18.04: VM fails to start due to management_network assuming non-existent interface eth0

[TJ]

In trying to launch a standard ubuntu/bionic64 Vagrant box on 18.04 LTS I found it fails with the report:

Call to virDomainCreateWithFlags failed: Unable to get index for interface eth0: No such device"

which is directly from libvirt (same error in syslog).

For many hours I thought it was due to my not getting a config.vm.network :public_network, ... statement correct in Vagrantfile. Finally I commented it out and the error still occurred!

At that point I realised the culprit is the 'hidden' management_network configuration. I dived into the source-code and see that there is no attribute exposed to set the @device value and therefore it uses the default of "eth0":

lib/vagrant-libvirt/action/create_network_interfaces.rb:85: @device = iface_configuration.fetch(:dev, 'eth0')

This is becoming even more of an issue now that most distributions are using systemd/udevd > v197 "Predictable Network Interface Names" ( https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/ ).

It is definitely something to be solved in this module rather than requiring what could be disruptive changes to the host configuration via kernel command-line net.ifnames=0 biosdevname=0.

This needs documenting as a gotcha up-front for newcomers especially (in docs and in the generated Vagrantfile) and an attribute provided to over-ride the interface device name at a minimum.

It would be preferable to detect the host's interface. Manually that is achieved with:

brctl show $(virsh net-info vagrant-libvirt | awk '/^Bridge:/{print $2}' ) | awk 'NR != 1{print $4}'

I did try working around it using an alias, but that isn't exposed in any useful way as an alternate name, as in:

sudo ip link set virbr1-nic alias eth0

Upstream have added an attribute to control @device since:

commit 7f50ca2
Author: qazokm <email address hidden>
Date: Fri Feb 23 12:09:49 2018 +0800

Add basic networking support for qemu session

which has added this among many other additions:

@management_network_device = 'virbr0' if @management_network_device == UNSET_VALUE
@system_uri = 'qemu:///system' if @system_uri == UNSET_VALUE

[TJ (tj)]

I'm currently working around it by bind-mounting the upstream source over the Ubuntu package thus:

mkdir -p /home/all/SourceCode/vagrant
cd /home/all/Sourcecode/vagrant
git clone https://github.com/vagrant-libvirt/vagrant-libvirt.git
sudo mount --bind /home/all/SourceCode/vagrant/vagrant-libvirt /usr/share/rubygems-integration/all/gems/vagrant-libvirt-0.0.43

After this I can use management_network_device in the Vagrantfile. E.g.:

Vagrant.configure("2") do |config|

        config.vm.define :tactyle do |tactyle|

                tactyle.vm.box = "ubuntu/bionic"

                tactyle.vm.network :public_network,
                        :dev => "virbr0",
                        :mode => "bridge",
                        :type => "bridge"

                # tactyle.vm.synced_folder "../data", "/vagrant_data"
                tactyle.ssh.insert_key = false

                tactyle.vm.hostname = "tactyle"
                tactyle.vm.provider :libvirt do |libvirt|
                        # volatile memory in MegaBytes
                        libvirt.memory = 1024
                        # virtual CPU qty
                        libvirt.cpus = 4
                        # headless system (no video output)
                        libvirt.graphics_type = "none"
                        # UK keyboard
                        libvirt.keymap = "en-gb"
                        # VM architecture requires 64-bit
                        libvirt.machine_arch = "x86_64"
                        # pass the host's /dev/random device to the guest
                        libvirt.random :model => "random"
                        # set the management interface
                        libvirt.management_network_device = 'virbr1'
                end

Alas, although the VM now starts, it fails in the initramfs and drops to the shell due to the root fs block device missing!