nf_nat_proto_gre.c

Go to the documentation of this file.

/*
 * nf_nat_proto_gre.c
 *
 * NAT protocol helper module for GRE.
 *
 * GRE is a generic encapsulation protocol, which is generally not very
 * suited for NAT, as it has no protocol-specific part as port numbers.
 *
 * It has an optional key field, which may help us distinguishing two
 * connections between the same two hosts.
 *
 * GRE is defined in RFC 1701 and RFC 1702, as well as RFC 2784
 *
 * PPTP is built on top of a modified version of GRE, and has a mandatory
 * field called "CallID", which serves us for the same purpose as the key
 * field in plain GRE.
 *
 * Documentation about PPTP can be found in RFC 2637
 *
 * (C) 2000-2005 by Harald Welte <[email protected]>
 *
 * Development of this code funded by Astaro AG (http://www.astaro.com/)
 *
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>

#include <net/netfilter/nf_nat.h>
#include <net/netfilter/nf_nat_l4proto.h>
#include <linux/netfilter/nf_conntrack_proto_gre.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Harald Welte <[email protected]>");
MODULE_DESCRIPTION("Netfilter NAT protocol helper module for GRE");

/* generate unique tuple ... */
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)
{
    static u_int16_t key;
    __be16 *keyptr;
    unsigned int min, i, range_size;

    /* If there is no master conntrack we are not PPTP,
       do not change tuples */
    if (!ct->master)
        return;

    if (maniptype == NF_NAT_MANIP_SRC)
        keyptr = &tuple->src.u.gre.key;
    else
        keyptr = &tuple->dst.u.gre.key;

    if (!(range->flags & NF_NAT_RANGE_PROTO_SPECIFIED)) {
        pr_debug("%p: NATing GRE PPTP\n", ct);
        min = 1;
        range_size = 0xffff;
    } else {
        min = ntohs(range->min_proto.gre.key);
        range_size = ntohs(range->max_proto.gre.key) - min + 1;
    }

    pr_debug("min = %u, range_size = %u\n", min, range_size);

    for (i = 0; ; ++key) {
        *keyptr = htons(min + key % range_size);
        if (++i == range_size || !nf_nat_used_tuple(tuple, ct))
            return;
    }

    pr_debug("%p: no NAT mapping\n", ct);
    return;
}

/* manipulate a GRE packet according to maniptype */
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)
{
    const struct gre_hdr *greh;
    struct gre_hdr_pptp *pgreh;

    /* pgreh includes two optional 32bit fields which are not required
     * to be there.  That's where the magic '8' comes from */
    if (!skb_make_writable(skb, hdroff + sizeof(*pgreh) - 8))
        return false;

    greh = (void *)skb->data + hdroff;
    pgreh = (struct gre_hdr_pptp *)greh;

    /* we only have destination manip of a packet, since 'source key'
     * is not present in the packet itself */
    if (maniptype != NF_NAT_MANIP_DST)
        return true;
    switch (greh->version) {
    case GRE_VERSION_1701:
        /* We do not currently NAT any GREv0 packets.
         * Try to behave like "nf_nat_proto_unknown" */
        break;
    case GRE_VERSION_PPTP:
        pr_debug("call_id -> 0x%04x\n", ntohs(tuple->dst.u.gre.key));
        pgreh->call_id = tuple->dst.u.gre.key;
        break;
    default:
        pr_debug("can't nat unknown GRE version\n");
        return false;
    }
    return true;
}

static const struct nf_nat_l4proto gre = {
    .l4proto        = IPPROTO_GRE,
    .manip_pkt      = gre_manip_pkt,
    .in_range       = nf_nat_l4proto_in_range,
    .unique_tuple       = gre_unique_tuple,
#if defined(CONFIG_NF_CT_NETLINK) || defined(CONFIG_NF_CT_NETLINK_MODULE)
    .nlattr_to_range    = nf_nat_l4proto_nlattr_to_range,
#endif
};

static int __init nf_nat_proto_gre_init(void)
{
    return nf_nat_l4proto_register(NFPROTO_IPV4, &gre);
}

static void __exit nf_nat_proto_gre_fini(void)
{
    nf_nat_l4proto_unregister(NFPROTO_IPV4, &gre);
}

module_init(nf_nat_proto_gre_init);
module_exit(nf_nat_proto_gre_fini);

void nf_nat_need_gre(void)
{
    return;
}
EXPORT_SYMBOL_GPL(nf_nat_need_gre);