]> TUD Dresden University of Technology

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NeuralAgent GmbH

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TUD Dresden University of Technology & Barkhausen Institut

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Applications CoRE Internet-Draft CoRE CoAP DNS This document defines a protocol for exchanging DNS messages over the Constrained Application Protocol (CoAP). These CoAP messages can be protected by (D)TLS-Secured CoAP (CoAPS) or Object Security for Constrained RESTful Environments (OSCORE) to provide encrypted DNS message exchange for constrained devices in the Internet of Things (IoT). About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the CoRE Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document defines DNS over CoAP (DoC), a protocol to send DNS queries and get DNS responses over the Constrained Application Protocol (CoAP) . Each DNS query-response pair is mapped into a CoAP message exchange. Each CoAP message can be secured by DTLS or Object Security for Constrained RESTful Environments (OSCORE) but also TLS to ensure message integrity and confidentiality. The application use case of DoC is inspired by DNS over HTTPS (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 RFC Ed.: Please remove the reference and replace it with throughout the document in case 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 . 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 , which solves the Path MTU problem of DNS over DTLS (see ); 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 or DNS over QUIC ), DoC allows for lightweight message protection based on OSCORE.

Basic DoC architecture | DoC |--- --- --- --->| DNS | |Client|<----------------|Server|<--- --- --- ---| Infrastructure | +------+ CoAP response +------+ DNS response '---------------' [DNS response] \ /\ / '-----DNS over CoAP----' '--DNS over UDP/HTTPS/QUIC/...--' ]]>

The most important components of DoC can be seen in : 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 , DNS over HTTPS , or DNS over QUIC 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 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 .

Terminology and Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 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 . As such, the DoC server communicates with an "upstream DNS infrastructure" or a single "upstream DNS server". A "DoC resource" is a CoAP resource 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 . describes that "Constrained RESTful Environments (CoRE)" realize the Representational State Transfer (REST) architecture in a suitable form for such constrained nodes. The terms "payload" and "body" are used as defined in . 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 .

Selection of a DoC Server While there is no path specified for the DoC resource, it is RECOMMENDED to use the root path "/" to keep the CoAP requests small. The DoC client needs to know the DoC server and the DoC resource at the DoC server. Possible options to assure this could be manual configuration of a Uniform Resource Identifier (URI) or Constrained Resource Identifier (CRI) , or automatic configuration, e.g., using a CoRE resource directory , DHCP or Router Advertisement options , or discovery of designated resolvers . Automatic configuration SHOULD only be done from a trusted source.

Discovery by Resource Type For discovery of the DoC resource through a link mechanism that allows describing a resource type (e.g., the Resource Type attribute in ), this document introduces the resource type "core.dns". It can be used to identify a generic DNS resolver that is available to the client.

Discovery using SVCB Resource Records or DNR A DoC server can also be discovered using Service Binding (SVCB) Resource Records (RR) or Discovery of Network-designated Resolvers (DNR) Service Parameters . defines the Application-Layer Protocol Negotiation (ALPN) ID for CoAP over TLS servers and 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 and Ephemeral Diffie-Hellman Over COSE (EDHOC) (with COSE abbreviating "Concise Binary Object Notation (CBOR) Object Signing and Encryption" ) for security. Specifying an alternate discovery mechanism is out of scope of this specification. provides further exploration of the challenges here. This document is not an SVCB mapping document for the CoAP schemes as defined in . A full SVCB mapping is being prepared in , 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 : Note that this restricts the length of each docpath-segment to at most 255 octets. Paths with longer segments cannot be advertised with the "docpath" SvcParam and are thus NOT RECOMMENDED for the path to the DoC resource. The presentation format value of "docpath" SHALL be a comma-separated list () 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 (). The wire-format value for "docpath" consists of 0 or more sequences of octets prefixed by their respective length as a single octet. We call this single octets the length octet. The length octet and the corresponding sequence form a length-value pair. These length-value pairs are concatenated to form the SvcParamValue. These pairs MUST exactly fill the SvcParamValue; otherwise, the SvcParamValue is malformed. Each such length-value pair represents one segment of the absolute path to the DoC resource. The root path "/" is represented by 0 length-value pairs, i.e., SvcParamValue length 0. Note that this format uses the same encoding as the "alpn" SvcParam and can reuse the decoders and encoders for that SvcParam with the adaption that a length of zero is allowed. As long as each docpath-segment is of length 0 and 24 octets, it is easily transferred into the path representation in CRIs by masking each length octet with the CBOR text string major type 3 (0x60 as an octet, see ). 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 ). 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, 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) , 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 .

