A Concise Binary Object Representation (CBOR) of DNS Messages

3.1. Domain Name Representation

Domain names are represented in their commonly known string format (e.g., "example.org", 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 "([^.]+\.)*[^.]+"

3.2. DNS Queries

DNS queries are encoded as CBOR arrays containing up to 3 entries in the following order: An optional transaction ID (as unsigned integer), the name (as text string, see Section 3.1), an optional record type (as unsigned integer), and an optional record class (as unsigned integer).¶

If the transaction ID is elided, the value 0 is assumed. It MUST be included and set to an unpredictable value less 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).¶

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. If a record class is required, the record type MUST also be provided.¶

The representation of a DNS query is defined in Figure 2.¶

type-spec = (
  record-type: uint,
  ? record-class: uint,
)
dns-question = (
  ? id: uint,
  name: domain-name,
  ? type-spec,
)
dns-query = [dns-question]

3.3. Standard DNS Resource Records (RRs)

DNS resource records are encoded as CBOR arrays containing 2 to 5 entries in the following order: An optional name (as text string, see Section 3.1), a TTL (as unsigned integer), an optional record type (as unsigned integer), an optional record class (as unsigned integer), and lastly a record data entry (as byte string or text string).¶

If the first element of the resource record is a string, the first element is a name. If the name is elided, the name from the query, either derived from transport context or the provided question section, see Section 3.4, is assumed. 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. If a record class is required, the record type MUST also be provided.¶

The byte format of the record data 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 and only 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 bytes of data, because the byte representation of DNS names requires both an additional byte to define the length of the first name component as well as a 0 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.¶

The representation of a DNS resource records is defined in Figure 3.¶

rr = (
  ? name: domain-name,
  ttl: uint,
  ? type-spec,
  rdata: bstr / domain-name,
)
dns-rr = [rr]

3.4. DNS Responses

DNS responses are encoded as a CBOR array containing up to 5 entries. The first entry MAY be an unsigned integer, representing the transaction ID of the response. If 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. The remaining 4 entries are arrays:¶

If only 1 array is included, then this is the DNS answer section represented as an array of one or more DNS Resource Records (see Section 3.3).¶

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

If 3 arrays are included, then 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 3.2 and both answer and additional sections are represented each as an array of one or more DNS Resource Records (see Section 3.3).¶

If 4 arrays are included, then 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 3.3).¶

extra-sections = (
  ? authority: [+ dns-rr],
  additional: [+ dns-rr],
)
sections = ((
  ? id: uint,
  answer: [+ dns-rr],
) // (
  ? id: uint,
  question: dns-query,
  answer: [+ dns-rr],
  ? extra-sections,
))
dns-response = [sections]

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" 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"). 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

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., "example.org" 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", "example.org", 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 (shared and argument table as one) 126 bytes:¶

    • [
        [
          h'20010db800000000000000000000',
          "_coap._udp.local" "example.org", 3600, 28, 2
        ],
        [
          [simple(1), 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')]
          ]
        ]
      ]
      

      ¶

5.1. Media Type Registration

This document registers two media type for the serialization format of DNS messages in CBOR. They follow the procedures specified in [RFC6838].¶

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 types "application/dns+cbor" and "application/dns+cbor-packed"" specified in Section 5.1:¶

A.1. DNS Queries

A DNS query of the record AAAA in class IN for name "example.org" is represented in CBOR extended diagnostic notation (EDN) (see Section 8 in [RFC7049] and Appendix G in [RFC8610]) as follows:¶

["example.org"]

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

["example.org", 1]

A query of ANY record for that name is represented as¶

["example.org", 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 [RFC7049] 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 ["example.org"] 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:¶

[[["example.org", 300, h'20010db8000000000000000000000001']]]

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

[["example.org"], [[300, h'20010db8000000000000000000000001']]]

To represent a minimal response of an A record with TTL 3600 seconds for the IPv4 address 192.0.2.1, a minimal response to ["example.org", 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 ["example.org", 255, 255] could be¶

[
  ["example.org", 12, 1],
  [[3600, "_coap._udp.local"]],
  [
    [3600, 2, "ns1.example.org"],
    [3600, 2, "ns2.example.org"]
  ],
  [
    [
      "_coap._udp.local", 3600, 28,
      h'20010db8000000000000000000000001'
    ],
    [
      "_coap._udp.local", 3600, 28,
      h'20010db8000000000000000000000002'
    ],
    [
      "ns1.example.org", 3600, 28,
      h'20010db8000000000000000000000035'
    ],
    [
      "ns2.example.org", 3600, 28,
      h'20010db8000000000000000000003535'
    ]
  ]
]

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 example.org domain, ns1.example.org at 2001:db8::35 and ns2.example.org at 2001.db8::3535. Each of the transmitted records has a TTL of 3600 seconds.¶

B.1. 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¶