DNS

The Domain Name Service (DNS) protocol searches for resources using a database distributed among different name servers.

The DNS message header structure is shown in the following illustration:

16	21						28	32 bits
ID	Q	Query	A	T	R	V	B	Rcode
Question count	Answer count
Authority count	Additional count
DNS message header structure

ID
16-bit field used to correlate queries and responses.

Q
1-bit field that identifies the message as a query or response.

Query
4-bit field that describes the type of message:

0	Standard query (name to address).
1	Inverse query (address to name).
2	Server status request.

A
Authoritative Answer. 1-bit field. When set to 1, identifies the response as one made by an authoritative name server.

T
Truncation. 1-bit field. When set to 1, indicates the message has been truncated.

R
1-bit field. Set to 1 by the resolve to request recursive service by the name server.

V
1-bit field. Signals the availability of recursive service by the name server.

B
3-bit field. Reserved for future use. Must be set to 0.

RCode
Response Code. 4-bit field that is set by the name server to identify the status of the query:

0	No error condition.
1	Unable to interpret query due to format error.
2	Unable to process due to server failure.
3	Name in query does not exist.
4	Type of query not supported.
5	Query refused.

Question count
16-bit field that defines the number of entries in the question section.

Answer count
16-bit field that defines the number of resource records in the answer section.

Authority count
16-bit field that defines the number of name server resource records in the authority section.

Additional count
16-bit field that defines the number of resource records in the additional records section.