Dns On Windows Server 1002003 [Electronic resources] نسخه متنی

A.3 Resource Record Data

A.3.1 Data Format

In addition to 2- and 4-octet integer values, resource record
data can contain domain-names or
character-strings.

Domain names in messages are expressed in terms of a sequence of
labels. Each label is represented as a one octet length field
followed by that number of octets. Since every domain name ends with
the null label of the root, a domain name is terminated by a length
byte of zero. The high order two bits of every length octet must be
zero, and the remaining six bits of the length field limit the label
to 63 octets or less.

In order to reduce the size of messages, the domain system utilizes a
compression scheme which eliminates the repetition of domain names in
a message. In this scheme, an entire domain name or a list of labels
at the end of a domain name is replaced with a pointer to a prior
occurrence of the same name.

The pointer takes the form of a two octet sequence:

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| 1  1|                OFFSET                   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

The first two bits are ones. This allows a pointer to be
distinguished from a label, since the label must begin with two zero
bits because labels are restricted to 63 octets or less. (The 10 and
01 combinations are reserved for future use.) The OFFSET field
specifies an offset from the start of the message (i.e., the first
octet of the ID field in the domain header). A zero offset specifies
the first byte of the ID field, etc.

character-string is a single length
octet

followed by that number of characters.
character-string is treated as binary
information, and can be up to 256 characters in length (including the
length octet).