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Linux/net/ipv4/netfilter/nf_nat_proto_gre.c

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  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
 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
 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 
132 static int __init nf_nat_proto_gre_init(void)
133 {
134         return nf_nat_l4proto_register(NFPROTO_IPV4, &gre);
135 }
136 
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 
145 void nf_nat_need_gre(void)
146 {
147         return;
148 }
149 EXPORT_SYMBOL_GPL(nf_nat_need_gre);
150 

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