https://blog.offline-net.com/2011/10/22/ubuntu-11-10-%ef%bc%9avirtualbox-%e3%81%aeubuntu%e3%82%92%e3%82%a2%e3%83%83%e3%83%97%e3%82%b0%e3%83%ac%e3%83%bc%e3%83%89%e3%81%99%e3%82%8b/