Installation

This is currently easiest to set up (to avoid backporting, see below) on a Debian Sid box with libvirt installed:

sudo apt-get install vagrant
vagrant plugin install vagrant-libvirt

After this, the Tails branch feature/vagrant-libvirt makes everything work like when Vagrant was using Virtualbox, except basebox building (see below).

Issues (and their workarounds) with vagrant-libvirt

Packaging

vagrant-libvirt is not packaged in Debian yet, so we need to RFP it, and then have it backported to Wheezy.

Also, some of vagrant-libvirt's dependencies are too old in Wheezy:

ruby-fog >= 1.10.0: Sid has 1.12.1 and backporting is trivial (i.e. just rebuild in a Wheezy chroot).
vagrant >= 1.1: Sid has 1.2.2, backporting is trivial but also requires packporting the follwing dependencies:
- ruby-childprocess >= 0.3.7: Sid has 0.3.9 and backporting is trivial.
- ruby-net-scp >= 1.1.0: Sid has 1.1.1 and backporting is almost trivial; tests are failing due to errors loading ruby-mocha. This can probably be fixed in debian/patches/disable-rubygems-in-tests.patch by requiring mocha instead of mocha/setup or similar (see the difference between this patch in the Wheezy and Sid sources). It of course builds if tests are disabled (i.e. DEB_BUILD_OPTIONS=nocheck).
- ruby-net-ssh >= 2.6.6: Sid has 2.6.8, and backporting is identical to backporting ruby-net-scp, described above.

Syncing (Tails' Git and built images)

When using the libvirt provider in Vagrant, "synced folders" are simply rsync:ed into the VM's filesystem, unlike with Vagrant with Virtualbox, which uses its shared folder feature. The rsync:ing only happens at VM start (not at VM stop; Vagrant specifies this as legit behaviour, so it's not a bug), so any changes made to Tails' git after that are not pulled into /amnesia.git (a VM restart is required), and the built image can't be synced to the host using /vagrant (remember, syncing back to host never happens). The current implementation therefore doesn't rely on synced folders but instead:

manually uploads Tails' git before starting the build using Vagrant's upload feature.
manually downloads the built image after building is complete using Vagrant's download feature.

Proper synced folders (bidirectional, continously "syncing", like Virtualbox' shared folders) would be preferable (faster, less disk usage (size of Tails' .git) in build guest, less code complexity, etc.), so we'd like this upstream issue to get fixed.

It should be noted that the libvirt provider does support shares using NFS, which would give synced folders with the properties we want, but this requires configuring an NFS server on the host box, which makes this approach too complicated.

Get rid of the Vagrant "verified downloads" monkeypatch

We monkeypatch Vagrant to verify the integrity of the downloaded basebox in vagrant/lib/vagrant_verified_download.rb. Due to heavy code reorganizing of Vagrant beween 1.0.x and 1.2.x, the monkeypatch isn't very pretty or convenient any more. Although the current one works, it's very fragile, but so are monkeypatches in general. A proper patch has been written and submitted upstream (issue, pull request) and, if accepted, we hopefully can get the next Vagrant containing the patch backported to Wheezy.

Basebox building

The squeeze definition on Tails branch feature/vagrant-libvirt can be successfully built using KVM (veewee kvm build squeeze) with the current release of veewee (0.3.7, installed per our current instructions), but:

veewee kvm export is not implemented.
the box image is created in the raw format, so it consumes the full 10 GiB disk space, and is incompatible with vagrant-libvirt, which only supports qcow2.

Both of these issues are fixed in veewee's Git, but getting it to run/install brought gem dependency hell onto my machine, so I've been unable to test it. Hopefully this will be easier after next realse (so there's a Ruby gem). Having the next version of veewee in Debian (and backported to Wheezy) would of course be ideal (RFP).

Until a vagrant-libvirt-compatible veewee is released, basebox building won't be fully automatic using rake any more. One will have to pick either of steps 1.{a,b,c} below to create a disk image, and then step 2 to package it into a .box (personally I went with 1c + 2 as that doesn't require any fiddling with Ruby gems):

1.a How to fix images built with veewee 0.3.7 using KVM

For now we can manually convert the raw image created by veewee like this (note: squeeze.img below can be found in your libvirt pool's directory):

qemu-img convert -c -O qcow2 squeeze.img box.img

1.b How to fix images built with veewee using Virtualbox

Since Virtualbox currently is better supported (both be veewee and our Rakefile) this may be an easier approach. The disk image can be extracted from the exported .box with tar xf (or tar xzf), and if we assume it's named box.vmdk we can make it compatible with vagrant-libvirt like this:

qemu-img convert -c -O qcow2 box.vmdk box.img

1.c How to fix our old squeeze.box

We can actually use the disk image box-disk1.vmdk from our old squeeze.box, unpack it with tar xf). While libvirt/KVM supports the vmdk format I recommend converting it to a better supported one (I/O errors are common in libvirt/KVM when using vmdk):

qemu-img convert -c -O qcow2 box-disk1.vmdk box-disk1.img

Next, setup a x86_64 domain in libvirt and boot it with a virtual hard drive backed by box-disk1.img, and install the new dependencies, which actually only is rsync, and while we're at it we might just as well update the system and take steps to reduce the image size:

unset HISTFILE
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install rsync
sudo apt-get remove virtualbox*
sudo apt-get autoremove
sudo apt-get clean
sudo rm -rf /var/lib/apt/lists/*
sudo rm -f /var/lib/dhcp/*.leases
sudo dd if=/dev/zero of=/EMPTY bs=1M
sudo rm -f /EMPTY
date | sudo tee /etc/vagrant_box_build_time
sudo halt

Note that even though our box-disk1.img is in qcow2 format already, we want to re-convert it using qemu-img to reduce the image size (that's why we run dd in the VM above):

qemu-img convert -c -O qcow2 box-disk1.img box.img

2 How to package the .box

This is how we package box.img from any of the previous steps into a vagrant-libvirt-compatible .box:

echo 'Vagrant.require_plugin "vagrant-libvirt"' > Vagrantfile
cat > metadata.json << END
{
  "provider"     : "libvirt",
  "format"       : "qcow2",
  "virtual_size" : 10
}
END
tar czf squeeze-libvirt.box ./metadata.json  ./Vagrantfile ./box.img

To test it, temporarily set (overwriting existing values):

config.vm.box_url = 'file:///path/to/squeeze-libvirt.box'
config.vm.box_checksum = '<sha256 checksum of squeeze-libvirt.box>'

in vagrant/Vagrantfile.