DNS over CoAP (DoC)

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Table of Contents

1. 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 [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. Constrained IoT devices may be restricted in memory, power consumption, link layer frame sizes, throughput, and latency. They may only have a handful kilobytes of both RAM and ROM. They may sleep for long durations of time, after which they need to refresh the named resources they know about. Name resolution in such scenarios must take into account link layer frame sizes of only a few hundred bytes, bit rates in the magnitute of kilobits per second, and latencies of several seconds [RFC7228].¶

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.¶

2. 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 [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.¶

3. 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 [RFC9463] or discovery of designated resolvers [RFC9462]. Automatic configuration SHOULD only be done from a trusted source.¶

Support for SVCB Resource Records [RFC9460], [RFC9461] or DNR Service Parameters [RFC9463] are not specified in this document. [I-D.lenders-core-dnr] explores solutions for CoAP for these mechanisms.¶

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.¶

4. Basic Message Exchange

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]

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]

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]

5. CoAP/CoRE Integration

6. 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.¶

7. 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 [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".¶

8. Security Considerations

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.¶

9. IANA Considerations

Appendix A. Evaluation

The authors of this document presented the design, implementation, and analysis of DoC in their paper "Securing Name Resolution in the IoT: DNS over CoAP" [DoC-paper].¶

Appendix B. Change Log

Acknowledgments

The authors of this document want to thank Carsten Bormann, Ben Schwartz, Marco Tiloca, and Tim Wicinski for their feedback and comments.¶

Authors' Addresses