DNS over CoAP (DoC)
draft-ietf-core-dns-over-coap-13

CoRE                                                       M. S. Lenders
Internet-Draft                                                TU Dresden
Intended status: Standards Track                               C. Amsüss
Expires: 4 September 2025                                               
                                                             C. Gündoğan
                                                        NeuralAgent GmbH
                                                           T. C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                        TU Dresden & Barkhausen Institut
                                                            3 March 2025

                          DNS over CoAP (DoC)
                    draft-ietf-core-dns-over-coap-13

Abstract

   This document defines a protocol for exchanging DNS messages over the
   Constrained Application Protocol (CoAP).  These CoAP messages can be
   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).

About This Document

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

   The latest revision of this draft can be found at https://core-
   wg.github.io/draft-dns-over-coap/draft-ietf-core-dns-over-coap.html.
   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-core-dns-over-coap/.

   Discussion of this document takes place on the CoRE Working Group
   mailing list (mailto:core@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/core/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/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.

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   This Internet-Draft will expire on 4 September 2025.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Selection of a DoC Server . . . . . . . . . . . . . . . . . .   5
     3.1.  Discovery by Resource Type  . . . . . . . . . . . . . . .   6
     3.2.  Discovery using SVCB Resource Records or DNR  . . . . . .   6
   4.  Basic Message Exchange  . . . . . . . . . . . . . . . . . . .   7
     4.1.  The "application/dns-message" Content-Format  . . . . . .   7
     4.2.  DNS Queries in CoAP Requests  . . . . . . . . . . . . . .   8
       4.2.1.  Request Format  . . . . . . . . . . . . . . . . . . .   8
       4.2.2.  Support of CoAP Caching . . . . . . . . . . . . . . .   8
       4.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  DNS Responses in CoAP Responses . . . . . . . . . . . . .   9
       4.3.1.  Response Codes and Handling DNS and CoAP errors . . .   9
       4.3.2.  Support of CoAP Caching . . . . . . . . . . . . . . .  10
       4.3.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .  11
   5.  CoAP/CoRE Integration . . . . . . . . . . . . . . . . . . . .  12
     5.1.  DNS Push  . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Mapping DoC to DoH  . . . . . . . . . . . . . . . . . . .  13
   6.  Considerations for Unprotected Use  . . . . . . . . . . . . .  13

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   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  13
     7.1.  DoC Client  . . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  DoC Server  . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  16
     9.2.  DNS Service Bindings (SVCB) Registry  . . . . . . . . . .  16
     9.3.  Resource Type (rt=) Link Target Attribute Values
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Evaluation . . . . . . . . . . . . . . . . . . . . .  21
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  21
     B.1.  Since [draft-ietf-core-dns-over-coap-12]  . . . . . . . .  21
     B.2.  Since draft-ietf-core-dns-over-coap-10  . . . . . . . . .  22
     B.3.  Since draft-ietf-core-dns-over-coap-09  . . . . . . . . .  22
     B.4.  Since draft-ietf-core-dns-over-coap-08  . . . . . . . . .  23
     B.5.  Since draft-ietf-core-dns-over-coap-07  . . . . . . . . .  23
     B.6.  Since draft-ietf-core-dns-over-coap-06  . . . . . . . . .  23
     B.7.  Since draft-ietf-core-dns-over-coap-05  . . . . . . . . .  23
     B.8.  Since draft-ietf-core-dns-over-coap-04  . . . . . . . . .  23
     B.9.  Since draft-ietf-core-dns-over-coap-03  . . . . . . . . .  24
     B.10. Since draft-ietf-core-dns-over-coap-02  . . . . . . . . .  24
     B.11. Since draft-ietf-core-dns-over-coap-01  . . . . . . . . .  24
     B.12. Since draft-ietf-core-dns-over-coap-00  . . . . . . . . .  25
     B.13. Since draft-lenders-dns-over-coap-04  . . . . . . . . . .  25
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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 can be
   secured by DTLS [RFC6347] [RFC9147] 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.  Constrained IoT devices may be
   restricted in memory, power consumption, link layer frame sizes,
   throughput, and latency.  They may only have a handful kilobytes of
   both RAM and ROM.  They may sleep for long durations of time, after
   which they need to refresh the named resources they know about.  Name

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   resolution in such scenarios must take into account link layer frame
   sizes of only a few hundred bytes, bit rates in the magnitude of
   kilobits per second, and latencies of several seconds [RFC7228]
   [I-D.ietf-iotops-7228bis].

