Example: CentOS Stream image

This example shows you how to install a CentOS Stream image and focuses mainly on CentOS Stream 9. Because the CentOS installation process might differ across versions, the installation steps might differ if you use a different version of CentOS.

Note

This is just an example, adjust paths and commands according to your environment

Download a CentOS install ISO

• Navigate to the CentOS mirrors page.

• Choose one of the mirrors and navigate to 9-stream/BaseOS/x86_64/iso. Download a ISO, choose boot to download packages during install, otherwise choose dvd

Start the installation process

Start the installation process using either the virt-manager or the virt-install command as described in the Tools: libvirt and virsh/virt-manager page.

virt-install

If you use the virt-install command, do not forget to connect your VNC client to the virtual machine.

The command should look something like this:

$ sudo virt-install --virt-type kvm --name my-centos --ram 2048 \
  --network network=default \
  --graphics vnc,listen=0.0.0.0 --noautoconsole \
  --os-type=linux --os-variant=centos-stream9 \
  --location=~/Downloads/CentOS-Stream-9-20240819.0-x86_64-dvd1.iso

virt-manager

Note

See here for libvirt wiki about new VM creation in virt-manager

When creating a new VM from the downloaded ISO, virt-manager should automatically detect the OS. If it fails, manually select CentOS Stream as OS. Default settings should be fine.

Step through the installation

Note

This guide focuses on the steps specific to create a OpenStack image, for a general overview of installation process see CentOS Documentation

In Installation Summary follow the instructions below.

DHCP and hostname

In Network & Host Name ensure that Ethernet is on and in Configure.../IPv4 Settings the Method is set to Automatic (DHCP). The same page allows for host name selection - leave it to default as the cloud-init package will be installed later.

Select installation option

In Software Selection choose what to install, the default is Server with GUI, the smallest choice is Minimal Install.

Create a working user

Configure a root password in Root Password, as this will be needed later to finalize the installation. By default it will be then blocked by cloud-init. It is also possible to create an adiministrator user, as it will be later deleted by virt-sysprep.

Detach the CD-ROM and reboot

virt-install

To eject a disk by using the virsh command, libvirt requires that you attach an empty disk at the same target that the CD-ROM was previously attached, which may be hda. You can confirm the appropriate target using the virsh dumpxml vm-image command.

$ sudo virsh dumpxml my-centos
<domain type='kvm' id='19'>
  <name>centos</name>
...
    <disk type='block' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='hda' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
    </disk>
...
</domain>

Run the following commands from the host to eject the disk and reboot using virsh, as root.

$ sudo virsh attach-disk --type cdrom --mode readonly my-centos "" hda
$ sudo virsh reboot my-centos

virt-manager

If you are using virt-manager, the commands above will work, but you can also use the GUI to detach and reboot.

Finalize installation

Install the ACPI service

To enable the hypervisor to reboot or shutdown an instance, you must install and run the acpid service on the guest system.

Log in to the CentOS guest and run the following commands to install the ACPI service and configure it to start when the system boots:

# dnf install acpid
# systemctl enable acpid

Configure to fetch metadata

Note

Check cloud-init documentation for more information

An instance must interact with the metadata service to perform several tasks on start up. For example, the instance must get the ssh public key and run the user data script. To ensure that the instance performs these tasks, install the cloud-init utility.

The cloud-init package automatically fetches the public key from the metadata server and places the key in an account. Install cloud-init inside the CentOS guest by running:

# dnf install cloud-init

The account varies by distribution. On CentOS Stream virtual machines, the account is called cloud-user.

You can change the name of the account used by cloud-init by editing the /etc/cloud/cloud.cfg file and adding a line with a different user. For example, to configure cloud-init to put the key in an account named admin, use the following syntax in the configuration file:

users:
  - name: admin
    (...)

Cloud-init alternatives

Warning

This method is not recommended as only gets the ssh public key from the metadata server. It does not get user data, which is optional data that can be passed by the user when requesting a new instance. User data is often used to run a custom script when an instance boots.

If you are not able to install the cloud-init package in your image, to fetch the ssh public key and add it to the root account, edit the /etc/rc.d/rc.local file and add the following lines before the line touch /var/lock/subsys/local:

if [ ! -d /root/.ssh ]; then
  mkdir -p /root/.ssh
  chmod 700 /root/.ssh
fi

# Fetch public key using HTTP
ATTEMPTS=30
FAILED=0
while [ ! -f /root/.ssh/authorized_keys ]; do
  curl -f http://169.254.169.254/latest/meta-data/public-keys/0/openssh-key \
    > /tmp/metadata-key 2>/dev/null
  if [ \$? -eq 0 ]; then
    cat /tmp/metadata-key >> /root/.ssh/authorized_keys
    chmod 0600 /root/.ssh/authorized_keys
    restorecon /root/.ssh/authorized_keys
    rm -f /tmp/metadata-key
    echo "Successfully retrieved public key from instance metadata"
    echo "*****************"
    echo "AUTHORIZED KEYS"
    echo "*****************"
    cat /root/.ssh/authorized_keys
    echo "*****************"
  fi
done

Note

Some VNC clients replace the colon (:) with a semicolon (;) and the underscore (_) with a hyphen (-). Make sure to specify http: and not http;. Make sure to specify authorized_keys and not authorized-keys.

Note

With this method, previous CentOS versions needed to disable the zeroconf route, this is not needed as of EL8.

Allow partitions to resize

In order for the root partition to properly resize, install the cloud-utils-growpart package, which contains the proper tools to allow the disk to resize using cloud-init.

# dnf install cloud-utils-growpart

Configure console

Warning

Actually this does not work even following https://access.redhat.com/solutions/3443551

For the openstack console log command to work properly, instance must be configured to sent output to a serial console.

1. Edit the /etc/default/grub file and append console=tty0 console=ttyS0,115200n8 to GRUB_CMDLINE_LINUX.

   For example:

   ...
   GRUB_CMDLINE_LINUX="resume=/dev/mapper/cs-swap rd.lvm.lv=cs/root rd.lvm.lv=cs/swap console=tty0 console=ttyS0,115200n8"
   

2. Then regenerate GRUB configuration:

   # grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline
   

Customize

If you are building an image manually, you are probably interested in customizing it, now it’s time to apply your edits.

Shut down the instance

Shutdown the VM:

# shutdown now

Clean up

The image needs to be cleaned up of details such as the MAC address - the tool used is virt-sysprep, part of libguestfs

sudo virt-sysprep -d my-centos

Resize the image

Resize and compress the image to remove unused space (the image will then use available space after initialization) using virt-sparsify, part of libguestfs

sudo virt-sparsify --compress /var/lib/libvirt/images/my-centos.qcow2 ~/upload.qcow2

Image is complete

The resized image is now ready to be uploaded using openstack image create. For more information, see the python-openstackclient command list.

Undefine the libvirt domain

Now that you can upload the image to the Image service, you no longer need to have this virtual machine image managed by libvirt.

$ sudo virsh undefine my-centos

Or if you used virt-manager, delete it through the GUI.