CoRE M. S. Lenders
Internet-Draft TU Dresden
Intended status: Standards Track C. Amsüss
Expires: 4 September 2025
C. Gündoğan
NeuralAgent GmbH
T. C. Schmidt
HAW Hamburg
M. Wählisch
TU Dresden & Barkhausen Institut
3 March 2025
DNS over CoAP (DoC)
draft-ietf-core-dns-over-coap-13
Abstract
This document defines a protocol for exchanging DNS messages over the
Constrained Application Protocol (CoAP). These CoAP messages can be
protected by DTLS-Secured CoAP (CoAPS) or Object Security for
Constrained RESTful Environments (OSCORE) to provide encrypted DNS
message exchange for constrained devices in the Internet of Things
(IoT).
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://core-
wg.github.io/draft-dns-over-coap/draft-ietf-core-dns-over-coap.html.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-core-dns-over-coap/.
Discussion of this document takes place on the CoRE Working Group
mailing list (mailto:core@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/core/. Subscribe at
https://www.ietf.org/mailman/listinfo/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/draft-dns-over-coap.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 4 September 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Selection of a DoC Server . . . . . . . . . . . . . . . . . . 5
3.1. Discovery by Resource Type . . . . . . . . . . . . . . . 6
3.2. Discovery using SVCB Resource Records or DNR . . . . . . 6
4. Basic Message Exchange . . . . . . . . . . . . . . . . . . . 7
4.1. The "application/dns-message" Content-Format . . . . . . 7
4.2. DNS Queries in CoAP Requests . . . . . . . . . . . . . . 8
4.2.1. Request Format . . . . . . . . . . . . . . . . . . . 8
4.2.2. Support of CoAP Caching . . . . . . . . . . . . . . . 8
4.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . 8
4.3. DNS Responses in CoAP Responses . . . . . . . . . . . . . 9
4.3.1. Response Codes and Handling DNS and CoAP errors . . . 9
4.3.2. Support of CoAP Caching . . . . . . . . . . . . . . . 10
4.3.3. Examples . . . . . . . . . . . . . . . . . . . . . . 11
5. CoAP/CoRE Integration . . . . . . . . . . . . . . . . . . . . 12
5.1. DNS Push . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. Mapping DoC to DoH . . . . . . . . . . . . . . . . . . . 13
6. Considerations for Unprotected Use . . . . . . . . . . . . . 13
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7. Implementation Status . . . . . . . . . . . . . . . . . . . . 13
7.1. DoC Client . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. DoC Server . . . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9.1. CoAP Content-Formats Registry . . . . . . . . . . . . . . 16
9.2. DNS Service Bindings (SVCB) Registry . . . . . . . . . . 16
9.3. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Evaluation . . . . . . . . . . . . . . . . . . . . . 21
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 21
B.1. Since [draft-ietf-core-dns-over-coap-12] . . . . . . . . 21
B.2. Since draft-ietf-core-dns-over-coap-10 . . . . . . . . . 22
B.3. Since draft-ietf-core-dns-over-coap-09 . . . . . . . . . 22
B.4. Since draft-ietf-core-dns-over-coap-08 . . . . . . . . . 23
B.5. Since draft-ietf-core-dns-over-coap-07 . . . . . . . . . 23
B.6. Since draft-ietf-core-dns-over-coap-06 . . . . . . . . . 23
B.7. Since draft-ietf-core-dns-over-coap-05 . . . . . . . . . 23
B.8. Since draft-ietf-core-dns-over-coap-04 . . . . . . . . . 23
B.9. Since draft-ietf-core-dns-over-coap-03 . . . . . . . . . 24
B.10. Since draft-ietf-core-dns-over-coap-02 . . . . . . . . . 24
B.11. Since draft-ietf-core-dns-over-coap-01 . . . . . . . . . 24
B.12. Since draft-ietf-core-dns-over-coap-00 . . . . . . . . . 25
B.13. Since draft-lenders-dns-over-coap-04 . . . . . . . . . . 25
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This document defines DNS over CoAP (DoC), a protocol to send DNS
[RFC1035] queries and get DNS responses over the Constrained
Application Protocol (CoAP) [RFC7252]. Each DNS query-response pair
is mapped into a CoAP message exchange. Each CoAP message can be
secured by DTLS [RFC6347] [RFC9147] or Object Security for
Constrained RESTful Environments (OSCORE) [RFC8613] to ensure message
integrity and confidentiality.
