DNS over CoAP (DoC)
draft-ietf-core-dns-over-coap-20
| Document | Type | Active Internet-Draft (core WG) | |
|---|---|---|---|
| Authors | Martine Sophie Lenders , Christian Amsüss , Cenk Gündoğan , Thomas C. Schmidt , Matthias Wählisch | ||
| Last updated | 2025-09-16 | ||
| Replaces | draft-lenders-dns-over-coap | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
TSVART IETF Last Call review
(of
-15)
by Tommy Pauly
Ready w/nits
ARTART IETF Last Call review
(of
-15)
by Todd Herr
Ready w/nits
DNSDIR Early review
(of
-01)
by Tim Wicinski
On the right track
|
||
| Additional resources |
DoC client implementation in RIOT
DoC server implementation in Python Working Group Repo Mailing list discussion |
||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Marco Tiloca | ||
| Shepherd write-up | Show Last changed 2025-04-03 | ||
| IESG | IESG state | RFC Ed Queue | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Mike Bishop | ||
| Send notices to | marco.tiloca@ri.se | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack | ||
| IANA expert review state | Expert Reviews OK | ||
| IANA expert review comments | The CoAP Content-Formats and DNS SVCB Service Parameter Keys (SvcParamKeys) registrations have been approved. | ||
| RFC Editor | RFC Editor state | EDIT | |
| Details |
draft-ietf-core-dns-over-coap-20
CoRE M. S. Lenders
Internet-Draft TU Dresden
Intended status: Standards Track C. Amsüss
Expires: 20 March 2026
C. Gündoğan
NeuralAgent GmbH
T. C. Schmidt
HAW Hamburg
M. Wählisch
TU Dresden & Barkhausen Institut
16 September 2025
DNS over CoAP (DoC)
draft-ietf-core-dns-over-coap-20
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 (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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Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 20 March 2026.
Copyright Notice
Copyright (c) 2025 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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Conventions . . . . . . . . . . . . . . . . . 5
3. Selection of a DoC Server . . . . . . . . . . . . . . . . . . 6
3.1. Discovery by Resource Type . . . . . . . . . . . . . . . 7
3.2. Discovery using SVCB Resource Records or DNR . . . . . . 7
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . 9
4. Basic Message Exchange . . . . . . . . . . . . . . . . . . . 11
4.1. The "application/dns-message" Content-Format . . . . . . 11
4.2. DNS Queries in CoAP Requests . . . . . . . . . . . . . . 11
4.2.1. Request Format . . . . . . . . . . . . . . . . . . . 11
4.2.2. Support of CoAP Caching . . . . . . . . . . . . . . . 12
4.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. DNS Responses in CoAP Responses . . . . . . . . . . . . . 12
4.3.1. Response Codes and Handling DNS and CoAP errors . . . 13
4.3.2. Support of CoAP Caching . . . . . . . . . . . . . . . 13
4.3.3. Examples . . . . . . . . . . . . . . . . . . . . . . 14
5. Interaction with other CoAP and CoRE Features . . . . . . . . 15
5.1. DNS Push Notifications and CoAP Observe . . . . . . . . . 15
5.2. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3. Mapping DoC to DoH . . . . . . . . . . . . . . . . . . . 16
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6. Considerations for Unprotected Use . . . . . . . . . . . . . 17
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 17
7.1. DoC Client . . . . . . . . . . . . . . . . . . . . . . . 17
7.2. DoC Server . . . . . . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. CoAP Content-Formats Registry . . . . . . . . . . . . . . 19
9.2. DNS Service Bindings (SVCB) Registry . . . . . . . . . . 20
9.3. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 20
10. Operational Considerations . . . . . . . . . . . . . . . . . 20
10.1. Co-existence of different DNS and CoAP transports . . . 20
10.2. Redirects . . . . . . . . . . . . . . . . . . . . . . . 21
10.3. Proxy Hop-Limit . . . . . . . . . . . . . . . . . . . . 21
10.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 21
10.5. DNS Extensions . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Evaluation . . . . . . . . . . . . . . . . . . . . . 26
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 27
B.1. Since draft-ietf-core-dns-over-coap-18 . . . . . . . . . 27
B.2. Since draft-ietf-core-dns-over-coap-17 . . . . . . . . . 27
B.3. Since draft-ietf-core-dns-over-coap-16 . . . . . . . . . 29
B.4. Since draft-ietf-core-dns-over-coap-15 . . . . . . . . . 29
B.5. Since draft-ietf-core-dns-over-coap-14 . . . . . . . . . 30
B.6. Since draft-ietf-core-dns-over-coap-13 . . . . . . . . . 30
B.7. Since draft-ietf-core-dns-over-coap-12 . . . . . . . . . 30
B.8. Since draft-ietf-core-dns-over-coap-10 . . . . . . . . . 31
B.9. Since draft-ietf-core-dns-over-coap-09 . . . . . . . . . 31
B.10. Since draft-ietf-core-dns-over-coap-08 . . . . . . . . . 32
