/*
 * ip_nat_proto_gre.c - Version $Revision: 1.1 $
 *
 * 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) 2000-2002 by Harald Welte <laforge@gnumonks.org>
 *
 * Development of this code funded by Astaro AG (http://www.astaro.com/)
 *
 */

#include <linux/config.h>
#include <linux/module.h>
#include <linux/ip.h>
#include <linux/netfilter_ipv4/ip_nat.h>
#include <linux/netfilter_ipv4/ip_nat_rule.h>
#include <linux/netfilter_ipv4/ip_nat_protocol.h>
#include <linux/netfilter_ipv4/ip_conntrack_proto_gre.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
MODULE_DESCRIPTION("Netfilter NAT protocol helper module for GRE");

#define DEBUGP(format, args...) printk(KERN_DEBUG __FILE__ ":" __FUNCTION__ \
                                     ": " format, ## args)
/* is key in given range between min and max */
static int
gre_in_range(const struct ip_conntrack_tuple *tuple,
             enum ip_nat_manip_type maniptype,
             const union ip_conntrack_manip_proto *min,
             const union ip_conntrack_manip_proto *max)
{
        return ntohl(tuple->src.u.gre.key) >= ntohl(min->gre.key)
                && ntohl(tuple->src.u.gre.key) <= ntohl(max->gre.key);
}

/* generate unique tuple ... */
static int
gre_unique_tuple(struct ip_conntrack_tuple *tuple,
                 const struct ip_nat_range *range,
                 enum ip_nat_manip_type maniptype,
                 const struct ip_conntrack *conntrack)
{
        u_int32_t min, i, range_size;
        u_int32_t key = 0, *keyptr;

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

        if (!(range->flags & IP_NAT_RANGE_PROTO_SPECIFIED)) {

                switch (tuple->dst.u.gre.version) {
                case 0:
                        DEBUGP("NATing GRE version 0 (ct=%p)\n",
                                conntrack);
                        min = 1;
                        range_size = 0xffffffff;
                        break;
                case GRE_VERSION_PPTP:
                        DEBUGP("%p: NATing GRE PPTP\n",
                                conntrack);
                        min = 1;
                        range_size = 0xffff;
                        break;
                default:
                        printk(KERN_WARNING "nat_gre: unknown GRE version\n");
                        return 0;
                        break;
                }

        } else {
                min = ntohl(range->min.gre.key);
                range_size = ntohl(range->max.gre.key) - min + 1;
        }

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

        for (i = 0; i < range_size; i++, key++) {
                *keyptr = htonl(min + key % range_size);
                if (!ip_nat_used_tuple(tuple, conntrack))
                        return 1;
        }

        DEBUGP("%p: no NAT mapping\n", conntrack);

        return 0;
}

/* manipulate a GRE packet according to maniptype */
static void
gre_manip_pkt(struct iphdr *iph, size_t len,
              const struct ip_conntrack_manip *manip,
              enum ip_nat_manip_type maniptype)
{
        struct gre_hdr *greh = (struct gre_hdr *)((u_int32_t *)iph+iph->ihl);
        struct gre_hdr_pptp *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 == IP_NAT_MANIP_DST) {
                /* key manipulation is always dest */
                switch (greh->version) {
                case 0:
                        if (!greh->key) {
                                DEBUGP("can't nat GRE w/o key\n");
                                break;
                        }
                        if (greh->csum) {
                                /* FIXME: Never tested this code... */
                                *(gre_csum(greh)) =
                                        ip_nat_cheat_check(~*(gre_key(greh)),
                                                        manip->u.gre.key,
                                                        *(gre_csum(greh)));
                        }
                        *(gre_key(greh)) = manip->u.gre.key;
                        break;
                case GRE_VERSION_PPTP:
                        DEBUGP("call_id -> 0x%04x\n",
                                ntohl(manip->u.gre.key));
                        pgreh->call_id = htons(ntohl(manip->u.gre.key));
                        break;
                default:
                        DEBUGP("can't nat unknown GRE version\n");
                        break;
                }
        }
}

/* print out a nat tuple */
static unsigned int
gre_print(char *buffer,
          const struct ip_conntrack_tuple *match,
          const struct ip_conntrack_tuple *mask)
{
        unsigned int len = 0;

        if (mask->dst.u.gre.version)
                len += sprintf(buffer + len, "version=%d ",
                                ntohs(match->dst.u.gre.version));

        if (mask->dst.u.gre.protocol)
                len += sprintf(buffer + len, "protocol=0x%x ",
                                ntohs(match->dst.u.gre.protocol));

        if (mask->src.u.gre.key)
                len += sprintf(buffer + len, "srckey=0x%x ",
                                ntohl(match->src.u.gre.key));

        if (mask->dst.u.gre.key)
                len += sprintf(buffer + len, "dstkey=0x%x ",
                                ntohl(match->src.u.gre.key));

        return len;
}

/* print a range of keys */
static unsigned int
gre_print_range(char *buffer, const struct ip_nat_range *range)
{
        if (range->min.gre.key != 0
            || range->max.gre.key != 0xFFFF) {
                if (range->min.gre.key == range->max.gre.key)
                        return sprintf(buffer, "key 0x%x ",
                                        ntohl(range->min.gre.key));
                else
                        return sprintf(buffer, "keys 0x%u-0x%u ",
                                        ntohl(range->min.gre.key),
                                        ntohl(range->max.gre.key));
        } else
                return 0;
}

/* nat helper struct */
static struct ip_nat_protocol gre =
        { { NULL, NULL }, "GRE", IPPROTO_GRE,
          gre_manip_pkt,
          gre_in_range,
          gre_unique_tuple,
          gre_print,
          gre_print_range
        };

static int __init init(void)
{
        if (ip_nat_protocol_register(&gre))
                return -EIO;

        return 0;
}

static void __exit fini(void)
{
        ip_nat_protocol_unregister(&gre);
}

module_init(init);
module_exit(fini);