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Linux SSH authentication with Google

Today I decided to play around with the Google two factor authentication method on one of my Linux VMs hosted in Azure.
What this means?
Well, my idea was to have a VM that I ususaly login with my SSH key automatically, to prompt me for a code that changes every 30 seconds.
Google Authenticator offers just that.
So, what do we need?

  • a phone running IOS, Android or Windows (I used my Android phone);
  • an Azure Account – you can get one for free here: https://azure.microsoft.com/en-us/free/ (Disclaimer: There’s no refferer included in the link.). You can use, however, your own, locally installed, Linux box or VM, on whichever platform you like or, if you like working in cloud, you can go for whichever cloud provider you like. I just happened to use Azure, for this demo.
  • a linux box (phisical, virtual) to which you have SSH access and “sudo” rights.

So, just to be sure I won’t lock myself out from a production VM, I started by creating a new VM in Azure, using the beautiful and simple Az Cli 2.0

You can obtain the same result by just “pointing & clicking” in the Azure portal following this tutorial: https://docs.microsoft.com/en-us/azure/virtual-machines/linux/quick-create-portal

The commands in Az Cli to create my VM was:

# creating the resource group 1st
$ az group create --name myResourceGroup --location westeurope
# creating the VM 
$ az vm create --resource-group myResourceGroup --location westeurope --name testVM --image UbuntuLTS --admin-username testuser --generate-ssh-keys

The result will look like (I obfuscated some confidential details, but the essential is there):

AZ Cli create resource group and Linux VM

AZ Cli create resource group and Linux VM

I used a Ubuntu 16.04 but this should work on most Debian based distribution, with small changes. I’ll update the instructions, in time, for other distributions too.

Now, as soon the VM is up & running, you should be able to connect from the same box you ran the AZ Cli commands:

$ ssh testuser@testVM_IpAddressHere

Remember, we created our VM with the testuser and the option to connect via SSH-key that we generated during VM creation.

The SSH key is now saved on the Linux Box we ran the AZ Cli script from, under our user /home/user/.ssh/id_rsa (the private key) and /home/user/.ssh/id_rsa.pub (the public key). If you plan to use other boxes to connect to this VM, make sure you have a copy of the keys on the box you need.

Now, from the account that needs to authenticate (e.g. testuser):

# Install the google authenticator
testuser@testVM:~$ sudo apt-get install libpam-google-authenticator
Reading package lists... Done
Building dependency tree
Reading state information... Done
libpam-google-authenticator is already the newest version (20130529-2).
0 upgraded, 0 newly installed, 0 to remove and 12 not upgraded.
#modify /etc/pam.d/sshd and comment out "@include common-auth" and add on the next line "auth required pam_google_authenticator.so"
testuser@testVM:~$ vi /etc/pam.d/sshd
# Modify /etc/ssh/sshd_config and change ChallengeResponseAuthentication to yes, PasswordAuthentication to no and add, at the end of the file, AuthenticationMethods publickey,keyboard-interactive
testuser@testVM:~$ vi /etc/ssh/sshd_config
testuser@testVM:~$ grep 'common-auth\|google' /etc/pam.d/sshd
#@include common-auth
auth required pam_google_authenticator.so
testuser@testVM:~$ grep 'Challenge\|Password\|AuthenticationMethods' /etc/ssh/sshd_config
ChallengeResponseAuthentication yes
PasswordAuthentication no
AuthenticationMethods publickey,keyboard-interactive
testuser@testVM:~$ 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/testuser@testVM%3Fsecret%3ABCO4WERTP3TZ34L

Your new secret key is: ABCO4WERTP3TZ34L
Your verification code is 423457
Your emergency scratch codes are:
12345675
22345674
32345673
42345672
52345671

Do you want me to update your "/home/testuser/.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
testuser@testVM:~$ sudo service ssh restart

 

google authentiator

Google Authenticator

Now, on your Android phone go to the Google Play and look for Google Authenticator and install it.

 

Open it on your phone and create a new account (e.g. testVM) and add the code generated above (e.g. ABCO4WERTP3TZ34L). This will generate you a new code every 30 seconds.

Connect from a new console to the testVM:

marin@Azure:~$ ssh testuser@testVM_IPAddressHere
Authenticated with partial success.
# Type the code generated on your phone by Google Authenticator
Verification code:
Welcome to Ubuntu 16.04.3 LTS (GNU/Linux 4.11.0-1016-azure x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

  Get cloud support with Ubuntu Advantage Cloud Guest:
    http://www.ubuntu.com/business/services/cloud

12 packages can be updated.
5 updates are security updates.


Last login: Fri Dec 22 14:49:50 2017 from 192.*.*.*
testuser@testVM:~$

Everything looks fine 🙂

Enjoy!

Disclaimer:

I’m not responsible in any way if you lock yourself out from your VM by following this tutorial. Everything you do based on this tutorial is solely your responsibility!

However, if it happens, in Azure you can follow this tutorial to reset the SSH access to your VM.

Marin Nedea
Marin Nedea
I'm passionate about open source software and technologies. In my spare time I build simple and functional websites from scratch, using PHP+HTML5+CSS3+MySQL and when I'm bored, I write simple PHP_CLI or bash scripts to play around on my Linux machine.

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