计算传输层效验和

#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>

#include <net/ip.h>
#include <net/tcp.h>

unsigned int sg_nf_in(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;
    u16 checksum;
        
    if ((iph = ip_hdr(skb)) == NULL)
        goto out;

    skb_pull(skb, ip_hdrlen(skb));
    skb_reset_transport_header(skb);
    skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr, skb->len, IPPROTO_TCP, 0);
    checksum = __skb_checksum_complete(skb);    

    printk(KERN_INFO "checksum = %u\n", checksum);
out:
    return NF_ACCEPT;
}


static struct nf_hook_ops sg_nf_ops = {
    .hook      = sg_nf_in,
    .owner     = THIS_MODULE,
    .pf     = PF_INET,
    .hooknum = NF_INET_LOCAL_IN,
};

static int __init main_init(void)
{
    if (nf_register_hook(&sg_nf_ops) != 0)
        goto out;

    return 0;
out:
    return -1;
}

static void __exit main_exit(void)
{
    nf_unregister_hook(&sg_nf_ops);
}

module_init(main_init);
module_exit(main_exit);
MODULE_LICENSE("GPL");