Re: [Qemu-devel] Connecting vde and LAN

[Henrik Nordstrom]

On Sun, 10 Jul 2005, Jim C. Brown wrote:

> Yes, but if one was running multiple qemu guests, it would be cleaner to
> implement this thru vde_switch instead of in qemu directly. That way eth0 is
> intercepted by only a single process. (Also no worries about having permissions
> to intercept packets and etc for qemu - just run vde_switch as root and qemu
> as a regular user.)

I didn't mean to implement this in qemu directly, but rather in a helper 
similar to vdeq, but the primary method should be via vde_switch. Having 
this shortcut helper is a future extension.

Inspired by this discussion I now have a prototype vde_packet glue for 
vde_switch at http://www.henriknordstrom.net/examples/vde_packet.c, 
connecting the vde_switch to the LAN via a packet socket.

Light testing looks promising, but there is some known issues which may 
make this approach unfeasible for the intended target:

a) vde_switch 1.5.8 is buggy and bounces packets back the same port they 
came from if the destination MAC is known to reside there, causing a loop 
and near meltdown of the host.  vde_switch should drop the packet in this 
case. The fix is trivial: if (target == port) return; at a suitable place 
near the top of port.c:send_dst(). I haven't looked if this may have been 
fixed in a later vde_switch release but I think not.

b) can't talk to the host itself. This is due to the packets going 
directly to the wire and never really "seen" by the host stack. Not sure 
yet if there is an easy way out, but I suppose it may be possible to set 
up a dummy tap with the same MAC and IP address as the base Ethernet 
device and duplicate broadcasts and packet directed to the host there, 
obviously assuming the administrator does not block this in firewalling..

Regards
Henrik