ip_gre.c

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/*
 *      Linux NET3:     GRE over IP protocol decoder. 
 *
 *      Authors: Alexey Kuznetsov (kuznet@ms2.inr.ac.ru)
 *
 *      This program is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU General Public License
 *      as published by the Free Software Foundation; either version
 *      2 of the License, or (at your option) any later version.
 *
 */

#include <linux/config.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/sched.h>
#include <linux/kernel.h>
#include <asm/uaccess.h>
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <linux/in.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <linux/if_arp.h>
#include <linux/mroute.h>
#include <linux/init.h>
#include <linux/in6.h>
#include <linux/inetdevice.h>
#include <linux/igmp.h>

#include <net/sock.h>
#include <net/ip.h>
#include <net/icmp.h>
#include <net/protocol.h>
#include <net/ipip.h>
#include <net/arp.h>
#include <net/checksum.h>

#ifdef CONFIG_IPV6
#include <net/ipv6.h>
#include <net/ip6_fib.h>
#include <net/ip6_route.h>
#endif

/*
   Problems & solutions
   --------------------

   1. The most important issue is detecting local dead loops.
   They would cause complete host lockup in transmit, which
   would be "resolved" by stack overflow or, if queueing is enabled,
   with infinite looping in net_bh.

   We cannot track such dead loops during route installation,
   it is infeasible task. The most general solutions would be
   to keep skb->encapsulation counter (sort of local ttl),
   and silently drop packet when it expires. It is the best
   solution, but it supposes maintaing new variable in ALL
   skb, even if no tunneling is used.

   Current solution: t->recursion lock breaks dead loops. It looks 
   like dev->tbusy flag, but I preferred new variable, because
   the semantics is different. One day, when hard_start_xmit
   will be multithreaded we will have to use skb->encapsulation.



   2. Networking dead loops would not kill routers, but would really
   kill network. IP hop limit plays role of "t->recursion" in this case,
   if we copy it from packet being encapsulated to upper header.
   It is very good solution, but it introduces two problems:

   - Routing protocols, using packets with ttl=1 (OSPF, RIP2),
     do not work over tunnels.
   - traceroute does not work. I planned to relay ICMP from tunnel,
     so that this problem would be solved and traceroute output
     would even more informative. This idea appeared to be wrong:
     only Linux complies to rfc1812 now (yes, guys, Linux is the only
     true router now :-)), all routers (at least, in neighbourhood of mine)
     return only 8 bytes of payload. It is the end.

   Hence, if we want that OSPF worked or traceroute said something reasonable,
   we should search for another solution.

   One of them is to parse packet trying to detect inner encapsulation
   made by our node. It is difficult or even impossible, especially,
   taking into account fragmentation. TO be short, tt is not solution at all.

   Current solution: The solution was UNEXPECTEDLY SIMPLE.
   We force DF flag on tunnels with preconfigured hop limit,
   that is ALL. :-) Well, it does not remove the problem completely,
   but exponential growth of network traffic is changed to linear
   (branches, that exceed pmtu are pruned) and tunnel mtu
   fastly degrades to value <68, where looping stops.
   Yes, it is not good if there exists a router in the loop,
   which does not force DF, even when encapsulating packets have DF set.
   But it is not our problem! Nobody could accuse us, we made
   all that we could make. Even if it is your gated who injected
   fatal route to network, even if it were you who configured
   fatal static route: you are innocent. :-)



   3. Really, ipv4/ipip.c, ipv4/ip_gre.c and ipv6/sit.c contain
   practically identical code. It would be good to glue them
   together, but it is not very evident, how to make them modular.
   sit is integral part of IPv6, ipip and gre are naturally modular.
   We could extract common parts (hash table, ioctl etc)
   to a separate module (ip_tunnel.c).

   Alexey Kuznetsov.
 */

static int ipgre_tunnel_init(struct device *dev);

/* Fallback tunnel: no source, no destination, no key, no options */

static int ipgre_fb_tunnel_init(struct device *dev);

static struct device ipgre_fb_tunnel_dev = {
        NULL, 0x0, 0x0, 0x0, 0x0, 0, 0, 0, 0, 0, NULL, ipgre_fb_tunnel_init,
};

static struct ip_tunnel ipgre_fb_tunnel = {
        NULL, &ipgre_fb_tunnel_dev, {0, }, 0, 0, 0, 0, 0, 0, 0, {"gre0", }
};

/* Tunnel hash table */

/*
   4 hash tables:

   3: (remote,local)
   2: (remote,*)
   1: (*,local)
   0: (*,*)

   We require exact key match i.e. if a key is present in packet
   it will match only tunnel with the same key; if it is not present,
   it will match only keyless tunnel.

