How to modify struct sk_buff

Question

I have to write a vpn module. First of all, I have wrote a kernel module that modifies all the incoming and outgoing TCP packets. It uses netfilter hooks. For the incoming packets, I have modified the bytes between (struct sk_buff)->data and (struct sk_buff)->tail pointers by incrementing them by one. For the outgoing packets, I have modified the bytes between (struct sk_buff)->data and (struct sk_buff)->tail pointers by decrementing them by one.

However, I tried to establish a TCP connection between localhost and localhost (by means of netcat) and I had not succeeded. Can you tell me what I am doing wrong? Need I modify some other fields from the struct sk_buff structure?

Answer 1

Is it possible to implement my simple vpn module only from kernel space?(thus without using special libraies such as libnetfilter_queue)?

Thank you.

Answer 2

Yes, you can do this without using libnetfilter. But given the limited information that you've provided about your project it's hard to give a good recommendation as to how to go about fixing your issue. Here's some references that should help.

1) I would recommend you take a look at the TUN/TAP interface driver APIs. This will allow you to implement your code in application space rather than kernel. See openvpn for a great example of this type of VPN.

If you're interested in doing more advanced kernel space hooking...
2) Check this article out on hooking into netfilter netfilter kernel hooks