openvpn-auth-azure-ad

State: Proof of concept!

openvpn-auth-azure-ad is an external service connects to the openvpn management interface and handle the authentication of connecting users against Azure AD.

OpenVPN version 2.4 is required. 2.5 is not tested yet.

Tested environment

Python

• Python 3.8 / 3.10

Server

• OpenVPN 2.5.6

Client

• OpenVPN Community Client for Windows 2.5.7
• Tunnelblick 3.8.7a

Authenticators

Currently, openvpn-auth-azure-ad supports 2 authentication method against Azure AD:

• device authorization grant flow
• Resource Owner Password Credentials grant (not recommend, see docs for limitations)

Additionally, if enabled openvpn-auth-azure-ad supports OpenVPNs auth-token mechanismus to allow users to bypass then authenticator above on re-authentications, e.g. due reneg-sec.

Installation

via pip

# pip install openvpn-auth-azure-ad

For install pip on your system, see pip docs.

To run a persistent daemon, you copy the systemd unit file to /etc/systemd/system, then run

# systemctl enable openvpn-auth-azure-ad
# systemctl start openvpn-auth-azure-ad

via docker

https://hub.docker.com/r/jkroepke/openvpn-auth-azure-ad

# docker run --rm \
    -v <path of openvpn mgmt socket>:/openvpn/management.sock
    -v /etc/openvpn-auth-azure-ad/config.conf:/etc/openvpn-auth-azure-ad/config.conf \
    -e AAD_CLIENT_ID= \
    -e OPENVPN_AAD_AUTH_SOCKET_PATH=/openvpn/management.sock \
    -e OPENVPN_AAD_AUTH_PASSWORD= \
    jkroepke/openvpn-auth-azure-ad

Usage

Args that start with '--' (eg. -V) can also be set in a config file (/etc/openvpn-auth-azure-ad/config.conf or ~/.openvpn-auth-azure-ad or specified via -c). Config file syntax allows: key=value, flag=true, stuff=[a,b,c] (for details, see syntax at https://goo.gl/R74nmi). If an arg is specified in more than one place, then commandline values override environment variables which override config file values which override defaults.

usage: openvpn-auth-azure-ad.py [-h] [-c CONFIG] [-V] [-t THREADS] [-a AUTHENTICATORS] [--auth-token] [--auth-token-lifetime AUTH_TOKEN_LIFETIME] [--remember-user] [--verify-openvpn-client]
                                [--verify-openvpn-client-id-token-claim] [-H OPENVPN_HOST] [-P OPENVPN_PORT] [-s OPENVPN_SOCKET] [-p OPENVPN_PASSWORD] [--openvpn-release-hold] --client-id CLIENT_ID
                                [--token-authority TOKEN_AUTHORITY] [--graph-endpoint GRAPH_ENDPOINT] [--prometheus] [--prometheus-listen-addr PROMETHEUS_LISTEN_ADDR]
                                [--prometheus-listen-port PROMETHEUS_LISTEN_PORT] [--log-level LOG_LEVEL]

optional arguments:
  -h, --help            show this help message and exit
  -c CONFIG, --config CONFIG
                        path of config file [env var: AAD_CONFIG_PATH]
  -V, --version         show program's version number and exit
  -t THREADS, --threads THREADS
                        Amount of threads to handle authentication [env var: AAD_THREAD_COUNT]

OpenVPN User Authentication:
  -a AUTHENTICATORS, --authenticators AUTHENTICATORS
                        Enable authenticators. Multiple authenticators can be separated with comma [env var: AAD_AUTHENTICATORS]
  --auth-token          Use auth token to re-authenticate clients [env var: AAD_AUTH_TOKEN]
  --auth-token-lifetime AUTH_TOKEN_LIFETIME
                        Lifetime of auth tokens in seconds [env var: AAD_AUTH_TOKEN_LIFETIME]
  --remember-user       If user authenticated once, the users refresh token is used to reauthenticate silently if possible. [env var: AAD_REMEMBER_USER]
  --verify-openvpn-client
                        Check if openvpn client common_name matches Azure AD token claim [env var: AAD_VERIFY_OPENVPN_CLIENT]
  --verify-openvpn-client-id-token-claim
                        AAD id_token claim used for client verification [env var: AAD_VERIFY_OPENVPN_CLIENT_ID_TOKEN_CLAIM]

