I am writing a kernel module that actually modifies the TCP header(both at sending and receiving) and I need to re-calculate the header because of that. I am capturing the packets using netfilter, so I have access to the whole SKB, I am using Linux 2.6.35-22 Looking into the source code of the TCP/IP, I found this function void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb); The comment above it says this routing computes the IPv4 TCP checksum Trying to use this function, I called it passing to it skb->sk and skb With no success so far. So my question is, how do I correctly calculate the TCP checksum? I know there's a psudoheader that must be created, and added to the calculation but there must be a function in the kernel I can call that already does that. Thanks in advance, Adel