test.json

{
  "builders": [{
    "type"                   : "virtualbox",
    "iso_url"                : "http://releases.ubuntu.com/12.10/ubuntu-12.10-server-amd64.iso",
    "iso_checksum"           : "4bd3270bde86d7e4e017e3847a4af485",
    "iso_checksum_type"      : "md5",
    "guest_additions_attach" : true,
    "guest_os_type"          : "Ubuntu_64",
    "ssh_username"           : "packer",
    "ssh_wait_timeout"       : "30s",
    "boot_command"           : [
      "<esc><esc><enter><wait>",
      "/install/vmlinuz noapic ",
      "preseed/url=http://{{ .HTTPIP }}:{{ .HTTPPort }}/preseed.cfg ",
      "debian-installer=en_GB auto locale=en_GB kbd-chooser/method=uk ",
      "hostname={{ .Name }} ",
      "fb=false debconf/frontend=noninteractive ",
      "keyboard-configuration/modelcode=SKIP keyboard-configuration/layout=GB ",
      "keyboard-configuration/variant=GB console-setup/ask_detect=false ",
      "initrd=/install/initrd.gz -- <enter>"
    ],
    "vboxmanage"             : [
      ["modifyvm", "{{.Name}}", "--memory", "1024"]
    ],
    "shutdown_command"       : "shutdown -P now"
  }]
}