Introduction

Virtualization with Kernel-based Virtual Machine (KVM) on CentOS offers powerful networking capabilities that allow you to create and manage virtual networks for your virtual machines (VMs). Proper networking configuration is essential for ensuring seamless communication between VMs and the external network. In this article, we’ll explore various aspects of networking configuration in KVM on CentOS.

Understanding Networking Modes

Before diving into the configuration details, it’s important to understand the different networking modes available in KVM:

1. Bridged Networking: Bridged networking connects VMs directly to your physical network, making them appear as separate devices with their own IP addresses. This mode is ideal when VMs need to be part of your local network, just like physical machines.
2. NAT (Network Address Translation): NAT networking creates a private network for VMs, and the host acts as a router, performing network address translation. VMs share the host’s IP address for external communication. This mode is suitable for isolated VMs that don’t need direct access to the external network.
3. Host-only Networking: In host-only networking, VMs can communicate with each other and the host but don’t have access to the external network. It’s useful for creating isolated development or testing environments.

Setting Up Bridged Networking

1. Prerequisites

• Ensure that your physical network has a DHCP server or a static IP address configuration.
• Identify the network interface you want to bridge (e.g., eth0).

2. Configure the Host

• Edit the network configuration file (/etc/sysconfig/network-scripts/ifcfg-eth0).
• Set the BRIDGE parameter to define the bridge interface (e.g., BRIDGE=br0).

3. Create the Bridge Interface

• Create a new bridge interface (br0) using the brctl command or by installing the bridge-utils package.
• Assign the bridge a static IP address or configure it to use DHCP.

4. Configure VMs

• Edit the XML configuration file of each VM (/etc/libvirt/qemu/vm_name.xml).
• Specify the network interface as bridge and set it to use br0 in the VM’s XML.

5. Restart Networking

• Restart the network service on the host to apply the changes (systemctl restart network).

Configuring NAT Networking

1. Prerequisites

• Ensure that the libvirt default network (virbr0) is available.
• Disable or stop the default network (virbr0) if it’s running.

2. Create a New Virtual Network

• Use virsh or virt-manager to create a new virtual network, specifying NAT as the mode.

3. Configure VMs

• When creating or editing VMs, select the newly created NAT network as the network source.

Using Host-Only Networking

1. Create a Host-Only Network

• Similar to NAT networking, create a new virtual network in virsh or virt-manager, but set it to host-only mode.

2. Configure VMs

• Assign the host-only network to VMs that need to communicate with each other and the host.

Security Considerations

• Implement firewall rules and security measures to protect your VMs and host.
• Disable unused network interfaces and services on the host for improved security.

Conclusion

Proper networking configuration in KVM on CentOS is crucial for achieving optimal performance and connectivity for your virtual machines. Whether you opt for bridged, NAT, or host-only networking, understanding the networking modes and following best practices will help you create a virtualized environment that meets your specific needs.