Secure SSH using Google Authenticator
There are several things you can do to secure SSH login. One of them is to use Google Authenticator and create a two step authentication on your CentOS VPS. Google Authenticator gives you an extra layer of security by generating time based one-time passwords (TOTP) on your smartphone that you must enter along with your username and password in order to login to the server via SSH (two step authentication).
In today’s blog article we will explain how to install Google Authenticator from source, configure ans secure SSH with google authenticator.
First of all update your CentOS virtual server
yum -y update
Next, install the ‘pam-devel‘ package which allows you to set authentication policies without having to recompile programs that handle authentication.
yum -y install pam-devel
Make sure that ntpd is installed and running because the TOTP security tokens are time sensitive
yum -y install ntp /etc/init.d/ntpd start chkconfig ntpd on
Download and unpack the Google authenticator package
cd /opt/ wget https://google-authenticator.googlecode.com/files/libpam-google-authenticator-1.0-source.tar.bz2 tar -xvzf libpam-google-authenticator-1.0-source.tar.bz2 cd libpam-google-authenticator-1.0
Compile and install the Google authenticator module
make make install
Now, run the Google authenticator on your server and answer each of the questions
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/root@YOURHOSTNAME%3Fsecret%3DWYD4YCGEE5N4M3LA Your new secret key is: WYD4YCGEE5N4M3LA Your verification code is 188127 Your emergency scratch codes are: 60086389 28918071 88502143 60873576 90892542 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) 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
Open the URL given after answering the first question and scan the QR code using the Google Authenticator application on your smartphone. That’s all. A new verification code will be generated every 30 seconds.
Now you will need to enable Google authenticator for SSH logins. Open PAM’s configuration file
And add the following line at the top
auth required pam_google_authenticator.so
Open the SSH configuration file and ensure that the ‘ChallengeResponseAuthentication‘ line is set to yes
Save the changes and restart the SSH service:
service sshd restart
Now, every time you try to SSH into your server you will be prompted to enter the verification code displayed in your Google Authenticator application.
login as: Verification code: Password:
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