Internet-Draft dns+cbor March 2023
Lenders, et al. Expires 14 September 2023 [Page]
Intended Status:
Standards Track
M. S. Lenders
FU Berlin
C. Bormann
Universität Bremen TZI
T. C. Schmidt
HAW Hamburg
M. Wählisch
FU Berlin

A Concise Binary Object Representation (CBOR) of DNS Messages


This document specifies a compressed data format of DNS messages using the Concise Binary Object Representation [RFC8949]. The primary purpose is to keep DNS messages small in constrained networks.

About This Document

This note is to be removed before publishing as an RFC.

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This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 14 September 2023.

1. Introduction

In constrained networks [RFC7228], the link layer may restrict the payload sizes to only a few hundreds bytes. Encrypted DNS resolution, such as DNS over HTTPS (DoH) [RFC8484] or DNS over CoAP (DoC) [I-D.ietf-core-dns-over-coap], may lead to DNS message sizes that exceed this limit, even when implementing header compression such as 6LoWPAN IPHC [RFC6282] or SCHC [RFC8724], [RFC8824].

Although adoption layers such as 6LoWPAN [RFC4944] or SCHC [RFC8724] offer fragmentation to comply with small MTUs, fragmentation should be avoided in constrained networks, because fragmentation combined with high packet loss multiplies the loss. As such, a compression format for DNS messages is needed.

This document specifies a compressed data format for DNS messages. DNS messages are encoded in Concise Binary Object Representation (CBOR) [RFC8949] and, additionally, unnecessary or redundant information is removed. To use the outcome of this specification in DoH and DoC, this document also specifies a Media Type header for DoH and a Content-Format option for DoC.

2. Terminology

CBOR types (unsigned integer, byte string, text string, arrays, etc.) are used as defined in [RFC8949].

TBD DNS server and client.

A DNS query is a message that queries DNS information from an upstream DNS resolver.

The term "constrained networks" is used as defined in [RFC7228].

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. CBOR Representations (application/dns+cbor)

To keep overhead minimal, a DNS message is represented as CBOR arrays. All CBOR items used in this specification are of definite length. CBOR arrays that do not follow the length definitions of this or follow-up specifications, MUST be silently ignored. It is assumed that DNS query and DNS response are distinguished message types and that the query can be mapped to the response by the transport protocol of choice. To define the representation of binary objects we use the Concise Data Definition Language (CDDL) [RFC8610].

If, for any reason, a DNS message is not representable in the CBOR format specified in this document, a fallback to the another DNS message format, e.g., the classic DNS wire format, MUST always be possible.

3.1. Domain Name Representation

Domain names are represented in their commonly known string format (e.g., "", see Section 2.3.1 in [RFC1035]) and in IDNA encoding [RFC5890] as a text string. For the purpose of this document, domain names remain case-insensitive as specified in [RFC1035].

The representation of a domain name is defined in Figure 1.

domain-name = tstr .regexp "([^.]+\.)*[^.]+"
Figure 1: Domain Name Definition

3.2. Standard DNS Resource Records (RRs)

DNS resource records are encoded either in their binary form as a byte string or as CBOR arrays containing 2 to 5 entries in the following order:

  1. An optional name (as text string, see Section 3.1),
  2. A TTL (as unsigned integer),
  3. An optional record type (as unsigned integer),
  4. An optional record class (as unsigned integer), and lastly
  5. A record data entry (as unsigned integer, negative integer, byte string, or text string).

If the first item of the resource record is a text string, it is its name. If the name is elided, the name is derived from the question section of the message. For responses, the question section is either taken from the query (see Section 3.3) or provided with the response see Section 3.4. The query may be derived from the transport context.

If the record type is elided, the record type from the question is assumed. If record class is elided, the record class from the question is assumed. When a record class is required, the record type MUST also be provided.

The byte format of the record data as a byte string follows the wire format as specified in Section 3.3 [RFC1035] (or other specifications of the respective record type). Note that this format does not include the RDLENGTH field from [RFC1035] as this value is encoded in the length field of the CBOR byte string.

If the record data represents a domain name (e.g., for CNAME or PTR records), the record data MAY be represented as a text string as specified in Section 3.1. This can save 1 byte of data, because the byte representation of DNS names requires both an additional byte to define the length of the first name component and well as a zero byte at the end of the name. With CBOR on the other hand only 1 byte is required to define type and length of the text string up until a string length of 23 characters. Likewise, if the record data is purely a numerical value, it can be expressed as either an unsigned or negative integer.

