



Internet Engineering Task Force (IETF)                     M. S. Lenders
Request for Comments: 9953                                    TU Dresden
Category: Standards Track                                      C. Amsüss
ISSN: 2070-1721                                                         
                                                             C. Gündoğan
                                                        NeuralAgent GmbH
                                                           T. C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                        TU Dresden & Barkhausen Institut
                                                              March 2026


                          DNS over CoAP (DoC)

Abstract

   This document defines a protocol for exchanging DNS queries (OPCODE
   0) over the Constrained Application Protocol (CoAP).  These CoAP
   messages can be protected by (D)TLS-Secured CoAP or Object Security
   for Constrained RESTful Environments (OSCORE) to provide encrypted
   DNS message exchange for constrained devices in the Internet of
   Things (IoT).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9953.

Copyright Notice

   Copyright (c) 2026 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
   2.  Terminology and Conventions
   3.  Selection of a DoC Server
     3.1.  Discovery by Resource Type
     3.2.  Discovery Using SVCB Resource Records or DNR
       3.2.1.  Examples
   4.  Basic Message Exchange
     4.1.  The "application/dns-message" Content-Format
     4.2.  DNS Queries in CoAP Requests
       4.2.1.  Request Format
       4.2.2.  Support of CoAP Caching
       4.2.3.  Example
     4.3.  DNS Responses in CoAP Responses
       4.3.1.  Response Codes and Handling DNS and CoAP Errors
       4.3.2.  Support of CoAP Caching
       4.3.3.  Examples
   5.  Interaction with Other CoAP and CoRE Features
     5.1.  DNS Push Notifications and CoAP Observe
     5.2.  OSCORE
     5.3.  Mapping DoC to DoH
   6.  Considerations for Unprotected Use
   7.  Security Considerations
   8.  IANA Considerations
     8.1.  CoAP Content-Formats Registry
     8.2.  DNS SVBC Service Parameter Keys (SvcParamKeys) Registry
     8.3.  Resource Type (rt=) Link Target Attribute Values Registry
   9.  Operational Considerations
     9.1.  Coexistence of Different DNS and CoAP Transports
     9.2.  Redirects
     9.3.  Proxy Hop Limit
     9.4.  Error Handling
     9.5.  DNS Extensions
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Evaluation
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document defines DNS over CoAP (DoC), a protocol to send DNS
   [STD13] queries and get DNS responses over the Constrained
   Application Protocol (CoAP) [RFC7252] using OPCODE 0 (Query).  Each
   DNS query-response pair is mapped into a CoAP message exchange.  Each
   CoAP message can be secured by any combination of DTLS 1.2 or newer
   [RFC6347] [RFC9147], TLS 1.3 or newer [RFC8323] [RFC8446], 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 (DoH)
   [RFC8484].  However, DoC aims for 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
   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]
   [RFC7228bis].

   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 (1) block-wise transfer [RFC7959], which solves the Path MTU
   problem of DNS over DTLS (see [RFC8094], Section 5), (2) CoAP
   proxies, which provide an additional level of caching, and (3) reuse
   of data structures for application traffic and DNS information, which
   saves memory on constrained devices.

   To avoid the 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 message protection based on
   OSCORE.

              . FETCH coaps://[2001:db8::1]/
             /
            /
          CoAP request
+------+  [DNS query]  +------+------+   DNS query     .--------------.
| DoC  |-------------->| DoC  | DNS  |--- --- --- --->|      DNS       |
|Client|<--------------|Server|Client|<--- --- --- ---| 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 can be
   the authoritative name server for the queried record or a DNS client
   (i.e., a stub or recursive resolver) that resolves DNS information by
   using other DNS transports such as DNS over UDP [STD13], 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.  A DoC server MAY also serve
   as a DNSSEC validator to provide DNSSEC validation to the more
   constrained DoC clients.

   Note that this specification is distinct from DoH because the CoAP-
   specific FETCH method [RFC8132] is used.  A benefit of using this
   method is having the DNS query in the body such as when using the
   POST method, but with the same caching advantages of responses to
   requests that use the GET method.  Having the DNS query in the body
   means that there is no need for extra base64 encoding, which would
   increase code complexity and message sizes.  Also, this allows for
   the block-wise transfer of queries [RFC7959].

