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Terraform

Terraform aws_iam_policy_document Multiple Conditions Syntax

By Filip on 10/07/2024

Learn how to use multiple conditions effectively within the `aws_iam_policy_document` data block in Terraform to craft precise and secure IAM policies.

Terraform aws_iam_policy_document Multiple Conditions Syntax

Table of Contents

Introduction

When working with Terraform's aws_iam_policy_document data source, defining multiple conditions within a single statement for an IAM policy requires a structured approach. AWS IAM policies use a JSON-based format where conditions are specified within a "Condition" block. Each "Condition" block, even with multiple key-value pairs, evaluates as a single condition. To illustrate, let's consider an example where we want to permit an action based on the request's IP range and the use of HTTPS. We'll then discuss the limitations of this approach and explore alternative methods for handling more complex scenarios involving "OR" logic or conditions across multiple keys.

Step-by-Step Guide

To define multiple conditions within a single statement in an IAM policy document using Terraform's aws_iam_policy_document data source, you need to understand how AWS structures these conditions.

AWS IAM uses a JSON-based policy language. Within a policy statement, the "Condition" block allows you to specify criteria for when the policy should take effect. To achieve the effect of multiple conditions, you would nest conditions within a single "Condition" block using operators like "StringEquals" or "Bool". Each key within "StringEquals" or "Bool" represents a single condition.

Let's illustrate with an example. Suppose you want to allow an action only if the request comes from a specific IP range ("1.2.3.4/32") and the request uses HTTPS:

data "aws_iam_policy_document" "example" {
  statement {
    actions = [ "s3:GetObject" ]
    resources = [ "arn:aws:s3:::examplebucket/*" ]

    condition {
      test = "StringEquals"
      values = ["https"]
      variable = "aws:SecureTransport"
    }

    condition {
      test = "IpAddress"
      values = ["1.2.3.4/32"]
      variable = "aws:SourceIp"
    }
  }
}

In this example, we have two separate "condition" blocks within the "statement" block. This signifies that both conditions must be met for the statement to be true and the action to be allowed. The first condition checks if the request is using HTTPS, and the second condition checks if the source IP address of the request falls within the specified range.

It's important to note that while you can have multiple "condition" blocks within a "statement", each "condition" block itself evaluates as a single condition.

Directly expressing "OR" logic between different condition keys within a single "condition" block is not supported by the aws_iam_policy_document data source in Terraform.

If you need to implement complex logic with "OR" conditions or conditions spanning multiple keys, you might need to explore alternative approaches:

  1. Multiple Statements: Create separate statements within your policy document, each handling a different combination of conditions. This can lead to more verbose policies but provides flexibility.

  2. Custom Policy Documents: Instead of using aws_iam_policy_document, define the entire policy document as a JSON string within your Terraform code. This gives you full control over the policy structure but requires careful manual management.

Remember to consult the AWS IAM documentation for detailed information on policy syntax, available condition keys, and best practices.

Code Example

The code defines two examples of AWS IAM policies using Terraform. The first example demonstrates achieving "OR" logic by using multiple statements within the policy document, allowing access if either HTTPS is used or the request comes from a specific IP address. The second example showcases defining a custom policy document as a JSON string for more complex logic, checking for both HTTPS and whether the source IP address falls within specified ranges.

This example demonstrates how to achieve "OR" logic by using multiple statements within the policy document.

data "aws_iam_policy_document" "example" {
  statement {
    actions = [ "s3:GetObject" ]
    resources = [ "arn:aws:s3:::examplebucket/*" ]

    condition {
      test = "StringEquals"
      values = ["https"]
      variable = "aws:SecureTransport"
    }
  }

  statement {
    actions = [ "s3:GetObject" ]
    resources = [ "arn:aws:s3:::examplebucket/*" ]

    condition {
      test = "IpAddress"
      values = ["1.2.3.4/32"]
      variable = "aws:SourceIp"
    }
  }
}

resource "aws_iam_policy" "example" {
  name   = "example"
  policy = data.aws_iam_policy_document.example.json
}

In this example, we have two separate statements. The first statement allows access only if the request uses HTTPS. The second statement allows access only if the request comes from the specified IP address. Since these are separate statements, the effect is an "OR" - the action will be allowed if either statement is true.

