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DNS over CoAP (DoC)
draft-lenders-dns-over-coap-04

Document Type Active Internet-Draft (individual)
Authors Martine Sophie Lenders , Christian Amsüss , Cenk Gündoğan , Thomas C. Schmidt , Matthias Wählisch
Last updated 2022-07-11
Replaces draft-lenders-dns-over-coaps
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Intended RFC status (None)
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Additional resources GitHub Repository
Stream Stream state (No stream defined)
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draft-lenders-dns-over-coap-04
CoRE                                                       M. S. Lenders
Internet-Draft                                                 FU Berlin
Intended status: Standards Track                               C. Amsüss
Expires: 12 January 2023                                                
                                                             C. Gündoğan
                                                           T. C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                                               FU Berlin
                                                            11 July 2022

                          DNS over CoAP (DoC)
                     draft-lenders-dns-over-coap-04

Abstract

   This document defines a protocol for sending DNS messages over the
   Constrained Application Protocol (CoAP).  These CoAP messages are
   protected by DTLS-Secured CoAP (CoAPS) or Object Security for
   Constrained RESTful Environments (OSCORE) to provide encrypted DNS
   message exchange for constrained devices in the Internet of Things
   (IoT).

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/anr-bmbf-pivot/draft-dns-over-coap.

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/.

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   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 12 January 2023.

Copyright Notice

   Copyright (c) 2022 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 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Selection of a DoC Server . . . . . . . . . . . . . . . . . .   4
   4.  Basic Message Exchange  . . . . . . . . . . . . . . . . . . .   4
     4.1.  The "application/dns-message" Content-Format  . . . . . .   5
     4.2.  DNS Queries in CoAP Requests  . . . . . . . . . . . . . .   5
       4.2.1.  Request Format  . . . . . . . . . . . . . . . . . . .   5
       4.2.2.  Support of CoAP Caching . . . . . . . . . . . . . . .   5
       4.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  DNS Responses in CoAP Responses . . . . . . . . . . . . .   6
       4.3.1.  Response Codes and Handling DNS and CoAP errors . . .   6
       4.3.2.  Support of CoAP Caching . . . . . . . . . . . . . . .   6
       4.3.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  CoAP/CoRE Integration . . . . . . . . . . . . . . . . . . . .   7
     5.1.  DoC Server Considerations . . . . . . . . . . . . . . . .   8
     5.2.  Observing the DNS Resource  . . . . . . . . . . . . . . .   8
     5.3.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Considerations for Unencrypted Use  . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  New "application/dns-message" Content-Format  . . . . . .   8
     8.2.  New "core.dns" Resource Type  . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10

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   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  11
     A.1.  Since draft-lenders-dns-over-coap-03  . . . . . . . . . .  11
     A.2.  Since draft-lenders-dns-over-coap-02  . . . . . . . . . .  12
     A.3.  Since draft-lenders-dns-over-coap-01  . . . . . . . . . .  12
     A.4.  Since draft-lenders-dns-over-coap-00  . . . . . . . . . .  13
     A.5.  Since draft-lenders-dns-over-coaps-00 . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   This document defines DNS over CoAP (DoC), a protocol to send DNS
   [RFC1035] queries and get DNS responses over the Constrained
   Application Protocol (CoAP) [RFC7252].  Each DNS query-response pair
   is mapped into a CoAP message exchange.  Each CoAP message is secured
   by DTLS [RFC6347] or Object Security for Constrained RESTful
   Environments (OSCORE) [RFC8613] to ensure message integrity and
   confidentiality.

   The application use case of DoC is inspired by DNS over HTTPS
   [RFC8484] (DoH).  DoC, however, aims for the deployment in the
   constrained Internet of Things (IoT), which usually conflicts with
   the requirements introduced by HTTPS.

   To prevent TCP and HTTPS resource requirements, constrained IoT
   devices could use DNS over DTLS [RFC8094].  In contrast to DNS over
   DTLS, DoC utilizes CoAP features to mitigate drawbacks of datagram-
   based communication.  These features include: block-wise transfer,
   which solves the Path MTU problem of DNS over DTLS (see [RFC8094],
   section 5); CoAP proxies, which provide an additional level of
   caching; re-use of data structures for application traffic and DNS
   information, which saves memory on constrained devices.

   To prevent resource requirements of DTLS or TLS on top of UDP (e.g.,
   introduced by DNS over QUIC [RFC9250]), DoC allows for lightweight
   end-to-end payload encryption based on OSCORE.

