Conditional Property in AWS CloudFormation

Question

We've got an AWS CloudFormation template for creating some EC2 instances. Some of those however require a specific PrivateIpAddress and I'm struggling to figure out how to incorporate that to the template.

For now I've got a template parameter PrivateIP and a creating a Condition RequestedPrivateIP. So far so good. However I can't figure out how to incorporate it to the AWS::EC2::Instance resource specification. I tried this:

 "PrivateIpAddress": { "Fn::If": [ "RequestedPrivateIP", { "Ref": "PrivateIP" }, "" <-- This doesn't work ] }, 

But that fails when RequestedPrivateIP is false with

CREATE_FAILED AWS::EC2::Instance NodeInstance Invalid addresses: [] 

Any idea how to optionally assign a static Private IP and if not specified leave it to AWS to set a dynamic one?

Answer 1

i would change the structure to:

"PrivateIpAddress": { "Fn::If": [ "RequestedPrivateIP", { "Ref": "PrivateIP" }, {"Ref" : "AWS::NoValue" } ] } 

the AWS::NoValue is there to give you the else option for your if statement. http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/pseudo-parameter-reference.html

Answer 2

Since it seems like the PrivateIpAddress property does not support an empty string as it value, I would suggest creating two separate resources of your AWS::EC2::Instance. One of them will have your condition RequestedPrivateIP while the other one should have the same condition but negated, e.g. DidNotRequestPrivateIP.

"InstanceWithPrivateIp": { "Type": "AWS::EC2::Instance", "Condition": "RequestedPrivateIP", "Properties": { [...] } }, "InstanceWithoutPrivateIp": { "Type": "AWS::EC2::Instance", "Condition": "DidNotRequestPrivateIP", "Properties": { [...] } }