v: 3
title: "DNS over CoAP (DoC)" abbrev: DoC docname: draft-ietf-core-dns-over-coap-02 category: std submissiontype: IETF
area: Applications workgroup: CoRE keyword: Internet-Draft
author:
- name: Martine Sophie Lenders org: Freie Universität Berlin abbrev: FU Berlin street: Takustrasse 9 city: Berlin code: D-14195 country: Germany email: [email protected]
- name: Christian Amsüss email: [email protected]
- name: Cenk Gündoğan org: Huawei Technologies street: Riesstrasse 25 city: Munich code: D-80992 country: Germany email: [email protected]
- name: Thomas C. Schmidt org: HAW Hamburg street: Berliner Tor 7 city: Hamburg code: D-20099 country: Germany email: [email protected]
- name: Matthias Wählisch org: Freie Universität Berlin abbrev: FU Berlin street: Takustrasse 9 city: Berlin code: D-14195 country: Germany email: [email protected]
normative: RFC1035: dns RFC7228: constr-nodes RFC7252: coap RFC7641: coap-observe RFC7959: coap-blockwise RFC8132: coap-fetch RFC8613: oscore RFC9147: dtls13
informative: RFC3986: uri RFC6690: core-link-format RFC8765: dns-push RFC8094: dodtls RFC8484: doh RFC9176: core-rd RFC9250: doq RFC8499: dns-terminology I-D.ietf-add-dnr: dnr I-D.ietf-core-href: cri
--- 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
This document defines DNS over CoAP (DoC), a protocol to send DNS {{-dns}} queries and get DNS responses over the Constrained Application Protocol (CoAP) {{-coap}}. Each DNS query-response pair is mapped into a CoAP message exchange. Each CoAP message is secured by DTLS {{-dtls13}} or Object Security for Constrained RESTful Environments (OSCORE) {{-oscore}} to ensure message integrity and confidentiality.
The application use case of DoC is inspired by DNS over HTTPS {{-doh}} (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 {{-dodtls}}. 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 {{-dodtls}}, 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 {{-doq}}), DoC allows for lightweight end-to-end payload encryption based on OSCORE.
. FETCH coaps://[2001:db8::1]/
/
/
CoAP request
+------+ [DNS query] +------+ DNS query .---------------.
| DoC |---------------->| DoC |--- --- --- --->| DNS |
|Client|<----------------|Server|<--- --- --- ---| Infrastructure |
+------+ CoAP response +------+ DNS response '---------------'
[DNS response]
\ /\ /
'-----DNS over CoAP----' '--DNS over UDP/HTTPS/QUIC/...--'
{: #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 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}}, DNS over HTTPS {{-doh}}, or DNS over QUIC {{-doq}} 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.
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 {{-dns-terminology}} or a DNS recursive resolver {{-dns-terminology}}.
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 {{-constr-nodes}}.
The terms "CoAP payload" and "CoAP body" are used as defined in {{-coap-blockwise}}, Section 2.
{::boilerplate bcp14-tagged}
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 {{-uri}} or CRI {{-cri}}, or automatic configuration, e.g., using a CoRE resource directory {{-core-rd}}, DHCP or Router Advertisement options {{-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 {{-core-link-format}}), the resource type "core.dns" can be used to identify a generic DNS resolver that is available to the client.
This document defines a CoAP Content-Format number for the Internet media type "application/dns-message". This media type is defined as in {{-doh}} Section 6, i.e., a single DNS message encoded in the DNS on-the-wire format {{-dns}}. 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 {{-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 {{-dns}} 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 {{-coap-fetch}} 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".
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 {{-coap-fetch}} 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 {{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.
A DNS response indicates either success or failure in the Response code of the DNS header (see {{-dns}} 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.
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 {{-coap}}, 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 {{-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 {{-dns-push}} 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 {{-dns-push}} 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 {{-dns-push}} to close its subscription to the resource record as well.
It is RECOMMENDED to carry DNS messages end-to-end encrypted using OSCORE {{-oscore}}. The exchange of the security context is out of scope of this document.
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. For unencrypted DoC usage the ID field MUST not be set to a fixed value as suggested in {{sec:req-caching}}, but changed with every query.
When using unencrypted CoAP (see {{sec:unencrypted-coap}}), setting the ID of a DNS message to 0 as specified in {{sec:req-caching}} opens the DNS cache of a DoC client to cache poisoning attacks via response spoofing. Because of that, this documents requires the ID to be changed with every query when CoAP is not secured (see {{sec:unencrypted-coap}}).
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 {{-coap}}, corresponding to the "application/dns-message" media type from the "Media Types" registry:
Media-Type: application/dns-message
Encoding: -
Id: TBD
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 {{-core-link-format}}.
Attribute Value: core.dns
Description: DNS over CoAP resource.
Reference: [TBD-this-spec] {{selection-of-a-doc-server}}
--- back
The authors of this document provide two reference implementations, a DoC client implementation available in the IoT operating system RIOT and a DoC server implementation in Python.
- 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
- 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
- Removed change log of draft-lenders-dns-over-coap
{:unnumbered}
The authors of this document want to thank Ben Schwartz and Tim Wicinski for their feedback and comments.