//v2 netfilter hooks example
//For any packet, get the ip header and check the protocol field
//if the protocol number equal to UDP (17), log in var/log/messages
//default action of module to let all packets through

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

static struct nf_hook_ops nfho;   //net filter hook option struct
struct sk_buff *sock_buff;
struct udphdr *udp_header;        //udp header struct (not used)
struct iphdr *ip_header;          //ip header struct

static unsigned int hook_func(unsigned int hooknum,
                              struct sk_buff **skb,
                              const struct net_device *in,
                              const struct net_device *out,
                              int (*okfn)(struct sk_buff *))
{
        sock_buff = *skb;
        ip_header = (struct iphdr *)skb_network_header(sock_buff);    //grab network header using accessor
        if(!sock_buff) { return NF_ACCEPT;}
        if (ip_header->protocol==17) {
                udp_header = (struct udphdr *)skb_transport_header(sock_buff);  //grab transport header
                printk(KERN_INFO "got udp packet \n");     //log we’ve got udp packet to /var/log/messages
                return NF_DROP;
        }
        return NF_ACCEPT;
}

static int __init init_main(void)
{
        nfho.hook = hook_func;
        nfho.hooknum = 1;
        nfho.pf = PF_INET;
        nfho.priority = NF_IP_PRI_FIRST;
        nf_register_hook(&nfho);
        return 0;
}

static void __exit cleanup_main(void)
{
        nf_unregister_hook(&nfho);
}

module_init(init_main);
module_exit(cleanup_main);