Examples RFC Ed.: Since the number for "docpath" was not assigned at the time of writing, we used the hex ff 0a (in decimal 65290; from the private use range of SvcParamKeys) throughout this section. Before publication, please replace ff 0a with the hexadecimal representation of the final value assigned by IANA in this section. 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): 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): Second, an examples for the path "/n/s" (the last 8 bytes ff 0a 00 04 01 6e 01 73): If the server also provides DNS over HTTPS, "dohpath" and "docpath" MAY co-exist:

Basic Message Exchange

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 , i.e., a single DNS message encoded in the DNS on-the-wire format . 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 , in its response to a query with an OPCODE that it does not implement (see also ).

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 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 is not needed.

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 , 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 ). A DoC server MUST be able to parse requests of Content-Format "application/dns-message".

Support of CoAP Caching The DoC client SHOULD set the ID field of the DNS header to 0 to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to the same DNS queries with a cache entry. This ensures that the CoAP Cache-Key (see ) 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.

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 the "application/dns-message" Content-Format. >Header<<- opcode: QUERY, status: NOERROR, id: 0 ;; flags: rd ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ARCOUNT: 0 ;; QUESTION SECTION: ;example.org. IN AAAA ]]>

DNS Responses in CoAP Responses Each DNS query-response pair is mapped to a CoAP request-response operation. DNS responses are provided in the body of the CoAP response, i.e., it is also possible to transfer them using block-wise transfer . A DoC server MUST be able to produce responses in the "application/dns-message" Content-Format (for details, see ) 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.

Response Codes and Handling DNS and CoAP errors A DNS response indicates either success or failure in the RCODE of the DNS header (see ). 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.

Support of CoAP Caching For reliability and energy saving measures, content decoupling (such as en-route caching on proxies) takes a far greater role than it does in HTTP. Likewise, CoAP makes it possible to use cache validation to refresh stale cache entries to reduce the number of large response messages. For cache validation, CoAP implementations regularly use hashing over the message content for ETag generation (see ). As such, the approach to guarantee the same cache key for DNS responses as proposed in DoH () 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 ) 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.

Examples The following example illustrates the response to the query "example.org. IN AAAA record", with recursion turned on. Successful responses carry one answer record including address 2001:db8:1:0:1:2:3:4 and TTL 79689. A successful response: >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. >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 , 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. >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.

Interaction with other CoAP and CoRE Features

DNS Push Notifications and CoAP Observe DNS Push Notifications 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 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.

OSCORE It is RECOMMENDED to carry DNS messages protected using OSCORE 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. describes a method to allow cache retrieval of OSCORE responses and discusses the corresponding implications on message sizes and security properties.

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 () and the TTL rewriting () 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.

Considerations for Unprotected Use The use of DoC without confidentiality and integrity protection is NOT RECOMMENDED. Without secure communication, many possible attacks need to be evaluated in the context of the application's threat model. This includes known threats for unprotected DNS and CoAP . While DoC does not use the random ID of the DNS header (see ), 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.

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 . 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 , "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.

DoC Client The authors of this document provide a DoC client implementation available in the IoT operating system RIOT.

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

DoC Server The authors of this document provide a DoC server implementation in Python.

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

Security Considerations General CoAP security considerations () apply to DoC. DoC also inherits the security considerations of the protocols used for secure communication, e.g., OSCORE () or DTLS ( and ). Additionally, DoC uses request patterns that require the maintenance of long-lived security contexts. provides insights on what can be done when those are resumed from a new endpoint. Though DTLS v1.2 was obsoleteted by DTLS v1.3 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 ), setting the ID of a DNS message to 0 as specified in 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 ). 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 ), 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 . 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.

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.

