Scenario: A new Linux box has been installed with Vagrant, and now I must git clone a non-open repository to the server, so I need to forward my SSH keys to the root user on the virtual server.
1) Add the following line to your Vagrantfile:
config.ssh.forward_agent = true
2) Once you’ve “vagrant ssh”‘d to the server:
$ sudo -E -s su
..to become root with your forwarded key.