Secure your logins with two-factor authentication
Secure Remote Access
There's no dearth of tricks to harden an SSH connection. For example, you can change its default port and restrict access by IP addresses. The most commonly suggested mechanism is to switch to key-based authentication instead of passwords, but this still presents a single point of failure. To make it more difficult for an attacker to compromise an SSH connection, you can also enable two-factor authentication for remote logins (Figure 6).
The procedure for adding OTP-based logins to an SSH connection is similar to the one for local logins. The only difference is that you need to install and setup the Google Authenticator PAM module on the remote SSH machine instead of the local machine. Begin by SSHing into the remote server as the user you wish to protect such as ssh bodhi@remote-machine
.
Now open another SSH connection to the machine and use this second connection to make changes to SSH's configuration. When you restart the SSH service on the remote machine, it won't close open connections. So the first connection acts as a fail-safe and ensures you won't lock yourself out in case of any accidental misconfiguration.
After you've logged into the remote machine, follow the steps described earlier to first install the Google Authenticator PAM module and then use the helper script to generate the 16 digit key for the remote user. Then add the details of this remote SSH user in the Google Authenticator mobile app.
Next, you can edit SSH's PAM configuration file on this remote machine with nano /etc/pam.d/sshd
. Scroll down to the bottom and add the following lines to the file:
# Secure SSH with OTPs auth required pam_google_authenticator.so nullok
Just like earlier, nullok
tells PAM that this authentication method is optional. This allows users without a Google Authenticator key to still log in using their SSH password. Remember that this is just a fail safe to prevent you from being locked out in case something goes wrong with the setup process. However, once you've tested it successfully, generate a key for all SSH users and delete nullok
from the end of this line to make logins via OTP mandatory.
After editing SSH's PAM file, it's time to configure SSH to support this kind of authentication. Open the SSH configuration file for editing with:
nano /etc/ssh/sshd_config
Look for the line that reads ChallengeResponseAuthentication
and change its value from no to yes. If the line doesn't exist, make sure you add it manually. Save and close the file, and then restart SSH to reload the configuration files with:
systemctl restart sshd.service
When you now re-establish the SSH connection, in addition to the remote user's password, you'll also be prompted for the Google Authenticator code.
Two-factor authentication is relatively straightforward to roll out but it takes some getting used to. However, you can rest easy knowing that you've increased the security of your computer by making it virtually impossible for crackers to brute force their way into your home directory.
Infos
- Google Authenticator: https://en.wikipedia.org/wiki/Google_Authenticator
- FreeOTP app: https://freeotp.github.io/
- The TOTP protocol: https://en.wikipedia.org/wiki/Time-based_One-time_Password_Algorithm
« Previous 1 2
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Endless OS 6 has Arrived
After more than a year since the last update, the latest release of Endless OS is now available for general usage.
-
Fedora Asahi 40 Remix Available for Macs with Apple Silicon
If you've been anticipating KDE's Plasma 6 for your Apple Silicon-powered Mac, then you're in luck.
-
Red Hat Adds New Deployment Option for Enterprise Linux Platforms
Red Hat has re-imagined enterprise Linux for an AI future with Image Mode.
-
OSJH and LPI Release 2024 Open Source Pros Job Survey Results
See what open source professionals look for in a new role.
-
Proton 9.0-1 Released to Improve Gaming with Steam
The latest release of Proton 9 adds several improvements and fixes an issue that has been problematic for Linux users.
-
So Long Neofetch and Thanks for the Info
Today is a day that every Linux user who enjoys bragging about their system(s) will mourn, as Neofetch has come to an end.
-
Ubuntu 24.04 Comes with a “Flaw"
If you're thinking you might want to upgrade from your current Ubuntu release to the latest, there's something you might want to consider before doing so.
-
Canonical Releases Ubuntu 24.04
After a brief pause because of the XZ vulnerability, Ubuntu 24.04 is now available for install.
-
Linux Servers Targeted by Akira Ransomware
A group of bad actors who have already extorted $42 million have their sights set on the Linux platform.
-
TUXEDO Computers Unveils Linux Laptop Featuring AMD Ryzen CPU
This latest release is the first laptop to include the new CPU from Ryzen and Linux preinstalled.