Why can user vagrant do "sudo su -" without entering password?

Question

I am a new student about vagrant. I just found that I can change to root by "sudo su -" without entering a password when inside the box supervised by Vagrant. Curiously I checked the "/etc/sudoers" and found nothing about user vagrant. Can anyone kindly explain this?

Answer 1

/etc/sudoers include another file which contains the NOPASSWD line.

vagrant@ubuntu-12:~$ sudo cat /etc/sudoers.d/vagrant 
%vagrant ALL=NOPASSWD:ALL

Answer 2

Based on http://docs.vagrantup.com/v2/boxes/base.html: "Many aspects of Vagrant expect the default SSH user to have passwordless sudo configured. This lets Vagrant configure networks, mount synced folders, install software, and more."

As moodh mentioned,

vagrant ALL=(ALL) NOPASSWD: ALL

will allow "password-less" sudo.