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[linux-2.6] / 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 struct ipt_entry *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);