/*

  *    Called once on startup.

  */

 static struct packet_type arp_packet_type __read_mostly = {

     .type =    cpu_to_be16(ETH_P_ARP),

     .func =    arp_rcv,

 };

 /*

  *    Receive an arp request from the device layer.

  */

 static int arp_rcv(struct sk_buff *skb, struct net_device *dev,

            struct packet_type *pt, struct net_device *orig_dev)

 {

     const struct arphdr *arp;

     /* do not tweak dropwatch on an ARP we will ignore */

     /* 不支持ARP，非本机包，回环包，不处理 */

     if (dev->flags & IFF_NOARP ||

         skb->pkt_type == PACKET_OTHERHOST ||

         skb->pkt_type == PACKET_LOOPBACK)

         goto consumeskb;

     /* skb如果是共享的，则克隆一个 */

     skb = skb_share_check(skb, GFP_ATOMIC);

     if (!skb)

         goto out_of_mem;

     /* ARP header, plus 2 device addresses, plus 2 IP addresses.  */

     /* 检查arp包完整性 */

     if (!pskb_may_pull(skb, arp_hdr_len(dev)))

         goto freeskb;

     /* 获取arp头 */

     arp = arp_hdr(skb);

     /* 地址长度和协议长度检查 */

     if (arp->ar_hln != dev->addr_len || arp->ar_pln != )

         goto freeskb;

     /* 清空neighCB */

     memset(NEIGH_CB(skb), , sizeof(struct neighbour_cb));

     /* 经过NF的ARP_IN钩子点，然后调用arp_process */

     return NF_HOOK(NFPROTO_ARP, NF_ARP_IN,

                dev_net(dev), NULL, skb, dev, NULL,

                arp_process);

 consumeskb:

     consume_skb(skb);

     return NET_RX_SUCCESS;

 freeskb:

     kfree_skb(skb);

 out_of_mem:

     return NET_RX_DROP;

 }