The application use case of DoC is inspired by DNS over HTTPS
[RFC8484] (DoH). DoC, however, aims for the deployment in the
constrained Internet of Things (IoT), which usually conflicts with
the requirements introduced by HTTPS. Constrained IoT devices may be
restricted in memory, power consumption, link layer frame sizes,
throughput, and latency. They may only have a handful kilobytes of
both RAM and ROM. They may sleep for long durations of time, after
which they need to refresh the named resources they know about. Name
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resolution in such scenarios must take into account link layer frame
sizes of only a few hundred bytes, bit rates in the magnitude of
kilobits per second, and latencies of several seconds [RFC7228]
[I-D.ietf-iotops-7228bis].
In order not to be burdened by the resource requirements of TCP and
HTTPS, constrained IoT devices could use DNS over DTLS [RFC8094]. In
contrast to DNS over DTLS, DoC can take advantage of CoAP features to
mitigate drawbacks of datagram-based communication. These features
include: block-wise transfer [RFC7959], which solves the Path MTU
problem of DNS over DTLS (see [RFC8094], Section 5); CoAP proxies,
which provide an additional level of caching; re-use of data
structures for application traffic and DNS information, which saves
memory on constrained devices.
To avoid resource requirements of DTLS or TLS on top of UDP (e.g.,
introduced by DNS over DTLS [RFC8094] or DNS over QUIC [RFC9250]),
DoC allows for lightweight payload encryption based on OSCORE.
. FETCH coaps://[2001:db8::1]/
/
/
CoAP request
+------+ [DNS query] +------+ DNS query .---------------.
| DoC |---------------->| DoC |--- --- --- --->| DNS |
|Client|<----------------|Server|<--- --- --- ---| Infrastructure |
+------+ CoAP response +------+ DNS response '---------------'
[DNS response]
\ /\ /
'-----DNS over CoAP----' '--DNS over UDP/HTTPS/QUIC/...--'
Figure 1: Basic DoC architecture
The most important components of DoC can be seen in Figure 1: a DoC
client tries to resolve DNS information by sending DNS queries
carried within CoAP requests to a DoC server. That DoC server is a
DNS client (i.e., a stub or recursive resolver) that resolves DNS
information by using other DNS transports such as DNS over UDP
[RFC1035], DNS over HTTPS [RFC8484], or DNS over QUIC [RFC9250] when
communicating with the upstream DNS infrastructure. Using that
information, the DoC server then replies to the queries of the DoC
client with DNS responses carried within CoAP responses.
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Note that this specification is distinct from DoH, since the CoRE-
specific FETCH method [RFC8132] is used. This was done to take
benefit from having the DNS query in the body as with POST, but still
having the caching advantages we would gain with GET. Having the DNS
query in the body means we do not need extra base64 encoding, which
would increase code complexity and message sizes. We are also able
to transfer a query block-wise.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
A server that provides the service specified in this document is
called a "DoC server" to differentiate it from a classic "DNS
server". A DoC server acts either as a DNS stub resolver [RFC8499]
or a DNS recursive resolver [RFC8499]. As such, the DoC server
communicates with an "upstream DNS infrastructure" or a single
"upstream DNS server".
A client using the service specified in this document to retrieve the
DNS information is called a "DoC client".
The term "constrained nodes" is used as defined in [RFC7228].
The terms "payload" and "body" are used as defined in [RFC7959],
Section 2. Note, that without block-wise transfer the terms
"payload" and "body" are to be understood as equal.
3. Selection of a DoC Server
While there is no path specified for the DoC resource, it is
RECOMMENDED to use the root path "/" to keep the CoAP requests small.
The DoC client needs to know the DoC server and the DoC resource at
the DoC server. Possible options to assure this could be manual
configuration of a URI [RFC3986] or CRI [I-D.ietf-core-href], or
automatic configuration, e.g., using a CoRE resource directory
[RFC9176], DHCP or Router Advertisement options [RFC9463], or
discovery of designated resolvers [RFC9462]. Automatic configuration
SHOULD only be done from a trusted source.
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3.1. Discovery by Resource Type
For discovery of a the DoC resource through a link mechanism that
allows describing a resource type (e.g., the Resource Type Attribute
in [RFC6690]), this document introduces the resource type "core.dns".
It can be used to identify a generic DNS resolver that is available
to the client.
3.2. Discovery using SVCB Resource Records or DNR
A DoC server can also be discovered using SVCB Resource Records (RR)
[RFC9460] [RFC9461] or DNR Service Parameters [RFC9463].
