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EC2 DescribeNetworkInterfaces Permission Error: Fix & Causes

By Filip on 10/05/2024

Troubleshoot AWS errors: Learn how to resolve the "provided execution role does not have permissions to call DescribeNetworkInterfaces on EC2" issue.

EC2 DescribeNetworkInterfaces Permission Error: Fix & Causes

Table of Contents

Introduction

When working with AWS Lambda functions within a Virtual Private Cloud (VPC), you might encounter the error "The provided execution role does not have permissions to call ... on EC2." This error indicates that your Lambda function lacks the necessary permissions to interact with your VPC resources. To resolve this, you need to modify the IAM role associated with your Lambda function. The IAM role requires permissions to manage network interfaces, which are essential for communication within the VPC. You can either attach the AWS managed policy "AWSLambdaVPCAccessExecutionRole" or create a custom policy that grants the required permissions. After updating the role, redeploy or invoke your Lambda function. Remember to follow security best practices by adhering to the principle of least privilege and configuring appropriate security groups. If the error persists, review your IAM role configuration, VPC settings, and security group rules.

Step-by-Step Guide

  1. Understand the Error: The error message "The provided execution role does not have permissions to call ... on EC2" means your AWS Lambda function lacks the necessary permissions to interact with your Virtual Private Cloud (VPC). This usually happens when you try to deploy a Lambda function within a VPC without granting it the required network access.

  2. Lambda, VPC, and IAM Roles: When a Lambda function runs inside a VPC, it needs network interfaces to interact with other resources within that VPC. These interfaces are managed by AWS EC2. An IAM role is used to grant your Lambda function the permissions it needs.

  3. Solution: Modify the IAM Role:

    • Open the IAM Console: Navigate to the IAM service in your AWS Management Console.
    • Find the Role: Locate the IAM role that's associated with your Lambda function. The role name is often something like "lambda_vpc_execution_role" or similar.
    • Attach Policy (Option 1: AWS Managed Policy):
      • Click on the role to open its details.
      • Go to the "Permissions" tab.
      • Click on "Attach policies."
      • Search for "AWSLambdaVPCAccessExecutionRole" and select it.
      • Click "Attach policy."
    • Attach Policy (Option 2: Custom Policy):
      • If you need more granular control, create a custom IAM policy:
        • Go to "Policies" in the IAM console.
        • Click "Create policy."
        • Use the JSON editor and add the following permissions (replace resources with your specific VPC, subnet IDs, etc.): 
          {
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:CreateNetworkInterface",
        "ec2:DescribeNetworkInterfaces",
        "ec2:DeleteNetworkInterface"
      ],
      "Resource": [
        "*" 
      ]
    }
  ]
}
        • Review and create the policy.
        • Attach this custom policy to your Lambda function's IAM role.

  4. Retest Your Lambda Function: After updating the IAM role, try redeploying or invoking your Lambda function. It should now have the necessary permissions to operate within your VPC.

Important Considerations:

  • Principle of Least Privilege: Always grant the minimum necessary permissions. Avoid using "*" for resources in your IAM policies unless absolutely necessary.
  • Security Groups: Ensure your VPC's security groups allow the necessary traffic between your Lambda function and other resources it needs to access.
  • Troubleshooting: If the error persists, double-check your IAM role configuration, VPC settings, and security group rules. AWS CloudTrail logs can be helpful for debugging permission issues.

Code Example

This Python code demonstrates how to use the AWS SDK for Python (Boto3) to grant a Lambda function access to a VPC. It defines a custom IAM policy with permissions for network interface creation, deletion, and IP address assignment within the specified VPC. The code then creates this policy in IAM, retrieves its ARN, and attaches it to a designated Lambda function's execution role. This enables the Lambda function to interact with resources within the VPC.

This example demonstrates how to create a custom IAM policy and attach it to a Lambda function's execution role using the AWS SDK for Python (Boto3).

import boto3

# Replace with your AWS account ID
aws_account_id = "123456789012"

# Replace with your VPC and subnet IDs
vpc_id = "vpc-0a1b2c3d4e5f6g7h8i"
subnet_ids = ["subnet-0a1b2c3d4e5f6g7h8i", "subnet-9j8k7l6m5n4p3o2q1r0"]

# Create an IAM client
iam = boto3.client('iam')

# Define the custom IAM policy
custom_policy = {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "ec2:CreateNetworkInterface",
                "ec2:DescribeNetworkInterfaces",
                "ec2:DeleteNetworkInterface"
            ],
            "Resource": [
                f"arn:aws:ec2:*:{aws_account_id}:network-interface/*",
            ]
        },
        {
            "Effect": "Allow",
            "Action": [
                "ec2:AssignPrivateIpAddresses",
                "ec2:UnassignPrivateIpAddresses"
            ],
            "Resource": [
                f"arn:aws:ec2:*:{aws_account_id}:network-interface/*"
            ]
        }
    ]
}

# Create the custom policy in IAM
response = iam.create_policy(
    PolicyName='LambdaVpcAccessPolicy',
    PolicyDocument=json.dumps(custom_policy)
)

policy_arn = response['Policy']['Arn']

