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Cisco ASA SSL VPN for AnyConnect

Last Updated: October 17th, 2019

Duo integrates with your Cisco ASA VPN to add two-factor authentication to any VPN login.


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 does not feature the interactive Duo Prompt for web-based logins, but does capture client IP informations for use with Duo policies, such as geolocation and authorized networks.

If you need inline self-service enrollment and the Duo Prompt for web-based VPN logins, refer to the ASA LDAPS SSL VPN Instructions.

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.

Connectivity Requirements

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.

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, and Duo policy settings and how to apply them. You'll need to pre-enroll your users in Duo using one of our available methods before they can log in using this configuration. 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 if necessary, and then contact Duo's cloud service for secondary authentication.

Locate (or set up) a system on which you will install the Duo Authentication Proxy. The proxy supports these operating systems:

  • Windows Server 2012 or later (Server 2016+ recommended)
  • CentOS 7 or later (CentOS 8+ recommended)
  • Red Hat Enterprise Linux 7 or later (RHEL 8+ recommended)
  • Ubuntu 16.04 or later (Ubuntu 18.04+ recommended)
  • Debian 7 or later (Debian 9+ recommended)

Then you'll need to:

  1. Sign up for a Duo account.
  2. Log in to the Duo Admin Panel and navigate to Applications.
  3. Click Protect an Application and locate Cisco RADIUS VPN 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.
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!

Install the Duo Authentication 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).

  1. Download the most recent Authentication Proxy for Windows from Note that the actual filename will reflect the version e.g. duoauthproxy-5.5.0.exe. View checksums for Duo downloads here.
  2. Launch the Authentication Proxy installer on the target Windows server as a user with administrator rights and follow the on-screen prompts.

To perform a silent install on Windows, issue the following from an elevated command prompt after downloading the installer (replacing version with the actual version you downloaded):

duoauthproxy-version.exe /S
  1. Ensure that Perl and a compiler toolchain are installed. On most recent RPM-based distributions — like Fedora, RedHat Enterprise, and CentOS — you can install these by running (as root):

    $ yum install gcc make libffi-devel perl zlib-devel diffutils

    On Debian-derived systems, install these dependencies by running (as root):

    $ apt-get install build-essential libffi-dev perl zlib1g-dev
  2. Download the most recent Authentication Proxy for Unix from From the command line you can use curl or wget to download the file, like $ wget --content-disposition Depending on your download method, the actual filename may reflect the version e.g. duoauthproxy-5.5.0-src.tgz. View checksums for Duo downloads here.

  3. Extract the Authentication Proxy files and build it as follows:

    $ tar xzf duoauthproxy-latest-src.tgz
    $ cd duoauthproxy-version-src
    $ make
  4. Install the authentication proxy (as root):

    $ cd duoauthproxy-build
    $ ./install

    Follow the prompts to complete the installation. The installer creates a user to run the proxy service and a group to own the log directory and files. You can accept the default user and group names or enter your own.

If you ever need to uninstall the proxy, run /opt/duoauthproxy/uninstall.

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:

Operating System Authentication
Proxy Version
Windows v5.0.0 and later C:\Program Files\Duo Security Authentication Proxy\conf\authproxy.cfg
Windows v4.0.2 and earlier C:\Program Files (x86)\Duo Security Authentication Proxy\conf\authproxy.cfg
Linux All /opt/duoauthproxy/conf/authproxy.cfg

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:


Individual properties beneath a section appear as:


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. 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

To use Active Directory/LDAP as your primary authenticator, add an [ad_client] section to the top of your config file. Add the following properties to the section:



The hostname or IP address of your domain controller.


The username of a domain account that has permission to bind to your directory and perform searches. We recommend creating a service account that has read-only access.


The password corresponding to service_account_username. If you're on Windows and would like to encrypt this password, see Encrypting Passwords in the full Authentication Proxy documentation.


The LDAP distinguished name (DN) of an Active Directory container or organizational unit (OU) containing all of the users you wish to permit to log in. For example:




The hostname or IP address of a secondary/fallback domain controller. You can add additional domain controllers as host_3, host_4, etc.


To further restrict access, specify the LDAP distinguished name (DN) of a security group that contains the users who should be able to log in as direct group members. Nested groups are not supported. Users who are not direct members of the specified group will not pass primary authentication. Example:


Starting with Authentication Proxy v3.2.0, the security_group_dn may be the DN of an AD user's primarygroup. Prior versions do not support primary groups.


LDAP attribute found on a user entry which will contain the submitted username. In most Active Directory configurations, it should not be necessary to change this option from the default value. OpenLDAP directories may use "uid" or another attribute for the username, which should be specified with this option.

Default: "sAMAccountName"

For example:


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


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:



The IP address of your RADIUS server. You can add backup servers with host_2, host_3, etc.


A secret to be shared between the Authentication Proxy and your existing RADIUS server. If you're on Windows and would like to encrypt this secret, see Encrypting Passwords in the full Authentication Proxy documentation.



The authentication port on your RADIUS server. Use port_2, port_3, etc. to specify ports for the backup servers.



If this option is set to true, all RADIUS attributes set by the primary authentication server will be copied into RADIUS responses sent by the proxy.

Default: false

For example:


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_auto] section and add the properties listed below. If you've already set up the Duo Authentication Proxy for a different RADIUS Auto application, append a number to the section header to make it unique, like [radius_server_auto2].



Your Duo integration key, obtained from the details page for the application in the Duo Admin Panel.


