title: How to debug "Vagrant cannot forward the specified ports on this VM" message
body: <p>I'm trying to start a Vagrant instance and getting the following message:</p>
  
  <pre><code>Vagrant cannot forward the specified ports on this VM, since they
  would collide with another VirtualBox virtual machine's forwarded
  ports! The forwarded port to 4567 is already in use on the host
  machine.
  
  To fix this, modify your current projects Vagrantfile to use another
  port. Example, where '1234' would be replaced by a unique host port:
  
    config.vm.forward_port 80, 1234
  </code></pre>
  
  <p>I opened VirtualBox, but I don't have any running boxes at the moment, so I'm stumped. How can I figure out which process is listening on 4567? Is there a way to list all Vagrant boxes running on my machine? </p>
  
  <p>Thanks,
  Kevin</p>