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KVM and Hosting

| 3 min read

🧾 Working with KVM Virtual Machines and Creating Networks (NAT, Isolated, Bridged) + Hosting a Local Repository Server

KVM (Kernel-based Virtual Machine) combined with libvirt and virt-manager provides a powerful and flexible virtualization platform for Linux users. One of the most important aspects of virtualization is networking — giving VMs internet, isolating them, or letting them behave like physical machines on your LAN.

This article explains:

  • How KVM networking works
  • How to create different network types
  • How to connect VMs to them
  • How to host a local repository server and register VMs with it

1. Understanding KVM/libvirt Networking

KVM uses libvirt to create and manage software-defined networks. These show up as virtual bridges (e.g., virbr0, virbr1, virbr3).

There are three primary network types:

1. NAT Network (Default Mode)

  • VM gets internet access
  • VM can access host
  • But host cannot connect to VM directly
  • VM sits behind NAT like a home router

Bridge: virbr0 Good for: General-purpose VMs

2. Isolated Network

  • No internet
  • Only communication within the isolated network
  • VM can access host, host can access VM
  • Offline labs, secure internal networks

Good for: Offline repositories, training labs, secure testing

3. Bridged Network (Direct LAN Access)

  • VM behaves like a physical machine on the LAN
  • Gets IP from your WiFi/router
  • Other LAN devices can reach VM

Good for: Servers, shared environments

2. Creating Networks in libvirt

Networks are defined using XML files and created with virsh.

A. Creating a NAT Network with Internet Access

Create NAT network named natnet:

cat <<EOF | sudo tee /tmp/natnet.xml
<network>
  <name>natnet</name>
  <forward mode='nat'/>
  <bridge name='virbr10' stp='off' delay='0'/>
  <ip address='192.168.250.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.250.100' end='192.168.250.200'/>
    </dhcp>
  </ip>
</network>
EOF

Load & activate:

sudo virsh net-define /tmp/natnet.xml
sudo virsh net-start natnet
sudo virsh net-autostart natnet

Check:

sudo virsh net-list --all

B. Creating an Isolated Network

cat <<EOF | sudo tee /tmp/isolated.xml
<network>
  <name>isolatedlab</name>
  <bridge name='virbr20'/>
  <ip address='192.168.200.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.200.100' end='192.168.200.200'/>
    </dhcp>
  </ip>
</network>
EOF

Enable:

sudo virsh net-define /tmp/isolated.xml
sudo virsh net-start isolatedlab
sudo virsh net-autostart isolatedlab

C. Creating a Bridged Network (LAN VM IP)

Note: Bridging over WiFi sometimes requires macvtap instead of a true bridge.

Example bridged XML:

cat <<EOF | sudo tee /tmp/br0.xml
<network>
  <name>hostbridge</name>
  <forward mode='bridge'/>
  <bridge name='br0'/>
</network>
EOF

Enable:

sudo virsh net-define /tmp/br0.xml
sudo virsh net-start hostbridge
sudo virsh net-autostart hostbridge

3. Assigning Networks to VMs (virt-manager)

Open virt-manager → VM → Shut Down → NIC → Network source:

✔ NAT → natnet ✔ Isolated → isolatedlab ✔ Bridged → hostbridge

Select Device model: virtio for best performance.

Start VM, get DHCP:

sudo dhclient enp1s0

4. Hosting a Local Offline Repository Server

Many enterprise labs require offline YUM/DNF repositories. You can host one on the host using Apache.

A. Install Apache

sudo dnf install -y httpd
sudo systemctl enable --now httpd

B. Create a repo directory and extract ISO

Mount RHEL ISO:

sudo mkdir /mnt/rheliso
sudo mount -o loop RHEL8.iso /mnt/rheliso

Copy content to web directory:

sudo mkdir -p /var/www/html/rhel8
sudo cp -r /mnt/rheliso/* /var/www/html/rhel8/
sudo restorecon -Rv /var/www/html

Test:

curl http://localhost/rhel8/

5. Making the Repo Reachable from VMs

If VM is on NAT network natnet:

Host IP = 192.168.250.1 VM IP = something like 192.168.250.100

Inside VM test connectivity:

ping 192.168.250.1
curl http://192.168.250.1/rhel8/

It should show the directory listing.

6. Creating a Local YUM Repository File

Inside VM:

sudo tee /etc/yum.repos.d/rhel8-local.repo <<EOF
[BaseOS]
name=RHEL 8 BaseOS Local
baseurl=http://192.168.250.1/rhel8/BaseOS
enabled=1
gpgcheck=0

[AppStream]
name=RHEL 8 AppStream Local
baseurl=http://192.168.250.1/rhel8/AppStream
enabled=1
gpgcheck=0
EOF

Save → Clean cache:

sudo dnf clean all
sudo dnf makecache

Now you can install packages offline from your host server:

sudo dnf install httpd
sudo dnf install tmux

7. Summary

✔ NAT network

VM gets internet, can reach host repo.

✔ Isolated network

VM cannot reach internet, good for labs.

✔ Bridged network

VM gets IP on LAN like physical machine.

✔ Apache repo server

Host serves /var/www/html/rhel8/ → VM uses it as offline repo.

✔ VM registers to repo using .repo file