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Chapter 10. Network Configuration

This page provides an introduction to the common networking configurations used by libvirt based applications. This information applies to all hypervisors, whether Xen, KVM or another. 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 guests are not accessible by default from external hosts. You must enable either Network Address Translation (NAT) or a network bridge to allow external hosts access to network services on guests.

10.1. 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'/>


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'/>