Example: CentOS image
This example shows you how to install a CentOS image and focuses mainly on CentOS 7. Because the CentOS installation process might differ across versions, the installation steps might differ if you use a different version of CentOS.
Download a CentOS install ISO
- Navigate to the CentOS mirrors page.
- Click one of the
HTTPlinks in the right-hand column next to one of the mirrors.
- Click the folder link of the CentOS version that you want to use. For example,
- Click the
- Click the
x86_64/folder link for 64-bit images.
- Click the netinstall ISO image that you want to download. For example,
CentOS-7-x86_64-NetInstall-1611.isois a good choice because it is a smaller image that downloads missing packages from the Internet during installation.
Start the installation process
Start the installation process using either the
virt-manager or the
virt-install command as described previously. If you use the
virt-install command, do not forget to connect your VNC client to the virtual machine.
- The name of your virtual machine image is
centos; you need this name when you use
virshcommands to manipulate the state of the image.
- You saved the netinstall ISO image to the
If you use the
virt-install command, the commands should look something like this:
# qemu-img create -f qcow2 /tmp/centos.qcow2 10G # virt-install --virt-type kvm --name centos --ram 1024 \ --disk /tmp/centos.qcow2,format=qcow2 \ --network network=default \ --graphics vnc,listen=0.0.0.0 --noautoconsole \ --os-type=linux --os-variant=centos7.0 \ --location=/data/isos/CentOS-7-x86_64-NetInstall-1611.iso
Step through the installation
At the initial Installer boot menu, choose the
Install CentOS 7 option. After the installation program starts, choose your preferred language and click
Continue to get to the installation summary. Accept the defaults.
Change the Ethernet status
The default Ethernet setting is
OFF. Change the setting of the Ethernet form
ON. In particular, ensure that
IPv4 Settings' Method is
Automatic (DHCP), which is the default.
The installer allows you to choose a host name. The default (
localhost.localdomain) is fine. You install the
cloud-init package later, which sets the host name on boot when a new instance is provisioned using this image.
Point the installer to a CentOS web server
Depending on the version of CentOS, the net installer requires the user to specify either a URL or the web site and a CentOS directory that corresponds to one of the CentOS mirrors. If the installer asks for a single URL, a valid URL might be
Consider using other mirrors as an alternative to
If the installer asks for web site name and CentOS directory separately, you might enter:
- Web site name:
- CentOS directory:
See CentOS mirror page to get a full list of mirrors, click on the
HTTP link of a mirror to retrieve the web site name of a mirror.
If prompted about which type of devices your installation uses, choose
Virtio Block Device.
Partition the disks
There are different options for partitioning the disks. The default installation uses LVM partitions, and creates three partitions (
swap), which works fine. Alternatively, you might want to create a single ext4 partition that is mounted to
/, which also works fine.
If unsure, use the default partition scheme for the installer. While no scheme is inherently better than another, having the partition that you want to dynamically grow at the end of the list will allow it to grow without crossing another partition's boundary.
Select installation option
Step through the installation, using the default options. The simplest thing to do is to choose the
Minimal Install install, which installs an SSH server.
Set the root password
During the installation, remember to set the root password when prompted.
Detach the CD-ROM and reboot
Wait until the installation is complete.
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.
# virsh dumpxml 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. If you are using
virt-manager, the commands below will work, but you can also use the GUI to detach and reboot it by manually stopping and starting.
# virsh attach-disk --type cdrom --mode readonly centos "" hda # virsh reboot centos
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 as root to the CentOS guest and run the following commands to install the ACPI service and configure it to start when the system boots:
# yum install acpid # systemctl enable acpid
Configure to fetch metadata
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, use one of these methods:
- Install a
cloud-initRPM, which is a port of the Ubuntu cloud-init package. This is the recommended approach.
- Modify the
/etc/rc.localfile to fetch desired information from the metadata service, as described in the next section.
Use cloud-init to fetch the public key
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:
# yum install cloud-init
The account varies by distribution. On CentOS-based virtual machines, the account is called
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 (...)
Install cloud-utils-growpart to 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.
# yum install cloud-utils-growpart
Write a script to fetch the public key (if no cloud-init)
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
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
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
The previous script 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.
As the OpenStack metadata service is compatible with version 2009-04-04 of the Amazon EC2 metadata service, consult the Amazon EC2 documentation on Using Instance Metadata <http://docs.amazonwebservices.com/AWSEC2/2009-04-04/UserGuide/ AESDG-chapter-instancedata.html> for details on how to get user data.
Disable the zeroconf route
For the instance to access the metadata service, you must disable the default zeroconf route:
# echo "NOZEROCONF=yes" >> /etc/sysconfig/network
nova console-log command to work properly on CentOS 7, you might need to do the following steps:
/etc/default/grubfile and configure the
GRUB_CMDLINE_LINUXoption. Delete the
rhgb quietand add
console=tty0 console=ttyS0,115200n8to the option.
... GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=cl/root rd.lvm.lv=cl/swap console=tty0 console=ttyS0,115200n8"
Run the following command to save the changes:
# grub2-mkconfig -o /boot/grub2/grub.cfg Generating grub configuration file ... Found linux image: /boot/vmlinuz-3.10.0-229.14.1.el7.x86_64 Found initrd image: /boot/initramfs-3.10.0-229.14.1.el7.x86_64.img Found linux image: /boot/vmlinuz-3.10.0-229.4.2.el7.x86_64 Found initrd image: /boot/initramfs-3.10.0-229.4.2.el7.x86_64.img Found linux image: /boot/vmlinuz-3.10.0-229.el7.x86_64 Found initrd image: /boot/initramfs-3.10.0-229.el7.x86_64.img Found linux image: /boot/vmlinuz-0-rescue-605f01abef434fb98dd1309e774b72ba Found initrd image: /boot/initramfs-0-rescue-605f01abef434fb98dd1309e774b72ba.img done
Shut down the instance
From inside the instance, run as root:
Clean up (remove MAC address details)
The operating system records the MAC address of the virtual Ethernet card in locations such as
/etc/sysconfig/network-scripts/ifcfg-eth0 during the instance process. However, each time the image boots up, the virtual Ethernet card will have a different MAC address, so this information must be deleted from the configuration file.
There is a utility called
virt-sysprep, that performs various cleanup tasks such as removing the MAC address references. It will clean up a virtual machine image in place:
# virt-sysprep -d centos
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. Use the
virsh undefine vm-image command to inform libvirt:
# virsh undefine centos
Image is complete
The underlying image file that you created with the
qemu-img create command is ready to be uploaded. For example, you can upload the
/tmp/centos.qcow2 image to the Image service by using the
openstack image create command. For more information, see the Create or update an image.