Duo RADIUS Two-Factor Authentication with RADIUS Challenge Text Prompt for Cisco ASA SSL VPNs

Overview

The Cisco AnyConnect RADIUS instructions support push, phone call, or passcode authentication for AnyConnect desktop and mobile client connections that use SSL encryption. 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.

The SAML VPN instructions feature inline enrollment and the interactive Duo Prompt for both web-based VPN logins and AnyConnect 4.6+ client logins. This deployment option features Duo Single Sign-On, our cloud-hosted SAML 2.0 identity provider. Primary and Duo secondary authentication occur at the identity provider, not at the ASA itself.

Please refer to the Duo for Cisco AnyConnect VPN with ASA or Firepower overview to learn more about the different options for protecting ASA logins with Duo MFA.

If you need to protect connections that use Cisco's desktop VPN client (IKE encryption), use our Cisco IPSec instructions.

Before starting, make sure that Duo is compatible with your Cisco ASA device. Log on to your Cisco ASDM interface and verify that your Cisco ASA firmware is version 8.3 or later.

To integrate Duo with your Cisco ASA SSL VPN, 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.

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.

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

To integrate Duo with your Cisco ASA 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 Cisco ASA 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 Cisco RADIUS 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 Cisco ASA SSL VPN

Next, we'll set up the Authentication Proxy to work with your Cisco ASA 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.

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 Cisco ASA

Add the Duo RADIUS server

1. Navigate to AAA/Local Users → AAA Server Groups, click Add, and fill out the form:

   Setting	Value
   AAA Server Group	Duo-RADIUS
   Protocol	RADIUS

   

   Click OK to create the new AAA server group.

2. Select the Duo-RADIUS group you just added. In the Add AAA Server dialog, enter the following information:

   Setting	Value
   Interface Name	The ASA interface where your Duo Authentication Proxy can be reached.
   Server Name or IP Address	The hostname or IP address of your Duo Authentication Proxy.
   Timeout	60 seconds should be sufficient to complete authentication; see the FAQ item about timeouts.
   Server Accounting Port	1813 (Technically this setting does not matter because the Duo Authentication Proxy does not support RADIUS Accounting).
   Retry Interval	10 seconds
   Server Secret Key	Shared Secret used in Authentication Proxy configuration.
   Microsoft CHAPv2 Capable	Unchecked

   

   Click OK, and then OK to save the new server.

3. You can verify connectivity to the Duo RADIUS server now. With the Duo AAA server group you just created selected, click the Test button.

4. On the "Test AAA Server" form, select Authentication.

5. Enter the username of user that exists in Duo and has a valid authentication device (like a phone or token).

6. In the "Password" field, enter the password for that user. If the user only has token authenticators available, you can append a comma followed by a passcode to the password, like password,123456. Click OK.

7. If the user is set up for Duo Push or phone call authentication, approve the Duo authentication request.

8. A new form pops up letting you know if the test was successful or failed.

Change the SSL VPN Authentication Method to Duo

1. Navigate to Network (Client) Access → AnyConnect Connection Profiles
2. Select the connection profile to which you want to add two-factor authentication and click Edit.
3. In the Authentication section of the Basic profile settings page select Duo-RADIUS from the AAA Server Group list.
4. Uncheck the Use LOCAL if Server Group fails option.
5. Click OK then click Apply.

Click Save to write all changes to the ASA device memory.

Configure AnyConnect Timeout

It's a good idea to increase the AnyConnect Authentication Timeout so that users have enough time to use Duo Push or phone callback.

1. Navigate to Configuration → Remote Access VPN → Network (Client) Access → AnyConnect Client Profile and Click Edit.
2. In the left menu, navigate to Preferences (Part 2).
3. Scroll to the bottom of the page and modify the Authentication Timeout (seconds) setting to 60 seconds.
4. Click OK.
5. Click Apply to activate the new AnyConnect Client settings.
6. Click Save to write this change to the ASA device memory.

This timeout setting will take effect after each client successfully logs into the VPN after applying the new profile.

If you find that AnyConnect client connections disconnect after about 12 seconds please see the following FAQ: Why is the AnyConnect client connection attempt disconnecting after 12 seconds when I have increased the timeout?

Test Your Setup

Launch the AnyConnect client and select the VPN profile that now uses Duo RADIUS authentication. When prompted, enter your primary login credentials.

After you enter your username and password, a second AnyConnect 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.

In this AnyConnect client example, one could type push, push1, or 1 to send a Duo push request to send a Duo push request to the phone with the 1311 number.

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

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 AnyConnect 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

SSL Browser Logins

If you also permit browser-based SSL VPN access via Clientless SSL VPN Access connection profiles then you can also configure a SSL VPN connection profile to use the Duo RADIUS server group you added for authentication. If you do this, then 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.

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 Cisco Frequently Asked Questions (FAQ) page or try searching our Cisco Knowledge Base articles or Community discussions. For further assistance, contact Support.

Network Diagram

1. Primary authentication initiated to Cisco ASA
2. Cisco ASA sends authentication request to the Duo 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. Cisco ASA VPN access granted