Shared Namespaces

🧠 First: What is a Namespace in Linux?

A namespace in Linux is a boundary or an isolation mechanism.

It decides what a process is allowed to see on the system.

Think of it like:

A private room inside a house.
You only see what’s inside your room, not the whole house.

Containers rely heavily on Linux namespaces.

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🧩 Types of Linux Namespaces

Some key ones used by containers:

Namespace	What it isolates	Example
NET	Network	Each container gets its own virtual network stack
PID	Processes	Each container sees only its own processes
IPC	Inter-process communication	Shared memory, semaphores
UTS	Hostname	Each container can have its own hostname
MNT	Filesystems	Each container has its own root filesystem

Kubernetes Pods mainly use NETWORK + IPC + UTS namespaces.

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🚀 Now: What is a Shared Namespace in a Pod?

• A Pod is NOT a container.
• A Pod is a wrapper that can contain multiple containers, and those containers share some namespaces.

That means:

👉 All containers inside the same Pod can see the same network, hostname, and IPC resources.

This is why we say:

Containers inside a Pod share namespaces.

Let’s break it down.

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🌐 1. Shared NETWORK Namespace (Most Important)

What it means:

All containers inside a Pod:

• Share the same IP address
• Share the same network interface
• Can talk to each other using localhost
• Can use the same ports

Example:

Pod has two containers:

• Container A runs on port 8080
• Container B runs on port 9000

Both can communicate like this:

Container A → http://localhost:9000
Container B → http://localhost:8080

They don’t need a Service to talk to each other.

👉 This is why we say: Pods = shared network namespace.

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💾 2. Shared STORAGE (Volumes)

Containers inside one Pod share:

• mounted volumes
• shared directories

Example:

/shared-data

Both containers can read/write to the same folders.

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🔧 3. Shared UTS Namespace (Hostname)

Containers inside a Pod share the same hostname.

If you run:

hostname

in container A and container B → you get the same result.

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🔄 4. Shared IPC Namespace

IPC = Inter-Process Communication.

Containers can:

• share memory
• send signals
• use semaphores

This is optional and depends on Pod settings.

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🎯 Simple Real-World Analogy

🏠 A Pod = a shared apartment

🚪 Containers = roommates

🌐 Network namespace = shared WiFi

All containers use the same IP → like one internet connection.

🧱 Volumes = shared storage room

All containers can use the same shared folder.

🔇 Namespaces = private walls

Containers cannot see outside the Pod’s isolation.

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🧠 Summary

Namespace = a sandbox that isolates what a container can see.

Shared Namespace = multiple containers inside the same Pod share the same sandbox.

So:

• Each Pod gets one IP, not each container
• Containers inside a Pod share the same networking environment
• Containers inside a Pod can talk using localhost