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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
18MODULE_AUTHOR("Harald Welte <laforge@netfilter.org>");
19MODULE_DESCRIPTION("IP tables TTL modification module");
20MODULE_LICENSE("GPL");
21
22static unsigned int
23ipt_ttl_target(struct sk_buff *skb,
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(skb, skb->len))
33 return NF_DROP;
34
35 iph = ip_hdr(skb);
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
65static 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
83static 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
93static int __init ipt_ttl_init(void)
94{
95 return xt_register_target(&ipt_TTL);
96}
97
98static void __exit ipt_ttl_fini(void)
99{
100 xt_unregister_target(&ipt_TTL);
101}
102
103module_init(ipt_ttl_init);
104module_exit(ipt_ttl_fini);
105