How to Quickly Add Multiple Security Groups to an EC2 Instance in Terraform

Learn how to effectively add multiple security groups from a data function output to EC2 instances using Terraform in this comprehensive guide.
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This video is based on the question https://stackoverflow.com/q/65658771/ asked by the user 'Johnboy7171' ( https://stackoverflow.com/u/14979614/ ) and on the answer https://stackoverflow.com/a/65659447/ provided by the user 'Marcin' ( https://stackoverflow.com/u/248823/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Adding Multiple Security Groups to an EC2 Instance in Terraform

When working with Amazon Web Services (AWS) and Terraform, one common challenge developers face is managing security groups efficiently. Specifically, you might encounter a situation where you need to apply multiple security groups to an EC2 instance—drawing from both shared groups and newly created ones. In this post, we will tackle the problem of adding multiple security groups from a data function output and provide a clear solution you can implement in your Terraform code.

Understanding the Problem

Suppose you're setting up an EC2 instance using Terraform, and you’ve defined a couple of security groups that are tagged in a specific way—such as shared-services-sg1 and shared-services-sg2. Additionally, you're creating a security group named john_app_sec_group2. The challenge arises when you try to reference these multiple security groups within the configuration for your EC2 instance.

Your goal is to combine the security groups retrieved through the data source with your newly created security group. Here’s a simplified view of your initial Terraform configuration:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

The fundamental issue in your initial configuration is with how the security group IDs are referenced. The data source you created is named shared, but you attempted to reference it incorrectly in the vpc_security_group_ids list. The corrected way to add the multiple security groups looks like this:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Solution

Data Source Usage: The data.aws_security_groups.shared.ids call refers to all security group IDs that match the filters you applied to the data source. Since you're using a wildcard in your filtering (i.e., shared-services-sg*), it will dynamically retrieve any matching security groups.

Combining with New Security Group: The corrected concat function combines the IDs retrieved from your data with the ID of your newly created security group, john_app_sec_group2. By placing these together in one list, you ensure that your EC2 instance can reference multiple security groups effectively.

Conclusion

By following the solution discussed in this guide, you'll be able to seamlessly add multiple security groups to your EC2 instance using Terraform. Ensuring that your vpc_security_group_ids line correctly references the data source will save you time and help avoid configuration errors. Now you can confidently manage security group associations in your Terraform configurations!

If you have further questions or require additional assistance with Terraform and AWS, feel free to reach out. Happy coding!

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