Add source of draft-ietf-core-dns-over-coap-00

draft-ietf-core-dns-over-coap-00.md

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+---
+title: "DNS over CoAP (DoC)"
+abbrev: DoC
+docname: draft-ietf-core-dns-over-coap-00
+category: std
+submissiontype: IETF
+
+ipr: trust200902
+area: Applications
+workgroup: CoRE
+keyword: Internet-Draft
+
+stand_alone: yes
+smart_quotes: no
+pi: [toc, sortrefs, symrefs]
+
+author:
+ -  name: Martine Sophie Lenders
+    org: Freie Universität Berlin
+    abbrev: FU Berlin
+    email: [email protected]
+ -  name: Christian Amsüss
+    email: [email protected]
+ -  name: Cenk Gündoğan
+    org: HAW Hamburg
+    email: [email protected]
+ -  name: Thomas C. Schmidt
+    org: HAW Hamburg
+    email: [email protected]
+ -  name: Matthias Wählisch
+    org: Freie Universität Berlin
+    abbrev: FU Berlin
+    email: [email protected]
+
+normative:
+
+informative:
+
+
+--- abstract
+
+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).
+
+--- middle
+
+Introduction
+============
+
+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 {{!RFC6347}} 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 end-to-end payload encryption based on OSCORE.
+
+~~~ drawing
+
+                - FETCH coaps://[2001:db8::1]/
+               /
+              /
+             CoAP request
++--------+   [DNS query]   +--------+   DNS query    +--------+
+|  DoC   |---------------->|  DoC   |...............>|  DNS   |
+| Client |<----------------| Server |<...............| Server |
++--------+  CoAP response  +--------+  DNS response  +--------+
+            [DNS response]
+
+~~~
+{: #fig-overview-arch title="Basic DoC architecture"}
+
+
+The most important components of DoC can be seen in {{fig-overview-arch}}: 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 {{?RFC7228}}.
+
+The terms "CoAP payload" and "CoAP body" are used as defined in {{!RFC7959}}.
+
+{::boilerplate bcp14-tagged}
+
+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 {{?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.
+
+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.
+
+Basic Message Exchange
+======================
+
+The "application/dns-message" Content-Format    {#sec:content-format}
+--------------------------------------------
+This document defines the Internet media type "application/dns-message" for
+the CoAP Content-Format. 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.
+
+DNS Queries in CoAP Requests
+----------------------------
+
+A DoC client encodes a single DNS query in one or more CoAP request messages the CoAP FETCH {{!RFC8132}} 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 {{!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 {{sec:content-format}}).
+A DoC server MUST be able to parse requests of Content-Format "application/dns-message".
+
+### Support of CoAP Caching {#sec:req-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 {{!RFC8132}} 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.
+
+    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]
+
+
+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 {{sec:content-format}}) 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 {{!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.
+
+### Support of CoAP Caching {#sec:resp-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 {{!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.
+
+
+<!--
+With short responses, a usable ETag might be almost as long as the response.
+With long-lived responses, the client does not need to revalidate often.
+With responses large enough to be fragmented,
+it's best practice for servers to set an ETag anyway.
+As specified in {{!RFC7252}} and {{!RFC8132}}, if the response associated with
+the ETag is still valid, the response uses the "2.03 Valid" code and consequently
+carries no payload.
+-->
+
+### 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:
+
+    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.
+
+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 {{?RFC1034}} 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. {{?RFC8490}}, 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 {{!RFC7641}}.
+
+OSCORE
+------
+It is RECOMMENDED to carry DNS messages end-to-end encrypted using OSCORE {{?RFC8611}}.
+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 {{!RFC7252}}, 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 {{?RFC6690}}.
+
+Attribute Value: core.dns
+
+Description: DNS over CoAP resource.
+
+Reference: [TBD-this-spec] {{selection-of-a-doc-server}}
+
+
+--- back
+
+Change Log
+==========
+
+# Acknowledgments
+{:numbered="false"}
+
+TODO acknowledge.