How to debug “Vagrant cannot forward the specified ports on this VM” message

Question

I\'m trying to start a Vagrant instance and getting the following message:

Vagrant cannot forward the specified ports on this VM, since they
would collide wi         


        

Answer 1

Watch out, your Vagrantfile is not the only one being used when bringing up a Vagrant box/instance.

When you get this:

~/dev/vagrant user$ vagrant reload
Vagrant cannot forward the specified ports on this VM, since they
would collide with some other application that is already listening
on these ports. The forwarded port to 8001 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.network :forwarded_port, guest: 8001, host: 1234

Sometimes, Vagrant will attempt to auto-correct this for you. In this
case, Vagrant was unable to. This is usually because the guest machine
is in a state which doesn't allow modifying port forwarding.
~/dev/vagrant user$ 

You are actually not only using the Vagrantfile from ~/dev/vagrant but also the one from your "box" distribution .box file which is typically located here:

~/.vagrant.d/boxes/trusty/0/virtualbox/Vagrantfile

And if you have a look at it you'll see it has plenty of default port mappings:

$ cat ~/.vagrant.d/boxes//trusty/0/virtualbox/Vagrantfile
$script = <