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Building virtual machines with vmbuilder

After installing qemu-kvm, libvirt-bin, bridge-utils and ubuntu-vm-builder, set up a bridge that virtual machines can attach to: 

auto lo
iface lo inet loopback

auto enp2s0
iface enp2s0 inet manual

auto br0
iface br0 inet static
      address 192.168.0.8
      netmask 255.255.255.0
      gateway 192.168.0.1
      nameserver 127.0.0.1
      bridge_ports enp2s0
      bridge_stp off
      bridge_fd 0
      bridge_maxwait 0

Then /etc/init.d/networking restart.

Install apt-cacher and give it the following config in /etc/apt-cacher/apt-cacher.conf: 

group = www-data
user = www-data
daemon_addr = 192.168.0.8
path_map = ubuntu archive.ubuntu.com/ubuntu
allowed_hosts = 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
ubuntu_release_names = trusty

The vm build script, using the local cache: 

#!/bin/sh

if [ $# -lt 2 ]; then
    echo "usage: HOSTNAME IP"
    exit 1
else
    HOSTNAME=$1
    IP=$2
fi

firstboot=`mktemp`
echo "#!/bin/bash" >>$firstboot
echo "rm -f /etc/resolvconf/resolv.conf.d/{original,base,head,tail}" >>$firstboot
echo "reboot" >>$firstboot
chmod +x $firstboot

qemu-img create -f qcow2 -o preallocation=falloc $HOME/vms/$HOSTNAME-rootdisk 4096M

vmbuilder kvm ubuntu \
  --suite trusty \
  --verbose \
  --libvirt qemu:///system \
  --destdir $HOME/vms/$HOSTNAME/ \
  --install-mirror http://192.168.0.8:3142/ubuntu \
  --mirror http://192.168.0.8:3142/ubuntu \
  --raw $HOME/vms/$HOSTNAME-rootdisk \
  --rootsize 4096 \
  --swapsize 0 \
  --mem 128 \
  --cpus 1 \
  --hostname $HOSTNAME \
  --bridge br0 \
  --ip $IP\
  --mask 255.255.255.0 \
  --gw 192.168.0.1 \
  --dns 192.168.0.8 \
  --lang en_US.UTF-8 \
  --timezone UTC \
  --user ubuntu \
  --name Ubuntu \
  --pass ubuntu \
  --ssh-user-key $HOME/.ssh/id_rsa.pub \
  --addpkg linux-image-generic \
  --addpkg openssh-server \
  --addpkg sudo \
  --firstboot $firstboot

rm $firstboot
rmdir $HOME/vms/$HOSTNAME/

virsh autostart $HOSTNAME
virsh start $HOSTNAME

Edit: To inspect and edit the virtual machine disk image, use guestfish, part of the libguestfs project: 

$ sudo apt-get install libguestfs-tools
$ guestfish --rw --add testserver-rootdisk

Welcome to guestfish, the guest filesystem shell for
editing virtual machine filesystems and disk images.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

><fs> run
100% ⟦▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒⟧ 00:00
><fs> list-filesystems
/dev/sda1: ext4
><fs> mount /dev/sda1 /
><fs> emacs /etc/resolv.conf
><fs> exit

Edit: Disk image pre-allocation can be done with QEMU-provided tools, which may or may not be a more kosher approach. Must investigate. 

qemu-img create -f qcow2 -o preallocation=falloc $HOME/vms/$HOSTNAME-rootdisk 32768M

Edit: Fix nameserver enforcement in build script.

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