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);