daddr = &ih->daddr;    ((u8 *)saddr)[2] ^= 1; /* change the third octet (class C) */    ((u8 *)daddr)[2] ^= 1;    ih->check = 0;         /* and rebuild the checksum (ip needs it) */    ih->check = ip_fast_csum((unsigned char *)ih,ih->ihl);    if (dev == snull_devs)        PDEBUGG("%08x:%05i --> %08x:%05i\n",               ntohl(ih->saddr),ntohs(((struct tcphdr *)(ih+1))->source),               ntohl(ih->daddr),ntohs(((struct tcphdr *)(ih+1))->dest));    else        PDEBUGG("%08x:%05i <-- %08x:%05i\n",               ntohl(ih->daddr),ntohs(((struct tcphdr *)(ih+1))->dest),               ntohl(ih->saddr),ntohs(((struct tcphdr *)(ih+1))->source));    /*     * Ok, now the packet is ready for transmission: first simulate a     * receive interrupt on the twin device, then  a     * transmission-done on the transmitting device     */    dest = snull_devs + (dev==snull_devs ? 1 : 0);    privp = (struct snull_priv *)dest->priv;    privp->status = SNULL_RX_INTR;    privp->rx_packetlen = len;    privp->rx_packetdata = buf;    snull_interrupt(0, dest, NULL);    privp = (struct snull_priv *)dev->priv;    privp->status = SNULL_TX_INTR;    privp->tx_packetlen = len;    privp->tx_packetdata = buf;    if (lockup && ((privp->stats.tx_packets + 1) % lockup) == 0) {        /* Simulate a dropped transmit interrupt */        netif_stop_queue(dev);        PDEBUG("Simulate lockup at %ld, txp %ld\n", jiffies,                        (unsigned long) privp->stats.tx_packets);    }    else        snull_interrupt(0, dev, NULL);}/* * Transmit a packet (called by the kernel) */int snull_tx(struct sk_buff *skb, struct net_device *dev){    int len, retval=0;    char *data;    struct snull_priv *priv = (struct snull_priv *)dev->priv;#ifndef LINUX_24    if (dev->tbusy || skb == NULL) {        PDEBUG("tint for %p, tbusy %ld, skb %p\n", dev, dev->tbusy, skb);        snull_tx_timeout (dev);        if (skb == NULL)            return 0;    }#endif    len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;    data = skb->data;    dev->trans_start = jiffies; /* save the timestamp */    /* Remember the skb, so we can free it at interrupt time */    priv->skb = skb;    /* actual deliver of data is device-specific, and not shown here */    snull_hw_tx(data, len, dev);    return retval; /* zero == done; nonzero == fail */}/* * Deal with a transmit timeout. */void snull_tx_timeout (struct net_device *dev){    struct snull_priv *priv = (struct snull_priv *)dev->priv;    PDEBUG("Transmit timeout at %ld, latency %ld\n", jiffies,                    jiffies - dev->trans_start);    priv->status = SNULL_TX_INTR;    snull_interrupt(0, dev, NULL);    priv->stats.tx_errors++;    netif_wake_queue(dev);}/* * Ioctl commands  */int snull_ioctl(struct net_device *dev, struct ifreq *rq, int cmd){     PDEBUG("ioctl\n");    return 0;}/* * Return statistics to the caller */struct net_device_stats *snull_stats(struct net_device *dev){    struct snull_priv *priv = (struct snull_priv *)dev->priv;    return &priv->stats;}/* * This function is called to fill up an eth header, since arp is not * available on the interface */#ifndef LINUX_20int snull_rebuild_header(struct sk_buff *skb){    struct ethhdr *eth = (struct ethhdr *) skb->data;    struct net_device *dev = skb->dev;        memcpy(eth->h_source, dev->dev_addr, dev->addr_len);    memcpy(eth->h_dest, dev->dev_addr, dev->addr_len);    eth->h_dest[ETH_ALEN-1]   ^= 0x01;   /* dest is us xor 1 */    return 0;}#else /* LINUX_20 */int snull_rebuild_header(void *buff, struct net_device *dev, unsigned long dst,                    struct sk_buff *skb){    struct ethhdr *eth = (struct ethhdr *)buff;    memcpy(eth->h_source, dev->dev_addr, dev->addr_len);    memcpy(eth->h_dest, dev->dev_addr, dev->addr_len);    eth->h_dest[ETH_ALEN-1]   ^= 0x01;   /* dest is us xor 1 */    return 0;}#endif /* LINUX_20 */int snull_header(struct sk_buff *skb, struct net_device *dev,                unsigned short type, void *daddr, void *saddr, unsigned len){    struct ethhdr *eth = (struct ethhdr *)skb_push(skb,ETH_HLEN);    eth->h_proto = htons(type);    memcpy(eth->h_source, saddr ? saddr : dev->dev_addr, dev->addr_len);    memcpy(eth->h_dest,   daddr ? daddr : dev->dev_addr, dev->addr_len);    eth->h_dest[ETH_ALEN-1]   ^= 0x01;   /* dest is us xor 1 */    return (dev->hard_header_len);}/* * The "change_mtu" method is usually not needed. * If you need it, it must be like this. */int snull_change_mtu(struct net_device *dev, int new_mtu){    unsigned long flags;    spinlock_t *lock = &((struct snull_priv *) dev->priv)->lock;        /* check ranges */    if ((new_mtu < 68) || (new_mtu > 1500))        return -EINVAL;    /*     * Do anything you need, and the accept the value     */    spin_lock_irqsave(lock, flags);    dev->mtu = new_mtu;    spin_unlock_irqrestore(lock, flags);    return 0; /* success */}/* * The init function (sometimes called probe). * It is invoked by register_netdev() */int snull_init(struct net_device *dev){#if 0    /*     * Make the usual checks: check_region(), probe irq, ...  -ENODEV     * should be returned if no device found.  No resource should be     * grabbed: this is done on open().      */#endif    /*      * Then, assign other fields in dev, using ether_setup() and some     * hand assignments     */    ether_setup(dev); /* assign some of the fields */    dev->open            = snull_open;    dev->stop            = snull_release;    dev->set_config      = snull_config;    dev->hard_start_xmit = snull_tx;    dev->do_ioctl        = snull_ioctl;    dev->get_stats       = snull_stats;    dev->change_mtu      = snull_change_mtu;      dev->rebuild_header  = snull_rebuild_header;    dev->hard_header     = snull_header;#ifdef HAVE_TX_TIMEOUT    dev->tx_timeout     = snull_tx_timeout;    dev->watchdog_timeo = timeout;#endif    /* keep the default flags, just add NOARP */    dev->flags           |= IFF_NOARP;    dev->hard_header_cache = NULL;      /* Disable caching */    /*     * Then, allocate the priv field. This encloses the statistics     * and a few private fields.     */    dev->priv = kmalloc(sizeof(struct snull_priv), GFP_KERNEL);    if (dev->priv == NULL)        return -ENOMEM;    memset(dev->priv, 0, sizeof(struct snull_priv));    spin_lock_init(& ((struct snull_priv *) dev->priv)->lock);    return 0;}/* * The devices */#ifdef LINUX_24struct net_device snull_devs[2] = {    { init: snull_init, },  /* init, nothing more */    { init: snull_init, }};#else /* pre-2.4 */char snull_names[16];struct net_device snull_devs[2] = {    {        name: snull_names,        init: snull_init,  /* init function */    },    {        name: snull_names+8,        init: snull_init,            /* init function */    }};#endif /* LINUX_24 *//* * Finally, the module stuff */int snull_init_module(void){    int result, i, device_present = 0;    snull_eth = eth; /* copy the cfg datum in the non-static place */    if (!snull_eth) { /* call them "sn0" and "sn1" */        strcpy(snull_devs[0].name, "sn0");        strcpy(snull_devs[1].name, "sn1");    } else { /* use automatic assignment */        strcpy(snull_devs[0].name, "eth%d");        strcpy(snull_devs[1].name, "eth%d");    }    for (i=0; i<2;  i++)        if ( (result = register_netdev(snull_devs + i)) )            printk("snull: error %i registering device \"%s\"\n",                   result, snull_devs[i].name);        else device_present++;#ifndef SNULL_DEBUG    EXPORT_NO_SYMBOLS;#endif    return device_present ? 0 : -ENODEV;}void snull_cleanup(void){    int i;       for (i=0; i<2;  i++) {        kfree(snull_devs[i].priv);        unregister_netdev(snull_devs + i);    }    return;}module_init(snull_init_module);module_exit(snull_cleanup);