This is my VM startup script:
#First, take the the main IF down, create a bridge, add the main IF to the bridge, setup IP info. then run this:
#PS: Doesn't seem to work with a wireless main IF...

qemu-system-x86_64 -net nic,vlan=0,model=ne2k_pci,macaddr=00:00:00:33:22:11 \
-net tap,vlan=0,ifname=tap0,script=qemu-ifup -hda vdisk-slack12.img -m 400

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here's my qemu-ifup script:
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#!/bin/sh
echo "Executing /etc/qemu-ifup"
echo "Bringing up $1 for bridged mode..."
/sbin/ifconfig $1 0.0.0.0 promisc up
echo "Adding $1 to br0..."
sudo /usr/sbin/brctl addif br0 $1
sleep 2
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