KVM Hypervisor on Bare Metal

KVM Overview

KVM or Kernel-based Virtual Machine is a complete open source virtualization solution for Linux on x86 hardware. It supports CPUs that come with virtualization extensions (Intel VT or AMD-V).

KVM provides the environment to run multiple virtual machines running Linux or Windows images.

Each virtual machine has virtualized devices such as:

  • a network card
  • a disk
  • a graphics adapter, etc.

Requirements

Installing the KVM Hypervisor and a First Guest

1 . Connect to your Bare Metal via SSH.

2 . Update the software already installed on the system to the latest available release:

apt update && apt upgrade

3 . All required packages for KVM are available in Ubuntu’s default repository. Install them with apt:

apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virtinst  libguestfs-tools libvirt-bin

4 . Verify that the CPU supports hardware virtualization, by executing kvm-ok.

If the CPU supports hardware virtualization, the following message is displayed:

INFO: /dev/kvm exists
KVM acceleration can be used

If the following message appears, hardware virtualization is not supported:

INFO: Your CPU does not support KVM extensions
KVM acceleration can NOT be used

It is still possible to run virtual machines, but they will run much slower without the KVM extensions.

5 . Create a directory to store the virtual disk images by running the following command:

mkdir -p /data/kvm/images

Start the installation of the virtual machine by running virt-install with the following parameters to install an Ubuntu Bionic Beaver (18.04) VM:

virt-install --name ubuntu1804 --ram 4096 --disk path=/data/kvm/images/ubuntu1804-2.img,size=10 --vcpus 2 --os-type linux --os-variant ubuntu18.04 --network bridge=virbr0 --graphics none --console pty,target_type=serial --location 'http://fr.archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/' --extra-args 'console=ttyS0,115200n8 serial'

The different flags have the following meaning:

  • --name: Specifies the name of the VM. Choose an easy to remember name for the instance.
  • --ram: Specifies the amount of RAM of the VM in MB. In the example the VM will be provisioned with 4096 MB of RAM.
  • --disk path: Specifies the path to the disk image and with the size parameter the size in gigabytes of the image (in the example 10GB).
  • --os-type: Specifies the OS type of the VM (i.e. linux or windows)
  • --os-variant: This parameter is used to optimize the guest configuration for a specific operating system variant (i.e. ubuntu18.04).
  • --network bridge: Specifies the network bridge interface to use
  • --graphics none: No graphical console will be added to the VM
  • --console: Specifies the characteristics of the serial console
  • --location: The location of the of the installation source for the VM. It can either be a:
    • DIRECTORY - Path to a local directory containing an installable distribution image
    • nfs://host/path - An NFS server location containing an installable distribution image
    • http://host/path - An HTTP server location containing an installable distribution image
    • ftp://host/path - An FTP server location containing an installable distribution image
  • --extra-args - Specifies additional kernel command line arguments to pass to the installer when performing an installation from --location.

The output of the installer will be redirected to the console. Complete the installation by following the tasks requested by the installer.

6 . Once the installation has been completed, return to a shell on the host and shutdown the virtual machine:

virsh shutdown ubuntu1804

7 . Mount the disk image as follows:

guestmount -d ubuntu1804 -i /mnt

8 . Create a symlink to redirect the output to the serial console of the virtual machine:

ln -s /mnt/lib/systemd/system/getty@.service /mnt/etc/systemd/system/getty.target.wants/getty@ttyS0.service

Unmount the virtual disk image:

umount /mnt

9 . Start the virtual machine again, after a few seconds the Ubuntu login appears:

virsh start ubuntu1804 --console

Log into the virtual machine and install the required software. The VM has configured its network automatically with DHCP and is able to communicate with the outside world by passing all traffic over the virtual bridge. However it is not possible to receive external connections with on the virtual machine until now.

