The Structure Of Internet Host Names | How Does A DNS Server Work | Cashing In DNS | DNS Message Format | Useful Links

DNS Message Format   All communications inside the domain protocol are carried in a single format called a message. The top-level format of the message is divided into five sections, shown here:         IDENTIFICATION FLAGS   NUMBER OF QUESTIONS NUMBER OF ANSVER RRs Header  12 bytes NUMBER OF AUTHORITY RRs NUMBER OF ADDITIONAL RRs   QUESTIONS   ANSWERS   AUTHORITY   ADDITIONAL INFORMATION   0----------------------------------15-16------------------------------31 Fig. 3   DNS message format 1. HEADER SECTION:     IDENTIFICATION: is set by the client and returned by the server.      The 16-bit PARAMETER consists of: Bit Meaning 0 Operation: 0 = Query, 1 = Response 1-4 Query type: 0 = Standard, 1 = Inverse, 2 = server status request. 5 Set if answer authoritative 6 Set if message truncated 7 Set if recursion desired 8 Set if recursion available 9-11 Reserved 12-15 Response type: 0 = No error, 1 = Format error, 2 = Server Failure, 3 = Name does not exit     NUMBER OF ....: give each a count of entries in the corresponding sections in the message.

2. QUESTION SECTION   Contains queries for which answers are desired. The client fills in only the question section; the server returns the question and answers with its response. Each question has Query Domain Name followed by Query Type and Query Class fields. 3. ANSWER, AUTHORITY, ADDITIONAL INFORMATION   Sections consist of a set of resource records that describe domain names and mappings. Each resource record describes one name.