How can run a desktop GUI application inside my headless VirtualBox that was launched via Vagrant ?

I already had an explanation on how to set up Vagrant with VirtualBox. I assume you already have all that set up.

There is an

X-Server on the host

You need to have an X-Server on you host-machine.

If you run a desktop Linux system as your host as well, then you already have an X Server.

On Mac OSX you can install XQuartz

When writing this article I have not tried it on MS Windows, but as I recall I used Xming at one of my clients.

Enable X-forwarding

You need to enable X-forwarding in the guest operating system. Probably the best is to do it via the Vagrant configuration file Vagrantfile:

  config.ssh.forward_x11 = true

ssh into the box

Instead of using vagrant ssh we'll need to use the ssh command to access the guest operating system. Run vagrant ssh-config on the host in order to find out the configuration details.

$ vagrant ssh-config

Host default
  User ubuntu
  Port 2222
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  PasswordAuthentication no
  IdentityFile /Users/gabor/work/.vagrant/private_key
  IdentitiesOnly yes
  LogLevel FATAL
  ForwardX11 yes

From this we can get the User, the HostName, the Port, and the location of the IdentityFile that holds the private key we need to use.

In addition we need to supply the -X flag that tells ssh to use the X-forwarding.

ssh ubuntu@ -p 2222 -i /Users/gabor/work/.vagrant/private_key -X

Then you can already start desktop GUI applications.

Traditionally xclock and xeyes were used to test this as they are really simple X-based applications, but if you cannot install either of those, you might have something like xarclock.

Once you know you can launch x applications in the guest and see them on the host, I recommend creating and alias for the command by adding this to your ~/.bashrc or ~/.bash_profile in your host. (Assuming Linux or OSX)

alias vssh='ssh ubuntu@ -p 2222 -i /Users/gabor/work/.vagrant/private_key -X'

and the reloading it using source.

That way the connection will be just a vssh away.