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Diffstat (limited to 'Documentation/networking/tuntap.txt')
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diff --git a/Documentation/networking/tuntap.txt b/Documentation/networking/tuntap.txt new file mode 100644 index 00000000000..ec3d109d787 --- /dev/null +++ b/Documentation/networking/tuntap.txt @@ -0,0 +1,147 @@ +Universal TUN/TAP device driver. +Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com> + + Linux, Solaris drivers + Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com> + + FreeBSD TAP driver + Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com> + + Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net> + +1. Description + TUN/TAP provides packet reception and transmission for user space programs. + It can be seen as a simple Point-to-Point or Ethernet device, which, + instead of receiving packets from physical media, receives them from + user space program and instead of sending packets via physical media + writes them to the user space program. + + In order to use the driver a program has to open /dev/net/tun and issue a + corresponding ioctl() to register a network device with the kernel. A network + device will appear as tunXX or tapXX, depending on the options chosen. When + the program closes the file descriptor, the network device and all + corresponding routes will disappear. + + Depending on the type of device chosen the userspace program has to read/write + IP packets (with tun) or ethernet frames (with tap). Which one is being used + depends on the flags given with the ioctl(). + + The package from http://vtun.sourceforge.net/tun contains two simple examples + for how to use tun and tap devices. Both programs work like a bridge between + two network interfaces. + br_select.c - bridge based on select system call. + br_sigio.c - bridge based on async io and SIGIO signal. + However, the best example is VTun http://vtun.sourceforge.net :)) + +2. Configuration + Create device node: + mkdir /dev/net (if it doesn't exist already) + mknod /dev/net/tun c 10 200 + + Set permissions: + e.g. chmod 0700 /dev/net/tun + if you want the device only accessible by root. Giving regular users the + right to assign network devices is NOT a good idea. Users could assign + bogus network interfaces to trick firewalls or administrators. + + Driver module autoloading + + Make sure that "Kernel module loader" - module auto-loading + support is enabled in your kernel. The kernel should load it on + first access. + + Manual loading + insert the module by hand: + modprobe tun + + If you do it the latter way, you have to load the module every time you + need it, if you do it the other way it will be automatically loaded when + /dev/net/tun is being opened. + +3. Program interface + 3.1 Network device allocation: + + char *dev should be the name of the device with a format string (e.g. + "tun%d"), but (as far as I can see) this can be any valid network device name. + Note that the character pointer becomes overwritten with the real device name + (e.g. "tun0") + + #include <linux/if.h> + #include <linux/if_tun.h> + + int tun_alloc(char *dev) + { + struct ifreq ifr; + int fd, err; + + if( (fd = open("/dev/net/tun", O_RDWR)) < 0 ) + return tun_alloc_old(dev); + + memset(&ifr, 0, sizeof(ifr)); + + /* Flags: IFF_TUN - TUN device (no Ethernet headers) + * IFF_TAP - TAP device + * + * IFF_NO_PI - Do not provide packet information + */ + ifr.ifr_flags = IFF_TUN; + if( *dev ) + strncpy(ifr.ifr_name, dev, IFNAMSIZ); + + if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){ + close(fd); + return err; + } + strcpy(dev, ifr.ifr_name); + return fd; + } + + 3.2 Frame format: + If flag IFF_NO_PI is not set each frame format is: + Flags [2 bytes] + Proto [2 bytes] + Raw protocol(IP, IPv6, etc) frame. + +Universal TUN/TAP device driver Frequently Asked Question. + +1. What platforms are supported by TUN/TAP driver ? +Currently driver has been written for 3 Unices: + Linux kernels 2.2.x, 2.4.x + FreeBSD 3.x, 4.x, 5.x + Solaris 2.6, 7.0, 8.0 + +2. What is TUN/TAP driver used for? +As mentioned above, main purpose of TUN/TAP driver is tunneling. +It is used by VTun (http://vtun.sourceforge.net). + +Another interesting application using TUN/TAP is pipsecd +(http://perso.enst.fr/~beyssac/pipsec/), an userspace IPSec +implementation that can use complete kernel routing (unlike FreeS/WAN). + +3. How does Virtual network device actually work ? +Virtual network device can be viewed as a simple Point-to-Point or +Ethernet device, which instead of receiving packets from a physical +media, receives them from user space program and instead of sending +packets via physical media sends them to the user space program. + +Let's say that you configured IPX on the tap0, then whenever +the kernel sends an IPX packet to tap0, it is passed to the application +(VTun for example). The application encrypts, compresses and sends it to +the other side over TCP or UDP. The application on the other side decompresses +and decrypts the data received and writes the packet to the TAP device, +the kernel handles the packet like it came from real physical device. + +4. What is the difference between TUN driver and TAP driver? +TUN works with IP frames. TAP works with Ethernet frames. + +This means that you have to read/write IP packets when you are using tun and +ethernet frames when using tap. + +5. What is the difference between BPF and TUN/TAP driver? +BFP is an advanced packet filter. It can be attached to existing +network interface. It does not provide a virtual network interface. +A TUN/TAP driver does provide a virtual network interface and it is possible +to attach BPF to this interface. + +6. Does TAP driver support kernel Ethernet bridging? +Yes. Linux and FreeBSD drivers support Ethernet bridging. |