Reading 4-bit value from unsigned pointer to int: TCP data offset reading impaired

Question

00000   45 00 02 87 8a 97 40 00  40 06 af d7 7f 00 00 01    E.....@.@.......
00016   7f 00 00 01 e0 56 00 50  0d 46 70 a8 0c e2 41 70    .....V.P.Fp...Ap
00032   80 18 01 01 b9 70 00 00  01 01 08 0a 00 64 fa 32    .....p.......d.2

This is how Wireshark/tcpdump/my code represents a part of TCP packet stored in dump file. Third line's first value, 80, it a correct TCP data offset of 32 bytes, or 8 32-bit (4 byte) words.

However, when (in my code), struct tcp_hdr is created over this payload (including ip header offset etc), the result of reading my_tcphdr->doff is 6, (equivalent for 24 bytes).

Suppose that reading such data from structure somehow isn't enough healthy.

How can I read this 4-bit (0xF0?) value to some int, given the (known as correct) pointer: unsigned char *doff = tcp_payload+MAGIC_16?

nos nos · Accepted Answer · 2011-01-30T22:04:41

You'd extract the byte and shift it 4 places to the right.

unsigned char *doff = tcp_payload+MAGIC_16
unsigned char doffval = (*doff) >>4;

As of why your struct tcp_hdr didn't "work", you should post that. Perhaps it's using the wrong endian, or the struct get padded so it doesn't match the on wire format.

Reading 4-bit value from unsigned pointer to int: TCP data offset reading impaired

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