2.  Terminology and Conventions

   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 as either a DNS stub resolver or a DNS
   recursive resolver [BCP219].  As such, the DoC server communicates
   with an "upstream DNS infrastructure" or a single "upstream DNS
   server".  A "DoC resource" is a CoAP resource [RFC7252] at the DoC
   server that DoC clients can target in order to send a DNS query in a
   CoAP request.

   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].
   [RFC6690] describes that Constrained RESTful Environments (CoRE)
   realize the Representational State Transfer (REST) architecture
   [REST] in a suitable form for such constrained nodes.

   A DoC server can provide Observe capabilities as defined in
   [RFC7641], Section 1.2.  As part of that, it administers a "list of
   observers".  DoC clients using these capabilities are "observers" as
   defined in [RFC7641], Section 1.2.  A "notification" is a CoAP
   response message with an Observe option; see [RFC7641], Section 4.2.

   The terms "payload" and "body" are used as defined in [RFC7959],
   Section 2.  Note that, when block-wise transfer is not used, the
   terms "payload" and "body" are to be understood as equal.

   In the examples in this document, the binary payload and resource
   records are shown in a hexadecimal representation as well as a human-
   readable format for better readability.  However, in the actual
   message sent and received, they are encoded in the binary message
   format defined in [STD13].

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 (1) manual
   configuration of a Uniform Resource Identifier (URI) [RFC3986] or
   Constrained Resource Identifier (CRI) [CRI] or (2) automatic
   configuration, e.g., using a CoRE resource directory [RFC9176], DHCP
   or Router Advertisement options [RFC9463], or discovery of designated
   resolvers [RFC9462].  Automatic configuration MUST only be done from
   a trusted source.

3.1.  Discovery by Resource Type

   For discovery of 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 Service Binding (SVCB)
   Resource Records (RRs) [RFC9460] [RFC9461] resolved via another DNS
   service (e.g., provided by an unencrypted local resolver) or
   Discovery of Network-designated Resolvers (DNR) Service Parameters
   [RFC9463] via DHCP or Router Advertisements.  [RFC8323] defines the
   Application-Layer Protocol Negotiation (ALPN) ID for CoAP over TLS
   servers and [RFC9952] defines the ALPN ID for CoAP over DTLS servers.
   DoC servers that use only OSCORE [RFC8613] and Ephemeral Diffie-
   Hellman Over COSE (EDHOC) [RFC9528] (COSE stands for "Concise Binary
   Object Notation (CBOR) Object Signing and Encryption" [RFC9052]) to
   support security cannot be discovered using these SVCB RR or DNR
   mechanisms.  Specifying an alternate discovery mechanism is out of
   the scope of this document.

   This document is not an SVCB mapping document for the CoAP schemes as
   defined in Section 2.4.3 of [RFC9460].  A full SVCB mapping is
   specified in [TRANSPORT-IND].  It generalizes mechanisms for all CoAP
   services.  This document introduces only the discovery of DoC
   services.

   This document specifies "docpath" as a single-valued Service
   Parameter Key (SvcParamKey) that is mandatory for DoC SVCB records.
   If the "docpath" SvcParamKey is absent, the service should not be
   considered a valid DoC service.

   The docpath is divided up into segments of the absolute path to the
   DoC resource (docpath-segment), each a sequence of 1-255 octets.  In
   ABNF [RFC5234]:

   docpath-segment = 1*255OCTET

   Note that this restricts the length of each docpath-segment to at
   most 255 octets.  Paths with longer segments cannot be advertised
   with the "docpath" SvcParam and are thus NOT RECOMMENDED for the path
   to the DoC resource.

   The presentation format value of "docpath" SHALL be a comma-separated
   list (Appendix A.1 of [RFC9460]) of 0 or more docpath-segments.  The
   root path "/" is represented by 0 docpath-segments, i.e., an empty
   list.  The same considerations apply for the "," and "\" characters
   in docpath-segments for zone-file implementations and the alpn-ids in
   an "alpn" SvcParam (Section 7.1.1 of [RFC9460]).

   The wire-format value for "docpath" consists of 0 or more sequences
   of octets prefixed by their respective length as a single octet.  We
   call this single octet the length octet.  The length octet and the
   corresponding sequence form a length-value pair.  These length-value
   pairs are concatenated to form the SvcParamValue.  These pairs MUST
   exactly fill the SvcParamValue; otherwise, the SvcParamValue is
   malformed.  Each such length-value pair represents one segment of the
   absolute path to the DoC resource.  The root path "/" is represented
   by 0 length-value pairs, i.e., SvcParamValue length 0.

