]> Freie Universität Berlin

m.lenders@fu-berlin.de

christian@amsuess.com

HAW Hamburg

cenk.guendogan@haw-hamburg.de

HAW Hamburg

t.schmidt@haw-hamburg.de

Freie Universität Berlin

m.waehlisch@fu-berlin.de

Applications CoRE Internet-Draft This document defines a protocol for sending DNS messages over the Constrained Application Protocol (CoAP). These CoAP messages are protected by DTLS-Secured CoAP (CoAPS) or Object Security for Constrained RESTful Environments (OSCORE) to provide encrypted DNS message exchange for constrained devices in the Internet of Things (IoT). Discussion Venues Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (core@ietf.org), which is archived 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 is secured by DTLS or Object Security for Constrained RESTful Environments (OSCORE) to ensure message integrity and confidentiality. The application use case of DoC is inspired by DNS over HTTPS (DoH). DoC, however, aims for the deployment in the constrained Internet of Things (IoT), which usually conflicts with the requirements introduced by HTTPS. To prevent TCP and HTTPS resource requirements, constrained IoT devices could use DNS over DTLS . In contrast to DNS over DTLS, DoC utilizes CoAP features to mitigate drawbacks of datagram-based communication. These features include: block-wise transfer, which solves the Path MTU problem of DNS over DTLS (see , section 5); CoAP proxies, which provide an additional level of caching; re-use of data structures for application traffic and DNS information, which saves memory on constrained devices. To prevent resource requirements of DTLS or TLS on top of UDP (e.g., introduced by DNS over QUIC ), DoC allows for lightweight end-to-end payload encryption based on OSCORE.

Basic DoC architecture | DoC |...............>| DNS | | Client |<----------------| Server |<...............| Server | +--------+ CoAP response +--------+ DNS response +--------+ [DNS response] ]]>

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 may or may not resolve that DNS information itself by using other DNS transports with an upstream DNS server. The DoC server then replies to the DNS queries with DNS responses carried within CoAP responses.

Terminology A server that provides the service specified in this document is called a "DoC server" to differentiate it from a classic "DNS server". Correspondingly, a client using this protocol to retrieve the DNS information is called a "DoC client". The term "constrained nodes" is used as defined in . The terms "CoAP payload" and "CoAP body" are used as defined in . 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.

Selection of a DoC Server In this document, it is assumed that the DoC client knows the DoC server and the DNS resource at the DoC server. Possible options could be manual configuration of a URI or CRI , or automatic configuration, e.g., using a CoRE resource directory , DHCP or Router Advertisement options . Automatic configuration SHOULD only be done from a trusted source. When discovering the DNS resource through a link mechanism that allows describing a resource type (e.g., the Resource Type Attribute in ), the resource type "core.dns" can be used to identify a generic DNS resolver that is available to the client.

Basic Message Exchange

The "application/dns-message" Content-Format This document defines the Internet media type "application/dns-message" for the CoAP Content-Format. This media type is defined as in Section 6, 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" format.

DNS Queries in CoAP Requests A DoC client encodes a single DNS query in one or more CoAP request messages the CoAP FETCH method. Requests SHOULD include an Accept option to indicate the type of content that can be parsed in the response. The CoAP request SHOULD be carried in a Confirmable (CON) message, if the transport used does not provide reliable message exchange.

Request Format When sending a CoAP request, a DoC client MUST include the DNS query in the body of the CoAP request. As specified in Section 2.3.1, the type of content of the body MUST be indicated using the Content-Format option. This document specifies the usage of Content-Format "application/dns-message" (details see ). 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 always to 0 to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to the same DNS queries with a cache entry. This ensures that the CoAP Cache-Key (see Section 2) does not change when multiple DNS queries for the same DNS data, carried in CoAP requests, are issued.

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

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

Response Codes and Handling DNS and CoAP errors A DNS response indicates either success or failure in the Response code of the DNS header (see Section 4.1.1). It is RECOMMENDED that CoAP responses that carry any valid DNS response use a "2.05 Content" response code. CoAP responses use non-successful response codes MUST NOT contain a DNS response and MUST only be used on errors in the CoAP layer or when a request does not fulfill the requirements of the DoC protocol. Communication errors with a DNS server (e.g., timeouts) SHOULD be indicated by including a SERVFAIL DNS response in a successful CoAP response. A DoC client might try to repeat a non-successful exchange unless otherwise prohibited. The DoC client might also decide to repeat a non-successful exchange with a different URI, for instance, when the response indicates an unsupported Content-Format.

Support of CoAP Caching The DoC server MUST ensure that any sum of the Max-Age value of a CoAP response and any TTL in the DNS response is less or equal to the corresponding TTL received from an upstream DNS server. This also includes the default Max-Age value of 60 seconds (see , section 5.10.5) when no Max-Age option is provided. The DoC client MUST then add the Max-Age value of the carrying CoAP response to all TTLs in a DNS response on reception and use these calculated TTLs for the associated records. The RECOMMENDED algorithm to assure the requirement for the DoC is to set the Max-Age option of a response to the minimum TTL of a DNS response and to subtract this value from all TTLs of that DNS response. This prevents expired records unintentionally being served from an intermediate CoAP cache. Additionally, it allows for the ETag value for cache validation, if it is based on the content of the response, not to change even if the TTL values are updated by an upstream DNS cache. If only one record set per DNS response is assumed, a simplification of this algorithm is to just set all TTLs in the response to 0 and set the TTLs at the DoC client to the value of the Max-Age option.

