Tutorial: Create a basic Lambda@Edge function - Amazon CloudFront

Tutorial: Create a basic Lambda@Edge function

This tutorial shows you how to get started with Lambda@Edge by creating and configuring a sample Node.js function that runs in CloudFront. This example adds HTTP security headers to a response when CloudFront retrieves a file. (This can improve security and privacy for a website.)

You don't need your own website for this tutorial. However, when you choose to create your own Lambda@Edge solution, you follow similar steps and select from the same options.

Step 1: Sign up for an AWS account

If you haven't already done so, sign up for an AWS account. For more information, see Sign up for an AWS account.

Step 2: Create a CloudFront distribution

Before you create the example Lambda@Edge function, you must have a CloudFront environment to work with that includes an origin to serve content from.

For this example, you create a CloudFront distribution that uses an Amazon S3 bucket as the origin for the distribution. If you already have an environment to use, you can skip this step.

To create a CloudFront distribution with an Amazon S3 origin
  1. Create an Amazon S3 bucket with a file or two, such as image files, for sample content. For help, follow the steps in Upload your content to Amazon S3. Make sure that you set permissions to grant public read access to the objects in your bucket.

  2. Create a CloudFront distribution and add your S3 bucket as an origin, by following the steps in Create a CloudFront web distribution. If you already have a distribution, you can add the bucket as an origin for that distribution instead.

    Tip

    Make a note of your distribution ID. Later in this tutorial when you add a CloudFront trigger for your function, you must choose the ID for your distribution in a dropdown list—for example, E653W22221KDDL.

Step 3: Create your function

In this step, you create a Lambda function from a blueprint template in the Lambda console. The function adds code to update security headers in your CloudFront distribution.

To create a Lambda function
  1. Sign in to the AWS Management Console and open the AWS Lambda console at https://console.aws.amazon.com/lambda/.

    Important

    Make sure that you're in the US-East-1 (N. Virginia) AWS Region (us-east-1). You must be in this Region to create Lambda@Edge functions.

  2. Choose Create function.

  3. On the Create function page, choose Use a blueprint, and then filter for the CloudFront blueprints by entering cloudfront in the search field.

    Note

    CloudFront blueprints are available only in the US-East-1 (N. Virginia) Region (us-east-1).

  4. Choose the Modify HTTP response header blueprint as the template for your function.

  5. Enter the following information about your function:

    • Function name – Enter a name for your function.

    • Execution role – Choose how to set the permissions for your function. To use the recommended basic Lambda@Edge permissions policy template, choose Create a new role from AWS policy templates.

    • Role name – Enter a name for the role that the policy template creates.

    • Policy templates – Lambda automatically adds the policy template Basic Lambda@Edge permissions because you chose a CloudFront blueprint as the basis for your function. This policy template adds execution role permissions that allow CloudFront to run your Lambda function for you in CloudFront locations around the world. For more information, see Set up IAM permissions and roles for Lambda@Edge.

  6. Choose Create function at the bottom of the page.

  7. In the Deploy to Lambda@Edge pane that appears, choose Cancel. (For this tutorial, you must modify the function code before deploying the function to Lambda@Edge.)

  8. Scroll down to the Code source section of the page.

  9. Replace the template code with a function that modifies the security headers that your origin returns. For example, you could use code similar to the following:

    'use strict'; exports.handler = (event, context, callback) => { //Get contents of response const response = event.Records[0].cf.request; const headers = response.headers; //Set new headers headers['strict-transport-security'] = [{key: 'Strict-Transport-Security', value: 'max-age= 63072000; includeSubdomains; preload'}]; headers['content-security-policy'] = [{key: 'Content-Security-Policy', value: "default-src 'none'; img-src 'self'; script-src 'self'; style-src 'self'; object-src 'none'"}]; headers['x-content-type-options'] = [{key: 'X-Content-Type-Options', value: 'nosniff'}]; headers['x-frame-options'] = [{key: 'X-Frame-Options', value: 'DENY'}]; headers['x-xss-protection'] = [{key: 'X-XSS-Protection', value: '1; mode=block'}]; headers['referrer-policy'] = [{key: 'Referrer-Policy', value: 'same-origin'}]; //Return modified response callback(null, response); };
  10. Choose File, Save to save your updated code.

  11. Choose Deploy.

Proceed to the next section to add a CloudFront trigger to run the function.

Step 4: Add a CloudFront trigger to run the function

Now that you have a Lambda function to update security headers, configure the CloudFront trigger to run your function to add the headers in any response that CloudFront receives from the origin for your distribution.

To configure the CloudFront trigger for your function
  1. In the Lambda console, on the Function overview page for your function, choose Add trigger.

