Traditional OpenVPN authentication typically relies on:

• Username/password credentials
• Certificate-based authentication
• Two-factor authentication (2FA) methods

While Google 2FA exists as mentioned, native OAuth/OpenID Connect integration would provide better SSO capabilities for Google Workspace environments.

Here are three viable approaches to implement this:

1. OAuth Proxy with PAM Module

This method uses an OAuth proxy server that authenticates users against Google's OAuth endpoint, then passes the authentication to OpenVPN via PAM:

# oauth2-proxy configuration example
provider = "google"
email_domains = ["yourdomain.com"]
upstreams = ["file:///var/run/openvpn.sock"]
client_id = "YOUR_CLIENT_ID.apps.googleusercontent.com"
client_secret = "YOUR_CLIENT_SECRET"
cookie_secret = "SECURE_RANDOM_STRING"

2. Certificate-Based Auth with Google Identity

Similar to your GitLab reference, we can implement certificate-based auth where:

1. Users authenticate via Google OAuth
2. System issues short-lived client certificates
3. OpenVPN accepts these certificates

Example OpenVPN server config:

client-cert-not-required
username-as-common-name
plugin /usr/lib/openvpn/openvpn-plugin-auth-pam.so openvpn

3. Custom Auth Script Using Google's API

For more control, you can write a custom auth script:

#!/bin/bash
# auth_script.sh
GOOGLE_TOKEN=$1
USER_EMAIL=$(curl -s https://www.googleapis.com/oauth2/v1/userinfo?access_token=$GOOGLE_TOKEN | jq -r '.email')

if [[ $USER_EMAIL =~ @yourdomain\.com$ ]]; then
  exit 0
else
  exit 1
fi

Key considerations when implementing this:

• Token validation latency
• Certificate revocation handling
• Session timeout alignment between Google and OpenVPN
• Mobile client compatibility

If building in-house seems complex, consider:

• Twingate (supports Google Workspace integration)
• Pritunl (enterprise OpenVPN with OAuth support)
• Perimeter 81 (cloud-based VPN with SSO)

When implementing any OAuth solution:

# Always:
- Use PKCE for OAuth flows
- Validate token signatures
- Implement proper scope validation
- Set appropriate token lifetimes
- Monitor authentication logs

---

Traditional OpenVPN authentication methods (PAM, LDAP, or local auth) don't meet modern SSO requirements for enterprises using Google Workspace. While Google's 2FA adds security, it doesn't provide true OAuth/SAML integration that IT teams increasingly demand.

We can achieve this through three approaches:

1. OAuth2 Proxy Middleware
   [Client] -> [OAuth2 Proxy] -> [OpenVPN Auth Script]

2. SAML SP Module
   [OpenVPN] <-SAML-> [Google IdP]

3. Custom PAM Module
   [OpenVPN PAM] -> [python-social-auth] -> [Google OAuth]

This is the most maintainable approach using oauth2-proxy:

# oauth2-proxy.cfg
provider = "google"
email_domains = ["yourdomain.com"]
client_id = "YOUR_GOOGLE_CLIENT_ID"
client_secret = "YOUR_CLIENT_SECRET"
cookie_secret = "SECURE_RANDOM_STRING"
upstreams = "file:///etc/openvpn/auth-control.sock"

Then configure OpenVPN to use the auth script:

# server.conf
script-security 2
auth-user-pass-verify "/path/to/oauth_verify.py" via-file

For enterprises using Google as IdP:

# Using mod_auth_mellon for Apache
<Location /vpn-auth>
    MellonEnable "info"
    MellonSPPrivateKeyFile /etc/apache2/mellon/sp.key
    MellonSPCertFile /etc/apache2/mellon/sp.crt
    MellonIdPMetadataFile /etc/apache2/mellon/google_metadata.xml
    MellonEndpointPath /mellon
    MellonVariable "cookie"
</Location>

For certificate + OAuth combination:

# openvpn-custom-auth.py
def verify_google_token(token):
    # Use google-auth library to verify
    from google.oauth2 import id_token
    from google.auth.transport import requests
    
    try:
        idinfo = id_token.verify_oauth2_token(
            token, requests.Request(), CLIENT_ID)
        if idinfo['iss'] not in ['accounts.google.com', 'https://accounts.google.com']:
            raise ValueError("Wrong issuer")
        return idinfo['email']
    except:
        return None

• Always validate token audience (client_id)
• Implement proper session timeout (match Google token expiry)
• Use dedicated OAuth credentials for VPN service
• Monitor failed authentication attempts