1// SPDX-License-Identifier: GPL-2.0-only
2/* module that allows mangling of the arp payload */
3#include <linux/module.h>
4#include <linux/netfilter.h>
5#include <linux/netfilter_arp/arpt_mangle.h>
6#include <net/sock.h>
7
8MODULE_LICENSE("GPL");
9MODULE_AUTHOR("Bart De Schuymer <bdschuym@pandora.be>");
10MODULE_DESCRIPTION("arptables arp payload mangle target");
11
12static unsigned int
13target(struct sk_buff *skb, const struct xt_action_param *par)
14{
15 const struct arpt_mangle *mangle = par->targinfo;
16 const struct arphdr *arp;
17 unsigned char *arpptr;
18 int pln, hln;
19
20 if (skb_ensure_writable(skb, write_len: skb->len))
21 return NF_DROP;
22
23 arp = arp_hdr(skb);
24 arpptr = skb_network_header(skb) + sizeof(*arp);
25 pln = arp->ar_pln;
26 hln = arp->ar_hln;
27 /* We assume that pln and hln were checked in the match */
28 if (mangle->flags & ARPT_MANGLE_SDEV) {
29 if (ARPT_DEV_ADDR_LEN_MAX < hln ||
30 (arpptr + hln > skb_tail_pointer(skb)))
31 return NF_DROP;
32 memcpy(arpptr, mangle->src_devaddr, hln);
33 }
34 arpptr += hln;
35 if (mangle->flags & ARPT_MANGLE_SIP) {
36 if (ARPT_MANGLE_ADDR_LEN_MAX < pln ||
37 (arpptr + pln > skb_tail_pointer(skb)))
38 return NF_DROP;
39 memcpy(arpptr, &mangle->u_s.src_ip, pln);
40 }
41 arpptr += pln;
42 if (mangle->flags & ARPT_MANGLE_TDEV) {
43 if (ARPT_DEV_ADDR_LEN_MAX < hln ||
44 (arpptr + hln > skb_tail_pointer(skb)))
45 return NF_DROP;
46 memcpy(arpptr, mangle->tgt_devaddr, hln);
47 }
48 arpptr += hln;
49 if (mangle->flags & ARPT_MANGLE_TIP) {
50 if (ARPT_MANGLE_ADDR_LEN_MAX < pln ||
51 (arpptr + pln > skb_tail_pointer(skb)))
52 return NF_DROP;
53 memcpy(arpptr, &mangle->u_t.tgt_ip, pln);
54 }
55 return mangle->target;
56}
57
58static int checkentry(const struct xt_tgchk_param *par)
59{
60 const struct arpt_mangle *mangle = par->targinfo;
61
62 if (mangle->flags & ~ARPT_MANGLE_MASK ||
63 !(mangle->flags & ARPT_MANGLE_MASK))
64 return -EINVAL;
65
66 if (mangle->target != NF_DROP && mangle->target != NF_ACCEPT &&
67 mangle->target != XT_CONTINUE)
68 return -EINVAL;
69 return 0;
70}
71
72static struct xt_target arpt_mangle_reg __read_mostly = {
73 .name = "mangle",
74 .family = NFPROTO_ARP,
75 .target = target,
76 .targetsize = sizeof(struct arpt_mangle),
77 .checkentry = checkentry,
78 .me = THIS_MODULE,
79};
80
81static int __init arpt_mangle_init(void)
82{
83 return xt_register_target(target: &arpt_mangle_reg);
84}
85
86static void __exit arpt_mangle_fini(void)
87{
88 xt_unregister_target(target: &arpt_mangle_reg);
89}
90
91module_init(arpt_mangle_init);
92module_exit(arpt_mangle_fini);
93

source code of linux/net/ipv4/netfilter/arpt_mangle.c