I wanted to enable Two Factor Authentication (2FA) on an SSH server, and be able to use either my YubiKey, or Google Authenticator if I don’t have my YubiKey with me.
Setting this up was pretty easy, I started with Google Authenticator
Google Authenticator libpam
- Install Google Authenticator libpam either from source or use a package provided by your distribution
# google-authenticator Do you want authentication tokens to be time-based (y/n) y https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/[email protected]%3Fsecret%3DXXXXXXXXXXXXXXXX%26issuer%3Ddb Your new secret key is: XXXXXXXXXXXXXXXX Your verification code is 123456 Your emergency scratch codes are: 12345671 12345672 12345673 12345674 12345675 Do you want me to update your "/root/.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) n
- Configure PAM, your milage will vary depending on how your distribution configures PAM, but in my case I changed
# cat /etc/pam.d/sshd auth include system-remote-login account include system-remote-login password include system-remote-login session include system-remote-login #
# cat /etc/pam.d/sshd auth include system-remote-login auth required pam_google_authenticator.so account include system-remote-login password include system-remote-login session include system-remote-login #
- Test it and ensure it works (keep a root shell open in case you’re locked out!)
# ssh 192.168.12.21 Password: Verification code: host ~ #
- Install Yubico Pam from source or use a package provided by your distribution
- Configure PAM, I changed /etc/pam.d/sshd to:
# cat !$ cat /etc/pam.d/sshd auth include system-remote-login auth [success=done new_authtok_reqd=ok default=ignore] pam_yubico.so id=16 nullok auth required pam_google_authenticator.so account include system-remote-login password include system-remote-login session include system-remote-login
Et voila, now when I SSH to this server and enter my password, I can use by YubiKey to complete the two factor authentication if I have it to hand:
# ssh 192.168.12.21 Password: YubiKey for `root': host ~ #
and if I don’t, I simply hit enter at the YubiKey prompt and complete two factor with Google Authenticator.
# ssh 192.168.12.21 Password: YubiKey for `root': Verification code: host ~ #
The key to how this works is setting
default=ignore, which means that if Yubico PAM authentication succeeds, the authentication process finishes without going onto
pam_google_authenticator.so. If Yubico PAM fails, it is ignored and continues to Google Authenticator.