In CloudFormation, store a Subnet ID in Parameter Store

Question

I have a CloudFormation template to create a VPC and add a subnet and security group to it. I need to store the new security group's ID and the subnet's ID in AWS Parameter Store.

This works fine for the security group using Value: !GetAtt [securityGroup, GroupId] in the , but I can't figure out how to reference the subnet's ID in a way that works with Parameter Store.

Here are the relevant parts of the CloudFormation:

Resources:

### Subnet resource ###

  subnet1:
    Type: AWS::EC2::Subnet
    Properties:
      AvailabilityZone: !Ref SubnetAvailabilityZone1
      CidrBlock: 10.2.0.0/16
      VpcId: !Ref requesterVpc

### Parameter Store resource ###

  subnet1Id:
    Type: AWS::SecretsManager::Secret
    Properties:
      Description: Lambda Subnet ID
      Name: /lambda-vpc/subnet/1
      Value: !GetAtt [subnet1, id] # <---------- this doesn't work        (Error A below)
      Value: !Ref subnet1          # <---------- this doesn't work either (Error B below)

Error A

Attempting to get the 'id' attribute of the subnet understandably gives the error

resource subnet1 does not support attribute type id in Fn::GetAtt.

I also tried "SubnetId" to no avail. Looking at the subnet's CF documentation shows there's no attribute for the subnet's ID returnable with Fn::GetAtt.

Error B

Attempting to get the subnet ID using Fn::Ref doesn't work either. Even though the documentation says "Ref returns the ID of the subnet", trying to update the CF stack gives this error when it attempts to create/update the "subnet1Id" parameter:

Property validation failure: [Encountered unsupported properties in {/}: [Value]]

amacrobert amacrobert · Accepted Answer · 2020-12-30T17:13:03

The issue was the AWS resource type I was using for the parameters.

I was using AWS::SecretsManager::Secret, but I needed AWS::SSM::Parameter. After switching to that and adding Type: String to Properties, !Ref subnet1 worked.

In CloudFormation, store a Subnet ID in Parameter Store

Error A

Error B

1 Answers