"NAT interface virbr0 is missing, please install libvirt"

[dschultzca]

Describe the bug
Adding a NAT node results in the message "NAT interface virbr0 is missing, please install libvirt". Similar to issue 3404.
I got this after moving a project from a GNS3 VM instance on VMWare to a GNS3 VM instance on KVM.

GNS3 version and operating system

• OS: Ubuntu 22.04
• GNS3 version: 2.2.54
• GNS3 VM on KVM

Steps to reproduce the behavior

1. I was starting with a clean install of Ubuntu 22.04
2. I then installed virt-manager
3. Rebooted the system to ensure all was running from a fresh start
4. Downloaded GNS3.VM.KVM.2.2.54.zip
5. Extracted the two QCOW2 files and copied them to /var/lib/libvirt/images/ on my host
6. Started virt-manager and created a new VM and connected the two GNS3 QCOW2 images as the drives
7. Booted the GNS3 VM
8. Opened a web browser to GNS3 at http://192.168.122.x/ (the address listed on the booted VM)
9. Created a new project
10. Added a VPCS node successfully
11. Added a NAT node and got the error "NAT interface virbr0 is missing, please install libvirt"

Screenshots
This is the error message from step 11 above

Opening a console on the VM and checking the interface list ip addr reveals there's no virbr0 entry.

If you check the libvirt default network entry (/etc/libvirt/qemu/networks/default.xml) on both the HOST and in the VM you will see they are the same, i.e.: they use the same /24 network.

Checking the journal in the VM I found a non-decscript message:

Feb 17 10:21:49 gns3vm libvirtd[615]: libvirt version: 10.0.0, package: 10.0.0-2ubuntu8.4~backport20.04.202409061918~ubuntu20.04.1 (Ubuntu)
Feb 17 10:21:49 gns3vm libvirtd[615]: hostname: gns3vm
Feb 17 10:21:49 gns3vm libvirtd[615]: internal error: Network is already in use by interface eth0

To Resolve the Problem
In the VM using the command sudo virsh net-edit default I changed the network address to be different from that of the host. After saving the change I was able to activate the default network using the command sudo virsh net-start default. I verified the net was active with sudo virsh net-list --all and ip addr.

With this change I was able to add a NAT node to my project, start the VPCS node, dhcp an address and ping an Internet host from the node.

My suggestion to fix this problem would be to modify the GNS3 VM /etc/libvirt/qemu/networks/default.xml network to use a different /24 network than the KVM host uses before building the VM for release. Please ensure it doesn't conflict with any of the other hypervisor's default network addresses either.