Examples The following examples illustrate the replies to the query "example.org. IN AAAA record", recursion turned on. Successful responses carry one answer record including address 2001:db8:1::1:2:3:4 and TTL 58719. A successful response: When a DNS error (SERVFAIL in this case) is noted in the DNS response, the CoAP response still indicates success: When an error occurs on the CoAP layer, the DoC server SHOULD respond with an appropriate CoAP error, for instance "4.15 Unsupported Content-Format" if the Content-Format option in the request was not set to "application/dns-message" and the Content-Format is not otherwise supported by the server.

CoAP/CoRE Integration

DoC Server Considerations In the case of CNAME records in a DNS response, a DoC server SHOULD follow common DNS resolver behavior by resolving a CNAME until the originally requested resource record type is reached. This reduces the number of message exchanges within an LLN. The DoC server SHOULD send compact answers, i.e., additional or authority sections in the DNS response should only be sent if needed or if it is anticipated that they help the DoC client to reduce additional queries.

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

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

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

Security Considerations TODO Security

IANA Considerations

New "application/dns-message" Content-Format IANA is requested to assign CoAP Content-Format ID for the DNS message media type in the "CoAP Content-Formats" sub-registry, within the "CoRE Parameters" registry , corresponding the "application/dns-message" media type from the "Media Types" registry: Media-Type: application/dns-message Encoding: - Id: TBD Reference: [TBD-this-spec]

New "core.dns" Resource Type IANA is requested to assign a new Resource Type (rt=) Link Target Attribute, "core.dns" in the "Resource Type (rt=) Link Target Attribute Values" sub-registry, within the "CoRE Parameters" register . Attribute Value: core.dns Description: DNS over CoAP resource. Reference: [TBD-this-spec]

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. 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. 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] 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. 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. 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. 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) 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. Informative References 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. 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 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. 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. 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] Universität Bremen TZI Fraunhofer SIT The Constrained Resource Identifier (CRI) is a complement to the Uniform Resource Identifier (URI) that serializes the URI components in Concise Binary Object Representation (CBOR) instead of a sequence of characters. This simplifies parsing, comparison and reference resolution in environments with severe limitations on processing power, code size, and memory size. The present revision -10 of this draft contains an experimental addition that allows representing user information (https://alice@chains.example) in the URI authority component. This feature lacks test vectors and implementation experience at the time of writing and requires discussion. 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. Orange Akamai Citrix Systems, Inc. Open-Xchange Microsoft The document specifies new DHCP and IPv6 Router Advertisement options to discover encrypted DNS resolvers (e.g., DNS-over-HTTPS, DNS-over- TLS, 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 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] This RFC is the revised basic definition of The Domain Name System. It obsoletes RFC-882. This memo describes the domain style names and their used for host address look up and electronic mail forwarding. It discusses the clients and servers in the domain name system and the protocol used between them. This document defines a new DNS OPCODE for DNS Stateful Operations (DSO). DSO messages communicate operations within persistent stateful sessions using Type Length Value (TLV) syntax. Three TLVs are defined that manage session timeouts, termination, and encryption padding, and a framework is defined for extensions to enable new stateful operations. This document updates RFC 1035 by adding a new DNS header OPCODE that has both different message semantics and a new result code. This document updates RFC 7766 by redefining a session, providing new guidance on connection reuse, and providing a new mechanism for handling session idle timeouts. This document defines extensions to the MPLS Label Switched Path (LSP) Ping and Traceroute mechanisms as specified in RFC 8029. The extensions allow the MPLS LSP Ping and Traceroute mechanisms to discover and exercise specific paths of Layer 2 (L2) Equal-Cost Multipath (ECMP) over Link Aggregation Group (LAG) interfaces. Additionally, a mechanism is defined to enable the determination of the capabilities supported by a Label Switching Router (LSR). This document updates RFC 8029.

Change Log

Since draft-lenders-dns-over-coap-03

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

Since draft-lenders-dns-over-coap-02

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

Since draft-lenders-dns-over-coap-01

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

Since draft-lenders-dns-over-coap-00

• Soften Content-Format requirements in and
• Clarify "CoAP payload"/"CoAP body" terminology
• Fix nits and typos

Since draft-lenders-dns-over-coaps-00

• Clarification in abstract that both DTLS and OSCORE can be used as secure transport
• Restructuring of :
  • Add dedicated on Content-Format
  • Add overview table about usable and required features for request method types to
  • Add dedicated and on caching requirements for CoAP requests and responses
• Fix nits and typos

Acknowledgments TODO acknowledge.