"SSH keys (and certs) are just another secret to be lost, stolen, or shared"
Duo Unix - Two-Factor Authentication for SSH (login_duo)


      


      

      

      
        Duo can be easily added to any Unix system to protect remote (SSH) logins with the addition of a simple login_duo module. It has been tested on Linux (RedHat, Fedora, CentOS, Debian, Ubuntu, Amazon Linux), BSD (FreeBSD, NetBSD, OpenBSD), Solaris, and AIX. The code is open-source and available on GitHub.
      


      Duo Network Gateway provides SSH access to internal hosts without a VPN or jump host with trusted endpoint detection and session awareness. Learn more.
Overview
Duo can be enabled on any Unix system with the addition of a simple login_duo utility.
We recommend deploying our pam_duo module with Pluggable Authentication Modules (PAM) support instead of login_duo in most scenarios for the most secure and customizable experience, especially if port forwarding and tunneling is used in your environment.


  Connectivity Requirements
  This application communicates with Duo's service on SSL TCP port 443. 
  Firewall configurations that restrict outbound access to Duo's service with rules using destination IP addresses or IP address ranges aren't recommended, since these may change over time to maintain our service's high availability. If your organization requires IP-based rules, please review Duo Knowledge Base article 1337.
  Effective June 30, 2023, Duo no longer supports TLS 1.0 or 1.1 connections or insecure TLS/SSL cipher suites. See Duo Knowledge Base article 7546 for additional guidance.
  

FIPS Support
Duo Unix is FIPS-compliant as of version 1.10.4 when run on any machine that has an operating system-wide FIPS mode (like CentOS/RedHat 7, Ubuntu 16.04, etc.). No additional flags or options are required.
Walkthrough Video




First Steps

Before moving on to the deployment steps, it's a good idea to familiarize yourself with Duo administration concepts and features like options for applications, available methods for enrolling Duo users, and Duo policy settings and how to apply them. See all Duo Administrator documentation.

Then you'll need to:


   Sign up for a Duo account.
  Log in to the Duo Admin Panel and navigate to Applications → Protect an Application.
  Locate UNIX Application in the applications list. Click Protect to get your integration key, secret key, and API hostname. You'll need this information to complete your setup. See Protecting Applications for more information about protecting applications in Duo and additional application options.
  If you plan to build Duo Unix from source, download the latest version of the duo_unix tarball (view checksum). From the command line you can use curl or wget to download the file, like $ wget --content-disposition https://dl.duosecurity.com/duo_unix-latest.tar.gz.



Treat your secret key like a password
The security of your Duo application is tied to the security of your secret key (skey). Secure it as you would any sensitive credential. Don't share it with unauthorized individuals or email it to anyone under any circumstances!


Build and Install from Source

Install login_duo Prerequisites
OpenSSL development headers and libraries are required for login_duo. When compiling on SUSE/SLES, the zlib package is also necessary.

You also need a compiler like gcc installed on your system to build Duo Unix.
Install login_duo
Once the required dependencies are built and installed, build and install duo_unix.


Extract the downloaded tarball for duo_unix and change to the extracted directory (note your actual extracted directory name reflects the actual version downloaded; the example syntax below references version 2.0.4). View checksums for Duo downloads here.
 $ tar zxf duo_unix-latest.tar.gz
 $ cd duo_unix-2.0.4


Build and install duo_unix.
 $ ./configure --prefix=/usr && make && sudo make install
For advanced build options, see the README file in the source tarball.


Once installed, proceed to Duo configuration.


Install from Linux Packages

To more easily install and maintain Duo Unix deployments, we've built Linux packages for some popular Linux distributions. Duo tests these packages against the specific listed versions of their respective distributions.
Please test all packages thoroughly prior to deploying them into your environment to ensure a great experience. Note that we exclusively provide Duo Unix install packages at pkg.duosecurity.com and cannot guarantee packages obtained from other sources.
When installing Duo Unix from packages, there is no need to also install the build-from-source prerequisites on the target systems.
To download the packages, you'll need Duo's GPG key. The GPG key verifies the Duo Unix package for currently supported OS distributions and versions.
We updated the Duo GPG key for packages on supported distros on May 18, 2020. If you installed Duo Unix from packages before May 2020, be sure to update your previously imported GPG key using command for your distro the before the next time you install or upgrade Duo Unix.
The current Duo GPG key expires in May 2030.
OS distributions identified as no longer supported in the distro-specific packages sections use a previous GPG key. We won't replace or update the GPG key on these EOL versions when it expires, and urge you to update to a supported OS.

  
        CentOS Stream
        CentOS Linux
        Fedora        
        Ubuntu
        Red Hat Enterprise Linux
        Debian
        Amazon Linux
  

CentOS Stream
Tested against 9 64-bit
IMPORTANT:

CentOS Stream 8 reached end of life on May 31, 2024. Duo Unix 2.0.3 was the last release with CentOS Stream 8 support.

Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/CentOSStream/$releasever/$basearch
enabled=1
gpgcheck=1
Execute the following shell commands for CentOS Stream 8 and later:
# rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
# yum install duo_unix



CentOS Linux
Consider migrating to CentOS Stream.
IMPORTANT:


CentOS 5 reached end of life on March 31, 2017. Duo Unix 1.10.5 was the last release with CentOS 5 support.


CentOS 6 reached end of life on November 30, 2020. Duo Unix 1.11.4 was the last release with CentOS 6 support.


CentOS 7 reached end of life on June 30, 2024. Duo Unix 2.0.3 was the last release with CentOS 7 support.


CentOS 8 reached end of life on December 31, 2021. Duo Unix 1.12.0 was the last release with CentOS 8 support.


Please review the Duo End of Sale, Last Date of Support, and End of Life Policy.
Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/CentOS/$releasever/$basearch
enabled=1
gpgcheck=1
Execute the following shell commands for CentOS 7 and later:
# rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
# yum install duo_unix



Fedora
Tested against Fedora 39 and 40 64-bit
IMPORTANT:


Fedora 34 reached end of life on June 7, 2022. Duo Unix 2.0.2 was the last release with Fedora 34 support.


Fedora 37 reached end of life on November 14, 2023. Duo Unix 2.0.2 was the last release with Fedora 37 support.


Fedora 38 reached end of life on May 31, 2024. Duo Unix 2.0.3 was the last release with Fedora 38 support.


Please review the Duo End of Sale, Last Date of Support, and End of Life Policy.
Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/Fedora/$releasever/$basearch
enabled=1
gpgcheck=1
Execute the following shell commands for Fedora 34 and later:
# rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
# yum install duo_unix



Ubuntu
Tested against 24.04 noble 64-bit, 22.04 jammy 64-bit, and 20.04 focal 64-bit
IMPORTANT:


Ubuntu 12.04 reached end of life in April 2017. Duo Unix 1.11.3 was the last release with Ubuntu 12.04 support.


Ubuntu 14.04 reached end of life in April 2019. Duo Unix 1.12.0 was the last release with Ubuntu 14.04 support.


Ubuntu 16.04 reached end of life in April 2021. Duo Unix 1.12.0 was the last release with Ubuntu 16.04 support.


Ubuntu 18.04 reached end of life in May 2023. Duo Unix 2.0.1 was the last release with Ubuntu 18.04 support.


Please review the Duo End of Sale, Last Date of Support, and End of Life Policy.
Create /etc/apt/sources.list.d/duosecurity.list with the following contents:
deb [arch=amd64] https://pkg.duosecurity.com/Ubuntu bionic main
or
deb [arch=amd64] https://pkg.duosecurity.com/Ubuntu focal main
or
deb [arch=amd64] https://pkg.duosecurity.com/Ubuntu jammy main
or
deb [arch=amd64] https://pkg.duosecurity.com/Ubuntu noble main
Execute the following shell commands for Ubuntu 18.04 and 20.04:
# curl -s https://duo.com/DUO-GPG-PUBLIC-KEY.asc | sudo apt-key add -
# apt-get update && apt-get install duo-unix

Execute the following shell commands for Ubuntu 22.04 and later:
# curl -s https://duo.com/DUO-GPG-PUBLIC-KEY.asc | sudo gpg --dearmor -o  /etc/apt/trusted.gpg.d/duo.gpg
# apt-get update && apt-get install duo-unix


Warning: Upgrading Ubuntu from 20.04 to 22.04 can break Duo Unix
Upgrading from 20.04 to 22.04 with do-release-upgrade requires use of the --allow-third-party flag. Without this flag do-release-upgrade disables third-party libraries, including those used by Duo Unix. This could prevent logins after upgrading.
Please see the Duo Knowledge Base article How do I upgrade Ubuntu from version 20.04 to 22.04 with Duo Unix installed? for more information, including recovery steps if you have already performed the upgrade without allowing third-party libraries.



Red Hat Enterprise Linux
Tested against 8.0 64-bit and 7.0 64-bit.
IMPORTANT:


Red Hat 5 reached full end of life on November 30, 2020. Duo Unix 1.10.5 was the last release with Red Hat 5 support.


Red Hat 6 entered extended life on November 30, 2020. Duo Unix 1.11.4 was the last release with Red Hat 6 support.


Please review the Duo End of Sale, Last Date of Support, and End of Life Policy.
Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/RedHat/$releasever/$basearch
enabled=1
gpgcheck=1
Execute the following shell commands for Red Hat 6 and later:
# rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
# yum install duo_unix



Debian
Tested against 12.0 32/64-bit and 11.0 32/64-bit.
IMPORTANT:


Debian 6 reached end of life on May 31, 2015. Duo Unix 1.9.18 was the last release with Debian 6 support.


Debian 7 reached end of life on April 26, 2016. Duo Unix 1.11.1 was the last release with Debian 7 support.


Debian 8 reached end of life on June 30, 2020. Duo Unix 1.11.4 was the last release with Debian 8 support.


Debian 9 reached end of life on June 30, 2022. Duo Unix 2.0.1 was the last release with Debian 9 support.


