Fetch IPv6 address of EC2 instance for creating AAAA DNS record

cloudformation-ipv6-address-via-custom-resource

# large parts taken from https://aws.amazon.com/blogs/mt/four-ways-to-retrieve-any-aws-service-property-using-aws-cloudformation-part-1/
AWSTemplateFormatVersion: '2010-09-09'
Parameters:
    ExampleServiceInstanceName:
    Default: Example
    Description: Instance and Host Name
    Type: String
Resources:
  ExampleService:
    Properties:
      ImageId: ami-09e0d6fdf60750e33  # Ubuntu Server 20.04 LTS (HVM), SSD Volume Type, (64-bit Arm)
      InstanceType: t4g.small
      KeyName: example.key
      SubnetId: subnet-123456789  # <- insert your subnet here
    Type: AWS::EC2::Instance
  ExampleDnsRecordAAAA:
    Properties:
      Comment: A record for the Example server.
      HostedZoneId: Z1234567890    # <- insert your hosted zone id here
      Name:
        Fn::Join:
        - .
        - - !Ref ExampleServiceInstanceName
          - Z1234567890             # <- insert your hosted zone id here
      ResourceRecords:
      - Fn::GetAtt: CustomIpv6Resource.Ipv6Address
      TTL: 900
      Type: AAAA
    Type: AWS::Route53::RecordSet
  
  #
  # CustomResource for fetching IPv6 address
  #
  LambdaBasicExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Statement:
        - Effect: Allow
          Principal:
            Service: lambda.amazonaws.com
          Action: sts:AssumeRole
      Path: /
      Policies:
        - PolicyName: CustomLambdaEC2DescribePolicy
          PolicyDocument:
            Version: '2012-10-17'
            Statement:
              - Effect: Allow
                Action:
                  - logs:CreateLogGroup
                  - logs:CreateLogStream
                  - logs:PutLogEvents
                Resource: arn:aws:logs:*:*:*
              - Effect: Allow
                Action:
                  - ec2:DescribeNetworkInterfaces
                Resource: '*'

  CustomIpv6Resource:
    Type: AWS::CloudFormation::CustomResource
    Properties:
      ServiceToken: !GetAtt 'CustomFunction.Arn'
      ResourceRef: !Ref ExampleService

  CustomFunction:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.lambda_handler
      Description: "Retrieves IPv6 address of EC2 instance"
      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)

          def lambda_handler(event, context):
            logger.info('got event {}'.format(event))
            try:
              responseData = {}
              if event['RequestType'] == 'Delete':
                logger.info('Incoming RequestType: Delete operation')
                cfnresponse.send(event, context, cfnresponse.SUCCESS, {})
              if event['RequestType'] in ["Create", "Update"]:
                # 1. retrieve resource reference ID or Name
                ResourceRef=event['ResourceProperties']['ResourceRef']
                # 2. retrieve boto3 client
                client = boto3.client('ec2')
                # 3. Invoke describe/retrieve function using ResourceRef
                response = response=client.describe_network_interfaces(Filters=[{'Name':'attachment.instance-id', 'Values':[ResourceRef] }])
                # 4. Parse and return required attributes
                responseData = {}
                responseData['Ipv6Address'] = response.get('NetworkInterfaces')[0]['Ipv6Addresses'][0]['Ipv6Address']
                logger.info('Retrieved IPv6 address!')
                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)