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Fadi Hamwi
Fadi Hamwi

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Dynamic Challenge in openVPN

#openvpn #challenge #iot #programming

Dynamic Challenge-Response Authentication in OpenVPN

TL;DR — OpenVPN's dynamic challenge-response mechanism lets a VPN server send a real-time prompt to the client during the authentication handshake — think OTP codes, hardware token PINs, or MFA push confirmations. By enabling the management interface on both client and server, you can programmatically intercept the challenge, deliver it to the user, collect their response, and feed it back into the connection flow. Challenges travel as a base64-encoded string in a specific >PASSWORD notification, and responses are injected with the username-password management command. The result is a standards-compatible, scriptable second factor that doesn't require patching OpenVPN itself.

Syllabus

  1. What is OpenVPN?
  2. What is the Challenge-Response Model?
  3. How OpenVPN Implements Dynamic Challenge-Response
  4. The Format of Challenges and Responses in OpenVPN
  5. Enabling the Management Interface

1. What is OpenVPN?

In a nutshell openVPN establishes an encrypted tunnel between two endpoints. All traffic within that tunnel is treated as if it were on a private LAN, regardless of the underlying public network. (like any other VPN).
It's widely used because of Cross-platform support and most importantly the flexible authentication.

The downside is the nature of the single-threaded connection.

2. What is the Challenge-Response Model?

The challenge-response model is a class of authentication protocols wildly used in IoT in which one party (the verifier — here, the VPN server) issues an unpredictable value called a challenge to another party (the claimant — here, the VPN client/user). The claimant must compute or retrieve or sign the challenge to produce a response and send it back. Now the verifier will have to verify the response (this verification differ depending on what authentication strategy we are using) and then Authentication succeeds only if the response satisfies the verifier's expectation.

  Server                          Client
    │                               │
    │──── 1. Send Challenge ────────▶│
    │                               │  (user sees OTP prompt, consults
    │                               │   hardware token, push app, etc.)
    │◀─── 2. Send Response ─────────│
    │                               │
    │  3. Verify Response           │
    │     ✓ → Grant Access          │
    │     ✗ → Reject                │
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Why Challenge-Response Instead of a Static Password?

For security reasons obviously no need to explain more than that :)

3. How OpenVPN Implements Dynamic Challenge-Response

OpenVPN's implementation is called Dynamic Challenge/Response (DCR) and works within the existing --auth-user-pass credential flow and most importantly you need to enable --auth-retry infinite on the client side we will explain why in a minute. Here's the big picture:

3.1 The Dynamic Challenge Flow 

Client → sends username + password ( username can be mac, pass could be public key)
       → Server script returns a CHALLENGE string in a CRV format
       → Client re-sends the encoded response
       → Server script validates final response → ACCEPT/DENY
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3.2 The Role of the Management Interface

The OpenVPN Management Interface is a telnet-like TCP socket (or Unix domain socket) that exposes runtime control of the OpenVPN daemon. When dynamic challenge is in play, the management interface is the bridge between the daemon and your application (in case you're using external authentication entity):

  • On the server: used to monitor connection state, push client-deny or client-auth decisions, and optionally inject per-client environment variables.
  • On the client: receives the >PASSWORD:CR_TEXT notification containing the base64-encoded challenge text, and accepts the username-password command to supply the response.

4. The Format of Challenges and Responses in OpenVPN

4.1 The Challenge Notification (Client-Side Management Interface)

When the server's auth script signals a dynamic challenge, the client's management interface emits:

>PASSWORD:Need 'Auth' username/password
SC:<flags>:<BASE64-CHALLENGE-TEXT>
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Breaking this down:

>PASSWORD:Need 'Auth' CRV1:E,R:VW50ZXIgZGVpbiBPVFA=
            ▲         ▲ ▲ ▲
            │         │ │ └── Base64-encoded challenge string
            │         │ └──── Flags (see below)
            │         └────── Literal prefix "CRV1::" for dynamic challenge
            └──────────────── Management interface event prefix
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4.2 Challenge Flags

The flags field is a comma-separated list immediately after SC::

Flag Meaning
E Echo — the client UI should display the response as the user types (e.g. it is not a secret)
R Required — the challenge must be answered; the connection cannot proceed without a response

Examples:

  • CRV1:E,R: — echo the input, response required
  • CRV1:R: — mask input like a password, response required
  • CRV1:: — optional challenge, mask input

4.3 Decoding the Challenge Text

The challenge text after the final : is Base64-encoded. Decode it to get the human-readable prompt:

echo "VW50ZXIgZGVpbiBPVFA=" | base64 --decode
# Output: Enter your OTP
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4.4 The Response Format

Responses to a dynamic challenge use a specially encoded password field. The client must send:

SCRV1::<BASE64-PASSWORD>::<BASE64-RESPONSE>
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Field Description
SCRV1: Static Challenge Response Version 1 — literal prefix
<BASE64-PASSWORD> The user's regular password, Base64-encoded
<BASE64-RESPONSE> The user's OTP/challenge answer, Base64-encoded

Example:

# User's regular password: "s3cr3tP@ss"
# User's OTP response:     "847291"

echo -n "s3cr3tP@ss" | base64   # → czNjcjN0UEBzcw==
echo -n "847291"     | base64   # → ODQ3Mjkx

# Final password field sent to server:
SCRV1:czNjcjN0UEBzcw==:ODQ3Mjkx
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The server-side --auth-user-pass-verify script receives this as the $password environment variable and must parse the SCRV1: prefix to split out the two components.

4.5 Signaling a Challenge from the Server

The auth script communicates a challenge back to the client by writing a file whose name is taken from $auth_control_file and whose contents follow this format:

CRV1:<flags>:<state_id>:<base64_username>:<challenge_text>
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Field Description
CRV1: Challenge-Response Version 1
<flags> E (echo), R (required), or combined
<state_id> Opaque string, echoed back in the response for stateful auth backends
<base64_username> The username, Base64-encoded
<challenge_text> Human-readable prompt shown to the user (plain text here)

Example file content written by the auth script:

CRV1:R:session-token-abc123:am9obg==:Enter your OTP token
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5. Enabling the Management Interface

5.1 Server Configuration (server.conf) 

dev tun

auth-user-pass  

management 127.0.0.1 7505
management-client-auth
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5.2 Client Configuration (client.ovpn) 

# --- Core client settings ---
client
dev tun

auth-user-pass # can be removed if server has it
auth-retry infinite # this is the most important because when we get the challenge from server we will disconnect the session and open a new one and send the encoded response with it via the correct format.

management 127.0.0.1 7505
management-query-passwords # important
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useful links:
openvpn forum
management interface docs
*Written for OpenVPN 2.5+ / 2.6. The CRV1 and SCRV1 formats are stable across these versions. Always consult the OpenVPN man page for the authoritative specification.

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