   In order not to be burdened by the resource requirements of TCP and
   HTTPS, constrained IoT devices could use DNS over DTLS [RFC8094].  In
   contrast to DNS over DTLS, DoC can take advantage of CoAP features to
   mitigate drawbacks of datagram-based communication.  These features
   include: block-wise transfer [RFC7959], 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 avoid resource requirements of DTLS or TLS on top of UDP (e.g.,
   introduced by DNS over DTLS [RFC8094] or DNS over QUIC [RFC9250]),
   DoC allows for lightweight payload encryption based on OSCORE.

                 . FETCH coaps://[2001:db8::1]/
                /
               /
              CoAP request
   +------+   [DNS query]   +------+   DNS query     .---------------.
   | DoC  |---------------->| DoC  |--- --- --- --->|      DNS        |
   |Client|<----------------|Server|<--- --- --- ---| Infrastructure  |
   +------+  CoAP response  +------+  DNS response   '---------------'
             [DNS response]
      \                        /\                                 /
       '-----DNS over CoAP----'  '--DNS over UDP/HTTPS/QUIC/...--'

                      Figure 1: Basic DoC architecture

   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 is a
   DNS client (i.e., a stub or recursive resolver) that resolves DNS
   information by using other DNS transports such as DNS over UDP
   [RFC1035], DNS over HTTPS [RFC8484], or DNS over QUIC [RFC9250] when
   communicating with the upstream DNS infrastructure.  Using that
   information, the DoC server then replies to the queries of the DoC
   client with DNS responses carried within CoAP responses.

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   Note that this specification is distinct from DoH, since the CoRE-
   specific FETCH method [RFC8132] is used.  This was done to take
   benefit from having the DNS query in the body as with POST, but still
   having the caching advantages we would gain with GET.  Having the DNS
   query in the body means we do not need extra base64 encoding, which
   would increase code complexity and message sizes.  We are also able
   to transfer a query block-wise.

2.  Terminology

   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.

   A server that provides the service specified in this document is
   called a "DoC server" to differentiate it from a classic "DNS
   server".  A DoC server acts either as a DNS stub resolver [RFC8499]
   or a DNS recursive resolver [RFC8499].  As such, the DoC server
   communicates with an "upstream DNS infrastructure" or a single
   "upstream DNS server".

   A client using the service specified in this document to retrieve the
   DNS information is called a "DoC client".

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

   The terms "payload" and "body" are used as defined in [RFC7959],
   Section 2.  Note, that without block-wise transfer the terms
   "payload" and "body" are to be understood as equal.

3.  Selection of a DoC Server

   While there is no path specified for the DoC resource, it is
   RECOMMENDED to use the root path "/" to keep the CoAP requests small.

   The DoC client needs to know the DoC server and the DoC resource at
   the DoC server.  Possible options to assure this 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 [RFC9463], or
   discovery of designated resolvers [RFC9462].  Automatic configuration
   SHOULD only be done from a trusted source.

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3.1.  Discovery by Resource Type

   For discovery of a the DoC resource through a link mechanism that
   allows describing a resource type (e.g., the Resource Type Attribute
   in [RFC6690]), this document introduces the resource type "core.dns".
   It can be used to identify a generic DNS resolver that is available
   to the client.

3.2.  Discovery using SVCB Resource Records or DNR

   A DoC server can also be discovered using SVCB Resource Records (RR)
   [RFC9460] [RFC9461] or DNR Service Parameters [RFC9463].
   [I-D.ietf-core-coap-dtls-alpn] provides solutions to discover CoAP
   over (D)TLS servers using the "alpn" SvcParam.
   [I-D.lenders-core-dnr] provides a problem statement for service
   bindings discovery for OSCORE and EDHOC.

