In this article, I will show you how to tighten your SSH server with a simple two factor authentication by using Google Authenticator. Google Authenticator helps you to secure the SSH access to the server by a two-factor authentication

Step 1:

First, I would recommend checking whether the NTP (Network Time Protocol) is running to ensure the server’s time is synchronized as Google Authenticator uses time-based codes/tokens that will fail if server time is not correct. So, check the NTP status. If NTP service is already running in the server, go to google-authenticator installation in . If not, follow the instruction in the step given below.Check whether the NTPD service is running:

pgrep ntpd 3102 

If nothing shows up, NTP is not running. So, need to install it:

yum install ntp

Ensure that the NTP is configured to start automatically during the system startup.

chkconfig ntpd on

Sync the time:


Start the NTP service

service ntpd start
pgrep ntpd

Step 2:

Install open source Google Authenticator PAM module.If ‘yum install google-authenticator’ says there is no package available try to install the EPEL repo for Centos.

 cd /tmp
 yum install wget -y
 wget # rpm -Uvh remi-release-6*.rpm epel-release-6*.rpm

Then verify the EPEL repo exists:

 yum repolist 
epel            Extra Packages for Enterprise Linux 6 – x86_64

Finally, install Google Authenticator:

 yum --enablerepo=epel install google-authenticator

Step 3:

vim /etc/pam.d/sshd

uncomment #auth substack password-auth .add the auth required pam_google_authenticator.So # auth substack password-auth account required pam_nologin. So The Password-auth rule is disabled because you require a key plus a Google Authenticator code, not a password. Now the file looks like this:

Step 4:

The next step is to update the sshd configuration file in /etc/ssh/sshd_config. Change the ChallengeResponseAuthentication line to `yes’.

Then you have to add the following line to use your public key and two-factor authentication. ‘ AuthenticationMethods publickey,keyboard-interactive ‘

 vim /etc/ssh/sshd_config

Step 5:

Next, you should configure the users who should be demanded a two-factor authentication.


Step 6:

Once you have created a Secret key, restart sshd service to apply your changes.

service sshd restart

Step 7:

Smartphone App Installation. Then you have to install Google authenticator app in your mobile (Android, Iphone, Windows, Blackberry). You can enter the provided key and then add any account name, enter the verification code and then click add or just scanning the QR CODE to add account.

Step 8:

Finally test the service by connecting with SSH to the server to see if it will require verification code. Of course it works.

If this article help, just share it to your friends.

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