B.11. Since draft-ietf-core-dns-over-coap-07 . . . . . . . . . 32
B.12. Since draft-ietf-core-dns-over-coap-06 . . . . . . . . . 32
B.13. Since draft-ietf-core-dns-over-coap-05 . . . . . . . . . 32
B.14. Since draft-ietf-core-dns-over-coap-04 . . . . . . . . . 33
B.15. Since draft-ietf-core-dns-over-coap-03 . . . . . . . . . 33
B.16. Since draft-ietf-core-dns-over-coap-02 . . . . . . . . . 33
B.17. Since draft-ietf-core-dns-over-coap-01 . . . . . . . . . 33
B.18. Since draft-ietf-core-dns-over-coap-00 . . . . . . . . . 34
B.19. Since draft-lenders-dns-over-coap-04 . . . . . . . . . . 34
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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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 DTLS 1.2 or newer [RFC6347] [RFC9147]
as well as Object Security for Constrained RESTful Environments
(OSCORE) [RFC8613] but also TLS 1.3 or newer [RFC8323] [RFC8446] to
ensure message integrity and confidentiality.
The application use case of DoC is inspired by DNS over HTTPS
[RFC8484] (DoH). DoC, however, 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]
[I-D.ietf-iotops-7228bis]
// RFC Ed.: Please remove the [RFC7228] reference and replace it with
// [I-D.ietf-iotops-7228bis] throughout the document in case
// [I-D.ietf-iotops-7228bis] becomes an RFC before publication..
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 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.
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. 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 authoritive 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 DNSSEC validator to provide DNSSEC validation to the more
constrained DoC clients.
Note that this specification is distinct from DoH, since the CoAP-
specific FETCH method [RFC8132] is used. This has the benefit of
having the DNS query in the body as when using the POST method, but
still with the same caching advantages of responses to requests that
use the GET method. Having the DNS query in the body means that we
do not need 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 either as a DNS stub resolver or a DNS
recursive resolver [BCP219]. As such, the DoC server communicates
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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 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 in that case. 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.
For better readability, 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. In the actual
message sent and received, however, 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 manual
configuration of a Uniform Resource Identifier (URI) [RFC3986] or
Constrained Resource Identifier (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
MUST only be done from a trusted source.
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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 (RR) [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 [I-D.ietf-core-coap-dtls-alpn] defines the ALPN ID for
CoAP over DTLS servers. DoC servers that use only OSCORE [RFC8613]
and Ephemeral Diffie-Hellman Over COSE (EDHOC) [RFC9528] (with COSE
abbreviating "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 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 [I-D.ietf-core-transport-indication]. It generalizes
mechanisms for all CoAP services. This document introduces only the
discovery of DoC services.
This document specifies "docpath" as a single-valued 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 devided 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
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list. The same considerations for the "," and "" characters in
docpath-segments for zone-file implementations as for the alpn-ids in
an "alpn" SvcParam apply (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 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 is of length 0 and 24 octets, it is easily transferred into
the path representation in CRIs [I-D.ietf-core-href] 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, they just make the
translation harder, as they require to make space for the longer
delimiters, in turn requiring to move octets.
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, going through 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
[I-D.ietf-core-coap-dtls-alpn]. 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
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from the SVCB RR (see [RFC9461]), or (3) from 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 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 infuence the Uri-Path option. That option is
only sent in the plaintext of an encrytped (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 [I-D.ietf-core-transport-indication].
