227 lines
8 KiB
Text
227 lines
8 KiB
Text
Universal TUN/TAP device driver.
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Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
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Linux, Solaris drivers
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Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
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FreeBSD TAP driver
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Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>
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Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>
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1. Description
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TUN/TAP provides packet reception and transmission for user space programs.
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It can be seen as a simple Point-to-Point or Ethernet device, which,
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instead of receiving packets from physical media, receives them from
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user space program and instead of sending packets via physical media
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writes them to the user space program.
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In order to use the driver a program has to open /dev/net/tun and issue a
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corresponding ioctl() to register a network device with the kernel. A network
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device will appear as tunXX or tapXX, depending on the options chosen. When
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the program closes the file descriptor, the network device and all
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corresponding routes will disappear.
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Depending on the type of device chosen the userspace program has to read/write
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IP packets (with tun) or ethernet frames (with tap). Which one is being used
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depends on the flags given with the ioctl().
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The package from http://vtun.sourceforge.net/tun contains two simple examples
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for how to use tun and tap devices. Both programs work like a bridge between
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two network interfaces.
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br_select.c - bridge based on select system call.
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br_sigio.c - bridge based on async io and SIGIO signal.
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However, the best example is VTun http://vtun.sourceforge.net :))
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2. Configuration
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Create device node:
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mkdir /dev/net (if it doesn't exist already)
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mknod /dev/net/tun c 10 200
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Set permissions:
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e.g. chmod 0666 /dev/net/tun
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There's no harm in allowing the device to be accessible by non-root users,
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since CAP_NET_ADMIN is required for creating network devices or for
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connecting to network devices which aren't owned by the user in question.
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If you want to create persistent devices and give ownership of them to
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unprivileged users, then you need the /dev/net/tun device to be usable by
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those users.
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Driver module autoloading
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Make sure that "Kernel module loader" - module auto-loading
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support is enabled in your kernel. The kernel should load it on
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first access.
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Manual loading
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insert the module by hand:
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modprobe tun
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If you do it the latter way, you have to load the module every time you
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need it, if you do it the other way it will be automatically loaded when
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/dev/net/tun is being opened.
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3. Program interface
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3.1 Network device allocation:
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char *dev should be the name of the device with a format string (e.g.
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"tun%d"), but (as far as I can see) this can be any valid network device name.
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Note that the character pointer becomes overwritten with the real device name
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(e.g. "tun0")
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#include <linux/if.h>
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#include <linux/if_tun.h>
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int tun_alloc(char *dev)
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{
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struct ifreq ifr;
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int fd, err;
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if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
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return tun_alloc_old(dev);
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memset(&ifr, 0, sizeof(ifr));
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/* Flags: IFF_TUN - TUN device (no Ethernet headers)
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* IFF_TAP - TAP device
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*
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* IFF_NO_PI - Do not provide packet information
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*/
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ifr.ifr_flags = IFF_TUN;
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if( *dev )
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strncpy(ifr.ifr_name, dev, IFNAMSIZ);
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if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
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close(fd);
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return err;
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}
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strcpy(dev, ifr.ifr_name);
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return fd;
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}
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3.2 Frame format:
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If flag IFF_NO_PI is not set each frame format is:
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Flags [2 bytes]
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Proto [2 bytes]
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Raw protocol(IP, IPv6, etc) frame.
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3.3 Multiqueue tuntap interface:
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From version 3.8, Linux supports multiqueue tuntap which can uses multiple
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file descriptors (queues) to parallelize packets sending or receiving. The
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device allocation is the same as before, and if user wants to create multiple
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queues, TUNSETIFF with the same device name must be called many times with
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IFF_MULTI_QUEUE flag.
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char *dev should be the name of the device, queues is the number of queues to
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be created, fds is used to store and return the file descriptors (queues)
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created to the caller. Each file descriptor were served as the interface of a
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queue which could be accessed by userspace.
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#include <linux/if.h>
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#include <linux/if_tun.h>
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int tun_alloc_mq(char *dev, int queues, int *fds)
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{
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struct ifreq ifr;
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int fd, err, i;
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if (!dev)
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return -1;
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memset(&ifr, 0, sizeof(ifr));
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/* Flags: IFF_TUN - TUN device (no Ethernet headers)
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* IFF_TAP - TAP device
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*
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* IFF_NO_PI - Do not provide packet information
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* IFF_MULTI_QUEUE - Create a queue of multiqueue device
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*/
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ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_MULTI_QUEUE;
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strcpy(ifr.ifr_name, dev);
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for (i = 0; i < queues; i++) {
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if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
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goto err;
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err = ioctl(fd, TUNSETIFF, (void *)&ifr);
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if (err) {
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close(fd);
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goto err;
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}
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fds[i] = fd;
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}
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return 0;
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err:
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for (--i; i >= 0; i--)
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close(fds[i]);
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return err;
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}
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A new ioctl(TUNSETQUEUE) were introduced to enable or disable a queue. When
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calling it with IFF_DETACH_QUEUE flag, the queue were disabled. And when
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calling it with IFF_ATTACH_QUEUE flag, the queue were enabled. The queue were
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enabled by default after it was created through TUNSETIFF.
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fd is the file descriptor (queue) that we want to enable or disable, when
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enable is true we enable it, otherwise we disable it
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#include <linux/if.h>
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#include <linux/if_tun.h>
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int tun_set_queue(int fd, int enable)
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{
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struct ifreq ifr;
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memset(&ifr, 0, sizeof(ifr));
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if (enable)
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ifr.ifr_flags = IFF_ATTACH_QUEUE;
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else
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ifr.ifr_flags = IFF_DETACH_QUEUE;
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return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
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}
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Universal TUN/TAP device driver Frequently Asked Question.
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1. What platforms are supported by TUN/TAP driver ?
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Currently driver has been written for 3 Unices:
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Linux kernels 2.2.x, 2.4.x
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FreeBSD 3.x, 4.x, 5.x
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Solaris 2.6, 7.0, 8.0
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2. What is TUN/TAP driver used for?
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As mentioned above, main purpose of TUN/TAP driver is tunneling.
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It is used by VTun (http://vtun.sourceforge.net).
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Another interesting application using TUN/TAP is pipsecd
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(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
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implementation that can use complete kernel routing (unlike FreeS/WAN).
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3. How does Virtual network device actually work ?
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Virtual network device can be viewed as a simple Point-to-Point or
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Ethernet device, which instead of receiving packets from a physical
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media, receives them from user space program and instead of sending
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packets via physical media sends them to the user space program.
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Let's say that you configured IPX on the tap0, then whenever
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the kernel sends an IPX packet to tap0, it is passed to the application
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(VTun for example). The application encrypts, compresses and sends it to
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the other side over TCP or UDP. The application on the other side decompresses
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and decrypts the data received and writes the packet to the TAP device,
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the kernel handles the packet like it came from real physical device.
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4. What is the difference between TUN driver and TAP driver?
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TUN works with IP frames. TAP works with Ethernet frames.
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This means that you have to read/write IP packets when you are using tun and
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ethernet frames when using tap.
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5. What is the difference between BPF and TUN/TAP driver?
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BPF is an advanced packet filter. It can be attached to existing
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network interface. It does not provide a virtual network interface.
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A TUN/TAP driver does provide a virtual network interface and it is possible
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to attach BPF to this interface.
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6. Does TAP driver support kernel Ethernet bridging?
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Yes. Linux and FreeBSD drivers support Ethernet bridging.
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