DNS.Message.EDNS0.Option.Cookie (DNS v0.4.1)

EDNS0.Option.Cookie

The DNS COOKIE option is an OPT RR [RFC6891] option that can be included in the RDATA portion of an OPT RR in DNS requests and responses. The option length varies, depending on the circumstances in which it is being used. There are two cases, as described below. Both use the same OPTION-CODE; they are distinguished by their length.

In a request sent by a client to a server when the client does not know the server's cookie, its length is 8, consisting of an 8-byte Client Cookie, as shown in Figure 1.

                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        OPTION-CODE = 10      |       OPTION-LENGTH = 8        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-    Client Cookie (fixed size, 8 bytes)              -+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 1: COOKIE Option, Unknown Server Cookie

In a request sent by a client when a Server Cookie is known, and in all responses to such a request, the length is variable -- from 16 to 40 bytes, consisting of an 8-byte Client Cookie followed by the variable-length (8 bytes to 32 bytes) Server Cookie, as shown in Figure 2. The variability of the option length stems from the variable-length Server Cookie. The Server Cookie is an integer number of bytes, with a minimum size of 8 bytes for security and a maximum size of 32 bytes for convenience of implementation.

                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        OPTION-CODE = 10      |   OPTION-LENGTH >= 16, <= 40   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-    Client Cookie (fixed size, 8 bytes)              -+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  /       Server Cookie  (variable size, 8 to 32 bytes)           /
  /                                                               /
  +-+-+-+-...

         Figure 2: COOKIE Option, Known Server Cookie

Summary

Types

t()

Functions

from_iodata(arg)

new(arg)

Types

t()

@type t() :: %DNS.Message.EDNS0.Option.Cookie{
  code: DNS.Message.EDNS0.OptionCode.t(),
  data: {client_cookie :: binary(), server_cookie :: binary() | nil},
  length: 8 | 16..40
}

Functions

from_iodata(arg)

new(arg)

@spec new({binary(), binary() | nil}) :: t()