vtun 虚拟网卡创建过程

只分析udp，

单独运行clinet，处于connect状态，并没有运行到tunnel这个步骤，单独运行client时没出现虚拟网卡；

单独运行server，处于listen状态，也没运行到tunnel这个步骤，所以单独运行server也不会出现虚拟新网卡。

 

当server端开启，处于监听状态，这时client请求连接，认证成功后client和server才会进入tunnel执行打开虚拟网卡，

以及读写发送等后续操作。

 

为深入理解虚拟网卡的建立过程，得深入学习套接字的阻塞和非阻塞问题。

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int tunnel(struct vtun_host *host) {     ......     if ( (host->persist == VTUN_PERSIST_KEEPIF) && (host->loc_fd >= 0) )     //接口是否已经打开         interface_already_open = 1;                                                 if( host->dev ){...}                                                      //判断虚拟设备类型     if(!interface_already_open)                                                 //虚拟网卡没有打开，则打开之     {         ...         switch(host->flags & VTUN_TYPE_MASK)         {             ...             case VTUN_TUN:                 if( (fd[0]=tun_open(dev)) < 0 )                                 //打开虚拟网卡，获取描述符                 {                     vtun_syslog(LOG_ERR,"Can't allocate tun device %s. %s(%d)", dev, strerror(errno), errno);                     return -1;                 }                 break;         }         host->loc_fd = fd[0];                                                 //虚拟设备文件描述符存在host->loc_fd中     }     switch( host->flags & VTUN_PROT_MASK )                                     //初始化协议，tcp还是udp     {         ......         case VTUN_UDP:             if( (opt = udp_session(host)) == -1){.....}                         //进行udp的socket创建等操作             proto_write = udp_write;             proto_read = udp_read;     }     switch( (pid=fork()) ){...}                                                 //建立子进程     switch( host->flags & VTUN_TYPE_MASK )                                     //根据虚拟设备类型，选择相应虚拟设备读写方式     {         .....         case VTUN_TUN:             set_title("%s tun %s", host->host, dev);             dev_read  = tun_read;             dev_write = tun_write;             break;     }     opt = linkfd(host);                             ....... }