Advanced ALB Listener Rules with Terraform: HTTP Header Routing & Redirects

In this part of my ongoing Terraform AWS Application Load Balancer (ALB) series, we focus on implementing advanced listener rules specifically HTTP Header-Based Routing and smart redirection techniques using query string and host header conditions.

These patterns are especially helpful in scenarios like:

• Routing requests in multi-tenant environments
• Handling marketing query parameters
• Redirecting traffic from legacy domains to modern services

Let’s walk through the key changes introduced in this configuration.

🔁 1. HTTP Header-Based Routing

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"]
    }
  }]
}

🔍 Explanation:

• http_header condition: Matches requests based on the value of a custom HTTP header.
• Flexible matching: Accepts multiple possible values.
• Sticky sessions: Maintains session affinity by routing the same client to the same target group for 1 hour.
• Use Case: Ideal for multi-tenant APIs or separating environments like staging vs. production.

🔁 2. Query String-Based Redirect

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

🔍 Explanation:

• Redirects any request with a query string like ?website=aws-eks.
• Sends the user to https://rezaops.com/aws-eks/.
• HTTP 302: A temporary redirect that preserves the original method.
• Use Case: Great for marketing campaigns, tracking parameters, or simplified short links.

🔁 3. Host Header-Based Redirect

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

🔍 Explanation:

• This rule looks at the Host header from the request.
• If the subdomain matches legacy.example.rezaops.com, traffic is redirected to a consolidated path.
• Use Case: Excellent for sunsetting subdomains or migrating legacy apps.

✅ What Stayed the Same

• Target groups mytg1 and mytg2: No change in health checks or port settings.
• Instance attachments: Reused existing EC2 configurations using aws_lb_target_group_attachment.

This ensures full backward compatibility while extending functionality.

🧩 Closing

This update brings powerful customization to your ALB setup using HTTP headers, query parameters, and host headers to route and redirect users exactly where you need them.