| Internet-Draft | DoC | July 2023 |
| Lenders, et al. | Expires 11 January 2024 | [Page] |
This document defines a protocol for sending DNS messages over the Constrained Application Protocol (CoAP). These CoAP messages are protected by DTLS-Secured CoAP (CoAPS) or Object Security for Constrained RESTful Environments (OSCORE) to provide encrypted DNS message exchange for constrained devices in the Internet of Things (IoT).¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list ([email protected]), which is archived at https://mailarchive.ietf.org/arch/browse/core/.¶
Source for this draft and an issue tracker can be found at https://github.com/core-wg/draft-dns-over-coap.¶
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This document defines DNS over CoAP (DoC), a protocol to send DNS [RFC1035] queries and get DNS responses over the Constrained Application Protocol (CoAP) [RFC7252]. Each DNS query-response pair is mapped into a CoAP message exchange. Each CoAP message is secured by DTLS [RFC9147] or Object Security for Constrained RESTful Environments (OSCORE) [RFC8613] to ensure message integrity and confidentiality.¶
The application use case of DoC is inspired by DNS over HTTPS [RFC8484] (DoH). DoC, however, aims for the deployment in the constrained Internet of Things (IoT), which usually conflicts with the requirements introduced by HTTPS.¶
To prevent TCP and HTTPS resource requirements, constrained IoT devices could use DNS over DTLS [RFC8094]. In contrast to DNS over DTLS, DoC utilizes CoAP features to mitigate drawbacks of datagram-based communication. These features include: block-wise transfer, which solves the Path MTU problem of DNS over DTLS (see [RFC8094], section 5); CoAP proxies, which provide an additional level of caching; re-use of data structures for application traffic and DNS information, which saves memory on constrained devices.¶
To prevent resource requirements of DTLS or TLS on top of UDP (e.g., introduced by DNS over QUIC [RFC9250]), DoC allows for lightweight payload encryption based on OSCORE.¶
The most important components of DoC can be seen in Figure 1: A DoC client tries to resolve DNS information by sending DNS queries carried within CoAP requests to a DoC server. That DoC server is a DNS client (i.e., a stub or recursive resolver) that resolves DNS information by using other DNS transports such as DNS over UDP [RFC1035], DNS over HTTPS [RFC8484], or DNS over QUIC [RFC9250] when communicating with the upstream DNS infrastructure. Using that information, the DoC server then replies to the queries of the DoC client with DNS responses carried within CoAP responses.¶
Note that this specification is 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.¶
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 [RFC8499] or a DNS recursive resolver [RFC8499].¶
A client using the service specified in this document to retrieve the DNS information is called a "DoC client".¶
The term "constrained nodes" is used as defined in [RFC7228].¶
The terms "CoAP payload" and "CoAP body" are used as defined in [RFC7959], 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
In this document, it is assumed that the DoC client knows the DoC server and the DNS resource at the DoC server. Possible options could be manual configuration of a URI [RFC3986] or CRI [I-D.ietf-core-href], or automatic configuration, e.g., using a CoRE resource directory [RFC9176], DHCP or Router Advertisement options [I-D.ietf-add-dnr]. 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 [RFC8323]¶
When discovering the DNS resource through a link mechanism that allows describing a resource type (e.g., the Resource Type Attribute in [RFC6690]), the resource type "core.dns" can be used to identify a generic DNS resolver that is available to the client.¶
While there is no path specified it is RECOMMENDED to use the root path "/" for the DNS resource to keep the CoAP requests small.¶
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 [RFC8484] Section 6, i.e., a single DNS message encoded in the DNS on-the-wire format [RFC1035]. Both DoC client and DoC server MUST be able to parse contents in the "application/dns-message" format.¶
A DoC client encodes a single DNS query in one or more CoAP request messages that use the CoAP FETCH [RFC8132] 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 [RFC1035] is not needed.¶
When sending a CoAP request, a DoC client MUST include the DNS query in the body of the CoAP request. As specified in [RFC8132] Section 2.3.1, the type of content of the body MUST be indicated using the Content-Format option. This document specifies the usage of Content-Format "application/dns-message" (details see Section 4.1). A DoC server MUST be able to parse requests of Content-Format "application/dns-message".¶
The DoC client SHOULD set the ID field of the DNS header always to 0 to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to the same DNS queries with a cache entry. This ensures that the CoAP Cache-Key (see [RFC8132] Section 2) does not change when multiple DNS queries for the same DNS data, carried in CoAP requests, are issued.¶
