Using cloud custodian to ensure compliance across AWS resources


In this post, I will describe my experiments with using Cloud Custodian to perform various tasks usually falling into the bucket of compliance and sometimes convention. I encourage you to take a look at some of the example policies.

Some of the areas I will cover are resource tagging and unused resources across multiple AWS accounts.

Installation and setup

Cloud Custodian is a Python 3 application, so you will need that installed. A docker image is also available to be used. In this post, I will assume a host installation with the CLI command custodian used to invoke the application.

We will also need to ensure that we have the AWS CLI configuration setup correctly. I am using the AWS_PROFILE environment variable to point to the configuration I want to use with custodian.

Non-actionable policies and reporting

The policies we will write and focus on in this section will have no actions associated with them. We will only use them as a discovery mechanism and then we will use the reporting functionality of cloud custodian to obtain a summarized report.

Ensuring certain tags exist

Let’s say that we want to find out all the S3 buckets which do not have certain tags defined. Cloud custodian requires us to specify this requirement as a policy YAML file which looks like as follows:


Create a new file called, s3.yaml and paste in the following contents:

  - name: s3-tag-policy
    resource: aws.s3
      - or:
        - "tag:Project": absent
        - "tag:Environment": absent
        - "tag:Provisioner": absent

Now, let’s run custodian from the same directory as the above policy file:

$ custodian run s3.yaml --output-dir=. 
2020-04-28 15:26:39,312: custodian.policy:INFO policy:s3-tag-policy resource:aws.s3 region:eu-central-1 count:13 time:27.72

The above command has invoked cloud custodian with the above policy definition and created a few files with presumably the result of the query in a new sub-directory s3-tag-policy in the current directory specified via --output-dir.

We will next use the report sub-command to summarize the results for us:

$ custodian report s3.yaml --output-dir=. --format grid
| Name                                    | CreationDate              |
| bucket 1                                | 2020-03-18T05:44:33+00:00 |
| bucket 2                                | 2020-03-18T05:44:33+00:00 |


Let’s now write a policy for checking the tagging across EC2 resources. Create a new file ec2.yaml and save it in the same directory as the above:

  - name: ec2-tag-policy
    resource: aws.ec2
      - or:
        - "tag:Project": absent
        - "tag:Environment": absent
        - "tag:Provisioner": absent


Next, let write a policy for checking the tagging across RDS resources. Create a new file rds.yaml and save it in the same directory as the above:

  - name: rds-tag-policy
    resource: aws.rds
      - or:
        - "tag:Project": absent
        - "tag:Environment": absent
        - "tag:Provisioner": absent

Running with all the policies

Let’s now run cloud custodian with all the above policies and across multiple AWS regions:

$ custodian run --output-dir=. ec2.yaml s3.yaml rds.yaml -r ap-southeast-2 -r eu-central-1 -r eu-west-1

2020-04-29 09:40:25,067: custodian.policy:INFO policy:ec2-tag-policy resource:aws.ec2 region:ap-southeast-2 count:3 time:0.35
2020-04-29 09:40:43,447: custodian.policy:INFO policy:s3-tag-policy resource:aws.s3 region:ap-southeast-2 count:13 time:18.38
2020-04-29 09:40:43,615: custodian.policy:INFO policy:rds-tag-policy resource:aws.rds region:ap-southeast-2 count:0 time:0.16
2020-04-29 09:40:46,425: custodian.policy:INFO policy:ec2-tag-policy resource:aws.ec2 region:eu-central-1 count:7 time:2.79
2020-04-29 09:41:14,221: custodian.policy:INFO policy:s3-tag-policy resource:aws.s3 region:eu-central-1 count:13 time:27.79
2020-04-29 09:41:17,455: custodian.policy:INFO policy:rds-tag-policy resource:aws.rds region:eu-central-1 count:0 time:3.23
2020-04-29 09:41:18,893: custodian.policy:INFO policy:ec2-tag-policy resource:aws.ec2 region:eu-west-1 count:0 time:1.42
2020-04-29 09:41:46,373: custodian.policy:INFO policy:s3-tag-policy resource:aws.s3 region:eu-west-1 count:13 time:27.48
2020-04-29 09:41:49,368: custodian.policy:INFO policy:rds-tag-policy resource:aws.rds region:eu-west-1 count:0 time:2.99

Now, let’s run the report sub-command. We can only pass policy files of the same resource type to report. Hence, we will invoke it separately for each resource type which also corresponds to the separate policy files in our case:

For EC2:

$ custodian report --output-dir=. ec2.yaml -r ap-southeast-2 -r eu-central-1 -r eu-west-1 --format grid
| CustodianDate              | InstanceId          | tag:Name                                   | InstanceType   | LaunchTime                | VpcId        | PrivateIpAddress   | Region         |
| 2020-04-29 09:40:46.423450 | i-0912121           | Instance Name                              | c4.large       | 2018-10-29T00:30:38+00:00 | vpc-0c6d8b65 |       | eu-central-1   |

For S3:

$ custodian report --output-dir=. s3.yaml -r ap-southeast-2 -r eu-central-1 -r eu-west-1 --format grid
| Name                                    | CreationDate              | Region    |
| cg-foo-bar                    | 2020-03-04T05:57:34+00:00 | eu-west-1 |

For RDS:

$ custodian report --output-dir=. rds.yaml -r ap-southeast-2 -r eu-central-1 -r eu-west-1 --format grid
| DBInstanceIdentifier   | DBName   | Engine   | EngineVersion   | MultiAZ   | AllocatedStorage   | StorageEncrypted   | PubliclyAccessible   | InstanceCreateTime   | Region   |

Multi-account/Organizational Invocation

To run custodian against multiple AWS accounts under the same organization, we will use another application which is part of the cloud custodian project - c7n-org. In my case, in the same virtual environment, I installed c7n-org using pip install c7n-org.

The next step is to create a account config file using the script pointed to from here which will allow generation using the AWS Organizations API:

$ python  -f accounts.yaml

The generated accounts.yaml will look like:

- account_id: 1212121212121
  name: acccount1
  role: arn:aws:iam::1212121212121:role/OrganizationAccountAccessRole
  - path:/SomePath
- account_id: 42312121477
  name: acccount2
  role: arn:aws:iam::42312121477:role/OrganizationAccountAccessRole
  - path:/SomePath1

Next, we will need to have the IAM role OrganizationAccountAccessRole in each of the member accounts. For accounts which were created as a member account, this IAM role already exists. For accounts which were invited, we will need to create the IAM role manually.

The key features for the Role are:

Once we have created the IAM roles, we will run c7n-org as follows:

$ c7n-org run -c accounts.yaml -s output -u ec2.yaml  -r ap-southeast-2 -r eu-central-1 -r eu-west-1

We can now use the report sub-command to summarize the results:

$ c7n-org report -c accounts.yaml -s output -u ec2.yaml  -r ap-southeast-2 -r eu-central-1 -r eu-west-1 
Acc1,eu-central-1,ec2-tag-policy,2020-04-30 16:07:05.109961,i-0e979a763cfcd3d82,website-application,t3.medium,2020-04-20T03:56:31+00:00,vpc-ce7b04a5,

As compared to custodian, c7-org run doesn’t support multiple policy files and c7n-org report doesn’t support the grid format.

Putting custodian to use