回环网卡驱动

loopback.c

/*
 * INET        An implementation of the TCP/IP protocol suite for the LINUX
 *        operating system. INET is implemented using the  BSD Socket
 *        interface as the means of communication with the user level.
 *
 *        Pseudo-driver for the loopback interface.
 *
 * Version:    @(#)loopback.c    1.0.4b    08/16/93
 *
 * Authors:    Ross Biro
 *        Fred N. van Kempen, <[e-mail]>
 *        Donald Becker, <[e-mail]>
 *
 *        Alan Cox    :    Fixed oddments for NET3.014
 *        Alan Cox    :    Rejig for NET3.029 snap #3
 *        Alan Cox    :     Fixed NET3.029 bugs and sped up
 *        Larry McVoy    :    Tiny tweak to double performance
 *        Alan Cox    :    Backed out LMV's tweak - the linux mm
 *                    can't take it...
 * Michael Griffith: Don't bother computing the checksums
 * on packets received on the loopback
 * interface.
 *        Alexey Kuznetsov:    Potential hang under some extreme
 *                    cases removed.
 *
 *        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/kernel.h>
#include <linux/jiffies.h>
#include <linux/module.h>
#include <linux/interrupt.h>
#include <linux/fs.h>
#include <linux/types.h>
#include <linux/string.h>
#include <linux/socket.h>
#include <linux/errno.h>
#include <linux/fcntl.h>
#include <linux/in.h>
#include <linux/init.h>

#include <asm/system.h>
#include <asm/uaccess.h>
#include <asm/io.h>

#include <linux/inet.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/skbuff.h>
#include <linux/ethtool.h>
#include <net/sock.h>
#include <net/checksum.h>
#include <linux/if_ether.h>    /* For the statistics structure. */
#include <linux/if_arp.h>    /* For ARPHRD_ETHER */
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/percpu.h>
#include <net/net_namespace.h>


unsigned long bytes = 0;
unsigned long packets = 0;


static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
{
    skb_orphan(skb);
    skb->protocol = eth_type_trans(skb,dev);

    bytes += skb->len;
    packets++;

    netif_rx(skb);

    return 0;
}

static struct net_device_stats *loopback_get_stats(struct net_device *dev)
{
    struct net_device_stats *stats = &dev->stats;

    stats->rx_packets = packets;
    stats->tx_packets = packets;
    stats->rx_bytes = bytes;
    stats->tx_bytes = bytes;
    return stats;
}

static const struct net_device_ops loopback_ops = {
    .ndo_start_xmit= loopback_xmit,
    .ndo_get_stats = loopback_get_stats,
};

/*
 * The loopback device is special. There is only one instance
 * per network namespace.
 */
static void loopback_setup(struct net_device *dev)
{


    dev->mtu        = (16 * 1024) + 20 + 20 + 12;//有
    dev->flags        = IFF_LOOPBACK;//有
    dev->priv_flags     &= ~IFF_XMIT_DST_RELEASE;
    dev->header_ops        = &eth_header_ops;//有
    dev->netdev_ops        = &loopback_ops;//有
}

/* Setup and register the loopback device. */
static __net_init int loopback_net_init(struct net *net)
{
    struct net_device *dev;
    int err;
    err = -ENOMEM;
    dev = alloc_netdev(0, "lo", loopback_setup);
    if (!dev)
        goto out;

    err = register_netdev(dev);
    if (err)
        goto out_free_netdev;

    net->loopback_dev = dev;
    return 0;


out_free_netdev:
    free_netdev(dev);
out:
    if (net == &init_net)
        panic("loopback: Failed to register netdevice: %d\n", err);
    return err;
}

static __net_exit void loopback_net_exit(struct net *net)
{
    struct net_device *dev = net->loopback_dev;

    unregister_netdev(dev);
}

/* Registered in net/core/dev.c */
struct pernet_operations __net_initdata loopback_net_ops = {
       .init = loopback_net_init,
       .exit = loopback_net_exit,
};

编译内核代码，并在开发板上运行，可通过ifconfig命令查看回环网卡信息，也可通过ping命令ping通，下图是不加那一行
ping不通的图，