[qemu] Create a complete subnet of virtual machines

Let's create a virtual network with qemu/kvm and vde :)

First, create a virtual switch where you will plug your virtual machines.

vde_switch -s /tmp/switch1.ctl -m 666 --daemon --mgmt /tmp/switch1.mgmt 

The -m option is here to set the flags so as to let everyone use the emulated switch. This is useful if you declare a switch with a link to a tap device.

Without the --daemon option, the vde_switch doesn't go in background and provides us a shell to configure its internals. This shell doesn't support history and readline commodities, so a better option is to use the --daemon mode coupled with the --mgmt option, this provide a socket where the virtual switch will listen for configuration input.

 
vdeterm /tmp/switch1.mgmt 

This command will give you a fully fledged shell. (You know you deserve it)

Finally the -s option is the most important as it is the interface you have to use to connect something to this virtual switch.

Congratulation, there is now an emulated empty switch running on your computer !

You can connect a vm to this switch with the following command:

qemu-system-x86_64 -drive file=your_image.qcow2,if=virtio --enable-kvm -net nic,model=virtio,macaddr=52:54:00:12:34:57 -net vde,sock=/tmp/switch1.ctl -cpu host -smp 4 -m 1G -vga qxl

The important thing is the -net vde option here.
Your vm should have registered to the vde_switch.

You can find out by using the command port/print in the vdeterm. You should get something like that:

 



The VMs you plug like this will be able to communicate with each other.

To enable the VMs to have an internet access, you can set up a NAT. This can be
done using slirp.

slirpvde -s /tmp/switch1.ctl --dhcp

That's it, it handles DNS, dhcp, name it, it does it :)
However, with this setup, you won't be able to access the VMs from the host, if you try and add a tap interface to the switch, it will get a dhcp and dns setup, it will get a default route rule. This will conflict with the nat and the ip forwarding won't work because the path are not correct.

You can prevent that by not using the dhcp server at all whith the tap interface. You can also use the tap interface as gateway for the VMs and enable the ip forwarding and masquerade nat rules instead of using the slirp tool (I will describe this in another post)

Sources: 

• http://wiki.v2.cs.unibo.it/wiki/index.php/VDE_Basic_Networking
• https://wiki.archlinux.org/index.php/QEMU#Networking_with_VDE2
• http://wiki.qemu.org/Documentation/Networkin
• https://xkcd.com/350/