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,
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 virsh commands 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
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:
See CentOS mirror page
to get a full list of mirrors, click on the
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
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
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
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:
cloud-initRPM, which is a port of the Ubuntu cloud-init package. This is the recommended approach.
/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.
cloud-init inside the CentOS guest by
# 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
by editing the
/etc/cloud/cloud.cfg file and adding a line
with a different user. For example, to configure
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,
/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
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 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
For the 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
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
image to the Image service by using the openstack image create
command. For more information, see the
python-openstackclient command list.