Taken from the KVM docs here...

https://wiki.debian.org/KVM#Libvirt_default_network



If you use libvirt to manage your VMs, libvirt provides a NATed bridged network named "default" that allows the host to communicate with the guests. This network is available only for the system domains (that is VMs created by root or using the qemu:///system connection URI). VMs using this network end up in 192.168.122.1/24 and DHCP is provided to them via dnsmasq. This network is not automatically started. To start it use:

virsh --connect=qemu:///system net-start default

To make the default network start automatically use:

virsh --connect=qemu:///system net-autostart default

Hi Fred.

I would probably remove virt-manager, reboot, and then try installing it again, it sounds like something is amiss with it.. and at least then you know it should be 'default'

I did that. So far so good, this seems to be the solution.

After starting the PC this morn, I tried virt-man again. The issue is back.

Launch virt-manager then highlight the VM and choose 'Edit -> Connection Details' next window make sure 'Autoconnect' is ticked, and on
'Virtual Networks' tab make sure 'Autostart on Boot' it ticked and the network is using NAT

I know I have actually set a different Network up than the default one created before to fix such issues;

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Hi FF. Here's what I see. I changed nothing. I don't see a virtnet option.

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XML view after running the temporary fix of 'sudo adduser $USER libvirt && sudo reboot'.

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VirtNet was the extra Network I created myself, and gave my own IP range... The 'Default' is the 192.168.122.0/24 network, and, like you are now finding, for me it didn't work reliably. I tried removing this default network, and setting it up again, but it didn't help as it always wanted to created the same subnet 192.168.122.0/24, so I just created a new network I called virtNet and gave it the subnet of 192.168.100.0/24 and use this one instead, and it 'just works' and is available as soon as the VM boots up.

Like yourself, I got fed up of having to 'fix' the default Network interface every time I wanted to use a VM..

virt-manager -> Edit -> Virtual Machine Details -> Add Hardware -> Network and then create a new NAT interface

Does it matter which device model I use - e.g. e1000?

I use virtio

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Still no change.

Today, my internet connection to the house was on and off. I had ran the usual temp fix of 'sudo adduser $USER libvirt && sudo reboot' before the internet starting being sporadic. The VM had internet for a while then none (along with my host and all other devices in the house). When the home internet came back online - the internet on the VMs was gone but working again on the host.