QEMU Networking Notes

 

QEMU Networking Notes

Last updated 2010-07-08

There are a number of different mechanisms for tackling networking in qemu (I use kvm and qemu interchangeably).

Direct Bridge

A direct bridge gives qemu complete access to the network. This example is based on Debian.

You need tun support:

sudo modprobe tun
echo tun >> /etc/modules

Install tunctl:

sudo apt-get install uml-utilities

Setup /etc/network/interfaces to establish the bridge:

# The primary network interface
#iface eth0 inet dhcp

# qemu bridge
iface br0 inet dhcp
bridge_ports eth0
bridge_fd 1
bridge_hello 1
bridge_stp off

Setup /etc/qemu-ifup to add the qemu instance to the bridge:

#!/bin/sh

# bridge
echo "Executing /etc/qemu-ifup"
echo "Bringing up $1 for bridged mode..."
sudo /sbin/ifconfig $1 0.0.0.0 promisc up
echo "Adding $1 to br0..."
sudo /usr/sbin/brctl addif br0 $1
sleep 2
sudo chmod 755 /etc/qemu-ifup

Finally, start the instance:

sudo ifup br0
sudo tunctl -u ${USER} tun0

kvm -m 512 -hda hda.qcow2 -boot c -no-acpi -net nic,vlan=0 \
    -net tap,vlan=0,ifname=tun0,script=/etc/qemu-ifup

Standard qemu MAC prefix is 52:54:00.

Note: Each instance started will need its own tun and MAC address.

Using Proxy Arp

By using proxy arp you can bridge to a fixed IP address. One nice thing about using proxy arp is that you can bridge to a wireless NIC, which you cannot do with direct bridging as described above.

This method requires that the VM have static IPs.

Instead of steps, here is a script, kvmnetworking, to do the bridging:

#!/bin/bash

kvm_if_up(){
    tunctl -u ${USER} -t tap0

    sysctl net.ipv4.ip_forward=1
    sysctl net.ipv4.conf.wlan0.proxy_arp=1

    sysctl net.ipv4.conf.tap0.proxy_arp=1

    ip link set dev tap0 up

    route add -host 192.168.22.125 dev tap0
}

kvm_if_down(){

    sysctl net.ipv4.ip_forward=0
    sysctl net.ipv4.conf.wlan0.proxy_arp=0

    sysctl net.ipv4.conf.tap0.proxy_arp=0

    ip link set dev tap0 down

    tunctl -d tap0
}

if [ $EUID -ne 0 ]; then
  echo "This script must be run as root" 1>&2
  exit 1
else

    case "$1" in

        start)
            kvm_if_up
            ;;
        stop)
            kvm_if_down
            ;;
        *)
            echo "Usage: $0 {start|stop}"
            ;;
    esac

fi

exit 0

Start the instance:

sudo kvm_networking start

kvm -m 512 -boot c -hda hda.qcow2 -no-acpi \
    -net nic,vlan=0,macaddr=52:54:00:00:00:01 \
    -net tap,ifname=tap0,script=no,vlan=0

Note: Replace wlan0 with whatever interface you want to use.