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Slack Webhook

TL;DR

To integrate Slack webhooks with ngrok:

  1. Launch your local webhook. npm start
  2. Launch ngrok. ngrok http 3000
  3. Configure Slack webhooks with your ngrok URL.
  4. Secure your webhook requests with verification.

This guide covers how to use ngrok to integrate your localhost app with Slack using Webhooks. Slack webhooks can be used to notify an external application whenever specific events occur in your Slack account. Slack requires your application to be available through an HTTPS endpoint.

By integrating ngrok with Slack, you can:

  • Develop and test Slack webhooks locally, eliminating the time in deploying your development code to a public environment and setting it up in HTTPS.
  • Inspect and troubleshoot requests from Slack in real-time via the inspection UI and API.
  • Modify and Replay Slack Webhook requests with a single click and without spending time reproducing events manually in your Slack account.
  • Secure your app with Slack validation provided by ngrok. Invalid requests are blocked by ngrok before reaching your app.

Step 1: Start your app​

For this tutorial, we'll use the sample NodeJS app available on GitHub.

To install this sample, run the following commands in a terminal:

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This will get the project installed locally.

Now you can launch the app by running the following command: npm run startSlack

The app runs by default on port 3000.

You can validate that the app is up and running by visiting http://localhost:3000. The application logs request headers and body in the terminal and a message in the browser.

Step 2: Launch ngrok​

Once your app runs successfully on localhost, let's get it on the internet securely using ngrok!

  1. If you're not an ngrok user yet, just sign up for ngrok for free.

  2. Download the ngrok agent.

  3. Go to the ngrok dashboard and copy your Authtoken. Tip: The ngrok agent uses the auth token to log into your account when you start a tunnel.

  4. Start ngrok by running the following command:

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  5. ngrok will display a URL where your localhost application is exposed to the internet (copy this URL for use with Slack). ngrok agent running

Step 3: Integrate Slack​

To register a webhook with your Slack account, follow the instructions below:

  1. Access the Slack Web app and sign in using your Slack account. Tip: There is no need to open the slack app on your desktop for now. Continue with the use Slack in your browser option.

  2. In the same browser tab, access the Slack API portal and then click Create an App. Tip: If you already have some apps created, click ****.

  3. In the Create an app window, click From scratch, provide the App Name, select a workspace under Pick a workspace to develop your app in, and then click Create App.

  4. In the Basic Information page, expand Add features and functionality and then click Event Subscriptions.

  5. In the Event Subscriptions page, click the Enable Events slider to turn it on. In the Request URL field, enter the URL provided by the ngrok agent to expose your application to the internet (i.e., https://1a2b-3c4d-5e6f-7g8h-9i0j.ngrok.app). Slack Request URL

    Note: Slack makes a one-time call to your app. It sends a challenge parameter as part of the request body and expects the app to respond with this value. See the Slack documentation for more information.

  6. On the same page, expand Subscribe to events on behalf of users, click Add Workspace Event, select message.im, and then click Save Changes. Tip: More about Slack event types here.

  7. In the left menu, click Install App, click Install to Workspace, and then click Allow to allow your application to access your workspace.

Run Webhooks with Slack and ngrok​

Because you've subscribed to the message.im event and installed your app to your slack workspace, you now can direct message any person in the workspace to make Slack call your localhost application:

  1. Access the Slack Web app or open the slack app on your desktop, and verify that your application appears under Apps in the left menu.

  2. Select one person in your workspace and send the person a message. Alternatively, you can select the Slackbot, write Hello Slack bot! in the message field, and then send it.

Confirm your localhost app receives notifications about the message.

Tip: Slack sends different request body contents and headers depending on the trigger event.

Inspecting requests​

ngrok's Traffic Inspector captures all requests made through your ngrok endpoint to your localhost app. Click on any request to view detailed information about both the request and response.

info

By default, accounts only collect traffic metadata to avoid exposing secrets. 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

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. Modify the request (optional): 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.

Secure webhook requests​

The ngrok signature webhook verification feature allows ngrok to assert that requests from your Slack webhook are the only traffic allowed to make calls to your localhost app.

Note: This ngrok feature is limited to 500 validations per month on free ngrok accounts. For unlimited, upgrade to Pro or Enterprise.

This is a quick step to add extra protection to your application.

  1. In the Basic Information page for your Slack app, click Show for the Signing Secret and copy the value that appears.

  2. Create a file named slack_policy.yml with the following content, replacing {your signing secret} with your Signing Secret from Slack:

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  3. Restart your ngrok agent by running the command:

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  4. In your Slack app, select the Slackbot, write Hello Slack bot! in the message field, and then send it.

Verify that your local application receives the request and logs information to the terminal.