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
36 MODULE_LICENSE("GPL");
37 MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
38 MODULE_DESCRIPTION("Netfilter NAT protocol helper module for GRE");
39
40 /* generate unique tuple ... */
41 static void
gre_unique_tuple(const struct nf_nat_l3proto * l3proto,struct nf_conntrack_tuple * tuple,const struct nf_nat_range * range,enum nf_nat_manip_type maniptype,const struct nf_conn * ct)42 gre_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 */
84 static bool
gre_manip_pkt(struct sk_buff * skb,const struct nf_nat_l3proto * l3proto,unsigned int iphdroff,unsigned int hdroff,const struct nf_conntrack_tuple * tuple,enum nf_nat_manip_type maniptype)85 gre_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
122 static 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
nf_nat_proto_gre_init(void)132 static int __init nf_nat_proto_gre_init(void)
133 {
134 return nf_nat_l4proto_register(NFPROTO_IPV4, &gre);
135 }
136
nf_nat_proto_gre_fini(void)137 static void __exit nf_nat_proto_gre_fini(void)
138 {
139 nf_nat_l4proto_unregister(NFPROTO_IPV4, &gre);
140 }
141
142 module_init(nf_nat_proto_gre_init);
143 module_exit(nf_nat_proto_gre_fini);
144
nf_nat_need_gre(void)145 void nf_nat_need_gre(void)
146 {
147 return;
148 }
149 EXPORT_SYMBOL_GPL(nf_nat_need_gre);
150