A tutorial from Make Tech Easier describes how to use Google-Authenticator to enable two-factor authentication for an SSH connection on Ubuntu Linux.
How to Enable Two-Factor Authentication for SSH Connection
By itself, SSH is a secure way of connecting to a remote machine. However, if you are still keen to add additional security to your SSH connection, you can add two-factor authentication to be prompted to enter a random verification code when you connect via SSH. We previously showed you how to do so on various social networks and show you here how to add two-factor authentication to your SSH connection. Note: this instruction is based on the Ubuntu server. If you are using another distribution, some of the commands may vary.
How to Enable Two-Factor Authentication for SSH Connection
