Скачать или смотреть How to Concat Strings and List Variable in Terraform

Learn how to concatenate CIDR blocks in Terraform using data sources and custom lists effectively.
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Uniting CIDR Blocks in Terraform: A Comprehensive Guide

When working with infrastructure as code (IaC), one common task is to manage various CIDR blocks necessary for your network configurations. If you're using Terraform and facing challenges with passing a list of CIDR blocks to a module, you’re not alone. Many users struggle with how to efficiently concatenate these blocks into a single list.

The Problem

You may have two types of CIDR blocks to manage:

CIDR Blocks from Data Sources: These are generated from existing subnets in your cloud environment.

Custom CIDR Blocks: This is a list you define based on your specific requirements or the environment you are working in.

Let's consider this real-world example. Suppose you are trying to pass a list of CIDR blocks in the format of ["0.0.0.0/23", "0.0.0.1/23"] to a module. The challenge arises when you also have additional CIDR blocks from subnets, and you need to merge them into a comprehensive list.

The Solution

To solve the concatenation challenge, you can utilize the powerful setunion function in Terraform. Here’s a step-by-step approach to create a combined list of CIDR blocks.

Step 1: Define Your Variables

Start by defining a variable for your custom CIDR blocks. This will allow the specification to change depending on the environment.

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Step 2: Retrieve Subnet CIDR Blocks

Use the data block to fetch your CIDR blocks from the existing subnets. Here’s an example of how to do that:

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Step 3: Combine Lists Using setunion

Next, create a locals block to combine both the custom CIDR blocks and the CIDR blocks obtained from your data sources. The setunion function is ideal for this as it ensures unique entries, but it can be replaced by concat() if you want to keep duplicate entries.

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Step 4: Output the Concatenated List

Finally, provide an output to easily reference the combined list of CIDR blocks:

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Example Output

When you run your Terraform configuration with the command:

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You should see an output similar to this:

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Accessing the Combined List

Whenever you need to reference the concatenated list, simply use local.all_cidrs within your module, and you’ll have access to all the required CIDR blocks.

Conclusion

By following these steps, you can effortlessly concatenate strings and lists in Terraform to match your infrastructure needs. Whether you have a mix of custom and existing network configurations, this method ensures your Terraform scripts remain clean, understandable, and efficient. Happy coding!