Mutual TLS

---

Overview

This module performs mutual TLS (mTLS) authentication when the ngrok edge terminates TLS on incoming connections to your HTTP endpoint. The client must present a valid TLS certificate that is signed by one of the specified CAs or the connection will be rejected.

ngrok injects headers into upstream HTTP requests with information about the client certificate presented.

Mutual TLS is supported on both HTTP and TLS endpoints.

Quickstart

Agent CLI

ngrok http 80 --mutual-tls-cas /path/to/cas.pem

Agent Configuration File

tunnels:
  example:
    proto: http
    addr: 80
    mutual_tls_cas: /path/to/cas.pem

SSH

note

Mutual TLS is not supported via SSH.

Go SDK

import (
    "context"
    "crypto/x509"
    "encoding/pem"
    "net"
    "os"

    "golang.ngrok.com/ngrok"
    "golang.ngrok.com/ngrok/config"
)

func listenMutualTLS(ctx context.Context) net.Listener {
    caBytes, _ := os.ReadFile("/path/to/cas.pem")
    der, _ := pem.Decode(caBytes)
    certs, _ := x509.ParseCertificates(der.Bytes)

    listener, _ := ngrok.Listen(ctx,
        config.HTTPEndpoint(
            config.WithMutualTLSCA(certs...),
        ),
        ngrok.WithAuthtokenFromEnv(),
    )

    return listener
}

Rust SDK

use ngrok::prelude::*;

async fn start_tunnel() -> anyhow::Result<impl Tunnel> {
    let ca_cert: &[u8] = load_bytes!("/path/to/cas.pem");

    let sess = ngrok::Session::builder()
        .authtoken_from_env()
        .connect()
        .await?;

    let tun = sess
        .http_endpoint()
        .mutual_tlsca(ca_cert.into())
        .listen()
        .await?;

    println!("Listening on URL: {:?}", tun.url());

    Ok(tun)
}

Kubernetes Ingress Controller

note

Mutual TLS is not yet supported with the Kubernetes Ingress Controller.

You can +1 the GitHub issue tracking its implementation.

Edges

Mutual TLS is a supported Edge module. When the Mutual TLS module is configured via an Edge, you must specify one or more references to Certificate Authority objects.

The Mutual TLS Edge module is applied to the edge directly and not to any individual route. This is because Mutual TLS is enforced before any HTTP processing can begin.

• Mutual TLS Edge Module API Resource
• Certificate Authority API Resource

Try it out

This example assumes you have an x509 private key and certificate encoded as PEM files called client-key.pem and client-cert.pem, respectively. The certificate must be signed by one of the CA certificates you provided to the Mutual TLS module.

Run curl with the following command:

curl --client client-cert.pem --key client-key.pem https://yourapp.ngrok.app

curl has a shortcut to pass a single file if the private key and certificate are concatenated together.

cat client-cert.pem client-key.pem > client-cert-and-key.pem
curl --cert client-cert-and-key.pem https://yourapp.ngrok.app

Behavior

Multiple CAs

You may specify multiple CAs to be used for mTLS authentication. A connection is considered authenticated if it presents a certificate signed by any of the specified CAs. Most agents allow you to specify multiple CAs by simply specifying a PEM file that contains multiple x509 CA certificates concatenated together. A file like that might look like:

-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

CA Basic Constraint

x509 certificates contain a basic constraint attribute called cA which defines whether or not the certificate may be used as a CA.

ngrok will refuse to accept a certificate as an mTLS certificate authority unless this constraint is set to true.

See RFC 5280 4.2.1.9.

Reference

Upstream Headers

When the Mutual TLS module is enabled, ngrok will insert headers into the HTTP request sent to your upstream server with details about the client certificate that was presented for the Mutual TLS handshake.

Header Name	Value
ngrok-mtls-subject-cn	The client certificate subject common name
ngrok-mtls-subject-alt-name-dns	The client certificate's DNS subject alternative names
ngrok-mtls-email-addresses	The client certificate's email subject alternative names
ngrok-mtls-serial-number	The client certificate's serial number

Events

When the mutual TLS module is enabled, it populates the following fields in http_request_complete.v0 events.

Fields
tls.client_cert.serial_number
tls.client_cert.subject.cn

No event data is captured when the module is enabled on TLS endpoints.

Errors

If the client does not present any certificate or it does not present a valid certificate signed by the CA, the TLS handshake will be aborted. Because the connection is aborted during the TLS handshake, the client will not receive an HTTP error response or an ngrok error code.

Instead, the TLS connection aborts with a TLS error as defined by RFC 5246. The most common alert code returned for a failed mutual TLS handshake is code 42 (bad_certificate) which most TLS implementations will report with the error string string "bad certificate".

Licensing

Mutual TLS is available on the Enterprise plan only.