AWS ALB HTTP Header-Based Routing and Advanced Redirects with Terraform

AWS ALB HTTP Header-Based Routing and Advanced Redirects with Terraform

Welcome back to my Terraform for AWS series focused on helping beginner DevOps engineers! 👋

In the previous part of this series, we implemented host-based routing for multiple applications behind an ALB. In this article, we take it further with HTTP Header-Based Routing and Redirect Rules.

This builds directly on our ALB module and expands the listener rules.

🔄 Key Changes Introduced in This Part

1. Listener Rules – HTTP Header-Based Routing

We replaced host-based rules with http_header-based routing. Instead of routing based on domain names, now traffic is routed based on a specific HTTP header called custom-header.

myapp1-rule = {
  actions = [{
    type = "weighted-forward"
    target_groups = [{
      target_group_key = "mytg1"
      weight = 1
    }]
    stickiness = {
      enabled  = true
      duration = 3600
    }
  }]
  conditions = [{
    http_header = {
      http_header_name = "custom-header"
      values = ["app-1", "app1", "my-app-1"]
    }
  }]
}

This block creates a routing rule that checks the HTTP header custom-header for values like app-1, app1, or my-app-1. If matched, traffic is forwarded to the target group mytg1. A similar block exists for mytg2 with header values app-2, app2, and my-app-2.

This is useful in multi-tenant apps or for routing based on custom API headers.

2. Redirect Rules Based on Query String and Host Header

We added two advanced redirect rules that demonstrate ALB’s flexibility:

A. Redirect by Query String

my-redirect-query = {
  priority = 3
  actions = [{
    type        = "redirect"
    status_code = "HTTP_302"
    host        = "rezaops.com"
    path        = "/dummy-query-redirect"
    protocol    = "HTTPS"
  }]
  conditions = [{
    query_string = {
      key   = "website"
      value = "aws-eks"
    }
  }]
}

➡️ If a request contains ?website=aws-eks, it will be redirected to https://rezaops.com/dummy-query-redirect. This is helpful for search-based or campaign-based routing.

B. Redirect by Host Header

my-redirect-hh = {
  priority = 4
  actions = [{
    type        = "redirect"
    status_code = "HTTP_302"
    host        = "rezaops.com"
    path        = "/dummy-host-header-redirect"
    protocol    = "HTTPS"
  }]
  conditions = [{
    host_header = {
      values = ["azure-aks11.rezaops.com"]
    }
  }]
}

➡️ This rule checks for the host header azure-aks11.rezaops.com and performs a redirect to a dummy path on rezaops.com. You can use this technique to guide users from deprecated subdomains to new paths or services.

These two examples show how to apply precise and user-friendly redirection logic at the load balancer level.

✅ What Stayed the Same

The ALB module version, target groups, and EC2 attachments remain unchanged:

• Target groups mytg1 and mytg2 are still configured with HTTP health checks.
• We use aws_lb_target_group_attachment to attach private EC2s to these target groups.

This consistency ensures zero disruption while adding new rules and behaviors.

📘 Summary

This part demonstrated how to:

• Route traffic using custom HTTP headers
• Configure advanced redirects using query strings and host headers

These techniques allow more granular control in multi-app, multi-tenant, or redirection-heavy environments.

📢 Coming Up Next:
Our next topic covers ALB-based redirects triggered by HTTP headers and query strings..

Stay tuned, and let me know your thoughts or questions in the comments!