Hoping this might be useful for someone else, since Google didn’t turn up the answer for me.

I got a vagrant-packaged CentOS box from a colleague, and starting it up warned me that the VirtualBox Guest Additions were out of date. Fixing things involved these steps:

  1. Get VirtualBox to “Install Guest Additions” (from the Devices menu of the running VM). Don’t be fooled: VirtualBox has not, in fact, installed the guest additions at this point.1
  2. What it has done is added the ISO as if it were a cdrom device: you have to mount it and run an install script. This is in the VirtualBox docs, but everything I found assumed the ISO would be on device /dev/cdrom. In my case, though, /dev/cdrom was very handily attached to the cdrom device on my host machine. (Vagrant setup did it? VirtualBox did it by default? Not sure.)
  3. Turns out the device you want is /dev/cdrom1 (yeah, obvious in retrospect). Mount that, run the install script, done.

Notes:

  1. Yes, this is a rather astonishing failure of UI design. But one that is fairly well-known, googlable, and not the root of the problem here. [↪]