[I-D.ietf-core-coap-dtls-alpn] provides solutions to discover CoAP
over (D)TLS servers using the "alpn" SvcParam.
[I-D.lenders-core-dnr] provides a problem statement for service
bindings discovery for OSCORE and EDHOC.
This document specifies "docpath" as a single-valued SvcParamKey
whose value MUST be a CBOR sequence of 0 or more text strings (see
[RFC8742] and [RFC8949]), delimited by the length of the
SvcParamValue field (in octets). If the SvcParamValue ends within a
CBOR text string, the SVCB RR MUST be considered as malformed. As a
text format, e.g., in DNS zone files, the CBOR diagnostic notation
(see Section 8 of [RFC8949] and [I-D.ietf-cbor-edn-literals]) of that
CBOR sequence can be used.
Note that this specifically does not surround the text string
sequence with a CBOR array or a similar CBOR data item. This path
format was chosen to coincide with the path representation in CRIs
[I-D.ietf-core-href]. Furthermore, it is easily transferable into a
sequence of CoAP Uri-Path options by mapping the initial byte of any
present CBOR text string (see [RFC8949], Section 3) into the Option
Delta and Option Length of the CoAP option, provided these CBOR text
strings are all of a length between 0 and 12 octets (see [RFC7252],
Section 3.1). Likewise, it can be transferred into a URI path-
abempty form (see [RFC3986], Section 3.3) by replacing the initial
byte of any present CBOR text string with the "/" character, provided
these CBOR text strings are all of a length less than 24 octets and
do not contain bytes that need escaping.
To use the service binding from an SVCB RR, the DoC client MUST send
a DoC request constructed from the SvcParams including "docpath". A
rough construction algorithm could be as follows, going through the
provided records in order of their priority.
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* If the "alpn" SvcParam value for the service is "coap", construct
a CoAP request for CoAP over TLS. If it is "co", construct a CoAP
request for CoAP over DTLS. Any other SvcParamKeys specifying a
CoAP transport are out of the scope of this document.
* The destination address for the request should be taken from
additional information about the target, e.g., from an AAAA record
associated with the target name or from an "ipv6hint" SvcParam
value, or, as a fallback, by querying an address for the target
name of the SVCB record.
* The destination port for the address is taken from the "port"
SvcParam value, if present. Otherwise, take the default port of
the CoAP transport.
* Set the target name of SVCB record in the Uri-Host option.
* For each element in the CBOR sequence of the "docpath" SvcParam
value, add a Uri-Path option to the request.
* If a "port" SvcParam value is provided or if a port was queried,
and if either differs from the default port of the transport or
the destination port selected above, set that port in the Uri-Port
option.
* If the request constructed this way receives a response, use the
same SVCB record for construction of future DoC queries. If not,
or if the endpoint becomes unreachable, repeat with the SVCB
record with the next highest priority.
A more generalized construction algorithm can be found in
[I-D.ietf-core-transport-indication].
4. Basic Message Exchange
4.1. The "application/dns-message" Content-Format
This document defines a CoAP Content-Format identifier for the
Internet media type "application/dns-message" to be the mnemonic 553
— based on the port assignment of DNS. This media type is defined as
in Section 6 of [RFC8484], i.e., a single DNS message encoded in the
DNS on-the-wire format [RFC1035]. Both DoC client and DoC server
MUST be able to parse contents in the "application/dns-message"
format. For the purposes of this document, only OPCODE 0 (Query) is
supported for DNS messages. Future work might provide specifications
and considerations for other values of OPCODE. Unless another error
takes precedence, a DoC server uses RCODE = 4, NotImp [RFC1035], in
its response when it receives a query with an OPCODE that it does not
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implement (see also Section 4.3.3).
4.2. DNS Queries in CoAP Requests
A DoC client encodes a single DNS query in one or more CoAP request
messages that use the CoAP FETCH [RFC8132] method. Requests SHOULD
include an Accept option to indicate the type of content that can be
parsed in the response.
Since CoAP provides reliability at the message layer (e.g., through
Confirmable messages) the retransmission mechanism of the DNS
protocol as defined in [RFC1035] is not needed.
4.2.1. Request Format
When sending a CoAP request, a DoC client MUST include the DNS query
in the body of the CoAP request. As specified in Section 2.3.1 of
[RFC8132], the type of content of the body MUST be indicated using
the Content-Format option. This document specifies the usage of
Content-Format "application/dns-message" (for details, see
Section 4.1). A DoC server MUST be able to parse requests of
Content-Format "application/dns-message".
