Virt builder quick provision script

Until now, I was using snap-guest tool together with Foreman to spawn my development/test virtual machines. I have been carefully monitoring (virt-builder)[] tool in recent Fedoras and it looks like the version from Fedora 20 is finally usable for my scenarios (thanks Rich for fixing some annoying bugs for me).

Therefore I am happy to announce new tool called fvb which stands for Foreman-virt-builder. This little script is based around two little commands virt-builder/virt-install and it has grown to something bigger over last couple of months. The usage is simple:

fvb --install-deps

The above command should install all required dependencies on Red Hat systems. Note Fedora 20+ is required to get this working since this needs some latest and greatest features from virt-builder.

fvb -n my-nightly-foreman -f
fvb -n rc-foreman -- "FOREMAN_REPO=rc" "MYVAR=value"
fvb -n stable-foreman -- "FOREMAN_REPO=releases/1.4"
fvb -n bz981123 -- "KOJI_BUILD=http://mykoji.blah/taskinfo?taskID=97281"
fvb -n my-own-script --script fb-xyz.bats -- BATS_REPOOWNER=lzap BATS_BRANCH=test

You may notice download of centos-6.xz file into ~/.cache/virt-builder. This will only happen once, or when new CentOS minor release is uploaded. The template image is handled by virt-builder project itself.

The script works the following way:

  • Generates MAC address from hypervisor hostname and guest hostname (hash) so it does not change when you provision again.
  • Removes existing guest from libvirt (if --force is given).
  • Generates random IP address.
  • Allocates DHCP and DNS entry with the MAC and IP in libvirt network (“default”) overwriting existing entries. This is compatible with virsh Foreman provider (and also works the similar way).
  • Installs your public SSH key on the image.
  • Creates a new VM image based on CentOS6 using virt-builder.
  • Configures a firstboot script:
    • install git
    • install bats framework
    • install foreman-bats scripts
    • run script provided by --script option (by default fb-install-foreman.bats)
  • Spawns the new image with virt-install (with nested kvm enabled if possible).
  • Refreshes dnsmasq configuration.
  • Waits until DHCP daemon (dnsmasq) returns the IP address.
  • Calls fortune command.

Dnsmasq configuration (optional)

By default dnsmasq is installed on Fedora and Red Hat systems, but the daemon is turned off and it is only used by libvirt. What I usually do is I have it configured as a caching DNS daemon on my laptop. It not only speeds up DNS queries (web browsing is much faster on all connections), but it also allows me to redirect DNS queries for different domains to different DNS servers. For example:

# grep -v ^# /etc/dnsmasq.conf | sort -u

# cat /etc/resolv.conf

# cat /etc/resolv.dnsmasq

# cat /etc/dnsmasq.d/local.lan

In the configuration above, you can see that dnsmasq is configured to read nameservers from a separate file (/etc/resolv.dnsmasq) where it has Google DNS public servers and to respond on localhost and to act like a simple DNS caching daemon. The system is configured to take advantage of that.

The last bit is configuration for local.lan domain (default --domain for fvb script). Libvirt keeps host names in this file and since this is in dnsmasq format, we can read it directly.

What does it mean? Well, if you configure everything according to this, once you spawn a VM called “test”, the box named “test.local.lan” will be immediately available, so you can do this:

ssh root@test.local.lan

Or even visit this:


That’s kinda cool, isn’t it?

Notes to redhatters

If you are not a redhatter, skip to the next section. If you want this setup, make sure you have something like this:

# cat /etc/dnsmasq.d/

This configuration adds separate handling for domain which goes to internal servers (these are dummy values). Also note that I want to use public DNS for vpn concentrators and web site (so I am able to connect initially and also the site works when not connected).

This way you can enjoy advantage of fvb script, fast public DNS lookups and fast internal lookups.

What next?

The fvb script is just a little prototype. It lives here and I might move it into a separate repository eventually.

The script can be used for anything, you can provide different distribution (use virt-builder -l to list all the base images available to you) and provide any command with --script that should be executed. For example to deploy Katello Foreman plugin (which we sometimes call “foretello”), do:

fvb -n knightly --script 'git clone && cd katello-deploy && ./setup.rb centos6'

The script accepts any shell variables and values after the -- option, so you are able to pass anything to your own scripts when needed.

Please send me patches and comments. If you like it, I can rename the project and generalize it a bit so it is useful for others as well.

Nested virtualization

I have one extra trick for you. If you do

echo “options kvm-intel nested=1″ | sudo tee /etc/modprobe.d/kvm-intel.conf

on the host machine and restart, you should be able to use KVM inside KVM. The fvb script spawns machines with required nesting options turned on, so you can configure Foreman/Katello for provisioning there.


usage: fvb options -- VARIABLE1=value1 ...

Script for building images with Foreman preinstalled using virt-builder and
bats suite on local libvirt host. The script also modifies dnsmasq
configuration and sends SIGHUP to refresh so the hostname is instantly
available. The root password is "redhat" but you should have your public
ssh key installed by default.

  --help | -h
        Show this message

  --name | -n
        Image name, default: nightly

  --distro | -d
        Distribution base image for virt-builder, default: centos-6

  --force | -f
        Overwrite target image and VM if they exist

        Do not use sudo for building and running VM - you will need to
        set --image-dir accordingly too when running under regular user.

  --image-dir [path]
        Target images path
        Default: /var/lib/libvirt/images

  --domain [domain]
        Domain suffix like "", default: local.lan

  --subnet [subnet]
        Subnet to be used for writing DHCP/DNS configuration
        Default: 192.168.122 (note there is no suffix or period)

  --pub-key [key]
        Install this public ssh key
        Default: /home/you/.ssh/

  --script [name]
        BATS script to execute during first boot (can be any shell command)
        Default: fb-install-foreman.bats

        Install required dependencies

Next time!

15 May 2014 | linux | fedora