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. 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

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,
  [question-section],
  ? extra-sections,
)
dns-query = [query-sections]

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]

3.5. EDNS(0)

TBD, do we need special formatting here? Yes! We'll use tags. Other special regcords may be needed as well.¶

4.1. Media Type Negotiation

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.¶

4.2. DNS Representation 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.¶

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.¶

4.3. Compression

How the compressor constructs the packing table, i.e., how the compression is applied, is out of scope of this document. Several potential compression algorithms were evaluated in [TBD].¶

4.4. Discussion

The DNS-specific specification for packed CBOR merges the argument and shared item table into one packed table. This differentiates it from the Basic Packed CBOR format specified in [I-D.ietf-cbor-packed].¶

In DNS compression only affix compression, i.e. straight/inverse referencing, and shared value referencing are needed for DNS compression, but no further functions. Since for those types of references, the arguments of the affix compression and the shared values do not collide—shared values are just affixes with an empty rump—we only need one table. Using this specific constrained for DNS, allows us to save the additional bytes that would be required for the 113 tag and the extra array in the Basic Packed CBOR format.¶

Compression of queries is not specified, as apart from EDNS(0) (TBD), they only consist of one question most of the time.¶

7.1. 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].¶

7.2. 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 7.1:¶

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 [RFC8949] 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 [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 ["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-02]

• Add Discussion section and note on compression¶

B.2. Since [draft-lenders-dns-cbor-01]

• Use MIME type parameter for packed instead of own MIME type¶
• Update definitions to accommodate for TID and flags, as well as more sections in query¶
• Clarify fallback to wire-format¶

B.3. 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¶