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_ipv4/ip_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, const struct net_device *in,
  24                 const struct net_device *out, unsigned int hooknum,
  25                 const void *targinfo, void *userinfo)
  26 {
  27         struct iphdr *iph;
  28         const struct ipt_TTL_info *info = targinfo;
  29         u_int16_t diffs[2];
  30         int new_ttl;
  31
  32         if (!skb_make_writable(pskb, (*pskb)->len))
  33                 return NF_DROP;
  34
  35         iph = (*pskb)->nh.iph;
  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                 diffs[0] = htons(((unsigned)iph->ttl) << 8) ^ 0xFFFF;
  58                 iph->ttl = new_ttl;
  59                 diffs[1] = htons(((unsigned)iph->ttl) << 8);
  60                 iph->check = csum_fold(csum_partial((char *)diffs,
  61                                                     sizeof(diffs),
  62                                                     iph->check^0xFFFF));
  63         }
  64
  65         return IPT_CONTINUE;
  66 }
  67
  68 static int ipt_ttl_checkentry(const char *tablename,
  69                 const void *e,
  70                 void *targinfo,
  71                 unsigned int targinfosize,
  72                 unsigned int hook_mask)
  73 {
  74         struct ipt_TTL_info *info = targinfo;
  75
  76         if (targinfosize != IPT_ALIGN(sizeof(struct ipt_TTL_info))) {
  77                 printk(KERN_WARNING "ipt_TTL: targinfosize %u != %Zu\n",
  78                                 targinfosize,
  79                                 IPT_ALIGN(sizeof(struct ipt_TTL_info)));
  80                 return 0;
  81         }
  82
  83         if (strcmp(tablename, "mangle")) {
  84                 printk(KERN_WARNING "ipt_TTL: can only be called from "
  85                         "\"mangle\" table, not \"%s\"\n", tablename);
  86                 return 0;
  87         }
  88
  89         if (info->mode > IPT_TTL_MAXMODE) {
  90                 printk(KERN_WARNING "ipt_TTL: invalid or unknown Mode %u\n",
  91                         info->mode);
  92                 return 0;
  93         }
  94
  95         if ((info->mode != IPT_TTL_SET) && (info->ttl == 0))
  96                 return 0;
  97
  98         return 1;
  99 }
 100
 101 static struct ipt_target ipt_TTL = {
 102         .name           = "TTL",
 103         .target         = ipt_ttl_target,
 104         .checkentry     = ipt_ttl_checkentry,
 105         .me             = THIS_MODULE,
 106 };
 107
 108 static int __init init(void)
 109 {
 110         return ipt_register_target(&ipt_TTL);
 111 }
 112
 113 static void __exit fini(void)
 114 {
 115         ipt_unregister_target(&ipt_TTL);
 116 }
 117
 118 module_init(init);
 119 module_exit(fini);