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MLu
Asked: 2014-11-19 15:38:08 +0800 CST

Conditional Property in AWS CloudFormation

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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?

amazon-web-services

2 Answers

  1. Best Answer

    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

    • 23

  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": {
        [...]
    }
}
    • 0