how to update gentoo

1. emerge --sync
2. emerge -uND world
3. emerge --depclean
To clean no use package.
4. revdep -rebuild
To re-compile packages which dynamic linking should be updated.
Loop this step until there are no warnings.

tun

猜測tun應該是這樣讓userspace 的程式可以跟kernel溝通，qemu的程式碼很漂亮...........