QEMU/Networking

QEMU supports networking by emulating some popular network cards (NICs), and establishing virtual LANs (VLAN). There are four ways that QEMU guests can be connected: user mode, socket redirection, Tap and VDE networking.

User mode networking

If no network options are specified, QEMU will default to emulating a single Intel e1000 PCI card with a user-mode network stack that bridges to the host's network. The following three command lines are equivalent:

qemu -m 256 -hda disk.img &
qemu -m 256 -hda disk.img -net nic -net user &
qemu-system-i386 -m 256 -hda disk.img -netdev user,id=network0 -device e1000,netdev=network0,mac=52:54:00:12:34:56 &

To use this network setup with the Linux kernel, you must set the configuration option CONFIG_E1000=y when compiling.

The -net option is superseded by -netdev in newer QEMU versions.

The guest OS will see an E1000 NIC with a virtual DHCP server on 10.0.2.2 and will be allocated an address starting from 10.0.2.15. A virtual DNS server will be accessible on 10.0.2.3, and a virtual SAMBA file server (if present) will be accessible on 10.0.2.4 allowing you to access files on the host via SAMBA file shares.

User mode networking is great for allowing access to network resources, including the Internet. In particular, it allows ssh from the guest to the host. By default, however, it acts as a firewall and does not permit any incoming traffic. It also doesn't support protocols other than TCP and UDP - so, for example, ping and other ICMP utilities won't work.

Redirecting ports

To allow network connections to the guest OS under user mode networking, you can redirect a port on the host OS to a port on the guest OS. This is useful for supporting file sharing, web servers and SSH servers from the guest OS.

Here is how to set up QEMU with a Windows XP guest sharing files and web pages under user mode networking. TCP port 5555 on the host is redirected to the guest's port 80 (the web server) and TCP port 5556 on the host is redirected to the guest's port 445 (Windows networking):

qemu -m 256 -hda disk.img -redir tcp:5555::80 -redir tcp:5556::445 &
...
mkdir -p /mnt/qemu
mount -t cifs //localhost/someshare /mnt/qemu -o user=test,pass=test,dom=workgroup,port=5556
firefox http://localhost:5555/

NB: When sharing folders from guest to host via Windows networking, you must specify a password for the user that mount will use to login; if you try to use no password, mount will fail with an I/O error.

TAP interfaces

QEMU can use TAP interfaces to provide full networking capability for the guest OS. This can be useful when the guest OS is running several network services and must be connected to via standard ports; where protocols other than TCP and UDP are required; and where multiple instances of QEMU need to connect to each other (although this can also be achieved in user mode networking via port redirects, or via sockets).

In QEMU 1.1 and newer the network bridge helper can set tun/tap up for you without the need for additional scripting.

For older versions, setting up a TAP interface is a bit more complicated than user mode networking. It requires installing virtual private networking (VPN) on the host OS, and then establishing a bridge between the host's networking and the virtual network.

Here's how to do it on Fedora 8 with static IP address assignment. The procedure should be very similar on other Linux distros, and probably not too different on other *nix systems.

TAP/TUN device

According to tuntap.txt, we create the TAP/TUN device first:

   $ sudo mkdir /dev/net
   $ sudo mknod /dev/net/tun c 10 200
   $ sudo /sbin/modprobe tun

qemu-ifup

First, set up a script to create the bridge and bring up the TAP interface. We'll call this script /etc/qemu-ifup.

#!/bin/sh 
# 
# script to bring up the tun device in QEMU in bridged mode 
# first parameter is name of tap device (e.g. tap0)
#
# some constants specific to the local host - change to suit your host
#
ETH0IPADDR=192.168.0.3
MASK=255.255.255.0
GATEWAY=192.168.0.1
BROADCAST=192.168.0.255
#
# First take eth0 down, then bring it up with IP address 0.0.0.0 
#
/sbin/ifdown eth0
/sbin/ifconfig eth0 0.0.0.0 promisc up
#
# Bring up the tap device (name specified as first argument, by QEMU)
#
/usr/sbin/openvpn --mktun --dev $1 --user `id -un`
/sbin/ifconfig $1 0.0.0.0 promisc up
#
# create the bridge between eth0 and the tap device
#
/usr/sbin/brctl addbr br0
/usr/sbin/brctl addif br0 eth0
/usr/sbin/brctl addif br0 $1
# 
# only a single bridge so loops are not possible, turn off spanning tree protocol
#
/usr/sbin/brctl stp br0 off 
# 
# Bring up the bridge with ETH0IPADDR and add the default route 
#
/sbin/ifconfig br0 $ETH0IPADDR netmask $MASK broadcast $BROADCAST
/sbin/route add default gw $GATEWAY
#
# stop firewall - comment this out if you don't use Firestarter
#
/sbin/service firestarter stop 

qemu-ifdown

You will also need a script to reset your networking after QEMU exits. To be consistent, we'll call it /etc/qemu-ifdown.

