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Chapter 6. Virtual Networks

6.1. Overview
6.2. Listing networks
6.3. Lifecycle control
6.4. Network configuration

6.1. Overview

A virtual network provides a method for connecting the network devices of one or more guest domains within a single host. The virtual network can either:
  • Remain isolated to the host; or
  • Allow routing of traffic off-node via the active network interfaces of the host OS. This includes the option to apply NAT to IPv4 traffic.
A virtual network is represented by the virNetwork object and has two unique identifiers.
  • name: short string, unique amongst all virtual network on a single host, both running and inactive. For maximum portability between hypervisors, applications should only use the characters a-Z,0-9,-,_ in names.
  • UUID: 16 unsigned bytes, guaranteed to be unique amongst all virtual networks on any host. RFC 4122 defines the format for UUIDs and provides a recommended algorithm for generating UUIDs with guaranteed uniqueness.
A virtual network may be transient or persistent. A transient virtual network is started upon creation, and can only be managed while it is running on the host. When taken offline, all trace of it will disappear. A persistent virtual network has its configuration maintained in a data store on the host, in an implementation defined format. Thus when a persistent network is brought offline, it is still possible to manage its inactive configuration. A transient network can be turned into a persistent network on the fly by defining a configuration for it.
After installation of libvirt, every host will get a single virtual network instance called 'default', which provides DHCP services to guests and allows NAT'd IP connectivity to the host's interfaces. This service is of most use to hosts with intermittent network connectivity. For example, laptops using wireless networking. This network is configured to start automatically.