   All keysless packets, if not matched configured keyless tunnels
   will match fallback tunnel.
 */

#define HASH_SIZE  16
#define HASH(addr) ((addr^(addr>>4))&0xF)

static struct ip_tunnel *tunnels[4][HASH_SIZE];

#define tunnels_r_l     (tunnels[3])
#define tunnels_r       (tunnels[2])
#define tunnels_l       (tunnels[1])
#define tunnels_wc      (tunnels[0])

/* Given src, dst and key, find approriate for input tunnel. */

static struct ip_tunnel * ipgre_tunnel_lookup(u32 remote, u32 local, u32 key)
{
        unsigned h0 = HASH(remote);
        unsigned h1 = HASH(key);
        struct ip_tunnel *t;

        for (t = tunnels_r_l[h0^h1]; t; t = t->next) {
                if (local == t->parms.iph.saddr && remote == t->parms.iph.daddr) {
                        if (t->parms.i_key == key && (t->dev->flags&IFF_UP))
                                return t;
                }
        }
        for (t = tunnels_r[h0^h1]; t; t = t->next) {
                if (remote == t->parms.iph.daddr) {
                        if (t->parms.i_key == key && (t->dev->flags&IFF_UP))
                                return t;
                }
        }
        for (t = tunnels_l[h1]; t; t = t->next) {
                if (local == t->parms.iph.saddr ||
                     (local == t->parms.iph.daddr && MULTICAST(local))) {
                        if (t->parms.i_key == key && (t->dev->flags&IFF_UP))
                                return t;
                }
        }
        for (t = tunnels_wc[h1]; t; t = t->next) {
                if (t->parms.i_key == key && (t->dev->flags&IFF_UP))
                        return t;
        }
        if (ipgre_fb_tunnel_dev.flags&IFF_UP)
                return &ipgre_fb_tunnel;
        return NULL;
}

static struct ip_tunnel **ipgre_bucket(struct ip_tunnel *t)
{
        u32 remote = t->parms.iph.daddr;
        u32 local = t->parms.iph.saddr;
        u32 key = t->parms.i_key;
        unsigned h = HASH(key);
        int prio = 0;

        if (local)
                prio |= 1;
        if (remote && !MULTICAST(remote)) {
                prio |= 2;
                h ^= HASH(remote);
        }

        return &tunnels[prio][h];
}

static void ipgre_tunnel_link(struct ip_tunnel *t)
{
        struct ip_tunnel **tp = ipgre_bucket(t);

        t->next = *tp;
        wmb();
        *tp = t;
}

static void ipgre_tunnel_unlink(struct ip_tunnel *t)
{
        struct ip_tunnel **tp;

        for (tp = ipgre_bucket(t); *tp; tp = &(*tp)->next) {
                if (t == *tp) {
                        *tp = t->next;
                        synchronize_bh();
                        break;
                }
        }
}

static struct ip_tunnel * ipgre_tunnel_locate(struct ip_tunnel_parm *parms, int create)
{
        u32 remote = parms->iph.daddr;
        u32 local = parms->iph.saddr;
        u32 key = parms->i_key;
        struct ip_tunnel *t, **tp, *nt;
        struct device *dev;
        unsigned h = HASH(key);
        int prio = 0;

        if (local)
                prio |= 1;
        if (remote && !MULTICAST(remote)) {
                prio |= 2;
                h ^= HASH(remote);
        }
        for (tp = &tunnels[prio][h]; (t = *tp) != NULL; tp = &t->next) {
                if (local == t->parms.iph.saddr && remote == t->parms.iph.daddr) {
                        if (key == t->parms.i_key)
                                return t;
                }
        }
        if (!create)
                return NULL;

        MOD_INC_USE_COUNT;
        dev = kmalloc(sizeof(*dev) + sizeof(*t), GFP_KERNEL);
        if (dev == NULL) {
                MOD_DEC_USE_COUNT;
                return NULL;
        }
        memset(dev, 0, sizeof(*dev) + sizeof(*t));
        dev->priv = (void*)(dev+1);
        nt = (struct ip_tunnel*)dev->priv;
        nt->dev = dev;
        dev->name = nt->parms.name;
        dev->init = ipgre_tunnel_init;
        memcpy(&nt->parms, parms, sizeof(*parms));
        if (dev->name[0] == 0) {
                int i;
                for (i=1; i<100; i++) {
                        sprintf(dev->name, "gre%d", i);
                        if (dev_get(dev->name) == NULL)
                                break;
                }
                if (i==100)
                        goto failed;
                memcpy(parms->name, dev->name, IFNAMSIZ);
        }
        if (register_netdevice(dev) < 0)
                goto failed;

        ipgre_tunnel_link(nt);
        /* Do not decrement MOD_USE_COUNT here. */
        return nt;

failed:
        kfree(dev);
        MOD_DEC_USE_COUNT;
        return NULL;
}

static void ipgre_tunnel_destroy(struct device *dev)
{
        ipgre_tunnel_unlink((struct ip_tunnel*)dev->priv);

        if (dev != &ipgre_fb_tunnel_dev) {
                kfree(dev);
                MOD_DEC_USE_COUNT;
        }
}


void ipgre_err(struct sk_buff *skb, unsigned char *dp, int len)
{
#ifndef I_WISH_WORLD_WERE_PERFECT

/* It is not :-( All the routers (except for Linux) return only
   8 bytes of packet payload. It means, that precise relaying of
   ICMP in the real Internet is absolutely infeasible.

   Moreover, Cisco "wise men" put GRE key to the third word
   in GRE header. It makes impossible maintaining even soft state for keyed
   GRE tunnels with enabled checksum. Tell them "thank you".

