net/ipv4/netfilter/ipt_TTL.c

   1 /* TTL modification target for IP tables
   2  * (C) 2000,2005 by Harald Welte <laforge@netfilter.org>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License version 2 as
   6  * published by the Free Software Foundation.
   7  *
   8  */
   9
  10 #include <linux/module.h>
  11 #include <linux/skbuff.h>
  12 #include <linux/ip.h>
  13 #include <net/checksum.h>
  14
  15 #include <linux/netfilter/x_tables.h>
  16 #include <linux/netfilter_ipv4/ipt_TTL.h>
  17
  18 MODULE_AUTHOR("Harald Welte <laforge@netfilter.org>");
  19 MODULE_DESCRIPTION("Xtables: IPv4 TTL field modification target");
  20 MODULE_LICENSE("GPL");
  21
  22 static unsigned int
  23 ttl_tg(struct sk_buff *skb, const struct net_device *in,
  24        const struct net_device *out, unsigned int hooknum,
  25        const struct xt_target *target, const void *targinfo)
  26 {
  27         struct iphdr *iph;
  28         const struct ipt_TTL_info *info = targinfo;
  29         int new_ttl;
  30
  31         if (!skb_make_writable(skb, skb->len))
  32                 return NF_DROP;
  33
  34         iph = ip_hdr(skb);
  35
  36         switch (info->mode) {
  37                 case IPT_TTL_SET:
  38                         new_ttl = info->ttl;
  39                         break;
  40                 case IPT_TTL_INC:
  41                         new_ttl = iph->ttl + info->ttl;
  42                         if (new_ttl > 255)
  43                                 new_ttl = 255;
  44                         break;
  45                 case IPT_TTL_DEC:
  46                         new_ttl = iph->ttl - info->ttl;
  47                         if (new_ttl < 0)
  48                                 new_ttl = 0;
  49                         break;
  50                 default:
  51                         new_ttl = iph->ttl;
  52                         break;
  53         }
  54
  55         if (new_ttl != iph->ttl) {
  56                 csum_replace2(&iph->check, htons(iph->ttl << 8),
  57                                            htons(new_ttl << 8));
  58                 iph->ttl = new_ttl;
  59         }
  60
  61         return XT_CONTINUE;
  62 }
  63
  64 static bool
  65 ttl_tg_check(const char *tablename, const void *e,
  66              const struct xt_target *target, void *targinfo,
  67              unsigned int hook_mask)
  68 {
  69         const struct ipt_TTL_info *info = targinfo;
  70
  71         if (info->mode > IPT_TTL_MAXMODE) {
  72                 printk(KERN_WARNING "ipt_TTL: invalid or unknown Mode %u\n",
  73                         info->mode);
  74                 return false;
  75         }
  76         if (info->mode != IPT_TTL_SET && info->ttl == 0)
  77                 return false;
  78         return true;
  79 }
  80
  81 static struct xt_target ttl_tg_reg __read_mostly = {
  82         .name           = "TTL",
  83         .family         = AF_INET,
  84         .target         = ttl_tg,
  85         .targetsize     = sizeof(struct ipt_TTL_info),
  86         .table          = "mangle",
  87         .checkentry     = ttl_tg_check,
  88         .me             = THIS_MODULE,
  89 };
  90
  91 static int __init ttl_tg_init(void)
  92 {
  93         return xt_register_target(&ttl_tg_reg);
  94 }
  95
  96 static void __exit ttl_tg_exit(void)
  97 {
  98         xt_unregister_target(&ttl_tg_reg);
  99 }
 100
 101 module_init(ttl_tg_init);
 102 module_exit(ttl_tg_exit);