OpenVPN Management Interface settings:
  -H OPENVPN_HOST, --openvpn-host OPENVPN_HOST
                        Host of OpenVPN management interface. [env var: OPENVPN_AAD_AUTH_HOST]
  -P OPENVPN_PORT, --openvpn-port OPENVPN_PORT
                        Port of OpenVPN management interface. [env var: OPENVPN_AAD_AUTH_PORT]
  -s OPENVPN_SOCKET, --openvpn-socket OPENVPN_SOCKET
                        Path of socket or OpenVPN management interface. [env var: OPENVPN_AAD_AUTH_SOCKET_PATH]
  -p OPENVPN_PASSWORD, --openvpn-password OPENVPN_PASSWORD
                        Passwort for OpenVPN management interface. [env var: OPENVPN_AAD_AUTH_PASSWORD]
  --openvpn-release-hold
                        Release hold on OpenVPN Server if --management-hold is enabled [env var: OPENVPN_AAD_AUTH_RELEASE_HOLD]

Azure AD settings:
  --client-id CLIENT_ID
                        Client ID of application. [env var: AAD_CLIENT_ID]
  --token-authority TOKEN_AUTHORITY
                        A URL that identifies a token authority. It should be of the format https://login.microsoftonline.com/your_tenant. By default, we will use https://login.microsoftonline.com/organizations
                        [env var: AAD_TOKEN_AUTHORITY]
  --graph-endpoint GRAPH_ENDPOINT
                        Endpoint of the graph API. See: https://developer.microsoft.com/en-us/graph/graph-explorer [env var: AAD_GRAPH_ENDPOINT]

Prometheus settings:
  --prometheus          Enable prometheus statistics [env var: AAD_PROMETHEUS_ENABLED]
  --prometheus-listen-addr PROMETHEUS_LISTEN_ADDR
                        prometheus listen addr [env var: AAD_PROMETHEUS_LISTEN_HOST]
  --prometheus-listen-port PROMETHEUS_LISTEN_PORT
                        prometheus statistics [env var: AAD_PROMETHEUS_PORT]
  --log-level LOG_LEVEL
                        Configure the logging level. [env var: AAD_LOG_LEVEL]

Register an app with AAD

See: https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app

TL;DR

1. Login as admin into tenant
2. Open App registrations in Azure AD admin center
3. Click new registration
4. Pick a name, chose a "Supported account types"-option. Let the redirect uri blank and click register.
5. Copy the client-id. You need the client-id as configuration option for openvpn-auth-azure-ad.
6. Click on Authentication on the left menu
7. "Add a platform", pick Mobile and desktop applications and chose the "MSAL only" option.
8. On Advanced settings, set "Treat application as a public client" to yes.

Required settings on OpenVPN configuration files

server.conf

Use auth-gen-token only on OpenVPN 2.5+. It conflicts with --auth-token.

management socket-name unix [pw-file]
management-hold
management-client-auth

See Reference manual for OpenVPN for detailed management settings.

If no client certificate are required

If your setup does not require certificates, the following options are required:

username-as-common-name

client.conf

auth-user-pass
auth-retry interact

auth-user-pass is always required otherwise dynamic challenges will not work.

Prometheus support

openvpn-auth-azure-ad has some built-in prometheus support to collect some statistics about authenticators. By default, the prometheus endpoint listen on port 9723.

Related projects

• https://github.com/CyberNinjas/openvpn-auth-aad
• https://github.com/stilljake/openvpn-azure-ad-auth

Copyright and license

© 2022 Jan-Otto Kröpke (jkroepke)

Licensed under the MIT License