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1.. SPDX-License-Identifier: GPL-2.0
2.. include:: <isonum.txt>
3
4===============================
5Universal TUN/TAP device driver
6===============================
7
8Copyright |copy| 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
9
10 Linux, Solaris drivers
11 Copyright |copy| 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
12
13 FreeBSD TAP driver
14 Copyright |copy| 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>
15
16 Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>
17
181. Description
19==============
20
21 TUN/TAP provides packet reception and transmission for user space programs.
22 It can be seen as a simple Point-to-Point or Ethernet device, which,
23 instead of receiving packets from physical media, receives them from
24 user space program and instead of sending packets via physical media
25 writes them to the user space program.
26
27 In order to use the driver a program has to open /dev/net/tun and issue a
28 corresponding ioctl() to register a network device with the kernel. A network
29 device will appear as tunXX or tapXX, depending on the options chosen. When
30 the program closes the file descriptor, the network device and all
31 corresponding routes will disappear.
32
33 Depending on the type of device chosen the userspace program has to read/write
34 IP packets (with tun) or ethernet frames (with tap). Which one is being used
35 depends on the flags given with the ioctl().
36
37 The package from http://vtun.sourceforge.net/tun contains two simple examples
38 for how to use tun and tap devices. Both programs work like a bridge between
39 two network interfaces.
40 br_select.c - bridge based on select system call.
41 br_sigio.c - bridge based on async io and SIGIO signal.
42 However, the best example is VTun http://vtun.sourceforge.net :))
43
442. Configuration
45================
46
47 Create device node::
48
49 mkdir /dev/net (if it doesn't exist already)
50 mknod /dev/net/tun c 10 200
51
52 Set permissions::
53
54 e.g. chmod 0666 /dev/net/tun
55
56 There's no harm in allowing the device to be accessible by non-root users,
57 since CAP_NET_ADMIN is required for creating network devices or for
58 connecting to network devices which aren't owned by the user in question.
59 If you want to create persistent devices and give ownership of them to
60 unprivileged users, then you need the /dev/net/tun device to be usable by
61 those users.
62
63 Driver module autoloading
64
65 Make sure that "Kernel module loader" - module auto-loading
66 support is enabled in your kernel. The kernel should load it on
67 first access.
68
69 Manual loading
70
71 insert the module by hand::
72
73 modprobe tun
74
75 If you do it the latter way, you have to load the module every time you
76 need it, if you do it the other way it will be automatically loaded when
77 /dev/net/tun is being opened.
78
793. Program interface
80====================
81
823.1 Network device allocation
83-----------------------------
84
85``char *dev`` should be the name of the device with a format string (e.g.
86"tun%d"), but (as far as I can see) this can be any valid network device name.
87Note that the character pointer becomes overwritten with the real device name
88(e.g. "tun0")::
89
90 #include <linux/if.h>
91 #include <linux/if_tun.h>
92
93 int tun_alloc(char *dev)
94 {
95 struct ifreq ifr;
96 int fd, err;
97
98 if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
99 return tun_alloc_old(dev);
100
101 memset(&ifr, 0, sizeof(ifr));
102
103 /* Flags: IFF_TUN - TUN device (no Ethernet headers)
104 * IFF_TAP - TAP device
105 *
106 * IFF_NO_PI - Do not provide packet information
107 */
108 ifr.ifr_flags = IFF_TUN;
109 if( *dev )
110 strscpy_pad(ifr.ifr_name, dev, IFNAMSIZ);
111
112 if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
113 close(fd);
114 return err;
115 }
116 strcpy(dev, ifr.ifr_name);
117 return fd;
118 }
119
1203.2 Frame format
121----------------
122
123If flag IFF_NO_PI is not set each frame format is::
124
125 Flags [2 bytes]
126 Proto [2 bytes]
127 Raw protocol(IP, IPv6, etc) frame.
128
1293.3 Multiqueue tuntap interface
130-------------------------------
131
132From version 3.8, Linux supports multiqueue tuntap which can uses multiple
133file descriptors (queues) to parallelize packets sending or receiving. The
134device allocation is the same as before, and if user wants to create multiple
135queues, TUNSETIFF with the same device name must be called many times with
136IFF_MULTI_QUEUE flag.