Forwarding Ports to Guests with IPtables

By default virtual machines, that are connected via a virtual network with <forward mode='nat'/> can make any outgoing connection they want. Incoming connections are possible from the hypervisor host itself and other virtual machines connected to the same virtual network. To be able to make services running on a guest, connected to a virtual network, publicly available on the Internet, a port forwarding is required. This means the host of the guest machine, forwards all requests sent to a specific port of the public IP to the internal IP of the guest to make services running on the virtual machine available for external users. This is done by configuring IPtables rules for each service that shall be exposed on the Internet.

1 . Get the list of available networks, by running the following command:

virsh net-list

The command returns a list like the following example:

Name                 State      Autostart     Persistent
----------------------------------------------------------
default              active     yes           yes

2 . Dump the configuration of the default network by running the following command:

virsh net-dumpxml default

The command returns a XML formatted list as the following example:

<network>
  <name>default</name>
  <uuid>0f951a2a-c69d-46ea-87ee-a9a78e48624f</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:28:e1:3e'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>

The DHCP range is clearly indicated and uses the IPs between 192.168.122.2 and 192.168.122.254.

4 . Retrieve the MAC address of the virtual machine in preparation of assigning an IP statically, by running virsh dumpxml VM_NAME, i.e.:

virsh dumpxml ubuntu1804 | grep -i '<mac address>'

The command returns the MAC address of the virtual machine. Take a note of it, as it is required in the following step:

<mac address='52:54:00:27:7a:45'/>

5 . Edit the default network configuration with the following command:

virsh net-edit default

6 . Add this line line after the <range …> section. Make sure to edit the MAC-address and the name to the values of your virtual machine:

<host mac='52:54:00:27:7a:45' name='ubuntu1804' ip='192.168.122.222'/>

7 . Shutdown the virtual machine, disable and re-enable DHCP, restart the libvirt-bin service and restart the virtual machine:

virsh shutdown ubuntu1804
virsh net-destroy default
virsh net-start default
systemctl restart libvirt-bin
virsh start ubuntu1804

8 . Verify that the virtual machine is up by sending a ping to it:

ping -a 192.168.122.222

If everything went well, the guest will reply on the ping command:

root@vm-host:~# ping -a 192.168.122.222
PING 192.168.122.222 (192.168.122.222) 56(84) bytes of data.
64 bytes from 192.168.122.222: icmp_seq=1 ttl=64 time=0.358 ms
64 bytes from 192.168.122.222: icmp_seq=2 ttl=64 time=0.283 ms
64 bytes from 192.168.122.222: icmp_seq=3 ttl=64 time=0.317 ms
64 bytes from 192.168.122.222: icmp_seq=4 ttl=64 time=0.306 ms

--- 192.168.122.222 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 5112ms
rtt min/avg/max/mdev = 0.283/0.402/0.816/0.186 ms

9 . Configure a port forwarding that forwards the port 5678 from the host machine to port 22 on the guest machine, to be able to SSH into it:

# Enable connections from the outside
iptables -I FORWARD -o virbr1 -d  192.168.122.222 -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 5678 -j DNAT --to 192.168.122.222:22

# Masquerade local subnet
iptables -I FORWARD -o virbr1 -d  192.168.122.222 -j ACCEPT
iptables -t nat -A POSTROUTING -s 192.168.122.0/24 -j MASQUERADE
iptables -A FORWARD -o virbr1 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i virbr1 -o ens2 -j ACCEPT
iptables -A FORWARD -i virbr1 -o lo -j ACCEPT

The interface vibr1 with the subnet 192.168.122.0/24 is the one used in the virtual network. The interface named ens2 is the public Internet interface of the host machine. Notice that this name may differ depending on your hardware and the OS version used.

10 . Access your VM via SSH by running the following command:

ssh user@PUBLIC_IP_HOST_MACHINE -p 5678

Once logged into the virtual machine you can configure additional service like web or email server applications and expose them on the Internet by adding IPtables rules.

For more information about configuring virtual machines with KVM, refer to the official documentation.

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