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. 260