Duo Unix - Two-Factor Authentication for SSH

Duo can be easily added to any Unix system to protect remote (SSH) or local logins. It has been tested on Linux (RedHat, Fedora, CentOS, Debian, Ubuntu, Gentoo), BSD (FreeBSD, NetBSD, OpenBSD, MacOS X), Solaris, HP-UX, and AIX.

Integration Video


Duo can be enabled on any Unix system with the addition of a simple login_duo utility or pam_duo PAM module. The code is open-source and available on GitHub.

First Steps

Before starting:

  1. Sign up for a Duo account
  2. Create a new UNIX Integration to get an integration key, secret key, and API hostname. (See Getting Started for help.)

Connectivity Requirements

This integration communicates with Duo’s service on TCP port 443. Also, we do not recommend locking down your firewall to individual IP addresses, since these may change over time to maintain our service’s high availability.


1. Set up login_duo

OpenSSL development headers and libraries are required for login_duo, so you’ll want to install those first. libpam is also a required dependency for pam_duo. When compiling on SUSE/SLES, the zlib package is also necessary.

Dependency Platform Installation
OpenSSL FreeBSD, NetBSD Installed by default
Debian, Ubuntu apt-get install libssl-dev
Red Hat, Fedora, CentOS yum install openssl-devel
SUSE/SLES zypper install libopenssl-devel
Solaris pkg install openssl
HP-UX, AIX 3rd party packages or source build
libpam FreeBSD, NetBSD, Mac OS X, Solaris, HP-UX, AIX Installed by default
Debian, Ubuntu apt-get install libpam-dev
Red Hat, Fedora, CentOS yum install pam-devel
SUSE/SLES zypper install pam-devel
zlib SUSE/SLES zypper install zlib-devel

Click here to download the latest version of duo_unix (checksum here). Then build and install:

$ tar zxf duo_unix-1.9.10.tar.gz
$ cd duo_unix-1.9.10
$ ./configure --prefix=/usr && make && sudo make install

For advanced build options, see the README file in the source tarball.

Once duo_unix is installed, edit login_duo.conf (in /etc/duo or /etc/security) to add your integration key, secret key, and API hostname:

; Duo integration key
; Duo secret key
; Duo API hostname

The login_duo binary is marked setuid in order to read the protected login_duo.conf configuration file. However, privileges are dropped immediately after, so the privilege escalation attack surface is minimal.

2. 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, you’ll be given an enrollment link and prompted to enroll:

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.

3. 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:

ForceCommand /usr/sbin/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 by UID or group in login_duo.conf. For example:

group = wheel

If you’d like to enable Duo for 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


p>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.

Configuration Options

The login_duo.conf and pam_duo.conf configuration files use the INI format. They can take the following options:

Key Required? Description
ikey Required Your integration key
skey Required Your secret key
host Required Your API hostname (i.e.
group Optional Skip Duo authentication for users not in a specific Unix group
failmode Optional

On service or configuration errors that prevent Duo authentication, fail “safe” (allow access) or “secure” (deny access). The default is “safe”.

pushinfo Optional Include information such as the command to be executed in the Duo Push message. Either “yes” or “no”. The default is “no”.
http_proxy Optional Experimental support for the standard http_proxy environment variable (honored by wget, curl, etc.).
autopush Optional

Either “yes” or “no”. Default is “no”. If “yes”, Duo Unix will automatically send a push login request to the user’s phone, falling back on a phone call if push is unavailable. If “no”, the user will be prompted to choose an authentication method.

When configured with autopush = yes, we recommend setting prompts = 1.

motd Optional

Print the contents of /etc/motd to screen after a succesful login. Either “yes” or “no”. The default is “no”.

This option is only available for login_duo.

prompts Optional

If a user fails to authenticate with a second factor, Duo Unix will prompt the user to authenticate again. This option sets the maximum number of prompts that Duo Unix will display before denying access. Must be 1, 2, or 3. Default is 3.

For example, when prompts = 1, the user will have to successfully authenticate on the first prompt, whereas if prompts = 2, if the user enters incorrect information at the initial prompt, he/she will be prompted to authenticate again.

When configured with autopush = yes, we recommend setting prompts = 1.

accept_env_factor Optional

Look for factor selection or passcode in the $DUO_PASSCODE environment variable before prompting the user for input. When $DUO_PASSCODE is non-empty, it will override autopush.

