AWS ARN Explained: Amazon Resource Name Beginners Guide

In this blog, I discuss concepts, tips, and tricks related to AWS ARNs. I will also explain how to create ARN URLs for specific AWS resources.

I have also added all the important links to AWS resources to quickly build the ARNs you need.

What is ARN in AWS?

Amazon Resource Names (ARNs) are unique identifiers assigned to individual AWS resources. It can be an ec2 instance, EBS Volumes, S3 bucket, load balancers, VPCs, route tables, etc.

An ARN looks like the following for an ec2 instance.

arn:aws:ec2:us-east-1:4575734578134:instance/i-054dsfg34gdsfg38

Why are ARNs Important?

ARNs play a crucial role in IAM policies. You will end up using ARNs if you follow the standard security best practices for IAM roles and policies.

ARNs have the following key use cases.

They are used in IAM policies for granting restricted granular access to resources. One example is to allow a specific IAM user to access only specific ec2 instances.
It can be used in Infrastructure as Code scripts and API calls to refer to other resources. You can see a practical example in the Terraform IAM role creation blog.

If you did not understand the above points, don't worry, we will look at those with practical examples in the following topics.

AWS ARN format

In most cases, you can build the ARN URL yourself following the below format.

arn:aws:service:region:account-id:resource-id
arn:aws:service:region:account-id:resource-type/resource-id
arn:aws:service:region:account-id:resource-type:resource-id

In the above formats, towards the end, you can see the difference in the formats which changes as per the resource types.

Here are the arn examples for all three formats.

S3 ARN Example: S3 has a flat hierarchy of buckets and associated objects. Here is what an S3 ARN would look like

arn:aws:s3:::devopscube-bucket

EC2 ARN Example: ec2 service has sub resource-types like image, security groups etc. The following example uses the instance resource-type.

arn:aws:ec2:us-east-1:4575734578134:instance/i-054dsfg34gdsfg38

Lambda ARN Example: Lambda functions can have multiple versions. Here the version is the qualifier. To have arn of the specific Lambda version, you need to mention the version number at the last as shown below

arn:aws:lambda:us-east-1:4575734578134:function:api-fucntion:1

Components of an ARN

ARN structure consists of several components and the arn structures used in AWS are given below

arn:partition:service:region:account-id:resource-id
arn:partition:service:region:account-id:resource-type/resource-id
arn:partition:service:region:account-id:resource-type:resource-id

The components are listed below in detail

ARN - Every arn begins with this prefix.
Partition - This specifies the region in which the AWS account is in, AWS is the partion for most resources.
There are also other particion, aws-us-gov for US government resources and aws-cn for resources in China.
Service - The service specifies the AWS service to which the resource is belong to. For example it will be arn:aws:ec2: for EC2 and arn:aws:s3: for S3.
Region - This component specifies the region in which the resource is located.
It is only available for regional resources like EC2 and will not be available for global resources like IAM.
Account ID - Specifies the account ID of the AWS account in which the resource is in.
Resource Type - This component is not available for all resources, it is only available for resources which are inside a service with multiple resources.
For example, EC2 has multiple resources like Instance, AMI, etc,.. Resource Type is used for these resources.
Resource ID - This is a unique identity for resources within the AWS account.

How to get the ARNs of AWS resources?

If you are getting started with AWS, you may find it difficult to put together the correct arn URL for a resource.

You can find the syntax for ARNs for all the AWS services here.

For example, in that AWS document, if you go ec2 resource from the list, and scroll down to the "Resource Types Defined by Amazon EC2" section, you will find the reference for all the sub rsource types for ec2 as shown below.

aws arn reference document — Click to view in HD

Another way is to use the aws policy generator.

In the policy generator, when you select the policy resource, it will automatically show the arn suggestion as shown below. You just need to add resource information.

ARN Wildcards

ARN definition supports wildcards. You will need a wildcard in many use cases.

Let's say you want an IAM policy that allows access to all objects in a single bucket. For this, you can have a wildcard arn like below.

arn:aws:s3:::my-data-bucket/*

Here is an example of using wildcard arn in an IAM policy. This policy allows all actions for dcubebucket S3 bucket.

{
	"Version": "2012-10-17",
	"Statement": [{
		"Sid": "Stmt1596173683332",
		"Action": "s3:*",
		"Effect": "Allow",
		"Resource": [
			"arn:aws:s3:::dcubebucket",
			"arn:aws:s3:::dcubebucket/*"
		]
	}]
}

Here is another example policy that allows limited access to all emr clusters.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "Stmt1596174145144",
      "Action": [
        "elasticmapreduce:AddInstanceFleet",
        "elasticmapreduce:AddTags",
        "elasticmapreduce:DescribeCluster",
        "elasticmapreduce:DescribeEditor",
        "elasticmapreduce:DescribeJobFlows",
        "elasticmapreduce:DescribeSecurityConfiguration"
      ],
      "Effect": "Allow",
      "Resource": "arn:aws:elasticmapreduce:*:*:cluster/*"
    }
  ]
}

ARN Paths

In some cases using wildecard is not recommend monstly in IAM policies which can give access to evey resiurces comes under the wildcard, this is where path comes in handy.

Paths are used to identify specific resources inside a resource type and it is a way to structure resources within AWS services that support hierarchical resource naming.

Paths are specified at the end of the arns, and each path is separated by ( / ) based on its path structure.

For example, if you want to give access to a group of users to a specific folder inside the bucket while restricting access to all other folders on a bucket, you can use arn paths to give access only to a specific folder

arn:aws:s3:::example-bucket/folder

Here is an example of using arn path in an IAM policy. This policy allows the user to access only the folder dev inside the dcubebucket S3 bucket.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::dcubebucket/dev/*",
      "Condition": {
        "StringLike": {
          "aws:userid": "12345678910/development/*"
        }
      }
    }
  ]
}

This policy gives access to users who come under development on the folder dev inside the S3 bucket dcubebucket.

Getting ARN from AWS CLI

You can get the ARNs of specific resources from the CLI.

For all IAM roles, policies, and users, you can get the ARN from the CLI by describing it.

Here is an example of getting arn of a role.

aws iam get-role --role-name EMR_DefaultRole

Here is the output with the arn.

Like this, you can try describing the resource and see if it outputs the arn.

Getting ARN from AWS Console

You can get the arn of IAM resources directly from the AWS console.

Just browse to the specific resource and you will find the related arn at the top as shown below.

Getting ARN as Output in Cloudformation

If you are using Cloudformation, you can get the resource arn in the output with the function Fn::GetAtt

Here is an example syntax of getting the arn of a Lambda function.

resources:
  Outputs:
    LamdaFunctionArn:
      Export:
        Name: MyFucntionARN
      Value:
        Fn::GetAtt: MyLambdaFunction.Arn

Getting ARNs of Principal

When you create an S3 bucket policy, SNS topic, VPC endpoint, and SQS policy, you need to specify a principal parameter.

Principal Is the trusted source specified as an ARN in the policy to allow or deny access to the resource

For example, if you want an S3 bucket to be accessed only by a specific user, you will specify the user arn as Principal in the policy.

If you use a Policy generator to create these policies, you will see the principal option as shown below.

Primarily you specify the user, account, and role arn as the principal. There are other Principal sources as well. Please see this AWS document to learn more.