alb-rule

A CloudFormation custom resource for creating ALB redirection rules.

At the time this was created, AWS had introduced redirect rules on ALBs but had not yet made them available in CloudFormation. The boto3 Python library does support it however.

Here is a how to create a Lambda-backed CloudFormation custom resource that can be used to implement redirect rules.

Update: CloudFormation now supports ALB Listener Rules, so this is no longer necessary

Lambda Function

This is a Python 2.7 function. All Python libraries it uses are available in the standard Lambda environment.

To build the Lambda zip file:

cd lambda
zip -r /tmp/AlbListenerRule .

Copy /tmp/AlbListenerRule.zip to your S3 bucket.

CloudFormation Example

The example CloudFormation template demonstrates how to use the Lambda-backed Custom Resource. It creates:-

• A Lambda function with and appropriate IAM role.
• An ALB with HTTP and HTTPS listeners.
• A default target group for the listeners.
• Some example redirect rules for the ALB.
  • Redirect to HTTP to HTTPS.
  • Redirect the /images path to a CloudFront distribution.

Rule Parameters

Parameters to create a rule are as required by the boto3 elb2 create_rule method. Only the Conditions, Actions, ListenerArn and Priority are used. They are defined in CloudFormation YAML in the same format that the boto3 method expects them.

For example:-

Conditions:
  - Field: host-header
    Values:
      - "*.*"
Actions:
  - Type: redirect
    RedirectConfig:
      Protocol: HTTPS
      Port: 443
      StatusCode: HTTP_301
ListenerArn: !Ref HTTPlistener
Priority: 10

• Multiple Conditions can be specified. All must be true for the rule to apply. The Field value can be only 'host-header' or 'path-pattern'.
• Multiple Actions can be specified. If there is more than one, an Order element is required in each. The last Action must be of Type 'forward' or 'fixed-response'.