   Well, I wonder, rfc1812 was written by Cisco employee,
   what the hell these idiots break standrads established
   by themself???
 */

        struct iphdr *iph = (struct iphdr*)dp;
        u16          *p = (u16*)(dp+(iph->ihl<<2));
        int grehlen = (iph->ihl<<2) + 4;
        int type = skb->h.icmph->type;
        int code = skb->h.icmph->code;
        struct ip_tunnel *t;
        u16 flags;

        flags = p[0];
        if (flags&(GRE_CSUM|GRE_KEY|GRE_SEQ|GRE_ROUTING|GRE_VERSION)) {
                if (flags&(GRE_VERSION|GRE_ROUTING))
                        return;
                if (flags&GRE_KEY) {
                        grehlen += 4;
                        if (flags&GRE_CSUM)
                                grehlen += 4;
                }
        }

        /* If only 8 bytes returned, keyed message will be dropped here */
        if (len < grehlen)
                return;

        switch (type) {
        default:
        case ICMP_PARAMETERPROB:
                return;

        case ICMP_DEST_UNREACH:
                switch (code) {
                case ICMP_SR_FAILED:
                case ICMP_PORT_UNREACH:
                        /* Impossible event. */
                        return;
                case ICMP_FRAG_NEEDED:
                        /* Soft state for pmtu is maintained by IP core. */
                        return;
                default:
                        /* All others are translated to HOST_UNREACH.
                           rfc2003 contains "deep thoughts" about NET_UNREACH,
                           I believe they are just ether pollution. --ANK
                         */
                        break;
                }
                break;
        case ICMP_TIME_EXCEEDED:
                if (code != ICMP_EXC_TTL)
                        return;
                break;
        }

        t = ipgre_tunnel_lookup(iph->daddr, iph->saddr, (flags&GRE_KEY) ? *(((u32*)p) + (grehlen>>2) - 1) : 0);
        if (t == NULL || t->parms.iph.daddr == 0 || MULTICAST(t->parms.iph.daddr))
                return;

        if (t->parms.iph.ttl == 0 && type == ICMP_TIME_EXCEEDED)
                return;

        if (jiffies - t->err_time < IPTUNNEL_ERR_TIMEO)
                t->err_count++;
        else
                t->err_count = 1;
        t->err_time = jiffies;
        return;
#else
        struct iphdr *iph = (struct iphdr*)dp;
        struct iphdr *eiph;
        u16          *p = (u16*)(dp+(iph->ihl<<2));
        int type = skb->h.icmph->type;
        int code = skb->h.icmph->code;
        int rel_type = 0;
        int rel_code = 0;
        int rel_info = 0;
        u16 flags;
        int grehlen = (iph->ihl<<2) + 4;
        struct sk_buff *skb2;
        struct rtable *rt;

        if (p[1] != __constant_htons(ETH_P_IP))
                return;

        flags = p[0];
        if (flags&(GRE_CSUM|GRE_KEY|GRE_SEQ|GRE_ROUTING|GRE_VERSION)) {
                if (flags&(GRE_VERSION|GRE_ROUTING))
                        return;
                if (flags&GRE_CSUM)
                        grehlen += 4;
                if (flags&GRE_KEY)
                        grehlen += 4;
                if (flags&GRE_SEQ)
                        grehlen += 4;
        }
        if (len < grehlen + sizeof(struct iphdr))
                return;
        eiph = (struct iphdr*)(dp + grehlen);

        switch (type) {
        default:
                return;
        case ICMP_PARAMETERPROB:
                if (skb->h.icmph->un.gateway < (iph->ihl<<2))
                        return;

                /* So... This guy found something strange INSIDE encapsulated
                   packet. Well, he is fool, but what can we do ?
                 */
                rel_type = ICMP_PARAMETERPROB;
                rel_info = skb->h.icmph->un.gateway - grehlen;
                break;

        case ICMP_DEST_UNREACH:
                switch (code) {
                case ICMP_SR_FAILED:
                case ICMP_PORT_UNREACH:
                        /* Impossible event. */
                        return;
                case ICMP_FRAG_NEEDED:
                        /* And it is the only really necesary thing :-) */
                        rel_info = ntohs(skb->h.icmph->un.frag.mtu);
                        if (rel_info < grehlen+68)
                                return;
                        rel_info -= grehlen;
                        /* BSD 4.2 MORE DOES NOT EXIST IN NATURE. */
                        if (rel_info > ntohs(eiph->tot_len))
                                return;
                        break;
                default:
                        /* All others are translated to HOST_UNREACH.
                           rfc2003 contains "deep thoughts" about NET_UNREACH,
                           I believe, it is just ether pollution. --ANK
                         */
                        rel_type = ICMP_DEST_UNREACH;
                        rel_code = ICMP_HOST_UNREACH;
                        break;
                }
                break;
        case ICMP_TIME_EXCEEDED:
                if (code != ICMP_EXC_TTL)
                        return;
                break;
        }

        /* Prepare fake skb to feed it to icmp_send */
        skb2 = skb_clone(skb, GFP_ATOMIC);
        if (skb2 == NULL)
                return;
        dst_release(skb2->dst);
        skb2->dst = NULL;
        skb_pull(skb2, skb->data - (u8*)eiph);
        skb2->nh.raw = skb2->data;

