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.
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.
This application communicates with Duo's service on 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 this Duo KB article.
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:
duo_unix
(view checksum). From the command line you can use curl
or wget
to download the file, like $ wget https://dl.duosecurity.com/duo_unix-latest.tar.gz
.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!
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.
login_duo
PrerequisitesOpenSSL development headers and libraries are required for login_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, Amazon Linux | yum install openssl-devel |
|
SUSE/SLES | zypper install libopenssl-devel |
|
Solaris | pkg install openssl |
|
AIX | 3rd party packages or source build | |
zlib | SUSE/SLES | zypper install zlib-devel |
You also need a compiler like gcc
installed on your system to build Duo Unix.
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 1.11.4). View checksums for Duo downloads here.
$ tar zxf duo_unix-latest.tar.gz
$ cd duo_unix-1.11.4
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 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.
[duo]
; Duo integration key
ikey = INTEGRATION_KEY
; Duo secret key
skey = SECRET_KEY
; Duo API hostname
host = API_HOSTNAME
You may also add optional Duo configuration options to login_duo.conf
. See the table in Duo Configuration Options for all available settings.
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.
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 $HOME/bin
) without root access — just specify the location of login_duo.conf
with the -c
flag.
To more easily install and maintain Duo Unix deployments, we've built Linux packages for a variety of popular distributions. These packages are tested against the specific listed versions of their respective distributions. Please test all packages thoroughly prior to deploying them into your environment to ensure a great experience.
To download the packages, you'll need Duo's GPG key. The GPG key verifies the Duo Unix package for currently supported OS distributions and versions.
We updated the Duo GPG key for packages on supported distros on May 18, 2020. If you installed Duo before May 2020, use these commands to update the GPG key before the next time you install or upgrade Duo Unix.
The current Duo GPG key expires in May 2030.
CentOS and Red Hat Enterprise Linux (RHEL)
rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
Ubuntu and Debian
curl -s https://duo.com/DUO-GPG-PUBLIC-KEY.asc | sudo apt-key add -
OS distributions identified as no longer supported in the distro-specific packages sections use a previous GPG key. We won't replace or update the GPG key on these EOL versions when it expires, and urge you to update to a supported OS.
CentOS
Tested against 8.0 64-bit, 7.1 64-bit, 6.7 32/64-bit, and 5.11 32/64-bit
IMPORTANT: Duo Unix 1.11.0 is the last release with CentOS 5 support.
Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/CentOS/$releasever/$basearch
enabled=1
gpgcheck=1
Execute the following shell commands for Centos 6 and later:
# rpm --import https://duo.com/DUO-GPG-PUBLIC-KEY.asc
# yum install duo_unix
Execute the following shell commands for Centos 5:
This GPG key expires in August 2020.
# rpm --import https://duo.com/RPM-GPG-KEY-DUO
# yum install duo_unix
Ubuntu
Tested against 20.04 focal
64-bit, 18.04 bionic
64-bit, 16.04 xenial
32/64-bit, 14.04.3 trusty
32/64-bit, and 12.04.5 precise
32/64-bit.
Create /etc/apt/sources.list.d/duosecurity.list with the following contents:
deb https://pkg.duosecurity.com/Ubuntu precise main
or
deb https://pkg.duosecurity.com/Ubuntu trusty main
or
deb https://pkg.duosecurity.com/Ubuntu xenial main
or
deb [arch=amd64] https://pkg.duosecurity.com/Ubuntu bionic main
or
deb [arch=amd64] https://pkg.duosecurity.com/Ubuntu focal main
Execute the following shell commands for Ubuntu 14.04 and later:
# curl -s https://duo.com/DUO-GPG-PUBLIC-KEY.asc | sudo apt-key add -
# apt-get update && apt-get install duo-unix
Execute the following shell commands for Ubuntu 12.04:
This GPG key expires in October 2024.