4.2.2. Support of CoAP Caching
The DoC client SHOULD set the ID field of the DNS header always to 0
to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to
the same DNS queries with a cache entry. This ensures that the CoAP
Cache-Key (see [RFC8132], Section 2) does not change when multiple
DNS queries for the same DNS data, carried in CoAP requests, are
issued.
4.2.3. Examples
The following example illustrates the usage of a CoAP message to
resolve "example.org. IN AAAA" based on the URI
"coaps://[2001:db8::1]/". The CoAP body is encoded in "application/
dns-message" Content Format. For better readability, we provide the
payload in a human-readable format. In the actual message, however,
it would be encoded in the binary message format (bytes printed in
hexadecimal representation) defined in [RFC1035].
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FETCH coaps://[2001:db8::1]/
Content-Format: application/dns-message
Accept: application/dns-message
Payload (binary):
00 00 01 20 00 01 00 00 00 00 00 00 07 65 78 61
6d 70 6c 65 03 6f 72 67 00 00 1c 00 01
Payload (human-readable):
;; ->>Header<<- opcode: QUERY, status: NOERROR, id: 0
;; flags: rd ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ARCOUNT: 0
;; QUESTION SECTION:
;example.org. IN AAAA
4.3. DNS Responses in CoAP Responses
Each DNS query-response pair is mapped to a CoAP request-response
operation. DNS responses are provided in the body of the CoAP
response. A DoC server MUST be able to produce responses in the
"application/dns-message" Content-Format (for details, see
Section 4.1) when requested. A DoC client MUST be able to understand
responses in "application/dns-message" format when it does not send
an Accept option. Any other response format than "application/dns-
message" MUST be indicated with the Content-Format option by the DoC
server.
4.3.1. Response Codes and Handling DNS and CoAP errors
A DNS response indicates either success or failure in the RCODE of
the DNS header (see Section 4.1.1 of [RFC1035]). It is RECOMMENDED
that CoAP responses that carry a parseable DNS response use a 2.05
(Content) response code.
CoAP responses using non-successful response codes MUST NOT contain a
DNS response and MUST only be used for errors in the CoAP layer or
when a request does not fulfill the requirements of the DoC protocol.
Communication errors with an upstream DNS server (e.g., timeouts)
MUST be indicated by including a DNS response with the appropriate
RCODE in a successful CoAP response, i.e., using a 2.xx response
code.
A DoC client might try to repeat a non-successful exchange unless
otherwise prohibited. The DoC client might also decide to repeat a
non-successful exchange with a different URI, for instance, when the
response indicates an unsupported Content-Format.
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4.3.2. Support of CoAP Caching
For reliability and energy saving measures, content decoupling, i.e.,
en-route caching on proxies, takes a far greater role than it does,
e.g., in HTTP. Likewise, CoAP makes it possible to use cache
validation to refresh stale cache entries to reduce the amount of
large response messages. For cache validation, CoAP implementations
regularly use hashing over the message content for ETag generation.
As such, the approach to guarantee the same cache key for DNS
responses as proposed in DoH ([RFC8484], Section 5.1) is not
sufficient and needs to be updated so that the TTLs in the response
are more often the same regardless of query time.
The DoC server MUST ensure that any sum of the Max-Age value of a
CoAP response and any TTL in the DNS response is less or equal to the
corresponding TTL received from an upstream DNS server. This also
includes the default Max-Age value of 60 seconds (see Section 5.10.5
of [RFC7252]) when no Max-Age option is provided. The DoC client
MUST then add the Max-Age value of the carrying CoAP response to all
TTLs in a DNS response on reception and use these calculated TTLs for
the associated records.
The RECOMMENDED algorithm for a DoC server to meet the requirement
for DoC is as follows: Set the Max-Age option of a response to the
minimum TTL of a DNS response and subtract this value from all TTLs
of that DNS response. This prevents expired records unintentionally
being served from an intermediate CoAP cache. Additionally, if the
ETag for cache validation is based on the content of the response, it
allows that ETag not to change. This then remains the case even if
the TTL values are updated by an upstream DNS cache. If only one
record set per DNS response is assumed, a simplification of this
algorithm is to just set all TTLs in the response to 0 and set the
TTLs at the DoC client to the value of the Max-Age option.
If shorter caching periods are plausible, e.g., if the RCODE of the
message indicates an error that should only be cached for a minimal
duration, the value for the Max-Age option SHOULD be set accordingly.
