Chapter 6. Network Configuration This chapter provides an introduction to the common networking configurations used by libvirt-based guest virtual machines. Red Hat Enterprise Linux 7 supports the following networking setups for virtualization: virtual networks using Network Address Translation ( NAT ) directly allocated physical devices using PCI device assignment directly allocated virtual functions using PCIe SR-IOV bridged networks You must enable NAT, network bridging or directly assign a PCI device to allow external hosts access to network services on guest virtual machines. 6.1. Network Address Translation (NAT) with libvirt One of the most common methods for sharing network connections is to use Network Address Translation (NAT) forwarding (also known as virtual networks). Host Configuration Every standard libvirt installation provides NAT-based connectivity to virtual machines as the default virtual network. Verify that it is available with the virsh net-list --all command. If it is missing, the following can be used in the XML configuration file (such as /etc/libvirtd/qemu/myguest.xml) for the guest: The default network is defined from /etc/libvirt/qemu/networks/default.xml Mark the default network to automatically start: Start the default network: Once the libvirt default network is running, you will see an isolated bridge device. This device does not have any physical interfaces added. The new device uses NAT and IP forwarding to connect to the physical network. Do not add new interfaces. libvirt adds iptables rules which allow traffic to and from guest virtual machines attached to the virbr0 device in the INPUT , FORWARD , OUTPUT and POSTROUTING chains. libvirt then attempts to enable the ip_forward parameter. Some other applications may disable ip_forward , so the best option is to add the following to /etc/sysctl.conf . Guest Virtual Machine Configuration Once the host configuration is complete, a guest virtual machine can be connected to the virtual network based on its name. To connect a guest to the 'default' virtual network, the following can be used in the XML configuration file (such as /etc/libvirtd/qemu/myguest.xml ) for the guest: Note Defining a MAC address is optional. If you do not define one, a MAC address is automatically generated and used as the MAC address of the bridge device used by the network. Manually setting the MAC address may be useful to maintain consistency or easy reference throughout your environment, or to avoid the very small chance of a conflict.

Commands:
virsh net-list --all Name State Autostart ----------------------------------------- default active yes
ll /etc/libvirt/qemu/ total 12 drwx------. 3 root root 4096 Nov 7 23:02 networks -rw-------. 1 root root 2205 Nov 20 01:20 r6.4.xml -rw-------. 1 root root 2208 Nov 8 03:19 r6.xml
virsh net-autostart default Network default marked as autostarted
virsh net-start default Network default started
brctl show bridge name bridge id STP enabled interfaces virbr0 8000.000000000000 yes
net.ipv4.ip_forward = 1
<interface type='network'> <source network='default'/> </interface>
<interface type='network'> <source network='default'/> <mac address='00:16:3e:1a:b3:4a'/> </interface>