Linux netfilter example--1

#include <linux/module.h>#include <linux/init.h>#include <linux/netfilter.h>#include <linux/netfilter_ipv4.h>#include <linux/kernel.h>MODULE_LICENSE("Dual BSD/GPL");static struct nf_hook_ops nfho;         //struct holding set of hook function options//function to be called by hookunsigned 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 *)){  printk(KERN_INFO "packet dropped\n");                                             //log to var/log/messages  return NF_DROP;                                                                   //drops the packet}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);