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
 108  3.3 Multiqueue tuntap interface:
 109
 110  From version 3.8, Linux supports multiqueue tuntap which can uses multiple
 111  file descriptors (queues) to parallelize packets sending or receiving. The
 112  device allocation is the same as before, and if user wants to create multiple
 113  queues, TUNSETIFF with the same device name must be called many times with
 114  IFF_MULTI_QUEUE flag.
 115
 116  char *dev should be the name of the device, queues is the number of queues to
 117  be created, fds is used to store and return the file descriptors (queues)
 118  created to the caller. Each file descriptor were served as the interface of a
 119  queue which could be accessed by userspace.
 120
 121  #include <linux/if.h>
 122  #include <linux/if_tun.h>
 123
 124  int tun_alloc_mq(char *dev, int queues, int *fds)
 125  {
 126      struct ifreq ifr;
 127      int fd, err, i;
 128
 129      if (!dev)
 130          return -1;
 131
 132      memset(&ifr, 0, sizeof(ifr));
 133      /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
 134       *        IFF_TAP   - TAP device
 135       *
 136       *        IFF_NO_PI - Do not provide packet information
 137       *        IFF_MULTI_QUEUE - Create a queue of multiqueue device
 138       */
 139      ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_MULTI_QUEUE;
 140      strcpy(ifr.ifr_name, dev);
 141
 142      for (i = 0; i < queues; i++) {
 143          if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
 144             goto err;
 145          err = ioctl(fd, TUNSETIFF, (void *)&ifr);
 146          if (err) {
 147             close(fd);
 148             goto err;
 149          }
 150          fds[i] = fd;
 151      }
 152
 153      return 0;
 154  err:
 155      for (--i; i >= 0; i--)
 156          close(fds[i]);
 157      return err;
 158  }
 159
 160  A new ioctl(TUNSETQUEUE) were introduced to enable or disable a queue. When
 161  calling it with IFF_DETACH_QUEUE flag, the queue were disabled. And when
 162  calling it with IFF_ATTACH_QUEUE flag, the queue were enabled. The queue were
 163  enabled by default after it was created through TUNSETIFF.
 164
 165  fd is the file descriptor (queue) that we want to enable or disable, when
 166  enable is true we enable it, otherwise we disable it
 167
 168  #include <linux/if.h>
 169  #include <linux/if_tun.h>
 170
 171  int tun_set_queue(int fd, int enable)
 172  {
 173      struct ifreq ifr;
 174
 175      memset(&ifr, 0, sizeof(ifr));
 176
 177      if (enable)
 178         ifr.ifr_flags = IFF_ATTACH_QUEUE;
 179      else
 180         ifr.ifr_flags = IFF_DETACH_QUEUE;
 181
 182      return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
 183  }
 184
 185Universal TUN/TAP device driver Frequently Asked Question.
 186   
 1871. What platforms are supported by TUN/TAP driver ?
 188Currently driver has been written for 3 Unices:
 189   Linux kernels 2.2.x, 2.4.x 
 190   FreeBSD 3.x, 4.x, 5.x
 191   Solaris 2.6, 7.0, 8.0
 192
 1932. What is TUN/TAP driver used for?
 194As mentioned above, main purpose of TUN/TAP driver is tunneling. 
 195It is used by VTun (http://vtun.sourceforge.net).
 196
 197Another interesting application using TUN/TAP is pipsecd
 198(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
 199implementation that can use complete kernel routing (unlike FreeS/WAN).
 200
 2013. How does Virtual network device actually work ? 
 202Virtual network device can be viewed as a simple Point-to-Point or
 203Ethernet device, which instead of receiving packets from a physical 
 204media, receives them from user space program and instead of sending 
 205packets via physical media sends them to the user space program. 
 206
 207Let's say that you configured IPX on the tap0, then whenever 
 208the kernel sends an IPX packet to tap0, it is passed to the application
 209(VTun for example). The application encrypts, compresses and sends it to 
 210the other side over TCP or UDP. The application on the other side decompresses
 211and decrypts the data received and writes the packet to the TAP device, 
 212the kernel handles the packet like it came from real physical device.
 213
 2144. What is the difference between TUN driver and TAP driver?
 215TUN works with IP frames. TAP works with Ethernet frames.
 216
 217This means that you have to read/write IP packets when you are using tun and
 218ethernet frames when using tap.
 219
 2205. What is the difference between BPF and TUN/TAP driver?
 221BPF is an advanced packet filter. It can be attached to existing
 222network interface. It does not provide a virtual network interface.
 223A TUN/TAP driver does provide a virtual network interface and it is possible
 224to attach BPF to this interface.
 225
 2266. Does TAP driver support kernel Ethernet bridging?
 227Yes. Linux and FreeBSD drivers support Ethernet bridging. 
 228
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