Duo Two-Factor Authentication for FortiGate SSL VPN and FortiClient with RADIUS Challenge Text Prompt for VPN Client Access

Overview

The FortiGate SSL VPN and FortiClient RADIUS instructions support push, phone call, or passcode authentication for web-based or FortiClient clients. This configuration offers a text-based Duo prompt over RADIUS Challenge, and captures client IP information for use with Duo policies, such as geolocation and authorized networks.

To integrate Duo with your FortiGate, you will need to install a local proxy service on a machine within your network. This Duo proxy server also acts as a RADIUS server — there's usually no need to deploy a separate additional RADIUS server to use Duo.

If you are already running a Duo Authentication Proxy server in your environment, you can generally use that existing host for additional applications, appending the new configuration sections to the current config.

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.

You should already have a working primary authentication configuration for your Fortinet FortiGate SSL VPN users before you begin to deploy Duo.

To integrate Duo with your Fortinet FortiGate SSL VPN, you will need to install a local Duo proxy service on a machine within your network. This Duo proxy server will receive incoming RADIUS requests from your Fortinet FortiGate SSL VPN, contact your existing local LDAP/AD or RADIUS server to perform primary authentication, and then contact Duo's cloud service for secondary authentication.

If you are already running a Duo Authentication Proxy server in your environment, you can use that existing host for additional applications, appending the new configuration sections to the current config. You don't have to set up a new Authentication Proxy server for each application you create. However, there are some cases where it might make sense for you to deploy a new proxy server for a new application, like if you want to co-locate the Duo proxy with the application it will protect in the same data center.

We recommend the following 64-bit operating systems for the system hosting the Duo Authentication Proxy:

• Windows Server 2016 or later
• CentOS Stream 9 or later
• Fedora 39 or later
• Red Hat Enterprise Linux 7 or later
• Rocky Linux 9 or later
• Ubuntu 20.04 LTS or later
• Debian 11 or later

The Duo End of Sale, Last Date of Support, and End of Life Policy states that Duo does not offer support for integrations running on operating system versions beyond the vendor’s stated Last Date of Support date.

See detailed Authentication Proxy operating system performance recommendations in the Duo Authentication Proxy Reference.

We do not recommend installing the Duo Authentication Proxy on the same Windows server that acts as your Active Directory domain controller or one with the Network Policy Server (NPS) role. If you must co-locate the Duo Authentication Proxy with these services, be prepared to resolve potential LDAP or RADIUS port conflicts between the Duo service and your pre-existing services.

Then you'll need to:

1. Sign up for a Duo account.
2. Log in to the Duo Admin Panel and navigate to Applications → Protect an Application.
3. Locate the entry for Fortinet FortiGate SSL VPN with a protection type of "2FA" 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.

Install the Duo Authentication Proxy

If you will reuse an existing Duo Authentication Proxy server for this new application, you can skip the install steps and go to Configure the Proxy.

The Duo Authentication Proxy can be installed on a physical or virtual host. We recommend a system with at least 1 CPU, 200 MB disk space, and 4 GB RAM (although 1 GB RAM is usually sufficient). See additional Authentication Proxy performance recommendations in the Duo Authentication Proxy Reference.

We do not recommend installing the Duo Authentication Proxy on the same Windows server that acts as your Active Directory domain controller or one with the Network Policy Server (NPS) role. If you must co-locate the Duo Authentication Proxy with these services, be prepared to resolve potential LDAP or RADIUS port conflicts between the Duo service and your pre-existing services.

duoauthproxy-version.exe /S

duoauthproxy-version.exe /S /exclude-auth-proxy-manager

sudo ./duoauthproxy-build/install --install-dir /opt/duoauthproxy --service-user duo_authproxy_svc --log-group duo_authproxy_grp --create-init-script yes

Configure the Proxy

After the installation completes, you will need to configure the proxy.

The Duo Authentication Proxy configuration file is named authproxy.cfg, and is located in the conf subdirectory of the proxy installation. With default installation paths, the proxy configuration file will be located at:

Note that as of v4.0.0, the default file access on Windows for the conf directory is restricted to the built-in Administrators group during installation.

The configuration file is formatted as a simple INI file. Section headings appear as:

[section]

Individual properties beneath a section appear as:

name=value

The Authentication Proxy may include an existing authproxy.cfg with some example content. For the purposes of these instructions, however, you should delete the existing content and start with a blank text file.

Duo Authentication Proxy Manager

The Duo Authentication Proxy Manager is a Windows utility for managing the Authentication Proxy installation on the Windows server where you install the Authentication Proxy. The Proxy Manager comes with Duo Authentication Proxy for Windows version 5.6.0 and later.

The Proxy Manager cannot manage remote Duo Authentication Proxy servers, nor can you install the Proxy Manager as a stand-alone application. There is no Proxy Manager available for Linux. The Proxy Manager only functions as part of a local Duo Authentication Proxy installation on Windows servers.

