A Practical Guide to SSH Tunnels: Local and Remote Port Forwarding
SSH port forwarding explained in a clean and visual way. How to use local and remote port forwarding. What sshd settings may need to be adjusted. How to memorize the right flags.
Materials by Ivan Velichko
Explore tutorials, challenges, courses, and more published by this author.
Tutorials by Ivan Velichko
See all →How Servers Work: A Hands-On Introduction to TCP Sockets
Learn how servers actually work by building a tiny TCP server and client from scratch. A hands-on introduction to sockets, TCP, and the network programming model every backend, DevOps, and platform engineer should go through at least once.
Tutorial on Containers
How Container Registries Work: Pushing and Pulling Images By Hand
Container registries look simple until you need to debug what was actually pushed, why a pull picked the wrong image, or why deleting a tag didn't remove anything. Learn how registries work by pushing, pulling, inspecting, and deleting image data directly through the Registry API.
Tutorial on Containers
What Is a Standard Container: Diving Into the OCI Runtime Spec
Are all containers just Linux processes? How does the OCI runtime specification define a container? What platforms do OCI containers support? What is a Docker container? Linux containers vs. Virtual Machine containers.
Tutorial on Containers
How Container Images Actually Work: Layers, Configs, Manifests, Indexes, and More
A practical deep dive into container image internals that will help you build a clear mental model of how images are composed, identified, stored, and distributed across registries.
How Kubernetes Reinvented Virtual Machines - In a Good Sense
How Virtual Machines were used to deploy services. What old problems containers solve and what new problems create. How Kubernetes used containers to recreate Virtual Machines in a better way?
Challenges by Ivan Velichko
See all →Docker 101: Publish All Exposed Ports on Random Host Ports to Avoid Port Conflicts
Can you get several builds of an app running side by side without port conflicts, so you can test them simultaneously?
Docker 101: Publish a Redis Container Port on the Private Network Only
A common setup for small deployments: two servers (VM or bare-metal), each with a public and a private IP. The app listens on the public port but wants to talk to Redis over the private network. Can you set it up?
Docker 101: Reach a Container Without Publishing a Port
Explore how containers are addressed on Docker's default bridge network and practice accessing a containerized service directly from the Docker host using its IP address.
Docker 101: Publish a PostgreSQL Container Port on Localhost Only
Learn how to keep a database available to an app running on your workstation without exposing its port to other machines on the network.
Docker 101: Publish a Container Port to Access the App from the Host
A web app is listening inside a container, but the browser can't reach its IP address. Recreate the container so the app is available on port 80 of the Docker host.
Call an HTTP API with curl: Creating, Updating, and Deleting Resources
Practice calling HTTP APIs from the command line: send JSON bodies with POST, PUT, and PATCH, upload a file, delete a resource, and read the status codes the server answers with.
Courses by Ivan Velichko
See all →Work-in-Progress
Hands-On Firecracker: microVMs From First Principles to Production
A hands-on introduction to Firecracker that takes you from booting your first microVM to building secure, production-ready sandbox infrastructure. Learn how to launch and control microVMs, wire up networking, share data efficiently, and harden execution - the essential skills for running autonomous agents and untrusted workloads safely at scale.
Computer Networking Fundamentals For Developers, DevOps, and Platform Engineers
A heavily illustrated introduction to the first two layers of Computer Networking: Ethernet and IP. A must for any developer or DevOps engineer who wants to build solid understanding of computer networking before they move to the higher-level protocols like TCP, UDP, and even HTTP.
Work-in-Progress
An Alternative Introduction to Dagger: Understanding the Core Concepts
Build a mental model of how Dagger works by migrating an example software project from traditional script-based automation to a Dagger-powered development workflow, peeking under the hood of the new technology on the way.
Course on Containers
Work-in-Progress
How (and Why) to Use containerd from the Command Line
Explore containerd with ctr, its default command-line client, try nerdctl as a potential Docker alternative, and learn how to debug Kubernetes Pods with crictl.