# curl -s https://duo.com/DUO-GPG-PUBLIC-KEY-OLD.asc | sudo apt-key add -
# apt-get update && apt-get install duo-unix
Red Hat Enterprise Linux
Tested against 8.0 64-bit, 7.0 64-bit, 6.4 32/64-bit, and 5.10 32/64-bit.
IMPORTANT: Duo Unix 1.11.0 is the last release with RHEL 5 support.
Create /etc/yum.repos.d/duosecurity.repo with the following contents:
[duosecurity]
name=Duo Security Repository
baseurl=https://pkg.duosecurity.com/RedHat/$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
Execute the following shell commands for Red Hat 5:
This GPG key expires in August 2020.
# rpm --import https://duo.com/RPM-GPG-KEY-DUO
# yum install duo_unix
Debian
Tested against 10.0 32/64-bit, 7.9 32/64-bit, 9.5 32/64-bit, 8.2 32/64-bit, and 6.0.10 32/64-bit.
IMPORTANT: Duo Unix 1.11.1 is the last release with Debian 7 support and 1.9.19 is the last release with Debian 6 support.
Create /etc/apt/sources.list.d/duosecurity.list with the following contents:
deb https//pkg.duosecurity.com/Debian squeeze main
or
deb https://pkg.duosecurity.com/Debian wheezy main
or
deb https://pkg.duosecurity.com/Debian jessie main
or
deb https://pkg.duosecurity.com/Debian stretch main
or
deb https://pkg.duosecurity.com/Debian buster main
Execute the following shell commands for Debian 8 and later:
# 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 Debian 6 or 7:
This GPG key expires in August 2020.
# curl -s https://duo.com/APT-GPG-KEY-DUO | sudo apt-key add -
# apt-get update && apt-get install duo-unix
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.
The login_duo.conf
configuration files use the INI format. It 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. api-XXXXXXXX.duosecurity.com
|
||||||||
groups
|
Optional |
If specified, Duo authentication is required only for users whose primary group or supplementary group list matches one of the space-separated pattern lists. A pattern consists of zero or more non-whitespace characters, " A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark ("
|
||||||||
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 |
Use the specified HTTP proxy, same format as the |
||||||||
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. Note that this effectively disables passcode authentication. If "no", the user will be prompted to choose an authentication method. When configured with |
||||||||
motd
|
Optional |
Print the contents of This option is only available for |
||||||||
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 When configured with |
||||||||
accept_env_factor
|
Optional |
Look for factor selection or passcode in the The SSH client will need Example:
You can also add a number to the end of these factor names if you have more than one device registered. For example, push2 will send a login request to your second phone, phone3 will call your third phone, etc. Default is This option is only available for |
||||||||
fallback_local_ip
|
Optional |
Duo Unix reports the IP address of the authorizing user, for the purposes of authorization and network allow/deny. If Duo Unix cannot detect the IP address of the client, setting If you are using Authorized Networks, enabling this option could cause unauthorized logins if the local IP is listed in the allow list. |
||||||||
https_timeout
|
Optional |
Set to the number of seconds to wait for HTTPS responses from Duo Security. If Duo Security takes longer than the configured number of seconds to respond to the preauth API call, the configured failmode is triggered. Other network operations such as DNS resolution, TCP connection establishment, and the SSL handshake have their own independent timeout and retry logic. Default is 0, which disables the HTTPS timeout. |
||||||||
send_gecos
|
Optional |
Sends the entire GECOS field as the Duo username. Default is If you specify |
||||||||
gecos_username_pos
|
Optional |
Specify this option to select what position from the GECOS field will be used as the username. Positions are separated by whatever you specify in For example, if the
Then setting If not configured, the GECOS field is not parsed for the username. Overrides |
||||||||
gecos_delim
|
Optional |
Specify this option to change the default value of the GECOS delimiter from a comma to another character. The new delimiter specified must be exactly one character, and must a valid punctuation character other than a colon ( For example, if the
Then setting If not configured, the default comma ( |
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
.