KVM + Libvirt Setup Guide (with Cloud-Init and Bridged Networking)

This guide outlines a reproducible, step-by-step workflow for provisioning a virtual machine using KVM, QEMU, Libvirt, and Cloud-Init on Ubuntu 24.04. The setup includes configuring a bridged network for LAN access, creating cloud-init files for initial user configuration, and managing the VM using virt-install and SSH.

1. Install KVM, QEMU, and Libvirt

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Start by updating your system and installing required virtualization tools:

1sudo apt update
2sudo apt install -y qemu-kvm libvirt-daemon-system libvirt-clients virtinst bridge-utils cloud-image-utils

Add your user to the necessary groups:

1sudo usermod -aG libvirt,kvm $USER

Log out and back in (or reboot) to apply group changes.

Verify virtualization support:

1lscpu | grep -i virtualization
2lsmod | grep kvm
3qemu-system-x86_64 --version

2. Prepare Directories and Download Cloud Image

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1mkdir -p ~/muffins/{images,cloudinit}

Navigate and download the Ubuntu cloud image:

1cd ~/muffins/images
2wget https://cloud-images.ubuntu.com/noble/current/noble-server-cloudimg-amd64.img -O ubuntu-24.04-cloudimg.qcow2

Create a working copy for the VM:

1cp ubuntu-24.04-cloudimg.qcow2 muffins-vm01.qcow2

3. Configure Bridged Networking Using Netplan

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If your system is using NetworkManager as the renderer (common on Ubuntu Desktop or systems with GUI), you can configure a network bridge using Netplan with NetworkManager support.

3.1 Create the Netplan file

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1sudo vim /etc/netplan/01-br0.yaml

Paste the following (update eno1 if needed):

 1network:
 2  version: 2
 3  renderer: NetworkManager
 4  ethernets:
 5    eno1:
 6      dhcp4: false
 7      dhcp6: false
 8  bridges:
 9    br0:
10      interfaces: [eno1]
11      dhcp4: true
12      dhcp6: false
13      parameters:
14        stp: false
15        forward-delay: 0

3.2 Apply changes

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1sudo netplan apply

3.3 Verify network

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1ip a
2bridge link

Expected: eno1 has no IP, br0 receives a LAN IP.

4. Adjust Permissions for Libvirt

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By default, Libvirt runs its QEMU processes under a dedicated system user (typically libvirt-qemu on Ubuntu). That user needs permission to:

• Traverse your home directory path (for example /home/abraham)
• Read the VM disk image and cloud-init ISO under ~/muffins

If these permissions are missing, you will see errors when creating or starting the VM, such as:

• Could not open disk image
• Permission denied on the qcow2 file or seed ISO

Instead of loosening permissions on your entire home directory with chmod, you can use POSIX Access Control Lists (ACLs) to grant just enough access to the libvirt-qemu user.

4.1 Grant Execute Permission on Your Home Directory

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This allows the libvirt-qemu user to traverse /home/$USER without granting read access to all files:

1sudo setfacl -m u:libvirt-qemu:--x /home/$USER

The --x means:

• No read (r) or write (w) permission
• Execute (x) only, which for directories means “can enter / traverse this directory”

4.2 Grant Read/Execute on the VM Directory

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Now allow Libvirt to read files under your ~/muffins directory, where images and cloud-init files live:

1sudo setfacl -R -m u:libvirt-qemu:r-x /home/$USER/muffins

Here r-x means:

• Read (r) allowed
• Execute (x) allowed (traverse directories)
• No write (w) permission

The -R flag applies this ACL recursively to the directory tree.

4.3 What Effect This Has

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After these commands:

• Libvirt can see and read:
  • ~/muffins/images/muffins-vm01.qcow2
  • ~/muffins/cloudinit/muffins-vm01-seed.iso
• Libvirt cannot modify those files (no write permission granted).
• Your other directories under $HOME are not globally opened up; access is limited to what you explicitly granted with ACLs.

This strikes a balance between:

• Making Libvirt work reliably with VM files stored in your home directory, and
• Avoiding overly permissive chmod 777-style changes on $HOME.

5. Create Cloud-Init Configuration

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Cloud-Init allows you to pass configuration to the VM at first boot, including the hostname, authorized SSH keys, and default user account.

You’ll create two files in your ~/muffins/cloudinit directory:

• user-data — contains Cloud-Init user setup instructions
• meta-data — provides instance metadata such as the hostname and ID

These files will be packaged into a seed ISO for the VM to read during boot.

5.1 Create user-data

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Navigate to the Cloud-Init folder and create the user-data file:

1cd ~/muffins/cloudinit
2vim user-data

Enter insert mode (i) and paste the following:

 1#cloud-config
 2hostname: muffins-vm01
 3users:
 4  - name: abraham
 5    sudo: ALL=(ALL) NOPASSWD:ALL
 6    groups: users, admin
 7    home: /home/abraham
 8    shell: /bin/bash
 9    lock_passwd: false
10    ssh_authorized_keys:
11      - ssh-ed25519 AAAA...yourkeyhere
12chpasswd:
13  expire: false

Save and exit: press Esc, type :wq, and press Enter.