This value might be 0, but if the DoC server knows that the error
will persist, greater values are also conceivable, depending on the
projected duration of the error. Same goes for DNS responses that
for any reason do not carry any records with a TTL.
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4.3.3. Examples
The following example illustrates the response to the query
"example.org. IN AAAA record", recursion turned on. Successful
responses carry one answer record including address
2001:db8:1::1:2:3:4 and TTL 79689. As in Section 4.2.3, we use a
human-readable format for the payload, but skip the binary conversion
this time.
A successful response:
2.05 Content
Content-Format: application/dns-message
Max-Age: 58719
Payload (human-readable):
;; ->>Header<<- opcode: QUERY, status: NOERROR, id: 0
;; flags: qr rd ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ARCOUNT: 0
;; QUESTION SECTION:
;example.org. IN AAAA
;; ANSWER SECTION:
;example.org. 79689 IN AAAA 2001:db8:1::1:2:3:4
When a DNS error—NxDomain (RCODE = 3) for "does.not.exist" in this
case—is noted in the DNS response, the CoAP response still indicates
success.
2.05 Content
Content-Format: application/dns-message
Payload (human-readable):
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 0
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;does.not.exist. IN AAAA
As described in Section 4.1, a DoC server uses NotImp (RCODE = 4) if
it does not support an OPCODE—a DNS Update (OPCODE = 5) for
"example.org" in this case.
2.05 Content
Content-Format: application/dns-message
Payload (human-readable):
;; ->>Header<<- opcode: UPDATE, status: NOTIMP, id: 0
;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ARCOUNT: 0
;; QUERY SECTION:
;example.org. IN AAAA
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When an error occurs at the CoAP layer, the DoC server SHOULD respond
with an appropriate CoAP error, for instance 4.15 (Unsupported
Content-Format) if the Content-Format option in the request was not
set to "application/dns-message" and the Content-Format is not
otherwise supported by the server.
4.15 Unsupported Content-Format
[no payload]
5. CoAP/CoRE Integration
5.1. DNS Push
DNS Push requires additional overhead, which conflicts with
constrained resources. This is the reason why it is RECOMMENDED to
use CoAP Observe [RFC7641] instead of DNS Push in the DoC domain.
The DoC server SHOULD provide Observe capabilities on its DoC
resource and do as follows.
If the CoAP request indicates that the DoC client wants to observe a
resource record, a DoC server MAY use a DNS Subscribe message
[RFC8765] instead of a classic DNS query to fetch the information on
behalf of a DoC client. If this is not supported by the DoC server,
it MUST act as if the resource were not observable.
Whenever the DoC server receives a DNS Push message [RFC8765] from
the DNS infrastructure for an observed resource record, the DoC
server sends an appropriate Observe response to the DoC client.
If no more DoC clients observe a resource record for which the DoC
server has an open subscription, the DoC server MUST use a DNS
Unsubscribe message [RFC8765] to close its subscription to the
resource record as well.
A DoC server can still provide Observe capabilities to its DoC
resource without providing this proxying to DNS Push, e.g., if it
receives new information on a record through other means.
5.2. OSCORE
It is RECOMMENDED to carry DNS messages protected using OSCORE
[RFC8613] between the DoC client and the DoC server. The
establishment and maintenance of the OSCORE Security Context is out
of the scope of this document.
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If cache retrieval of OSCORE responses is desired, it can be
achieved, for instance, by using the method defined in
[I-D.amsuess-core-cachable-oscore]. This has, however, implications
on message sizes and security properties, which are compiled in that
document.
5.3. Mapping DoC to DoH
This document provides no specification on how to map between DoC and
DoH, e.g., at a CoAP-to-HTTP-proxy. In fact, such a direct mapping
is NOT RECOMMENDED: rewriting the FETCH method (Section 4.2) and the
TTL rewriting (Section 4.3.2) as specified in this draft would be
non-trivial. It is RECOMMENDED to use a DNS forwarder to map between
DoC and DoH, as would be the case for mapping between any other pair
of DNS transports.
6. Considerations for Unprotected Use
The use of DoC without confidentiality and integrity protection is
NOT RECOMMENDED. Without confidential communication, many possible
attacks need to be evaluated in the context of the application's
threat model. This includes known threats for unprotected DNS
[RFC3833] [RFC7626] and CoAP Section 11 of [RFC7252]. But there is
also an attack that is mitigated even by unprotected DNS over UDP:
The random ID of the DNS header affords some protection against off-
path cache poisoning attacks. Note, however, that this particular
threat can also be mitigated by using random large token values in
the CoAP request.