        /* Try to guess incoming interface */
        if (ip_route_output(&rt, eiph->saddr, 0, RT_TOS(eiph->tos), 0)) {
                kfree_skb(skb2);
                return;
        }
        skb2->dev = rt->u.dst.dev;

        /* route "incoming" packet */
        if (rt->rt_flags&RTCF_LOCAL) {
                ip_rt_put(rt);
                rt = NULL;
                if (ip_route_output(&rt, eiph->daddr, eiph->saddr, eiph->tos, 0) ||
                    rt->u.dst.dev->type != ARPHRD_IPGRE) {
                        ip_rt_put(rt);
                        kfree_skb(skb2);
                        return;
                }
        } else {
                ip_rt_put(rt);
                if (ip_route_input(skb2, eiph->daddr, eiph->saddr, eiph->tos, skb2->dev) ||
                    skb2->dst->dev->type != ARPHRD_IPGRE) {
                        kfree_skb(skb2);
                        return;
                }
        }

        /* change mtu on this route */
        if (type == ICMP_DEST_UNREACH && code == ICMP_FRAG_NEEDED) {
                if (rel_info > skb2->dst->pmtu) {
                        kfree_skb(skb2);
                        return;
                }
                skb2->dst->pmtu = rel_info;
                rel_info = htonl(rel_info);
        } else if (type == ICMP_TIME_EXCEEDED) {
                struct ip_tunnel *t = (struct ip_tunnel*)skb2->dev->priv;
                if (t->parms.iph.ttl) {
                        rel_type = ICMP_DEST_UNREACH;
                        rel_code = ICMP_HOST_UNREACH;
                }
        }

        icmp_send(skb2, rel_type, rel_code, rel_info);
        kfree_skb(skb2);
#endif
}

int ipgre_rcv(struct sk_buff *skb, unsigned short len)
{
        struct iphdr *iph = skb->nh.iph;
        u8     *h = skb->h.raw;
        u16    flags = *(u16*)h;
        u16    csum = 0;
        u32    key = 0;
        u32    seqno = 0;
        struct ip_tunnel *tunnel;
        int    offset = 4;

        if (flags&(GRE_CSUM|GRE_KEY|GRE_ROUTING|GRE_SEQ|GRE_VERSION)) {
                /* - Version must be 0.
                   - We do not support routing headers.
                 */
                if (flags&(GRE_VERSION|GRE_ROUTING))
                        goto drop;

                if (flags&GRE_CSUM) {
                        csum = ip_compute_csum(h, len);
                        offset += 4;
                }
                if (flags&GRE_KEY) {
                        key = *(u32*)(h + offset);
                        offset += 4;
                }
                if (flags&GRE_SEQ) {
                        seqno = ntohl(*(u32*)(h + offset));
                        offset += 4;
                }
        }

        if ((tunnel = ipgre_tunnel_lookup(iph->saddr, iph->daddr, key)) != NULL) {
                skb->mac.raw = skb->nh.raw;
                skb->nh.raw = skb_pull(skb, h + offset - skb->data);
                memset(&(IPCB(skb)->opt), 0, sizeof(struct ip_options));
                skb->ip_summed = 0;
                skb->protocol = *(u16*)(h + 2);
                skb->pkt_type = PACKET_HOST;
#ifdef CONFIG_NET_IPGRE_BROADCAST
                if (MULTICAST(iph->daddr)) {
                        /* Looped back packet, drop it! */
                        if (((struct rtable*)skb->dst)->key.iif == 0)
                                goto drop;
                        tunnel->stat.multicast++;
                        skb->pkt_type = PACKET_BROADCAST;
                }
#endif

                if (((flags&GRE_CSUM) && csum) ||
                    (!(flags&GRE_CSUM) && tunnel->parms.i_flags&GRE_CSUM)) {
                        tunnel->stat.rx_crc_errors++;
                        tunnel->stat.rx_errors++;
                        goto drop;
                }
                if (tunnel->parms.i_flags&GRE_SEQ) {
                        if (!(flags&GRE_SEQ) ||
                            (tunnel->i_seqno && (s32)(seqno - tunnel->i_seqno) < 0)) {
                                tunnel->stat.rx_fifo_errors++;
                                tunnel->stat.rx_errors++;
                                goto drop;
                        }
                        tunnel->i_seqno = seqno + 1;
                }
                tunnel->stat.rx_packets++;
                tunnel->stat.rx_bytes += skb->len;
                skb->dev = tunnel->dev;
                dst_release(skb->dst);
                skb->dst = NULL;
                netif_rx(skb);
                return(0);
        }
        icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PROT_UNREACH, 0);

drop:
        kfree_skb(skb);
        return(0);
}

static int ipgre_tunnel_xmit(struct sk_buff *skb, struct device *dev)
{
        struct ip_tunnel *tunnel = (struct ip_tunnel*)dev->priv;
        struct net_device_stats *stats = &tunnel->stat;
        struct iphdr  *old_iph = skb->nh.iph;
        struct iphdr  *tiph;
        u8     tos;
        u16    df;
        struct rtable *rt;                      /* Route to the other host */
        struct device *tdev;                    /* Device to other host */
        struct iphdr  *iph;                     /* Our new IP header */
        int    max_headroom;                    /* The extra header space needed */
        int    gre_hlen;
        u32    dst;
        int    mtu;