   This document specifies "docpath" as a single-valued SvcParamKey
   whose value MUST be a CBOR sequence of 0 or more text strings (see
   [RFC8742] and [RFC8949]), delimited by the length of the
   SvcParamValue field (in octets).  If the SvcParamValue ends within a
   CBOR text string, the SVCB RR MUST be considered as malformed.  As a
   text format, e.g., in DNS zone files, the CBOR diagnostic notation
   (see Section 8 of [RFC8949] and [I-D.ietf-cbor-edn-literals]) of that
   CBOR sequence can be used.

   Note that this specifically does not surround the text string
   sequence with a CBOR array or a similar CBOR data item.  This path
   format was chosen to coincide with the path representation in CRIs
   [I-D.ietf-core-href].  Furthermore, it is easily transferable into a
   sequence of CoAP Uri-Path options by mapping the initial byte of any
   present CBOR text string (see [RFC8949], Section 3) into the Option
   Delta and Option Length of the CoAP option, provided these CBOR text
   strings are all of a length between 0 and 12 octets (see [RFC7252],
   Section 3.1).  Likewise, it can be transferred into a URI path-
   abempty form (see [RFC3986], Section 3.3) by replacing the initial
   byte of any present CBOR text string with the "/" character, provided
   these CBOR text strings are all of a length less than 24 octets and
   do not contain bytes that need escaping.

   To use the service binding from an SVCB RR, the DoC client MUST send
   a DoC request constructed from the SvcParams including "docpath".  A
   rough construction algorithm could be as follows, going through the
   provided records in order of their priority.

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   *  If the "alpn" SvcParam value for the service is "coap", construct
      a CoAP request for CoAP over TLS.  If it is "co", construct a CoAP
      request for CoAP over DTLS.  Any other SvcParamKeys specifying a
      CoAP transport are out of the scope of this document.

   *  The destination address for the request should be taken from
      additional information about the target, e.g., from an AAAA record
      associated with the target name or from an "ipv6hint" SvcParam
      value, or, as a fallback, by querying an address for the target
      name of the SVCB record.

   *  The destination port for the address is taken from the "port"
      SvcParam value, if present.  Otherwise, take the default port of
      the CoAP transport.

   *  Set the target name of SVCB record in the Uri-Host option.

   *  For each element in the CBOR sequence of the "docpath" SvcParam
      value, add a Uri-Path option to the request.

   *  If a "port" SvcParam value is provided or if a port was queried,
      and if either differs from the default port of the transport or
      the destination port selected above, set that port in the Uri-Port
      option.

   *  If the request constructed this way receives a response, use the
      same SVCB record for construction of future DoC queries.  If not,
      or if the endpoint becomes unreachable, repeat with the SVCB
      record with the next highest priority.

   A more generalized construction algorithm can be found in
   [I-D.ietf-core-transport-indication].

4.  Basic Message Exchange

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

   This document defines a CoAP Content-Format identifier for the
   Internet media type "application/dns-message" to be the mnemonic 553
   — based on the port assignment of DNS.  This media type is defined as
   in Section 6 of [RFC8484], 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.  For the purposes of this document, only OPCODE 0 (Query) is
   supported for DNS messages.  Future work might provide specifications
   and considerations for other values of OPCODE.  Unless another error
   takes precedence, a DoC server uses RCODE = 4, NotImp [RFC1035], in
   its response when it receives a query with an OPCODE that it does not

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   implement (see also Section 4.3.3).

4.2.  DNS Queries in CoAP Requests

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

   Since CoAP provides reliability at the message layer (e.g., through
   Confirmable messages) the retransmission mechanism of the DNS
   protocol as defined in [RFC1035] is not needed.

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 Section 2.3.1 of
   [RFC8132], 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" (for 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.  For better readability, we provide the
   payload in a human-readable format.  In the actual message, however,
   it would be encoded in the binary message format (bytes printed in
   hexadecimal representation) defined in [RFC1035].

