Networking

libvirt networking

NAT forwarding

Every standard libvirt installation provides NAT based connectivity to virtual machines out of the box. This is the so called 'default virtual network'. You can verify that it is available with

# virsh net-list --all
Name                 State      Autostart 
-----------------------------------------
default              active     yes

If it is missing, then the example XML config can be reloaded & activated

# virsh net-define /usr/share/libvirt/networks/default.xml
Network default defined from /usr/share/libvirt/networks/default.xml
# virsh net-autostart default
Network default marked as autostarted
# virsh net-start default
Network default started

When the libvirt default network is running, you will see an isolated bridge device. This device explicitly does *NOT* have any physical interfaces added, since it uses NAT + forwarding to connect to outside world. Do not add interfaces

# brctl show
bridge name	bridge id		STP enabled	interfaces
virbr0		8000.000000000000	yes