`libvirt.uplink` fails after upgrading from ubuntu 20.04 to 24.04

[sdargoeuves]

Describe the bug

When using libvirt.uplink in a topology to connect an ASAv firewall to a physical NIC so I can create my VPN to the cloud, vagrant fails immediately after domain creation with:

Error while attaching new device to domain. Call to virDomainAttachDeviceFlags failed:
unsupported configuration: setting MTU on interface type direct is not supported yet

Environment

• Host OS: Ubuntu 24.04 (upgraded in-place from 20.04)
• libvirt: 10.0.0
• QEMU: 8.2.2
• vagrant-libvirt: 0.12.2
• netlab: latest dev

OS/libvirt version context

This was working correctly on Ubuntu 20.04. The issue appeared after upgrading to Ubuntu 24.04 which ships libvirt 10.0.0. The situation on Ubuntu 22.04 is unknown.

Root cause

In netsim/templates/provider/libvirt/libvirt-bridge.j2, :libvirt__mtu => 9500 is set unconditionally for all interface types, including public/direct (macvtap) interfaces created by libvirt.uplink:

                  :libvirt__mtu => 9500,
                  :autostart => true,
                  :auto_config => false

libvirt 10.x rejects the <mtu> element on type='direct' interfaces. Older libvirt silently ignored it.

Note: setting mtu: 1500 on the affected links in the topology did not help, probably because it would be applied as configuration after the node is up during the netlab initial part.

Fix applied

In libvirt-bridge.j2, wrap the :libvirt__mtu line so it is only emitted for private (bridge) networks, not public/direct ones:

{% if not pubnet %}
                  :libvirt__mtu => 9500,
{% endif %}

This fix has been tested on Ubuntu 24.04 with libvirt 10.0.0 using libvirt.uplink and resolves the issue.

Potential side effects

I am not certain whether skipping :libvirt__mtu on public interfaces has any unintended consequences, and I have not tested libvirt.public.

I will create the PR for further discussion.