/*

   发送arp请求

   ARP请求的目的邻居项

   缓存在该邻居项中的待发送报文，来获取该skb的源IP地址

  */

 static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb)

 {

     __be32 saddr = ;

     u8 dst_ha[MAX_ADDR_LEN], *dst_hw = NULL;

     struct net_device *dev = neigh->dev;

     __be32 target = *(__be32 *)neigh->primary_key;

     int probes = atomic_read(&neigh->probes);

     struct in_device *in_dev;

     struct dst_entry *dst = NULL;

     rcu_read_lock();

     /* 获取IP配置块 */

     in_dev = __in_dev_get_rcu(dev);

     if (!in_dev) {

         rcu_read_unlock();

         return;

     }

     /* 选取源地址 */

     switch (IN_DEV_ARP_ANNOUNCE(in_dev)) {

     default:

     case :        /* By default announce any local IP */

         /* 使用任意本地地址 */

         if (skb && inet_addr_type_dev_table(dev_net(dev), dev,

                       ip_hdr(skb)->saddr) == RTN_LOCAL)

             saddr = ip_hdr(skb)->saddr;

         break;

     case :        /* Restrict announcements of saddr in same subnet */

         /* 在同一子网 */

         if (!skb)

             break;

         /* 获取skb的源ip */

         saddr = ip_hdr(skb)->saddr;

         /* 源地址是本地接口地址 */

         if (inet_addr_type_dev_table(dev_net(dev), dev,

                          saddr) == RTN_LOCAL) {

             /* saddr should be known to target */

             /* 判断是否在同一子网 */

             if (inet_addr_onlink(in_dev, target, saddr))

                 break;

         }

         saddr = ;

         break;

     case :        /* Avoid secondary IPs, get a primary/preferred one */

         /* 禁止从IP，获取主IP */

         break;

     }

     rcu_read_unlock();

     /* 源地址为空，选择一个地址 */

     if (!saddr)

         saddr = inet_select_addr(dev, target, RT_SCOPE_LINK);

     /* 计算是否使用完单播发送次数 */

     probes -= NEIGH_VAR(neigh->parms, UCAST_PROBES);

     /* 未使用完 */

     if (probes < ) {

         if (!(neigh->nud_state & NUD_VALID))

             pr_debug("trying to ucast probe in NUD_INVALID\n");

         /* 目的硬件地址 */

         neigh_ha_snapshot(dst_ha, neigh, dev);

         dst_hw = dst_ha;

     }

     /* 使用完 */

     else {

         /* 计算是否使用完应用程序ARPD发送次数 */

         probes -= NEIGH_VAR(neigh->parms, APP_PROBES);

         /* 未使用完 */

         if (probes < ) {

             /* 应用程序ARPD请求 */

             neigh_app_ns(neigh);

             return;

         }

     }

     /* 获取目的路由缓存 */

     if (skb && !(dev->priv_flags & IFF_XMIT_DST_RELEASE))

         dst = skb_dst(skb);

     /* 发送arp请求包 */

     arp_send_dst(ARPOP_REQUEST, ETH_P_ARP, target, dev, saddr,

              dst_hw, dev->dev_addr, NULL, dst);

 }