  2. For Trigger configuration, choose CloudFront.

  3. Choose Deploy to Lambda@Edge.

  4. In the Deploy to Lambda@Edge pane, under Configure CloudFront trigger, enter the following information:

    • Distribution – The CloudFront distribution ID to associate with your function. In the dropdown list, choose the distribution ID.

    • Cache behavior – The cache behavior to use with the trigger. For this example, leave the value set to *, which means your distribution's default cache behavior. For more information, see Cache behavior settings in the Distribution settings reference topic.

    • CloudFront event – The trigger that specifies when your function runs. We want the security headers function to run whenever CloudFront returns a response from the origin. In the dropdown list, choose Origin response. For more information, see Add triggers for a Lambda@Edge function.

  5. Select the Confirm deploy to Lambda@Edge check box.

  6. Choose Deploy to add the trigger and replicate the function to AWS locations worldwide.

  7. Wait for the function to replicate. This typically takes several minutes.

    You can check to see if replication is finished by going to the CloudFront console and viewing your distribution. Wait for the distribution status to change from Deploying to a date and time, which means that your function has been replicated. To verify that the function works, follow the steps in the next section.

Step 5: Verify that the function runs

Now that you've created your Lambda function and configured a trigger to run it for a CloudFront distribution, check to make sure that the function is accomplishing what you expect it to. In this example, we check the HTTP headers that CloudFront returns, to make sure that the security headers are added.

To verify that your Lambda@Edge function adds security headers
  1. In a browser, enter the URL for a file in your S3 bucket. For example, you might use a URL similar to https://d111111abcdef8.cloudfront.net/image.jpg.

    For more information about the CloudFront domain name to use in the file URL, see Customize the URL format for files in CloudFront.

  2. Open your browser's Web Developer toolbar. For example, in your browser window in Chrome, open the context (right-click) menu, and then choose Inspect.

  3. Choose the Network tab.

  4. Reload the page to view your image, and then choose an HTTP request on the left pane. You see the HTTP headers displayed in a separate pane.

  5. Look through the list of HTTP headers to verify that the expected security headers are included in the list. For example, you might see headers similar to those shown in the following screenshot.

    HTTP headers list with the expected security headers highlighted.

If the security headers are included in your headers list, great! You've successfully created your first Lambda@Edge function. If CloudFront returns errors or there are other issues, continue to the next step to troubleshoot the issues.

Step 6: Troubleshoot issues

If CloudFront returns errors or doesn't add the security headers as expected, you can investigate your function's execution by looking at CloudWatch Logs. Be sure to use the logs stored in the AWS location that is closest to the location where the function is executed.

For example, if you view the file from London, try changing the Region in the CloudWatch console to Europe (London).

To examine CloudWatch logs for your Lambda@Edge function
  1. Sign in to the AWS Management Console and open the CloudWatch console at https://console.aws.amazon.com/cloudwatch/.

  2. Change Region to the location that is shown when you view the file in your browser. This is where the function is executing.

  3. In the left pane, choose Logs to view the logs for your distribution.

For more information, see Monitor CloudFront metrics with Amazon CloudWatch.

Step 7: Clean up your example resources

If you created an Amazon S3 bucket and CloudFront distribution just for this tutorial, delete the AWS resources that you allocated so that you no longer accrue charges. After you delete your AWS resources, any content that you added is no longer available.

Tasks

Delete the S3 bucket

Before you delete your Amazon S3 bucket, make sure that logging is disabled for the bucket. Otherwise, AWS continues to write logs to your bucket as you delete it.

To disable logging for a bucket
  1. Open the Amazon S3 console at https://console.aws.amazon.com/s3/.

  2. Select your bucket, and then choose Properties.

  3. From Properties, choose Logging.

  4. Clear the Enabled check box.

  5. Choose Save.

Now, you can delete your bucket. For more information, see Deleting a bucket in the Amazon Simple Storage Service Console User Guide.

Delete the Lambda function

For instructions to delete the Lambda function association and optionally the function itself, see Delete Lambda@Edge functions and replicas.

Delete the CloudFront distribution

Before you delete a CloudFront distribution, you must disable it. A disabled distribution is no longer functional and does not accrue charges. You can enable a disabled distribution at any time. After you delete a disabled distribution, it's no longer available.

To disable and delete a CloudFront distribution
  1. Open the CloudFront console at https://console.aws.amazon.com/cloudfront/v4/home.

  2. Select the distribution that you want to disable, and then choose Disable.

  3. When prompted for confirmation, choose Yes, Disable.

  4. Select the disabled distribution, and then choose Delete.

  5. When prompted for confirmation, choose Yes, Delete.

Related information

Now that you have a basic idea of how Lambda@Edge functions work, learn more by reading the following: