root/net/ipv4/netfilter/ipt_TTL.c

DEFINITIONS

1. ttl_tg
2. ttl_tg_check
3. ttl_tg_init
4. ttl_tg_exit

/* TTL modification target for IP tables
 * (C) 2000,2005 by Harald Welte <laforge@netfilter.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <net/checksum.h>

#include <linux/netfilter/x_tables.h>
#include <linux/netfilter_ipv4/ipt_TTL.h>

MODULE_AUTHOR("Harald Welte <laforge@netfilter.org>");
MODULE_DESCRIPTION("Xtables: IPv4 TTL field modification target");
MODULE_LICENSE("GPL");

static unsigned int
ttl_tg(struct sk_buff *skb, const struct xt_target_param *par)
{
        struct iphdr *iph;
        const struct ipt_TTL_info *info = par->targinfo;
        int new_ttl;

        if (!skb_make_writable(skb, skb->len))
                return NF_DROP;

        iph = ip_hdr(skb);

        switch (info->mode) {
                case IPT_TTL_SET:
                        new_ttl = info->ttl;
                        break;
                case IPT_TTL_INC:
                        new_ttl = iph->ttl + info->ttl;
                        if (new_ttl > 255)
                                new_ttl = 255;
                        break;
                case IPT_TTL_DEC:
                        new_ttl = iph->ttl - info->ttl;
                        if (new_ttl < 0)
                                new_ttl = 0;
                        break;
                default:
                        new_ttl = iph->ttl;
                        break;
        }

        if (new_ttl != iph->ttl) {
                csum_replace2(&iph->check, htons(iph->ttl << 8),
                                           htons(new_ttl << 8));
                iph->ttl = new_ttl;
        }

        return XT_CONTINUE;
}

static bool ttl_tg_check(const struct xt_tgchk_param *par)
{
        const struct ipt_TTL_info *info = par->targinfo;

        if (info->mode > IPT_TTL_MAXMODE) {
                printk(KERN_WARNING "ipt_TTL: invalid or unknown Mode %u\n",
                        info->mode);
                return false;
        }
        if (info->mode != IPT_TTL_SET && info->ttl == 0)
                return false;
        return true;
}

static struct xt_target ttl_tg_reg __read_mostly = {
        .name           = "TTL",
        .family         = NFPROTO_IPV4,
        .target         = ttl_tg,
        .targetsize     = sizeof(struct ipt_TTL_info),
        .table          = "mangle",
        .checkentry     = ttl_tg_check,
        .me             = THIS_MODULE,
};

static int __init ttl_tg_init(void)
{
        return xt_register_target(&ttl_tg_reg);
}

static void __exit ttl_tg_exit(void)
{
        xt_unregister_target(&ttl_tg_reg);
}

module_init(ttl_tg_init);
module_exit(ttl_tg_exit);