CoAP Content-Formats Registry IANA is requested to assign a CoAP Content-Format ID for the "application/dns-message" media type in the "CoAP Content-Formats" registry, within the "Constrained RESTful Environments (CoRE) Parameters" registry group , corresponding to the "application/dns-message" media type from the "Media Types" registry (see ). Content Type: application/dns-message Content Coding: - ID: 553 (suggested) Reference: and [RFC-XXXX],

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 . Values for SvcParamKeys

Number	Name	Meaning	Change Controller	Reference
10 (suggested)	docpath	DNS over CoAP resource path	IETF	[RFC-XXXX],

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],

References Normative References This RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System. It obsoletes RFC-883. This memo documents the details of the domain name client - server communication. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. The Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred. Instead of relying on IP fragmentation, this specification extends basic CoAP with a pair of "Block" options for transferring multiple blocks of information from a resource representation in multiple request-response pairs. In many important cases, the Block options enable a server to be truly stateless: the server can handle each block transfer separately, with no need for a connection setup or other server-side memory of previous block transfers. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252. The methods defined in RFC 7252 for the Constrained Application Protocol (CoAP) only allow access to a complete resource, not to parts of a resource. In case of resources with larger or complex data, or in situations where resource continuity is required, replacing or requesting the whole resource is undesirable. Several applications using CoAP need to access parts of the resources. This specification defines the new CoAP methods, FETCH, PATCH, and iPATCH, which are used to access and update parts of a resource. The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or Transport Layer Security (TLS). This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport. This document defines a protocol for sending DNS queries and getting DNS responses over HTTPS. Each DNS query-response pair is mapped into an HTTP exchange. This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. This document describes the Concise Binary Object Representation (CBOR) Sequence format and associated media type "application/cbor-seq". A CBOR Sequence consists of any number of encoded CBOR data items, simply concatenated in sequence. Structured syntax suffixes for media types allow other media types to build on them and make it explicit that they are built on an existing media type as their foundation. This specification defines and registers "+cbor-seq" as a structured syntax suffix for CBOR Sequences. The Domain Name System (DNS) was designed to return matching records efficiently for queries for data that are relatively static. When those records change frequently, DNS is still efficient at returning the updated results when polled, as long as the polling rate is not too high. But, there exists no mechanism for a client to be asynchronously notified when these changes occur. This document defines a mechanism for a client to be notified of such changes to DNS records, called DNS Push Notifications. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. TUD Dresden University of Technology HAW Hamburg TUD Dresden University of Technology & Barkhausen Institut This document specifies an Application-Layer Protocol Negotiation (ALPN) ID for transport-layer-secured Constrained Application Protocol (CoAP) services. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document. This document updates RFC 2308 by clarifying the definitions of "forwarder" and "QNAME". It obsoletes RFC 8499 by adding multiple terms and clarifications. Comprehensive lists of changed and new definitions can be found in Appendices A and B. Although the DNS Security Extensions (DNSSEC) have been under development for most of the last decade, the IETF has never written down the specific set of threats against which DNSSEC is designed to protect. Among other drawbacks, this cart-before-the-horse situation has made it difficult to determine whether DNSSEC meets its design goals, since its design goals are not well specified. This note attempts to document some of the known threats to the DNS, and, in doing so, attempts to measure to what extent (if any) DNSSEC is a useful tool in defending against these threats. This memo provides information for the Internet community. This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] The Internet Protocol Suite is increasingly used on small devices with severe constraints on power, memory, and processing resources, creating constrained-node networks. This document provides a number of basic terms that have been useful in the standardization work for constrained-node networks. DNS queries and responses are visible to network elements on the path between the DNS client and its server. These queries and responses can contain privacy-sensitive information, which is valuable to protect. This document proposes the use of Datagram Transport Layer Security (DTLS) for DNS, to protect against passive listeners and certain active attacks. As latency is critical for DNS, this proposal also discusses mechanisms to reduce DTLS round trips and reduce the DTLS handshake size. The proposed mechanism runs over port 853. This document describes the privacy issues associated with the use of the DNS by Internet users. It provides general observations about typical current privacy practices. It is intended to be an analysis of the present situation and does not prescribe solutions. This document obsoletes RFC 7626. In many Internet of Things (IoT) applications, direct discovery of resources is not practical due to sleeping nodes or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which contains information about resources held on other servers, allowing lookups to be performed for those resources. The input to an RD is composed of links, and the output is composed of links constructed from the information stored in the RD. This document specifies the web interfaces that an RD supports for web servers to discover the RD and to register, maintain, look up, and remove information on resources. Furthermore, new target attributes useful in conjunction with an RD are defined. This document describes the use of QUIC to provide transport confidentiality for DNS. The encryption provided by QUIC has similar properties to those provided by TLS, while QUIC transport eliminates the head-of-line blocking issues inherent with TCP and provides more efficient packet-loss recovery than UDP. DNS over QUIC (DoQ) has privacy properties similar to DNS over TLS (DoT) specified in RFC 7858, and latency characteristics similar to classic DNS over UDP. This specification