection MAY especially be needed when a DNS client is using the
   Do53 and the resolver would like to upgrade the DNS client session to
   a more secure session.  This MAY require a specific ERROR code that
   will request the DNS client to perform service discovery.

   It is expected that DRDP MUST always be available via Do53.  However,
   this does not mean that a resolver is expected to implement the Do53
   sub type service for a resolving service.

11.  Security Considerations

11.1.  Use of protected channel is RECOMMENDED

   When available, it is recommended to chose a protected version of the
   rdns service.  More specifically, the use of end-to-end protection
   ensures that the DNS client is connected to the expected platform and



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   that its traffic cannot be intercepted on path.  Typically, the
   selection of resolver on the Internet (and not on your ISP network)
   and the use of a non protected channel enables an attacker to monitor
   your DNS traffic.  The similar observation remains true if you are
   connected to the resolver of your ISP.  It is commonly believed that
   trusting your ISP (that is your first hop) makes encryption
   unecessary.  Trusting your ISP is mandatory in any case, but the
   associated level of trust with an protected channel is restricted to
   the operation of the DNS platform.  With non protected channel the
   trust is extended to any segment between the DNS client and the
   resolver, which is consequently larger.  The use of a protected
   channel is recommended as it will prevent anyone on path to monitor
   your traffic.

11.2.  DNSSEC is RECOMMENDED

   The exchanges SHOULD be protected with DNSSEC to ensure integrity of
   the information between the authoritative servers and the DNS client.
   Without DNSSEC protection, DNS messages may be tampered typically
   when they are transmitted over an unprotected channel either between
   the DNS client and the resolver or between the resolver and the
   authoritative servers.  The messages may be tampered by an online
   attacker intercepting the messages or by the intermediary devices.
   It is important to realize that protection provided by TLS is limited
   to the channel between the DNS client and the resolver.  There are a
   number of cases were the trust in the resolver is not sufficient
   which justify the generalization of the use of DNSSEC.  The following
   examples are illustrative and are intended to be exhaustive.

   First, the discovery exchanges may happen over an unprotected
   channel, in which case, the messages exchanged may be tampered by
   anyone on-path between the DNS client and the resolver as well as
   between the resolver and the authoritative servers - including the
   resolver.  When TLS is used between the DNS client and the resolver,
   this does not necessarily mean the DNS client trusts the resolver.
   Typically, the TLS session may be established with a self-signed
   certificate in which case the session is basically protected by a
   proof-of-ownership.  In other cases, the session may be established
   based on Certificate Authorities (CA) that have been configured into
   the TLS client, but that are not necessarily trusted by the DNS
   client.  In such cases, the connected resolver may be used to
   discover resolvers from another domain.  In this case, the resolver
   is probably interacting with authoritative servers using untrusted
   and unprotected channels.  Integrity protection relies on DNSSEC.







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11.3.  TLSA is RECOMMENDED

   When TLS is used to protect the DNS exchanges, certificates or
   fingerprint SHOULD be provided to implement trust into the
   communication between the DNS client and the resolver.  The TLS
   session and the association of the private key to a specific identity
   can be based on two different trust model.  The Web PKI that will
   rely on CA provisioned in the TLS library or the TA provided to the
   DNS client.  A DNS client SHOULD be able to validate the trust of a
   TLS session based on the DNSSEC trust model using DANE.

   When the DNS client is protecting its session to the resolver via
   TLS, the DNS client may initiate an TLS session that is not validated
   by a CA or a TLSA RRsets.  The DNS client MUST proceed to the
   discovery process and validate the certificate match the TLSA RRset.
   In case of mismatch the DNS client MUST abort the session.

12.  Privacy Considerations

   When the discovery protocol is performed using a resolver that
   belongs to one domain for another domain, or over an unprotected
   channel, the DNS client must be conscious that its is revealing to
   the resolver its intention to use another resolver.  More
   specifically, suppose an resolver is complying some legal
   requirements that DNS traffic must be unencrypted.  Using this
   resolver to perform a resolver discovery reveals the intention of
   potentially using alternative resolvers.  Alternatively, narrowing
   down the discovery over a specific sub type of resolver (DoT, or DoH)
   may reveal to that resolver the type of communication.  As result,
   when performing a discovery over a domain that differs to the domain
   the resolver belongs to, it is RECOMMENDED to request the SRV RRsets
   associated to all different sub type of proposed services.