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

   Payload (human-readable):
     ;; ->>Header<<- opcode: QUERY, status: NOERROR, id: 0
     ;; flags: rd ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ARCOUNT: 0

     ;; QUESTION SECTION:
     ;example.org.             IN      AAAA

4.3.  DNS Responses in CoAP Responses

   Each DNS query-response pair is mapped to a CoAP 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 (for details, see
   Section 4.1) when requested.  A DoC client MUST be able to 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 RCODE of
   the DNS header (see Section 4.1.1 of [RFC1035]).  It is RECOMMENDED
   that CoAP responses that carry a parseable DNS response use a 2.05
   (Content) response code.

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

   Communication errors with an upstream DNS server (e.g., timeouts)
   MUST be indicated by including a DNS response with the appropriate
   RCODE in a successful CoAP response, i.e., using a 2.xx response
   code.

   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.

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4.3.2.  Support of CoAP Caching

   For reliability and energy saving measures, content decoupling, i.e.,
   en-route caching on proxies, takes a far greater role than it does,
   e.g., in HTTP.  Likewise, CoAP makes it possible to use cache
   validation to refresh stale cache entries to reduce the amount of
   large response messages.  For cache validation, CoAP implementations
   regularly use hashing over the message content for ETag generation.
   As such, the approach to guarantee the same cache key for DNS
   responses as proposed in DoH ([RFC8484], Section 5.1) is not
   sufficient and needs to be updated so that the TTLs in the response
   are more often the same regardless of query time.

   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 Section 5.10.5
   of [RFC7252]) 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.

   The RECOMMENDED algorithm for a DoC server to meet the requirement
   for DoC is as follows: Set the Max-Age option of a response to the
   minimum TTL of a DNS response and subtract this value from all TTLs
   of that DNS response.  This prevents expired records unintentionally
   being served from an intermediate CoAP cache.  Additionally, if the
   ETag for cache validation is based on the content of the response, it
   allows that ETag not to change.  This then remains the case 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.

   If shorter caching periods are plausible, e.g., if the RCODE of the
   message indicates an error that should only be cached for a minimal
   duration, the value for the Max-Age option SHOULD be set accordingly.
   This value might be 0, but if the DoC server knows that the error
   will persist, greater values are also conceivable, depending on the
   projected duration of the error.  Same goes for DNS responses that
   for any reason do not carry any records with a TTL.

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4.3.3.  Examples

   The following example illustrates the response 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 79689.  As in Section 4.2.3, we use a
   human-readable format for the payload, but skip the binary conversion
   this time.

   A successful response:

   2.05 Content
   Content-Format: application/dns-message
   Max-Age: 58719
   Payload (human-readable):
     ;; ->>Header<<- opcode: QUERY, status: NOERROR, id: 0
     ;; flags: qr rd ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ARCOUNT: 0

     ;; QUESTION SECTION:
     ;example.org.                 IN      AAAA
     ;; ANSWER SECTION:
     ;example.org.         79689   IN      AAAA    2001:db8:1::1:2:3:4

   When a DNS error—NxDomain (RCODE = 3) for "does.not.exist" in this
   case—is noted in the DNS response, the CoAP response still indicates
   success.

  2.05 Content
  Content-Format: application/dns-message
  Payload (human-readable):
    ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 0
    ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

    ;; QUESTION SECTION:
    ;does.not.exist.              IN      AAAA

   As described in Section 4.1, a DoC server uses NotImp (RCODE = 4) if
   it does not support an OPCODE—a DNS Update (OPCODE = 5) for
   "example.org" in this case.

   2.05 Content
   Content-Format: application/dns-message
   Payload (human-readable):
     ;; ->>Header<<- opcode: UPDATE, status: NOTIMP, id: 0
     ;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ARCOUNT: 0

     ;; QUERY SECTION:
     ;example.org.                 IN      AAAA

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   When an error occurs at 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.

   4.15 Unsupported Content-Format
   [no payload]

5.  CoAP/CoRE Integration

5.1.  DNS Push

   DNS Push requires additional overhead, which conflicts with
   constrained resources.  This is the reason why it is RECOMMENDED to
   use CoAP Observe [RFC7641] instead of DNS Push in the DoC domain.
   The DoC server SHOULD provide Observe capabilities on its DoC
   resource and do as follows.

