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Cloud computing offers tremendous agility, flexibility, and scalability. However, with great power comes great responsibility, especially when operating in environments like Amazon Web Services (AWS), where resources are easy to spin up but just as easy to misconfigure or forget. For developers and engineering teams responsible for cloud infrastructure, maintaining security compliance, implementing cost controls, and automating cloud governance is not just a best practice, it's a necessity.
Cloud Custodian is a powerful open-source rules engine designed to manage your AWS environment through policy-as-code. It enables you to automatically enforce security rules, eliminate wasteful resource spending, and build continuous compliance workflows into your DevOps pipelines. Custodian makes it easy to define your policies in YAML, validate and test them, and run them in both scheduled and event-driven modes. The best part? It aligns with how developers already work, declaratively, collaboratively, and code-first.
In this blog, we’ll take a deep dive into how you can use Cloud Custodian to build a secure, cost-efficient, and well-governed AWS infrastructure. This guide is tailored for developers, DevOps engineers, cloud architects, and SREs who want to implement scalable policy automation in a programmatic and sustainable way.
At its core, Cloud Custodian enables cloud resource management by defining JSON or YAML-based policies that describe your governance requirements. These policies include conditions and actions, executed when those conditions are met. The Custodian engine supports AWS natively (as well as Azure and GCP) and allows fine-grained control over more than 100+ AWS resource types including EC2, S3, RDS, IAM, Lambda, and many more.
Where traditional governance often relies on manual reviews, ad hoc scripts, or third-party dashboards, Cloud Custodian brings consistency, repeatability, and version control into the governance process. For security teams, it offers an automated way to enforce encryption, access policies, and monitoring. For FinOps teams, it identifies and shuts down unused or over-provisioned resources. And for developers, it becomes a key part of the CI/CD workflow, ensuring environments remain compliant without slowing down innovation.
To get started with Cloud Custodian in AWS, you'll need a basic Python development environment. Custodian is written in Python and is installed using pip. After installation, configuration is done via YAML files, and execution is performed using the custodian command-line tool. Here's what the general setup process looks like:
By integrating this setup into your local dev environment or CI/CD workflows, you create a consistent and predictable framework for managing AWS infrastructure in line with organizational policies.
Security is one of the most critical aspects of managing a cloud environment. In AWS, small misconfigurations can lead to massive data leaks or compliance violations. Cloud Custodian helps by offering automated policy enforcement, so developers can focus on building without constantly worrying about security reviews. Here are a few high-impact ways to enforce security using Custodian:
S3 buckets often become targets for misconfiguration, especially in large-scale or dynamic environments. With Custodian, you can define policies to automatically detect when a bucket is publicly accessible, either through ACLs or misconfigured bucket policies, and take corrective action.
You can schedule the policy to run daily or have it triggered in near-real-time by AWS CloudTrail events. This level of proactive S3 monitoring ensures your data stays protected at all times. Notifications can be sent to the security team or even the original bucket creator for transparency.
Encrypting data at rest is not just a best practice, it’s a compliance mandate for many industries. Custodian makes it easy to check whether EBS volumes are encrypted and automatically stop or tag instances associated with unencrypted volumes. This applies to both root and attached volumes and can be used in tandem with launch-time validation policies to prevent the creation of unencrypted storage in the first place.
By automating EBS encryption checks and remediation, Custodian helps organizations meet compliance standards like HIPAA, PCI-DSS, and GDPR without manual review.
Cloud Custodian allows you to scan all security groups across your AWS environment and identify ingress rules that expose your services to the public internet. For example, you can set up filters to find rules that allow SSH or RDP access (ports 22 and 3389) from 0.0.0.0/0, then either remove those rules or notify the resource owner.
In a zero-trust or least-privilege environment, these kinds of policies are essential. They allow developers to test freely in dev accounts but ensure compliance in production through automatic controls.
IAM misconfigurations often lead to privilege escalations or security breaches. Custodian lets you define policies that detect overly permissive IAM roles, such as those granting *:* access, or users and roles without MFA enabled.
These policies can revoke access, disable accounts, or send alerts, all while generating logs for security audits. Over time, this leads to a more secure, auditable, and controlled IAM environment, without adding friction to the developer experience.
Beyond security, one of the most compelling use cases for Custodian is its ability to enforce cost control policies automatically. In dynamic AWS environments, it’s easy for teams to forget to shut down dev resources or over-provision compute instances. Custodian helps by identifying waste and taking automated action.
Custodian policies can detect EC2 instances running with no network activity, RDS databases with no connections, or unattached EBS volumes sitting idle. These resources not only consume costs but also clutter dashboards and increase surface area.
With Custodian, you can define thresholds (e.g., no activity for 7 days) and automatically stop, snapshot, or delete those resources. You can also tag them with warning labels or notify users ahead of cleanup.
Tagging is essential for understanding who owns what in your AWS environment and for attributing costs to the right teams or projects. Custodian policies can enforce required tags at resource creation time, notify users when tags are missing, and even auto-tag resources with account, region, or user metadata.
This ensures your cost reporting, chargeback, and forecasting processes are based on accurate and complete data.
Not all workloads need high-performance compute instances. Many AWS environments are littered with over-sized EC2 instances or provisioned RDS nodes that far exceed usage requirements. Custodian allows you to identify such over-provisioned resources based on metrics like CPU utilization, memory usage, or storage throughput.
Once identified, these resources can be flagged for review, tagged for optimization, or even automatically downsized during off-peak hours. This leads to more efficient resource use and reduced monthly bills.
Custodian policies are defined using a clean, human-readable YAML format. This makes them easy to read, review, and version control. Developers can collaborate on policy development just like they would for application code, using Git workflows and code reviews.
This aligns perfectly with infrastructure-as-code practices, making Custodian a natural fit for teams already using tools like Terraform or AWS CDK.
Cloud Custodian supports multiple execution modes:
This flexibility means Custodian can run wherever it’s needed, during CI/CD, in the cloud, or on a developer's machine.
Large organizations often use multiple AWS accounts for isolation and security. Custodian supports multi-account deployments using c7n-org, a companion tool that allows you to run the same policy across many accounts and regions in parallel.
Policies can be centrally managed, updated via GitOps workflows, and distributed using role assumptions. This ensures consistency in governance while allowing teams to remain autonomous.
Cloud Custodian allows developers to own and automate governance at the pace of cloud. It brings repeatability, compliance, and cost optimization into the software development lifecycle, all without introducing complex overhead or slowing down releases.
By empowering teams to define security, cost, and operational policies as code, Custodian closes the gap between compliance and engineering. Whether you're securing data, optimizing spend, or scaling operations across accounts, Cloud Custodian helps you build a safer, leaner, and more compliant AWS environment.
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