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 0700 /dev/net/tun
43     if you want the device only accesible 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     Make sure that "Kernel module loader" - module auto-loading support is enabled
49     in your kernel.
50
51     Add the following line to the /etc/modules.conf:
52	alias char-major-10-200 tun
53     and run
54        depmod -a
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/), an 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?
144BFP 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