7. Implementation Status
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
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According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
// RFC Ed.: Please remove this section before publication. When
// deleting this section, please also remove RFC7942 from the
// references of this document.
7.1. DoC Client
The authors of this document provide a DoC client implementation
available in the IoT operating system RIOT (https://doc.riot-os.org/
group__net__gcoap__dns.html).
Level of maturity: production
Version compatibility: draft-ietf-core-dns-over-coap-13
License: LGPL-2.1
Contact information: Martine S. Lenders <martine.lenders@tu-
dresden.de>
Last update of this information: September 2024
7.2. DoC Server
The authors of this document provide a DoC server implementation in
Python (https://github.com/anr-bmbf-pivot/aiodnsprox).
Level of maturity: production
Version compatibility: draft-ietf-core-dns-over-coap-13
License: MIT
Contact information: Martine S. Lenders <martine.lenders@tu-
dresden.de>
Last update of this information: September 2024
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8. Security Considerations
General CoAP security considerations in Section 11 of [RFC7252] apply
to DoC. Additionally, DoC uses request patterns that require the
maintenance of long-lived security contexts. Section 2.6 of
[I-D.ietf-core-corr-clar] provides insights on what can be done when
those are resumed from a new endpoint.
When using unprotected CoAP (see Section 6), setting the ID of a DNS
message to 0 as specified in Section 4.2.2 opens the DNS cache of a
DoC client to cache poisoning attacks via response spoofing. This
document requires an unpredictable CoAP token in each DoC query from
the client when CoAP is not secured to mitigate such an attack over
DoC (see Section 6).
For secure communication via DTLS or OSCORE the impact of a fixed ID
on security is limited, as both harden against injecting spoofed
responses. Consequently, the ID of the DNS message can be set to 0
without any concern in order to leverage the advantages of CoAP
caching.
A DoC client must be aware that the DoC server may communicate
unprotected with the upstream DNS infrastructure, e.g., using DNS
over UDP. DoC can only guarantee confidentiality and integrity of
communication between parties for which the security context is
exchanged. The DoC server may use another security context to
communicate upstream with both confidentiality and integrity (e.g.,
DNS over QUIC [RFC9250]) or with just integrity (e.g., DNSSEC
[RFC9364]), but, while recommended, this is opaque to the DoC client
on the protocol level.
A DoC client may not be able to perform DNSSEC validation, e.g., due
to code size constraints, or due to size of the responses. It may
trust its DoC server to perform DNSSEC validation; how that trust is
expressed is out of the scope of this document. A DoC client may be,
for instance, configured to use a particular credential by which it
recognizes an eligible DoC server. That information can also imply
trust in the DNSSEC validation by that server.
9. IANA Considerations
// RFC Ed.: throughout this section, please replace RFC-XXXX with the
// RFC number of this specification and remove this note.
This document has the following actions for IANA.
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9.1. CoAP Content-Formats Registry
IANA is requested to assign a CoAP Content-Format ID for the DNS
message media type in the "CoAP Content-Formats" registry, within the
"Constrained RESTful Environments (CoRE) Parameters" registry group
[RFC7252], corresponding to the "application/dns-message" media type
from the "Media Types" registry (see [RFC8484]).
Content Type: application/dns-message
Content Coding: -
ID: 553 (suggested)
Reference: [RFC8484][RFC-XXXX, Section 4.1]
9.2. DNS Service Bindings (SVCB) Registry
IANA is requested to add the following entry to the "Service
Parameter Keys (SvcParamKeys)" registry within the "DNS Service
Bindings (SVCB)" registry group. The definition of this parameter
can be found in Section 3.
+=============+=========+===============+============+============+
| Number | Name | Meaning | Change | Reference |
| | | | Controller | |
+=============+=========+===============+============+============+
| 10 | docpath | DNS over CoAP | IETF | [RFC-XXXX, |
| (suggested) | | resource path | | Section 3] |
+-------------+---------+---------------+------------+------------+
Table 1: Values for SvcParamKeys
9.3. Resource Type (rt=) Link Target Attribute Values Registry
IANA is requested to add a new Resource Type (rt=) Link Target
Attribute "core.dns" to the "Resource Type (rt=) Link Target
Attribute Values" registry within the "Constrained RESTful
Environments (CoRE) Parameters" registry group.
Value: core.dns
Description: DNS over CoAP resource.
Reference: [RFC-XXXX, Section 3]
10. References
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10.1. Normative References
[I-D.ietf-cbor-edn-literals]
Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
literals-16, 8 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-literals-16>.
