How to set a two-way networking with qemu/kvm

Here is an example for debian and kvm.

On a host machine:

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sudo kvm -net tap,script=/etc/kvm/kvm-ifup -net nic -m 256 -hda debian-lenny.img -curses
ifconfig tap0 192.168.5.1

On a virtual machine:

echo 'iface eth0 inet static
        address 192.168.5.2
        netmask 255.255.255.0
        gateway 192.168.5.1' >> /etc/network/interfaces