How to append data on a packet from kernel space?

Question

I am trying to append some data on a packet from kernel space. I have an echo client and server. I type in the command line like: ./client \"message\" and the server just

Answer 1

About one year ago for kernel 2.6.26 I did it like this:

// Do we need extra space?
if(len - skb_tailroom(skb) > 0){

  // Expand skb tail until we have enough room for the extra data
  if (pskb_expand_head(skb, 0, extra_data_len - skb_tailroom(skb), GFP_ATOMIC)) {
    // allocation failed. Do whatever you need to do
  }

  // Allocation succeeded

  // Reserve space in skb and return the starting point
  your_favourite_structure* ptr = (your_favourite_structure*) 
                                  skb_push(skb, sizeof(*ptr)); 

  // Now either set each field of your structure or memcpy into it.
  // Remember you can use a char*

}

Don't forget:

• Recalculate UDP checksum, because you changed data in the transported data.

• Change the field tot_len(total length) in the ip header, because you added data to the packet.

• Recalculate the IP header checksum, because you changed the tot_len field.

Extra note: This is just a simple thing. I see in your code you're allocating tmp as a 200 byte array and using that to store the data of your message. If you send a bigger packet you'll have a hard time debugging this as kernel crashes due to memory overflows are too painful.