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Chapter 11. Network Configuration

This chapter provides an introduction to the common networking configurations used by libvirt based guest virtual machines. For additional information, consult the libvirt network architecture documentation: http://libvirt.org/intro.html.
Red Hat Enterprise Linux 6 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.

11.1. Network Address Translation (NAT) with libvirt
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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. 

# virsh net-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
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If it is missing, the example XML configuration file can be reloaded and activated: 
# virsh net-define /usr/share/libvirt/networks/default.xml
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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
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Start the default network: 
# virsh net-start default
Network default started
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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. 
# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.000000000000       yes
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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. 
 net.ipv4.ip_forward = 1
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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 could be used in the XML configuration file (such as /etc/libvirtd/qemu/myguest.xml) for the guest: 

<interface type='network'>
   <source network='default'/>
</interface>
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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. 
<interface type='network'>
  <source network='default'/>
  <mac address='00:16:3e:1a:b3:4a'/>
</interface>
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