Internet-Draft DoC July 2022
Lenders, et al. Expires 12 January 2023 [Page]
Intended Status:
Standards Track
M. S. Lenders
FU Berlin
C. Amsüss
C. Gündoğan
HAW Hamburg
T. C. Schmidt
HAW Hamburg
M. Wählisch
FU Berlin

DNS over CoAP (DoC)


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

Discussion Venues

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

Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (, which is archived at

Source for this draft and an issue tracker can be found at

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 12 January 2023.

1. Introduction

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

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

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

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

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

Figure 1: Basic DoC architecture

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

2. Terminology

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

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

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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Selection of a DoC Server

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

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

4. Basic Message Exchange

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

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

4.2. DNS Queries in CoAP Requests

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

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

4.2.1. Request Format

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

4.2.2. Support of CoAP Caching

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

4.2.3. Examples

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

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

4.3. DNS Responses in CoAP Responses

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

4.3.1. Response Codes and Handling DNS and CoAP errors

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

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

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

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

4.3.2. Support of CoAP Caching

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

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

4.3.3. Examples

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

A successful response:

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

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

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

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

5. CoAP/CoRE Integration

5.1. DoC Server Considerations

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

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

5.2. Observing the DNS Resource

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


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

6. Considerations for Unencrypted Use

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

8. IANA Considerations

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

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

Media-Type: application/dns-message

Encoding: -


Reference: [TBD-this-spec]

8.2. New "core.dns" Resource Type

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

Attribute Value: core.dns

Description: DNS over CoAP resource.

Reference: [TBD-this-spec] Section 3

9. References

9.1. Normative References

Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, , <>.
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, , <>.
Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, , <>.

9.2. Informative References

Boucadair, M., Reddy, T., Wing, D., Cook, N., and T. Jensen, "DHCP and Router Advertisement Options for the Discovery of Network-designated Resolvers (DNR)", Work in Progress, Internet-Draft, draft-ietf-add-dnr-11, , <>.
Bormann, C. and H. Birkholz, "Constrained Resource Identifiers", Work in Progress, Internet-Draft, draft-ietf-core-href-10, , <>.
Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, , <>.
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <>.
Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, , <>.
Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, , <>.
Reddy, T., Wing, D., and P. Patil, "DNS over Datagram Transport Layer Security (DTLS)", RFC 8094, DOI 10.17487/RFC8094, , <>.
Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <>.
Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., Lemon, T., and T. Pusateri, "DNS Stateful Operations", RFC 8490, DOI 10.17487/RFC8490, , <>.
Akiya, N., Swallow, G., Litkowski, S., Decraene, B., Drake, J., and M. Chen, "Label Switched Path (LSP) Ping and Traceroute Multipath Support for Link Aggregation Group (LAG) Interfaces", RFC 8611, DOI 10.17487/RFC8611, , <>.
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, , <>.
Huitema, C., Dickinson, S., and A. Mankin, "DNS over Dedicated QUIC Connections", RFC 9250, DOI 10.17487/RFC9250, , <>.

Appendix A. Change Log

A.1. Since draft-lenders-dns-over-coap-03

  • Remove TBD on CoRE resource directory in Section 1. Optional usage of CoRE-RD is already discussed in Section 3
  • Remove TBD on "request text duplication" in Appendix A. This issue will be handled in a separate draft on a new CBOR-based content format
  • Add note on OSCORE in Section 5.3
  • Add note on Observe in Section 5.2
  • Change title from "DNS queries over CoAP" to "DNS over CoAP"
  • Mention "application/dns-message" parsing requirement in Section 4.1
  • Remove obvious CoAP behavior restatements from Section 4
  • Remove ETag specifications in Section 4.3.2
  • Caching: specify Max-Age / TTL calculation including simplified version in Section 4.3.2
  • Remove subsection on "Proxies and caching". All relevant things on caching were discussed in Section 4.2.2 and Section 4.3.2, considerations on proxy / DoC server behavior and late responses are general CoAP problems.
  • Be more precise when Confirmable (CON) messages SHOULD be used in Section 4.2

A.2. Since draft-lenders-dns-over-coap-02

  • Clarify server selection to be out-of-band and define "core.dns" resource type in Section 3 and Section 8.2
  • Add message manipulation considerations for DoC servers in Section 5.1
  • Update Considerations for Unencrypted Use in Section 6

A.3. Since draft-lenders-dns-over-coap-01

  • Remove GET and POST methods
  • Add note on ETag and response codes
  • Provide requirement conflict for DNS over QUIC
  • Clarify Content-Format / Accept handling

A.4. Since draft-lenders-dns-over-coap-00

A.5. Since draft-lenders-dns-over-coaps-00

  • Clarification in abstract that both DTLS and OSCORE can be used as secure transport
  • Restructuring of Section 4:

  • Fix nits and typos


TODO acknowledge.

Authors' Addresses

Martine Sophie Lenders
Freie Universität Berlin
Christian Amsüss
Cenk Gündoğan
HAW Hamburg
Thomas C. Schmidt
HAW Hamburg
Matthias Wählisch
Freie Universität Berlin