Shell Gyms by Ivan Velichko
See all → Shell Gym , on Linux
Linux 101: Create Files and Directories
Create files and directory structures from the Linux terminal. Practice using touch and mkdir, building nested directory trees, working with tricky names, and creating groups of files in one go.
Shell Gym , on Linux
Linux 101: List Files and Use Patterns
Inspect directory contents from the Linux terminal, read what a listing tells you, sort it to find the entry you need faster, and use shell patterns to name groups of files.
Shell Gym , on Linux
Linux 101: Navigate the Filesystem
Your third hands-on session in a Linux terminal. Learn how the Linux directory tree is organized and practice navigating it with absolute paths, relative paths, and common shell shortcuts.
Shell Gym , on Linux
Linux 101: Get Unstuck in the Shell
Your second hands-on session in a Linux terminal. Find out what a command name really runs, learn how to get help with --help and man pages, and stop or suspend commands that do not give the prompt back.
Shell Gym , on Linux
Linux 101: Meet the Linux Shell
Your first hands-on session in a Linux terminal. Practice running commands, reading their output, using arguments and options, checking whether commands succeed or fail, and chaining commands together. No prior Linux experience is required.
Skill Paths by Ivan Velichko
See all →Learn Linux by Fixing Broken Servers
Practice Linux troubleshooting on deliberately broken servers. Investigate vague failures, find the root cause, and make the system work again.
Master SSH Tunnels: Local and Remote Port Forwarding
Learn how to reach an internal port on a remote machine from the local system, jump through a bastion into a private VPC, expose a development server to the Internet through a reverse tunnel, publish a home network device, and spin up a full-fledged SOCKS proxy with nothing but a regular SSH connection.
Linux Storage 101: Drives, Partitions, and Mounts
Practice the most common Linux storage operations in a series of hands-on challenges: discover and mount drives, create partition tables, format partitions with different filesystems, map directories with bind mounts, and make mount points survive a reboot.
Docker 101: Pull and Push Container Images
Learn how to move container images between your machine and container registries. Practice pulling images by tag, digest, and platform; tagging and pushing them to public and private registries; mirroring images across repositories; shipping images into air-gapped environments; and publishing multi-platform images.
Docker 101: Build Container Images
Learn how to write Dockerfiles from scratch and build production-ready container images. Start with the basics of image building and progress through handling dependencies, compiling applications, inspecting image internals, and optimizing builds with multi-stage Dockerfiles. Then go deeper with advanced Dockerfile instructions, analyzing image internals, and producing multi-platform builds.
Docker 101: Run and Manage Containers
Go from zero to a confident Docker beginner. Learn how to run containers of all kinds, interact with them, and build a clear mental model of how Docker manages your applications behind the scenes.
Playgrounds by Ivan Velichko
See all →This playground was created by an iximiuz Labs user and is provided with no warranty.
HotROD + Jaeger on K3s
Jaeger's HotROD demo app pre-deployed on a 3-node K3s cluster, wired to a Jaeger v2 all-in-one over OTLP. Click a customer, then read the trace.
This playground was created by an iximiuz Labs user and is provided with no warranty.
OpenClaw on K3s with agent-sandbox + gVisor
A 3-node K3s cluster with gVisor (runsc), the kubernetes-sigs/agent-sandbox controllers, and the OpenClaw + gVisor example pre-deployed. OpenClaw's Control UI is available as a dedicated tab.
This playground was created by an iximiuz Labs user and is provided with no warranty.
OpenTelemetry Demo on k3s
A snapshot-backed three-node k3s cluster with the complete OpenTelemetry Demo, including Jaeger and Grafana.
This playground was created by an iximiuz Labs user and is provided with no warranty.
Dual-Network Router Lab
Five Ubuntu VMs across public and private subnets, joined by a dual-homed router. The private subnet has no direct Internet access.
This playground was created by an iximiuz Labs user and is provided with no warranty.
This playground was created by an iximiuz Labs user and is provided with no warranty.
Docker on Btrfs
Hands-on Docker on Btrfs: experiment with native snapshots, compression, and checksums; inspect setup, grow the filesystem live, and back up/restore images and containers.