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vpcDependency

A Lambda function which implements a Custom Resource for Cloud Formation that gets VPCs and subnets by name.

Installation

Create a Role with ./create-role.sh. This creates a new stack with the appropriate permissions for the function.

Deploy the lambda function with ./deploy-lambda.sh. Now the function can be used to get VPCs via a Cloud Formation Custom Resource.

Cloud Formation Usage

Use the function inside your Cloud Formation template by declaring a custom resource, Custom::VpcDependency.

The Custom::VpcDependency refers to a VpcName that is sent to the Lambda function and is used to lookup a VPC. If no VPC or more than one is found FAILED is returned.

It is also possible to send a filter parameter OnlyDefaultSubnets to limit the returned subnets to the ones which are default in their availability zones. This can be used to only get one subnet per zone.

The outputs from the Custom::VpcDependency can be referred with Fn:GetAtt.

Available values are: VpcId, CidrBlock, Subnets, SubnetIds, SubnetId0, SubnetId1,...

Example: "Fn::GetAtt": ["Vpc", "VpcId"]

Example Output

{
  VpcId: 'vpc-061bfc6c',
  State: 'available',
  CidrBlock: '172.31.0.0/16',
  DhcpOptionsId: 'dopt-1d1bfc77',
  InstanceTenancy: 'default',
  IsDefault: true,
  SubnetId0: 'subnet-07110265',
  SubnetId1: 'subnet-041bfc6e',
  SubnetId2: 'subnet-a68eb3d2',
  SubnetIds: 'subnet-07110265,subnet-041bfc6e,subnet-a68eb3d2',
  Subnets: ['subnet-07110265','subnet-041bfc6e','subnet-a68eb3d2']
}

Extended Example with Stack

"Parameters": {
  "VpcName": {
    "Description": "Name of VPC",
    "Type": "String",
    "Default": "default"
  }
},
"Resources": {
  "Vpc": {
    "Type": "Custom::VpcDependency",
    "Properties": {
      "ServiceToken": { "Fn::Join": [ "", [
        "arn:aws:lambda:",
        { "Ref": "AWS::Region" },
        ":",
        { "Ref": "AWS::AccountId" },
        ":function:vpcDependency"
      ] ] },
      "VpcName": { "Ref": "VpcName" },
      "OnlyDefaultSubnets": "true"
    }
  }
  "Outputs": {
    "VpcId": {
      "Value": {
        "Fn::GetAtt": [ "Vpc", "VpcId" ]
      },
      "Description": "Vpc Id"
    },
    "SubnetIds": {
      "Value": {
        "Fn::GetAtt": [ "Vpc", "SubnetIds" ]
      },
      "Description": "Comma-separated subnet ids"
    }

  }
}