Solution for missing /vagrant file system.

If you are messing around with Vagrant and VirtualBox, you will sometimes end up with a vm where the /vagrant shared file system does not mount properly.  Vagrant complains that the vboxsf kernel module is not loaded properly and gives you a mount command to run manually.  When you try to mount it manually with

sudo mount -t vboxfs -o uid=500,gid=500 vagrant /vagrant

You get this error:

/sbin/mount.vboxsf: mounting failed with the error: No such device

  According to this page, you have to reinstall the VBox guest additions with this command:

cd /opt/VBoxGuestAdditions-*/init
sudo ./vboxadd setup

However, for a generic box, it fails because gcc and the kernel sources are not installed.  So when you run into this problem on a Centos 6 box (specifically, a vm based on the bento/centos-6.8 vagrant box), here is the exact sequence of commands that fixes the problem:

cd /opt/VBoxGuestAdditions-*/init
sudo yum install gcc make kernel-devel
sudo ./vboxadd setup

Then rerun the special mount command to mount the /vagrant file system.