        if (tunnel->recursion++) {
                tunnel->stat.collisions++;
                goto tx_error;
        }

        if (dev->hard_header) {
                gre_hlen = 0;
                tiph = (struct iphdr*)skb->data;
        } else {
                gre_hlen = tunnel->hlen;
                tiph = &tunnel->parms.iph;
        }

        if ((dst = tiph->daddr) == 0) {
                /* NBMA tunnel */

                if (skb->dst == NULL) {
                        tunnel->stat.tx_fifo_errors++;
                        goto tx_error;
                }

                if (skb->protocol == __constant_htons(ETH_P_IP)) {
                        rt = (struct rtable*)skb->dst;
                        if ((dst = rt->rt_gateway) == 0)
                                goto tx_error_icmp;
                }
#ifdef CONFIG_IPV6
                else if (skb->protocol == __constant_htons(ETH_P_IPV6)) {
                        struct in6_addr *addr6;
                        int addr_type;
                        struct neighbour *neigh = skb->dst->neighbour;

                        if (neigh == NULL)
                                goto tx_error;

                        addr6 = (struct in6_addr*)&neigh->primary_key;
                        addr_type = ipv6_addr_type(addr6);

                        if (addr_type == IPV6_ADDR_ANY) {
                                addr6 = &skb->nh.ipv6h->daddr;
                                addr_type = ipv6_addr_type(addr6);
                        }

                        if ((addr_type & IPV6_ADDR_COMPATv4) == 0)
                                goto tx_error_icmp;

                        dst = addr6->s6_addr32[3];
                }
#endif
                else
                        goto tx_error;
        }

        tos = tiph->tos;
        if (tos&1) {
                if (skb->protocol == __constant_htons(ETH_P_IP))
                        tos = old_iph->tos;
                tos &= ~1;
        }

        if (ip_route_output(&rt, dst, tiph->saddr, RT_TOS(tos), tunnel->parms.link)) {
                tunnel->stat.tx_carrier_errors++;
                goto tx_error;
        }
        tdev = rt->u.dst.dev;

        if (tdev == dev) {
                ip_rt_put(rt);
                tunnel->stat.collisions++;
                goto tx_error;
        }

        df = tiph->frag_off;
        mtu = rt->u.dst.pmtu - tunnel->hlen;

        if (skb->protocol == __constant_htons(ETH_P_IP)) {
                if (skb->dst && mtu < skb->dst->pmtu && mtu >= 68)
                        skb->dst->pmtu = mtu;

                df |= (old_iph->frag_off&__constant_htons(IP_DF));

                if ((old_iph->frag_off&__constant_htons(IP_DF)) &&
                    mtu < ntohs(old_iph->tot_len)) {
                        icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu));
                        ip_rt_put(rt);
                        goto tx_error;
                }
        }
#ifdef CONFIG_IPV6
        else if (skb->protocol == __constant_htons(ETH_P_IPV6)) {
                struct rt6_info *rt6 = (struct rt6_info*)skb->dst;

                if (rt6 && mtu < rt6->u.dst.pmtu && mtu >= IPV6_MIN_MTU) {
                        if ((tunnel->parms.iph.daddr && !MULTICAST(tunnel->parms.iph.daddr)) ||
                            rt6->rt6i_dst.plen == 128) {
                                rt6->rt6i_flags |= RTF_MODIFIED;
                                skb->dst->pmtu = mtu;
                        }
                }

                if (mtu >= IPV6_MIN_MTU && mtu < skb->len - tunnel->hlen + gre_hlen) {
                        icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu, dev);
                        ip_rt_put(rt);
                        goto tx_error;
                }
        }
#endif

        if (tunnel->err_count > 0) {
                if (jiffies - tunnel->err_time < IPTUNNEL_ERR_TIMEO) {
                        tunnel->err_count--;

                        dst_link_failure(skb);
                } else
                        tunnel->err_count = 0;
        }

        skb->h.raw = skb->nh.raw;

        max_headroom = ((tdev->hard_header_len+15)&~15)+ gre_hlen;

        if (skb_headroom(skb) < max_headroom || skb_cloned(skb) || skb_shared(skb)) {
                struct sk_buff *new_skb = skb_realloc_headroom(skb, max_headroom);
                if (!new_skb) {
                        ip_rt_put(rt);
                        stats->tx_dropped++;
                        dev_kfree_skb(skb);
                        tunnel->recursion--;
                        return 0;
                }
                if (skb->sk)
                        skb_set_owner_w(new_skb, skb->sk);
                dev_kfree_skb(skb);
                skb = new_skb;
        }

        skb->nh.raw = skb_push(skb, gre_hlen);
        memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
        dst_release(skb->dst);
        skb->dst = &rt->u.dst;

        /*
         *      Push down and install the IPIP header.
         */

        iph                     =       skb->nh.iph;
        iph->version            =       4;
        iph->ihl                =       sizeof(struct iphdr) >> 2;
        iph->frag_off           =       df;
        iph->protocol           =       IPPROTO_GRE;
        iph->tos                =       tos;
        iph->daddr              =       rt->rt_dst;
        iph->saddr              =       rt->rt_src;

        if ((iph->ttl = tiph->ttl) == 0) {
                if (skb->protocol == __constant_htons(ETH_P_IP))
                        iph->ttl = old_iph->ttl;
#ifdef CONFIG_IPV6
                else if (skb->protocol == __constant_htons(ETH_P_IPV6))
                        iph->ttl = ((struct ipv6hdr*)old_iph)->hop_limit;
#endif
                else
                        iph->ttl = ip_statistics.IpDefaultTTL;
        }

