Querying

All communications inside of the domain protocol are carried in a single format called a message. The top level format of message is divided into 5 sections (some of which are empty in certain cases) shown below:

+---------------------+
|        Header       |
+---------------------+
|       Question      | the question for the name server
+---------------------+
|        Answer       | RRs answering the question
+---------------------+
|      Authority      | RRs pointing toward an authority
+---------------------+
|      Additional     | RRs holding additional information
+---------------------+

The header section is always present. The header includes fields that specify which of the remaining sections are present, and also specify whether the message is a query or a response, a standard query or some other opcode, etc.

The names of the sections after the header are derived from their use in standard queries. The question section contains fields that describe a question to a name server. These fields are a query type (QTYPE), a query class (QCLASS), and a query domain name (QNAME). The last three sections have the same format: a possibly empty list of concatenated resource records (RRs). The answer section contains RRs that answer the question; the authority section contains RRs that point toward an authoritative name server; the additional records section contains RRs which relate to the query, but are not strictly answers for the question.

A one bit field that specifies whether this message is a query (0), or a response (1).

• query: query to a name server
• response: response from a name server

A four bit field that specifies kind of query in this message. This value is set by the originator of a query and copied into the response.

• query: a standard query (QUERY)
• inverseQuery: an inverse query (IQUERY)
• statusRequest: a server status request (STATUS)
• notify: (NOTIFY)
• update: (UPDATE)

Undocumented

Response code - this 4 bit field is set as part of responses.