148 lines
5.5 KiB
Plaintext
148 lines
5.5 KiB
Plaintext
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.
|