describes the use of DoQ as a general-purpose transport for DNS and includes the use of DoQ for stub to recursive, recursive to authoritative, and zone transfer scenarios. This document describes the DNS Security Extensions (commonly called "DNSSEC") that are specified in RFCs 4033, 4034, and 4035, as well as a handful of others. One purpose is to introduce all of the RFCs in one place so that the reader can understand the many aspects of DNSSEC. This document does not update any of those RFCs. A second purpose is to state that using DNSSEC for origin authentication of DNS data is the best current practice. A third purpose is to provide a single reference for other documents that want to refer to DNSSEC. This document specifies the "SVCB" ("Service Binding") and "HTTPS" DNS resource record (RR) types to facilitate the lookup of information needed to make connections to network services, such as for HTTP origins. SVCB records allow a service to be provided from multiple alternative endpoints, each with associated parameters (such as transport protocol configuration), and are extensible to support future uses (such as keys for encrypting the TLS ClientHello). They also enable aliasing of apex domains, which is not possible with CNAME. The HTTPS RR is a variation of SVCB for use with HTTP (see RFC 9110, "HTTP Semantics"). By providing more information to the client before it attempts to establish a connection, these records offer potential benefits to both performance and privacy. The SVCB DNS resource record type expresses a bound collection of endpoint metadata, for use when establishing a connection to a named service. DNS itself can be such a service, when the server is identified by a domain name. This document provides the SVCB mapping for named DNS servers, allowing them to indicate support for encrypted transport protocols. This document defines Discovery of Designated Resolvers (DDR), a set of mechanisms for DNS clients to use DNS records to discover a resolver's encrypted DNS configuration. An Encrypted DNS Resolver discovered in this manner is referred to as a "Designated Resolver". These mechanisms can be used to move from unencrypted DNS to encrypted DNS when only the IP address of a resolver is known. These mechanisms are designed to be limited to cases where Unencrypted DNS Resolvers and their Designated Resolvers are operated by the same entity or cooperating entities. It can also be used to discover support for encrypted DNS protocols when the name of an Encrypted DNS Resolver is known. This document specifies new DHCP and IPv6 Router Advertisement options to discover encrypted DNS resolvers (e.g., DNS over HTTPS, DNS over TLS, and DNS over QUIC). Particularly, it allows a host to learn an Authentication Domain Name together with a list of IP addresses and a set of service parameters to reach such encrypted DNS resolvers. This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. Universität Bremen TZI Fraunhofer SIT The Constrained Resource Identifier (CRI) is a complement to the Uniform Resource Identifier (URI) that represents the URI components in Concise Binary Object Representation (CBOR) rather than as a sequence of characters. This approach simplifies parsing, comparison, and reference resolution in environments with severe limitations on processing power, code size, and memory size. This RFC updates RFC 7595 to add a note on how the "URI Schemes" registry of RFC 7595 cooperates with the "CRI Scheme Numbers" registry created by the present RFC. // (This "cref" paragraph will be removed by the RFC editor:) The // present revision –23 attempts to address the AD review comments; // it is submitted right before the I-D deadline in order to serve as // discussion input to IETF 123 even though not all discussions have // completed. In particular, the updated handling of zone // identifiers requires some additional scrutiny. TUD Dresden University of Technology The Constrained Application Protocol (CoAP, [RFC7252]) is available over different transports (UDP, DTLS, TCP, TLS, WebSockets), but lacks a way to unify these addresses. This document provides terminology and provisions based on Web Linking [RFC8288] and Service Bindings (SVCB, [RFC9460]) to express alternative transports available to a device, and to optimize exchanges using these. Universität Bremen TZI Ericsson Universitat Politecnica de Catalunya The Internet Protocol Suite is increasingly used on small devices with severe constraints on power, memory, and processing resources, creating constrained-node networks. This document provides a number of basic terms that have been useful in the standardization work for constrained-node networks. TUD Dresden University of Technology HAW Hamburg TUD Dresden University of Technology & Barkhausen Institut This document states problems when designing DNS SVCB records to discover endpoints that communicate over Object Security for Constrained RESTful Environments (OSCORE) [RFC8613]. As a consequence of learning about OSCORE, this discovery will allow a host to learn both CoAP servers and DNS over CoAP resolvers that use OSCORE to encrypt messages and Ephemeral Diffie-Hellman Over COSE (EDHOC) [RFC9528] for key exchange. Challenges arise because SVCB records are not meant to be used to exchange security contexts, which is required in OSCORE scenarios. RISE AB Group communication with the Constrained Application Protocol (CoAP) can be secured end-to-end using Group Object Security for Constrained RESTful Environments (Group OSCORE), also across untrusted intermediary proxies. However, this sidesteps the proxies' abilities to cache responses from the origin server(s). This specification restores cacheability of protected responses at proxies, by introducing consensus requests which any client in a group can send to one server or multiple servers in the same group. TU Dresden, Germany Unaffiliated, Vienna, Austria Huawei Technologies, Munich, Germany TU Dresden, Germany HAW Hamburg, Hamburg, Germany TU Dresden &amp; Barkhausen Institut, Dresden, Germany Association for Computing Machinery (ACM) Universität Bremen TZI RFC 7252 defines the Constrained Application Protocol (CoAP), along with a number of additional specifications, including RFC 7641, RFC 7959, RFC 8132, and RFC 8323. RFC 6690 defines the link format that is used in CoAP self-description documents. Some parts of the specification may be unclear or even contain errors that may lead to misinterpretations that may impair interoperability between different implementations. The present document provides corrections, additions, and clarifications to the RFCs cited; this document thus updates these RFCs. In addition, other clarifications related to the use of CoAP in other specifications, including RFC 7390 and RFC 8075, are also provided. University of California, Irvine This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. 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. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice.