The representation of a DNS resource records is defined in Figure 2.

type-spec = (
  record-type: uint,
  ? record-class: uint,
rr = (
  ? name: domain-name,
  ttl: uint,
  ? type-spec,
  rdata: int / bstr / domain-name,
dns-rr = [rr] / bstr
Figure 2: DNS Resource Record Definition

3.3. DNS Queries

DNS queries are encoded as CBOR arrays containing up to 5 entries in the following order:

  1. An optional transaction ID (as unsigned integer),
  2. An optional flag field (as unsigned integer),
  3. The question section (as array),
  4. An optional authority section (as array), and
  5. An optional additional section (as array)

If the first item of the query is an array, it is the question section, if it is an unsigned integer, it is the transaction ID. If the transaction ID is present and followed by another unsigned integer, that item is a flag field and maps to the header flags in [RFC1035] and the "DNS Header Flags" IANA registry including the QR flag and the Opcode. It MUST be lesser than 2^16.

If the transaction ID is elided, the value 0 is assumed, same for the flags. The transaction ID MUST be included and set to an unpredictable value lesser than 2^32, if the DNS transport can not ensure the prevention of DNS response spoofing. An example for such a transport is unencrypted DoC (see [I-D.ietf-core-dns-over-coap], Section 6).

The question section is encoded as a CBOR array containing up to 3 entries:

  1. The queried name (as text string, see Section 3.1),
  2. An optional record type (as unsigned integer), and
  3. An optional record class (as unsigned integer)

If the record type is elided, record type AAAA as specified in [RFC3596] is assumed. If the record class is elided, record class IN as specified in [RFC1035] is assumed. When a record class is required, the record type MUST also be provided.

The remainder of the query is either empty or MUST consist of up to two arrays. The first array, if present, encodes the authority section of the query as an array of DNS resource records (see Section 3.2) The second array, if present, encodes the additional section of the query as an array of DNS resource records (see Section 3.2)

The representation of a DNS query is defined in Figure 3.

query-id-flags = (
  id: uint .default 0,
  ? flags: uint .default 0,
question-section = (
  name: domain-name,
  ? type-spec,
extra-sections = (
  ? authority: [+ dns-rr],
  additional: [+ dns-rr],
query-sections = (
  ? query-id-flags,
  ? extra-sections,
dns-query = [query-sections]
Figure 3: DNS Query Definition

3.4. DNS Responses

DNS responses are encoded as a CBOR array containing up to 7 entries.

  1. An optional transaction ID (as unsigned integer),
  2. An optional flag field (as unsigned integer),
  3. An optional question section (as array, encoded as described in Section 3.3)
  4. The answer section (as array),
  5. An optional authority section (as array), and
  6. An optional additional section (as array)

If the CBOR array is a response to a query that contains a transaction ID, it MUST be included and set to the corresponding value present in the query. If it is not included, the transaction ID is implied to be 0.

If the CBOR array is a response to a query for which the flags indicate that flags are set in the response, they MUST be set accordingly and thus included in the response. If the flags are not included, the flags are implied to be 0x8000 (everything unset except for the QR flag).

If the response includes only 1 array, this is the DNS answer section represented as an array of one or more DNS Resource Records (see Section 3.2).

If the response includes 2 arrays, the first entry is a question section and the second entry is an answer section. The question section is encoded like as specified in Section 3.3, the answer section is represented as an array of one or more DNS Resource Records (see Section 3.2).

If the response includes 3 arrays, the first section is a question section, the second an answer section, and the third an additional section (TBD: back choice to favor additional section by empirical data). Again, the question section is encoded like a DNS query as specified in Section 3.3 and both answer and additional sections are represented each as an array of one or more DNS Resource Records (see Section 3.2).

If the response includes 4 arrays, the first section is a question section, the second an answer section, the third an authority section, and the fourth an additional section (TBD: back by empirical data). They follow the specification of 3 arrays in the answer. The authority section is also represented as an array of one or more DNS Resource Records (see Section 3.2).

response-id-flags = (
  id: uint .default 0,
  ? flags: uint .default 0x8000,
response-sections = ((
  ? response-id-flags,
  answer: [+ dns-rr],
) // (
  ? response-id-flags,
  question: [question-section],
  answer: [+ dns-rr],
  ? extra-sections,
dns-response = [response-sections]
Figure 4: DNS Response Definition

3.5. EDNS(0)

TBD, do we need special formatting here?

