Re: [Qemu-devel] tun/tap networking

[Paul Brook <paul@codesourcery.com>]

> I have some questions about the networking that I hope someone can answer.
> Qemu is able to use tun & tap devices. I've taken the tundev.c program,
> which opens a tun device and passes the fd to qemu, and compared it to
> the tapdev.c program (which qemu is also able to use) and there's very
> little difference to how it's opened. According to the little tun/tap
> documentation I understand, the tap descriptor should be providing
> ethernet frames instead of the IP packets [ethernet payloads] that tun
> should be providing. But qemu does not seem to differentiate between the
> two types of file descriptors passed by tundev and tapdev respectively,
> so I am a little confused how qemu can work with both types of fd's.

Qemu only uses tap devices (ie. ethernet frames).

> I'm interested in the handling of ethernet frames because I haven't been
> able to get the bridge to pass packets between added interfaces (yes,
> they're all up and promisc) and I'm not too thrilled with networking being
> bridged anyway, and it seems to me that if an fd were hooked up to a
> BPF capturing everything from the real ethernet device in promiscuous
> mode, and pushing out any raw frames it receives, that I could bypass
> the bridge and make it as if the emulator's virtual ethernet device is
> a real one. Or is there some reason this won't work? (after all, other
> products don't have this, there must be a reason right?)

The host doesn't see the packets set with BFP.

Paul