使用ioctl添加网关产生Network is unreachable的原因

代码如下

#define DBG_ERR(x...)        perror(x)
int net_add_gateway(in_addr_t addr)
{
    struct rtentry rt;
    int skfd;
    struct sockaddr_in sa = {
        sin_family:PF_INET,
        sin_port:0
    };

    /* Clean out the RTREQ structure. */
    memset((char *) &rt, 0, sizeof(struct rtentry));

    /* Fill in the other fields. */
    rt.rt_flags = (RTF_UP | RTF_GATEWAY);

    rt.rt_dst.sa_family = PF_INET;
    rt.rt_genmask.sa_family = PF_INET;

    sa.sin_addr.s_addr = addr;
    memcpy((char *) &rt.rt_gateway, (char *) &sa, sizeof(struct sockaddr));

    /* Create a socket to the INET kernel. */
    if ((skfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        DBG_ERR("socket error");
        return -1;
    }
    /* Tell the kernel to accept this route. */
    if (ioctl(skfd, SIOCADDRT, &rt) < 0) {
        DBG_ERR("net_add_gateway: ioctl SIOCADDRT");
        close(skfd);
        return -1;
    }
    /* Close the socket. */
    close(skfd);
    return (0);
}

在添加网关时,只要网关IP与IP地址不在同一个网段内,ioctl便会引起Network is unreachable错误.只要将ip、netmask、gateway设置对了即可.