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Using two factor authentication for users

Instead of relying on complex passwords for client certificates (that usually get written somewhere) this image provides support for two factor authentication with OTP devices.

The most common app that provides OTP generation is Google Authenticator (iOS and Android) you can download it and use this image to generate user configuration.

Usage

In order to enable two factor authentication the following steps are required.

  • Choose a more secure cipher to use because since OpenVPN 2.3.13 the default openvpn cipher BF-CBC will cause a renegotiated connection every 64 MB of data

  • Generate server configuration with -2 and -C $CIPHER options

      docker run -v $OVPN_DATA:/etc/openvpn --rm kylemanna/openvpn ovpn_genconfig -u udp://vpn.example.com -2 -C $CIPHER

  • Generate your client certificate (possibly without a password since you're using OTP)

      docker run -v $OVPN_DATA:/etc/openvpn --rm -it kylemanna/openvpn easyrsa build-client-full <user> nopass

  • Generate authentication configuration for your client. -t is needed to show QR code, -i is optional for interactive usage

      docker run -v $OVPN_DATA:/etc/openvpn --rm -it kylemanna/openvpn ovpn_otp_user <user>

The last step will generate OTP configuration for the provided user with the following options

google-authenticator --time-based --disallow-reuse --force --rate-limit=3 --rate-time=30 --window-size=3 \
    -l "${1}@${OVPN_CN}" -s /etc/openvpn/otp/${1}.google_authenticator

It will also show a shell QR code in terminal you can scan with the Google Authenticator application. It also provides a link to a google chart url that will display a QR code for the authentication.

Do not share QR code (or generated url) with anyone but final user, that is your second factor for authentication that is used to generate OTP codes

Here's an example QR code generated for an hypotetical user@example.com user.

Example QR Code

Generate client configuration for <user> and import it in OpenVPN client. On connection it will prompt for user and password. Enter your username and a 6 digit code generated by Authenticator app and you're logged in.

TL;DR

Under the hood this configuration will setup an openvpn PAM service configuration (/etc/pam.d/openvpn) that relies on the awesome Google Authenticator PAM module. In this configuration the auth part of PAM flow is managed by OTP codes and the account part is not enforced because you're likely dealing with virtual users and you do not want to create a system account for every VPN user.

ovpn_otp_user script will store OTP credentials under /etc/openvpn/otp/<user>.google_authentication. In this way when you take a backup OTP users are included as well.

Finally it will enable the openvpn plugin openvpn-plugin-auth-pam.so in server configuration and append the auth-user-pass directive in client configuration.

Debug

If something is not working you can verify your PAM setup with these commands

# Start a shell in container
docker run -v $OVPN_DATA:/etc/openvpn --rm -it kylemanna/openvpn bash
# Then in container you have pamtester utility already installed
which pamtester
# To check authentication use this command that will prompt for a valid code from Authenticator APP
pamtester -v openvpn <user> authenticate

In the last command <user> should be replaced by the exact string you used in the ovpn_otp_user command.

If you configured everything correctly you should get authenticated by entering a OTP code from the app.