Your Duo secret key, obtained from the details page for the application in the Duo Admin Panel. If you're on Windows and would like to encrypt the skey, see Encrypting Passwords in the full Authentication Proxy documentation.


Your Duo API hostname (e.g., obtained from the details page for the application in the Duo Admin Panel.


The IP address of your Cisco ASA SSL VPN. Only clients with configured addresses and shared secrets will be allowed to send requests to the Authentication Proxy.


A secret to be shared between the proxy and your Cisco ASA SSL VPN. If you're on Windows and would like to encrypt this secret, see Encrypting Passwords in the full Authentication Proxy documentation.


The mechanism that the Authentication Proxy should use to perform primary authentication. This should correspond with a "client" section elsewhere in the config file.


Use Active Directory for primary authentication. Make sure you have an [ad_client] section configured.


Use RADIUS for primary authentication. Make sure you have a [radius_client] section configured.


Do not perform primary authentication. Make sure you have a [duo_only_client] section configured.

This parameter is optional if you only have one "client" section. If you have multiple, each "server" section should specify which "client" to use.



Port on which to listen for incoming RADIUS Access Requests. If you have multiple RADIUS server sections you should use a unique port for each one.

Default: 1812


Either safe or secure:




In the event that Duo's service cannot be contacted, users' authentication attempts will be permitted if primary authentication succeeds.


In the event that Duo's service cannot be contacted, all users' authentication attempts will be rejected.

Default: safe


The IP address of your second Cisco ASA SSL VPN, if you have one. You can specify additional devices as as radius_ip_3, radius_ip_4, etc.


The secrets shared with your second Cisco ASA SSL VPN, if using one. You can specify secrets for additional devices as radius_secret_3, radius_secret_4, etc. If you're on Windows and would like to encrypt this secret, see Encrypting Passwords in the full Authentication Proxy documentation.

A completed config file that uses Active Directory should look something like:


Make sure to save your configuration file when done.


View video guides for proxy deployment at the Authentication Proxy Overview or see the Authentication Proxy Reference Guide for additional configuration options.

Start the Proxy

Open an Administrator command prompt and run:

net start DuoAuthProxy

Alternatively, open the Windows Services console (services.msc), locate "Duo Security Authentication Proxy Service" in the list of services, and click the Start Service button.

Authentication Proxy v5.1.0 and later includes the authproxyctl executable, which shows the connectivity tool output when starting the service. The installer adds the Authentication Proxy C:\Program Files\Duo Security Authentication Proxy\bin to your system path automatically, so you should not need to specify the full path to authproxyctl to run it.

From an administrator command prompt run:

authproxyctl start

If the service starts successfully, Authentication Proxy service output is written to the authproxy.log file, which can be found in the log subdirectory.

If you see an error saying that the "service could not be started", open the Application Event Viewer and look for an Error from the source "DuoAuthProxy". The traceback may include a "ConfigError" that can help you find the source of the issue.

Stop and restart the Authentication Proxy service by either clicking the Restart Service button in the Windows Services console or issuing these commands from an Administrator command prompt:

net stop DuoAuthProxy & net start DuoAuthProxy

To stop and restart the Authentication Proxy using authproxyctl, from an administrator command prompt run:

authproxyctl restart

Open a root shell and run:

# /opt/duoauthproxy/bin/authproxyctl start

To ensure the proxy started successfully, run:

# /opt/duoauthproxy/bin/authproxyctl status

Authentication Proxy service output is written to the authproxy.log file, which can be found in the log subdirectory.

To stop and restart the Authentication Proxy, open a root shell and run:

# /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.

Configure Your Cisco ASA

Add the Duo RADIUS server

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

    Setting Value
    AAA Server Group Duo-RADIUS
    Protocol RADIUS

    Add AAA Server Group

    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 Authentication Port 1812 (or whichever port specified in your authproxy.cfg file)
    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

    New RADIUS Server

    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) AccessAnyConnect 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. Change Authenticator
  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 ConfigurationRemote Access VPNNetwork (Client) AccessAnyConnect 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. AnyConnect Timeout
  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.

Test Your Setup

Launch the AnyConnect client and select the VPN profile that now uses Duo RADIUS authentication.

To test your setup, attempt to log in to your newly-configured system as a user enrolled in Duo with an authentication device.

When you enter your username and password, you will receive an automatic push or phone callback.

Alternatively you may add a comma (",") to the end of your password and append a Duo factor option:

push Perform Duo Push authentication. You can use Duo Push if you've installed and activated Duo Mobile on your device.
phone Perform phone callback authentication.
sms Send a new batch of SMS passcodes. Your authentication attempt will be denied. You can then authenticate with one of the newly-delivered passcodes.
A numeric passcode Log in using a passcode, either generated with Duo Mobile, sent via SMS, generated by your hardware token, or provided by an administrator. Examples: "123456" or "2345678".

For example, if you wanted to use a passcode to authenticate instead of Duo Push or a phone call, you would enter:

username: bob
password: hunter2,123456

If you wanted to use specify use of phone callback to authenticate instead of an automatic Duo Push request, you would enter:

username: bob
password: hunter2,phone

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.

AnyConnect Client with Duo RADIUS

Once you approve the Duo authentication request (or if you appended a valid passcode to your password for MFA), the AnyConnect client is connected to the VPN.

Logging into the web-based VPN via browser sends an automatic Duo request via push or phone call. You can also append a different Duo factor name or passcode to your password in the browser, just like you can in AnyConnect.


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