OCI containers

From the perspective of the System Administrator (SAs)

🧱 1. Foundational Concept

"OCI containers are a formalized, standardized way to run isolated processes on a Linux system, with predictable configuration and lifecycle semantics."

🔐 2. Parallels with Familiar Concepts

📦 3. OCI = Open Standard

• OCI (Open Container Initiative) defines:

  • Image Format – How container images are packaged and stored.
  • Runtime Specification – How containers are executed from images (e.g., via runc).
  • Distribution Specification – How images are shared over registries (e.g., docker pull/podman pull).

"Think of OCI as the 'POSIX' for containers. It's what makes runtimes like Docker, Podman, and containerd interoperate cleanly."

🚀 4. Why It Matters to SAs

• Standardized Deployment: Build once, run anywhere across Docker, Kubernetes, CRI-O, etc.
• Immutable Infrastructure: Container images are versioned, reproducible, and verifiable.
• Lifecycle Integration: You can plug containers into systemd, CI/CD pipelines, or even CRON jobs.
• Security Boundaries: SELinux/AppArmor, seccomp, and user namespaces offer defense-in-depth.

🛠 5. Common Tools Using OCI

• Docker/Podman: Build/run containers.
• containerd/cri-o: Runtime daemons used by Kubernetes.
• Kubernetes: Schedules and manages OCI-compliant containers.

🧪 6. Example Explanation

"An OCI container is just a process on the host that’s running in a constrained environment: a chrooted rootfs, with limited CPU/memory, no access to host PID or network unless configured. What makes it powerful is that the environment is described in a portable, declarative JSON file, and the image that backs it is versioned and shareable."

🧪 Hands-on: Running a Container with runc

Goal: Run /bin/sh from a busybox image inside a minimal OCI container using runc.

🔧 1. Pre-reqs

sudo dnf install -y runc  # or apt install runc

Make a working directory:

mkdir ~/runc-test && cd ~/runc-test

📦 2. Create a Root Filesystem

Extract it from the image (in cache else pulls it first):

mkdir -p rootfs
docker export $(docker create busybox) |tar -C rootfs -xvf -

🧾 3. Generate Default OCI Config

runc spec

• config.json

This creates a file called config.json. It’s the OCI Runtime Spec, describing how to run the container.

✏️ 4. Edit the config.json

Open in your favorite editor:

vi config.json

Change the following fields:

• process.args:

  "args": ["sh"]
  

• Optional (to drop privileges):

  "root": {
  "path": "rootfs",
  "readonly": false
  }
  

• Optional (to set hostname):

  "hostname": "runc-demo"
  

🚀 5. Run the Container

sudo runc run demo

You’ll be dropped into a shell (sh) inside the container. You can confirm isolation:

hostname        # should be "runc-demo"
ps              # only shows minimal PID tree
ls /            # comes from BusyBox

Type exit to leave.

🧠 Summary

• rootfs/ contains the filesystem.
• config.json is the OCI Runtime Spec.
• runc run <container-id> creates an isolated process from that spec.
• Nothing about registries, Docker, or Kubernetes is required—this is bare-metal containerization.