AWS CloudFormation: how do I refer to the default/main route table (that is created when a VPC is created) in a cloudformation template?

Question

I have a CloudFormation template that creates a custom VPC. The template creates the following resources - a VPC, an Internet Gateway, attaches the IGW to the VPC, and creates a Public Subnet. I want to add a route (destination 0.0.0.0/0, target IGW) to the Route Table that gets created as part of the VPC.

I have read through the cloudformation documentation for routes, route tables to figure out how to do this, but to no avail.

I can use the Fn::Ref function to refer to resources or parameters that are explicitly created as part of the template, but how do I refer to resources that get created inherently with the VPC?

Any insights on how to re-use the existing route table, NACL and Security Group are much appreciated.

Thanks,

Answer 1

Good job so far - you have your internet gateway, route table, and a public subnet. Now you need to create the route and attach the route table to the subnet if you haven't already done so. If you're using YAML it might look something like this:

 InternetGateway:
    Type: AWS::EC2::InternetGateway
    Properties:
      Tags:
        - Key: Name
          Value: !Ref EnvironmentName

  InternetGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      InternetGatewayId: !Ref InternetGateway
      VpcId: !Ref VPC

  PublicSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      AvailabilityZone: !Select [ 0, !GetAZs '' ]
      CidrBlock: !Ref PublicSubnet1CIDR
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} Public Subnet (AZ1)

  PublicRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} Public Routes

  DefaultPublicRoute:
    Type: AWS::EC2::Route
    DependsOn: InternetGatewayAttachment
    Properties:
      RouteTableId: !Ref PublicRouteTable
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway

  PublicSubnet1RouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      RouteTableId: !Ref PublicRouteTable
      SubnetId: !Ref PublicSubnet1

Answer 2

1. Don't use the default route table (see https://serverfault.com/questions/588904/aws-vpc-default-route-table-in-cloudformation)
2. You can get default security group as per https://serverfault.com/questions/544439/aws-cloudformation-vpc-default-security-group
3. And finally you can also get the DefaultNetworkAcl in the same as DefaultSecurityGroup above. See also https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-vpc.html)