]> Freie Universität Berlin

Takustrasse 9 Berlin D-14195 Germany m.lenders@fu-berlin.de

christian@amsuess.com

Huawei Technologies

Riesstrasse 25 Munich D-80992 Germany cenk.gundogan@huawei.com

HAW Hamburg

Berliner Tor 7 Hamburg D-20099 Germany t.schmidt@haw-hamburg.de

Freie Universität Berlin

Takustrasse 9 Berlin D-14195 Germany 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 payload encryption 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 disjunct from DoH since the CoRE-specific FETCH method is used. This was done to take benefit from having the DNS query in the payload as with POST, but still having the caching advantages we would gain with GET. Having the DNS query in the payload means we do not need extra base64 encoding, which would increase code complexity and message sizes. We are also able to transfer a query block-wise.

Terminology 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 . 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 . The terms "CoAP payload" and "CoAP body" are used as defined in , Section 2. 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. TBD DNR Service Parameters + SVCB Resource Records (also see #22):

• coap Exists already in TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry
• Never mandated for DTLS, should be kept for CoAP over TLS; DTLS needs its own ALPN ID
• SVCB: Also needs to considering OSCORE/EDHOC

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. While there is no path specified it is RECOMMENDED to use the root path "/" for the DNS resource to keep the CoAP requests small.

Basic Message Exchange

The "application/dns-message" Content-Format This document defines a CoAP Content-Format number 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, 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 that use the CoAP FETCH method. Requests SHOULD include an Accept option to indicate the type of content that can be parsed in the response. Since CoAP provides reliability of the message layer (e.g. CON) 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 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 For reliability and energy saving measures content decoupling and thus en-route caching on proxies takes a far greater role than it does, e.g., in HTTP. Likewise, CoAP utilizes cache validation to refresh stale cache entries without large messages which often uses hashing over the message content for ETag generation. As such, the approach to guarantee the same cache key for DNS responses as proposed in DoH (, 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 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

DNS Push DNS Push requires 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. 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 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 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.

OSCORE It is RECOMMENDED to carry DNS messages encrypted using OSCORE between the DoC client and the DoC server. The exchange of the security context is out of scope of this document.

Mapping DoC to DoH This document provides no specification how to map between DoC and DoH, e.g., at a CoAP-HTTP-proxy, and it 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 DNS transport.

Considerations for Unencrypted Use The use of DoC without a security mode of CoAP is NOT RECOMMENDED. Without a security mode, a large number of possible attacks need to be evaluate in the context of the application's threat model. This includes threats that are mitigated even by DNS over UDP: For example, the random ID of the DNS header afford some protection against off-path cache poisoning attacks---a threat that might be mitigated by using random large token values in the CoAP request.

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".

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

Level of maturity:

production

Version compability:

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

License:

LGPL-2.1

Contact information:

Martine Lenders <m.lenders@fu-berlin.de>

Last update of this information:

March 2023

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

Level of maturity:

production

Version compability:

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

License:

MIT

Contact information:

Martine Lenders <m.lenders@fu-berlin.de>

Last update of this information:

March 2023

Security Considerations When using unencrypted CoAP (see ), setting the ID of a DNS message to 0 as specified in opens open the DNS cache of a DoC client to cache poisoning attacks via response spoofing. This documents 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 encrypted usage with DTLS or OSCORE the impact of a fixed ID on security is limited, as both harden against injecting spoofed responses. Consequently, it is of little concern to leverage the benefits of CoAP caching by setting the ID to 0. TODO more 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 to the "application/dns-message" media type from the "Media Types" registry: Media-Type: application/dns-message Encoding: - Id: 553 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 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. 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. 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 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. 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 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] 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 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. 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 defines a protocol for sending DNS queries and getting DNS responses over HTTPS. Each DNS query-response pair is mapped into an HTTP exchange. 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. 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 sometimes 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 obsoletes RFC 7719 and updates RFC 2308. 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. Orange Nokia 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. 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 -12 of this draft adds a registry that is // intended to provide full coverage for representing a URI scheme // (and certain text strings used in their place) as negative // integers. 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.

Change Log

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 DoC are disjunct
• Clarify mapping between DoC and DoH
• Update considerations on unencrypted 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 Ben Schwartz and Tim Wicinski for their feedback and comments.