                   - FETCH coaps://[2001:db8::1]/
                  /
                 /
                CoAP request
   +--------+   [DNS query]   +--------+   DNS query    +--------+
   |  DoC   |---------------->|  DoC   |...............>|  DNS   |
   | Client |<----------------| Server |<...............| Server |
   +--------+  CoAP response  +--------+  DNS response  +--------+
               [DNS response]

                      Figure 1: Basic DoC architecture

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   The most important components of DoC can be seen in Figure 1: A DoC
   client tries to resolve DNS information by sending DNS queries
   carried within CoAP requests to a DoC server.  That DoC server may or
   may not resolve that DNS information itself by using other DNS
   transports with an upstream DNS server.  The DoC server then replies
   to the DNS queries with DNS responses carried within CoAP responses.

2.  Terminology

   A server that provides the service specified in this document is
   called a "DoC server" to differentiate it from a classic "DNS
   server".  Correspondingly, a client using this protocol to retrieve
   the DNS information is called a "DoC client".

   The term "constrained nodes" is used as defined in [RFC7228].

   The terms "CoAP payload" and "CoAP body" are used as defined in
   [RFC7959].

   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.

3.  Selection of a DoC Server

   In this document, it is assumed that the DoC client knows the DoC
   server and the DNS resource at the DoC server.  Possible options
   could be manual configuration of a URI [RFC3986] or CRI
   [I-D.ietf-core-href], or automatic configuration, e.g., using a CoRE
   resource directory [RFC9176], DHCP or Router Advertisement options
   [I-D.ietf-add-dnr].  Automatic configuration SHOULD only be done from
   a trusted source.

   When discovering the DNS resource through a link mechanism that
   allows describing a resource type (e.g., the Resource Type Attribute
   in [RFC6690]), the resource type "core.dns" can be used to identify a
   generic DNS resolver that is available to the client.

4.  Basic Message Exchange

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4.1.  The "application/dns-message" Content-Format

   This document defines the Internet media type "application/dns-
   message" for the CoAP Content-Format.  This media type is defined as
   in [RFC8484] Section 6, i.e., a single DNS message encoded in the DNS
   on-the-wire format [RFC1035].  Both DoC client and DoC server MUST be
   able to parse contents in the "application/dns-message" format.

4.2.  DNS Queries in CoAP Requests

   A DoC client encodes a single DNS query in one or more CoAP request
   messages the CoAP FETCH [RFC8132] method.  Requests SHOULD include an
   Accept option to indicate the type of content that can be parsed in
   the response.

   The CoAP request SHOULD be carried in a Confirmable (CON) message, if
   the transport used does not provide reliable message exchange.

4.2.1.  Request Format

   When sending a CoAP request, a DoC client MUST include the DNS query
   in the body of the CoAP request.  As specified in [RFC8132]
   Section 2.3.1, the type of content of the body MUST be indicated
   using the Content-Format option.  This document specifies the usage
   of Content-Format "application/dns-message" (details see
   Section 4.1).  A DoC server MUST be able to parse requests of
   Content-Format "application/dns-message".

4.2.2.  Support of CoAP Caching

   The DoC client SHOULD set the ID field of the DNS header always to 0
   to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to
   the same DNS queries with a cache entry.  This ensures that the CoAP
   Cache-Key (see [RFC8132] Section 2) does not change when multiple DNS
   queries for the same DNS data, carried in CoAP requests, are issued.

4.2.3.  Examples

   The following example illustrates the usage of a CoAP message to
   resolve "example.org.  IN AAAA" based on the URI
   "coaps://[2001:db8::1]/".  The CoAP body is encoded in "application/
   dns-message" Content Format.

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   FETCH coaps://[2001:db8::1]/
   Content-Format: application/dns-message
   Accept: application/dns-message
   Payload: 00 00 01 20 00 02 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            01 00 01                                        [binary]

4.3.  DNS Responses in CoAP Responses

   Each DNS query-response pair is mapped to a CoAP REST request-
   response operation.  DNS responses are provided in the body of the
   CoAP response.  A DoC server MUST be able to produce responses in the
   "application/dns-message" Content-Format (details see Section 4.1)
   when requested.  A DoC client MUST understand responses in
   "application/dns-message" format when it does not send an Accept
   option.  Any other response format than "application/dns-message"
   MUST be indicated with the Content-Format option by the DoC server.

4.3.1.  Response Codes and Handling DNS and CoAP errors

   A DNS response indicates either success or failure in the Response
   code of the DNS header (see [RFC1035] Section 4.1.1).  It is
   RECOMMENDED that CoAP responses that carry any valid DNS response use
   a "2.05 Content" response code.

