How to install and configure KVM on CentOS 7 or RHEL servers? How to install KVM tool on CentOS 7 and use cloud images to install guest virtual machine? These are the questions we seek to answer in this article. Stay with us.

Kernel-based Virtual Machine or KVM is an emulation software for CentOS or RHEL 7. KVM turns your server into a control center. In this article, we will show you how to install and manage a virtual environment with the help of KVM tool on Linux CentOS 7 or RHEL 7. Also, the method of installing and managing virtual machines (VM) in a physical server using CLI will be explained. Make sure that the simulation technology or VT is enabled in your server’s BIOS. Meanwhile, you can check Intel VT CPU support and AMD-V emulation using the following command.

$ lscpu | grep Virtualization
Virtualization: VT-x

Follow the steps below to install KVM tool on CentOS 7/RHEL 7 server.

Step 1) Install KVM tool

Type the following yum command.

# yum install qemu-kvm libvirt libvirt-python libguestfs-tools virt-install

Installing KVM tool on CentOS 7/RHEL 7 Linux server

Start the libvirtd service.

# systemctl enable libvirtd
# systemctl start libvirtd

Step 2) Verify KVM installation

Make sure the KVM module is loaded with lsmod and grep commands.

# lsmod | grep -i kvm

Step 3) Setting up the bridge network

A dhcpd-based bridge network is configured by libvirtd by default. You can check this issue using the following commands.

# brctl show
# virsh net-list

Default KVM network

All virtual machines (of guest type) only have network access to other VMs on the same server. Here, a private network 192.168.122.0/24 will be created for you. Check it out.

# virsh net-dumpxml default

If you want your virtual machines to be available to other servers on the LAN, you need to set up a bridge network on the server connected to the LAN. For this purpose, update the nic configuration file such as ifcfg-enp3s0 or em1.

# vi /etc/sysconfig/network-scripts/enp3s0

Add the following line.

BRIDGE=br0

Save and close the file in vi. Then edit and add the /etc/sysconfig/network-scripts/ifcfg-br0 file.

# vi /etc/sysconfig/network-scripts/ifcfg-br0

Add the following lines.

DEVICE="br0"
# I am getting ip from DHCP server #
BOOTPROTO="dhcp"
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
ONBOOT="yes"
TYPE="Bridge"
DELAY="0"

Restart the network service. Note that the ssh command will terminate and it is better to start over.

# systemctl restart NetworkManager

Check the issue with the brctl command.

# brctl show

Step 4) Create your first virtual machine.

Here we are going to create a VM on CentOS 7.x. First, get the latest version of the CentOS 7.x image using the wget command.

# cd /var/lib/libvirt/boot/
# wget https://mirrors.kernel.org/centos/7.4.1708/isos/x86_64/CentOS-7-x86_64-Minimal-1708.iso

Check ISO images

# wget https://mirrors.kernel.org/centos/7.4.1708/isos/x86_64/sha256sum.txt
# sha256sum -c sha256sum.txt

Creating a CentOS 7.x virtual machine

Here, we are going to create a CentOS 7.x virtual machine with 2 GB of RAM, two CPU cores, one nic and 40 GB of disk space. For this purpose we have:

# virt-install \
--virt-type=kvm \
--name centos7 \
--ram 2048 \
--vcpus=1 \
--os-variant=centos7.0 \
--cdrom=/var/lib/libvirt/boot/CentOS-7-x86_64-Minimal-1708.iso \
--network=bridge=br0,model=virtio \
--graphics vnc \
--disk path=/var/lib/libvirt/images/centos7.qcow2,size=40,bus=virtio,format=qcow2

To set vnc input from another terminal via ssh we have:

# virsh dumpxml centos7 | grep vnc
<graphics type='vnc' port='5901' autoport='yes' listen='127.0.0.1'>

Here you need to write down the port number or 5901. To access the remote vnc server, you need to use an SSH client to set up the tunnel and a VNC client. Through the client or desktop, type the SSH port forwarding command as follows.

1	`$ ssh [email protected] -L 5901:127.0.0.1:5901`

Once the ssh tunnel is established, you can point your VNC client to localhost 127.0.0.1 and port 5901 as follows.

VNC client reference

In this case, you should see the CentOS Linux guest installation page as below.

CentOS Linux installation welcome screen

Now just follow the instructions that appear on the screen and install CentOS 7. When the installation is finished, click the reboot button. The remote server has closed the connection with the VNC client. You can reconnect using the KVM tool to configure other parts of the server, including the SSH service or firewall.

