Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.
1 Universal TUN/TAP device driver. 2 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk[AT]yahoo[DOT]com> 3 4 Linux, Solaris drivers 5 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk[AT]yahoo[DOT]com> 6 7 FreeBSD TAP driver 8 Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin[AT]yahoo[DOT]com> 9 10 Revision of this document 2002 by Florian Thiel <florian.thiel[AT]gmx[DOT]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 Universal TUN/TAP device driver Frequently Asked Question. 109 110 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, 8.0 115 116 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). 119 120 Another interesting application using TUN/TAP is pipsecd 121 (http://perso.enst.fr/~beyssac/pipsec/), an userspace IPSec 122 implementation that can use complete kernel routing (unlike FreeS/WAN). 123 124 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. 129 130 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. 136 137 4. What is the difference between TUN driver and TAP driver? 138 TUN works with IP frames. TAP works with Ethernet frames. 139 140 This means that you have to read/write IP packets when you are using tun and 141 ethernet frames when using tap. 142 143 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. 148 149 6. Does TAP driver support kernel Ethernet bridging? 150 Yes. Linux and FreeBSD drivers support Ethernet bridging.