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" .

Change Log RFC Ed.: Please remove this section before publication.

Since draft-ietf-core-dns-over-coap-16

• Mention TLS as possible protection mechanism in abstract and intro
• Fix representation format in the docpath examples
• Make docpath wire-format paragraph easier to parse

Since draft-ietf-core-dns-over-coap-15

• Address Genart and Artart review:
  • Add editor's note about removing RFC7228 reference in case rfc7228bis comes out before publication
  • Address minor nits
  • Resolve less well-known abbreviations
  • Name default ports for "coap" and "co"
  • Add reasoning why we also consider DTLS v1.2 (RFC 6347)
  • Add illustrative reference for ETag generation
• Address DNS SVCB SvcParamKeys IANA expert review:
  • docpath: clarifications and examples
  • Rework representation format and wire-format of "docpath"
  • State that we don't do the full SVCB mapping
  • Explicitly do not limit what port= can do.
  • port limitations: We're not the SVCB mapping document
• Address Tsvart Review
  • Prefer ADN for Uri-Host; don't prescribe how it is set

Since draft-ietf-core-dns-over-coap-14

• Remove superfluous and confusing step in SVCB to request algorithm
• Address AD review:
  • Remove RFC8949 as CBOR diagnostic notation reference
  • CoRE-specific FETCH method -> CoAP-specific FETCH method
  • Remove double reference to BCP 219
  • Fix wording and references around SVCB records and ALPN
  • Move format description for examples to Terminology section
  • Retitle section 5 to "Interaction with other CoAP and CoRE Features"
  • Make prevention of poisoning attacks normative for unprotected CoAP
  • Provide specs on if the SHOULD on ID=0 does not apply
  • Make construction algorithm normative
  • Add definition for CoRE
  • General grammar, wording, and spelling cleanups

Since draft-ietf-core-dns-over-coap-13

• Address shepherd review

Since draft-ietf-core-dns-over-coap-12

• Address Esko's review
• Address Marcos's review
• Address Mikolai's review

Since 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

Since draft-ietf-core-dns-over-coap-09

• Update SVCB SvcParamKey
• Update corr-clar reference
• Add reference to DNS Update [RFC2136], clarify that it is currently not considered
• Add to security considerations: unprotected upstream DNS and DNSSEC

Since draft-ietf-core-dns-over-coap-08

• Update Cenk's Affiliation

Since 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

Since 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

Since draft-ietf-core-dns-over-coap-05

• Add references to relevant SVCB/DNR RFCs and drafts

Since draft-ietf-core-dns-over-coap-04

• Add note on cacheable OSCORE
• Address early IANA review

Since draft-ietf-core-dns-over-coap-03

• Amended Introduction with short contextualization of constrained environments
• Add on evaluation

Since draft-ietf-core-dns-over-coap-02

• Move implementation details to Implementation Status (in accordance with )
• 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

Since 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

Since 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)
• Replace layer violating statement for CON with statement of fact
• Add security considerations on ID=0

Since draft-lenders-dns-over-coap-04

• Removed change log of draft-lenders-dns-over-coap

Acknowledgments The authors of this document want to thank Mike Bishop, Carsten Bormann, 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.