A KVM network bridge is a virtual switch that connects virtual machines directly to the host's physical network, allowing VMs to obtain IP addresses from the same LAN as the host.

What is a KVM Bridge?

A KVM bridge is a software-based virtual switch that links the virtual network interface cards (VNICs) of KVM virtual machines to the host's physical network interface card (NIC). This setup allows VMs to appear as full network peers on the LAN, bypassing NAT translation, and enables direct communication with other devices on the network while receiving IP addresses from the same DHCP server as the host .

Benefits of Using a Bridge

• Direct LAN access: VMs can communicate with other devices on the same subnet without NAT overhead .
• Simplified networking: Eliminates complex port forwarding and NAT configurations .
• Flexible topology: Supports multiple VMs communicating with each other and external networks seamlessly .
• Production-ready: Suitable for environments where VMs need stable IP addresses and full network visibility .

How to Create a KVM Bridge

GUI Method (Virtual Machine Manager)

1. Open Virtual Machine Manager (VMM).
2. Navigate to Edit → Connection Details → Virtual Networks → New.
3. Choose Forwarding Mode: Bridge and select the physical NIC.
4. Assign the bridge to each VM under Settings → Network → Source → Select the bridge.
5. VMs will receive IP addresses from the LAN DHCP server .

Command-Line Method (NetworkManager)

1. Create a bridge interface: nmcli connection add type bridge ifname br0 con-name br0
2. Attach the physical NIC to the bridge: nmcli connection add type ethernet ifname ens3 master br0
3. Activate the bridge: nmcli connection up br0
4. Configure VMs to use br0 as their network source .

Using Open vSwitch (Advanced)

Open vSwitch (OVS) provides a production-quality, multilayer virtual switch for advanced network automation. It can attach to a dedicated NIC or allow routing through multiple host interfaces. This is ideal for large-scale deployments or when advanced features like VLANs, traffic monitoring, or distributed switching are required .

Considerations

• IP configuration: Move the host NIC's IP to the bridge to avoid network disruption .
• Persistence: GUI-created bridges may not persist across reboots on all distributions; command-line or OVS methods are more reliable .
• Security: Ensure firewall and network policies are configured to protect VMs exposed directly to the LAN. A KVM bridge provides a robust and flexible way to integrate virtual machines into your physical network, offering near-native network performance and simplified management for both small and large-scale deployments .