https://blog.offline-net.com/2011/06/26/ubuntu-10-10%ef%bc%9avirtualbox%e3%81%a7ssh%e6%8e%a5%e7%b6%9a%e3%81%a7%e3%81%8d%e3%82%8b%e3%82%88%e3%81%86%e3%81%ab%e3%81%99%e3%82%8b/