The following steps are system relevant (critical) and only recommended for advanced linux users. If the ssh configuration will fail, you won’t be able to login to your system via ssh anymore. The mandatory prerequisite is a ssh server that you can log on using private/public key only!
Install the software for 2FA (Two-Factor-Authentication) with googles authenticator app
sudo -s apt install libpam-google-authenticator
Leave the root-Shell and run the following command as your user
You will be asked for:
Do you want authentication tokens to be time-based (y/n) y
Do you want me to update your "~/.google_authenticator" file (y/n) y
Do you want to disallow multiple uses of the same authentication token? This restricts you to one login about every 30s, but it increases your chances to notice or even prevent man-in-the-middle attacks (y/n) y
By default, tokens are good for 30 seconds and in order to compensate for possible time-skew between the client and the server, we allow an extra token before and after the current time. If you experience problems with poor time synchronization, you can increase the window from its default size of 1:30min to about 4min. Do you want to do so (y/n) n
If the computer that you are logging into isn't hardened against brute-force login attempts, you can enable rate-limiting for the authentication module. By default, this limits attackers to no more than 3 login attempts every 30s. Do you want to enable rate-limiting (y/n) y
Change to the root-Shell again
and configure your ssh server
Change the file to
# PAM configuration for the Secure Shell service # Standard Un*x authentication. # @include common-auth auth required pam_google_authenticator.so ...
Save and quit (:wq!) the file.
Edit your SSH-config to examplarily mine
Port 22 Protocol 2 HostKey /etc/ssh/ssh_host_rsa_key HostKey /etc/ssh/ssh_host_dsa_key HostKey /etc/ssh/ssh_host_ecdsa_key HostKey /etc/ssh/ssh_host_ed25519_key UsePrivilegeSeparation yes KeyRegenerationInterval 3600 ServerKeyBits 1024 SyslogFacility AUTH LogLevel INFO LoginGraceTime 30 PermitRootLogin no StrictModes yes RSAAuthentication yes PubkeyAuthentication yes IgnoreRhosts yes RhostsRSAAuthentication no HostbasedAuthentication no IgnoreUserKnownHosts yes PermitEmptyPasswords no ChallengeResponseAuthentication yes PasswordAuthentication no X11Forwarding no X11DisplayOffset 10 PrintMotd no PrintLastLog no TCPKeepAlive yes Banner /etc/ssh/issue AcceptEnv LANG LC_* Subsystem sftp /usr/lib/openssh/sftp-server UsePAM yes AuthenticationMethods publickey,keyboard-interactive AllowUsers your_ssh_user(s)
Restart your ssh server and re-logon to your server. You will be prompt for a second factor.
Start your google authenticator app (on iOS or on android) and read your second factor.