   CoAP responses use non-successful response codes MUST NOT contain a
   DNS response and MUST only be used on errors in the CoAP layer or
   when a request does not fulfill the requirements of the DoC protocol.

   Communication errors with a DNS server (e.g., timeouts) SHOULD be
   indicated by including a SERVFAIL DNS response in a successful CoAP
   response.

   A DoC client might try to repeat a non-successful exchange unless
   otherwise prohibited.  The DoC client might also decide to repeat a
   non-successful exchange with a different URI, for instance, when the
   response indicates an unsupported Content-Format.

4.3.2.  Support of CoAP Caching

   The DoC server MUST ensure that any sum of the Max-Age value of a
   CoAP response and any TTL in the DNS response is less or equal to the
   corresponding TTL received from an upstream DNS server.  This also
   includes the default Max-Age value of 60 seconds (see [RFC7252],
   section 5.10.5) when no Max-Age option is provided.  The DoC client
   MUST then add the Max-Age value of the carrying CoAP response to all
   TTLs in a DNS response on reception and use these calculated TTLs for
   the associated records.

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   The RECOMMENDED algorithm to assure the requirement for the DoC is to
   set the Max-Age option of a response to the minimum TTL of a DNS
   response and to subtract this value from all TTLs of that DNS
   response.  This prevents expired records unintentionally being served
   from an intermediate CoAP cache.  Additionally, it allows for the
   ETag value for cache validation, if it is based on the content of the
   response, not to change even if the TTL values are updated by an
   upstream DNS cache.  If only one record set per DNS response is
   assumed, a simplification of this algorithm is to just set all TTLs
   in the response to 0 and set the TTLs at the DoC client to the value
   of the Max-Age option.

4.3.3.  Examples

   The following examples illustrate the replies to the query
   "example.org.  IN AAAA record", recursion turned on.  Successful
   responses carry one answer record including address
   2001:db8:1::1:2:3:4 and TTL 58719.

   A successful response:

   2.05 Content
   Content-Format: application/dns-message
   Max-Age: 58719
   Payload: 00 00 81 a0 00 01 00 01 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            1c 00 01 00 01 37 49 00 10 20 01 0d b8 00 01 00 [binary]
            00 00 01 00 02 00 03 00 04                      [binary]

   When a DNS error (SERVFAIL in this case) is noted in the DNS
   response, the CoAP response still indicates success:

   2.05 Content
   Content-Format: application/dns-message
   Payload: 00 00 81 a2 00 01 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01          [binary]

   When an error occurs on the CoAP layer, the DoC server SHOULD respond
   with an appropriate CoAP error, for instance "4.15 Unsupported
   Content-Format" if the Content-Format option in the request was not
   set to "application/dns-message" and the Content-Format is not
   otherwise supported by the server.

5.  CoAP/CoRE Integration

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5.1.  DoC Server Considerations

   In the case of CNAME records in a DNS response, a DoC server SHOULD
   follow common DNS resolver behavior [RFC1034] by resolving a CNAME
   until the originally requested resource record type is reached.  This
   reduces the number of message exchanges within an LLN.

   The DoC server SHOULD send compact answers, i.e., additional or
   authority sections in the DNS response should only be sent if needed
   or if it is anticipated that they help the DoC client to reduce
   additional queries.

5.2.  Observing the DNS Resource

   There are use cases where updating a DNS record might be necessary on
   the fly.  Examples of this include e.g.  [RFC8490], Section 4.1.2,
   but just saving messages by omitting the query for a subscribed name
   might also be valid.  As such, the DNS resource MAY be observable as
   specified in [RFC7641].

5.3.  OSCORE

   It is RECOMMENDED to carry DNS messages end-to-end encrypted using
   OSCORE [RFC8611].  The exchange of the security context is out of
   scope of this document.

6.  Considerations for Unencrypted Use

   While not recommended, DoC can be used without any encryption (e.g.,
   in very constrained environments where encryption is not possible or
   necessary).  It can also be used when lower layers provide secure
   communication between client and server.  In both cases, potential
   benefits of unencrypted DoC usage over classic DNS are e.g. block-
   wise transfer or alternative CoAP Content-Formats to overcome link-
   layer constraints.

7.  Security Considerations

   TODO Security

8.  IANA Considerations

8.1.  New "application/dns-message" Content-Format

   IANA is requested to assign CoAP Content-Format ID for the DNS
   message media type in the "CoAP Content-Formats" sub-registry, within
   the "CoRE Parameters" registry [RFC7252], corresponding the
   "application/dns-message" media type from the "Media Types" registry:

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   Media-Type: application/dns-message

   Encoding: -

   Id: TBD

   Reference: [TBD-this-spec]

8.2.  New "core.dns" Resource Type

   IANA is requested to assign a new Resource Type (rt=) Link Target
   Attribute, "core.dns" in the "Resource Type (rt=) Link Target
   Attribute Values" sub-registry, within the "CoRE Parameters" register
   [RFC6690].