5.2 Create meta-data

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Create the meta-data file:

1vim meta-data

Enter insert mode and paste:

1instance-id: muffins-vm01
2local-hostname: muffins-vm01

Save and exit with Esc, then :wq.

5.3 Build the Seed ISO

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Use cloud-localds to generate the seed ISO from your configuration files:

1sudo cloud-localds muffins-vm01-seed.iso user-data meta-data
2sudo chown libvirt-qemu:kvm muffins-vm01-seed.iso
3sudo chmod 640 muffins-vm01-seed.iso

The resulting muffins-vm01-seed.iso will be used during VM creation to apply your initial configuration.

6. Create the Virtual Machine with virt-install

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With your system prepared, cloud-init seed ISO created, and bridged networking configured, you’re now ready to create the virtual machine using virt-install.

virt-install is a command-line tool for creating VMs with Libvirt. It allows you to define CPU, memory, storage, network, and boot parameters in one command—making it ideal for provisioning repeatable VM environments.

6.1 Run virt-install

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Run the following command to define and start the VM:

 1sudo virt-install \
 2  --name muffins-vm01 \
 3  --ram 4096 \
 4  --vcpus 2 \
 5  --disk path=/home/$USER/muffins/images/muffins-vm01.qcow2,format=qcow2 \
 6  --disk path=/home/$USER/muffins/cloudinit/muffins-vm01-seed.iso,device=cdrom \
 7  --os-variant ubuntu24.04 \
 8  --network bridge=br0,model=virtio \
 9  --graphics none \
10  --import

6.2 Verify VM Creation

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Once virt-install completes, verify that your VM is running:

1sudo virsh list --all

If the VM isn’t running, inspect its logs:

1sudo virsh console muffins-vm01

Use Ctrl + ] to exit the console.

6.3 Common virt-install Troubleshooting

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• Disk permission errors:
  Make sure the cloud-init ISO and qcow2 image are readable by libvirt-qemu (see Section 4)

• Bridge not found:
  Check your network bridge exists with ip a and ensure its name matches (br0)

• Unsupported OS variant:
  List supported variants with:

  1osinfo-query os | grep ubuntu24
  

• Start VM manually:
  If the VM wasn’t started automatically, run:

  1sudo virsh start muffins-vm01
  

7. Access and Verify the Virtual Machine

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Once your VM has been created using virt-install, the next step is to verify that it is running correctly and that you can access it.

7.1 Check VM Status with virsh

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Use the virsh tool to view the list of running and stopped virtual machines:

1sudo virsh list --all

You should see an entry similar to:

Id   Name          State
-----------------------------
 1   muffins-vm01  running

To view interfaces associated with the VM:

1sudo virsh domiflist muffins-vm01

This command confirms that your VM is connected to the correct network (in this case, bridge br0).

7.2 Access the Console (Optional)

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You can open a serial console to verify boot messages or debug issues:

1sudo virsh console muffins-vm01

To exit the console, press:

Ctrl + ]

This is useful if:

• Cloud-init appears not to have run
• SSH is not responding
• The VM does not acquire a network lease

7.3 Find the VM’s IP Address

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Check cloud-init output during console login — it usually prints the network configuration. If not, you can:

• Check DHCP leases on your router

• Use virsh to inspect guest info:

  1sudo virsh domifaddr muffins-vm01
  

  (Note: This works only if the guest agent is installed.)

• Scan the local network with nmap:

  1sudo nmap -sn 192.168.2.0/24
  

  Look for a device with the hostname muffins-vm01 or a newly discovered IP.

7.4 SSH Into the Virtual Machine

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Once you find the VM’s IP address, you can SSH into it:

1ssh abraham@<vm-ip> -i ~/.ssh/id_ed25519

Replace <vm-ip> with the VM’s actual IP address.

If using a custom SSH key or password login is enabled in cloud-init, adjust your connection command as needed.

Summary

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In this guide, you set up a virtual machine on Ubuntu 24.04 using KVM, QEMU, and Libvirt with the following key components:

• Bridged networking using Netplan, enabling your VM to behave like a physical device on your LAN.
• Cloud-Init configuration to define a hostname, create a user with SSH access, and configure the VM at first boot.
• ISO-based provisioning using cloud-localds to build a seed image for cloud-init.
• VM creation via virt-install, specifying CPU, memory, storage, network, and initial boot parameters.
• Access and verification using virsh and SSH.

This workflow provides a reproducible and flexible method for building VMs suitable for DevOps testing, Kubernetes labs, or application deployments. With bridge networking and cloud-init in place, you can easily clone or automate this environment for larger-scale projects.