[I-D.ietf-core-coap-dtls-alpn]
Lenders, M. S., Amsüss, C., Schmidt, T. C., and M.
Wählisch, "ALPN ID Specification for CoAP over DTLS", Work
in Progress, Internet-Draft, draft-ietf-core-coap-dtls-
alpn-02, 3 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-dtls-alpn-02>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/rfc/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/rfc/rfc7641>.
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[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/rfc/rfc7959>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/rfc/rfc8132>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/rfc/rfc8613>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/rfc/rfc8742>.
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/rfc/rfc8765>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
10.2. Informative References
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[DoC-paper]
Lenders, M., Amsüss, C., Gündogan, C., Nawrocki, M.,
Schmidt, T., and M. Wählisch, "Securing Name Resolution in
the IoT: DNS over CoAP", Association for Computing
Machinery (ACM), Proceedings of the ACM on Networking vol.
1, no. CoNEXT2, pp. 1-25, DOI 10.1145/3609423, September
2023, <https://doi.org/10.1145/3609423>.
[I-D.amsuess-core-cachable-oscore]
Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
Progress, Internet-Draft, draft-amsuess-core-cachable-
oscore-10, 8 January 2025,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
cachable-oscore-10>.
[I-D.ietf-core-corr-clar]
Bormann, C., "Constrained Application Protocol (CoAP):
Corrections and Clarifications", Work in Progress,
Internet-Draft, draft-ietf-core-corr-clar-01, 18 December
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
core-corr-clar-01>.
[I-D.ietf-core-href]
Bormann, C. and H. Birkholz, "Constrained Resource
Identifiers", Work in Progress, Internet-Draft, draft-
ietf-core-href-18, 3 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
href-18>.
[I-D.ietf-core-transport-indication]
Amsüss, C. and M. S. Lenders, "CoAP Transport Indication",
Work in Progress, Internet-Draft, draft-ietf-core-
transport-indication-07, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
transport-indication-07>.
[I-D.ietf-iotops-7228bis]
Bormann, C., Ersue, M., Keränen, A., and C. Gomez,
"Terminology for Constrained-Node Networks", Work in
Progress, Internet-Draft, draft-ietf-iotops-7228bis-01, 8
January 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-iotops-7228bis-01>.
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[I-D.lenders-core-dnr]
Lenders, M. S., Amsüss, C., Schmidt, T. C., and M.
Wählisch, "Discovery of Network-designated OSCORE-based
Resolvers: Problem Statement", Work in Progress, Internet-
Draft, draft-lenders-core-dnr-05, 3 March 2025,
<https://datatracker.ietf.org/doc/html/draft-lenders-core-
dnr-05>.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, DOI 10.17487/RFC3833, August
2004, <https://www.rfc-editor.org/rfc/rfc3833>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/rfc6690>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/rfc/rfc7228>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<https://www.rfc-editor.org/rfc/rfc7626>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/rfc/rfc7942>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/rfc/rfc8094>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 8499, DOI 10.17487/RFC8499, January
2019, <https://www.rfc-editor.org/rfc/rfc8499>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/rfc/rfc9176>.
[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/rfc/rfc9250>.
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[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/rfc/rfc9364>.
[RFC9460] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.
[RFC9461] Schwartz, B., "Service Binding Mapping for DNS Servers",
RFC 9461, DOI 10.17487/RFC9461, November 2023,
<https://www.rfc-editor.org/rfc/rfc9461>.
[RFC9462] Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", RFC 9462,
DOI 10.17487/RFC9462, November 2023,
<https://www.rfc-editor.org/rfc/rfc9462>.
[RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
and T. Jensen, "DHCP and Router Advertisement Options for
the Discovery of Network-designated Resolvers (DNR)",
RFC 9463, DOI 10.17487/RFC9463, November 2023,
<https://www.rfc-editor.org/rfc/rfc9463>.
Appendix A. Evaluation
The authors of this document presented the design, implementation,
and analysis of DoC in their paper "Securing Name Resolution in the
IoT: DNS over CoAP" [DoC-paper].
Appendix B. Change Log
// RFC Ed.: Please remove this section before publication.