        ((u16*)(iph+1))[0] = tunnel->parms.o_flags;
        ((u16*)(iph+1))[1] = skb->protocol;

        if (tunnel->parms.o_flags&(GRE_KEY|GRE_CSUM|GRE_SEQ)) {
                u32 *ptr = (u32*)(((u8*)iph) + tunnel->hlen - 4);

                if (tunnel->parms.o_flags&GRE_SEQ) {
                        ++tunnel->o_seqno;
                        *ptr = htonl(tunnel->o_seqno);
                        ptr--;
                }
                if (tunnel->parms.o_flags&GRE_KEY) {
                        *ptr = tunnel->parms.o_key;
                        ptr--;
                }
                if (tunnel->parms.o_flags&GRE_CSUM) {
                        *ptr = 0;
                        *(__u16*)ptr = ip_compute_csum((void*)(iph+1), skb->len - sizeof(struct iphdr));
                }
        }

        iph->tot_len            =       htons(skb->len);
        iph->id                 =       htons(ip_id_count++);
        ip_send_check(iph);

        stats->tx_bytes += skb->len;
        stats->tx_packets++;
        ip_send(skb);
        tunnel->recursion--;
        return 0;

tx_error_icmp:
        dst_link_failure(skb);

tx_error:
        stats->tx_errors++;
        dev_kfree_skb(skb);
        tunnel->recursion--;
        return 0;
}

static int
ipgre_tunnel_ioctl (struct device *dev, struct ifreq *ifr, int cmd)
{
        int err = 0;
        struct ip_tunnel_parm p;
        struct ip_tunnel *t;

        MOD_INC_USE_COUNT;

        switch (cmd) {
        case SIOCGETTUNNEL:
                t = NULL;
                if (dev == &ipgre_fb_tunnel_dev) {
                        if (copy_from_user(&p, ifr->ifr_ifru.ifru_data, sizeof(p))) {
                                err = -EFAULT;
                                break;
                        }
                        t = ipgre_tunnel_locate(&p, 0);
                }
                if (t == NULL)
                        t = (struct ip_tunnel*)dev->priv;
                memcpy(&p, &t->parms, sizeof(p));
                if (copy_to_user(ifr->ifr_ifru.ifru_data, &p, sizeof(p)))
                        err = -EFAULT;
                break;

        case SIOCADDTUNNEL:
        case SIOCCHGTUNNEL:
                err = -EPERM;
                if (!capable(CAP_NET_ADMIN))
                        goto done;

                err = -EFAULT;
                if (copy_from_user(&p, ifr->ifr_ifru.ifru_data, sizeof(p)))
                        goto done;

                err = -EINVAL;
                if (p.iph.version != 4 || p.iph.protocol != IPPROTO_GRE ||
                    p.iph.ihl != 5 || (p.iph.frag_off&__constant_htons(~IP_DF)) ||
                    ((p.i_flags|p.o_flags)&(GRE_VERSION|GRE_ROUTING)))
                        goto done;
                if (p.iph.ttl)
                        p.iph.frag_off |= __constant_htons(IP_DF);

                if (!(p.i_flags&GRE_KEY))
                        p.i_key = 0;
                if (!(p.o_flags&GRE_KEY))
                        p.o_key = 0;

                t = ipgre_tunnel_locate(&p, cmd == SIOCADDTUNNEL);

                if (dev != &ipgre_fb_tunnel_dev && cmd == SIOCCHGTUNNEL &&
                    t != &ipgre_fb_tunnel) {
                        if (t != NULL) {
                                if (t->dev != dev) {
                                        err = -EEXIST;
                                        break;
                                }
                        } else {
                                unsigned nflags=0;

                                t = (struct ip_tunnel*)dev->priv;

                                if (MULTICAST(p.iph.daddr))
                                        nflags = IFF_BROADCAST;
                                else if (p.iph.daddr)
                                        nflags = IFF_POINTOPOINT;

                                if ((dev->flags^nflags)&(IFF_POINTOPOINT|IFF_BROADCAST)) {
                                        err = -EINVAL;
                                        break;
                                }
                                start_bh_atomic();
                                ipgre_tunnel_unlink(t);
                                t->parms.iph.saddr = p.iph.saddr;
                                t->parms.iph.daddr = p.iph.daddr;
                                t->parms.i_key = p.i_key;
                                t->parms.o_key = p.o_key;
                                memcpy(dev->dev_addr, &p.iph.saddr, 4);
                                memcpy(dev->broadcast, &p.iph.daddr, 4);
                                ipgre_tunnel_link(t);
                                end_bh_atomic();
                                netdev_state_change(dev);
                        }
                }