   If the CoAP request indicates that the DoC client wants to observe a
   resource record, a DoC server MAY use a DNS Subscribe message
   [RFC8765] instead of a classic DNS query to fetch the information on
   behalf of a DoC client.  If this is not supported by the DoC server,
   it MUST act as if the resource were not observable.

   Whenever the DoC server receives a DNS Push message [RFC8765] from
   the DNS infrastructure for an observed resource record, the DoC
   server sends an appropriate Observe response to the DoC client.

   If no more DoC clients observe a resource record for which the DoC
   server has an open subscription, the DoC server MUST use a DNS
   Unsubscribe message [RFC8765] to close its subscription to the
   resource record as well.

   A DoC server can still provide Observe capabilities to its DoC
   resource without providing this proxying to DNS Push, e.g., if it
   receives new information on a record through other means.

5.2.  OSCORE

   It is RECOMMENDED to carry DNS messages protected using OSCORE
   [RFC8613] between the DoC client and the DoC server.  The
   establishment and maintenance of the OSCORE Security Context is out
   of the scope of this document.

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   If cache retrieval of OSCORE responses is desired, it can be
   achieved, for instance, by using the method defined in
   [I-D.amsuess-core-cachable-oscore].  This has, however, implications
   on message sizes and security properties, which are compiled in that
   document.

5.3.  Mapping DoC to DoH

   This document provides no specification on how to map between DoC and
   DoH, e.g., at a CoAP-to-HTTP-proxy.  In fact, such a direct mapping
   is NOT RECOMMENDED: rewriting the FETCH method (Section 4.2) and the
   TTL rewriting (Section 4.3.2) as specified in this draft would be
   non-trivial.  It is RECOMMENDED to use a DNS forwarder to map between
   DoC and DoH, as would be the case for mapping between any other pair
   of DNS transports.

6.  Considerations for Unprotected Use

   The use of DoC without confidentiality and integrity protection is
   NOT RECOMMENDED.  Without confidential communication, many possible
   attacks need to be evaluated in the context of the application's
   threat model.  This includes known threats for unprotected DNS
   [RFC3833] [RFC7626] and CoAP Section 11 of [RFC7252].  But there is
   also an attack that is mitigated even by unprotected DNS over UDP:
   The random ID of the DNS header affords some protection against off-
   path cache poisoning attacks.  Note, however, that this particular
   threat can also be mitigated by using random large token values in
   the CoAP request.

7.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

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   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   // RFC Ed.: Please remove this section before publication.  When
   // deleting this section, please also remove RFC7942 from the
   // references of this document.

7.1.  DoC Client

   The authors of this document provide a DoC client implementation
   available in the IoT operating system RIOT (https://doc.riot-os.org/
   group__net__gcoap__dns.html).

   Level of maturity:  production

   Version compatibility:  draft-ietf-core-dns-over-coap-13

   License:  LGPL-2.1

   Contact information:  Martine S.  Lenders <martine.lenders@tu-
      dresden.de>

   Last update of this information:  September 2024

7.2.  DoC Server

   The authors of this document provide a DoC server implementation in
   Python (https://github.com/anr-bmbf-pivot/aiodnsprox).

   Level of maturity:  production

   Version compatibility:  draft-ietf-core-dns-over-coap-13

   License:  MIT

   Contact information:  Martine S.  Lenders <martine.lenders@tu-
      dresden.de>

   Last update of this information:  September 2024

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8.  Security Considerations

   General CoAP security considerations in Section 11 of [RFC7252] apply
   to DoC.  Additionally, DoC uses request patterns that require the
   maintenance of long-lived security contexts.  Section 2.6 of
   [I-D.ietf-core-corr-clar] provides insights on what can be done when
   those are resumed from a new endpoint.

   When using unprotected CoAP (see Section 6), setting the ID of a DNS
   message to 0 as specified in Section 4.2.2 opens the DNS cache of a
   DoC client to cache poisoning attacks via response spoofing.  This
   document requires an unpredictable CoAP token in each DoC query from
   the client when CoAP is not secured to mitigate such an attack over
   DoC (see Section 6).