   Note that this format uses the same encoding as the "alpn" SvcParam,
   and it can reuse the decoders and encoders for that SvcParam with the
   adaption that a length of zero is allowed.  As long as each docpath-
   segment has a length between 0 and 23 octets, inclusive, it is easily
   transferred into the path representation in CRIs [CRI] by masking
   each length octet with the CBOR text string major type 3 (0x60 as an
   octet; see [RFC8949]).  Furthermore, it is easily transferable into a
   sequence of CoAP Uri-Path options by mapping each length octet into
   the Option Delta and Option Length of the corresponding CoAP Uri-Path
   option, provided the docpath-segments 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 by replacing each length
   octet with the "/" character.  None of the abovementioned prevent
   longer docpath-segments than the considered ones; they just make the
   translation harder as space is required for the longer delimiters,
   which in turn require octets to be moved.

   To use the service binding from an SVCB RR or DNR Encrypted DNS
   option, the DoC client MUST send a DoC request constructed from the
   SvcParams including "docpath".  The construction algorithm for DoC
   requests is as follows, with the provided records in order of their
   priority.  For the purposes of this algorithm, the DoC client is
   assumed to be SVCB-optional (see Section 3 of [RFC9460]).

   *  If the "alpn" SvcParam value for the service is "coap", a CoAP
      request for CoAP over TLS MUST be constructed [RFC8323].  If it is
      "co", a CoAP request for CoAP over DTLS MUST be constructed
      [RFC9952].  Any other SvcParamKeys specifying a transport are out
      of the scope of this document.

   *  The destination address for the request SHOULD be taken from
      additional information about the target.  This may include (1) A
      or AAAA RRs associated with the target name and delivered with the
      SVCB RR (see [RFC9462]), (2) "ipv4hint" or "ipv6hint" SvcParams
      from the SVCB RR (see [RFC9461]), or (3) IPv4 or IPv6 addresses
      provided if DNR [RFC9463] is used.  As a fallback, an address MAY
      be queried for the target name of the SVCB record from another DNS
      service.

   *  The destination port for the request MUST be taken from the "port"
      SvcParam value, if present.  Otherwise, take the default port of
      the CoAP transport, e.g., with regards to this specification, the
      default is TCP port 5684 for "coap" or UDP port 5684 for "co".
      This document introduces no limitations on the ports that can be
      used.  If a malicious SVCB record allows its originator to
      influence message payloads, Section 12 of [RFC9460] recommends
      placing such restrictions in the SVCB mapping document.  The
      records used in this document only influence the Uri-Path option.
      That option is only sent in the plaintext of an encrypted (D)TLS
      channel and thus does not warrant any limitations.

   *  The request URI's hostname component MUST be the Authentication
      Domain Name (ADN) when obtained through DNR and MUST be the target
      name of the SVCB record when obtained through a _dns query (if
      AliasMode is used to the target name of the AliasMode record).
      This can be achieved efficiently by setting that name in TLS
      Server Name Indication (SNI) [RFC8446] or by setting the Uri-Host
      option on each request.

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

   *  If the request constructed this way receives a response, the same
      SVCB record MUST be used for construction of future DoC queries.
      If not, or if the endpoint becomes unreachable, the algorithm
      repeats with the SVCB RR or DNR Encrypted DNS option with the next
      highest Service Priority as a fallback (see Sections 2.4.1 and 3
      of [RFC9460]).

   A more generalized construction algorithm for any CoAP request can be
   found in [TRANSPORT-IND].

3.2.1.  Examples

   A typical SVCB resource record response for a DoC server at the root
   path "/" of the server looks like the following (the "docpath"
   SvcParam is the last 4 bytes 00 0a 00 00 in the binary):

   Resource record (binary):
    04 5f 64 6e 73 07 65 78 61 6d 70 6c 65 03 6f 72
    67 00 00 40 00 01 00 00 06 28 00 1e 00 01 03 64
    6e 73 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 00
    01 00 03 02 63 6f 00 0a 00 00

   Resource record (human-readable):
    _dns.example.org.  1576  IN SVCB 1 dns.example.org (
        alpn=co docpath )

   The root path is RECOMMENDED but not required.  Here are two examples
   where the "docpath" represents paths of varying depth.  First, "/dns"
   is provided in the following example (the last 8 bytes 00 0a 00 04 03
   64 6e 73):

