更新完Guest的内核之后，Guest下提示“ vboxclient the virtualbox kernel service is not running”。

Guest下执行：sudo /etc/init.d/vboxadd setup

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http://askubuntu.com/questions/642991/virtualbox-ubuntu-14-04-vboxclient-the-virtualbox-kernel-service-is-not-runnin

This usually happens when you update the Guest's Linux Kernel.
After each update you need to do

    For Host (installing server)

    sudo /etc/init.d/vboxdrv setup

    For Guest (installing additions)

    sudo /etc/init.d/vboxadd setup

The reason why inserting the VBox addition disk again works is because that's one of the things the installation procedure does as well.