1. Install KVM and Associated Dependencies

sudo apt install qemu-kvm libvirt-clients libvirt-daemon-system bridge-utils virtinst libvirt-daemon -y

2. Define the Network Bridge

nano br0.xml

Please add the following lines inside br0.xml

<network>  
  <name>br0</name>  
  <forward mode="bridge"/>  
  <bridge name="br0" />  
</network>

Press Ctrl+X and then y to save changes. Then issue the following commands:

sudo virsh net-define br0.xml  
sudo virsh net-start br0  
sudo virsh net-autostart br0

Assuming no errors, the br0 network is defined and ready. Use the following command to verify the status of br0:

sudo virsh net-list --all

The output should look similar to the provided example:

 Name      State    Autostart   Persistent
--------------------------------------------
 br0       active   yes         yes
 default   active   yes         yes

2.5 Virsh Default Network Parameters

You can also start the default virsh network if needed with the following commands:

sudo virsh net-start default  
sudo virsh net-autostart default  
sudo virsh net-list --all

The output should look similar to the example output shown below:

user@hostname:~$ sudo virsh net-list --all
 Name      State    Autostart   Persistent
--------------------------------------------
 br0       active   yes         yes
 default   active   yes         yes

3. Edit the Interfaces File and Bond br0 to Your Physical Network Interface(s)

Use the following command to edit /etc/network/interfaces:

sudo nano /etc/network/interfaces

Using nano, make the following changes to /etc/network/interfaces. NOTE: The name of the network interface(s) may be different. Take note and be sure to use the correct interface name(s) on your system. My interface was named eno1.

Example interfaces File

More details can be found here.

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# Bridge bound to eno1

auto br0
iface br0 inet static
        bridge_ports eno1
        address 10.0.0.10/24
        netmask 255.255.255.0
        gateway 10.0.0.1
        bridge_stp off
        bridge_maxwait 5
        dns-nameservers 1.1.1.1 8.8.8.8

When finished, press Ctrl+X followed by Y to save changes to /etc/network/interfaces. Use the following command to restart the network service and apply changes:

sudo systemctl restart networking

Verify the bridge is working and has an IP on your network:

ip a

The output of ip a should look similar to the example shown below. Example output has been truncated:

br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 9e:5e:88:25:07:cf brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.10/24 brd 10.0.0.255 scope global br0
       valid_lft forever preferred_lft forever
    inet6 fe80::9c5e:88ff:fe25:7cf/64 scope link
       valid_lft forever preferred_lft forever

4. A Bit More Housekeeping

Edit /etc/group and add your username to the libvirt group:

sudo nano /etc/group

Find libvirt:x:111: and then add your username:

libvirt:x:111:your-user-name-here

If you need to add additional users to the libvirt group, separate them with commas:

libvirt:x:111:user1,user2,user3

You can also add your username to the libvirt group with the following command:

sudo usermod -aG libvirt $(whoami)

Use the following command to verify membership to the libvirt group:

groups $(whoami)

The output should look similar to the example below:

user : user cdrom floppy sudo audio dip video plugdev users kvm netdev libvirt

To manage the VMs via GUI, install virt-manager on another machine with the following command:

sudo apt install virt-manager -y

Before setting up guest VMs on the hypervisor, use tools like wget, curl, or lynx to download images/ISO files into the /var/lib/libvirt/images/ folder.

If using another machine to manage your KVM hypervisor, it is advisable to copy your SSH ID before adding the connection in virt-manager:

ssh-copy-id user@remote-host

Follow the remaining prompts and connect to the hypervisor via IP address or hostname using virt-manager.

5. Finally (Optional Stuff)

IP Forwarding

For more complicated network setups, you may want to enable IPv4 forwarding using the following commands:

sudo nano /etc/sysctl.conf

Uncomment or add the following line to /etc/sysctl.conf:

net.ipv4.ip_forward=1

Then apply changes with:

sudo sysctl -p

Explanation: Enabling IP forwarding ensures that the bridge can properly forward traffic, which is crucial for networking performance and reliability in a virtualized environment.

Example of When You Might Need IP Forwarding

  1. Multiple Subnets: If your virtual machines are on a different subnet than your host machine and need to communicate with other subnets or the internet.
  2. NAT Configuration: If your host system provides internet access to VMs on a private network.
  3. Complex Network Topologies: In setups involving multiple networks and routers, where traffic needs to be forwarded between interfaces.

Good luck and happy hacking!