You have an existing VPC and you want to deploy Big-IP, and you want to add a default route to steer traffic to the ENI of the Big-IP after it's created.
We can easily use F5’s supported templates to deploy Big-IP in AWS with a CloudFormation Template (CFT) into this VPC, but what if you want to add some automation after the Big-IP is deployed? For example, what if you wanted to add a default route to steer traffic to the ENI of the Big-IP that was just created? You might think of these options:
This article explains option #3. Modify the automation for your other use cases.
The steps to follow when you need Lambda functionality as part of your CFT are:
I’ll explain how to do each of these and provide an example CFT at the end.
Include code such as below to create an IAM role that is appropriate for your Lambda function. Do not provide more permission than required to this role. Very public breaches have occurred as a result of mis-use of IAM roles, so by keeping your IAM roles limited in scope, you decrease your risk if it is mis-used. (In fact, I wrote some best practices to decrease this risk.) In our case, we’re allowing my Lambda function permissions over EC2 service.
Here’s where you’ll need to define the data you’ll pass to your Lambda function (if any), what you want Lambda to do, and what it should return (if anything). Then write your function in a supported language. I’ve chosen Python.
In this case, we’re updating a route in AWS, so we’re passing 3 things to the function: a CIDR range, an ENI id, and a RouteTable id. Upon creation, this Lambda function will update the specified route (CIDR) in the specified RouteTable, to point at the specified ENI. The Lambda function will also reference the IAM role we are creating.
Custom Resources can be used to represent things we cannot define with AWS CFT types normally, like EC2 instances and VPC’s. A Lambda-backed Custom Resource is just one example of this. Our Custom Resource will pass an Event Type to the Lambda function (either Create, Update, or Delete), and then we’ll pass Resource Properties that will provide the function with the CIDR, ENI id, and RouteTable id required. In our case, the Lambda function returns the output of the API call upon Create, but returns nothing upon Delete.
Still with me? String these together, and you have a Lambda function that is executed as part of your CFT. CloudFormation is smart enough to know that these depend upon values from other resources in the template, so they will be created after the EC2 instance is created. You could use a DependsOn statement to ensure this behavior, if you had other reasons to wait for something before Lambda execution.
Obviously you'll want to modify these to point at the ENI of a Big-IP device, but I've kept these templates very simple to show you just the updating of a route via Lambda. You may even want to modify the code of the Lambda function to do other things - go for it!
Enjoy. Use this article as a starting point, then think of cool use cases for Lambda functions. Prove them out and leave a comment to share your experience. And don’t forget -