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   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("IP tables TTL modification module");
  20 MODULE_LICENSE("GPL");
  21
  22 static unsigned int
  23 ipt_ttl_target(struct sk_buff **pskb,
  24                const struct net_device *in, const struct net_device *out,
  25                unsigned int hooknum, const struct xt_target *target,
  26                const void *targinfo)
  27 {
  28         struct iphdr *iph;
  29         const struct ipt_TTL_info *info = targinfo;
  30         int new_ttl;
  31
  32         if (!skb_make_writable(pskb, (*pskb)->len))
  33                 return NF_DROP;
  34
  35         iph = ip_hdr(*pskb);
  36
  37         switch (info->mode) {
  38                 case IPT_TTL_SET:
  39                         new_ttl = info->ttl;
  40                         break;
  41                 case IPT_TTL_INC:
  42                         new_ttl = iph->ttl + info->ttl;
  43                         if (new_ttl > 255)
  44                                 new_ttl = 255;
  45                         break;
  46                 case IPT_TTL_DEC:
  47                         new_ttl = iph->ttl - info->ttl;
  48                         if (new_ttl < 0)
  49                                 new_ttl = 0;
  50                         break;
  51                 default:
  52                         new_ttl = iph->ttl;
  53                         break;
  54         }
  55
  56         if (new_ttl != iph->ttl) {
  57                 nf_csum_replace2(&iph->check, htons(iph->ttl << 8),
  58                                               htons(new_ttl << 8));
  59                 iph->ttl = new_ttl;
  60         }
  61
  62         return XT_CONTINUE;
  63 }
  64
  65 static bool ipt_ttl_checkentry(const char *tablename,
  66                 const void *e,
  67                 const struct xt_target *target,
  68                 void *targinfo,
  69                 unsigned int hook_mask)
  70 {
  71         const struct ipt_TTL_info *info = targinfo;
  72
  73         if (info->mode > IPT_TTL_MAXMODE) {
  74                 printk(KERN_WARNING "ipt_TTL: invalid or unknown Mode %u\n",
  75                         info->mode);
  76                 return false;
  77         }
  78         if (info->mode != IPT_TTL_SET && info->ttl == 0)
  79                 return false;
  80         return true;
  81 }
  82
  83 static struct xt_target ipt_TTL __read_mostly = {
  84         .name           = "TTL",
  85         .family         = AF_INET,
  86         .target         = ipt_ttl_target,
  87         .targetsize     = sizeof(struct ipt_TTL_info),
  88         .table          = "mangle",
  89         .checkentry     = ipt_ttl_checkentry,
  90         .me             = THIS_MODULE,
  91 };
  92
  93 static int __init ipt_ttl_init(void)
  94 {
  95         return xt_register_target(&ipt_TTL);
  96 }
  97
  98 static void __exit ipt_ttl_fini(void)
  99 {
 100         xt_unregister_target(&ipt_TTL);
 101 }
 102
 103 module_init(ipt_ttl_init);
 104 module_exit(ipt_ttl_fini);