linux/Documentation/networking/tuntap.txt
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   1Universal TUN/TAP device driver.
   2Copyright (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
  121. 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
  362. 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 0666 /dev/net/tun
  43     There's no harm in allowing the device to be accessible by non-root users,
  44     since CAP_NET_ADMIN is required for creating network devices or for 
  45     connecting to network devices which aren't owned by the user in question.
  46     If you want to create persistent devices and give ownership of them to 
  47     unprivileged users, then you need the /dev/net/tun device to be usable by
  48     those users.
  49
  50  Driver module autoloading
  51
  52     Make sure that "Kernel module loader" - module auto-loading
  53     support is enabled in your kernel.  The kernel should load it on
  54     first access.
  55  
  56  Manual loading 
  57     insert the module by hand:
  58        modprobe tun
  59
  60  If you do it the latter way, you have to load the module every time you
  61  need it, if you do it the other way it will be automatically loaded when
  62  /dev/net/tun is being opened.
  63
  643. Program interface 
  65  3.1 Network device allocation:
  66
  67  char *dev should be the name of the device with a format string (e.g.
  68  "tun%d"), but (as far as I can see) this can be any valid network device name.
  69  Note that the character pointer becomes overwritten with the real device name
  70  (e.g. "tun0")
  71
  72  #include <linux/if.h>
  73  #include <linux/if_tun.h>
  74
  75  int tun_alloc(char *dev)
  76  {
  77      struct ifreq ifr;
  78      int fd, err;
  79
  80      if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
  81         return tun_alloc_old(dev);
  82
  83      memset(&ifr, 0, sizeof(ifr));
  84
  85      /* Flags: IFF_TUN   - TUN device (no Ethernet headers) 
  86       *        IFF_TAP   - TAP device  
  87       *
  88       *        IFF_NO_PI - Do not provide packet information  
  89       */ 
  90      ifr.ifr_flags = IFF_TUN; 
  91      if( *dev )
  92         strncpy(ifr.ifr_name, dev, IFNAMSIZ);
  93
  94      if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
  95         close(fd);
  96         return err;
  97      }
  98      strcpy(dev, ifr.ifr_name);
  99      return fd;
 100  }              
 101 
 102  3.2 Frame format:
 103  If flag IFF_NO_PI is not set each frame format is: 
 104     Flags [2 bytes]
 105     Proto [2 bytes]
 106     Raw protocol(IP, IPv6, etc) frame.
 107
 108Universal TUN/TAP device driver Frequently Asked Question.
 109   
 1101. What platforms are supported by TUN/TAP driver ?
 111Currently driver has been written for 3 Unices:
 112   Linux kernels 2.2.x, 2.4.x 
 113   FreeBSD 3.x, 4.x, 5.x
 114   Solaris 2.6, 7.0, 8.0
 115
 1162. What is TUN/TAP driver used for?
 117As mentioned above, main purpose of TUN/TAP driver is tunneling. 
 118It is used by VTun (http://vtun.sourceforge.net).
 119
 120Another interesting application using TUN/TAP is pipsecd
 121(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
 122implementation that can use complete kernel routing (unlike FreeS/WAN).
 123
 1243. How does Virtual network device actually work ? 
 125Virtual network device can be viewed as a simple Point-to-Point or
 126Ethernet device, which instead of receiving packets from a physical 
 127media, receives them from user space program and instead of sending 
 128packets via physical media sends them to the user space program. 
 129
 130Let's say that you configured IPX on the tap0, then whenever 
 131the kernel sends an IPX packet to tap0, it is passed to the application
 132(VTun for example). The application encrypts, compresses and sends it to 
 133the other side over TCP or UDP. The application on the other side decompresses
 134and decrypts the data received and writes the packet to the TAP device, 
 135the kernel handles the packet like it came from real physical device.
 136
 1374. What is the difference between TUN driver and TAP driver?
 138TUN works with IP frames. TAP works with Ethernet frames.
 139
 140This means that you have to read/write IP packets when you are using tun and
 141ethernet frames when using tap.
 142
 1435. What is the difference between BPF and TUN/TAP driver?
 144BPF is an advanced packet filter. It can be attached to existing
 145network interface. It does not provide a virtual network interface.
 146A TUN/TAP driver does provide a virtual network interface and it is possible
 147to attach BPF to this interface.
 148
 1496. Does TAP driver support kernel Ethernet bridging?
 150Yes. Linux and FreeBSD drivers support Ethernet bridging. 
 151
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