Learn more about using the Proxy Manager in the Duo Authentication Proxy Reference before you continue.

To launch the Proxy Manager utility:

• Open the Start Menu and go to Duo Security.
• Click the Duo Authentication Proxy Manager icon to launch the application. You must have administrative privileges on the Windows server and accept the prompt for elevation.
• The Proxy Manager launches and automatically opens the %ProgramFiles%\Duo Security Authentication Proxy\conf\authproxy.cfg file for editing.

Use the Proxy Manager editor on the left to make the authproxy.cfg changes in these instructions. As you type into the editor, the Proxy Manager will automatically suggest configuration options. Accepting these suggestions helps make sure you use the correct option syntax.

As you follow the instructions on this page to edit the Authentication Proxy configuration, you can click Validate to verify your changes (output shown on the right).

When you complete the Authentication Proxy configuration steps in this document, you can use the Save button to write your updates to authproxy.cfg, and then use the authproxy.cfg button to start the Authentication Proxy service before continuing on to the next configuration steps.

If you do not use the Proxy Manager to edit your configuration then we recommend using WordPad or another text editor instead of Notepad when editing the config file on Windows.

Configure the Proxy for Your Primary Authenticator

In this step, you'll set up the Proxy's primary authenticator — the system which will validate users' existing passwords. Determine which type of primary authentication you'll be using, and create either an Active Directory/LDAP [ad_client] client section, or a RADIUS [radius_client] section as follows.

Active Directory

Add an [ad_client] section if you'd like to use an Active Directory domain controller (DC) or LDAP-based directory server to perform primary authentication. This section accepts the following options:

Required

search_dn=DC=example,DC=com

Optional

security_group_dn=CN=DuoVPNUsers,OU=Groups,DC=example,DC=com

For example:

[ad_client]
host=1.2.3.4
host_2=1.2.3.5
service_account_username=duoservice
service_account_password=password1
search_dn=DC=example,DC=com
security_group_dn=CN=DuoVPNUsers,OU=Groups,DC=example,DC=com

For advanced Active Directory configuration, see the full Authentication Proxy documentation.

RADIUS

To use RADIUS as your primary authenticator, add a [radius_client] section to the top of your config file. Then add the following properties to the section:

Required

Optional

For example:

[radius_client]
host=1.2.3.4
host_2=1.2.3.5
secret=radiusclientsecret

In addition, make sure that the RADIUS server is configured to accept authentication requests from the Authentication Proxy.

For advanced RADIUS configuration, see the full Authentication Proxy documentation.

Configure the Proxy for Your Fortinet FortiGate SSL VPN

Next, we'll set up the Authentication Proxy to work with your Fortinet FortiGate SSL VPN. Create a [radius_server_challenge] section and add the properties listed below. If you've already set up the Duo Authentication Proxy for a different RADIUS Challenge application, append a number to the section header to make it unique, like [radius_server_challenge2].

Required

Optional

A completed config file, using Active Directory as the primary authenticator, should look something like:

[ad_client]
host=1.2.3.4
service_account_username=duoservice
service_account_password=password1
search_dn=cn=Users,dc=example,dc=com
 
[radius_server_challenge]
ikey=DIXXXXXXXXXXXXXXXXXX
skey=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
api_host=api-XXXXXXXX.duosecurity.com
failmode=safe
client=ad_client
radius_ip_1=5.6.7.8
radius_secret_1=radiussecret1
port=1812

Make sure to save your configuration file in your text editor — or validate and save in the Proxy Manager for Windows — when you're finished making changes.

Note: If you are running or planning to upgrade to FortiOS v7.2.10 or v7.4.5, you will need to configure the Duo Authentication Proxy to send the `message-authenticator` attribute in RADIUS replies.

Add this line to the [radius_server_challenge] configuration section of you authproxy.cfg file:

force_message_authenticator=true

Start the Proxy

net start DuoAuthProxy

authproxyctl start

net stop DuoAuthProxy & net start DuoAuthProxy

authproxyctl restart

# /opt/duoauthproxy/bin/authproxyctl start

# /opt/duoauthproxy/bin/authproxyctl status

# /opt/duoauthproxy/bin/authproxyctl restart

If you modify your authproxy.cfg configuration after initial setup, you'll need to stop and restart the Duo Authentication Proxy service or process for your change to take effect.

You can view information about your Authentication Proxy in the Authentication Proxy Dashboard.

Configure Your Fortinet FortiGate SSL VPN

Add a RADIUS Server

1. Log in to the Fortinet FortiGate administrative interface.
2. Click the User & Device section in the left navigation panel and navigate to Authentication → RADIUS Servers.
3. Click the Create New button to create a new RADIUS server.