The following example illustrates the usage of a CoAP message to resolve "example.org. IN AAAA" based on the URI "coaps://[2001:db8::1]/". The CoAP body is encoded in "application/dns-message" Content Format.¶
FETCH coaps://[2001:db8::1]/
Content-Format: application/dns-message
Accept: application/dns-message
Payload: 00 00 01 20 00 02 00 00 00 00 00 00 07 65 78 61 [binary]
6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
01 00 01 [binary]
¶
Each DNS query-response pair is mapped to a CoAP REST request-response operation. 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 Section 4.1) 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.¶
A DNS response indicates either success or failure in the Response code of the DNS header (see [RFC1035] Section 4.1.1). It is RECOMMENDED that CoAP responses that carry any valid DNS response use a "2.05 Content" response code.¶
CoAP responses use non-successful response codes MUST NOT contain 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.¶
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 ([RFC8484], section 5.1) is not sufficient and needs to be updated so that the TTLs in the response are more often the same regardless of query time.¶
The DoC server MUST ensure that 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 [RFC7252], 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.¶
The following examples illustrate the replies to the query "example.org. IN AAAA record", recursion turned on. Successful responses carry one answer record including address 2001:db8:1::1:2:3:4 and TTL 58719.¶
A successful response:¶
2.05 Content
Content-Format: application/dns-message
Max-Age: 58719
Payload: 00 00 81 a0 00 01 00 01 00 00 00 00 07 65 78 61 [binary]
6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
1c 00 01 00 01 37 49 00 10 20 01 0d b8 00 01 00 [binary]
00 00 01 00 02 00 03 00 04 [binary]
¶
When a DNS error (SERVFAIL in this case) is noted in the DNS response, the CoAP response still indicates success:¶
2.05 Content
Content-Format: application/dns-message
Payload: 00 00 81 a2 00 01 00 00 00 00 00 00 07 65 78 61 [binary]
6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 [binary]
¶
When an error occurs on the CoAP layer, the DoC server SHOULD respond with an appropriate CoAP error, for instance "4.15 Unsupported Content-Format" if the Content-Format option in the request was not set to "application/dns-message" and the Content-Format is not otherwise supported by the server.¶
DNS Push requires additional overhead, which conflicts with constrained resources, This is the reason why it is RECOMMENDED to use CoAP Observe [RFC7641] instead of DNS Push in the DoC domain.¶
If the CoAP request indicates that the DoC client wants to observe a resource record, a DoC server MAY use a DNS Subscribe message [RFC8765] 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 [RFC8765] 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 [RFC8765] to close its subscription to the resource record as well.¶
It is RECOMMENDED to carry DNS messages encrypted using OSCORE [RFC8613] between the DoC client and the DoC server. The exchange of the security context is out of scope of this document.¶
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 (Section 4.2) and the TTL rewriting (Section 4.3.2) as specified in this draft would be non-trivial. It is RECOMMENDED to use a DNS forwarder to map between DoC and DoH, as would be the case for mapping between any other DNS transport.¶
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.¶
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶
According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
The authors of this document provide a DoC client implementation available in the IoT operating system RIOT.¶
The authors of this document provide a DoC server implementation in Python.¶
When using unencrypted CoAP (see Section 6), setting the ID of a DNS message to 0 as specified in Section 4.2.2 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 Section 6).¶
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 is requested to assign CoAP Content-Format ID for the DNS message media type in the "CoAP Content-Formats" sub-registry, within the "CoRE Parameters" registry [RFC7252], corresponding to the "application/dns-message" media type from the "Media Types" registry:¶
Media-Type: application/dns-message¶
Encoding: -¶
Id: 553¶
Reference: [TBD-this-spec]¶
IANA is requested to assign a new Resource Type (rt=) Link Target Attribute, "core.dns" in the "Resource Type (rt=) Link Target Attribute Values" sub-registry, within the "CoRE Parameters" register [RFC6690].¶
Attribute Value: core.dns¶
Description: DNS over CoAP resource.¶
The authors of this document want to thank Ben Schwartz and Tim Wicinski for their feedback and comments.¶