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I'm studying about dns and would like to understand about this information, because I could not fully understand.

The header contains the following fields:

                                1  1  1  1  1  1
  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                      ID                       |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    QDCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ANCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    NSCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ARCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

I would to know what mean this numbers on top.

J. A
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1 Answers1

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The numbers across the top are simply the bit numbers within the 16 bit word, although as is common with the RFC series of documents they're ordered from most significant bit to least, instead of the (more intuitive) other way around.

So, for example, given an array data of octets containing that header, the ID would be:

 (data[0] << 8) | data[1]

and the QR bit would be the most significant bit of data[2]

Alnitak
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  • ok so -> `0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5` this is the 16bit word. and about -> `1 1 1 1 1 1` what this is? Thanks for your answer. – J. A Aug 09 '16 at 22:19
  • The '1's are just the "tens" for the "0 1 2 3 4 5" underneath them, so that they go from 0 .. 15 – Alnitak Aug 10 '16 at 08:37
  • and no, I don't know why the RFC author decided to write them vertically instead of horizontally. Next time I see him, I'll ask. – Alnitak Aug 10 '16 at 08:44