$ timedatectl status #!!!
$ sudo apt-get install libpam-google-authenticator
student@debian:~$ man google-authenticator
student@debian:~$ google-authenticator
student@debian:~$ google-authenticator -t -f -d -w 3 -e 5 -r 3 -R 30
Your new secret key is: NNNNNNNNNNNNNNNNNNNNNNNN
Enter code from app (-1 to skip): NNNNNN
Code confirmed
Your emergency scratch codes are:
...
NNNNNNNN
student@debian:~$ more .google_authenticator
...
debian:~# cat /etc/pam.d/sshd
...
# Enable MFA using Google Authenticator PAM
auth required pam_google_authenticator.so #nullok
debian:~# cat /etc/ssh/sshd_config
...
KbdInteractiveAuthentication yes
...
debian:~# systemctl reload sshd