Creating a Secure Media CDN with CloudFront and Lambda@Edge

In this series of posts, we are going to build a CloudFront distribution that serves your S3 Media, performs on-demand image transformation, and even authenticates incoming requests! But before that, let's just backtrack over a few concepts we will be using:

• CloudFront: AWS Content Delivery Network. We put it in front of our S3 bucket to provide a cache layer in dozens of points of presence around the world.

• Lambdas: AWS serverless functions that are provisioned on-demand.

• Lambda@Edge: Lambda functions that are attached to a CloudFront distribution in order to run a piece of code before or after CloudFront fetches a media from our origin (the S3 bucket).

• Cloudinary: Image transformation service. We will use it as a "custom origin" to on-demand transform images, but more on that in the next blog post.

• Terraform: Infrastructure as Code tool that will provide us an easy way to set up everything we need on AWS.

Note: we will not cover in these posts how to upload media files to the S3 or how to create a service to emit authentication credentials. We will solely focus on the delivery of the media part.

Getting started

Before we start anything, we will need Terraform installed, an AWS account set up and the credentials that Terraform will use to access this AWS account in order to create resources there, which can be done by either using AWS access key “env vars” or using the AWS CLI to authenticate, take a look here for more info.

Setting up S3 and CloudFront

You can go ahead and put all of the following code in a single main.tf file or separate them into files inside the same folder as you see fit. You can also terraform plan and apply after every bit or in a single sweep by the end of the post. You can see the full solution for this post in this GitHub repo. With everything ready, we start by creating an S3 bucket. We will use an aws_s3_bucket resource for that.

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You can see we need a unique bucket name there because S3 names must be unique across AWS as a whole, not only your account. We set the ACL (access control list) to private, meaning only you can see and edit the contents of S3. Don't worry, we will have other means to grant CloudFront read access to it. In the last part, we are defining the CORS rule, enabling different origins to perform GET and HEAD requests.

Now, we need to prevent public ACLs from ever being created or used, in order to ensure that no one can publicly access the contents of this S3 bucket. We can achieve this by using the aws_s3_bucket_public_access_block resource.

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With this, your bucket is now protected from any unwanted public access! But now you may be wondering, how does CloudFront access it in order to distribute the media files? Well, we grant such access using an Origin Access Identity, so let us create one:

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Awesome! So are we all set? Well, no. You see, this resource above alone does not really do anything. We just created a credential that can be used, but not only is it not being used, it also does not grant permission to anything. So now we need to:

1. Tell S3 to allow read access when a request uses this OAI to authenticate.

2. Tell CloudFront to use this OAI whenever requesting media to the origin (our S3 bucket)

We already have the S3 bucket, so let's start with the first one. How can we grant such permission? The answer is bucket policies! And we can do this on Terraform as follows:

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So what we are doing here is: we are defining a policy document JSON. That document tells our S3 bucket to allow the s3:GetObject (which means read to a single file) whenever the requester is identified as the OAI we created. And then we attach this policy document to our S3 using the aws_s3_bucket_policy resource so it is actually used by our bucket.

Now for the final bit, we need to create our CloudFront distribution keeping in mind that it needs to point to our S3 bucket while using the OAI to authenticate. All of this can be done using the aws_cloudfront_distribution resource.

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This was a long one but bear with me. First, we put a meaningful comment to know what this distribution is about, we also create it as enabled, do not attach any web ACL, and allow IPv6 requests. Then, we define our origin which means our S3 bucket. Inside the origin config, we tell CloudFront to use the origin access identity we created previously, as it has read access to our S3. We do not restrict traffic by region and we set the viewer certificate to the one that comes by default with CloudFront, meaning you will get a {randomId}.cloudfront.net by the end. If you want to use your own domain, you need to set up your SSL certificate here and use the alias attribute. Finally, we set up our default cache behavior. By default, requests will be allowed and cached when using the HTTP methods above, and we also allow compress and set the default and max TTL for 30 days. We allow query strings to be forwarded (this will be needed later when we have our Lambda@Edge), redirect HTTP traffic to HTTPS, and use our S3 origin (the only one we have anyway).

Now, if you have not done it yet, perform the following terraform commands (be sure to set up AWS credentials env vars beforehand):

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This will output all the actions we will perform, it should look like this:

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To actually perform these actions, you need to run terraform apply. This can take a while, and the very last log should display the URL of your CloudFront distribution (called cloudfront_domain_name). To test this, you can go to the S3 bucket in the AWS Console and upload any media file there. After this, you should be able to fetch it using https://{randomId}.cloudfront.net/{imagePath}, because using the plain S3 url http://{bucket-name}}.s3.{aws-region}.amazonaws.com/{imagePath} will not work!

You can check the full solution we went through up to this point here in this GitHub repo.

That is about it! We have an S3 media bucket with a CloudFront distribution working! In the next steps, we will see how to create a Lambda@Edge and attach them to our CloudFront distribution in order to have on-demand image transformations and authentication. See you part II!

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