原创 Universal TUN/TAP device driver.

http://www.kernel.org/pub/linux/kernel/people/marcelo/linux-2.4/Documentation/networking/tuntap.txt

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>
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 
  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.
  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:
     mknod /dev/net/tun c 10 200
  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:
	alias char-major-10-200 tun

     Run:
        depmod -a 
     Driver will be automatically loaded when application access /dev/net/tun.
3. Program interface 
  3.1 Network device allocation:
  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 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 ?
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?
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 ? 
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.
6. 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
to attach BPF to this interface.
8. Does TAP driver support kernel Ethernet bridging?
Yes. Linux and FreeBSD drivers support Ethernet bridging.