br0 is a network bridge interface commonly used in Linux systems. It functions as a virtual Layer 2 (Ethernet) switch, enabling multiple network interfaces—both physical and virtual—to communicate as if they were connected to the same physical network. The br0 interface is particularly useful in virtualization (e.g., KVM, QEMU, LXC), container environments, and advanced networking setups.

How the br0 Bridge Works

A bridge like br0 connects two or more network interfaces, forwarding packets based on MAC addresses, without performing any routing. All interfaces connected to the bridge act as if they are on the same physical switch. This makes br0 an ideal solution for integrating virtual machines or containers into the host’s network.

Example:

• The physical interface eth0 is added to the bridge br0.

• A virtual interface vnet0 from a VM is also connected to br0.

• Devices behind eth0 and vnet0 can communicate directly as if on the same LAN.

Benefits of Using br0

• Transparent Virtual Networking – Devices connected to br0 operate within the same broadcast domain.

• No Need for NAT – Guests can obtain IP addresses directly from the host’s DHCP server.

• Flexible Configuration – Easily integrates with VLANs, bonding, firewalls, and traffic control.

• Ideal for Virtualization – Allows virtual machines direct access to the physical network.

Creating br0 in Linux

To configure br0, you can use tools such as iproute2, netplan, systemd-networkd, or the legacy bridge-utils package.

Basic example using ip commands:

ip link add name br0 type bridge
ip link set dev br0 up
ip link set dev eth0 master br0
dhclient br0

This configuration creates a bridge named br0, adds the physical interface eth0 to it, and enables DHCP on br0.

Using br0 in Virtualization (KVM/QEMU)

When running virtual machines using KVM/QEMU or libvirt, br0 is often configured to allow VMs to access the external LAN network directly. This setup is preferable to NAT when you need full network integration—for example, when a VM must receive its own IP from the router.

Security Considerations

When working with bridges like br0, be mindful of:

• Firewall rules – Ensure you control inter-interface traffic properly.

• Spoofing protection – Prevent MAC and IP address spoofing, especially in multi-tenant environments.

The br0 interface is a vital tool for advanced networking scenarios in Linux, particularly for system administrators managing virtualization or complex networking environments. When properly configured, br0 provides a powerful and efficient way to bridge physical and virtual interfaces, maintaining full network performance and flexibility. Mastering br0 is essential for anyone building robust and scalable Linux network infrastructures.