   Attribute Value: core.dns

   Description: DNS over CoAP resource.

   Reference: [TBD-this-spec] Section 3

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/rfc/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/rfc/rfc2119>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7641>.

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   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/rfc/rfc7959>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/rfc/rfc8132>.

   [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/rfc/rfc8174>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/rfc/rfc8613>.

9.2.  Informative References

   [I-D.ietf-add-dnr]
              Boucadair, M., Reddy, 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-11, 11 July
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              add-dnr-11>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-10, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-10>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/rfc/rfc1034>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/rfc/rfc6690>.

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   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/rfc/rfc7228>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/rfc/rfc8094>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

   [RFC8490]  Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              RFC 8490, DOI 10.17487/RFC8490, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8490>.

   [RFC8611]  Akiya, N., Swallow, G., Litkowski, S., Decraene, B.,
              Drake, J., and M. Chen, "Label Switched Path (LSP) Ping
              and Traceroute Multipath Support for Link Aggregation
              Group (LAG) Interfaces", RFC 8611, DOI 10.17487/RFC8611,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8611>.

   [RFC9176]  Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/rfc/rfc9176>.

   [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/rfc/rfc9250>.

Appendix A.  Change Log

A.1.  Since draft-lenders-dns-over-coap-03
      (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
      coap-03)

   *  Remove TBD on CoRE resource directory in Section 1.  Optional
      usage of CoRE-RD is already discussed in Section 3

   *  Remove TBD on "request text duplication" in Appendix A.  This
      issue will be handled in a separate draft on a new CBOR-based
      content format

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   *  Add note on OSCORE in Section 5.3

   *  Add note on Observe in Section 5.2

   *  Change title from "DNS queries over CoAP" to "DNS over CoAP"

   *  Mention "application/dns-message" parsing requirement in
      Section 4.1

   *  Remove obvious CoAP behavior restatements from Section 4

   *  Remove ETag specifications in Section 4.3.2

   *  Caching: specify Max-Age / TTL calculation including simplified
      version in Section 4.3.2

   *  Remove subsection on "Proxies and caching".  All relevant things
      on caching were discussed in Section 4.2.2 and Section 4.3.2,
      considerations on proxy / DoC server behavior and late responses
      are general CoAP problems.

   *  Be more precise when Confirmable (CON) messages SHOULD be used in
      Section 4.2

A.2.  Since draft-lenders-dns-over-coap-02
      (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
      coap-02)

   *  Clarify server selection to be out-of-band and define "core.dns"
      resource type in Section 3 and Section 8.2

   *  Add message manipulation considerations for DoC servers in
      Section 5.1

   *  Update Considerations for Unencrypted Use in Section 6

A.3.  Since draft-lenders-dns-over-coap-01
      (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
      coap-01)

   *  Remove GET and POST methods

   *  Add note on ETag and response codes

   *  Provide requirement conflict for DNS over QUIC

   *  Clarify Content-Format / Accept handling

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A.4.  Since draft-lenders-dns-over-coap-00
      (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
      coap-00)

   *  Soften Content-Format requirements in Section 4.2.1 and
      Section 4.3

   *  Clarify "CoAP payload"/"CoAP body" terminology

   *  Fix nits and typos

A.5.  Since draft-lenders-dns-over-coaps-00
      (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
      coaps-00)

   *  Clarification in abstract that both DTLS and OSCORE can be used as
      secure transport

   *  Restructuring of Section 4:

      -  Add dedicated Section 4.1 on Content-Format

      -  Add overview table about usable and required features for
         request method types to Section 4.2

      -  Add dedicated Section 4.2.2 and Section 4.3.2 on caching
         requirements for CoAP requests and responses

   *  Fix nits and typos

Acknowledgments

   TODO acknowledge.

Authors' Addresses

   Martine Sophie Lenders
   Freie Universität Berlin
   Email: m.lenders@fu-berlin.de

   Christian Amsüss
   Email: christian@amsuess.com

   Cenk Gündoğan
   HAW Hamburg
   Email: cenk.guendogan@haw-hamburg.de

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   Thomas C. Schmidt
   HAW Hamburg
   Email: t.schmidt@haw-hamburg.de

   Matthias Wählisch
   Freie Universität Berlin
   Email: m.waehlisch@fu-berlin.de

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