B.1. Since [draft-ietf-core-dns-over-coap-12]
* Address Esko's review
* Address Marcos's review
* Address Mikolai's review
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B.2. Since draft-ietf-core-dns-over-coap-10
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-10)
* Replace imprecise or wrong terms:
- disjunct => distinct
- unencrypted CoAP => unprotected CoAP
- security mode => confidential communication
* Pull in definition of CBOR sequences and their EDN
* Fix broken external section references
* Define terminology for "upstream DNS infrastructure" and "upstream
DNS server"
* Fix wording on DNS error handling
* Clarify that any OpCode beyond 0 is not supported for now and
remove now redundant DNS Upgrade section as a consequence
* Clarify that the DoC/DoH mapping is what is NOT RECOMMENDED
* Avoid use of undefined term “CoAP resource identifier”
* Discuss Max-Age option value in an error case
* Add human-readable format to examples
* General language check pass
B.3. Since draft-ietf-core-dns-over-coap-09
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-09)
* Update SVCB SvcParamKey
* Update corr-clar reference
* Add reference to DNS Update [RFC2136]
(https://datatracker.ietf.org/doc/html/rfc2136), clarify that it
is currently not considered
* Add to security considerations: unprotected upstream DNS and
DNSSEC
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B.4. Since draft-ietf-core-dns-over-coap-08
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-08)
* Update Cenk's Affiliation
B.5. Since draft-ietf-core-dns-over-coap-07
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-07)
* Address IANA early review #1368678
* Update normative reference to CoAP over DTLS alpn SvcParam
* Add missing DTLSv1.2 reference
* Security considerations: Point into corr-clar-future
* Implementation Status: Update to current version
B.6. Since draft-ietf-core-dns-over-coap-06
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-06)
* Add "docpath" SVCB ParamKey definition
* IANA fixes
- Use new column names (see Errata 4954)
- Add reference to RFC 8484 for application/dns-message Media
Type
- IANA: unify self references
B.7. Since draft-ietf-core-dns-over-coap-05
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-05)
* Add references to relevant SVCB/DNR RFCs and drafts
B.8. Since draft-ietf-core-dns-over-coap-04
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-04)
* Add note on cacheable OSCORE
* Address early IANA review
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B.9. Since draft-ietf-core-dns-over-coap-03
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-03)
* Amended Introduction with short contextualization of constrained
environments
* Add Appendix A on evaluation
B.10. Since draft-ietf-core-dns-over-coap-02
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-02)
* Move implementation details to Implementation Status (in
accordance with [RFC7942])
* Recommend root path to keep the CoAP options small
* Set Content-Format for application/dns-message to 553
* SVCB/DNR: Move to Server Selection Section but leave TBD based on
DNSOP discussion for now
* Clarify that DoC and DoH are distinct
* Clarify mapping between DoC and DoH
* Update considerations on unprotected use
* Don't call OSCORE end-to-end encrypted
B.11. Since draft-ietf-core-dns-over-coap-01
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-01)
* Specify DoC server role in terms of DNS terminology
* Clarify communication of DoC to DNS infrastructure is agnostic of
the transport
* Add subsection on how to implement DNS Push in DoC
* Add appendix on reference implementation
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B.12. Since draft-ietf-core-dns-over-coap-00
(https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
coap-00)
* SVGify ASCII art
* Move section on "DoC Server Considerations" (was Section 5.1) to
its own draft (draft-lenders-dns-cns
(https://datatracker.ietf.org/doc/draft-lenders-dns-cns/))
* Replace layer violating statement for CON with statement of fact
* Add security considerations on ID=0
B.13. Since draft-lenders-dns-over-coap-04
(https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
coap-04)
* Removed change log of draft-lenders-dns-over-coap
Acknowledgments
The authors of this document want to thank Carsten Bormann, Esko
Dijk, Thomas Fossati, Mikolai Gütschow, Ben Schwartz, Marco Tiloca,
and Tim Wicinski for their feedback and comments.
Authors' Addresses
Martine Sophie Lenders
TUD Dresden University of Technology
Helmholtzstr. 10
D-01069 Dresden
Germany
Email: martine.lenders@tu-dresden.de
Christian Amsüss
Email: christian@amsuess.com
Cenk Gündoğan
NeuralAgent GmbH
Mies-van-der-Rohe-Straße 6
D-80807 Munich
Germany
Email: cenk.gundogan@neuralagent.ai
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Thomas C. Schmidt
HAW Hamburg
Berliner Tor 7
D-20099 Hamburg
Germany
Email: t.schmidt@haw-hamburg.de
Matthias Wählisch
TUD Dresden University of Technology & Barkhausen Institut
Helmholtzstr. 10
D-01069 Dresden
Germany
Email: m.waehlisch@tu-dresden.de
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