SSH – Two Factor Authentication

Many people are using Google Authenticator to secure their google apps such as gmail. However what if you wanted to be able to utilize two factor authentication (something you have, something you know) for your SSH logins? What if you want to protect yourself against accidently using weak passwords, which can lead to a successful brute force attack?

On both RedHat and Debian based systems, Google Authenticator’s one time passwords are pretty simple to implement. For the purposes of this guide, I’ll be using CentOS 6 and Ubuntu 12.04.

It should be noted that by using this guide, ALL your users (including root) will be required to use the google authenticator to SSH in unless you have SSH keys already in place. Please check with your administration teams before setting this up to ensure you don’t accidently disable their access, or lock yourself out from SSH!

Procedure

1. Install the module

# RedHat 6 based systems
rpm -ivh http://linux.mirrors.es.net/fedora-epel/6/x86_64/epel-release-6-7.noarch.rpm
yum install google-authenticator

# Debian based systems
aptitude install libpam-google-authenticator

2. Now update the /etc/pam.d/sshd file and add the following at the end of the ‘auth’ section:

auth required pam_google_authenticator.so

3. Then update your /etc/ssh/sshd_config

# Change
ChallengeResponseAuthentication no

# To
ChallengeResponseAuthentication yes

4. Restart sshd

# Redhat:  
service sshd restart

# Ubuntu:  
service ssh restart

5. Now, setup keys for your user

google-authenticator

It will ask you to update your ~/.google_authenticator file, answer yes to this question, and whatever you would like to use for the next three. Once complete, the following will be present to you:

    New Secret Key
    Verification Code
    Emergency Scratch Codes

You will use the new secret key for adding the account to your phone’s google authenticator app. The emergency scratch codes should be copied down and stored somewhere secure. They can be used if you ever lose your iphone, or otherwise need to get into your account without your phone’s google authenticator app.

Now when you log into your server using your user account, it will prompt you for your google auth token, followed by your normal password for the server. Any accounts that don’t have the this setup will not be allowed to log in.

Final thoughts

Remember, two factor authentication is only one part of a defense in depth strategy. No security management system is perfect, but each layer you add will help increase your solutions security footprint.

Using PAM to enforce access based on time

Sometimes there is a need to restrict user access based on time. This could be access to one particular service, or all PAM enabled services. A common example is to only allow access for the user ‘bob’ monday through friday between 9:00AM – 6:00PM. This can be enforced by utilizing the pam_time module.

The pam_time module is an account module type. No arguments are passed directly to the module, but instead all configuration takes place within /etc/security/time.conf.

The time.conf operates based on rules, and each rule uses the following syntax:

services;ttys;users;times

Example Rules
Restrict SSHD access for bob to weekdays between 9:00AM – 7:00PM

sshd;*;bob;Wk0900-1900

Restrict ALL access for bob to weekdays between 9:00AM – 5:00PM

*;*;bob;Wk0900-1700

Restrict ALL access for ALL users except root to weekdays between 9:00AM – 5:00PM

sshd;*;bob;Wk0900-1700

Restrict SSH access for ALL users except bob and jane to weekdays between 9:00AM – 5:00PM

sshd;*;!bob|!jane;Wk0900-1700

To only allow bob to access SSH on Tuesdays between 3:23PM and 4:24PM:

sshd;*;bob;Tu1523-1624

Below is all the available abbreviates for the days of the week:

Mo : Monday Fr : Friday Wd : Sa/Su
Tu : Tuesday Sa : Saturday wk : Mo/Tu/We/Th/Fr
We : Wenesday Su : Sunday
Th : Thursday Al : All Days

Installation And Configuration
In our example, I am going be setting this up on a CentOS 5.x server. For the restricted user, the following variables will be used:

username: bob
allowed access times: 9:00AM - 6:00PM
restricted services: SSHD

First, add the user and time restriction to /etc/security/time.conf:

sshd;*;bob;Wk0900-1800

Now, update the pam module for login and sshd. You are┬áincluding ‘account required pam_time.so‘. But┬áI’ll post entire file for reference

cat /etc/pam.d/sshd
#%PAM-1.0
auth required pam_sepermit.so
auth include password-auth
account required pam_time.so
account required pam_nologin.so
account include password-auth
password include password-auth
# pam_selinux.so close should be the first session rule
session required pam_selinux.so close
session required pam_loginuid.so
# pam_selinux.so open should only be followed by sessions to be executed in the user context
session required pam_selinux.so open env_params
session optional pam_keyinit.so force revoke
session include password-auth
cat /etc/pam.d/system-auth
#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth required pam_env.so
auth sufficient pam_unix.so nullok try_first_pass
auth requisite pam_succeed_if.so uid >= 500 quiet
auth required pam_deny.so
account required pam_time.so
account required pam_unix.so
account sufficient pam_localuser.so
account sufficient pam_succeed_if.so uid < 500 quiet
account required pam_permit.so
password requisite pam_cracklib.so try_first_pass retry=3 type=
password sufficient pam_unix.so md5 shadow nullok try_first_pass use_authtok
password required pam_deny.so
session optional pam_keyinit.so revoke
session required pam_limits.so
session [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid
session required pam_unix.so

And finally, restart SSH

service sshd restart