1/*
2 * nf_nat_proto_gre.c
3 *
4 * NAT protocol helper module for GRE.
5 *
6 * GRE is a generic encapsulation protocol, which is generally not very
7 * suited for NAT, as it has no protocol-specific part as port numbers.
8 *
9 * It has an optional key field, which may help us distinguishing two
10 * connections between the same two hosts.
11 *
12 * GRE is defined in RFC 1701 and RFC 1702, as well as RFC 2784
13 *
14 * PPTP is built on top of a modified version of GRE, and has a mandatory
15 * field called "CallID", which serves us for the same purpose as the key
16 * field in plain GRE.
17 *
18 * Documentation about PPTP can be found in RFC 2637
19 *
20 * (C) 2000-2005 by Harald Welte <laforge@gnumonks.org>
21 *
22 * Development of this code funded by Astaro AG (http://www.astaro.com/)
23 *
24 * (C) 2006-2012 Patrick McHardy <kaber@trash.net>
25 *
26 */
27
28#include <linux/module.h>
29#include <linux/skbuff.h>
30#include <linux/ip.h>
31
32#include <net/netfilter/nf_nat.h>
33#include <net/netfilter/nf_nat_l4proto.h>
34#include <linux/netfilter/nf_conntrack_proto_gre.h>
35
36MODULE_LICENSE("GPL");
37MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
38MODULE_DESCRIPTION("Netfilter NAT protocol helper module for GRE");
39
40/* generate unique tuple ... */
41static void
42gre_unique_tuple(const struct nf_nat_l3proto *l3proto,
43		 struct nf_conntrack_tuple *tuple,
44		 const struct nf_nat_range *range,
45		 enum nf_nat_manip_type maniptype,
46		 const struct nf_conn *ct)
47{
48	static u_int16_t key;
49	__be16 *keyptr;
50	unsigned int min, i, range_size;
51
52	/* If there is no master conntrack we are not PPTP,
53	   do not change tuples */
54	if (!ct->master)
55		return;
56
57	if (maniptype == NF_NAT_MANIP_SRC)
58		keyptr = &tuple->src.u.gre.key;
59	else
60		keyptr = &tuple->dst.u.gre.key;
61
62	if (!(range->flags & NF_NAT_RANGE_PROTO_SPECIFIED)) {
63		pr_debug("%p: NATing GRE PPTP\n", ct);
64		min = 1;
65		range_size = 0xffff;
66	} else {
67		min = ntohs(range->min_proto.gre.key);
68		range_size = ntohs(range->max_proto.gre.key) - min + 1;
69	}
70
71	pr_debug("min = %u, range_size = %u\n", min, range_size);
72
73	for (i = 0; ; ++key) {
74		*keyptr = htons(min + key % range_size);
75		if (++i == range_size || !nf_nat_used_tuple(tuple, ct))
76			return;
77	}
78
79	pr_debug("%p: no NAT mapping\n", ct);
80	return;
81}
82
83/* manipulate a GRE packet according to maniptype */
84static bool
85gre_manip_pkt(struct sk_buff *skb,
86	      const struct nf_nat_l3proto *l3proto,
87	      unsigned int iphdroff, unsigned int hdroff,
88	      const struct nf_conntrack_tuple *tuple,
89	      enum nf_nat_manip_type maniptype)
90{
91	const struct gre_hdr *greh;
92	struct gre_hdr_pptp *pgreh;
93
94	/* pgreh includes two optional 32bit fields which are not required
95	 * to be there.  That's where the magic '8' comes from */
96	if (!skb_make_writable(skb, hdroff + sizeof(*pgreh) - 8))
97		return false;
98
99	greh = (void *)skb->data + hdroff;
100	pgreh = (struct gre_hdr_pptp *)greh;
101
102	/* we only have destination manip of a packet, since 'source key'
103	 * is not present in the packet itself */
104	if (maniptype != NF_NAT_MANIP_DST)
105		return true;
106	switch (greh->version) {
107	case GRE_VERSION_1701:
108		/* We do not currently NAT any GREv0 packets.
109		 * Try to behave like "nf_nat_proto_unknown" */
110		break;
111	case GRE_VERSION_PPTP:
112		pr_debug("call_id -> 0x%04x\n", ntohs(tuple->dst.u.gre.key));
113		pgreh->call_id = tuple->dst.u.gre.key;
114		break;
115	default:
116		pr_debug("can't nat unknown GRE version\n");
117		return false;
118	}
119	return true;
120}
121
122static const struct nf_nat_l4proto gre = {
123	.l4proto		= IPPROTO_GRE,
124	.manip_pkt		= gre_manip_pkt,
125	.in_range		= nf_nat_l4proto_in_range,
126	.unique_tuple		= gre_unique_tuple,
127#if IS_ENABLED(CONFIG_NF_CT_NETLINK)
128	.nlattr_to_range	= nf_nat_l4proto_nlattr_to_range,
129#endif
130};
131
132static int __init nf_nat_proto_gre_init(void)
133{
134	return nf_nat_l4proto_register(NFPROTO_IPV4, &gre);
135}
136
137static void __exit nf_nat_proto_gre_fini(void)
138{
139	nf_nat_l4proto_unregister(NFPROTO_IPV4, &gre);
140}
141
142module_init(nf_nat_proto_gre_init);
143module_exit(nf_nat_proto_gre_fini);
144
145void nf_nat_need_gre(void)
146{
147	return;
148}
149EXPORT_SYMBOL_GPL(nf_nat_need_gre);
150