3.6. Name and Address Compression with Packed CBOR

If both DNS server and client support packed CBOR [I-D.ietf-cbor-packed], it MAY be used for name and address compression in DNS responses.

A DNS client uses media type "application/dns+cbor;packed=1" to negotiate (see, e.g., [RFC9110] or [RFC7252], Section 5.5.4) with the DNS server if the server supports packed CBOR. If it does, it MAY request the response to be in packed CBOR (media type "applicaton/dns+cbor;packed=1"). The server then SHOULD reply with the response in packed CBOR.

The representation of DNS responses in packed CBOR differs, in that responses are now represented as a CBOR array of two arrays. The first array is a packing table that is used both as shared item table and argument table (see [I-D.ietf-cbor-packed], Section 2.1), the second is the compressed response.

The representation of a packed DNS response is defined in Figure 5.

compr-dns-response = any /TBD; how to express packed CBOR in CDDL?/
packed-dns-response = [[pack-table], compr-dns-response]
pack-table = any
Figure 5: Definition of DNS messages in packed CBOR

If an index in the packing table is referenced with shared item reference ([I-D.ietf-cbor-packed], Section 2.2) a decoder uses the packing table as a shared item table. If an index in the packing table is referenced as an argument ([I-D.ietf-cbor-packed], Sections 2.3 and 4), a decoder uses the packing table as an argument table.

Discussion TBD:

  • For queries, as they are only one question, i.e. at most one value of each at most, compression is not necessary.
  • Address and name compression are mostly about affix compression (i.e. straight/inverse referencing)
    ==> For occasions were value is the affix (e.g., "" in ANY example in Appendix A.2) use shared item referencing to argument table to safe bytes (no extra shared item table, no, e.g., 216(""), just simple(0))

    • Example: Using Basic Packed CBOR ([I-D.ietf-cbor-packed], section 3.1):

      • 131 bytes (Basic Packed CBOR)
      • 200 bytes (plain CBOR, see Appendix A.2)
      • 194 bytes (wire-format)
      • 113(
            ["_coap._udp.local", "", 3600, 28, 2],
            [h'20010db800000000000000000000', simple(1)],
              [simple(1), 12, 1],
              [[simple(1), simple(0)]],
                [simple(1), simple(4), 217("ns1.")],
                [simple(1), simple(4), 217("ns2.")]
                [simple(0), simple(1), simple(3), 6(h'0001')],
                [simple(0), simple(1), simple(3), 6(h'0002')],
                [217("ns1."), simple(1), simple(3), 6(h'0035')],
                [217("ns2."), simple(1), simple(3), 6(h'3535')]

      vs. application/dns+cbor;packed=1 (shared and argument table as one) 126 bytes:

      • [
            "_coap._udp.local" "", 3600, 28, 2
            [simple(2), 12, 1],
            [[simple(3), simple(1)]],
              [simple(2), simple(5), 218("ns1.")],
              [simple(2), simple(5), 218("ns2.")]
              [simple(1), simple(3), simple(4), 6(h'0001')],
              [simple(1), simple(3), simple(4), 6(h'0002')],
              [218("ns1."), simple(3), simple(4), 6(h'0035')],
              [218("ns2."), simple(3), simple(4), 6(h'3535')]

3.6.1. Table Setup

TBD How to construct the packing table, here's a sketch:

  • Find most often used prefix and values

    • Probably some threshold needed, to prevent, e.g., 1 byte prefixes filling valuable table space
  • Sort descending by number of occurrences and length

    • Long prefixes should take precedence for index 0 for Tag 6 usage

5. IANA Considerations

5.1. Media Type Registration

This document registers a media type for the serialization format of DNS messages in CBOR. It follows the procedures specified in [RFC6838].

5.1.1. "application/dns+cbor"

Type name: application

Subtype name: dns+cbor

Required parameters: None

Optional parameters: packed

Encoding considerations: Must be encoded as using [RFC8949]. See [TBD-this-spec] for details.

Security considerations: See Section 4 of this draft

Interoperability considerations: TBD

Published specification: [TBD-this-spec]

Applications that use this media type: TBD DNS over X systems

Fragment Identifier Considerations: TBD

Additional information:

   Deprecated alias names for this type: N/A

   Magic number(s): N/A

   File extension(s): dnsc

   Macintosh file type code(s): none

Person & email address to contact for further information: Martine S. Lenders

Intended usage: COMMON

Restrictions on Usage: None?

