How do I add routes to the main route table in my Amazon VPC with CloudFormation?

Resolution

When you use CloudFormation to create an Amazon VPC, CloudFormation doesn't recognize the main route table that's created by default. You can't add or remove routes from the main route table because you can't reference the route table from your CloudFormation template.

To resolve this issue, use an AWS Lambda backed custom resource in the CloudFormation template.

To create a CloudFormation stack with a custom resource, use the following RouteTable-template.yml template:

AWSTemplateFormatVersion: 2010-09-09
Description: Template to add routes to default/main routetable of VPC
Resources:
  MyVPC:
    Type: 'AWS::EC2::VPC'
    Properties:
      CidrBlock: 10.0.0.0/16
      Tags:
        - Key: Env
          Value: Test
  LambdaIAMRole:
    Type: 'AWS::IAM::Role'
    DependsOn: MyVPC
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - lambda.amazonaws.com
            Action:
              - 'sts:AssumeRole'
      Path: /
      Policies:
        - PolicyName: root
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action:
                  - 'ec2:Describe*'
                Resource: '*'
              - Effect: Allow
                Action:
                  - 'logs:CreateLogGroup'
                  - 'logs:CreateLogStream'
                  - 'logs:PutLogEvents'
                Resource: 'arn:aws:logs:*:*:*'
  LambdaFunction:
    Type: 'AWS::Lambda::Function'
    Properties:
      Handler: index.lambda_handler
      Role: !GetAtt LambdaIAMRole.Arn
      Runtime: python3.9
      Timeout: 50
      Code:
        ZipFile: |
            from __future__ import print_function
            import json
            import boto3
            import urllib3
            import cfnresponse

            SUCCESS = "SUCCESS"
            FAILED = "FAILED"

            http = urllib3.PoolManager()

            print('Loading function')
            ec2 = boto3.client('ec2')

            def lambda_handler(event, context):
                print("Received event: " + json.dumps(event, indent=2))
                responseData={}
                try:
                    if event['RequestType'] == 'Delete':
                        print("Request Type:",event['RequestType'])
                        print("Delete Request - No Physical resources to delete")
                    elif event['RequestType'] == 'Create':
                        print("Request Type:",event['RequestType'])
                        VPCID=event['ResourceProperties']['VPCID']
                        RouteTableID=get_vpc(VPCID)
                        responseData={'RouteTableID':RouteTableID}
                        print("Sending response to custom resource")
                    elif event['RequestType'] == 'Update':
                        print("Request Type:",event['RequestType'])
                        VPCID=event['ResourceProperties']['VPCID']
                        RouteTableID=get_vpc(VPCID)
                        responseData={'RouteTableID':RouteTableID}
                        print("Sending response to custom resource")
                    responseStatus = 'SUCCESS'
                    print("responseStatus: " + responseStatus)
                    cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData, "CustomResourcePhysicalID")
                except Exception as e:
                    print('Failed to process:', e)
                    responseStatus = 'FAILURE'
                    responseData = {'Failure': 'Something bad happened.'}
                    cfnresponse.send(event, context, cfnresponse.FAILURE, responseData, "CustomResourcePhysicalID")

            def get_vpc(VPCID):
                response = ec2.describe_route_tables (
                  Filters=[
                    {
                      'Name': 'association.main',
                      'Values': [ 'true' ]
                    },
                    {
                      'Name': 'vpc-id',
                      'Values': [ VPCID ]
                    }
                  ]
                )
                print("Printing the VPC Route Table ID ....")
                RouteTableID=response['RouteTables'][0]['RouteTableId']
                print(RouteTableID)
                return RouteTableID

            def send(event, context, responseStatus, responseData, physicalResourceId=None, noEcho=False):
                responseUrl = event['ResponseURL']
                print(responseUrl)
                responseBody = {'Status': responseStatus,
                                'Reason': 'See the details in CloudWatch Log Stream: ' + context.log_stream_name,
                                'PhysicalResourceId': physicalResourceId or context.log_stream_name,
                                'StackId': event['StackId'],
                                'RequestId': event['RequestId'],
                                'LogicalResourceId': event['LogicalResourceId'],
                                'Data': responseData}
                json_responseBody = json.dumps(responseBody)
                print("Response body:\n" + json_responseBody)
                headers = {
                    'content-type' : '',
                    'content-length' : str(len(json_responseBody))
                }
                try:
                    response = http.request('PUT', responseUrl, headers=headers, body=json_responseBody)
                    print("Status code: " + response.reason)
                except Exception as e:
                    print("send(..) failed executing requests.put(..): " + str(e))


  Lambdatrigger:
    Type: 'Custom::RouteTableLambda'
    Properties:
      ServiceToken: !GetAtt LambdaFunction.Arn
      VPCID: !Ref MyVPC
  MyInternetGateway:
    Type: 'AWS::EC2::InternetGateway'
    Properties:
      Tags:
        - Key: Env
          Value: Test
  AttachGateway:
    Type: 'AWS::EC2::VPCGatewayAttachment'
    Properties:
      VpcId: !Ref MyVPC
      InternetGatewayId: !Ref MyInternetGateway
  MyRoute:
    Type: 'AWS::EC2::Route'
    Properties:
      RouteTableId: !GetAtt Lambdatrigger.RouteTableID

      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref MyInternetGateway
Outputs:
  RouteTableID:
    Value: !GetAtt Lambdatrigger.RouteTableID

The custom resource uses an Lambda function to retrieve the main route table ID. To reference the ID, use the GetAtt function in AWS::EC2::Route to add routes to the main route table. Or, open the CloudFormation console, and then choose Output. You can find the information under RouteTableID.

The following example shows the SUCCESS response body that the custom resource sends to the CloudFormation stack:

{  "Status": "SUCCESS",
  "Reason": "See the details in CloudWatch Log Stream: 2022/08/31/[$LATEST]c48b90efb3944c11ad3fb6e1ce5e1f45",
  "PhysicalResourceId": "CustomResourcePhysicalID",
  "StackId": "arn:aws:cloudformation:us-west-1:abcd:stack/VPC-RT/06c957b0-297e-11ed-afb5-02ca6fd67f8d",
  "RequestId": "55c0f2b8-3044-47f7-aba4-84502b4ef632",
  "LogicalResourceId": "Lambdatrigger",
  "NoEcho": false,
  "Data": {
    "RouteTableID": "rtb-0fba8d15701234567a"
  }
}