[tun/tap] An introduction of TUN/TAP

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

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 tunX or tapX device and all routes corresponding to it.

And also, here is a web site about Universal TUN/TAP device driver Frequently Asked Question.

http://vtun.sourceforge.net/tun/faq.html

The description of Universal TUN/TAP device driver

https://www.kernel.org/doc/Documentation/networking/tuntap.txt