Managing Secrets in Containers: Podman vs. Docker Secrets Explained

Secure Sensitive Data in Containerized Applications with Confidence

Why Secrets Matter in Containerization

Secrets management is a critical aspect of containerized applications. Sensitive information like API keys, passwords, and TLS certificates must be kept secure to prevent unauthorized access. Docker and Podman offer built-in mechanisms for managing secrets, enabling secure, dynamic injection of confidential data into running containers without compromising the container images or logs. While Docker requires Swarm mode to use secrets, Podman supports secrets in standalone containers, making it a more flexible alternative.

Key Differences Between Podman and Docker Secrets

Standalone Container Support

Docker requires Swarm mode to manage secrets, which limits its use to Swarm services only. Podman, however, supports secrets for both standalone and rootless containers, providing greater flexibility for small-scale deployments.

Deployment Mode

Docker secrets are available only when Swarm mode is enabled. In contrast, Podman secrets do not require any orchestration or special mode, making them simpler to use in diverse environments.

Secret Creation and Usage

• Docker uses docker secret create, allowing secrets to be created from input streams or files.

• Podman uses podman secret create, which reads directly from files for secret creation.

Both platforms mount secrets to containers at /run/secrets/, but their usage scopes differ: Docker restricts usage to Swarm services, while Podman allows secrets in any container.

Security

Docker and Podman handle secret management with different security approaches.

Docker Secrets Security:
In Docker, secrets are tightly integrated with Swarm mode. When a secret is created and distributed to nodes in the Swarm, Docker ensures that the secret is encrypted in transit between nodes and encrypted at rest on the Swarm manager node. Additionally, secrets are only accessible to the containers running within the corresponding service, preventing unauthorized access by other containers or users on the same host.

Podman Secrets Security:
Podman takes a simpler but effective approach by storing secrets as files directly on the host system. The security of Podman secrets depends on:

1. File System Permissions: Secrets are stored in directories with restricted permissions (accessible only by the root user or the appropriate Podman-managed users in rootless mode). This limits access to only privileged users and the containers that explicitly request the secrets.

2. Container Isolation: Podman uses the same container runtime as Docker but runs without a daemon, leveraging namespace-based isolation (user, mount, and network namespaces). This ensures that secrets mounted into one container are not accessible to other containers unless explicitly shared.

3. Host Security Configuration: Podman’s reliance on host-level security means that overall system security configurations, such as filesystem encryption, user permissions, and SELinux or AppArmor policies, contribute directly to protecting secrets.

Alternatives to Docker Secrets Without Swarm Mode

For users who prefer not to enable Docker Swarm, there are alternative methods for managing secrets:

1. Environment Variables: Although easy to use, they may expose secrets in process listings or logs.

2. Bind-Mounting Secret Files: Manually mount a file containing secrets into a container. This requires careful permission management to avoid exposure.

3. Docker Compose with External Secrets: Use docker-compose with external secrets management tools such as HashiCorp Vault, AWS Secrets Manager, or Azure Key Vault.

4. Docker Plugins for Secrets Management: Third-party plugins and libraries integrate with Docker to provide Swarm-free secret handling.

Managing Secrets in Docker

1. Enabling Secrets in Docker

Docker secrets require Swarm mode, which you can enable on your Docker host:

docker swarm init

2. Creating a Secret

Create a secret from a file or a string:

echo "mysecretdata" | docker secret create my_secret -

3. Listing Secrets

To check existing secrets:

docker secret ls

4. Using a Secret in a Service

Secrets are only available in services, not standalone containers:

docker service create --name my_service --secret my_secret nginx

5. Accessing the Secret in a Container

Inside the container, secrets are available as files under /run/secrets/:

docker exec -it my_service cat /run/secrets/my_secret

6. Removing a Secret

When no longer needed, remove a secret:

docker secret rm my_secret

Managing Secrets in Podman

1. Creating a Secret

Podman stores secrets on the host system:

echo "mysecretdata" > secret_file
podman secret create my_secret secret_file

2. Listing Secrets

To view all available secrets:

podman secret ls

3. Using a Secret with a Container

Attach a secret to a Podman container at runtime:

podman run --name my_container --secret my_secret nginx

4. Accessing the Secret in the Container

Secrets are mounted in /run/secrets/ by default:

podman exec -it my_container cat /run/secrets/my_secret

5. Removing a Secret

Delete a secret from Podman:

podman secret rm my_secret

Conclusion

Effective secret management is vital for securing sensitive data in containerized applications. While Docker’s secrets are robust for Swarm-based deployments, Podman’s flexible approach offers significant advantages for standalone containers. Depending on your use case, alternative solutions like environment variables, bind-mounting, or external secret managers can also enhance security. Understanding these tools allows developers to protect their applications against data exposure risks.