   Resource record (binary):
    04 5f 64 6e 73 07 65 78 61 6d 70 6c 65 03 6f 72
    67 00 00 40 00 01 00 00 00 55 00 22 00 01 03 64
    6e 73 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 00
    01 00 03 02 63 6f 00 0a 00 04 03 64 6e 73

   Resource record (human-readable):
    _dns.example.org.    85  IN SVCB 1 dns.example.org (
        alpn=co docpath=dns )

   Second, see an example for the path "/n/s" (the last 8 bytes 00 0a 00
   04 01 6e 01 73):

   Resource record (binary):
    04 5f 64 6e 73 07 65 78 61 6d 70 6c 65 03 6f 72
    67 00 00 40 00 01 00 00 06 6b 00 22 00 01 03 64
    6e 73 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 00
    01 00 03 02 63 6f 00 0a 00 04 01 6e 01 73

   Resource record (human-readable):
    _dns.example.org.   1643  IN SVCB 1 dns.example.org (
        alpn=co docpath=n,s )

   If the server also provides DNS over HTTPS, "dohpath" and "docpath"
   MAY coexist:

   Resource record (binary):
    04 5f 64 6e 73 07 65 78 61 6d 70 6c 65 03 6f 72
    67 00 00 40 00 01 00 00 01 ad 00 2c 00 01 03 64
    6e 73 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 00
    01 00 06 02 68 33 02 63 6f 00 07 00 07 2f 7b 3f
    64 6e 73 7d 00 0a 00 00

   Resource record (human-readable):
    _dns.example.org.   429  IN SVCB 1 dns.example.org (
        alpn=h3,co dohpath=/{?dns} docpath )

4.  Basic Message Exchange

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

   This document defines a CoAP Content-Format ID 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 [STD13].  Both DoC client and DoC server MUST be able to
   parse contents in the "application/dns-message" Content-Format.  This
   document only specifies OPCODE 0 (Query) for DNS over CoAP messages.
   Future documents can provide considerations for additional OPCODEs or
   extend its specification (e.g., by describing whether other CoAP
   codes need to be used for which OPCODE).  Unless another error takes
   precedence, a DoC server uses RCODE = 4, NotImp [STD13], in its
   response to a query with an OPCODE that it does not 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] request 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 [STD13] 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).

4.2.2.  Support of CoAP Caching

   The DoC client SHOULD set the ID field of the DNS header 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.  Apart from losing these caching benefits, there is no harm
   in not setting it to 0, e.g., when the query was received from
   somewhere else.  In any instance, a DoC server MUST copy the ID from
   the query in its response to that query.

4.2.3.  Example

   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 the
   "application/dns-message" Content-Format.

   FETCH coaps://[2001:db8::1]/
   Content-Format: 553 (application/dns-message)
   Accept: 553 (application/dns-message)
   Payload (binary):
    00 00 01 00 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; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 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, i.e., it is also possible to transfer them using block-wise
   transfer [RFC7959].  A DoC server MUST be able to produce responses
   in the "application/dns-message" Content-Format (for details, see
   Section 4.1) when requested.  The use of the Accept option in the
   request is OPTIONAL.  However, all DoC clients MUST be able to parse
   an "application/dns-message" response (see also Section 4.1).  Any
   response Content-Format other 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 [STD13]).  It is RECOMMENDED that CoAP responses
   that carry a parsable 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.  When an error occurs at the CoAP layer, e.g., if an unexpected
   request method or an unsupported Content-Format in the request are
   used, the DoC server SHOULD respond with an appropriate CoAP error.

   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

   For reliability and energy-saving measures, content decoupling (such
   as en-route caching on proxies) takes a far greater role than it does
   in HTTP.  Likewise, CoAP makes it possible to use cache validation to
   refresh stale cache entries to reduce the number of large response
   messages.  For cache validation, CoAP implementations regularly use
   hashing over the message content for ETag generation (see [RFC7252],
   Section 5.10.6).  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 the sum of the Max-Age value of a
   CoAP response and any TTL in the DNS response is less than 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.

   To meet the requirement for DoC, the RECOMMENDED algorithm for a DoC
   server 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 from
   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.  The same applies for DNS responses
   that, for any reason, do not carry any records with a TTL.

4.3.3.  Examples

   The following example illustrates the response to the query
   "example.org.  IN AAAA record", with recursion turned on.  Successful
   responses carry one answer record including the address
   2001:db8:1:0:1:2:3:4 and TTL 79689.