   For secure communication via DTLS or OSCORE the impact of a fixed ID
   on security is limited, as both harden against injecting spoofed
   responses.  Consequently, the ID of the DNS message can be set to 0
   without any concern in order to leverage the advantages of CoAP
   caching.

   A DoC client must be aware that the DoC server may communicate
   unprotected with the upstream DNS infrastructure, e.g., using DNS
   over UDP.  DoC can only guarantee confidentiality and integrity of
   communication between parties for which the security context is
   exchanged.  The DoC server may use another security context to
   communicate upstream with both confidentiality and integrity (e.g.,
   DNS over QUIC [RFC9250]) or with just integrity (e.g., DNSSEC
   [RFC9364]), but, while recommended, this is opaque to the DoC client
   on the protocol level.

   A DoC client may not be able to perform DNSSEC validation, e.g., due
   to code size constraints, or due to size of the responses.  It may
   trust its DoC server to perform DNSSEC validation; how that trust is
   expressed is out of the scope of this document.  A DoC client may be,
   for instance, configured to use a particular credential by which it
   recognizes an eligible DoC server.  That information can also imply
   trust in the DNSSEC validation by that server.

9.  IANA Considerations

   // RFC Ed.: throughout this section, please replace RFC-XXXX with the
   // RFC number of this specification and remove this note.

   This document has the following actions for IANA.

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9.1.  CoAP Content-Formats Registry

   IANA is requested to assign a CoAP Content-Format ID for the DNS
   message media type in the "CoAP Content-Formats" registry, within the
   "Constrained RESTful Environments (CoRE) Parameters" registry group
   [RFC7252], corresponding to the "application/dns-message" media type
   from the "Media Types" registry (see [RFC8484]).

   Content Type: application/dns-message

   Content Coding: -

   ID: 553 (suggested)

   Reference: [RFC8484][RFC-XXXX, Section 4.1]

9.2.  DNS Service Bindings (SVCB) Registry

   IANA is requested to add the following entry to the "Service
   Parameter Keys (SvcParamKeys)" registry within the "DNS Service
   Bindings (SVCB)" registry group.  The definition of this parameter
   can be found in Section 3.

    +=============+=========+===============+============+============+
    | Number      | Name    | Meaning       | Change     | Reference  |
    |             |         |               | Controller |            |
    +=============+=========+===============+============+============+
    | 10          | docpath | DNS over CoAP | IETF       | [RFC-XXXX, |
    | (suggested) |         | resource path |            | Section 3] |
    +-------------+---------+---------------+------------+------------+

                      Table 1: Values for SvcParamKeys

9.3.  Resource Type (rt=) Link Target Attribute Values Registry

   IANA is requested to add a new Resource Type (rt=) Link Target
   Attribute "core.dns" to the "Resource Type (rt=) Link Target
   Attribute Values" registry within the "Constrained RESTful
   Environments (CoRE) Parameters" registry group.

   Value: core.dns

   Description: DNS over CoAP resource.

   Reference: [RFC-XXXX, Section 3]

10.  References

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10.1.  Normative References

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
              Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
              literals-16, 8 January 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-literals-16>.

   [I-D.ietf-core-coap-dtls-alpn]
              Lenders, M. S., Amsüss, C., Schmidt, T. C., and M.
              Wählisch, "ALPN ID Specification for CoAP over DTLS", Work
              in Progress, Internet-Draft, draft-ietf-core-coap-dtls-
              alpn-02, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coap-dtls-alpn-02>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8742>.

   [RFC8765]  Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              RFC 8765, DOI 10.17487/RFC8765, June 2020,
              <https://www.rfc-editor.org/rfc/rfc8765>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8949>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9147>.

10.2.  Informative References

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   [DoC-paper]
              Lenders, M., Amsüss, C., Gündogan, C., Nawrocki, M.,
              Schmidt, T., and M. Wählisch, "Securing Name Resolution in
              the IoT: DNS over CoAP", Association for Computing
              Machinery (ACM), Proceedings of the ACM on Networking vol.
              1, no. CoNEXT2, pp. 1-25, DOI 10.1145/3609423, September
              2023, <https://doi.org/10.1145/3609423>.

