HOST            QEMU GUEST1
        +---------------+   +--------------+
        | 10.10.5.158   |   |              |
 LAN ---+---- eth0      |   |              |
        |               |   |              |    QEMU GUEST2
        |   +------+ +--+---+---- nic0     |   +--------------+
        |   | tap0---+  |   |192.168.100.5 |   |              |
        |   | tap1---+  |   +--------------+   |              |
        |   +------+ |  |                      |              |
        |     br0    +--+----------------------+---- nic0     |
        |192.168.100.254|                      |192.168.100.1 |
        +---------------+                      +--------------+

Needed packages:

tunctl (uml-utilities)
bridge-utilities
kvm

Setup the network:

Create a file call kvm-network with the following content and make it executeable.

#!/bin/bash

KVMNET_UID=1000
KVMNET_GID=$(grep kvm /etc/group | cut -d ':' -f 3)

# number of TUN/TAP devices to setup
NUM_OF_DEVICES=3

case $1 in
        start)
                modprobe kvm
                modprobe kvm_intel

                modprobe tun
                echo "Setting up bridge device br0"
                brctl addbr br0
                ifconfig br0 192.168.100.254 netmask 255.255.255.0 up
                for ((i=0; i < NUM_OF_DEVICES ; i++)); do
                        echo -n "Setting up "
                        tunctl -b -g ${KVMNET_GID} -t kvmnet$i
                        #tunctl -b -u ${KVMNET_UID} -t kvmnet$i
                        brctl addif br0 kvmnet$i
                        ifconfig kvmnet$i up 0.0.0.0 promisc
                done
                SuSEfirewall2 stop
                SuSEfirewall2
        ;;
        stop)
                for ((i=0; i < NUM_OF_DEVICES ; i++)); do
                        ifconfig kvmnet$i down
                        brctl delif br0 kvmnet$i
                        tunctl -d kvmnet$i
                done
                ifconfig br0 down
                brctl delbr br0
                SuSEfirewall2 stop
                SuSEfirewall2

                rmmod kvm_intel
                rmmod kvm
        ;;
        *)
                echo "Usage: $(basename $0) (start|stop)"
        ;;
esac

br0 is the gateway to the external network.

Setting up the firewall:

Edit /etc/sysconfig/SuSEfirewall and set the following variables:

FW_DEV_INT="br0 qtap0 qtap1 qtap2 qtap3 qtap4"
FW_ROUTE="yes"
FW_MASQUERADE="yes"
FW_MASQ_NETS="192.168.100.0/24"
FW_PROTECT_FROM_INT="no"

If you don't run a SUSE system use the following lines to setup masquerading:

echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

On the guest you have to set the default gateway to 192.168.100.254 which is the bridge br0 and take a look in /etc/resolv.conf to get the name servers. I run a Windows 2003 Server as a guest which is the dhcp and name server for the other guests (Vista, several Linux installations).

Setting up qemu

Guest 1:

#!/bin/bash

qemu-kvm /path/to/vm.img \
          -net nic,model=rtl8139,macaddr=52:54:00:12:34:56 \
          -net tap,ifname=qtap0,script=no \
          -m 256 \
          -smp 1 \
          -usb \
          -usbdevice tablet \
          -localtime

Guest 2:

#!/bin/bash

qemu-kvm /path/to/vm2.img \
          -net nic,model=rtl8139,macaddr=52:54:00:12:34:57 \
          -net tap,ifname=qtap1,script=no \
          -m 256 \
          -smp 1 \
          -usb \
          -usbdevice tablet \
          -localtime

Note that the VMs have different MAC addresses. It took me a long time to find why I couldn't ping from one guest to another By the way, one of the guests is running Vista, which runs smoothly on my machine with KVM.