Example 2: Custom Policy Document for Complex Logic

This example demonstrates how to define a custom policy document as a JSON string to achieve more complex logic.

resource "aws_iam_policy" "example" {
  name = "example"
  policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject"
      ],
      "Resource": [
        "arn:aws:s3:::examplebucket/*"
      ],
      "Condition": {
        "Bool": {
          "aws:SecureTransport": "true"
        },
        "IpAddress": {
          "aws:SourceIp": [
            "1.2.3.4/32",
            "192.168.1.0/24"
          ]
        }
      }
    }
  ]
}
EOF
}

In this example, we define the entire policy document as a JSON string. This allows us to express more complex conditions. Here, we are checking for both HTTPS and whether the source IP address falls within one of two specified ranges.

Remember to replace the example values with your actual requirements and to consult the AWS IAM documentation for a comprehensive understanding of policy syntax and available options.

Additional Notes

  • Each key-value pair within a "condition" block acts as an independent filter. Think of them as "ANDed" together. All key-value conditions within that block must be met for the condition to be true.
  • Limitations of aws_iam_policy_document:
    • The data source is designed for generating basic policy structures. It doesn't directly support complex logic like "OR" between different condition keys within a single "condition" block.
    • For more intricate scenarios, you'll need to switch to either multiple statements or manually crafting the policy JSON.
  • When to Choose Each Approach:
    • Multiple Statements: Best for straightforward "OR" logic where you have separate sets of conditions that should grant access.
    • Custom Policy Documents: Provides the most flexibility but requires a deeper understanding of IAM policy syntax and increases the potential for errors if not managed carefully.
  • Importance of AWS IAM Documentation: Always refer to the official AWS IAM documentation for the most up-to-date information on:
    • Supported condition keys (e.g., aws:SourceIp, aws:SecureTransport, etc.)
    • Valid values and formats for condition elements
    • IAM policy best practices to ensure your policies are secure and effective.
  • Terraform Best Practices:
    • Modularity: For larger policies, consider breaking them down into smaller, reusable modules to improve readability and maintainability.
    • Validation: Use tools like terraform validate and AWS's IAM policy simulator to catch errors early in your development process.
  • Security Considerations:
    • Principle of Least Privilege: Always grant only the necessary permissions. Avoid overly permissive policies.
    • Regular Reviews: Periodically review your IAM policies to ensure they still align with your security requirements and to remove any unnecessary permissions.

Summary

This article explains how to define multiple conditions within a single statement in an IAM policy document using Terraform's aws_iam_policy_document data source.

Key Points:

  • Multiple "condition" blocks: You can use multiple "condition" blocks within a single "statement" to define multiple conditions. All conditions must be met for the statement to be true.
  • No direct "OR" logic: You cannot directly express "OR" logic between different condition keys within a single "condition" block.
  • Alternatives for complex logic:
    • Multiple Statements: Create separate statements for different condition combinations.
    • Custom Policy Documents: Define the entire policy document as a JSON string for full control.

Example:

The provided example demonstrates allowing an action only if the request comes from a specific IP range and uses HTTPS. This is achieved by using two "condition" blocks within the "statement" block.

Important Notes:

  • Each "condition" block evaluates as a single condition.
  • Refer to AWS IAM documentation for detailed information on policy syntax and best practices.

Conclusion

In conclusion, managing multiple conditions within AWS IAM policies using Terraform's aws_iam_policy_document data source requires a clear understanding of how conditions are structured and evaluated. While this data source is useful for generating basic policy structures, it has limitations when it comes to expressing complex logic like "OR" conditions between different keys within a single "condition" block. For such scenarios, consider using multiple statements within your policy document or defining the entire policy as a custom JSON string. Remember to prioritize clarity and security by adhering to IAM best practices, leveraging the official AWS IAM documentation, and thoroughly testing your policies. By understanding these concepts and choosing the right approach for your specific needs, you can effectively manage complex access control requirements in your AWS infrastructure.

References

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