3.2.1. Examples
// RFC Ed.: Since the number for "docpath" was not assigned at the
// time of writing, we used the hex ff 0a (in decimal 65290; from the
// private use range of SvcParamKeys) throughout this section.
// Before publication, please replace ff 0a with the hexadecimal
// representation of the final value assigned by IANA in this
// section. Please remove this paragraph after that.
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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 ff 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 ff 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 ff 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 ff 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, an examples for the path "/n/s" (the last 8 bytes ff 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 ff 0a 00 04 01 6e 01 73
Resource record (human-readable):
_dns.example.org. 643 IN SVCB 1 dns.example.org (
alpn=co docpath=n,s )
If the server also provides DNS over HTTPS, "dohpath" and "docpath"
MAY co-exist:
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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 2b 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 ff 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 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 [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).
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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
it 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
a "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.
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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 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. 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.
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
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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. 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 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.
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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—a DNS Update (OPCODE = 5) for
"example.org" in this case.
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] provides 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 clients 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 to [RFC7641]. The DoC
server MAY subscribe to DNS push notifications for that record. This
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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 an 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 do so 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 means of obtaining current resource records.
This may happen through DNS Push, but 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.
[I-D.amsuess-core-cachable-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 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.
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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 of at least 2 bytes
length to mitigate this kind of poisoning attack.
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.
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
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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
8. 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 [RFC9147],
Section 11). 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.
Though DTLS v1.2 [RFC6347] was obsoleteted by DTLS v1.3 [RFC9147]
there are still 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).
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For secure communication via (D)TLS or OSCORE, an unpredictable ID
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]), but, 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 due to 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.
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.
9.1. CoAP Content-Formats Registry
IANA is requested to assign a CoAP Content-Format ID for the
"application/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)
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Reference: [RFC8484] and [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. Operational Considerations
10.1. Co-existence of different DNS and CoAP transports
Many DNS transports may co-exist 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.
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CoAP supports Confirmable and Non-Confirmable messages [RFC7252] to
deploy different levels of reliability. This document, however, 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.
10.2. Redirects
Application-layer redirects (e.g., HTTP) redirct 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 server.
10.3. Proxy Hop-Limit
Mistakes might lead to CoAP proxies forming infinite loops. Using
the CoAP Hop-Limit option [RFC8768] mitigates such loops.
10.4. 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 message 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.
10.5. DNS Extensions
DNS extensions that are specific to the choice of transport, such as
[RFC7828], are not applicable to DoC.
11. References
11.1. Normative References
[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-05, 11 August 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-dtls-alpn-05>.
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[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>.
[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/rfc/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/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>.
[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>.
[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/rfc/rfc8323>.
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[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>.
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/rfc/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/rfc/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/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>.
[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>.
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[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>.
11.2. 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>.
[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-11, 6 July 2025,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
cachable-oscore-11>.
[I-D.ietf-core-corr-clar]
Bormann, C., "Constrained Application Protocol (CoAP):
Corrections and Clarifications", Work in Progress,
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Internet-Draft, draft-ietf-core-corr-clar-02, 20 June
2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
core-corr-clar-02>.
[I-D.ietf-core-href]
Bormann, C. and H. Birkholz, "Constrained Resource
Identifiers", Work in Progress, Internet-Draft, draft-
ietf-core-href-24, 30 August 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
href-24>.
[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-09, 7 July 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
transport-indication-09>.
[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-02, 7
July 2025, <https://datatracker.ietf.org/doc/html/draft-
ietf-iotops-7228bis-02>.
[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/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>.
[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/rfc/rfc7828>.
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[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/rfc/rfc7858>.
[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>.
[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/rfc/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/rfc/rfc9052>.
[RFC9076] Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
DOI 10.17487/RFC9076, July 2021,
<https://www.rfc-editor.org/rfc/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/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>.
[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/rfc/rfc9528>.
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].
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Appendix B. Change Log
// RFC Ed.: Please remove this section before publication.
