net/ipv4/netfilter/nf_nat_proto_gre.c

   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_base_hdr *greh;
  92         struct pptp_gre_header *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 pptp_gre_header *)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
 107         switch (greh->flags & GRE_VERSION) {
 108         case GRE_VERSION_0:
 109                 /* We do not currently NAT any GREv0 packets.
 110                  * Try to behave like "nf_nat_proto_unknown" */
 111                 break;
 112         case GRE_VERSION_1:
 113                 pr_debug("call_id -> 0x%04x\n", ntohs(tuple->dst.u.gre.key));
 114                 pgreh->call_id = tuple->dst.u.gre.key;
 115                 break;
 116         default:
 117                 pr_debug("can't nat unknown GRE version\n");
 118                 return false;
 119         }
 120         return true;
 121 }
 122
 123 static const struct nf_nat_l4proto gre = {
 124         .l4proto                = IPPROTO_GRE,
 125         .manip_pkt              = gre_manip_pkt,
 126         .in_range               = nf_nat_l4proto_in_range,
 127         .unique_tuple           = gre_unique_tuple,
 128 #if IS_ENABLED(CONFIG_NF_CT_NETLINK)
 129         .nlattr_to_range        = nf_nat_l4proto_nlattr_to_range,
 130 #endif
 131 };
 132
 133 static int __init nf_nat_proto_gre_init(void)
 134 {
 135         return nf_nat_l4proto_register(NFPROTO_IPV4, &gre);
 136 }
 137
 138 static void __exit nf_nat_proto_gre_fini(void)
 139 {
 140         nf_nat_l4proto_unregister(NFPROTO_IPV4, &gre);
 141 }
 142
 143 module_init(nf_nat_proto_gre_init);
 144 module_exit(nf_nat_proto_gre_fini);
 145
 146 void nf_nat_need_gre(void)
 147 {
 148         return;
 149 }
 150 EXPORT_SYMBOL_GPL(nf_nat_need_gre);