   The absence of traffic that results from switching completely to a
   newly discovered resolver right after the discovery process provides
   an indication to the resolver the DNS client is switching to.  It is
   hard to make that switch unnoticed to the initial resolver and the
   DNS resolver MUST assume this will be noticed.  The information of
   switching may be limited by sharing the traffic between different
   resolvers, however, the traffic pattern associated to each resolver
   may also reveal the switch.  In addition, when the initial resolver
   is provided by the ISP, the ISP is also able to monitor the IP
   traffic and infer the switch.  As a result, the DNS client SHOULD
   assume the switch will be detected.

   With DoT or DoH, the selection of port 443 will make the traffic
   indistinguishable from HTTPS traffic.  This means that an observer
   will not be able to tell whether the traffic carries web traffic or



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   DNS traffic.  Note that it presents an interest if the server offers
   both a web service as well as a resolution service.  Note that many
   resolvers have a dedicated IP address for the resolution service, in
   which case, the information will be inferred from the IP address.
   Note also that traffic analysis may infer this as well.  Typically
   suppose an IP address hosts one or multiple web sites that are not
   popular as well as a resolving service.  If this IP address is
   associated frequent short size exchanges, it is likely that these
   exchanges will be DNS exchanges rather than Web traffic.  The size of
   the packet may also be used as well as many other patterns.  As a
   result, the use port 443 to hide the DNS traffic over web traffic
   should be considered as providing limited privacy.

13.  IANA Considerations

   This document requests the IANA the creation of the following
   underscored node names in the Underscored and Globally Scoped DNS
   Node Names registry https://www.iana.org/assignments/dns-parameters/
   dns-parameters.xhtml#dns-parameters-14

   RR Type | _NODE NAME | Reference
   --------+------------+----------
   SRVCB   | _dns       | RFC-TBD

   SvcParamKey | NAME         | Meaning                     | Reference
   ------------+--------------+-----------------------------+-----------
   7           | user-display | User friendly string (UTF8) | RFC-TBD
               |              | to represent the resolver   |
               | uri_template | URI template                |
               | auth_domain  | Domains the resolving       |
               |              | service is authoritative    |
               | filetring    | Filetring services provided |
               | ip_subnet    | ip ranges accepted.         |
               | dnssec       | DNSSEC validation enabled   |

14.  Acknowledgments

   We would like thank Mirja Kuehlewind as well as the GSMA IG for their
   comments.  We also thank Ted Hardie and Paul Hoffman for their feed
   backs regarding the dns schemes for DoT and DoH.
   We thank Ben Schwartz for the comments on the list size.  We thank
   Harald Alvestrand for its recommendation on having a model that
   enable multiple third party providers to provide their own list of
   resolving domains.  We thank Stephan Bortzmeyer, Ralf Weber, Chris
   Box for its clarifications.






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15.  Appendices

15.1.  DRDP Requirements

   This section lists the DRDP requirements.

   REQ 1: DRDP MUST enable a DNS client to discover the available
   resolving services with their associated characteristics in order to
   proceeds to a selection process.  The selection process takes
   resolving services identities and associated parameters and proceed
   to the selection.
   Any sort of resolving service selection is outside the scope of DRDP.

   REQ 2: While the discovery process is triggered by the DNS client, a
   third party MUST be able to provide necessary input information so a
   resolving service discovery process can be triggered within a
   specific context.
   Provisioning protocols to provide this information is not as per say
   in scope of DRDP.  DRDP defines the format of the format for such
   input as well as a set of such inputs.

   REQ 3: Any information used in DRDP MUST be authenticated by its
   owner.  In particular, the characteristics associated to the
   resolving service MUST be certified by the resolving service operator
   / owner and MUST be associated a validity period.  In addition, a
   third party providing a set of inputs MUST authenticate that set.

   REQ 4: Information associated to the resolving services is intended
   to enable the selection process that is assumed to match the end user
   or application policy.  The selection process is out of scope of
   DRDP.  Information may carry some characteristics of a resolving
   service or hints that will help the selection.  In particular an
   operator of multiple resolving service MUST be able to associate a
   preference to the proposed resolving services.  To ease automation of
   the selection as well as to make multiple applications benefit from
   DRDP the information MUST be carried over a standardized format.