   [I-D.amsuess-core-cachable-oscore]
              Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
              Progress, Internet-Draft, draft-amsuess-core-cachable-
              oscore-10, 8 January 2025,
              <https://datatracker.ietf.org/doc/html/draft-amsuess-core-
              cachable-oscore-10>.

   [I-D.ietf-core-corr-clar]
              Bormann, C., "Constrained Application Protocol (CoAP):
              Corrections and Clarifications", Work in Progress,
              Internet-Draft, draft-ietf-core-corr-clar-01, 18 December
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              core-corr-clar-01>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-18, 3 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-18>.

   [I-D.ietf-core-transport-indication]
              Amsüss, C. and M. S. Lenders, "CoAP Transport Indication",
              Work in Progress, Internet-Draft, draft-ietf-core-
              transport-indication-07, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              transport-indication-07>.

   [I-D.ietf-iotops-7228bis]
              Bormann, C., Ersue, M., Keränen, A., and C. Gomez,
              "Terminology for Constrained-Node Networks", Work in
              Progress, Internet-Draft, draft-ietf-iotops-7228bis-01, 8
              January 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-iotops-7228bis-01>.

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   [I-D.lenders-core-dnr]
              Lenders, M. S., Amsüss, C., Schmidt, T. C., and M.
              Wählisch, "Discovery of Network-designated OSCORE-based
              Resolvers: Problem Statement", Work in Progress, Internet-
              Draft, draft-lenders-core-dnr-05, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-lenders-core-
              dnr-05>.

   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
              Name System (DNS)", RFC 3833, DOI 10.17487/RFC3833, August
              2004, <https://www.rfc-editor.org/rfc/rfc3833>.

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

   [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>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,
              <https://www.rfc-editor.org/rfc/rfc7626>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/rfc/rfc7942>.

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 8499, DOI 10.17487/RFC8499, January
              2019, <https://www.rfc-editor.org/rfc/rfc8499>.

   [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>.

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   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,
              <https://www.rfc-editor.org/rfc/rfc9364>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

   [RFC9461]  Schwartz, B., "Service Binding Mapping for DNS Servers",
              RFC 9461, DOI 10.17487/RFC9461, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9461>.

   [RFC9462]  Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", RFC 9462,
              DOI 10.17487/RFC9462, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9462>.

   [RFC9463]  Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
              and T. Jensen, "DHCP and Router Advertisement Options for
              the Discovery of Network-designated Resolvers (DNR)",
              RFC 9463, DOI 10.17487/RFC9463, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9463>.

Appendix A.  Evaluation

   The authors of this document presented the design, implementation,
   and analysis of DoC in their paper "Securing Name Resolution in the
   IoT: DNS over CoAP" [DoC-paper].

Appendix B.  Change Log

   // RFC Ed.: Please remove this section before publication.

B.1.  Since [draft-ietf-core-dns-over-coap-12]

   *  Address Esko's review

   *  Address Marcos's review

   *  Address Mikolai's review

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B.2.  Since draft-ietf-core-dns-over-coap-10
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-10)

   *  Replace imprecise or wrong terms:

      -  disjunct => distinct

      -  unencrypted CoAP => unprotected CoAP

      -  security mode => confidential communication

   *  Pull in definition of CBOR sequences and their EDN

   *  Fix broken external section references

   *  Define terminology for "upstream DNS infrastructure" and "upstream
      DNS server"

   *  Fix wording on DNS error handling

   *  Clarify that any OpCode beyond 0 is not supported for now and
      remove now redundant DNS Upgrade section as a consequence

   *  Clarify that the DoC/DoH mapping is what is NOT RECOMMENDED

   *  Avoid use of undefined term “CoAP resource identifier”

   *  Discuss Max-Age option value in an error case

   *  Add human-readable format to examples

   *  General language check pass

B.3.  Since draft-ietf-core-dns-over-coap-09
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-09)

