没有/dev/tunX 设备文件。相反，您打开/dev/net/tun 并通过ioctl() 将其配置为“指向”tun0。为了展示基本过程，我将使用命令行工具ip tun tap 创建 TUN 接口，然后展示从该 TUN 设备读取的 C 代码。所以通过命令行创建tun接口：
sudo ip tuntap add mode tun dev tun0
ip addr add 10.0.0.0/24 dev tun0 # give it an ip
ip link set dev tun0 up # bring the if up
ip route get 10.0.0.2 # check that packets to 10.0.0.x are going through tun0
ping 10.0.0.2 # leave this running in another shell to be able to see the effect of the next example
现在我们创建了tun0。要从用户空间程序读取/写入此接口的数据包，您需要使用ioctl() 与/dev/net/tun 设备文件进行交互。这是一个示例，它将读取到达tun0 接口的数据包并打印大小：
#include <fcntl.h> /* O_RDWR */
#include <string.h> /* memset(), memcpy() */
#include <stdio.h> /* perror(), printf(), fprintf() */
#include <stdlib.h> /* exit(), malloc(), free() */
#include <sys/ioctl.h> /* ioctl() */
/* includes for struct ifreq, etc */
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/if.h>
#include <linux/if_tun.h>
int tun_open(char *devname)
{
struct ifreq ifr;
int fd, err;
if ( (fd = open("/dev/net/tun", O_RDWR)) == -1 ) {
perror("open /dev/net/tun");exit(1);
}
memset(&ifr, 0, sizeof(ifr));
ifr.ifr_flags = IFF_TUN;
strncpy(ifr.ifr_name, devname, IFNAMSIZ); // devname = "tun0" or "tun1", etc
/* ioctl will use ifr.if_name as the name of TUN
* interface to open: "tun0", etc. */
if ( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) == -1 ) {
perror("ioctl TUNSETIFF");close(fd);exit(1);
}
/* After the ioctl call the fd is "connected" to tun device specified
* by devname ("tun0", "tun1", etc)*/
return fd;
}
int main(int argc, char *argv[])
{
int fd, nbytes;
char buf[1600];
fd = tun_open("tun0"); /* devname = ifr.if_name = "tun0" */
printf("Device tun0 opened\n");
while(1) {
nbytes = read(fd, buf, sizeof(buf));
printf("Read %d bytes from tun0\n", nbytes);
}
return 0;
}