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Monthly Archives: September 2014
The Google authenticator app adds an easy to use true two factor login (rather than just an extra password, I’m looking at you online banking). It is time limited and each code is valid one for 30 seconds, however this can be changed.
There are other guides out there which describe the setup but these follow the assumption that you will be using passwords to login rather than public keys.
This method is tested for Ubuntu 14.04.1 but will likely work for any distro that has a recent release of openssh (v6.6+). The following commands assume you are logged in as root, if not prefix with
First step is to install the authenticator.
aptitude install libpam-google-authenticator
Next, setup the authenticator for your login or root.
$ /usr/bin/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/... [QR CODE] Your new secret key is: D3WF27FKWZA5TAO3 (not real :P) Your verification code is 123456 Your emergency scratch codes are: 12345678 12345678 12345678 12345678 12345678 Do you want me to update your "/home/sam/.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 isnt 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
You’ll need to either scan the provided QR code via the authenticator app or follow the provided url.
Now you are enrolled in two-step authentication, congrats.
Next is to update PAM to accept Google Authenticator tokens. Update
/etc/pam.d/sshd to include the follow at the top of the file (above @include common-auth).
#Google Authenticator auth sufficient pam_google_authenticator.so
Sufficient is used rather than required here because required would prompt for the password after the code is entered which is not what we want as public keys are being used.
The final step is to setup the ssh server to accept public keys and allow keyboard interaction. Open
/etc/ssh/sshd_config and make the following changes. But take a backup first.
# Add this if not present PubkeyAuthentication yes # This allows sshd to ask for the verification code # after the public key is accepted AuthenticationMethods publickey,keyboard-interactive # Change the following to yes, it is no by default ChallengeResponseAuthentication yes # Also disable password logins PasswordAuthentication no # This should be present, but add if not present UsePAM yes
Now restart the ssh server.
service ssh restart
Important: Don’t close your current session until you know the login is working correctly, as you may get locked out.
Time to test, open a new ssh connection to your box, you should now be greeted like this:
Using username "root". Authenticating with public key "my-public-key" from agent Further authentication required Using keyboard-interactive authentication. Verification code:
If this fails, you can restore the backup you made of the sshd config (you did make one?), remove the authenticator from the pam config. and restart ssh.