I'm working with AWS CloudFormation and need some help with a list of private subnets. I'm trying to configure my ECS task within a specific set of subnets using the following code:
```yaml
AwsVpcConfiguration:
Subnets: !Split [ ",", !Ref PrivateSubnetIds ]
AssignPublicIp: "Disabled"
SecurityGroups:
- !GetAtt ECSSecurityGroup.GroupId
```
My main question is about referencing the `PrivateSubnetIds` in the `ECSSecurityGroup` resource. I want to define allowed ports for each specified subnet, but I'm not sure how to effectively use the list of subnet IDs. Here's where I'm stuck:
```yaml
ECSSecurityGroup:
SecurityGroupIngress:
- CidrIp: "192.168.0.0/24" #CIDR for the first subnet
IpProtocol: "tcp"
...
- CidrIp: "192.168.4.0/24" #CIDR for the second subnet
...
```
Is there a way to reference the subnet IDs from `PrivateSubnetIds` in my security group definition, especially when the number of subnets may vary?
1 Answer
To reference specific IPs for each subnet in your security group, you can use the `!Select` function alongside `!Split`. For example, in your `ECSSecurityGroup`, you could do something like this:
```yaml
ECSSecurityGroup:
SecurityGroupIngress:
- CidrIp: !Select [ 0, !Split [ ",", !Ref PrivateSubnetIds ]]
IpProtocol: "tcp"
...
- CidrIp: !Select [ 1, !Split [ ",", !Ref PrivateSubnetIds ]]
...
```
This way, you can dynamically select the subnet CIDR block based on the index. Just make sure you adjust the indexing based on the order of your subnets!
Got it, thanks! But what if I'm dealing with a variable number of subnets? Is there a workaround for that?