在Linux,网络分为两个层，各自是网络堆栈协议支持层，以及接收和发送网络协议的设备驱动程序层。
网络堆栈是硬件中独立出来的部分。主要用来支持TCP/IP等多种协议，网络设备驱动层是连接网络堆栈协议层和网络硬件的中间层。
网络设备驱动程序的主要功能是:
（1）模块载入或内核启动相关的初始化处理
（2）清除模块时的处理
（3）网络设备的检索和探測
（4）网络设备的初始化和注冊
（5）打开或关闭网络设备
（6）发送网络数据
（7）接收网络数据
（8）中断处理(在发送完数据时。硬件向内核产生一个中断。告诉内核数据已经发送完成。在网络设备接收到数据时，也要发生一个中断，告诉内核。数据已经到达，请及时处理)
（9）超时处理
（10）多播处理
（11）网络设备的控制ioctl
而Linux网络设备驱动的主要功能就是网络设备的初始化，网络设备的配置，数据包的收发。
以下代码是关于虚拟硬件的网络驱动的样例
2、代码
#undef PDEBUG /* undef it, just in case */#ifdef SNULL_DEBUG# ifdef __KERNEL__ /* This one if debugging is on, and kernel space */# define PDEBUG(fmt, args...) printk( KERN_DEBUG "snull: " fmt, ## args)# else /* This one for user space */# define PDEBUG(fmt, args...) fprintf(stderr, fmt, ## args)# endif#else# define PDEBUG(fmt, args...) /* not debugging: nothing */#endif#undef PDEBUGG#define PDEBUGG(fmt, args...) /* nothing: it's a placeholder *//* These are the flags in the statusword */#define SNULL_RX_INTR 0x0001#define SNULL_TX_INTR 0x0002/* Default timeout period */#define SNULL_TIMEOUT 6 /* In jiffies */#include#include #include /* printk() */#include /* kmalloc() */#include /* error codes */#include /* size_t */#include /* mark_bh */#include #include /* struct device, and other headers */#include /* eth_type_trans */#include /* struct iphdr */#include /* struct tcphdr */#include #include #include #include #include static int lockup = 0;static int timeout = SNULL_TIMEOUT;struct net_device snull_devs[2];//这里定义两个设备，一个是snull0,一个是snull1//网络设备结构体,作为net_device->privstruct snull_priv { struct net_device_stats stats;//实用的统计信息 int status;//网络设备的状态信息。是发完数据包。还是接收到网络数据包 int rx_packetlen;//接收到的数据包长度 u8 *rx_packetdata;//接收到的数据 int tx_packetlen;//发送的数据包长度 u8 *tx_packetdata;//发送的数据 struct sk_buff *skb;//socket buffer结构体，网络各层之间传送数据都是通过这个结构体来实现的 spinlock_t lock;//自旋锁};void snull_tx_timeout (struct net_device *dev);//网络接口的打开函数int snull_open(struct net_device *dev) { printk("call snull_open/n"); memcpy(dev->dev_addr, "/0SNUL0", ETH_ALEN);//分配一个硬件地址，ETH＿ALEN是网络设备硬件地址的长度 netif_start_queue(dev);//打开传输队列，这样才干进行传输数据 return 0;}int snull_release(struct net_device *dev){ printk("call snull_release/n"); netif_stop_queue(dev); //当网络接口关闭的时候，调用stop方法。这个函数表示不能再发送数据 return 0;}//接包函数void snull_rx(struct net_device *dev, int len, unsigned char *buf){ struct sk_buff *skb; struct snull_priv *priv = (struct snull_priv *) dev->priv; /* * The packet has been retrieved from the transmission * medium. Build an skb around it, so upper layers can handle it */ skb = dev_alloc_skb(len+2);//分配一个socket buffer,而且初始化skb->data,skb->tail和skb->head if (!skb) { printk("snull rx: low on mem - packet dropped/n"); priv->stats.rx_dropped++; return; } skb_reserve(skb, 2); /* align IP on 16B boundary */ memcpy(skb_put(skb, len), buf, len);//skb_put是把数据写入到socket buffer /* Write metadata, and then pass to the receive level */ skb->dev = dev; skb->protocol = eth_type_trans(skb, dev);//返回的是协议号 skb->ip_summed = CHECKSUM_UNNECESSARY; //此处不校验 priv->stats.rx_packets++;//接收到包的个数＋1 priv->stats.rx_bytes += len;//接收到包的长度 netif_rx(skb);//通知内核已经接收到包。而且封装成socket buffer传到上层 return;} /* * The typical interrupt entry point *///中断处理。此程序中没有硬件，因此，没有真正的硬件中断，仅仅是模拟中断，在发送完网络数据包之后。会产生中断//用来通知内核已经发送完数据包，当新的数据包到达网络接口时，会发生中断。通知新的数据包已经到来了void snull_interrupt(int irq, void *dev_id, struct pt_regs *regs){ int statusword;//用来标识是发送完成还是接收到新的数据包 struct snull_priv *priv; /* * As usual, check the "device" pointer for shared handlers. * Then assign "struct device *dev" */ struct net_device *dev = (struct net_device *)dev_id; /* ... and check with hw if it's really ours */ if (!dev /*paranoid*/ ) return; /* Lock the device */ priv = (struct snull_priv *) dev->priv; spin_lock(&priv->lock); /* retrieve statusword: real netdevices use I/O instructions */ statusword = priv->status; if (statusword & SNULL_RX_INTR) {//假设是接收 /* send it to snull_rx for handling */ snull_rx(dev, priv->rx_packetlen, priv->rx_packetdata); } if (statusword & SNULL_TX_INTR) {//假设发送完成 /* a transmission is over: free the skb */ priv->stats.tx_packets++; priv->stats.tx_bytes += priv->tx_packetlen; dev_kfree_skb(priv->skb);//释放skb 套接字缓冲区 } /* Unlock the device and we are done */ spin_unlock(&priv->lock); return;}/* * Transmit a packet (low level interface) *///真正的处理的发送数据包//模拟从一个网络向还有一个网络发送数据包void snull_hw_tx(char *buf, int len, struct net_device *dev){ /* * This function deals with hw details. This interface loops * back the packet to the other snull interface (if any). * In other words, this function implements the snull behaviour, * while all other procedures are rather device-independent */ struct iphdr *ih;//ip头部 struct net_device *dest;//目标设备结构体。net_device存储一个网络接口的重要信息，是网络驱动程序的核心 struct snull_priv *priv; u32 *saddr, *daddr;//源设备地址与目标设备地址 /* I am paranoid. Ain't I? */ if (len < sizeof(struct ethhdr) + sizeof(struct iphdr)) { printk("snull: Hmm... packet too short (%i octets)/n", len); return; } /* * Ethhdr is 14 bytes, but the kernel arranges for iphdr * to be aligned (i.e., ethhdr is unaligned) */ ih = (struct iphdr *)(buf+sizeof(struct ethhdr)); saddr = &ih->saddr; daddr = &ih->daddr; //在同一台机器上模拟两个网络。不同的网段地址，进行发送网络数据包与接收网络数据包 ((u8 *)saddr)[2] ^= 1; /* change the third octet (class C) ^是位异或操作符把第三个部分的网络地址与1进行异或，因为同一网络的数据不进行转发*/ ((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);//假设dev是0，那么dest就是1,假设dev是1。那么dest是0 priv = (struct snull_priv *) dest->priv;//目标dest中的priv priv->status = SNULL_RX_INTR; priv->rx_packetlen = len; priv->rx_packetdata = buf; snull_interrupt(0, dest, NULL); priv = (struct snull_priv *) dev->priv; priv->status = SNULL_TX_INTR; priv->tx_packetlen = len; priv->tx_packetdata = buf; if (lockup && ((priv->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) priv->stats.tx_packets); } else snull_interrupt(0, dev, NULL);} /* * Transmit a packet (called by the kernel) *///发包函数int snull_tx(struct sk_buff *skb, struct net_device *dev){ int len; char *data; struct snull_priv *priv = (struct snull_priv *) dev->priv; if ( skb == NULL) { PDEBUG("tint for %p, skb %p/n", dev, skb); snull_tx_timeout (dev); if (skb == NULL) return 0; } len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;//ETH_ZLEN是所发的最小数据包的长度 data = skb->data;//将要发送的数据包中数据部分 dev->trans_start = jiffies; //保存当前的发送时间 priv->skb = skb; snull_hw_tx(data, len, dev);//真正的发送函数 return 0; /* Our simple device can not fail */}/* * Deal with a transmit timeout. *///一旦超出watchdog_timeo就会调用snull_tx_timeoutvoid snull_tx_timeout (struct net_device *dev){ printk("call snull_tx_timeout/n"); 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);//超时后发生中断 priv->stats.tx_errors++;//发送的错误数 netif_wake_queue(dev); //为了再次发送数据，调用此函数，又一次启动发送队列 return;} /* * 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;//得到统计资料信息}//设备初始化函数int snull_init(struct net_device *dev){ printk("call snull_init/n"); /* * Then, assign other fields in dev, using ether_setup() and some * hand assignments */ ether_setup(dev);//填充一些以太网中的设备结构体的项 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; dev->tx_timeout = snull_tx_timeout;//超时处理 dev->watchdog_timeo = timeout; /* keep the default flags, just add NOARP */ dev->flags |= IFF_NOARP; dev->hard_header_cache = NULL; /* Disable caching */ SET_MODULE_OWNER(dev); /* * Then, allocate the priv field. This encloses the statistics * and a few private fields. *///为priv分配内存 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;}struct net_device snull_devs[2] = { { init: snull_init, }, /* init, nothing more */ { init: snull_init, }};int snull_init_module(void){ int i,result=0; strcpy(snull_devs[0].name,"snull0");//net_device结构体中的name表示设备名 strcpy(snull_devs[1].name,"snull1");//即定义了两个设备，snull0与snull1 for (i=0; i<2; i++) if ( (result = register_netdev(snull_devs+i)) )//注冊设备 printk("snull: error %i registering device /"%s/"/n", result, snull_devs[i].name); return 0;}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);