This example shows you how to install a CentOS image and focuses mainly on CentOS 6.4. Because the CentOS installation process might differ across versions, the installation steps might differ if you use a different version of CentOS.html
Download a CentOS install ISOlinux
Navigate to the CentOS mirrors page.web
Click one of the HTTP
links in the right-hand column next to one of the mirrors.ubuntu
Click the folder link of the CentOS version that you want to use. For example, 6.4/
.centos
Click the isos/
folder link.bash
Click the x86_64/
folder link for 64-bit images.app
Click the netinstall ISO image that you want to download. For example, CentOS-6.4-x86_64-netinstall.iso
is a good choice because it is a smaller image that downloads missing packages from the Internet during installation.dom
Start the installation process using either virt-manager or virt-install as described in the previous section. If you use virt-install, do not forget to connect your VNC client to the virtual machine.ssh
Assume that:curl
The name of your virtual machine image is centos-6.4
; you need this name when you use virsh commands to manipulate the state of the image.
You saved the netinstall ISO image to the /data/isos
directory.
If you use virt-install, the commands should look something like this:
# qemu-img create -f qcow2 /tmp/centos-6.4.qcow2 10G # virt-install --virt-type kvm --name centos-6.4 --ram 1024 \ --disk /tmp/centos-6.4.qcow2,format=qcow2 \ --network network=default \ --graphics vnc,listen=0.0.0.0 --noautoconsole \ --os-type=linux --os-variant=rhel6 \ --extra-args="console=tty0 console=ttyS0,115200n8 serial" \ --location=/data/isos/CentOS-6.4-x86_64-netinstall.iso
At the initial Installer boot menu, choose the Install or upgrade an existing system option. Step through the installation prompts. Accept the defaults.
The default TCP/IP settings are fine. In particular, ensure that Enable IPv4 support is enabled with DHCP, which is the default.
Choose URL as the installation method.
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 http://mirror.umd.edu/centos/6/os/x86_64
.
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Note |
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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: mirror.umd.edu
CentOS directory: centos/6/os/x86_64
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 Basic Storage Devices.
The installer may ask 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.
There are different options for partitioning the disks. The default installation uses LVM partitions, and creates three partitions (/boot
, /
, 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 because no scheme is better than another.
Step through the installation, using the default options. The simplest thing to do is to choose the "Basic Server" install (may be called "Server" install on older versions of CentOS), which installs an SSH server.
When the installation has completed, the Congratulations, your CentOS installation is complete screen appears.
To eject a disk by using the virsh command, libvirt requires that you attach an empty disk at the same target that the CDROM was previously attached, which should be hdc
. You can confirm the appropriate target using the dom dumpxml vm-image
command.
# virsh dumpxml centos-6.4 <domain type='kvm'> <name>centos-6.4</name> ... <disk type='block' device='cdrom'> <driver name='qemu' type='raw'/> <target dev='hdc' 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-6.4 "" hdc # virsh destroy centos-6.4 # virsh start centos-6.4
When you boot for the first time after installation, you might be prompted about authentication tools. Select Exit. Then, log in as root.
To enable the hypervisor to reboot or shutdown an instance, you must install and run the acpid
service on the guest system.
Run the following commands inside the CentOS guest to install the ACPI service and configure it to start when the system boots:
# yum install acpid # chkconfig acpid on
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-init
RPM, which is a port of the Ubuntu cloud-init package. This is the recommended approach.
Modify /etc/rc.local
to fetch desired information from the metadata service, as described in the next section.
The cloud-init
package automatically fetches the public key from the metadata server and places the key in an account. You can install cloud-init
inside the CentOS guest by adding the EPEL repo:
# yum install http://download.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm # yum install cloud-init
The account varies by distribution. On Ubuntu-based virtual machines, the account is called ubuntu
. On Fedora-based virtual machines, the account is called ec2-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
, add this line to the configuration file:
user: admin
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
:
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|
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
"*****************"
done
|
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Note |
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Some VNC clients replace the colon ( |
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Note |
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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. |
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 6.x
, you might need to add the following lines to the /boot/grub/menu.lst
file:
serial --unit=0 --speed=115200 terminal --timeout=10 console serial # Edit the kernel line to add the console entries kernel console=tty0 console=ttyS0,115200n8...
The operating system records the MAC address of the virtual Ethernet card in locations such as /etc/sysconfig/network-scripts/ifcfg-eth0
and /etc/udev/rules.d/70-persistent-net.rules
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-6.4
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-6.4