How to build a RHEL 7.x virtual machine

First, you must have the rhel-server-7.3-x86_64-dvd.iso file saved. Here we create a RHEL 7.x virtual machine with 2 GB of RAM, two CPU cores, one nic and 40 GB of disk space.

# virt-install \
--virt-type=kvm \
--name rhel7 \
--memory=2048,maxmemory=4096 \
--vcpus=2 \
--os-variant=rhel7.3 \
--cdrom=/var/lib/libvirt/boot/rhel-server-7.3-x86_64-dvd.iso \
--network=bridge=virbr0,model=virtio \
--graphics vnc \
--disk path=/var/lib/libvirt/images/rhel7.qcow2,size=40,bus=virtio,format=qcow2

To configure vnc input from another terminal via ssh we have:

$ sudo virsh dumpxml rhel7 | grep vnc
<graphics type='vnc' port='5904' autoport='yes' listen='127.0.0.1'>

Here you need to write down the port number or 5904. To access the remote vnc server, you need to use an SSH client to set up the tunnel and a VNC client. Through the client or desktop, type the SSH port forwarding command as follows.

1	`$ ssh [email protected] -L 5904:127.0.0.1:5904`

Once the ssh tunnel is established, you can point the VNC client to localhost 127.0.0.1 and port 5904 to proceed with the next steps of the RHEL 7.x installation.

Step 5) Using cloud images

The installation method presented above can be suitable for educational purposes or a virtual machine. But if you want to use more number of virtual machines, it is better to try cloud images. It will be possible to edit “pre-production” cloud images according to your needs. For example, you can add a user, enter ssh keys, and set the time zone. Now let’s see how to create a virtual machine with 1 GB of RAM, 20 GB of disk space and a vCPU in CentOS 7 in this way.

Download CentOS 7 cloud image

# cd /var/lib/libvirt/boot
# wget http://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud.qcow2

Generating the required directories

# D=/var/lib/libvirt/images
# VM=centos7-vm1 ## vm name ##
# mkdir -vp $D/$VM
mkdir: created directory '/var/lib/libvirt/images/centos7-vm1'

Create meta-data file

# cd $D/$VM
# vi meta-data

Add the following in this file.

instance-id: centos7-vm1
local-hostname: centos7-vm1

Create user-data file

Here we want to log into the VM via ssh keys. Accordingly, first make sure that the ssh keys are in place.

# ssh-keygen -t ed25519 -C "VM Login ssh key"

ssh-keygen command

Now edit user-data as below.

# cd $D/$VM
# vi user-data

Add the following according to your settings.

#cloud-config
# Hostname management
preserve_hostname: False
hostname: centos7-vm1
fqdn: centos7-vm1.nixcraft.com
# Users
users:
- default
- name: vivek
groups: ['wheel']
shell: /bin/bash
sudo: ALL=(ALL) NOPASSWD:ALL
ssh-authorized-keys:
- ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIMP3MOF2ot8MOdNXCpHem0e2Wemg4nNmL2Tio4Ik1JY VM Login ssh key
# Configure where output will go
output:
all: ">> /var/log/cloud-init.log"
# configure interaction with ssh server
ssh_genkeytypes: ['ed25519', 'rsa']
# Install my public ssh key to the first user-defined user configured
# in cloud.cfg in the template (which is centos for CentOS cloud images)
ssh_authorized_keys:
- ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIMP3MOF2ot8MOdNXCpHem0e2Wemg4nNmL2Tio4Ik1JY VM Login ssh key
# set timezone for VM
timezone: Asia/Kolkata
# Remove cloud-init
runcmd:
- systemctl stop network && systemctl start network
- yum -y remove cloud-init

Copy the cloud image.

# cd $D/$VM
# cp /var/lib/libvirt/boot/CentOS-7-x86_64-GenericCloud.qcow2 $VM.qcow2

Create a 20 GB disk image

# cd $D/$VM
# export LIBGUESTFS_BACKEND=direct
# qemu-img create -f qcow2 -o preallocation=metadata $VM.new.image 20G
# virt-resize --quiet --expand /dev/sda1 $VM.qcow2 $VM.new.image

Set VM image disk size

Overwrite the resized image.

