TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 12-5 DNS message format
Header format
The header section is always present and has a fixed length of 12 bytes. The
other sections are of variable length.
ID A 16-bit identifier assigned by the program. This identifier is
copied in the corresponding reply from the name server and can
be used for differentiation of responses when multiple queries
are outstanding at the same time.
Parameters A 16-bit value in the following format (Table 12-3).
Table 12-3 Parameters
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Q
R
Op code A
A
Where:
QR Flag identifying a query (0) or a response(1).
Op code 4-bit field specifying the kind of query:
440 TCP/IP Tutorial and Technical Overview
T
C
R
D
R
A
0 Standard query (QUERY)
1 Inverse query (IQUERY)
2 Server status request (STATUS)
Zero Rcode