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SubjectKernel TUN/TAP Documentation rework
Hi!

The existing documentation for the tun/tap interface was outdated and in
some ways just plain wrong. It took me at least an evening to figure out
how to get tun/tap to work. I did a rework that should now be correct.

The documentation only applies to Linux! I feel it should be included in
both 2.4 and 2.5. I also send the patch to the author of tun/tap and the
"small fix" maintainers for 2.4 and 2.5. Hope, this is correct.

Greetings, Florian

--
Florian Thiel - Medienzentrum Kassel
Systembetreuung Internet- und Kommunikationstechnik
Kasseler Schulen am Netz - http://www.ksan.de
--- Documentation/networking/tuntap.txt.orig 2002-09-30 15:44:32.000000000 +0200
+++ Documentation/networking/tuntap.txt 2002-10-01 08:34:56.000000000 +0200
@@ -7,43 +7,71 @@
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 viewed as a simple Point-to-Point or Ethernet device, which
- instead of receiving packets from a physical media, receives them from
+ 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.

- When a program opens /dev/net/tun, driver creates and registers corresponding
- net device tunX or tapX. After a program closed above devices, driver will
- automatically delete tunXX or tapXX device and all routes corresponding to it.
-
- This package(http://vtun.sourceforge.net/tun) contains two simple example
- programs how to use tun and tap devices. Both programs works like
- bridge between two network interfaces.
+ 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 :))
+ 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 accesible 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.

- Add following line to the /etc/modules.conf:
+ Add the following line to the /etc/modules.conf:
alias char-major-10-200 tun
-
- Run:
+ and run
depmod -a
-
- Driver will be automatically loaded when application access /dev/net/tun.
+
+ 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;
@@ -79,65 +107,44 @@

Universal TUN/TAP device driver Frequently Asked Question.

-1. What is the TUN ?
-The TUN is Virtual Point-to-Point network device.
-TUN driver was designed as low level kernel support for
-IP tunneling. It provides to userland application
-two interfaces:
- - /dev/tunX - character device;
- - tunX - virtual Point-to-Point interface.
-
-Userland application can write IP frame to /dev/tunX
-and kernel will receive this frame from tunX interface.
-In the same time every frame that kernel writes to tunX
-interface can be read by userland application from /dev/tunX
-device.
-
-2. What is the TAP ?
-The TAP is a Virtual Ethernet network device.
-TAP driver was designed as low level kernel support for
-Ethernet tunneling. It provides to userland application
-two interfaces:
- - /dev/tapX - character device;
- - tapX - virtual Ethernet interface.
-
-Userland application can write Ethernet frame to /dev/tapX
-and kernel will receive this frame from tapX interface.
-In the same time every frame that kernel writes to tapX
-interface can be read by userland application from /dev/tapX
-device.
-
-3. What platforms are supported by TUN/TAP driver ?
+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

-4. What is TUN/TAP driver used for?
+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).

-5. How does Virtual network device actually work ?
+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
-kernel sends any IPX packet to tap0, it is passed to the application
-(VTun for example). Application encrypts, compresses and sends it to
-the other side over TCP or UDP. Application on other side decompress
-and decrypts them and write packet to the TAP device, kernel handles
-the packet like it came from real physical device.
+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.

-6. What is the difference between TUN driver and TAP driver?
+4. What is the difference between TUN driver and TAP driver?
TUN works with IP frames. TAP works with Ethernet frames.

-7. What is the difference between BPF and TUN/TAP driver?
-BFP is a advanced packet filter. It can be attached to existing
-network interface. It does not provide virtual network interface.
-TUN/TAP driver does provide virtual network interface and it is possible
+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.

-8. Does TAP driver support kernel Ethernet bridging?
+6. Does TAP driver support kernel Ethernet bridging?
Yes. Linux and FreeBSD drivers support Ethernet bridging.
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