4. On the New RADIUS Server page, enter the following information:

   Name	Duo RADIUS
   Type	Query
   Primary Server Name/IP	The IP address or FQDN of your Duo RADIUS proxy
   Primary Server Secret	The RADIUS secret configured on your Duo RADIUS proxy
   Authentication Scheme	Click the "Specify Authentication Protocol" radio button and select PAP from the drop-down menu

   
5. Click the OK button to create the new RADIUS server.

Configure a User Group

1. Click the User & Device section in the left navigation panel and navigate to User → User Groups.
2. If you have an existing user group, click on it to edit its settings. If you don't yet have a user group, click Create New to create one.

3. On the Edit User Group or New User Group page, enter the following information:

   Name	Duo SSL VPN
   Type	Firewall

4. Click the Create New button in the Remote groups section and select the Duo RADIUS remote server. You do not have to specify a group.

   
5. Click the OK button to save the user group settings.

Define an IPv4 Policy

1. Click the Policy & Objects section in the left navigation panel and navigate to IPv4 Policy.
2. Open the policy for your SSL-VPN for editing.
3. Click the plus icon under the Source to expand the Select Entries on interface on the right.
4. Click User on the "Select Entries" interface, and select the Duo user group that you created earlier. This adds the group you selected to the "Source" list.
5. Click OK to save the change and return to the list of IPv4 policies.

Configure timeout

The Fortinet appliance has a default timeout of 5 seconds, which will fail for anything other than a passcode authentication. The timeout can be increased from the Fortinet command line interface to resolve the issue. Duo recommends increasing the timeout to at least 60 seconds

1. Connect to the appliance CLI. Consult the documentation that accompanied your Fortinet device for more information.
2. Execute the following commands:
   config system global
set remoteauthtimeout 60
end


Test Your Setup

Launch your FortiClient application or access the SSL VPN login page in your browser. When prompted, enter your primary login credentials.

After you enter your username and password, a second VPN client window displays the Duo RADIUS challenge text prompt, listing your available factors (or an enrollment URL).

Duo two-factor login for jdoe

  Enter a passcode or select one of the following options:
  
  1. Duo Push to XXX-XXX-1234
  2. Phone call to XXX-XXX-1234
  3. SMS passcodes to XXX-XXX-1234
  
  Passcode or option (1-3)
  

In the text response area, type in the number that matches the option you want to use to log in. For example, to log in with Duo Push after receiving the above challenge prompt, enter 1, and then approve the Duo Push request you receive.

Instead of typing in the number that corresponds to the listed options, you may also enter a Duo factor option:

If you wanted to use Duo Push (rather than a passcode) to authenticate, you would enter:

push

You can also specify a number after the factor name if you have more than one device enrolled (as the automatic push or phone call goes to the first capable device attached to a user). So you can enter phone2 or push2 if you have two phones enrolled and you want the authentication request to go to the second phone.

The format of the Duo RADIUS text challenge prompt may change if you have multiple authentication devices enrolled in Duo, due to the character length limit of a RADIUS packet. If we detect that the user logging in has too many devices to list by number with full details the Duo proxy will automatically switch to a compact format, like this.

Choose a secondary factor from (`push1`, `phone1`, `sms1`) or enter passcode:

Respond to this prompt in the same way, by typing in either the name of a Duo factor listed or a number that matches a factor's position in the list.

Once you approve the Duo authentication request or enter a valid passcode, the AnyConnect client is connected to the VPN.

Logging into the web-based VPN via browser also shows the text-based RADIUS challenge screen where you can choose from your available factors or enter a passcode, just like you can in AnyConnect.

See the Challenge Mode End-User Instructions in the Duo User Guide for more details about the RADIUS challenge login experience.

If the RADIUS challenge response text doesn't display well in your client application you may need to set the optional prompt_format setting for [radius_server_challenge] to a value other than the default of console in your Duo Authentication Proxy's authproxy.cfg file. Be sure to restart the Authentication Proxy service after making the change.

Troubleshooting

Need some help? Review troubleshooting tips for the Authentication Proxy and try the connectivity tool included with Duo Authentication Proxy 2.9.0 and later to discover and troubleshoot general connectivity issues.

Also take a look at the SonicWall SRA Frequently Asked Questions (FAQ) page or try searching our SonicWall SRA Knowledge Base articles or Community discussions. For further assistance, contact Support.

Network Diagram

1. Primary authentication initiated to Fortinet Fortigate SSL VPN
2. Fortinet Fortigate SSL VPN sends authentication request to Duo Security’s authentication proxy
3. Primary authentication using Active Directory or RADIUS
4. Duo authentication proxy connection established to Duo Security over TCP port 443
5. Secondary authentication via Duo Security’s service
6. Duo Authentication Proxy receives authentication response
7. Fortinet Fortigate SSL VPN access granted