| Internet-Draft | DoC | July 2025 |
| Lenders, et al. | Expires 8 January 2026 | [Page] |
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).¶
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.¶
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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/.¶
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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 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 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.¶
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.¶
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].¶
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 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.¶
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 [RFC1035].¶
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 SHOULD only be done from a trusted source.¶
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.¶
A DoC server can also be discovered using Service Binding (SVCB) Resource Records (RR) [RFC9460] [RFC9461] or Discovery of Network-designated Resolvers (DNR) Service Parameters [RFC9463]. [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. Because the ALPN extension is only defined for (D)TLS, these mechanisms cannot be used for DoC servers which 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]) for security. Specifying an alternate discovery mechanism is out of scope of this specification. [I-D.lenders-core-dnr] provides further exploration of the challenges here.¶
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 being prepared in [I-D.ietf-core-transport-indication], generalizing mechanisms that are introduced in this document for discovery of DoC.¶
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-sperated list (Appendix A.1 of [RFC9460]) of 0 or more docpath-segments. The root path "/" is represented by 0 docpath-segments, i.e., an empty list. The same considerations for the "," and "" characters in docpath-segments for zone-file implementations as for the alpn-ids in an "alpn" SvcParam MAY 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, the length octet, and 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, 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.¶
If the "alpn" SvcParam value for the service is "coap", a CoAP request for CoAP over TLS MUST be constructed. If it is "co", a CoAP request for CoAP over DTLS MUST be constructed. 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, e.g., from an AAAA record associated with the target name or from an "ipv6hint" SvcParam value. As a fallback, an address MAY be queried for the target name of the SVCB record.¶
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 SHOULD be repeated with the SVCB record with the next highest priority.¶
A more generalized construction algorithm for any CoAP request can be found in [I-D.ietf-core-transport-indication].¶
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.¶
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:¶
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 )
¶
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" Content-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 to a query with an OPCODE that it does not implement (see also Section 4.3.3).¶
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 [RFC1035] is not needed.¶
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".¶
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.¶
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 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¶
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. A DoC client MUST be able to understand responses in the "application/dns-message" Content-Format when it does not send an Accept option. Any response Content-Format other than "application/dns-message" MUST be indicated with the Content-Format option by the DoC server.¶
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. 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.¶
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 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.¶
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, ARCOUNT: 0 ;; QUESTION SECTION: ;example.org. IN AAAA ;; ANSWER SECTION: ;example.org. 79689 IN AAAA 2001:db8:1:0:1:2:3:4¶
When a DNS error – NxDomain (RCODE = 3) for "does.not.exist" in this case – is noted in the DNS response, the CoAP response still indicates success.¶
2.05 Content Content-Format: 553 (application/dns-message) Payload (human-readable): ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 0 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;does.not.exist. IN AAAA¶
As described in Section 4.1, a DoC server uses NotImp (RCODE = 4) if it does not support an OPCODE—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, ARCOUNT: 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]¶
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. 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 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 DoC resource were not observable.¶
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.¶
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 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.¶
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.¶
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.¶
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 attacks.¶
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.¶
The authors of this document provide a DoC client implementation available in the IoT operating system RIOT.¶
The authors of this document provide a DoC server implementation in Python.¶
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) or DTLS ([RFC6347], Section 5 and [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).¶
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]), but, while recommended, this is opaque to the DoC client on the protocol level. Record integrity can also be ensured upstream using, e.g., DNSSEC [RFC9364].¶
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 server.¶
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.¶
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)¶
Reference: [RFC8484] and [RFC-XXXX], Section 4.1¶
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 Controller | Reference |
|---|---|---|---|---|
| 10 (suggested) | docpath | DNS over CoAP resource path | IETF | [RFC-XXXX], Section 3 |
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.¶
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].¶
RFC Ed.: Please remove this section before publication.¶
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:¶
Address Tsvart Review¶
Prefer ADN for Uri-Host; don't prescribe how it is set¶
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¶
Address shepherd review¶
Replace imprecise or wrong terms:¶
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¶
Update Cenk's Affiliation¶
Add references to relevant SVCB/DNR RFCs and drafts¶
Amended Introduction with short contextualization of constrained environments¶
Add Appendix A on evaluation¶
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¶
SVGify ASCII art¶
Move section on "DoC Server Considerations" (was Section 5.1) to its own draft (draft-lenders-dns-cns)¶
Replace layer violating statement for CON with statement of fact¶
Add security considerations on ID=0¶
Removed change log of draft-lenders-dns-over-coap¶
The authors of this document want to thank Mike Bishop, Carsten Bormann, Elwyn B. Davies, Esko Dijk, Thomas Fossati, Mikolai Gütschow, Todd Herr, Tommy Pauly, Ben Schwartz, Marco Tiloca, and Tim Wicinski for their feedback and comments.¶