#!/bin/sh 
# 
# Script to bring down and delete bridge br0 when QEMU exits 
# 
# Bring down eth0 and br0 
#
/sbin/ifdown eth0
/sbin/ifdown br0
/sbin/ifconfig br0 down 
# 
# Delete the bridge
#
/usr/sbin/brctl delbr br0 
# 
# bring up eth0 in "normal" mode 
#
/sbin/ifconfig eth0 -promisc
/sbin/ifup eth0 
#
# delete the tap device
#
/usr/sbin/openvpn --rmtun --dev $1
#
# start firewall again
# 
/sbin/service firestarter start 

Allowing users to call the scripts

In qemu 1.1 and above just use the helper program, which doesn't require any scripts and can be setuid root.

For older versions, the two scripts above need to be run as the superuser, so that they can modify the network settings of the system. The most convenient way to achieve that is to permit users of QEMU to call the scripts using the sudo command. To set this up, add the following to the file /etc/sudoers:

User_Alias QEMUERS = fred, john, milly, ...

Cmnd_Alias QEMU = /etc/qemu-ifup, /etc/qemu-ifdown

QEMUERS ALL=(ALL) NOPASSWD: QEMU

Starting QEMU with a TAP interface

Now create a script to start QEMU with a VLAN, and clean up after itself when it exits. This one uses tap0. Specifying script=no tells QEMU to just use the tap device without calling the scripts - we do this so that QEMU can be run as a regular user, not root.

#!/bin/sh 
sudo /etc/qemu-ifup tap0
qemu -m 256 -hda disk.img -net nic -net tap,ifname=tap0,script=no,downscript=no
sudo /etc/qemu-ifdown tap0

Run that script, and it will create a TAP interface, bridge it to eth0, run QEMU, and drop the bridge and TAP interface again on exit.

Windows Vista and later – Network Location

Windows Vista and later classify network connections as either public or private. The classification determines the firewall rules that will be applied to that connection. Windows maintains a list of known connections and if it finds a network connection which is not in that list it will prompt the user to indicate whether this is a "Home", "Work" or "Public" network. The network is identified by the MAC address of its default gateway, which QEMU seems to allocate randomly each time it starts. The result is that every time a Windows session is started with QEMU it pops up a window asking you to indicate the "network location". This is not normally a serious problem, but it can be annoying.

The solution is to force the netdev interface to always use the same MAC address. QEMU does not appear to provide an option to set this, but it can be set in the ifup script. Using Iproute2, which has superceded ifconfig, the command:

ip link set dev tapn address 52:54:00:12:34:56

will change the MAC address of the host-side interface to that given, which could be any legal MAC address that is unique within the local network.

Sockets

QEMU can connect multiple QEMU guest systems on a VLAN using TCP or UDP sockets.

described here

SMB server

If the host system has a SMB server installed (SAMBA/CIFS on *nix), QEMU can emulate a virtual SMB server for the guest system using the -smb option. Specify the folder to be shared, and it will be available to the guest as \\10.0.2.4\qemu (or you can put 10.0.2.4 into the hosts or lmhosts file as smbserver and map to \\smbserver\qemu).

qemu -m 256 disk.img -smb /usr/workspace/testing01

This isn't strictly necessary, because guests in QEMU can typically access SMB servers in the host environment. It can be quite useful, however, for setting up independent workspaces for each QEMU guest without needing to configure SMB shares for each one.

External links

• documentation[1]
• forum post on bridging QEMU to Linux
• Script to make virtual private networks without having to bridge your real network interface. Uses tap devices similar to here but doesn't touch your real network interfaces. Similar to vmnet1-vmnet7 in Vmware. Then, use iptables to configure your subnets like you would do with real physical private subnets on your LAN