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

   
MODULE_LICENSE("Dual BSD/GPL");

static struct nf_hook_ops nfho;         //struct holding set of hook function options


struct sk_buff *sock_buff;
struct udphdr *udp_header;          //udp header struct (not used)
struct iphdr *ip_header;

//function to be called by hook
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 hello_init(void)   
{   
  nfho.hook = hook_func;                       //function to call when conditions below met
  nfho.hooknum = NF_IP_PRE_ROUTING;            //called right after packet recieved, first hook in Netfilter
  nfho.pf = PF_INET;                           //IPV4 packets
  nfho.priority = NF_IP_PRI_FIRST;             //set to highest priority over all other hook functions
  nf_register_hook(&nfho);                     //register hook

  return 0; 
}   
static void hello_exit(void)   
{   
    nf_unregister_hook(&nfho);
}   
module_init(hello_init);   
module_exit(hello_exit); 

#KVER = /usr/src/linux-source-2.6.38  
KVER = /lib/modules/2.6.18-274.el5/build  
CURDIR = $(shell pwd)  
# Kernel modules  
obj-m := hello.o  
build: kernel_modules  
kernel_modules:  
        $(MAKE) -C $(KVER) M=$(CURDIR) modules  
clean:  
        $(MAKE) -C $(KVER) M=$(CURDIR) clean