W5500有时无法联网是何原因

internet down 状态，是在网络状态的标志位；该位的显示，是由 check_netdev_internet_up_work 处理的；通常哪个 netdev 设备连接状态发生变化就会去检测该 netdev 的状态。可能是这中间哪一步出现问题，导致 w5500 的网络状态标识错误了；
如果你一定要用 internet up 状态去选择网卡，sal_check_netdev_internet_up 一定要维护这个函数的添加工作队列的功能。不然，这个网络状态在错误状态时，并不会频繁更新<不准确>。
你的 RT-Thread 版本是多少，master 版本下应该不会有这个问题。

这看起来是link.rt-thread.org服务器不稳定导致的。一旦中间出现socket异常，整个网卡就一直处于INTERNET_DOWN状态。而且，开机出现的概率很大。

/* check SAL network interface device internet status */
static void check_netdev_internet_up_work(struct rt_work *work, void *work_data)
{
#define SAL_INTERNET_VERSION   0x00
#define SAL_INTERNET_BUFF_LEN  12
#define SAL_INTERNET_TIMEOUT   (2)
#define SAL_INTERNET_HOST      "link.rt-thread.org"
#define SAL_INTERNET_PORT      8101
#define SAL_INTERNET_MONTH_LEN 4
#define SAL_INTERNET_DATE_LEN  16
    int index, sockfd = -1, result = 0;
    struct sockaddr_in server_addr;
    struct hostent *host;
    struct timeval timeout;
    struct netdev *netdev = (struct netdev *)work_data;
    socklen_t addr_len = sizeof(struct sockaddr_in);
    char send_data[SAL_INTERNET_BUFF_LEN], recv_data = 0;
    struct rt_delayed_work *delay_work = (struct rt_delayed_work *)work;
    const char month[][SAL_INTERNET_MONTH_LEN] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
    char date[SAL_INTERNET_DATE_LEN];
    int moth_num = 0;
    struct sal_proto_family *pf = (struct sal_proto_family *) netdev->sal_user_data;
    const struct sal_socket_ops *skt_ops;
    if (work)
    {
        rt_free(delay_work);
    }
    /* get network interface socket operations */
    if (pf == RT_NULL || pf->skt_ops == RT_NULL)
    {
        result = -RT_ERROR;
        goto __exit;
    }
    host = (struct hostent *) pf->netdb_ops->gethostbyname(SAL_INTERNET_HOST);
    if (host == RT_NULL)
    {
        result = -RT_ERROR;
        goto __exit;
    }
    skt_ops = pf->skt_ops;
    if ((sockfd = skt_ops->socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        result = -RT_ERROR;
        goto __exit;
    }
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SAL_INTERNET_PORT);
    server_addr.sin_addr = *((struct in_addr *)host->h_addr);
    rt_memset(&(server_addr.sin_zero), 0, sizeof(server_addr.sin_zero));
    timeout.tv_sec = SAL_INTERNET_TIMEOUT;
    timeout.tv_usec = 0;
    /* set receive and send timeout */
    skt_ops->setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, (void *) &timeout, sizeof(timeout));
    skt_ops->setsockopt(sockfd, SOL_SOCKET, SO_SNDTIMEO, (void *) &timeout, sizeof(timeout));
    /* get build moth value*/
    rt_memset(date, 0x00, SAL_INTERNET_DATE_LEN);
    rt_snprintf(date, SAL_INTERNET_DATE_LEN, "%s", __DATE__);
    for (index = 0; index < sizeof(month) / SAL_INTERNET_MONTH_LEN; index++)
    {
        if (rt_memcmp(date, month[index], SAL_INTERNET_MONTH_LEN - 1) == 0)
        {
            moth_num = index + 1;
            break;
        }
    }
    /* not find build month */
    if (moth_num == 0 || moth_num > sizeof(month) / SAL_INTERNET_MONTH_LEN)
    {
        result = -RT_ERROR;
        goto __exit;
    }
    rt_memset(send_data, 0x00, SAL_INTERNET_BUFF_LEN);
    send_data[0] = SAL_INTERNET_VERSION;
    for (index = 0; index < netdev->hwaddr_len; index++)
    {
        send_data[index + 1] = netdev->hwaddr[index] + moth_num;
    }
    send_data[9] = RT_VERSION;
    send_data[10] = RT_SUBVERSION;
    send_data[11] = RT_REVISION;
    skt_ops->sendto(sockfd, send_data, SAL_INTERNET_BUFF_LEN, 0,
                    (struct sockaddr *)&server_addr, sizeof(struct sockaddr));
    result = skt_ops->recvfrom(sockfd, &recv_data, sizeof(recv_data), 0, (struct sockaddr *)&server_addr, &addr_len);
    if (result < 0)
    {
        goto __exit;
    }
    if (recv_data == RT_FALSE)
    {
        result = -RT_ERROR;
        goto __exit;
    }
__exit:
    if (result > 0)
    {
        LOG_D("Set network interface device(%s) internet status up.", netdev->name);
        netdev->flags |= NETDEV_FLAG_INTERNET_UP;
    }
    else
    {
        LOG_D("Set network interface device(%s) internet status down.", netdev->name);
        netdev->flags &= ~NETDEV_FLAG_INTERNET_UP;
    }
    if (sockfd >= 0)
    {
        skt_ops->closesocket(sockfd);
    }
}