   A successful response:

   2.05 Content
   Content-Format: 553 (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, ADDITIONAL: 0

    ;; QUESTION SECTION:
    ;example.org.                 IN      AAAA
    ;; ANSWER SECTION:
    ;example.org.         79689   IN      AAAA    2001:db8:1:0:1:2:3:4

   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: 553 (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 - in this case, it errors on a DNS
   Update (OPCODE = 5) for "example.org".

   2.05 Content
   Content-Format: 553 (application/dns-message)
   Payload (human-readable):
    ;; ->>Header<<- opcode: UPDATE, status: NOTIMP, id: 0
    ;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

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

   When an error occurs at the CoAP layer, the DoC server responds 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.  Interaction with Other CoAP and CoRE Features

5.1.  DNS Push Notifications and CoAP Observe

   DNS Push Notifications [RFC8765] provide the capability to
   asynchronously notify clients about resource record changes.
   However, it results in 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.
   This is particularly useful if a short-lived record is requested
   frequently.  The DoC server SHOULD provide Observe capabilities on
   its DoC resource and do as follows.

   If a DoC client wants to observe a resource record, a DoC server can
   respond with a notification and add the client to its list of
   observers for that resource in accordance with [RFC7641].  The DoC
   server MAY subscribe to DNS Push Notifications for that record.  This
   involves sending a DNS Subscribe message (see Section 6.2 of
   [RFC8765]), instead of a classic DNS query to fetch the information
   on behalf of the DoC client.

   After the list of observers for a particular DNS query has become
   empty (by explicit or implicit cancellation of the observation as per
   Section 3.6 of [RFC7641]), and no other reason to subscribe to that
   request is present, the DoC server SHOULD cancel the corresponding
   subscription.  This can involve sending a DNS Unsubscribe message or
   closing the session (see Section 6.4 of [RFC8765]).  As there is no
   CoAP observer anymore from the perspective of the DoC server, a
   failure to unsubscribe or close the session cannot be communicated
   back to any DoC observer.  As such, error handling (if any) needs to
   be resolved between the DoC server and the upstream DNS
   infrastructure.

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

   A server that responds with notifications (i.e., sends the Observe
   option) needs to have the means of obtaining current resource
   records.  This may happen through DNS Push or also by upstream
   polling or implicit circumstances (e.g., if the DoC server is the
   authoritative name server for the record and wants to notify about
   changes).

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.

   [CACHEABLE-OSCORE] describes a method to allow cache retrieval of
   OSCORE responses and discusses the corresponding implications on
   message sizes and security properties.

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.  Such a direct mapping is NOT
   RECOMMENDED: Rewriting the FETCH method (Section 4.2) and TTL
   (Section 4.3.2) as specified in this document 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 secure 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]
   [RFC9076] and CoAP (Section 11 of [RFC7252]).  While DoC does not use
   the random ID of the DNS header (see Section 4.2.2), equivalent
   protection against off-path poisoning attacks is achieved by using
   random large token values for unprotected CoAP requests.  If a DoC
   message is unprotected, it MUST use a random token with a length of
   at least 2 bytes to mitigate this kind of poisoning attack.

7.  Security Considerations

   General CoAP security considerations ([RFC7252], Section 11) apply to
   DoC.  DoC also inherits the security considerations of the protocols
   used for secure communication, e.g., OSCORE ([RFC8613], Section 12)
   as well as DTLS 1.2 or newer ([RFC6347], Section 5 and [RFC9147],
   Section 11).  Additionally, DoC uses request patterns that require
   the maintenance of long-lived security contexts.  Section 2.9 of
   [CoAP-CORR-CLAR] provides insights on what can be done when those are
   resumed from a new endpoint.

   Though DTLS v1.2 [RFC6347] was obsoleted by DTLS v1.3 [RFC9147],
   there are many CoAP implementations that still use v1.2 at the time
   of writing.  As such, this document also accounts for the usage of
   DTLS v1.2 even though newer versions are RECOMMENDED when using DTLS
   to secure CoAP.

   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 (D)TLS or OSCORE, an unpredictable ID to
   protect against spoofing is not necessary.  Both (D)TLS and OSCORE
   offer mechanisms to harden against injecting spoofed responses in
   their protocol design.  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]); however, while recommended, this is opaque
   to the DoC client on the protocol level.  Record integrity can also
   be ensured upstream using DNSSEC [BCP237].

