1 Universal TUN/TAP device driver.
2 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
5 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
8 Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>
10 Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>
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.
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.
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().
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 :))
38 mkdir /dev/net (if it doesn't exist already)
39 mknod /dev/net/tun c 10 200
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
50 Driver module autoloading
52 Make sure that "Kernel module loader" - module auto-loading
53 support is enabled in your kernel. The kernel should load it on
57 insert the module by hand:
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.
65 3.1 Network device allocation:
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
73 #include <linux/if_tun.h>
75 int tun_alloc(char *dev)
80 if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
81 return tun_alloc_old(dev);
83 memset(&ifr, 0, sizeof(ifr));
85 /* Flags: IFF_TUN - TUN device (no Ethernet headers)
86 * IFF_TAP - TAP device
88 * IFF_NO_PI - Do not provide packet information
90 ifr.ifr_flags = IFF_TUN;
92 strncpy(ifr.ifr_name, dev, IFNAMSIZ);
94 if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
98 strcpy(dev, ifr.ifr_name);
103 If flag IFF_NO_PI is not set each frame format is:
106 Raw protocol(IP, IPv6, etc) frame.
108 Universal TUN/TAP device driver Frequently Asked Question.
110 1. What platforms are supported by TUN/TAP driver ?
111 Currently 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
116 2. What is TUN/TAP driver used for?
117 As mentioned above, main purpose of TUN/TAP driver is tunneling.
118 It is used by VTun (http://vtun.sourceforge.net).
120 Another interesting application using TUN/TAP is pipsecd
121 (http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
122 implementation that can use complete kernel routing (unlike FreeS/WAN).
124 3. How does Virtual network device actually work ?
125 Virtual network device can be viewed as a simple Point-to-Point or
126 Ethernet device, which instead of receiving packets from a physical
127 media, receives them from user space program and instead of sending
128 packets via physical media sends them to the user space program.
130 Let's say that you configured IPX on the tap0, then whenever
131 the 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
133 the other side over TCP or UDP. The application on the other side decompresses
134 and decrypts the data received and writes the packet to the TAP device,
135 the kernel handles the packet like it came from real physical device.
137 4. What is the difference between TUN driver and TAP driver?
138 TUN works with IP frames. TAP works with Ethernet frames.
140 This means that you have to read/write IP packets when you are using tun and
141 ethernet frames when using tap.
143 5. What is the difference between BPF and TUN/TAP driver?
144 BPF is an advanced packet filter. It can be attached to existing
145 network interface. It does not provide a virtual network interface.
146 A TUN/TAP driver does provide a virtual network interface and it is possible
147 to attach BPF to this interface.
149 6. Does TAP driver support kernel Ethernet bridging?
150 Yes. Linux and FreeBSD drivers support Ethernet bridging.