| Internet-Draft | DoC | March 2022 |
| Lenders, et al. | Expires 8 September 2022 | [Page] |
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).¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on TODO¶
Source for this draft and an issue tracker can be found at https://github.com/anr-bmbf-pivot/draft-dns-over-coap.¶
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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 [I-D.ietf-dprive-dnsoquic]), 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]
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.¶
TBD: additional feature sets of CoAP/CoRE¶
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.¶
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 [I-D.ietf-core-resource-directory], 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.¶
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].¶
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.¶
To enable reliable message exchange, the CoAP request SHOULD be carried in a Confirmable (CON) message.¶
When sending a CoAP request, a DoC client MUST include the DNS query in the body (i.e. the payload, or the concatenated payloads) 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).¶
If block-wise transfer [RFC7959] is supported by the client, more than one CoAP request message MAY be used. If more than one CoAP request message is used to encode the DNS query, it must be chained together using the Block1 option in those CoAP requests.¶
The FETCH request is sent to the URI specified in Section 3.¶
A DoC server MUST be able to parse requests of Content-Format "application/dns-message".¶
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.¶
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.¶
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]
¶
Each DNS query-response pair is mapped to a CoAP REST request-response operation, which may consist of several CoAP request-response pairs if block-wise transfer is involved. DNS responses are provided in the body (i.e. the payload, or the concatenated payloads) of the CoAP response. A DoC server MUST indicate the type of content of the body using the Content-Format option, and 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.¶
If supported, a DoC server MAY transfer the DNS response in more than one CoAP responses using the Block2 option [RFC7959].¶
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 any payload and may 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.¶
It is RECOMMENDED to set the Max-Age option of a response to the minimum TTL in the Answer section of a DNS response. This prevents expired records unintentionally being served from a CoAP cache.¶
It is RECOMMENDED that DoC servers set an ETag option on large responses (TBD: more concrete guidance) that have a short Max-Age relative to the expected clients' caching time. Thus, clients that need to revalidate a response can do so using the established ETag mechanism. With responses large enough to be fragmented, it's best practice for servers to set an ETag anyway. As specified in [RFC7252] and [RFC8132], if the response associated with the ETag is still valid, the response uses the "2.03 Valid" code and consequently carries no payload.¶
The following examples illustrate the replies 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 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.¶
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.¶
TBD:¶
General CoAP proxy problem, but what to do when DoC server is a DNS proxy, response came not yet in but retransmission by DoC client was received (see Figure 2)¶
send empty ACK (maybe move to best practices appendix)¶
DoC client DoC proxy DNS server
| CoAP req [rt 1] | |
|------------------>| DNS query [rt 1] |
| |------------------->|
| CoAP req [rt 2] | |
|------------------>| DNS resp |
| CoAP resp |<-------------------|
|<------------------| |
| | |
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.¶
TODO Security¶
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: -¶
Id: TBD¶
Reference: [TBD-this-spec]¶
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.¶
TBD:¶
TODO acknowledge.¶