Author: Martine S. Lenders

Change controller: Martine S. Lenders

Provisional registrations? No

5.2. CoAP Content-Format Registration

IANA is requested to assign CoAP Content-Format ID for the new DNS message media types in the "CoAP Content-Formats" sub-registry, within the "CoRE Parameters" registry [RFC7252], corresponding the "application/dns+cbor" media type specified in Section 5.1:

5.2.1. "application/dns-cbor"

Media-Type: application/dns+cbor

Encoding: -


Reference: [TBD-this-spec]

5.2.2. "application/dns+cbor;packed=1"

Media-Type: application/dns+cbor;packed=1

Encoding: -


Reference: [TBD-this-spec]

6. References

6.1. Normative References

Bormann, C., "Packed CBOR", Work in Progress, Internet-Draft, draft-ietf-cbor-packed-08, , <>.
Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", STD 88, RFC 3596, DOI 10.17487/RFC3596, , <>.
Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, , <>.
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <>.
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, , <>.
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <>.

6.2. Informative References

Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C., and M. Wählisch, "DNS over CoAP (DoC)", Work in Progress, Internet-Draft, draft-ietf-core-dns-over-coap-01, , <>.
Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, , <>.
Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, , <>.
Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, , <>.
Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <>.
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. Zuniga, "SCHC: Generic Framework for Static Context Header Compression and Fragmentation", RFC 8724, DOI 10.17487/RFC8724, , <>.
Minaburo, A., Toutain, L., and R. Andreasen, "Static Context Header Compression (SCHC) for the Constrained Application Protocol (CoAP)", RFC 8824, DOI 10.17487/RFC8824, , <>.
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <>.

Appendix A. Examples

A.1. DNS Queries

A DNS query of the record AAAA in class IN for name "" is represented in CBOR extended diagnostic notation (EDN) (see Section 8 in [RFC8949] and Appendix G in [RFC8610]) as follows:


A query of an A record for the same name is represented as

[["", 1]]

A query of ANY record for that name is represented as

[["", 255, 255]]

A.2. DNS Responses

The responses to the examples provided in Appendix A.1 are shown below. We use the CBOR extended diagnostic notation (EDN) (see Section 8 in [RFC8949] and Appendix G in [RFC8610]).

To represent an AAAA record with TTL 300 seconds for the IPv6 address 2001:db8::1, a minimal response to [""] could be

[[[300, h'20010db8000000000000000000000001']]]

In this case, the name is derived from the query.

If the name or the context is required, the following response would also be valid:

[[["", 300, h'20010db8000000000000000000000001']]]

If the query can not be mapped to the response for some reason, a response would look like:

[[""], [[300, h'20010db8000000000000000000000001']]]

To represent a minimal response of an A record with TTL 3600 seconds for the IPv4 address, a minimal response to ["", 1] could be

[[300, h'c0000201']]

Note that here also the 1 of record type A can be elided, as this record type is specified in the question section.

Lastly, a response to ["", 255, 255] could be

  ["", 12, 1],
  [[3600, "_coap._udp.local"]],
    [3600, 2, ""],
    [3600, 2, ""]
      "_coap._udp.local", 3600, 28,
      "_coap._udp.local", 3600, 28,
      "", 3600, 28,
      "", 3600, 28,

This one advertises two local CoAP servers (identified by service name _coap._udp.local) at 2001:db8::1 and 2001:db8::2 and two nameservers for the domain, at 2001:db8::35 and at 2001.db8::3535. Each of the transmitted records has a TTL of 3600 seconds.

Appendix B. Change Log

B.1. Since [draft-lenders-dns-cbor-01]

  • Update definitions to accommodate for TID and flags, as well as more sections in query
  • Clarify fallback to wire-format

B.2. Since draft-lenders-dns-cbor-00

  • Add support for DNS transaction IDs
  • Name and Address compression utilizing CBOR-packed
  • Minor fixes to CBOR EDN and CDDL


TODO acknowledge.

  • Carsten Bormann

Authors' Addresses

Martine Sophie Lenders
Freie Universität Berlin
Carsten Bormann
Universität Bremen TZI
Thomas C. Schmidt
HAW Hamburg
Matthias Wählisch
Freie Universität Berlin