AWS::WAFv2::WebACLAssociation ResourceArn for Application Load Balancer in CloudFormation

Question

I have a CloudFormation template which creates an ElasticBeanstalk environment like this:

        "ApplicationEnvironment": {
            "Type": "AWS::ElasticBeanstalk::Environment",
            "Properties": {
                "ApplicationName": {
                    "Ref": "Application"
                },
                "SolutionStackName": "64bit Amazon Linux 2018.03 v2.11.2 running Java 8",
                "VersionLabel": { 
                    "Ref": "AppVersion"
                },
                "Tier": {
                    "Name": "WebServer",
                    "Type": "Standard"
                },
                "OptionSettings": [
                    ...
                    {
                        "Namespace": "aws:elasticbeanstalk:environment",
                        "OptionName": "EnvironmentType",
                        "Value": "LoadBalanced"
                    },
                    {
                        "Namespace": "aws:elasticbeanstalk:environment",
                        "OptionName": "LoadBalancerType",
                        "Value": "application"
                    },
                    ...

---
        "WAF": {
            "Type": "AWS::WAFv2::WebACL",
            "Properties": {
                "DefaultAction": {
                    "Type": "BLOCK"
                },              
                "Scope": "REGIONAL",
                "VisibilityConfig": {
                    "CloudWatchMetricsEnabled": "false",
                    "MetricName": { "Fn::Join": [ "", [ { "Ref": "AWS::StackName" }, "metric-waf" ] ] },
                    "SampledRequestsEnabled": "false"
                },
                "Rules": [
                    {
                        "Action" : {
                          "Type" : "BLOCK"
                        },
                        "Priority" : 0,
                        "Statement" : {
                            "ManagedRuleGroupStatement": {
                                "VendorName": "AWS",
                                "Name": "AWSManagedRulesCommonRuleSet"
                            }
                        }
                    }
                ]
            }
        },
        "WAFAssociation": {
            "Type" : "AWS::WAFv2::WebACLAssociation",
            "Properties" : {
                "ResourceArn" : ???,
                "WebACLArn" : { "Ref": "WAF" }
            }
        }

I intend to associate the Beanstalk ALB with the WebACL but have no idea how to refer to the application load balancer ARN that the template creates. I cannot just put a hardcoded ARN in since it always changes based on what the template creates.

Is there some way I can refer to the ALB ARN in the ResourceArn field? Or do I need to apply the WebACL somewhere in the Beanstalk Option Settings?

Answer 1

I think the only way would be through a custom resource which takes EB env name, uses describe_environment_resources API call to get the EB env info (including LA arn), and returns back to your stuck.

Below is a working example of such a resource which you could add to your template:

  LambdaBasicExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Statement:
        - Effect: Allow
          Principal:
            Service: lambda.amazonaws.com
          Action: sts:AssumeRole
      Path: /
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/AmazonEC2FullAccess
        - arn:aws:iam::aws:policy/AWSElasticBeanstalkFullAccess
        - arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole

  MyCustomResource:
    Type: Custom::GetEBLoadBalancerArn
    Properties:
      ServiceToken: !GetAtt 'MyCustomFunction.Arn'
      EBEnvName: !Ref MyEnv

  MyCustomFunction:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.lambda_handler
      Description: "Get ARN of EB Load balancer"
      Timeout: 30
      Role: !GetAtt 'LambdaBasicExecutionRole.Arn'
      Runtime: python3.7
      Code:
        ZipFile: |
          import json
          import logging
          import cfnresponse
          import boto3

          logger = logging.getLogger()
          logger.setLevel(logging.INFO)

          eb = boto3.client('elasticbeanstalk')
          ec2 = boto3.client('ec2')

          def lambda_handler(event, context):
            logger.info('got event {}'.format(event))  
            try:

              responseData = {}

              if event['RequestType'] in ["Create"]:                      

                eb_env_name = event['ResourceProperties']['EBEnvName']

                response = eb.describe_environment_resources(
                    EnvironmentName=eb_env_name
                )

                lb_arn = response['EnvironmentResources']['LoadBalancers'][0]['Name']

                logger.info(str(response['EnvironmentResources']['LoadBalancers'][0]['Name']))

                responseData = {
                  "LBArn": lb_arn
                }

                cfnresponse.send(event, context, 
                                 cfnresponse.SUCCESS, responseData)

              else:
                logger.info('Unexpected RequestType!') 
                cfnresponse.send(event, context, 
                                  cfnresponse.SUCCESS, responseData)

            except Exception as err:

              logger.error(err)
              responseData = {"Data": str(err)}
              cfnresponse.send(event,context, 
                               cfnresponse.FAILED,responseData)
            return    

Having the resource you would just use:

        "WAFAssociation": {
            "Type" : "AWS::WAFv2::WebACLAssociation",
            "Properties" : {
                "ResourceArn" : { "GetAtt": ["MyCustomResource", "LBArn"] },
                "WebACLArn" : { "Ref": "WAF" }
            }
        }