# Replace with your Lambda function's execution role name
lambda_role_name = "lambda_vpc_execution_role"

# Attach the custom policy to the Lambda function's execution role
iam.attach_role_policy(
    RoleName=lambda_role_name,
    PolicyArn=policy_arn
)

print(f"Custom policy attached to role: {lambda_role_name}")

Explanation:

  1. Import Boto3: Import the Boto3 library to interact with AWS services.
  2. Set Variables: Replace placeholders with your AWS account ID, VPC ID, and subnet IDs.
  3. Create IAM Client: Create an IAM client using Boto3.
  4. Define Custom Policy: Define the custom IAM policy in JSON format, granting the necessary permissions for VPC access.
  5. Create Policy in IAM: Use the create_policy method to create the custom policy in IAM.
  6. Get Policy ARN: Retrieve the ARN of the newly created policy.
  7. Attach Policy to Role: Use the attach_role_policy method to attach the custom policy to your Lambda function's execution role.

Remember:

  • Replace the placeholders with your actual values.
  • Ensure your Lambda function's execution role already exists.
  • This code snippet focuses on creating and attaching the custom policy. You'll need to integrate it into your Lambda function deployment process.

Additional Notes

Understanding the Error:

  • The error message itself might vary slightly, but it will always mention "execution role," "permissions," and likely "EC2" or "CreateNetworkInterface." This combination is a strong indicator of VPC access issues.
  • This error is particularly common during the initial setup of a Lambda function within a VPC or when deploying changes that affect its networking configuration.

Lambda, VPC, and IAM Roles:

  • Think of the IAM role as the "identity card" for your Lambda function. Without the right permissions on this card, the function can't perform actions within your AWS environment.
  • When a Lambda function is configured to run inside a VPC, it's essentially placed within a private network. Just like any other resource in that network, it needs the ability to create and manage network interfaces to communicate.

Solution: Modify the IAM Role:

  • AWS Managed Policy vs. Custom Policy: The AWS managed policy (AWSLambdaVPCAccessExecutionRole) is the easiest solution if it provides all the permissions your function needs. A custom policy gives you more granular control but requires a deeper understanding of IAM.
  • Resource Restrictions: In the custom policy example, replacing "*" with specific ARNs for your VPC and subnets enhances security by limiting the function's access.
  • Additional Permissions: Depending on what your Lambda function does within the VPC, you might need to add more permissions to the IAM role beyond just network interface management. For example, if it needs to access an RDS database, you'll need to grant it relevant RDS permissions.

Important Considerations:

  • Security Groups: Security groups act as firewalls for your VPC resources. Make sure the security group associated with your Lambda function allows inbound and outbound traffic on the necessary ports to/from other resources it needs to interact with.
  • Troubleshooting:
    • CloudTrail Logs: Enable CloudTrail logging for your Lambda function. This will record API calls made by the function, including any that were denied due to permission issues, helping you pinpoint the problem.
    • IAM Simulator: AWS provides an IAM simulator that lets you test policies and see what actions they allow or deny. This can be helpful for debugging complex permission scenarios.

Example: Granting Lambda VPC Access using AWS SDK for Python (Boto3):

  • This code snippet provides a practical example of how to automate the process of granting VPC access to your Lambda function using infrastructure-as-code principles.
  • Consider integrating this code into your deployment scripts or using tools like AWS CDK or Serverless Framework, which can handle IAM role and policy management as part of your deployment process.

Summary

This error occurs when a Lambda function deployed within a VPC lacks permission to interact with the VPC's network resources. Here's how to fix it:

Problem: Lambda functions in a VPC need network interfaces managed by EC2. Without proper IAM role permissions, they can't create or use these interfaces.

Solution: Modify the Lambda function's IAM role:

  1. Open the IAM Console and locate the role associated with your Lambda function.
  2. Attach the AWSLambdaVPCAccessExecutionRole managed policy for a quick solution.
  3. Alternatively, create a custom IAM policy with the following permissions (replace resources with your specific VPC and subnet IDs): 
    {
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:CreateNetworkInterface",
        "ec2:DescribeNetworkInterfaces",
        "ec2:DeleteNetworkInterface"
      ],
      "Resource": [
        "*" 
      ]
    }
  ]
}
  4. Attach the custom policy to your Lambda function's IAM role.

Retest your Lambda function. It should now have the necessary permissions.

Important Considerations:

  • Least Privilege: Grant only the necessary permissions.
  • Security Groups: Configure VPC security groups to allow required traffic.
  • Troubleshooting: Use CloudTrail logs to debug persistent permission issues.

Conclusion

By ensuring your Lambda function's IAM role has the necessary permissions to interact with your VPC, you can resolve the "The provided execution role does not have permissions to call ... on EC2" error and enable your Lambda functions to operate seamlessly within your VPC environment. Remember to adhere to security best practices, grant the minimum necessary permissions, and leverage AWS tools like CloudTrail and the IAM simulator for troubleshooting and policy validation.

References

The provided execution role d...

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