                if (t) {
                        err = 0;
                        if (cmd == SIOCCHGTUNNEL) {
                                t->parms.iph.ttl = p.iph.ttl;
                                t->parms.iph.tos = p.iph.tos;
                                t->parms.iph.frag_off = p.iph.frag_off;
                        }
                        if (copy_to_user(ifr->ifr_ifru.ifru_data, &t->parms, sizeof(p)))
                                err = -EFAULT;
                } else
                        err = (cmd == SIOCADDTUNNEL ? -ENOBUFS : -ENOENT);
                break;

        case SIOCDELTUNNEL:
                err = -EPERM;
                if (!capable(CAP_NET_ADMIN))
                        goto done;

                if (dev == &ipgre_fb_tunnel_dev) {
                        err = -EFAULT;
                        if (copy_from_user(&p, ifr->ifr_ifru.ifru_data, sizeof(p)))
                                goto done;
                        err = -ENOENT;
                        if ((t = ipgre_tunnel_locate(&p, 0)) == NULL)
                                goto done;
                        err = -EPERM;
                        if (t == &ipgre_fb_tunnel)
                                goto done;
                }
                err = unregister_netdevice(dev);
                break;

        default:
                err = -EINVAL;
        }

done:
        MOD_DEC_USE_COUNT;
        return err;
}

static struct net_device_stats *ipgre_tunnel_get_stats(struct device *dev)
{
        return &(((struct ip_tunnel*)dev->priv)->stat);
}

static int ipgre_tunnel_change_mtu(struct device *dev, int new_mtu)
{
        struct ip_tunnel *tunnel = (struct ip_tunnel*)dev->priv;
        if (new_mtu < 68 || new_mtu > 0xFFF8 - tunnel->hlen)
                return -EINVAL;
        dev->mtu = new_mtu;
        return 0;
}

#ifdef CONFIG_NET_IPGRE_BROADCAST
/* Nice toy. Unfortunately, useless in real life :-)
   It allows to construct virtual multiprotocol broadcast "LAN"
   over the Internet, provided multicast routing is tuned.


   I have no idea was this bicycle invented before me,
   so that I had to set ARPHRD_IPGRE to a random value.
   I have an impression, that Cisco could make something similar,
   but this feature is apparently missing in IOS<=11.2(8).
   
   I set up 10.66.66/24 and fec0:6666:6666::0/96 as virtual networks
   with broadcast 224.66.66.66. If you have access to mbone, play with me :-)

   ping -t 255 224.66.66.66

   If nobody answers, mbone does not work.

   ip tunnel add Universe mode gre remote 224.66.66.66 local <Your_real_addr> ttl 255
   ip addr add 10.66.66.<somewhat>/24 dev Universe
   ifconfig Universe up
   ifconfig Universe add fe80::<Your_real_addr>/10
   ifconfig Universe add fec0:6666:6666::<Your_real_addr>/96
   ftp 10.66.66.66
   ...
   ftp fec0:6666:6666::193.233.7.65
   ...

 */

static int ipgre_header(struct sk_buff *skb, struct device *dev, unsigned short type,
                        void *daddr, void *saddr, unsigned len)
{
        struct ip_tunnel *t = (struct ip_tunnel*)dev->priv;
        struct iphdr *iph = (struct iphdr *)skb_push(skb, t->hlen);
        u16 *p = (u16*)(iph+1);

        memcpy(iph, &t->parms.iph, sizeof(struct iphdr));
        p[0]            = t->parms.o_flags;
        p[1]            = htons(type);

        /*
         *      Set the source hardware address. 
         */
         
        if (saddr)
                memcpy(&iph->saddr, saddr, 4);

        if (daddr) {
                memcpy(&iph->daddr, daddr, 4);
                return t->hlen;
        }
        if (iph->daddr && !MULTICAST(iph->daddr))
                return t->hlen;
        
        return -t->hlen;
}

static int ipgre_open(struct device *dev)
{
        struct ip_tunnel *t = (struct ip_tunnel*)dev->priv;

        MOD_INC_USE_COUNT;
        if (MULTICAST(t->parms.iph.daddr)) {
                struct rtable *rt;
                if (ip_route_output(&rt, t->parms.iph.daddr,
                                    t->parms.iph.saddr, RT_TOS(t->parms.iph.tos), 
                                    t->parms.link)) {
                        MOD_DEC_USE_COUNT;
                        return -EADDRNOTAVAIL;
                }
                dev = rt->u.dst.dev;
                ip_rt_put(rt);
                if (dev->ip_ptr == NULL) {
                        MOD_DEC_USE_COUNT;
                        return -EADDRNOTAVAIL;
                }
                t->mlink = dev->ifindex;
                ip_mc_inc_group(dev->ip_ptr, t->parms.iph.daddr);
        }
        return 0;
}

static int ipgre_close(struct device *dev)
{
        struct ip_tunnel *t = (struct ip_tunnel*)dev->priv;
        if (MULTICAST(t->parms.iph.daddr) && t->mlink) {
                dev = dev_get_by_index(t->mlink);
                if (dev && dev->ip_ptr)
                        ip_mc_dec_group(dev->ip_ptr, t->parms.iph.daddr);
        }
        MOD_DEC_USE_COUNT;
        return 0;
}

