linux skb hook

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

static struct nf_hook_ops nfho;

static unsigned int hook_func(const struct nf_hook_ops *ops, struct sk_buff *skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *)) {
    printk(KERN_INFO "Packet received: %pI4 -> %pI4\n", &ip_hdr(skb)->saddr, &ip_hdr(skb)->daddr);
    return NF_ACCEPT;
}

static int __init skb_hook_init(void) {
    nfho.hook = hook_func;
    nfho.hooknum = NF_INET_PRE_ROUTING;
    nfho.pf = PF_INET;
    nfho.priority = NF_IP_PRI_FIRST;

    if (nf_register_hook(&nfho)) {
        printk(KERN_ERR "Failed to register skb hook\n");
        return -EAGAIN;
    }

    printk(KERN_INFO "skb hook registered\n");
    return 0;
}

static void __exit skb_hook_exit(void) {
    nf_unregister_hook(&nfho);
    printk(KERN_INFO "skb hook unregistered\n");
}

module_init(skb_hook_init);
module_exit(skb_hook_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("A simple skb hook module");