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Chapter 11. Network Configuration

11.1. Network address translation (NAT) with libvirt
11.2. Bridged networking with libvirt
This page provides an introduction to the common networking configurations used by libvirt based applications. For additional information consult the libvirt network architecture documentation.
The two common setups are "virtual network" or "shared physical device". The former is identical across all distributions and available out-of-the-box. The latter needs distribution specific manual configuration.
Network services on virtualized guests are not accessible by default from external hosts. You must enable either Network address translation (NAT) ir a network Bridge to allow external hosts access to network services on virtualized guests.

11.1. 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 know as virtual networks).
Host configuration
Every standard libvirt installation provides NAT based connectivity to virtual machines out of the box. This is the so called 'default virtual network'. Verify that it is available with the virsh net-list --all command.
# virsh net-list --all
Name                 State      Autostart 
-----------------------------------------
default              active     yes

If it is missing, the example XML configuration file can be reloaded and activated:
# virsh net-define /usr/share/libvirt/networks/default.xml

The default network is defined from /usr/share/libvirt/networks/default.xml
Mark the default network to automatically start:
# virsh net-autostart default
Network default marked as autostarted

Start the default network:
# virsh net-start default
Network default started

Once the libvirt default network is running, you will see an isolated bridge device. This device does not have any physical interfaces added, since it uses NAT and IP forwarding to connect to outside world. Do not add new interfaces.
# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.000000000000       yes

libvirt adds iptables rules which allow traffic to and from guests 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.
 net.ipv4.ip_forward = 1

Guest configuration
Once the host configuration is complete, a guest can be connected to the virtual network based on its name. To connect a guest to the 'default' virtual network, the following XML can be used in the guest:
<interface type='network'>
  <source network='default'/>
</interface>

Note

Defining a MAC address is optional. A MAC address is automatically generated if omitted. Manually setting the MAC address is useful in certain situations.
<interface type='network'>
  <source network='default'/>
  <mac address='00:16:3e:1a:b3:4a'/>
  </interface>