Two-Step Authentication for SSH on CentOS 6 Using Google Authenticator

Google Authenticator implements TOTP (timebased one-time-password) security tokens from RFC6238 via the Google mobile app Google Authenticator. The Authenticator provides a six digit one-time password users must provide in addition to their username and password to login, sometimes branded “two-step authentication”. Here, we install and configure a pluggable authentication module (PAM) which allows login using one-time passcodes.

Download and Install

At the time of this writing, only an old version of libpam-google-authenticator is available in the EPEL package repository. Hence, we are going to compile it from source. First, install prerequisites:

# yum install make gcc pam-devel

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
Starting ntpd:                                             [  OK  ]
# chkconfig  ntpd on

Then download and install libpam-google-authenticator from source:

# cd /tmp
# wget
# 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
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o google-authenticator.o google-authenticator.c
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o base32.o base32.c
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o hmac.o hmac.c
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o sha1.o sha1.c
gcc -g   -o google-authenticator google-authenticator.o base32.o hmac.o sha1.o  -ldl
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o pam_google_authenticator.o pam_google_authenticator.c
gcc -shared -g   -o pam_google_authenticator.o base32.o hmac.o sha1.o -lpam
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o demo.o demo.c
gcc -DDEMO --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o pam_google_authenticator_demo.o pam_google_authenticator.c
gcc -g   -rdynamic -o demo demo.o pam_google_authenticator_demo.o base32.o hmac.o sha1.o  -ldl
gcc -DTESTING --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden        \
              -o pam_google_authenticator_testing.o pam_google_authenticator.c
gcc -shared -g   -o pam_google_authenticator_testing.o base32.o hmac.o sha1.o -lpam
gcc --std=gnu99 -Wall -O2 -g -fPIC -c  -fvisibility=hidden  -o pam_google_authenticator_unittest.o pam_google_authenticator_unittest.c
gcc -g   -rdynamic -o pam_google_authenticator_unittest pam_google_authenticator_unittest.o base32.o hmac.o sha1.o -lc  -ldl
# make install
cp /lib64/security
cp google-authenticator /usr/local/bin

Set Up Google Authenticator

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|0&cht=qr&chl=otpauth://totp/user@server%3Fsecret%3DABCD12E3FGHIJKLMN
Your new secret key is: ABCD12E3FGHIJKLMN
Your verification code is 98765432
Your emergency scratch codes are:
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

These settings are stored in the user’s ~/.google_authenticator file.
Copy and paste the URL into your browser and scan the QR code that is displayed with the app Google Authenticator on your mobile device. If you can’t scan the QR code, then you can enter the information manually with the given secret key and verification code. A new verification code should be displayed every 30 seconds.
Emergency one-time use verification codes are also given for you to write down in a secure place in case you were to not have your mobile device with you.

Configure PAM

Have PAM require Google Authenticator for SSH authentication. Modify /etc/pam.d/sshd and add the line “auth required” at the top.

auth       required
auth       required
auth       include      password-auth
account    required
account    include      password-auth
password   include      password-auth
# close should be the first session rule
session    required close
session    required
# open should only be followed by sessions to be executed in the user context
session    required open env_params
session    optional force revoke
session    include      password-auth

This will require 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 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.

Configure the SSH Service

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

PasswordAuthentication yes
ChallengeResponseAuthentication yes
UsePAM yes

Restart the SSH service:

# 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: *******

If you have any problems, look in the /var/log/secure system log file.
If you have SELinux enabled, you may not be able to login, and get this error in /var/log/secure:

Jan  3 23:42:50 hostname sshd(pam_google_authenticator)[1654]: Failed to update secret file "/home/username/.google_authenticator"
Jan  3 23:42:50 hostname  sshd[1652]: error: PAM: Cannot make/remove an entry for the specified session for username from

This is probably due /home/username/.google_authenticator not having an appropriate Type Enforcement (TE):

# ls -Z /home/username/.google_authenticator
-r--------. username username unconfined_u:object_r:user_home_t:s0 /home/username/.google_authenticator

See here for additional information on this issue.

Skip Google Authenticator Authentication if Logging in from the Local Network

You may trust systems on you local network enough not not require that SSH connections from them use Google Authenticator. If so, modify /etc/pam.d/sshd so that it looks like this:

auth [success=1 default=ignore] accessfile=/etc/security/access-local.conf
auth       required

Then add the file /etc/security/access-local.conf with the contents:

# Google Authenticator can be skipped on local network
+ : ALL :
- : ALL : ALL

This assumes your local network is subnet

My System Configuration

  • CentOS 6.5 x86 64-bit
  • Google Authenticator libpam 1.0


8 thoughts on “Two-Step Authentication for SSH on CentOS 6 Using Google Authenticator”

  1. You’re doing God’s work, son!
    I can’t believe it just…works. Couldn’t figure out how to make it work with SELinux, so I had to disable it.
    Now to make it work with windows auth…

  2. Hi thanks for the tutorial!
    I’m getting an error where it asks for the Verification code, then i type it in….it asks for the password, i type it in…. then i get: Access Denied.
    It also blocks me to access with root or another standard user (It doesn’t ask me for verification code for other users, only password)
    Any thoughts?

  3. Jul 4 18:22:44 localhost sshd(pam_google_authenticator)[3792]: Failed to update secret file “/root/.google_authenticator”
    Jul 4 18:22:49 localhost sshd[3790]: error: PAM: Cannot make/remove an entry for the specified session for root from
    Jul 4 18:22:51 localhost sshd[3791]: Connection closed by
    I am getting these errors from /var/log/secure file. I have disabled SELinux at all but still can not connect via PAM it is continiously asking for password and verification code:
    sh@sh–work:~$ ssh root@
    Verification code:
    Verification code:

  4. You’re a star!
    Works a treat, am even using it just to authenticate external connections only and leaving internal users to use passwords only.
    Many thanks for a good, clear and concise set of instructions.

    1. Works perfectly. I have noticed one issue..Sometimes the verification prompt times out too quick. Is there a way to increase the timeout. Did google but could not find much. Thank you for the great info.

      1. Not working for me. All the time I got
        sshd(pam_google_authenticator)[15672]: Invalid verification code

  5. Hi,

    Thanks for your post. I have webmin installed on my server. When I enable the SSH 2FA, Webmin login stops working. Can you please give me any suggestion on how to fix this?

    Thank you so much.


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