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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	Universal TUN/TAP device driver.
2	Copyright (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	
12	1. 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	
36	2. 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	
64	3. 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	
185	Universal TUN/TAP device driver Frequently Asked Question.
186	   
187	1. What platforms are supported by TUN/TAP driver ?
188	Currently 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	
193	2. What is TUN/TAP driver used for?
194	As mentioned above, main purpose of TUN/TAP driver is tunneling. 
195	It is used by VTun (http://vtun.sourceforge.net).
196	
197	Another interesting application using TUN/TAP is pipsecd
198	(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
199	implementation that can use complete kernel routing (unlike FreeS/WAN).
200	
201	3. How does Virtual network device actually work ? 
202	Virtual network device can be viewed as a simple Point-to-Point or
203	Ethernet device, which instead of receiving packets from a physical 
204	media, receives them from user space program and instead of sending 
205	packets via physical media sends them to the user space program. 
206	
207	Let's say that you configured IPX on the tap0, then whenever 
208	the 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 
210	the other side over TCP or UDP. The application on the other side decompresses
211	and decrypts the data received and writes the packet to the TAP device, 
212	the kernel handles the packet like it came from real physical device.
213	
214	4. What is the difference between TUN driver and TAP driver?
215	TUN works with IP frames. TAP works with Ethernet frames.
216	
217	This means that you have to read/write IP packets when you are using tun and
218	ethernet frames when using tap.
219	
220	5. What is the difference between BPF and TUN/TAP driver?
221	BPF is an advanced packet filter. It can be attached to existing
222	network interface. It does not provide a virtual network interface.
223	A TUN/TAP driver does provide a virtual network interface and it is possible
224	to attach BPF to this interface.
225	
226	6. Does TAP driver support kernel Ethernet bridging?
227	Yes. Linux and FreeBSD drivers support Ethernet bridging. 
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