B.1. Since draft-ietf-core-dns-over-coap-18
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-18)
* Address Address Mohamed Boucadair's COMMENT:
- Add Operational Considerations Section
- Make SVCB references normative
- Remove redundant requirement on parsing application/dns-message
- Remove contradicting statement and outdated reference about
ALPN
- Add DNS client to Fig. 1
- Clarify recursion termination in the CoAP realm
- Clarify where addresses are coming from with DDR/DNR
* Address Gorry Fairhurst's follow-up DISCUSS:
- Refer to Observe terminology in Section 2
- Clarify registration
- Provide alternative observation examples
- Clarify that error handling is in the hands of the DoC server
B.2. Since draft-ietf-core-dns-over-coap-17
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-17)
* Address Roman Danyliw's COMMENT:
- Remove unused RFC8742 reference
* Address Vladimír Čunát's DNSDIR review
- Address Éric Vyncke' COMMENT:
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- Mention OPCODE 0 in Abstract and Introduction
- Reference to STD13 instead of RFC1035
- Provide extension pointers for future documents on other
OPCODES
- Use only singular for example section if there is only one
example
- Improvements on DNSSEC
- Hyphenate link-layer as modifier to frame
* Address Paul Wouters's DISCUSS and COMMENT:
- Remove unnecessary and confusing ad flag from query example
- Phrase full SVCB mapping sentence more neutrally
* Address Gorry Fairhurst's COMMENT:
- Add note (in addition to the RFC Ed.:) about paragraph removal
- Add references for "coap" and "co" ALPN to SvcParam algorithm
- Address Gorry's nits
* Address Gorry Fairhurst's DISCUSS:
- Update push notifications
- observation: Do not use normative language
* Address Orie Steele's COMMENT:
- Automatic configuration MUST only be done from a trusted source
- Remove confusing and unnecessary MAY
- Remove normative repeat of SvcParam algorithm by citing RFC
9461
- Fix wording around Accept option
* Address Deb Cooley's COMMENT:
- Group (D)TLS references
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- Automatic configuration MUST only be done from a trusted source
- Fix wording about unpredictable ID and spoofing
- Remove confusing "e.g."
B.3. Since draft-ietf-core-dns-over-coap-16
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-16)
* Mention TLS as possible protection mechanism in abstract and intro
* Fix representation format in the docpath examples
* Make docpath wire-format paragraph easier to parse
B.4. Since draft-ietf-core-dns-over-coap-15
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-15)
* Address Genart and Artart review:
- Add editor's note about removing RFC7228 reference in case
rfc7228bis comes out before publication
- Address minor nits
- Resolve less well-known abbreviations
- Name default ports for "coap" and "co"
- Add reasoning why we also consider DTLS v1.2 (RFC 6347)
- Add illustrative reference for ETag generation
* Address DNS SVCB SvcParamKeys IANA expert review:
- docpath: clarifications and examples
- Rework representation format and wire-format of "docpath"
- State that we don't do the full SVCB mapping
- Explicitly do not limit what port= can do.
- port limitations: We're not the SVCB mapping document
* Address Tsvart Review
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- Prefer ADN for Uri-Host; don't prescribe _how_ it is set
B.5. Since draft-ietf-core-dns-over-coap-14
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-14)
* Remove superfluous and confusing step in SVCB to request algorithm
* Address AD review:
- Remove RFC8949 as CBOR diagnostic notation reference
- CoRE-specific FETCH method -> CoAP-specific FETCH method
- Remove double reference to BCP 219
- Fix wording and references around SVCB records and ALPN
- Move format description for examples to Terminology section
- Retitle section 5 to "Interaction with other CoAP and CoRE
Features"
- Make prevention of poisoning attacks normative for unprotected
CoAP
- Provide specs on if the SHOULD on ID=0 does not apply
- Make construction algorithm normative
- Add definition for CoRE
- General grammar, wording, and spelling cleanups
B.6. Since draft-ietf-core-dns-over-coap-13
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-13)
* Address shepherd review
B.7. Since draft-ietf-core-dns-over-coap-12
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-12)
* Address Esko's review
* Address Marcos's review
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* Address Mikolai's review
B.8. 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.9. 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
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* Add to security considerations: unprotected upstream DNS and
DNSSEC
B.10. 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.11. 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.12. 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.13. 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
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B.14. 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
B.15. 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.16. 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.17. 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
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* Add subsection on how to implement DNS Push in DoC
* Add appendix on reference implementation
B.18. 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.19. 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 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.
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
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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
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