Linux netfilter example-1

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 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 *))
{
  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);