   A DoC client may not be able to perform DNSSEC validation, e.g., due
   to code size constraints or the 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.  For instance, a DoC
   client may be 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 DoC server.

8.  IANA Considerations

8.1.  CoAP Content-Formats Registry

   IANA has assigned a CoAP Content-Format ID for the "application/dns-
   message" media type in the "CoAP Content-Formats" registry in the
   "Constrained RESTful Environments (CoRE) Parameters" registry group
   [RFC7252]; this corresponds to the "application/dns-message" media
   type from the "Media Types" registry (see [RFC8484]).

           +=========================+=====+===================+
           | Content Type            | ID  | Reference         |
           +=========================+=====+===================+
           | application/dns-message | 553 | [RFC8484] and RFC |
           |                         |     | 9953, Section 4.1 |
           +-------------------------+-----+-------------------+

                      Table 1: CoAP Content-Format ID

8.2.  DNS SVBC Service Parameter Keys (SvcParamKeys) Registry

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

   +========+=========+===============+===================+===========+
   | Number | Name    | Meaning       | Change Controller | Reference |
   +========+=========+===============+===================+===========+
   | 10     | docpath | DNS over CoAP | IETF              | RFC 9953, |
   |        |         | resource path |                   | Section 3 |
   +--------+---------+---------------+-------------------+-----------+

                     Table 2: Value for SvcParamKeys

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

   IANA has added "core.dns" to the "Resource Type (rt=) Link Target
   Attribute Values" registry in the "Constrained RESTful Environments
   (CoRE) Parameters" registry group.

        +==========+========================+=====================+
        | Value    | Description            | Reference           |
        +==========+========================+=====================+
        | core.dns | DNS over CoAP resource | RFC 9953, Section 3 |
        +----------+------------------------+---------------------+

             Table 3: Resource Type (rt=) Link Target Attribute

9.  Operational Considerations

9.1.  Coexistence of Different DNS and CoAP Transports

   Many DNS transports may coexist on the DoC server, such as DNS over
   UDP [STD13], DNS over (D)TLS [RFC7858] [RFC8094], DNS over HTTPS
   [RFC8484], or DNS over QUIC [RFC9250].  In principle, transports
   employing channel or object security should be preferred.  In
   constrained scenarios, DNS over CoAP is preferable to DNS over DTLS.
   The final decision regarding the preference, however, heavily depends
   on the use case and is therefore left to the implementers or users
   and is not defined in this document.

   CoAP supports Confirmable and Non-Confirmable messages [RFC7252] to
   deploy different levels of reliability.  However, this document does
   not enforce any of these message types, as the decision on which one
   is appropriate depends on the characteristics of the network where
   DoC is deployed.

9.2.  Redirects

   Application-layer redirects (e.g., HTTP) redirect a client to a new
   server.  In the case of DoC, this leads to a new DNS server.  This
   new DNS server may provide different answers to the same DNS query
   than the previous DNS server.  At the time of writing, CoAP does not
   support redirection.  Future specifications of CoAP redirect may need
   to consider the impact of different results between previous and new
   DNS servers.

9.3.  Proxy Hop Limit

   Mistakes might lead to CoAP proxies forming infinite loops.  Using
   the CoAP Hop-Limit option [RFC8768] mitigates such loops.

9.4.  Error Handling

   Section 4.3.1 specifies that DNS operational errors should be
   reported back to a DoC client using the appropriate DNS RCODE.  If a
   DoC client did not receive any successful DNS messages from a DoC
   server for a while, it might indicate that the DoC server lost
   connectivity to the upstream DNS infrastructure.  The DoC client
   should handle this error case like a recursive resolver that lost
   connectivity to the upstream DNS infrastructure.  In case of CoAP
   errors, the usual mechanisms for CoAP response codes apply.

9.5.  DNS Extensions

   DNS extensions that are specific to the choice of transport, such as
   described in [RFC7828], are not applicable to DoC.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/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/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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/info/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/info/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/info/rfc7641>.

   [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/info/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/info/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/info/rfc8174>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

   [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>.

   [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/info/rfc8613>.

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

   [RFC8768]  Boucadair, M., Reddy.K, T., and J. Shallow, "Constrained
              Application Protocol (CoAP) Hop-Limit Option", RFC 8768,
              DOI 10.17487/RFC8768, March 2020,
              <https://www.rfc-editor.org/info/rfc8768>.

