Two Factor Authentication With OpenSSH

This material is work-in-progress ...

Do not follow instructions here until this notice is removed.
(Last edited by Larena on 12 Jun 2017.)

Note: Currently the packages required to follow this how-to are available only on edge (future AL3.7)

Using Google Authenticator

apk add google-authenticator openssh-server-pam

cat /etc/pam.d/base-auth

# basic PAM configuration for Alpine.

auth		required	pam_env.so
#auth		required	pam_unix.so	nullok_secure
auth		required	pam_nologin.so	successok
auth		required	/lib/security/pam_google_authenticator.so

cat /etc/ssh/sshd_config

UsePAM yes
AuthenticationMethods publickey,keyboard-interactive
ChallengeResponseAuthentication yes
PermitRootLogin yes

Note: This configuration does NOT allow password authentication. To allow password authentication append "password" to "AuthenticatioMethods"

As user root:

google-authenticator

Do you want authentication tokens to be time-based (y/n) y
https://www.google.com/<pruned>
Your new secret key is: <secret>
Your verification code is <pruned>
Your emergency scratch codes are:
  <pruned>
  <pruned>
  <pruned>
  <pruned>
  <pruned>

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) n

By default, tokens are good for 30 seconds. 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 +-1min (window size of 3) to about +-4min (window size of
17 acceptable tokens).
Do you want to do so? (y/n) y

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

Download Google Authenticator app and enter manually your <secret> key