137
138``char *dev`` should be the name of the device, queues is the number of queues
139to be created, fds is used to store and return the file descriptors (queues)
140created to the caller. Each file descriptor were served as the interface of a
141queue which could be accessed by userspace.
142
143::
144
145 #include <linux/if.h>
146 #include <linux/if_tun.h>
147
148 int tun_alloc_mq(char *dev, int queues, int *fds)
149 {
150 struct ifreq ifr;
151 int fd, err, i;
152
153 if (!dev)
154 return -1;
155
156 memset(&ifr, 0, sizeof(ifr));
157 /* Flags: IFF_TUN - TUN device (no Ethernet headers)
158 * IFF_TAP - TAP device
159 *
160 * IFF_NO_PI - Do not provide packet information
161 * IFF_MULTI_QUEUE - Create a queue of multiqueue device
162 */
163 ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_MULTI_QUEUE;
164 strcpy(ifr.ifr_name, dev);
165
166 for (i = 0; i < queues; i++) {
167 if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
168 goto err;
169 err = ioctl(fd, TUNSETIFF, (void *)&ifr);
170 if (err) {
171 close(fd);
172 goto err;
173 }
174 fds[i] = fd;
175 }
176
177 return 0;
178 err:
179 for (--i; i >= 0; i--)
180 close(fds[i]);
181 return err;
182 }
183
184A new ioctl(TUNSETQUEUE) were introduced to enable or disable a queue. When
185calling it with IFF_DETACH_QUEUE flag, the queue were disabled. And when
186calling it with IFF_ATTACH_QUEUE flag, the queue were enabled. The queue were
187enabled by default after it was created through TUNSETIFF.
188
189fd is the file descriptor (queue) that we want to enable or disable, when
190enable is true we enable it, otherwise we disable it::
191
192 #include <linux/if.h>
193 #include <linux/if_tun.h>
194
195 int tun_set_queue(int fd, int enable)
196 {
197 struct ifreq ifr;
198
199 memset(&ifr, 0, sizeof(ifr));
200
201 if (enable)
202 ifr.ifr_flags = IFF_ATTACH_QUEUE;
203 else
204 ifr.ifr_flags = IFF_DETACH_QUEUE;
205
206 return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
207 }
208
209Universal TUN/TAP device driver Frequently Asked Question
210=========================================================
211
2121. What platforms are supported by TUN/TAP driver ?
213
214Currently driver has been written for 3 Unices:
215
216 - Linux kernels 2.2.x, 2.4.x
217 - FreeBSD 3.x, 4.x, 5.x
218 - Solaris 2.6, 7.0, 8.0
219
2202. What is TUN/TAP driver used for?
221
222As mentioned above, main purpose of TUN/TAP driver is tunneling.
223It is used by VTun (http://vtun.sourceforge.net).
224
225Another interesting application using TUN/TAP is pipsecd
226(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
227implementation that can use complete kernel routing (unlike FreeS/WAN).
228
2293. How does Virtual network device actually work ?
230
231Virtual network device can be viewed as a simple Point-to-Point or
232Ethernet device, which instead of receiving packets from a physical
233media, receives them from user space program and instead of sending
234packets via physical media sends them to the user space program.
235
236Let's say that you configured IPv6 on the tap0, then whenever
237the kernel sends an IPv6 packet to tap0, it is passed to the application
238(VTun for example). The application encrypts, compresses and sends it to
239the other side over TCP or UDP. The application on the other side decompresses
240and decrypts the data received and writes the packet to the TAP device,
241the kernel handles the packet like it came from real physical device.
242
2434. What is the difference between TUN driver and TAP driver?
244
245TUN works with IP frames. TAP works with Ethernet frames.
246
247This means that you have to read/write IP packets when you are using tun and
248ethernet frames when using tap.
249
2505. What is the difference between BPF and TUN/TAP driver?
251
252BPF is an advanced packet filter. It can be attached to existing
253network interface. It does not provide a virtual network interface.
254A TUN/TAP driver does provide a virtual network interface and it is possible
255to attach BPF to this interface.
256
2576. Does TAP driver support kernel Ethernet bridging?
258
259Yes. Linux and FreeBSD drivers support Ethernet bridging.