Using KVM to virtualize guest operating systems under Linux is great, but the default networking configuration doesn’t allow incoming connections to the VM. This is not peculiar to KVM – this is the default for most (all?) virtualization technologies, including VirtualBox.
If, like me, you need to access you guest VM’s from the outside, then you need to setup bridge networking. There are plenty of resources on the web on how to do this, and this blog is simply a quick overview of the steps I took to enable bridge networking under KVM. It will serve as a reminder to my self of the steps I took, and possibly help others out as well.
Below is a list of the sites I used for setting up bridged networking with KVM under Linux:
https://help.ubuntu.com/community/NetworkConnectionBridge
http://drupal.bitfunnel.net/drupal/macosx-bridge-qemu
http://en.blog.guylhem.net/post/88201449689/running-qemu-with-tap0-and-nat-under-osx-109
http://superuser.com/questions/42249/how-to-bridge-two-ethernet-ports-on-mac-os-x
In a nutshell, setting up a bridge on Linux means installing the necessary bridge software and editing the /etc/network/interfaces file, both of which are described here:
https://help.ubuntu.com/community/NetworkConnectionBridge
If you’re like me, seeing a working config file gets you a long way, so below is my /etc/network/interfaces file. For reasons not related to bridging, my network is setup as a class B network, so you will probably have to adjust the various addresses accordingly:
$ cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
iface eth0 inet manual
auto br0
iface br0 inet static
address 172.16.0.190
network 172.16.0.0
netmask 255.255.0.0
broadcast 172.16.0.255
gateway 172.16.0.1
dns-search mydomain.com
dns-nameservers 172.16.0.10
bridge_ports eth0
bridge_stp off
bridge_fd 0
bridge_maxwait 0
Once bridging is setup on the Linux host, all that needs to be done is start QEMU with the correct network options, and then edit the guest network settings. The options to use for QEMU are:
-net nic,vlan=0
-net tap,vlan=0,ifname=tap0
Note that these options replace the existing QEMU network options, and are not to be used in conjunction with them.
It is these args that inform QEMU that you wish to use bridged networking.
To use bridge networking on Windows 7, add the above two arguments to your QEMU start line and then setup your network manually within Windows 7. For example, if you use this command for non-bridge networking:
qemu-system-x86_64 -enable-kvm -m 8192M -cpu host -drive file=/dev/zvol/rpool/kvm/windows7/disk0 -netdev user,id=vlan0 -net nic,model=e1000,netdev=vlan0 -smp 2
Then use this for bridged networking:
qemu-system-x86_64 -enable-kvm -m 8192M -cpu host -drive file=/dev/zvol/rpool/kvm/windows7/disk0 -net nic,vlan=0 -net tap,vlan=0,ifname=tap0 -smp 2
Note that this assumes you have previously setup bridge networking support in Linux. Also, you must run QEMU as root in order to use bridge networking.
To use bridge networking on OS X Mountain Lion, use the above two arguments for networking in your QEMU start line. For example:
qemu-system-x86_64 -enable-kvm -m 4096 -cpu core2duo -machine q35 -usb -device usb-kbd -device usb-mouse -device isa-applesmc,osk="ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc" -kernel ./chameleon_svn2534_boot -smbios type=2 -device ide-drive,bus=ide.2,drive=MacHDD -drive id=MacHDD,if=none,format=raw,file=/dev/zvol/rpool/kvm/mountain-lion/disk0 -net nic,vlan=0 -net tap,vlan=0,ifname=tap0 -monitor stdio -smp 2
And then setup your network manually within Mountain Lion using these commands:
sudo ifconfig bridge0 create sudo ifconfig bridge0 addm en0 sudo ifconfig bridge0 up
These commands aren’t necessary, but may be useful if you make a mistake:
sudo ifconfig bridge0 down sudo ifconfig bridge0 deletem en0 sudo ifconfig bridge0 destroy
To use bridge networking on OS X Mavericks, use the OS X Network utility as described in the above posts to add a bridge, making sure to add your Ethernet device (en0) to it. Then add use the two QEMU networking options above in your QEMU command-line along with the virtio driver. For example:
qemu-system-x86_64 -enable-kvm -m 4096 -cpu core2duo -machine q35 -usb -device usb-kbd -device usb-mouse -device isa-applesmc,osk="ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc" -kernel ./chameleon_svn2534_boot -smbios type=2 -device ide-drive,bus=ide.2,drive=MacHDD -drive id=MacHDD,if=none,format=raw,file=/dev/zvol/rpool/kvm/mavericks/disk0 -net nic,model=virtio,vlan=0 -net tap,vlan=0,ifname=tap0 -monitor stdio -smp 2