#endif

static void ipgre_tunnel_init_gen(struct device *dev)
{
        struct ip_tunnel *t = (struct ip_tunnel*)dev->priv;

        dev->destructor         = ipgre_tunnel_destroy;
        dev->hard_start_xmit    = ipgre_tunnel_xmit;
        dev->get_stats          = ipgre_tunnel_get_stats;
        dev->do_ioctl           = ipgre_tunnel_ioctl;
        dev->change_mtu         = ipgre_tunnel_change_mtu;

        dev_init_buffers(dev);

        dev->type               = ARPHRD_IPGRE;
        dev->hard_header_len    = LL_MAX_HEADER + sizeof(struct iphdr) + 4;
        dev->mtu                = 1500 - sizeof(struct iphdr) - 4;
        dev->flags              = IFF_NOARP;
        dev->iflink             = 0;
        dev->addr_len           = 4;
        memcpy(dev->dev_addr, &t->parms.iph.saddr, 4);
        memcpy(dev->broadcast, &t->parms.iph.daddr, 4);
}

static int ipgre_tunnel_init(struct device *dev)
{
        struct device *tdev = NULL;
        struct ip_tunnel *tunnel;
        struct iphdr *iph;
        int hlen = LL_MAX_HEADER;
        int mtu = 1500;
        int addend = sizeof(struct iphdr) + 4;

        tunnel = (struct ip_tunnel*)dev->priv;
        iph = &tunnel->parms.iph;

        ipgre_tunnel_init_gen(dev);

        /* Guess output device to choose reasonable mtu and hard_header_len */

        if (iph->daddr) {
                struct rtable *rt;
                if (!ip_route_output(&rt, iph->daddr, iph->saddr, RT_TOS(iph->tos), tunnel->parms.link)) {
                        tdev = rt->u.dst.dev;
                        ip_rt_put(rt);
                }

                dev->flags |= IFF_POINTOPOINT;

#ifdef CONFIG_NET_IPGRE_BROADCAST
                if (MULTICAST(iph->daddr)) {
                        if (!iph->saddr)
                                return -EINVAL;
                        dev->flags = IFF_BROADCAST;
                        dev->hard_header = ipgre_header;
                        dev->open = ipgre_open;
                        dev->stop = ipgre_close;
                }
#endif
        }

        if (!tdev && tunnel->parms.link)
                tdev = dev_get_by_index(tunnel->parms.link);

        if (tdev) {
                hlen = tdev->hard_header_len;
                mtu = tdev->mtu;
        }
        dev->iflink = tunnel->parms.link;

        /* Precalculate GRE options length */
        if (tunnel->parms.o_flags&(GRE_CSUM|GRE_KEY|GRE_SEQ)) {
                if (tunnel->parms.o_flags&GRE_CSUM)
                        addend += 4;
                if (tunnel->parms.o_flags&GRE_KEY)
                        addend += 4;
                if (tunnel->parms.o_flags&GRE_SEQ)
                        addend += 4;
        }
        dev->hard_header_len = hlen + addend;
        dev->mtu = mtu - addend;
        tunnel->hlen = addend;
        return 0;
}

#ifdef MODULE
static int ipgre_fb_tunnel_open(struct device *dev)
{
        MOD_INC_USE_COUNT;
        return 0;
}

static int ipgre_fb_tunnel_close(struct device *dev)
{
        MOD_DEC_USE_COUNT;
        return 0;
}
#endif

__initfunc(int ipgre_fb_tunnel_init(struct device *dev))
{
        struct ip_tunnel *tunnel = (struct ip_tunnel*)dev->priv;
        struct iphdr *iph;

        ipgre_tunnel_init_gen(dev);
#ifdef MODULE
        dev->open               = ipgre_fb_tunnel_open;
        dev->stop               = ipgre_fb_tunnel_close;
#endif

        iph = &ipgre_fb_tunnel.parms.iph;
        iph->version            = 4;
        iph->protocol           = IPPROTO_GRE;
        iph->ihl                = 5;
        tunnel->hlen            = sizeof(struct iphdr) + 4;

        tunnels_wc[0]           = &ipgre_fb_tunnel;
        return 0;
}


static struct inet_protocol ipgre_protocol = {
  ipgre_rcv,             /* GRE handler          */
  ipgre_err,             /* TUNNEL error control */
  0,                    /* next                 */
  IPPROTO_GRE,          /* protocol ID          */
  0,                    /* copy                 */
  NULL,                 /* data                 */
  "GRE"                 /* name                 */
};


/*
 *      And now the modules code and kernel interface.
 */

#ifdef MODULE
int init_module(void) 
#else
__initfunc(int ipgre_init(void))
#endif
{
        printk(KERN_INFO "GRE over IPv4 tunneling driver\n");

        ipgre_fb_tunnel_dev.priv = (void*)&ipgre_fb_tunnel;
        ipgre_fb_tunnel_dev.name = ipgre_fb_tunnel.parms.name;
#ifdef MODULE
        register_netdev(&ipgre_fb_tunnel_dev);
#else
        register_netdevice(&ipgre_fb_tunnel_dev);
#endif

        inet_add_protocol(&ipgre_protocol);
        return 0;
}

#ifdef MODULE

void cleanup_module(void)
{
        if ( inet_del_protocol(&ipgre_protocol) < 0 )
                printk(KERN_INFO "ipgre close: can't remove protocol\n");

        unregister_netdev(&ipgre_fb_tunnel_dev);
}

#endif

---