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This guide shows you how to use ngrok to receive Castle webhooks on your localhost app. By integrating ngrok with Castle, you can:
  • Develop and test Castle webhooks locally without deploying to a public environment or setting up HTTPS.
  • Inspect and troubleshoot requests from Castle in real time via the inspection UI and API.
  • Modify and replay Castle webhook requests with a single click instead of reproducing events manually in your Castle account.
  • Secure your app with Castle webhook validation provided by ngrok. Invalid requests are blocked by ngrok before reaching your app.

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What you’ll need

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1. Start your app

For this tutorial, you can use the sample Node.js app on GitHub. To install the sample, run the following in a terminal: 
git clone https://github.com/ngrok/ngrok-webhook-nodejs-sample.git
cd ngrok-webhook-nodejs-sample
npm install
Then start the app: 
npm start
The app runs on port 3000 by default. You can confirm it’s running by visiting http://localhost:3000. The app logs request headers and body in the terminal and shows a message in the browser.

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2. Expose your app with ngrok

Once your app is running locally, you’re ready to put it online securely using ngrok.
The ngrok agent uses your authtoken to authenticate when you start a tunnel.
  • Start ngrok: 
    ngrok http 3000
  • Copy the URL ngrok displays. Your app is now exposed at that URL for use with Castle.

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3. Configure Castle to send webhooks

Castle can send webhook requests to your app when events occur in your account. To register for those events:
  • Sign in to the Castle dashboard.
  • On the left menu, click the gear icon next to your name and then click Configuration.
  • On the Application page, click Webhooks in the top menu and then click Add a new Endpoint.
  • On the Set up a new webhook endpoint page, enter your ngrok URL in the ENDPOINT URL field (for example, https://1a2b-3c4d-5e6f-7g8h-9i0j.ngrok.app).
  • Select all events under EVENTS TO SUBSCRIBE and click Save.
  • On the Webhooks page, click the slider icon to activate the webhook.

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Run webhooks with Castle and ngrok

Castle sends different request body contents depending on the event. To trigger new calls from Castle to your app:
  • On the Application page, click Webhooks in the top menu and then click Test for your webhook.
  • On the Test the webhook endpoint popup, select an event in the EVENT field and click Send test webhook.
You can also create an event using the Castle SDK.
Confirm your localhost app receives the test event and logs both headers and body in the terminal.

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Inspecting requests

ngrok’s Traffic Inspector captures all requests made through your ngrok endpoint to your localhost app. Select any request to view detailed information about both the request and response.
To avoid exposing secrets, accounts only collect traffic metadata by default. You must enable full capture in the Observability section of your account settings to capture complete request and response data.
Use the traffic inspector to:
  • Validate webhook payloads and response data
  • Debug request headers, methods, and status codes
  • Troubleshoot integration issues without adding logging to your app

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Replaying requests

Test your webhook handling code without triggering new events from your service using the Traffic Inspector’s replay feature:
  1. Send a test webhook from your service to generate traffic in your Traffic Inspector.
  2. Select the request you want to replay in the traffic inspector.
  3. Choose your replay option:
    • Click Replay to send the exact same request again
    • Select Replay with modifications to edit the request before sending
  4. (Optional) Modify the request: Edit any part of the original request, such as changing field values in the request body.
  5. Send the request by clicking Replay.
Your local application will receive the replayed request and log the data to the terminal.

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Secure webhook requests

ngrok can verify that incoming requests are from your Castle webhook so only that traffic reaches your app.
Webhook verification is limited to 500 validations per month on free accounts. If you need more, you can upgrade to Hobbyist or Pay-as-you-go. See TPU Pricing for details.
To add verification:
  • In the Castle dashboard, go to Configuration and copy the value of the API SECRET field on the Application page.
  • Create a Traffic Policy file named castle_policy.yml. Replace {your api secret} with the value you copied: 
    on_http_request:
  - actions:
      - type: verify-webhook
        config:
          provider: castle
          secret: "{your api secret}"
  • Restart ngrok with the policy file: 
    ngrok http 3000 --traffic-policy-file castle_policy.yml
  • Test the webhook endpoint or create an event using the Castle SDK to trigger a request.
Your app should receive the request and log it in the terminal.