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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Universal 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:
David Woodhouseca6bb5d2006-06-22 16:07:52 -070042 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070049
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
Jason Wangf422d2a2013-03-05 19:10:26 +0000108 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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700185Universal 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
Frederik Schwarzer0211a9c2008-12-29 22:14:56 +0100198(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
Linus Torvalds1da177e2005-04-16 15:20:36 -0700199implementation 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?
Cal Peake3d79c332006-04-01 01:46:12 +0200221BPF is an advanced packet filter. It can be attached to existing
Linus Torvalds1da177e2005-04-16 15:20:36 -0700222network 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.