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 xt_target_param *par)
  24 {
  25         struct iphdr *iph;
  26         const struct ipt_TTL_info *info = par->targinfo;
  27         int new_ttl;
  28
  29         if (!skb_make_writable(skb, skb->len))
  30                 return NF_DROP;
  31
  32         iph = ip_hdr(skb);
  33
  34         switch (info->mode) {
  35                 case IPT_TTL_SET:
  36                         new_ttl = info->ttl;
  37                         break;
  38                 case IPT_TTL_INC:
  39                         new_ttl = iph->ttl + info->ttl;
  40                         if (new_ttl > 255)
  41                                 new_ttl = 255;
  42                         break;
  43                 case IPT_TTL_DEC:
  44                         new_ttl = iph->ttl - info->ttl;
  45                         if (new_ttl < 0)
  46                                 new_ttl = 0;
  47                         break;
  48                 default:
  49                         new_ttl = iph->ttl;
  50                         break;
  51         }
  52
  53         if (new_ttl != iph->ttl) {
  54                 csum_replace2(&iph->check, htons(iph->ttl << 8),
  55                                            htons(new_ttl << 8));
  56                 iph->ttl = new_ttl;
  57         }
  58
  59         return XT_CONTINUE;
  60 }
  61
  62 static bool ttl_tg_check(const struct xt_tgchk_param *par)
  63 {
  64         const struct ipt_TTL_info *info = par->targinfo;
  65
  66         if (info->mode > IPT_TTL_MAXMODE) {
  67                 printk(KERN_WARNING "ipt_TTL: invalid or unknown Mode %u\n",
  68                         info->mode);
  69                 return false;
  70         }
  71         if (info->mode != IPT_TTL_SET && info->ttl == 0)
  72                 return false;
  73         return true;
  74 }
  75
  76 static struct xt_target ttl_tg_reg __read_mostly = {
  77         .name           = "TTL",
  78         .family         = NFPROTO_IPV4,
  79         .target         = ttl_tg,
  80         .targetsize     = sizeof(struct ipt_TTL_info),
  81         .table          = "mangle",
  82         .checkentry     = ttl_tg_check,
  83         .me             = THIS_MODULE,
  84 };
  85
  86 static int __init ttl_tg_init(void)
  87 {
  88         return xt_register_target(&ttl_tg_reg);
  89 }
  90
  91 static void __exit ttl_tg_exit(void)
  92 {
  93         xt_unregister_target(&ttl_tg_reg);
  94 }
  95
  96 module_init(ttl_tg_init);
  97 module_exit(ttl_tg_exit);