Assumindo que sua configuração de sshd tenha PermitRootLogin yes
.
sudo grep "login as the ubuntu user" /root/.??*
No entanto, o link que Mike Scott forneceu é um que eu recomendo que você leia atentamente e tome cuidado.
ROOT SSH
Finally, if you wish to circumvent the Ubuntu security standard and revert to the old practice of allowing ssh and rsync as root, this command will open it up for a new instance of the official Ubuntu images:
ssh -i KEYPAIR.pem ubuntu@HOSTNAME 'sudo cp /home/ubuntu/.ssh/authorized_keys /root/.ssh/'
This is not recommended, but it may be a way to get existing EC2 automation code to continue working until you can upgrade to the sudo practices described above.
Eu mantenho os logins de SSH raiz desativados porque qualquer servidor voltado para o público com SSH ativado será danificado continuamente dia e noite por tentativas de login raiz de botnets criminosos.
Em outros lugares, as documentações avisam
Enabling the Root account is rarely necessary. Almost everything you need to do as administrator of an Ubuntu system can be done via sudo or gksudo. If you really need a persistent Root login, the best alternative is to simulate a Root login shell using the following command...
sudo -i