Documentation/networking/tuntap.txt - third_party/linux - Git at Google

 Universal TUN/TAP device driver.
 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>

   Linux, Solaris drivers
   Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>

   FreeBSD TAP driver
   Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>

   Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>

 1. Description
   TUN/TAP provides packet reception and transmission for user space programs.
   It can be seen as a simple Point-to-Point or Ethernet device, which,
   instead of receiving packets from physical media, receives them from
   user space program and instead of sending packets via physical media
   writes them to the user space program.

   In order to use the driver a program has to open /dev/net/tun and issue a
   corresponding ioctl() to register a network device with the kernel. A network
   device will appear as tunXX or tapXX, depending on the options chosen. When
   the program closes the file descriptor, the network device and all
   corresponding routes will disappear.

   Depending on the type of device chosen the userspace program has to read/write
   IP packets (with tun) or ethernet frames (with tap). Which one is being used
   depends on the flags given with the ioctl().

   The package from http://vtun.sourceforge.net/tun contains two simple examples
   for how to use tun and tap devices. Both programs work like a bridge between
   two network interfaces.
   br_select.c - bridge based on select system call.
   br_sigio.c  - bridge based on async io and SIGIO signal.
   However, the best example is VTun http://vtun.sourceforge.net :))

 2. Configuration
   Create device node:
      mkdir /dev/net (if it doesn't exist already)
      mknod /dev/net/tun c 10 200

   Set permissions:
      e.g. chmod 0666 /dev/net/tun
      There's no harm in allowing the device to be accessible by non-root users,
      since CAP_NET_ADMIN is required for creating network devices or for
      connecting to network devices which aren't owned by the user in question.
      If you want to create persistent devices and give ownership of them to
      unprivileged users, then you need the /dev/net/tun device to be usable by
      those users.

   Driver module autoloading

      Make sure that "Kernel module loader" - module auto-loading
      support is enabled in your kernel.  The kernel should load it on
      first access.

   Manual loading
      insert the module by hand:
         modprobe tun

   If you do it the latter way, you have to load the module every time you
   need it, if you do it the other way it will be automatically loaded when
   /dev/net/tun is being opened.

 3. Program interface
   3.1 Network device allocation:

   char *dev should be the name of the device with a format string (e.g.
   "tun%d"), but (as far as I can see) this can be any valid network device name.
   Note that the character pointer becomes overwritten with the real device name
   (e.g. "tun0")

   #include <linux/if.h>
   #include <linux/if_tun.h>

   int tun_alloc(char *dev)
   {
       struct ifreq ifr;
       int fd, err;

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

       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
        */
       ifr.ifr_flags = IFF_TUN;
       if( *dev )
          strncpy(ifr.ifr_name, dev, IFNAMSIZ);

       if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
          close(fd);
          return err;
       }
       strcpy(dev, ifr.ifr_name);
       return fd;
   }

   3.2 Frame format:
   If flag IFF_NO_PI is not set each frame format is:
      Flags [2 bytes]
      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 ?
 Currently driver has been written for 3 Unices:
    Linux kernels 2.2.x, 2.4.x
    FreeBSD 3.x, 4.x, 5.x
    Solaris 2.6, 7.0, 8.0

 2. What is TUN/TAP driver used for?
 As mentioned above, main purpose of TUN/TAP driver is tunneling.
 It is used by VTun (http://vtun.sourceforge.net).

 Another interesting application using TUN/TAP is pipsecd
 (http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
 implementation that can use complete kernel routing (unlike FreeS/WAN).

 3. How does Virtual network device actually work ?
 Virtual network device can be viewed as a simple Point-to-Point or
 Ethernet device, which instead of receiving packets from a physical
 media, receives them from user space program and instead of sending
 packets via physical media sends them to the user space program.

 Let's say that you configured IPv6 on the tap0, then whenever
 the kernel sends an IPv6 packet to tap0, it is passed to the application
 (VTun for example). The application encrypts, compresses and sends it to
 the other side over TCP or UDP. The application on the other side decompresses
 and decrypts the data received and writes the packet to the TAP device,
 the kernel handles the packet like it came from real physical device.

 4. What is the difference between TUN driver and TAP driver?
 TUN works with IP frames. TAP works with Ethernet frames.

 This means that you have to read/write IP packets when you are using tun and
 ethernet frames when using tap.

 5. What is the difference between BPF and TUN/TAP driver?
 BPF is an advanced packet filter. It can be attached to existing
 network interface. It does not provide a virtual network interface.
 A TUN/TAP driver does provide a virtual network interface and it is possible
 to attach BPF to this interface.

