Re: [Qemu-devel] make qemu use tap0 instead of tun0

[Jim C. Brown]

On Thu, Jul 07, 2005 at 03:55:30AM +0200, Herbert Poetzl wrote:
> 1.1 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.
> 
> 1.2 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.
> 
> (from http://vtun.sourceforge.net/tun/faq.html)
> 
> best,
> Herbert
> 

I know this. Actually that page seems out of date, as Linux's tuntap now sends
all accesses to /dev/net/tun regardless of the type of device (tun or tap).

qemu have never used tun devices, it only uses tap devices. In other words,
qemu has never read/write IP frames, it only deals with ethernet frames. This is
why having qemu use tun0 as the device name is misleading - it makes developers
believe that qemu is using a tun device (IP frames) even though it is really
using a tap device (ethernet frames).

The current NE2000 emulation (or any hardware nic emulation for that matter)
would not work if qemu used an actual tun device.