How to Add a Custom Header to Amazon AWS ELB Response

How to Add a Custom Header to Amazon AWS ELB Response

In the modern computing world, load balancers play a crucial role in distributing network traffic across multiple servers. Amazon’s Elastic Load Balancing (ELB) is a prime example of this technology. ELB automatically distributes incoming application traffic across multiple targets, ensuring that your applications are highly available and fault-tolerant.

In this article, we will focus on a specific feature of ELB: adding custom headers to ELB responses. Custom headers can help you troubleshoot, analyze traffic, enhance security, or personalize your services.

What Are Custom Headers?

Before we dive into the “how,” let’s take a moment to answer the question: “What are custom headers?” HTTP headers let the client and the server pass additional information with an HTTP request or response. A custom HTTP header is a header that you define for your specific needs.

Why Add Custom Headers?

There are several reasons why you might want to add custom headers. One common use case is to enhance security by adding headers such as Strict-Transport-Security, Content-Security-Policy, or X-Content-Type-Options. Another use case is to improve debugging and tracing by adding custom identifiers to your responses.

Adding Custom Headers to AWS ELB Response

Unfortunately, as of my knowledge cutoff in September 2021, AWS ELB does not support adding custom headers directly. However, there is a workaround. You can add custom headers using AWS Lambda functions in combination with the Application Load Balancer (ALB), a specific type of ELB.

Step 1: Create a Lambda Function

First, you need to create a Lambda function that will add the custom header. Here’s a simple example in Node.js:

exports.handler = async (event) => {
    const response = event.Records[0].cf.response;
    const headers = response.headers;

    headers['custom-header'] = [{
        key: 'Custom-Header',
        value: 'Custom Value',
    }];

    return response;
};

In this example, we’re adding a custom header called “Custom-Header” with a value of “Custom Value.”

Step 2: Create a Target Group for Your Lambda Function

Next, you need to create a target group for your Lambda function. In the AWS Management Console, navigate to the EC2 service, then to Load Balancing > Target Groups. Click “Create target group,” select “Lambda Function” as the target type, and select the Lambda function you created in step 1.

Step 3: Register Your Lambda Function as a Target

Now you need to register your Lambda function as a target. In the AWS Management Console, navigate to the EC2 service, then to Load Balancing > Target Groups. Select the target group you created in step 2, click the “Targets” tab, and click “Register targets.” Select your Lambda function and click “Include as pending below,” then “Register.”

Step 4: Create a Listener Rule for Your Application Load Balancer

Finally, you need to create a listener rule for your Application Load Balancer. In the AWS Management Console, navigate to the EC2 service, then to Load Balancing > Load Balancers. Select your load balancer and click the “Listeners” tab. Click “View/edit rules,” then “Insert rule.” Select “Forward to,” choose the target group you created in step 2, and click “Save.”

With these steps, your Application Load Balancer will now add the custom header to every response.

Conclusion

While AWS ELB might not support adding custom headers directly, you can achieve the same result with a simple workaround. Using AWS Lambda functions, you can customize your ELB responses to fit your specific needs.

Remember, while custom headers can enhance your services, they can also introduce potential security risks if not used correctly. Always validate and sanitize your headers to ensure the safety and efficiency of your system.

The world of AWS is vast and constantly evolving. Stay tuned for more tips and tricks to navigate this powerful cloud computing service.

Keywords: AWS ELB, Custom Headers, AWS Lambda, Application Load Balancer, HTTP Headers, AWS Management Console, Load Balancing

Tags: #AWS, #ELB, #AWSLambda, #HTTPHeaders, #LoadBalancing

About Saturn Cloud

Saturn Cloud is your all-in-one solution for data science & ML development, deployment, and data pipelines in the cloud. Spin up a notebook with 4TB of RAM, add a GPU, connect to a distributed cluster of workers, and more. Join today and get 150 hours of free compute per month.