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How to Secure SSH with Google Authenticator’s Two-Factor Authentication

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Want to secure your SSH server with easy-to-use two-factor authentication? Google provides the necessary software to integrate Google Authenticator’s time-based one-time password (TOTP) system with your SSH server. You’ll have to enter the code from your phone when you connect.

Google Authenticator doesn’t “phone home” to Google — all the work happens on your SSH server and your phone. In fact, Google Authenticator is completely open-source, so you can even examine its source code yourself.

Install Google Authenticator

To implement multifactor authentication with Google Authenticator, we’ll need the open-source Google Authenticator PAM module. PAM stands for “pluggable authentication module” – it’s a way to easily plug different forms of authentication into a Linux system.

Ubuntu’s software repositories contain an easy-to-install package for the Google Authenticator PAM module. If your Linux distribution doesn’t contain a package for this, you’ll have to download it from the Google Authenticator downloads page on Google Code and compile it yourself.

To install the package on Ubuntu, run the following command:

sudo apt-get install libpam-google-authenticator

(This will only install the PAM module on our system – we’ll have to activate it for SSH logins manually.)

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Create an Authentication Key

Log in as the user you’ll be logging in with remotely and run the google-authenticator command to create a secret key for that user.

Allow the command to update your Google Authenticator file by typing y. You’ll then be prompted with several questions that will allow you to restrict uses of the same temporary security token, increase the time window that tokens can be used for, and limit allowed acces attempts to hinder brute-force cracking attempts. These choices all trade some security for some ease-of-use.

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Google Authenticator will present you with a secret key and several “emergency scratch codes.” Write down the emergency scratch codes somewhere safe – they can only be used one time each, and they’re intended for use if you lose your phone.

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Enter the secret key in the Google Authenticator app on your phone (official apps are available for Android, iOS, and Blackberry). You can also use the scan barcode feature – go to the URL located near the top of the command’s output and you can scan a QR code with your phone’s camera.

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You’ll now have a constantly changing verification code on your phone.

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If you want to log in remotely as multiple users, run this command for each user. Each user will have their own secret key and their own codes.

Activate Google Authenticator

Next you’ll have to require Google Authenticator for SSH logins. To do so, open the /etc/pam.d/sshd file on your system (for example, with the sudo nano /etc/pam.d/sshd command) and add the following line to the file:

auth required pam_google_authenticator.so

Next, open the /etc/ssh/sshd_config file, locate the ChallengeResponseAuthentication line, and change it to read as follows:

ChallengeResponseAuthentication yes

(If the ChallengeResponseAuthentication line doesn’t already exist, add the above line to the file.)

Finally, restart the SSH server so your changes will take effect:

sudo service ssh restart

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You’ll be prompted for both your password and Google Authenticator code whenever you attempt to log in via SSH.

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Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.

  • Published 08/14/12

Comments (15)

  1. Jean-Francois Messier

    Excellent article. I use this authenticator for my main GMail account for a while now, and I really like the fact that I can have multiple secret keys and number generators on my phone. The trick is now to see whether I can replicate the secret key across multiple servers to have a single generated code to access multiple servers.

  2. foo

    I’m in love with Google’s 2-factor auth. I use it for Google account(obviously), LastPass, SSH, and once implemented…dropbox =D

  3. reklaimer

    Tried this under my ubuntu 10.10 server. Even when adding a repo manually that I found it still could not find the libpam-google-authenticator package. Maybe it just doesn’t work for 10.10?

  4. foobar

    Someone needs to add this to Tomato/DD-WRT

  5. Tim

    I had this on my SSH server for a little while… it worked really well… and then I went to connect with WinSCP… there isn’t a great implementation for connecting via SFTP.

  6. cpradio

    I’m with Tim, I can’t really use this without WinSCP support…. sigh

  7. Deltaray

    It seems that you’re using a libvte based terminal (gnome-terminal) in your example screenshots, since you are concerned enough about security to implement two factor auth, you really should check out this document, which documents a major security flaw in that library:

    http://climagic.org/bugreports/libvte-scrollback-written-to-disk.html

  8. Toto

    Interesting article, the idea looks interesting.

    Just a quick question, can you still have several way to connect to your ssh server at the same time?

    Basically, I’m usually connecting to my ssh server from my laptop or tablet, so I created a public/private pair of key (DSA), because it’s faster, more secure etc…

    I would like to use the google system but only if I’m not able to use my private key. Is there is a a way to do that?
    Thanks,

  9. Rearden

    For those that are concerned about your smart phone not have battery charge on being in an area that has no coverage, there is a simpler alternative at: http://taferno.sourceforge.net

    It uses simple USB flash drives and provides Two-Factor Authentication for:
    1. OpenVPN
    2. SSH
    3. Web Single Sign On

  10. Dave

    To both Tim and cpradio, not sure what you’re on about here. I’ve had Google Authenticator set-up for some time now on my SSH server, WinSCP seems to work fine for me. Just make sure you’re running the latest version. Failing that, if you use SSH keys, two factor authentication is bypassed. WinSCP supports both Challenge Response and SSH keys.

  11. Bobby

    A million thank-yous!

  12. Nanakos chrysostomos

    I think that the yubikey or softyubikey for gnome, kde, cinnamon and mate is a better solution. Please check them out. I solves all previous problems. Cheers!

  13. aselvan

    @Toto: if you don’t have your private key on a device you are trying to login from, and you have setup the two-factor authentication with google authenticator, sshd will do two factor authentication. You don’t have to do anything, it will work exactly like you want. I have the same need and it works correctly i.e. uses key based login where private keys are available and two factor when keys are not available.

  14. Toto

    @aselvan : Thanks for your input, I actually tried that and it worked like a charm.

    For those of you who are interested to do so, I recommend you to set the shared key as preferred login method.

  15. Andrew

    Hi

    I have two types of authentication on my server.
    One is used just for my account to administer the server (not root) that has public/private keys
    The second has just user and pass, but has shell access restricted, are locked in to using a shared ftp folder and are restricted from doing pretty much anything but ftp and http forwarding. (via a group policy in sshd)

    I can see that above toto had asked if it was possible to use either public keys or authenticator if not available. is it possible to have the authenticator AND the pub/private key for the standard account, while leaving the accounts in my restricted group untouched.

    its probably overkill, with what feels like 3 step protection, but i am curious if it is possible.

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