How to pass Private Ip of multiple EC2 to eachother in AWS Cloudformation.

Question

Hi there,
I have a use case where, I am initializing 3 EC2 instances in Cloudformation template.
And in each of these EC2 instances, I need to use the PrivateIp of all 3 EC2 instances created in template.
How can I achieve this?

Any help would be appreciated.
Thanks in advance!

Answer

Hi,

To achieve what you want you must one Fn::GetAtt with attribute PrivateIP of EC2 instance. See https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html

So, let's say your 3 EC2 instances have logical name Instance1, Instance2 and Instance3.

To refer to the private ip of each from any location in your template, you need to code (YAML): `!GetAtt InstanceN.PrivateIp` (short form) or `Fn::GetAtt: [ InstanceN, PrivateIp ]`

Best.

Didier

Answer

Hi.

If you need the private IP address in the user data script there is no need to pass it via CloudFormation. Instead use the Instance metadata endpoint to retrieve it.

```
TOKEN=`curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"`
IP = `curl -H "X-aws-ec2-metadata-token: $TOKEN" -v http://169.254.169.254/latest/meta-data/local-ipv4`
```

You can read more [here](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instancedata-data-retrieval.html) and [here](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html)

Answer

If it were me, this is what I would do:

In the CloudFormation template, add the instance private IP addresses to the Outputs section:
```
Outputs:
  InstanceAPrivateIP:
    Value: !GetAtt InstanceA.PrivateIp
  ...etc...
```

Next, when you launch the containers you can query the outputs of the stack by using the CloudFormation `DescribeStacks` API. You don't say what language you're using but from the CLI you could do this:
```
InstanceAIP=$(aws cloudformation describe-stacks --stack-name STACKNAMEHERE --query "Stacks[0].Outputs[?OutputKey=='InstanceAPrivateIP'].OutputValue" --output text)
  ...etc...
docker run -e 1st_EC2_IP=${InstaceAIP} -e 2nd_EC2_IP=${InstanceBIP} -e 3rd_EC2_IP=${InstanceCIP}
```

Note that you can query the stack output from within the container as well if that works for you. The requirement here is that you know the stack name when doing the query.

Alternately, you could store the IP addresses in ParameterStore and then retrieve them at runtime.

How to pass Private Ip of multiple EC2 to eachother in AWS Cloudformation.

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