   REQ 5: DRDP MUST be designed to be used indifferently by a DNS client
   using any DNS transport protocol (Do53, DoT, DoH, ...).  However,
   DRDP SHOULD be able to restrict the information retrieved to a
   certain type of resolving service, i.e. Do53, DoT, DoH.

   REQ 6: DRDP deployment MUST NOT be disruptive for the legacy DNS
   client or infrastructure and legacy client SHOULD be able to
   incrementally include DRDP.






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15.2.  Discovery of specific service instance

   To reduce the size of the messages, the DNS client MAY also prefer to
   query information of resolving services using a specific transport
   (DNS, DoT, DoH) that are designated as sub sets.  A DNS client MAY
   list the different subsets of that resolving domain with a PTR query.
   This document defines the following subsets _53._dns for DNS,
   _853._dns for DoT and _443.__dns for DoH.  Other subsets MAY be
   defined in the future.  A DNS client that does not understand a
   subset SHOULD ignore it and maybe proceed to the discovery as defined
   in Section 6.

   All subsets MUST share the same resolving domain and be listed with a
   PTR RRsets.  The DNS client MAY NOT performed a DNS query of type
   PTR, for example, if it has a previous knowledge of the existence of
   the subset or if indicated by its policy.  In this it MAY directly
   proceed to the SRVCB resolution.

   The same restrictions as defined in section Section 6 apply.

   Note that while the SvcFieldPriority indicates the priority within a
   subservice, this field MUST have a coherence across subservices.  The
   priority provided SHOULD be coherent with the case of a _dns SRVCB
   query of section Section 6.

   The figure below illustrates an example of zone file.  RRSIG and TLSA
   have been omitted for the purpose of clarity.

### Definition of the resolving service subsets
_dns.example.com PTR _53._dns.example.com
_dns.example.com PTR _853._dns.example.com
_dns.example.com PTR _443._dns.example.com

### services instances per service subset
_53._dns.example.com. 7200 IN SVCB 0 svc0.example.com.
svc0.example.com.    7200 IN SVCB 12 ( svc0.example.net.
                                       port="5353" user-display="Legacy Resolver" )
_853._dns.example.com.    7200 IN SVCB 0 svc1.example.com.
svc1.example.com.    7200 IN SVCB 1 ( svc1.example.net.  alpn="dot"
                                      port="5353" esniconfig="..."
                                      user-display="Preferred Example's Choice" )

_443_dns.example.com.    7200 IN SVCB 0 svc4.example.net.
svc4.example.com.    7200 IN SVCB 3 ( svc2.example.net. alpn="h2"
                                       port="5353" esniconfig="..." user-display= )
svc4.example.com.    7200 IN SVCB 2 ( svc3.example.net. alpn="h3"
                                      port="5353" esniconfig="..."
                                      user-display="Testing QUIC")



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16.  References

16.1.  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>.

   [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>.

   [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>.

   [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>.

16.2.  Informative References

   [curl]     "Publicly available servers", n.d.,
              <https://github.com/curl/curl/wiki/DNS-over-
              HTTPS#publicly-available-servers>.

   [dnsprivacy.org]
              "DNS Privacy Test Servers", n.d.,
              <https://dnsprivacy.org/wiki/display/DP/
              DNS+Privacy+Test+Servers#DNSPrivacyTestServers-
              Publicresolvers>.

   [I-D.btw-add-home]
              Boucadair, M., Reddy.K, T., Wing, D., and N. Cook,
              "Encrypted DNS Discovery and Deployment Considerations for
              Home Networks", draft-btw-add-home-07 (work in progress),
              July 2020.

   [moz.com]  "The Moz Top 500 Websites", n.d.,
              <https://moz.com/top500>.





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   [public-dns.info]
              "Public DNS Server List", n.d.,
              <https://public-dns.info/>.

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <https://www.rfc-editor.org/info/rfc7719>.

Author's Address

   Daniel Migault
   Ericsson
   8275 Trans Canada Route
   Saint Laurent, QC  4S 0B6
   Canada

   EMail: daniel.migault@ericsson.com


































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