   [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/info/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/info/rfc9147>.

   [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/info/rfc9460>.

   [RFC9461]  Schwartz, B., "Service Binding Mapping for DNS Servers",
              RFC 9461, DOI 10.17487/RFC9461, November 2023,
              <https://www.rfc-editor.org/info/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/info/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/info/rfc9463>.

   [RFC9952]  Lenders, M. S., Amsüss, C., Schmidt, T. C., and M.
              Wählisch, "Application-Layer Protocol Negotiation (ALPN)
              ID for CoAP over DTLS", RFC 9952, DOI 10.17487/RFC9952,
              March 2026, <https://www.rfc-editor.org/info/rfc9952>.

   [STD13]    Internet Standard 13,
              <https://www.rfc-editor.org/info/std13>.
              At the time of writing, this STD comprises the following:

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

              Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

10.2.  Informative References

   [BCP219]   Best Current Practice 219,
              <https://www.rfc-editor.org/info/bcp219>.
              At the time of writing, this BCP comprises the following:

              Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [BCP237]   Best Current Practice 237,
              <https://www.rfc-editor.org/info/bcp237>.
              At the time of writing, this BCP comprises the following:

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

   [CACHEABLE-OSCORE]
              Amsüss, C. and M. Tiloca, "End-to-End Protected and
              Cacheable Responses for the Constrained Application
              Protocol (CoAP) using Group Object Security for
              Constrained RESTful Environments (Group OSCORE)", Work in
              Progress, Internet-Draft, draft-ietf-core-cacheable-
              oscore-01, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              cacheable-oscore-01>.

   [CoAP-CORR-CLAR]
              Bormann, C., "Constrained Application Protocol (CoAP):
              Corrections and Clarifications", Work in Progress,
              Internet-Draft, draft-ietf-core-corr-clar-04, 19 March
              2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
              core-corr-clar-04>.

   [CRI]      Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-30, 21 November 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-30>.

   [DoC-paper]
              Lenders, M. S., Amsüss, C., Gündogan, C., Nawrocki, M.,
              Schmidt, T., and M. Wählisch, "Securing Name Resolution in
              the IoT: DNS over CoAP", 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>.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000,
              <https://www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.

   [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/info/rfc3833>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/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/info/rfc7228>.

   [RFC7228bis]
              Bormann, C., Ersue, M., Keränen, A., and C. Gomez,
              "Terminology for Constrained-Node Networks", Work in
              Progress, Internet-Draft, draft-ietf-iotops-7228bis-05, 14
              March 2026, <https://datatracker.ietf.org/doc/html/draft-
              ietf-iotops-7228bis-05>.

   [RFC7828]  Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828,
              DOI 10.17487/RFC7828, April 2016,
              <https://www.rfc-editor.org/info/rfc7828>.

   [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>.

   [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/info/rfc8094>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

   [RFC9076]  Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
              DOI 10.17487/RFC9076, July 2021,
              <https://www.rfc-editor.org/info/rfc9076>.

   [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/info/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/info/rfc9250>.

   [RFC9528]  Selander, G., Preuß Mattsson, J., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
              DOI 10.17487/RFC9528, March 2024,
              <https://www.rfc-editor.org/info/rfc9528>.

   [TRANSPORT-IND]
              Amsüss, C. and M. S. Lenders, "CoAP Transport Indication",
              Work in Progress, Internet-Draft, draft-ietf-core-
              transport-indication-09, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              transport-indication-09>.

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].

Acknowledgments

   The authors of this document want to thank Mike Bishop, Carsten
   Bormann, Mohamed Boucadair, Deb Cooley, Vladimír Čunát, Roman
   Danyliw, Elwyn B. Davies, Esko Dijk, Gorry Fairhurst, Thomas Fossati,
   Mikolai Gütschow, Todd Herr, Tommy Pauly, Jan Romann, Ben Schwartz,
   Orie Steele, Marco Tiloca, Éric Vyncke, Tim Wicinski, and Paul
   Wouters for their feedback and comments.

   This work was supported in parts by the German Federal Ministry of
   Research, Technology and Space (BMFTR) under the grant numbers
   16KIS1386K (TU Dresden) and 16KIS1387 (HAW Hamburg) within the
   research project PIVOT and under the grant numbers 16KIS1694K (TU
   Dresden) and 16KIS1695 (HAW Hamburg) within the research project
   C-ray4edge.

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


   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
