/*
	by hkpco(ChanAm Park)
	hkpco@korea.com
	http://hkpco.kr/

	cut off the MSN messenger
	using by netfilter module
*/

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/netdevice.h>
#include <linux/ip.h>
#include <linux/tcp.h>

#define MSG_CHK(x, y)	((x[y +0] == 'M') && (x[y +1] == 'S') && (x[y +2] == 'G'))
#define MSNP_CHK(x, y)	((x[y +0] == 'M') && (x[y +1] == 'S') && (x[y +2] == 'N') && (x[y +3] == 'P'))

unsigned int msn_cutting( unsigned int hook_no, struct sk_buff **pp_skb,
	const struct net_device *dev_in, const struct net_device *dev_out,
	int (*handler)(struct sk_buff *) )
{
	int pk_len, cnt = 0;
	unsigned char *pk_sig;

	struct iphdr *ip_h = (struct iphdr *)((*pp_skb)->data);
	struct tcphdr *tcp_h = (struct tcphdr *)((*pp_skb)->data + sizeof(struct iphdr));

	pk_len = (*pp_skb)->len - ((ip_h->ihl * 4) + (tcp_h->doff * 4));
	pk_sig = (unsigned char *)(tcp_h + tcp_h->doff * 4);

	if( pk_len >= 3 ) {
		do
		{
			if(MSG_CHK(pk_sig, cnt)) {
				printk( KERN_ALERT "MSN Detected - \"MSG\" string found\n" );

				return NF_DROP;
			}
		} while( (++cnt) <= (pk_len -3) );

		cnt = 0;
		if( pk_len >= 4 ) {
			do
			{
				if(MSNP_CHK(pk_sig, cnt)) {
					printk( KERN_ALERT "MSN Detected - \"MSNP\" string found\n" );

					return NF_DROP;
				}
			} while( (++cnt) <= (pk_len -4) );
		}
	}

	return NF_ACCEPT;
}

static struct nf_hook_ops netfilter = {
	.hook		= msn_cutting,
	.owner		= THIS_MODULE,
	.pf		= PF_INET,
	.hooknum  	= NF_IP_LOCAL_IN,
	.priority 	= NF_IP_PRI_FIRST
};

int __init hk_init( void )
{
	nf_register_hook( &netfilter );
	printk( KERN_ALERT "[MSN Cutting Module] Loaded\n" );

	return 0;
}

void __exit hk_exit( void )
{
	nf_unregister_hook( &netfilter );
	printk( KERN_ALERT "[MSN Cutting Module] Exit\n" );
}

module_init( hk_init );
module_exit( hk_exit );

MODULE_LICENSE( "GPL" );
MODULE_AUTHOR( "hkpco <ChanAm Park>" );