title: modify data packet netfilter
body: <p>This is 2 example:
  <a href="https://stackoverflow.com/questions/12529497/how-to-append-data-on-a-packet-from-kernel-space?rq=1">How to append data on a packet from kernel space?</a>
  <a href="https://stackoverflow.com/questions/12999548/how-to-route-the-splitted-packets-using-netfilter-hooks-in-kernel-space">How to route the splitted packets using netfilter hooks in kernel space</a>
  I just want change data coming server at hook LOCAL_IN, this is similar spllitted example.
  At append data example, that is ok. But splitted example, that is not work.
  I think problem is update length, checksum udp,ip packet(example: the value offset in calculating checksum at hook LOCAL_IN and LOCAL_OUT is different( int offset = skb_transport_offset(skb)) because when a packet goes in, packet is processed before go to udp layer).I try to alter htons -> ntohs but that is not work.</p>
  
  <p>Anyone have idea to solve? Thanks</p>