 6. Does TAP driver support kernel Ethernet bridging?
 Yes. Linux and FreeBSD drivers support Ethernet bridging.
	Universal TUN/TAP device driver.
	Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>

	Linux, Solaris drivers
	Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>

	FreeBSD TAP driver
	Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>

	Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>

	1. Description
	TUN/TAP provides packet reception and transmission for user space programs.
	It can be seen as a simple Point-to-Point or Ethernet device, which,
	instead of receiving packets from physical media, receives them from
	user space program and instead of sending packets via physical media
	writes them to the user space program.

	In order to use the driver a program has to open /dev/net/tun and issue a
	corresponding ioctl() to register a network device with the kernel. A network
	device will appear as tunXX or tapXX, depending on the options chosen. When
	the program closes the file descriptor, the network device and all
	corresponding routes will disappear.

	Depending on the type of device chosen the userspace program has to read/write
	IP packets (with tun) or ethernet frames (with tap). Which one is being used
	depends on the flags given with the ioctl().

	The package from http://vtun.sourceforge.net/tun contains two simple examples
	for how to use tun and tap devices. Both programs work like a bridge between
	two network interfaces.
	br_select.c - bridge based on select system call.
	br_sigio.c - bridge based on async io and SIGIO signal.
	However, the best example is VTun http://vtun.sourceforge.net :))

	2. Configuration
	Create device node:
	mkdir /dev/net (if it doesn't exist already)
	mknod /dev/net/tun c 10 200

	Set permissions:
	e.g. chmod 0666 /dev/net/tun
	There's no harm in allowing the device to be accessible by non-root users,
	since CAP_NET_ADMIN is required for creating network devices or for
	connecting to network devices which aren't owned by the user in question.
	If you want to create persistent devices and give ownership of them to
	unprivileged users, then you need the /dev/net/tun device to be usable by
	those users.

	Driver module autoloading

	Make sure that "Kernel module loader" - module auto-loading
	support is enabled in your kernel. The kernel should load it on
	first access.

	Manual loading
	insert the module by hand:
	modprobe tun

	If you do it the latter way, you have to load the module every time you
	need it, if you do it the other way it will be automatically loaded when
	/dev/net/tun is being opened.

	3. Program interface
	3.1 Network device allocation:

	char *dev should be the name of the device with a format string (e.g.
	"tun%d"), but (as far as I can see) this can be any valid network device name.
	Note that the character pointer becomes overwritten with the real device name
	(e.g. "tun0")

	#include <linux/if.h>
	#include <linux/if_tun.h>

	int tun_alloc(char *dev)
	{
	struct ifreq ifr;
	int fd, err;

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

	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
	*/
	ifr.ifr_flags = IFF_TUN;
	if( *dev )
	strncpy(ifr.ifr_name, dev, IFNAMSIZ);

	if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
	close(fd);
	return err;
	}
	strcpy(dev, ifr.ifr_name);
	return fd;
	}

	3.2 Frame format:
	If flag IFF_NO_PI is not set each frame format is:
	Flags [2 bytes]
	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 ?
	Currently driver has been written for 3 Unices:
	Linux kernels 2.2.x, 2.4.x
	FreeBSD 3.x, 4.x, 5.x
	Solaris 2.6, 7.0, 8.0

	2. What is TUN/TAP driver used for?
	As mentioned above, main purpose of TUN/TAP driver is tunneling.
	It is used by VTun (http://vtun.sourceforge.net).

	Another interesting application using TUN/TAP is pipsecd
	(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
	implementation that can use complete kernel routing (unlike FreeS/WAN).

	3. How does Virtual network device actually work ?
	Virtual network device can be viewed as a simple Point-to-Point or
	Ethernet device, which instead of receiving packets from a physical
	media, receives them from user space program and instead of sending
	packets via physical media sends them to the user space program.

	Let's say that you configured IPv6 on the tap0, then whenever
	the kernel sends an IPv6 packet to tap0, it is passed to the application
	(VTun for example). The application encrypts, compresses and sends it to
	the other side over TCP or UDP. The application on the other side decompresses
	and decrypts the data received and writes the packet to the TAP device,
	the kernel handles the packet like it came from real physical device.

	4. What is the difference between TUN driver and TAP driver?
	TUN works with IP frames. TAP works with Ethernet frames.

	This means that you have to read/write IP packets when you are using tun and
	ethernet frames when using tap.

	5. What is the difference between BPF and TUN/TAP driver?
	BPF is an advanced packet filter. It can be attached to existing
	network interface. It does not provide a virtual network interface.
	A TUN/TAP driver does provide a virtual network interface and it is possible
	to attach BPF to this interface.

	6. Does TAP driver support kernel Ethernet bridging?
	Yes. Linux and FreeBSD drivers support Ethernet bridging.