#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>

static unsigned int my_nf_hookfn(unsigned int hooknum,
                 struct sk_buff *skb,
                 const struct net_device *in,
                 const struct net_device *out,
                 int (*okfn)(struct sk_buff *))
{
    /* process packet */
    //...

    return NF_ACCEPT;
}

static struct nf_hook_ops my_nfho = {
    .owner       = THIS_MODULE,
    .hook        = my_nf_hookfn,
    .hooknum     = NF_IP_LOCAL_IN,
    .pf          = PF_INET,
    .priority    = NF_IP_PRI_FIRST
};

int __init my_hook_init(void)
{
    return nf_register_hook(&my_nfho);
}

void __exit my_hook_exit(void)
{
    nf_unregister_hook(&my_nfho);
}

module_init(my_hook_init);
module_exit(my_hook_exit);