netfilter and pskb

alok_sethi · 03-12-2006, 11:45 PM

Hi all
using net filter when i try to access the address of udp header using
temp =(__u32 *) ((*pskb)->h.uh)

and when i try to access the same address using data pointer

temp=(__u32 *)(*pskb)->data;
temp+=5;

value of temp in both cases comes out to be different.

in the second case the value returned by *temp is the correct udp header
but the value returned by *temp in first case is not correct

can some one please help me to find out what i am doing wrong
thanks

regards
alok

Mara · 03-14-2006, 05:09 PM

Try to print both values (addresses). My guess (a good one) is that (*pskb)->h.uh will give you a pointer to the beginning of the IP header. The thing is that h in sk_buff is an union. It has more possibilities than only uh. You should calculate the offset on your own. Remember, however, that just adding 5 may not be the right way to go if there are IP options (it's very rare but may happen). Use the value from IP header instead.