How to calculate TCP checksum [closed]

Question

I am using netfilter hook POST_ROUTING in a kernel module to capture packets.

I am doing NOTHING to the packets, not a single change. I simply check if it's a TCP and then re-calculate the checksum(later on, I need to change some fields so I want to get the checksum calculation right).

I re-calc the checksum and once I load the module, all TCP connection simply won't work. w3m for example gets stuck at Opening Socket.

I am calculating the TCP checksum using this:

tcph->check = tcp_v4_check(skb->len - ip_hdrlen(skb), iph->saddr, iph->daddr, csum_partial((char *)tcph, skb->len - ip_hdrlen(skb), 0));

Please, what's wrong with what I am doing?

Oh, and the kernel I am using is 2.6.35-22

EDIT: I'll include my hook function:

static unsigned int post_icmp_check(unsigned int hooknum,
                   struct sk_buff *skb,
                   const struct net_device *in,
                   const struct net_device *out,
                   int (*okfn)(struct sk_buff *))
{
    struct iphdr *iph;
    struct tcphdr *tcph;    
    iph = ip_hdr(skb);

    if(iph->saddr == in_aton("127.0.0.1") || iph->daddr == in_aton("127.0.0.1"))
        return NF_ACCEPT;

    if(iph->protocol == IPPROTO_TCP){
        tcph = tcp_hdr(skb);
        tcph->check = 0;
        tcph->check = tcp_v4_check(skb->len - ip_hdrlen(skb), iph->saddr, iph->daddr, csum_partial((char *)tcph, skb->len - ip_hdrlen(skb), 0));
    }
    return NF_ACCEPT;
}

Answer 1

IIRC, TCP CRC should include IPv4 header (12 octects sized "pseudo-header") as well. Do you?