https://blog.offline-net.com/2012/10/07/dtiserversmanvps%ef%bc%9anetwork-traffic-%e3%82%92%e3%82%b3%e3%83%b3%e3%82%bd%e3%83%bc%e3%83%ab%e3%81%a7%e8%a6%8b%e3%82%8biftop%e3%83%bbifstatus/