HI all!!
I am trying to do nat through my applictaion..But i am
not able to calculate the right checksum..The code i
am using for calculating checksum is the standarad
one..but i am not getting the right checksum..Just for
testing purposes, i set the checksum field of ip
header to 0 and calculated the checksum of ip header
without any changes..but i am getting a different
answer..
This is what i am doing:
/////Printing original checksum
printf(" CHECKSUM:%d ", ntohs(ip->check));
/////Recalculating the checksum of same pkt after
setting checksum field to 0
ip->check=0;
prinf("checksum %d",ntohs(csum_part(ip,iplen, 0)));
The code for calculating checksum i am using is:
u_int16_t csum_part(void *buffer, unsigned int len,
u_int16_t prevsum)
{
u_int32_t sum = 0;
u_int16_t *ptr = buffer;
while (len > 1) {
sum += *ptr++;
len -= 2;
}
if (len) {
union {
u_int8_t byte;
u_int16_t wyde;
} odd;
odd.wyde = 0;
odd.byte = *((u_int8_t *)ptr);
sum += odd.wyde;
}
sum = (sum >> 16) + (sum & 0xFFFF);
sum += prevsum;
return (sum + (sum >> 16));
}
Can somebody tell me where exactly is the problem..Why
am i not getting the right checksum,???????????///
Thanx in advance,
Paridhi
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