Problem: frequently the first command I type to my boxes is su -.
Question: how do I make vagrant ssh use the root user by default?
Solution:
Add the following to your Vagrantfile:
config.ssh.username = 'root'
config.ssh.password = 'vagrant'
config.ssh.insert_key = 'true'
When you vagrant ssh henceforth, you will login as root and should expect the following:
==> mybox: Waiting for machine to boot. This may take a few minutes...
mybox: SSH address: 127.0.0.1:2222
mybox: SSH username: root
mybox: SSH auth method: password
mybox: Warning: Connection timeout. Retrying...
mybox: Warning: Remote connection disconnect. Retrying...
==> mybox: Inserting Vagrant public key within guest...
==> mybox: Key inserted! Disconnecting and reconnecting using new SSH key...
==> mybox: Machine booted and ready!
Update 23-Jun-2015: This works for version 1.7.2 as well. Keying security has improved since 1.7.0; this technique overrides back to the previous method which uses a known private key. This solution is not intended to be used for a box that is accessible publicly without proper security measures done prior to publishing.
Reference: