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[Qemu-devel] tun/tap networking

From: Don Kitchen
Subject: [Qemu-devel] tun/tap networking
Date: Fri, 30 Sep 2005 15:13:21 -0700 (PDT)

I'm very pleased with qemu's price/performance ratio.

I've used that one "pricey" product at work, but it always seemed a bit
expensive for home users. But I only knew about some of the other emulators,
the ones that are so slow you wonder why didn't they warn you not to
bother downloading the thing to start with. But qemu has definately made
it past usability barrier.

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.

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?)


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