Key Takeaway

<aside> 💡

In this article, we explore how multiple network namespaces can communicate with each other through a Linux Bridge, using visual guides and hands-on experiments (with the ping command).

With a Linux Bridge, communication becomes possible between isolated or independent environments without the need to establish point-to-point connections for each one.

For a better understanding, please refer to the previous articles on Namespaces, VETH, and ARP.

• 1. Network Namespace and Virtual Ethernet (VETH)
• 2. Unveiling the Secrets of How IP and ARP Work </aside>

Linux Bridge

Introduction

A Linux Bridge is a virtual network device implemented within the Linux kernel that enables Layer 2 (Data Link Layer) packet forwarding between multiple network interfaces on the same host—similar to a physical switch.

It is primarily used to bridge different network interfaces—such as virtual machines, containers, or physical network interfaces—allowing them to communicate within the same Layer 2 broadcast domain.

How It Works

• A Linux Bridge functions as a software-based switch and operates via the Linux kernel’s bridge module.
• When a packet arrives at the bridge, it checks the destination MAC address and consults its internal forwarding table:
  • If the destination MAC is known, the packet is forwarded to the corresponding interface.
  • If unknown, the packet is broadcast to all bridged interfaces (except the one it came from).
• The bridge supports the Spanning Tree Protocol (STP) to prevent network loops and ensure network stability.

Practical Use Case

Imagine you have a server running multiple virtual machines that need to communicate with the external network:

1. Create a Linux bridge (e.g., br0).
2. Add the host's physical network interface (eth0) and the virtual machines' network interfaces (vnet0, vnet1) to br0.
3. Assign an IP address to br0 if Layer 3 communication is required.
4. Now, the virtual machines can communicate with the external network through br0 and with each other directly.

Userspace Tools

• bridge-utils: Contains the legacy brctl tool. Source code is available in the bridge-utils repository.
• iproute2: A modern network configuration suite. Source code can be found in the iproute2 repository.