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Universal TUN/TAP device driver.

Copyright (C) 1999-2000 Maxim Krasnyansky 

  Linux, Solaris drivers

  Copyright (C) 1999-2000 Maxim Krasnyansky 

  FreeBSD TAP driver

  Copyright (c) 1999-2000 Maksim Yevmenkin 

  Revision of this document 2002 by Florian Thiel 

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 0700 /dev/net/tun

     if you want the device only accesible by root. Giving regular users the

     right to assign network devices is NOT a good idea. Users could assign

     bogus network interfaces to trick firewalls or administrators.

  Driver module autoloading

     Make sure that "Kernel module loader" - module auto-loading support is enabled

     in your kernel.

     Add the following line to the /etc/modules.conf:

        alias char-major-10-200 tun

     and run

        depmod -a

  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 

  #include 

  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.

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/), an 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 IPX on the tap0, then whenever

the kernel sends an IPX 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?

BFP 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.