Debian 10 reached end of life on June 30, 2024. Duo Unix 2.0.3 was the last release with Debian 10 support.


Please review the Duo End of Sale, Last Date of Support, and End of Life Policy.
Create /etc/apt/sources.list.d/duosecurity.list with the following contents:
deb https://pkg.duosecurity.com/Debian stretch main
or
deb https://pkg.duosecurity.com/Debian buster main
or
deb https://pkg.duosecurity.com/Debian bullseye main
or
deb https://pkg.duosecurity.com/Debian bookworm main
Execute the following shell commands for Debian 9 and later:
# curl -s https://duo.com/DUO-GPG-PUBLIC-KEY.asc | sudo apt-key add -
# apt-get update && apt-get install duo-unix



Amazon Linux
Tested against Amazon Linux 2023.
Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/AmazonLinux/$releasever/$basearch
enabled=1
gpgcheck=1
Execute the following shell commands for Amazon Linux:
# rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
# yum install duo_unix



Once the Duo Unix package is installed, proceed to Duo configuration.
Duo Configuration
The login_duo.conf configuration file uses the INI format.
Once duo_unix is installed, edit login_duo.conf
(in /etc/duo or /etc/security) to add the integration key, secret key, and API hostname from your Duo Unix application.
You may also add optional Duo configuration options to login_duo.conf. See the table below for all available settings.
[duo]
; Duo integration key
ikey = INTEGRATION_KEY
; Duo secret key
skey = SECRET_KEY
; Duo API hostname
host = API_HOSTNAME
Duo Configuration Options

Example configuration file with additional options:
[duo]
ikey=DIXXXXXXXXXXXXXXXXXX
skey=X1hXztPX1rb1X71x1wXkpnmXXvqXXXqqj1XoXbbXu
host=api-xxxxxxxx.duosecurity.com
pushinfo=yes
autopush=yes

For more information, see the man page for login_duo.
Test Your Setup
Verify that Duo Unix works properly, and then enable it for logins.
Test login_duo
As a regular user, test login_duo manually by running
$ /usr/sbin/login_duo
On some systems, you may instead have to run /usr/local/sbin/login_duo.
If everything is set up correctly and your username doesn't exist in Duo, you'll be given an enrollment link:

Visit the URL, enroll your phone, and then try login_duo again, this time adding a command to run after authentication is complete:
$ /usr/sbin/login_duo echo 'YOU ROCK!'
You should see something like this:

If you are having trouble with these steps, or if you aren't getting a Duo login prompt, try running login_duo with the -d flag to enable debug output.
Enable login_duo
To protect remote access via SSH, use login_duo.
To enable two-factor authentication for any SSH login method (password, pubkey, etc.) for any user, edit your sshd_config (usually
in /etc or /etc/ssh) to add the following line:

This feature is only available with OpenSSH 4.4+.

ForceCommand /usr/sbin/login_duo
This ForceCommand directive instructs sshd to run login_duo (to perform two-factor authentication) before any other requested commands. However, according to the sshd documentation: "The command is invoked by using the user's login shell with the -c option." This means that shell rc files (e.g. .bashrc, .cshrc, etc.) execute before login_duo; if users can edit these files, they may be able to disable Duo authentication for their own accounts. Keep in mind that ForceCommand also disables command=. Mitigate these issues by deploying pam_duo instead of login_duo.
We strongly recommend that you disable PermitTunnel and AllowTcpForwarding in your sshd_config when using login_duo to protect SSH logins. Since OpenSSH sets up port forwarding and tunneling before Duo's two-factor challenge, an attacker may be able to access internal services via port forwarding before completing secondary authentication. Adding the following lines to your sshd_config will prevent this scenario:
PermitTunnel no
AllowTcpForwarding no
You can also optionally limit two-factor authentication to a subset of users whose primary or supplementary group is specified in
login_duo.conf. For example:
group = wheel
If you'd like to enable Duo only for specific accounts using SSH pubkeys, use the command option in those users' authorized_keys instead. For example, to verify each admin authorized to log into a shared root account, your ~/.ssh/authorized_keys might look something like this:
command="/usr/sbin/login_duo -f user1" ssh-dss FRP...FD== user1@company
command="/usr/sbin/login_duo -f user2" ssh-dss YUX...IO== user2@company
Now restart the SSH service.
This also works for user-local installations (e.g. in $HOME/bin) without root access — just specify the location of login_duo.conf with the -c flag.
Troubleshooting

Need some help? Take a look at the Duo UNIX  Frequently Asked Questions (FAQ) page or try searching our Duo UNIX Knowledge Base articles or Community discussions. For further assistance, contact Support.

If you open a support case with Duo, be sure to use the Duo Unix Support Tool to create a tarball you can send to the support engineer to aid with troubleshooting.
Network Diagram

SSH connection initiated
Primary authentication
Duo Unix connection established to Duo Security over TCP port 443
Secondary authentication via Duo Security’s service
Duo Unix receives authentication response
SSH session logged in




    