# cd $D/$VM
# mv $VM.new.image $VM.qcow2

Create a cloud-init ISO file

# mkisofs -o $VM-cidata.iso -V cidata -J -r user-data meta-data

Create a cloud-init ISO file

Create a pool

# virsh pool-create-as --name $VM --type dir --target $D/$VM
Pool centos7-vm1 created

Installing a CentOS 7 virtual machine

# cd $D/$VM
# virt-install --import --name $VM \
--memory 1024 --vcpus 1 --cpu host \
--disk $VM.qcow2,format=qcow2,bus=virtio \
--disk $VM-cidata.iso,device=cdrom \
--network bridge=virbr0,model=virtio \
--os-type=linux \
--os-variant=centos7.0 \
--graphics spice \
--noautoconsole

Delete unnecessary files

# cd $D/$VM
# virsh change-media $VM hda --eject --config
# rm meta-data user-data centos7-vm1-cidata.iso

Find KVM device IP address or dhcp address

# virsh net-dhcp-leases default

Creating a CentOS7 virtual machine

Log into the VM

Use the ssh command as follows.

1	`# ssh [email protected]`

Sample VM implementation

Useful commands

We review some useful commands for managing virtual machines.

List of all VMs

# virsh list --all

Get VM information

# virsh dominfo vmName
# virsh dominfo centos7-vm1

Stopping and shutting down a VM

# virsh shutdown centos7-vm1

Start the VM

# virsh start centos7-vm1

Set the VM to start automatically at system startup

# virsh autostart centos7-vm1

Reboot (soft and safe reboot) the VM

# virsh reboot centos7-vm1

Reboot (hard and insecure) VM

# virsh reset centos7-vm1

Remove the VM

# virsh shutdown centos7-vm1
# virsh undefine centos7-vm1
# virsh pool-destroy centos7-vm1
# D=/var/lib/libvirt/images
# VM=centos7-vm1
# rm -ri $D/$VM

To see the general list of virsh command types, we have:

# virsh help | less
# virsh help | grep reboot

Notes about virt-builder

It is possible to quickly build virtual machine images in CentOS 7 using the virt-builder command.

List of VM images

virt-builder --list
virt-builder --list | grep -i fedora
virt-builder --list | grep -i debian
virt-builder --list | grep -i ubuntu
virt-builder --list | grep -i freebsd

Creating Ubuntu 16.04 LTS virtual machine

# virt-builder ubuntu-16.04 \
--size=20G --format qcow2 -o /var/lib/libvirt/images/ncbz01-disk01.qcow2 \
--hostname ncbz01 --network --timezone Asia/Kolkata \
--firstboot-command "dpkg-reconfigure openssh-server" \
--edit '/etc/default/grub:
s/^GRUB_CMDLINE_LINUX_DEFAULT=.*/GRUB_CMDLINE_LINUX_DEFAULT="console=tty0 console=ttyS0,115200n8"/' \
--run-command update-grub

Sample output (note that the random root password will be displayed on the screen):

[   3.9] Downloading: http://libguestfs.org/download/builder/ubuntu-16.04.xz
######################################################################## 100.0%
[ 121.7] Planning how to build this image
[ 121.7] Uncompressing
[ 126.5] Resizing (using virt-resize) to expand the disk to 20.0G
[ 142.1] Opening the new disk
[ 143.8] Setting a random seed
[ 143.8] Setting the machine ID in /etc/machine-id
[ 143.8] Setting the hostname: ncbz01
[ 144.3] Setting the timezone: Asia/Kolkata
[ 144.3] Setting passwords
virt-builder: Setting random password of root to 1KTLsuVx14k989eq
[ 144.9] Finishing off
Output file: /var/lib/libvirt/images/ncbz01-disk01.qcow2
Output size: 20.0G
Output format: qcow2
Total usable space: 18.6G
Free space: 17.7G (94%)

Now that the image is created, it’s time to install the VM.

# virt-install --import --name ncbz01 \
--ram 2048 \
--vcpu 2 \
--disk path=/var/lib/libvirt/images/ncbz01-disk01.qcow2,format=qcow2 \
--os-variant ubuntu17.04 \
--network=bridge=virbr0,model=virtio \
--noautoconsole

Performance:

# virsh start ncbz01

Log into the VM

# virsh console ncbz01

Conclusion

In this article, you learned how to install and set up the KVM tool on servers with CentOS 7 or RHEL 7 Linux operating systems. We hope that this article has also received your attention.

Installing KVM on CentOS 7 Linux server