Hi, Glad to meet you all!
I got a problem in cksum computation for UDP packet. At the end of this mail, there is
a function copied from a textbook. I believe it is a universal cksum calculation function.
I captured a UDP packet on the net:
0x07, 0x2c, 0x00, 0x35, 0x00, 0x0c, 0x98, 0x8e, 0x64, 0x69, 0x72, 0x0a
The 6th and 7th element is the cksum computed by the sender's kernel. My question is that
when I use in_cksum to compute the cksum, it returns 0x1f22, different from 0x98, 0x8e.
What's wrong here? Is there any other algorithm to compute cksum?
Thank you in advance!
Qin An
==========================================
unsigned short in_cksum (unsigned short *ptr, int nbytes)
{
unsigned long sum; /* assumes long == 32 bits */
unsigned short oddbyte, answer; /* assumes short == 16 bits */
sum = 0;
while (nbytes > 1){
sum += *ptr++;
nbytes -= 2;
}
/* mop up an odd byte, if necessary */
if (nbytes == 1){
/* make sure top half is zero */
oddbyte = 0;
/* one byte only */
* ((unsigned char *) &oddbyte) = *(unsigned char *) ptr;
sum += oddbyte;
}
/* add back carry outs from top 16 bits to low 16 bits */
/* add high-16 to low-16 */
sum = (sum >> 16) + (sum & 0xffff);
/* add carry */
sum += (sum >> 16);
/* ones-compliment, then truncate to 16 bits */
answer = (unsigned short)(~sum);
return answer;
}
unsigned char s[] = {
0x07, 0x2c, 0x00, 0x35, 0x00, 0x0c, 0x98,
0x8e, 0x64, 0x69, 0x72, 0x0a};
int main (int argc, char *argv[])
{
int i;
s[6] = s[7] = 0;
i = in_cksum ((unsigned short *) &s[0], 12);
}
This archive was generated by hypermail 2b30 : Wed Jul 30 2003 - 07:55:05 PDT