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SSH With Two-Factor Authentication, Google Authenticator

May 5, 2015, Written by 0 comment

To protect your SSH server with an two-factor authentication, you can use the Google Authenticator PAM module. Each time you are connecting to your server via SSH with Google Authenticator PAM installed  you have to enter the code from your smartphone with Google Authenticator installed.

On Red Hat, CentOS and Fedora systems install the ‘pam-devel‘ package.

# yum install pam-devel make gcc-c++

Install Wget if you have not installed one yet

TOTP (timebased one-time-password) security tokens are time sensitive. Hence, make sure that your system has ntpd running, and is configured to start the service at boot

# service ntpd start
# chkconfig  ntpd on

Download and extract Google authenticator module under Home directory (assume you are already logged in home directory of root).

# cd /tmp
 # wget http://google-authenticator.googlecode.com/files/libpam-google-authenticator-1.0-source.tar.bz2
 # bunzip2 libpam-google-authenticator-1.0-source.tar.bz2
 # tar xf libpam-google-authenticator-1.0-source.tar
 # cd libpam-google-authenticator-1.0
 # make
 # make install
 cp pam_google_authenticator.so /lib64/security
 cp google-authenticator /usr/local/bin

Before configuring SSH, first set up Google Authenticator. Run “google-authenticator” as the user you wish to log in with via SSH. You will be prompted with a few questions.

Do you want me to update your "~/.google_authenticator" file (y/n) y
 
https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/[email protected]%3Fsecret%3DABCD12E3FGHIJKLMN
Your new secret key is: ABCD12E3FGHIJKLMN
Your verification code is 98765432
Your emergency scratch codes are:
  01444567
  32123245
  33330123
  23328901
  54444489
 
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) 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) y

This requires all users to use Google Authenticator for SSH authentication. To only require those users with Google Authenticator configured for their account (the ~/.google_authenticator file exists), then instead enter “auth required pam_google_authenticator.so nullok“.

The order in which you place items in this file matters. Given this configuration, you will first be prompted for your Google Authenticator verification code, then for your system account password when you SSH into the system.

Modify /etc/ssh/sshd_config. Verify these settings:

PasswordAuthentication yes
ChallengeResponseAuthentication yes
UsePAM yes
# service sshd restart

When you SSH into the system as a user configured for Google Authenticator, you will have to enter the verification code that is displayed in you Google Authenticator app, and then by your system password at the next prompt:

login as: root
Verification code: 01234567
Password: *******
martin

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