Projects…interjects…Part Deux!


Yesterday was a fantastic day trying to get the IP data field to be pointed to by in the skbuff structure. The documentation in the files did not help make things clearer.
The situation was where I was supposed to use the
unsigned char* data field in the skbuff structure to point to the IP data starting point.
Tried a lot of pointer math and the following finally worked:
IP Data pointer location:

unsigned char * ptr = sb->data +
int byte_size = ntohs(sb->nh.iph->tot_len) –

In fact, Vinay Reddy (vinayvinay@gmail.com) suggested something which I think was even better than the stuff that was working for me. He said the pointer value should be:

unsigned char * ptr = sb->nh.iph->raw + sb->nh.iph->ihl*4;

I think this actually grabs the gist of what I exactly want to do.
I *really* want to point with respect to the IP Header. I do not really care about where sb->data really points to so I guess Vinay’s method is much better. Haven’t implemented it so I really don’t know but sounds the most logical!

– Rajat


Brand New Day


It’s a brand new day with no novelty! Back to the lab today trying to now get access to the packet data to calculate the hash values. I suspect that inside netfilter’s sk_buff structure there’s an unsigned char* data field. This probably is exactly what I need to get the hash values. There’s this awesome link which has great information about sk_buff structure. The unsigned int len; has the size of the complete input data including the headers. I guess if this len value == size of the actual data for the IP header (which could be TCP header / UDP header / ICMP header) then if we are using chunks of this data to find hashes then the following algorithm could be used:

no_of_chunks = len / BYTE_SIZE_FOR_SIGN;

addendum = len % BYTE_SIZE_FOR_SIGN;

for (int i = 0; i < no_of_chunks; i++)
(i+1)*BYTE_SIZE_FOR_SIGN - 1 ,0));
no_of_chunks*BYTE_SIZE_FOR_SIGN+addendum, 0)

This are my initial thoughts let’s see how it works out!

Rajat’s Homepage


9 hrs…still capturing packets!


9 hrs down the line I’m still capturing packets, now comes the challenging part-to make the hash tables of raw data! My Rabin’s hash algo seems to be working but the only hiccup that could happen was when the negative hash values are obtained. I don’t know whether it is a bug in my code or what but I got negative values for Rabin’s Hash when I gave the string “this is test”! This is gonna test me for sure.