   *  Update SVCB SvcParamKey

   *  Update corr-clar reference

   *  Add reference to DNS Update [RFC2136]
      (https://datatracker.ietf.org/doc/html/rfc2136), clarify that it
      is currently not considered

   *  Add to security considerations: unprotected upstream DNS and
      DNSSEC

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B.4.  Since draft-ietf-core-dns-over-coap-08
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-08)

   *  Update Cenk's Affiliation

B.5.  Since draft-ietf-core-dns-over-coap-07
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-07)

   *  Address IANA early review #1368678

   *  Update normative reference to CoAP over DTLS alpn SvcParam

   *  Add missing DTLSv1.2 reference

   *  Security considerations: Point into corr-clar-future

   *  Implementation Status: Update to current version

B.6.  Since draft-ietf-core-dns-over-coap-06
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-06)

   *  Add "docpath" SVCB ParamKey definition

   *  IANA fixes

      -  Use new column names (see Errata 4954)

      -  Add reference to RFC 8484 for application/dns-message Media
         Type

      -  IANA: unify self references

B.7.  Since draft-ietf-core-dns-over-coap-05
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-05)

   *  Add references to relevant SVCB/DNR RFCs and drafts

B.8.  Since draft-ietf-core-dns-over-coap-04
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-04)

   *  Add note on cacheable OSCORE

   *  Address early IANA review

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B.9.  Since draft-ietf-core-dns-over-coap-03
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-03)

   *  Amended Introduction with short contextualization of constrained
      environments

   *  Add Appendix A on evaluation

B.10.  Since draft-ietf-core-dns-over-coap-02
       (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
       coap-02)

   *  Move implementation details to Implementation Status (in
      accordance with [RFC7942])

   *  Recommend root path to keep the CoAP options small

   *  Set Content-Format for application/dns-message to 553

   *  SVCB/DNR: Move to Server Selection Section but leave TBD based on
      DNSOP discussion for now

   *  Clarify that DoC and DoH are distinct

   *  Clarify mapping between DoC and DoH

   *  Update considerations on unprotected use

   *  Don't call OSCORE end-to-end encrypted

B.11.  Since draft-ietf-core-dns-over-coap-01
       (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
       coap-01)

   *  Specify DoC server role in terms of DNS terminology

   *  Clarify communication of DoC to DNS infrastructure is agnostic of
      the transport

   *  Add subsection on how to implement DNS Push in DoC

   *  Add appendix on reference implementation

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B.12.  Since draft-ietf-core-dns-over-coap-00
       (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
       coap-00)

   *  SVGify ASCII art

   *  Move section on "DoC Server Considerations" (was Section 5.1) to
      its own draft (draft-lenders-dns-cns
      (https://datatracker.ietf.org/doc/draft-lenders-dns-cns/))

   *  Replace layer violating statement for CON with statement of fact

   *  Add security considerations on ID=0

B.13.  Since draft-lenders-dns-over-coap-04
       (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
       coap-04)

   *  Removed change log of draft-lenders-dns-over-coap

Acknowledgments

   The authors of this document want to thank Carsten Bormann, Esko
   Dijk, Thomas Fossati, Mikolai Gütschow, Ben Schwartz, Marco Tiloca,
   and Tim Wicinski for their feedback and comments.

Authors' Addresses

   Martine Sophie Lenders
   TUD Dresden University of Technology
   Helmholtzstr. 10
   D-01069 Dresden
   Germany
   Email: martine.lenders@tu-dresden.de

   Christian Amsüss
   Email: christian@amsuess.com

   Cenk Gündoğan
   NeuralAgent GmbH
   Mies-van-der-Rohe-Straße 6
   D-80807 Munich
   Germany
   Email: cenk.gundogan@neuralagent.ai

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   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   D-20099 Hamburg
   Germany
   Email: t.schmidt@haw-hamburg.de

   Matthias Wählisch
   TUD Dresden University of Technology & Barkhausen Institut
   Helmholtzstr. 10
   D-01069 Dresden
   Germany
   Email: m.waehlisch@tu-dresden.de

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