The SSH client will need SendEnv DUO_PASSCODE in its configuration, and the SSH server will similarily need AcceptEnv DUO_PASSCODE.

Default is "no".

fallback_local_ip Optional

Duo Unix reports the IP address of the authorizing user, for the purposes of authorization and whitelisting. If Duo Unix cannot detect the IP address of the client, setting fallback_local_ip = yes will cause Duo Unix to send the IP address of the server it is running on.

If you are using IP whitelisting, enabling this option could cause unauthorized logins if the local IP is listed in the whitelist.

Example (fake) configuration file:


For more information, see the man page for login_duo.

PAM Configuration

You can deploy pam_duo instead of login_duo to provide a more comprehensive coverage of PAM-enabled applications (like su, sudo, ftpd, etc.).

First, configure, build, and install the application:

./configure --with-pam --prefix=/usr && make && sudo make install

Now modify your pam_duo.conf file to enter your keys and api-hostname. You should be able to find this file in /etc/duo/pam_duo.conf. This file takes the same configuration options as login_duo.conf, except where otherwise noted.

Next, you’ll need to modify your system’s PAM configuration to include a line like the following:

auth required

The location of this line and the specified control flag (eg. “required”, “requisite”, “sufficient”) varies. For most common configurations, place pam_duo directly after pam_unix (frequently found in common-auth or system-auth on Linux), set pam_unix’s control flag to “requisite”, and set pam_duo’s control flag to whatever pam_unix used to be.

If you want to use pam_duo with OpenSSH sshd, set both UsePAM and ChallengeResponseAuthentication to yes in your sshd_config. You should also set UseDNS to no so that PAM Duo is always passed the IP address of the connecting user, rather than the resolved hostname.

UsePAM yes
ChallengeResponseAuthentication yes
UseDNS no

We recommend leaving a root shell open while making any changes to your PAM configuration, in order to prevent accidently locking yourself out. Additionally, always make sure your PAM configuration works locally, before testing it with SSH logins.

The following are some PAM configuration examples for common Linux systems:

Ubuntu 10.04 LTS



auth [success=1 default=ignore] nullok_secure
auth    requisite


auth requisite nullok_secure
auth    [success=1 default=ignore]
auth    requisite

CentOS 5.5



auth required
auth    sufficient nullok try_first_pass
auth    requisite uid >= 500 quiet
auth    required


auth required
auth    requisite nullok try_first_pass
auth    sufficient
auth    requisite uid >= 500 quiet
auth    required




auth required 
auth    required try_first_pass likeauth nullok


auth required 
auth    requisite try_first_pass likeauth nullok 
auth    required

Linux Distribution Packages

To more easily install and maintain Duo Unix deployments, we’ve built Linux packages for a variety of popular distributions. These packages are tested 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.

Red Hat Enterprise Linux

Tested against 6.4 32/64-bit

Create /etc/yum.repos.d/duosecurity.repo with the following contents:

name=Duo Security Repository

Execute the following shell commands:

# rpm --import
# yum install duo_unix


Tested against 5.9 32/64-bit and 6.4 32/64-bit

Create /etc/yum.repos.d/duosecurity.repo with the following contents:

name=Duo Security Repository

Execute the following shell commands:

# rpm --import
# yum install duo_unix


Tested against 7.1 32/64-bit

Create /etc/apt/sources.list.d/duosecurity.list with the following contents:

deb wheezy main

Execute the following shell commands:

# curl -s | sudo apt-key add -
# apt-get update && apt-get install duo-unix


Tested against 12.04.3 32/64-bit

Create /etc/apt/sources.list.d/duosecurity.list with the following contents:

deb precise main

Execute the following shell commands:

# curl -s | sudo apt-key add -
# apt-get update && apt-get install duo-unix

Puppet Module

For environments that utilize Puppet we offer a module to deploy and manage Duo Unix. This module will handle configuration of our Linux package repositories, package installation, and configuration of either login_duo or pam_duo. If you’d like to contribute to its development, please visit our Github project page.

To install the module for deployment:

# puppet module install duosecurity/duo_unix

Please be sure to view the README for details on configuration options and example usage.