net: docs: document multiqueue tuntap API

     Proto [2 bytes]

     Raw protocol(IP, IPv6, etc) frame.

  3.3 Multiqueue tuntap interface:

  From version 3.8, Linux supports multiqueue tuntap which can uses multiple

  file descriptors (queues) to parallelize packets sending or receiving. The

  device allocation is the same as before, and if user wants to create multiple

  queues, TUNSETIFF with the same device name must be called many times with

  IFF_MULTI_QUEUE flag.

  char *dev should be the name of the device, queues is the number of queues to

  be created, fds is used to store and return the file descriptors (queues)

  created to the caller. Each file descriptor were served as the interface of a

  queue which could be accessed by userspace.

  #include <linux/if.h>

  #include <linux/if_tun.h>

  int tun_alloc_mq(char *dev, int queues, int *fds)

  {

      struct ifreq ifr;

      int fd, err, i;

      if (!dev)

          return -1;

      memset(&ifr, 0, sizeof(ifr));

      /* Flags: IFF_TUN   - TUN device (no Ethernet headers)

       *        IFF_TAP   - TAP device

       *

       *        IFF_NO_PI - Do not provide packet information

       *        IFF_MULTI_QUEUE - Create a queue of multiqueue device

       */

      ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_MULTI_QUEUE;

      strcpy(ifr.ifr_name, dev);

      for (i = 0; i < queues; i++) {

          if ((fd = open("/dev/net/tun", O_RDWR)) < 0)

             goto err;

          err = ioctl(fd, TUNSETIFF, (void *)&ifr);

          if (err) {

             close(fd);

             goto err;

          }

          fds[i] = fd;

      }

      return 0;

  err:

      for (--i; i >= 0; i--)

          close(fds[i]);

      return err;

  }

  A new ioctl(TUNSETQUEUE) were introduced to enable or disable a queue. When

  calling it with IFF_DETACH_QUEUE flag, the queue were disabled. And when

  calling it with IFF_ATTACH_QUEUE flag, the queue were enabled. The queue were

  enabled by default after it was created through TUNSETIFF.

  fd is the file descriptor (queue) that we want to enable or disable, when

  enable is true we enable it, otherwise we disable it

  #include <linux/if.h>

  #include <linux/if_tun.h>

  int tun_set_queue(int fd, int enable)

  {

      struct ifreq ifr;

      memset(&ifr, 0, sizeof(ifr));

      if (enable)

         ifr.ifr_flags = IFF_ATTACH_QUEUE;

      else

         ifr.ifr_flags = IFF_DETACH_QUEUE;

      return ioctl(fd, TUNSETQUEUE, (void *)&ifr);

  }

Universal TUN/TAP device driver Frequently Asked Question.

1. What platforms are supported by TUN/TAP driver ?