Network Bridge

Abstract

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• A software-based network device that operates at Layer 2 (Data Link layer) of the OSI Model
• Connects multiple network segments into a single broadcast domain
• Functions similarly to an Network Switch, making multiple networks behave as one
• Transparently forwards Ethernet frames between interfaces based on MAC addresses
• Implemented as a virtual network interface in the operating system

Core Concepts

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Virtual Interface

• Created and managed through operating system commands
• Appears as a network interface (e.g., bridge0)
• Can be dynamically created and destroyed
• Supports various physical and virtual interfaces

Layer 2 Communication

• Enables direct communication between devices
• No Layer 3 (IP) routing required
• Works at the Ethernet frame level

Bridge Architecture

      [bridge0]
      /      \
   [en0]    [en1]

• Bridge interface (bridge0) connects multiple physical interfaces
• Physical interfaces (en0, en1) represent network connections
• All connected interfaces operate as a single broadcast domain

Use Cases

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Virtualisation

• VM Networking

  • Guest VMs share host’s network interface
  • VMs appear as full LAN members
  • Enables seamless network integration

• Container Networking

  • Docker Bridge Network connects containers
  • Creates isolated network environments
  • Enables container-to-container communication

Network Management

• Internet Sharing

  • Bridges Wi-Fi and Ethernet interfaces
  • Enables connection sharing between networks
  • Common in macOS and Linux systems

• Traffic Analysis

  • Interface bridging for passive monitoring
  • Network traffic capture and analysis
  • Security monitoring and debugging

Implementation

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macOS Bridge Configuration

• Software Bridge Implementation

  • bridge0 as the primary bridge interface
  • Managed through ifconfig commands
  • Supports Ethernet and Wi-Fi interfaces

• Limitations

  • Best performance with Ethernet interfaces
  • Wi-Fi bridging may be unstable
  • Driver-specific restrictions may apply

Bridge Management

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• Interface Control

  # Create bridge
  ifconfig bridge0 create
   
  # Add interfaces
  ifconfig bridge0 addm en0 addm en1
   
  # Enable bridge
  ifconfig bridge0 up

• View bridge interface membership

  bridge link show
   
  # on Macos 
  ifconfig <interface_name> | grep member

  • Lists interfaces belonging to bridges
  • Shows bridge names (e.g., br-lan, br-guest)
  • Displays Spanning Tree Protocol (STP) state
  • Shows interface cost and priority
  • Indicates bridge port status

• List only bridge interfaces

  ifconfig -l | tr ' ' '\n' | grep ^bridge

  • Filters and displays only bridge interface names
  • Useful for quick identification of bridge interfaces
  • Output is clean, one interface per line

• Show detailed bridge configuration

  ifconfig bridge0

  • Displays complete configuration of specified bridge
  • Shows bridge parameters and settings
  • Includes interface statistics
  • Lists member interfaces

• List all interfaces with bridge details

  ifconfig | grep -A5 ^bridge

  • Shows all bridge interfaces with 5 lines of context
  • Includes interface status and configuration
  • Displays bridge-specific parameters
  • Shows member interface information