Documentation/networking/tuntap.txt

   1 Universal TUN/TAP device driver.
   2 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
   3
   4   Linux, Solaris drivers
   5   Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
   6
   7   FreeBSD TAP driver
   8   Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>
   9
  10   Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>
  11
  12 1. Description
  13   TUN/TAP provides packet reception and transmission for user space programs.
  14   It can be seen as a simple Point-to-Point or Ethernet device, which,
  15   instead of receiving packets from physical media, receives them from
  16   user space program and instead of sending packets via physical media
  17   writes them to the user space program.
  18
  19   In order to use the driver a program has to open /dev/net/tun and issue a
  20   corresponding ioctl() to register a network device with the kernel. A network
  21   device will appear as tunXX or tapXX, depending on the options chosen. When
  22   the program closes the file descriptor, the network device and all
  23   corresponding routes will disappear.
  24
  25   Depending on the type of device chosen the userspace program has to read/write
  26   IP packets (with tun) or ethernet frames (with tap). Which one is being used
  27   depends on the flags given with the ioctl().
  28
  29   The package from http://vtun.sourceforge.net/tun contains two simple examples
  30   for how to use tun and tap devices. Both programs work like a bridge between
  31   two network interfaces.
  32   br_select.c - bridge based on select system call.
  33   br_sigio.c  - bridge based on async io and SIGIO signal.
  34   However, the best example is VTun http://vtun.sourceforge.net :))
  35
  36 2. Configuration
  37   Create device node:
  38      mkdir /dev/net (if it doesn't exist already)
  39      mknod /dev/net/tun c 10 200
  40
  41   Set permissions:
  42      e.g. chmod 0700 /dev/net/tun
  43      if you want the device only accessible by root. Giving regular users the
  44      right to assign network devices is NOT a good idea. Users could assign
  45      bogus network interfaces to trick firewalls or administrators.
  46
  47   Driver module autoloading
  48
  49      Make sure that "Kernel module loader" - module auto-loading
  50      support is enabled in your kernel.  The kernel should load it on
  51      first access.
  52
  53   Manual loading
  54      insert the module by hand:
  55         modprobe tun
  56
  57   If you do it the latter way, you have to load the module every time you
  58   need it, if you do it the other way it will be automatically loaded when
  59   /dev/net/tun is being opened.
  60
  61 3. Program interface
  62   3.1 Network device allocation:
  63
  64   char *dev should be the name of the device with a format string (e.g.
  65   "tun%d"), but (as far as I can see) this can be any valid network device name.
  66   Note that the character pointer becomes overwritten with the real device name
  67   (e.g. "tun0")
  68
  69   #include <linux/if.h>
  70   #include <linux/if_tun.h>
  71
  72   int tun_alloc(char *dev)
  73   {
  74       struct ifreq ifr;
  75       int fd, err;
  76
  77       if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
  78          return tun_alloc_old(dev);
  79
  80       memset(&ifr, 0, sizeof(ifr));
  81
  82       /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
  83        *        IFF_TAP   - TAP device
  84        *
  85        *        IFF_NO_PI - Do not provide packet information
  86        */
  87       ifr.ifr_flags = IFF_TUN;
  88       if( *dev )
  89          strncpy(ifr.ifr_name, dev, IFNAMSIZ);
  90
  91       if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
  92          close(fd);
  93          return err;
  94       }
  95       strcpy(dev, ifr.ifr_name);
  96       return fd;
  97   }
  98
  99   3.2 Frame format:
 100   If flag IFF_NO_PI is not set each frame format is:
 101      Flags [2 bytes]
 102      Proto [2 bytes]
 103      Raw protocol(IP, IPv6, etc) frame.
 104
 105 Universal TUN/TAP device driver Frequently Asked Question.
 106
 107 1. What platforms are supported by TUN/TAP driver ?
 108 Currently driver has been written for 3 Unices:
 109    Linux kernels 2.2.x, 2.4.x
 110    FreeBSD 3.x, 4.x, 5.x
 111    Solaris 2.6, 7.0, 8.0
 112
 113 2. What is TUN/TAP driver used for?
 114 As mentioned above, main purpose of TUN/TAP driver is tunneling.
 115 It is used by VTun (http://vtun.sourceforge.net).
 116
 117 Another interesting application using TUN/TAP is pipsecd
 118 (http://perso.enst.fr/~beyssac/pipsec/), an userspace IPSec
 119 implementation that can use complete kernel routing (unlike FreeS/WAN).
 120
 121 3. How does Virtual network device actually work ?
 122 Virtual network device can be viewed as a simple Point-to-Point or
 123 Ethernet device, which instead of receiving packets from a physical
 124 media, receives them from user space program and instead of sending
 125 packets via physical media sends them to the user space program.
 126
 127 Let's say that you configured IPX on the tap0, then whenever
 128 the kernel sends an IPX packet to tap0, it is passed to the application
 129 (VTun for example). The application encrypts, compresses and sends it to
 130 the other side over TCP or UDP. The application on the other side decompresses
 131 and decrypts the data received and writes the packet to the TAP device,
 132 the kernel handles the packet like it came from real physical device.
 133
 134 4. What is the difference between TUN driver and TAP driver?
 135 TUN works with IP frames. TAP works with Ethernet frames.
 136
 137 This means that you have to read/write IP packets when you are using tun and
 138 ethernet frames when using tap.
 139
 140 5. What is the difference between BPF and TUN/TAP driver?
 141 BFP is an advanced packet filter. It can be attached to existing
 142 network interface. It does not provide a virtual network interface.
 143 A TUN/TAP driver does provide a virtual network interface and it is possible
 144 to attach BPF to this interface.
 145
 146 6. Does TAP driver support kernel Ethernet bridging?
 147 Yes. Linux and FreeBSD drivers support Ethernet bridging.