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Beginner KVM Networking Lab with Alpine Linux (Step-by-Step)

#kvm #devops #baremetal

🧪 Beginner KVM Networking Lab with Alpine Linux (Step-by-Step)

In this tutorial, we will:

  • Download Alpine Linux ISO
  • Create alpine1 VM
  • Install Alpine to disk
  • Verify NAT networking
  • Clone and create alpine2
  • Test VM-to-VM communication

All using KVM + libvirt on Ubuntu.

We use:

Alpine Linux
because it is extremely lightweight and perfect for networking labs.

🧱 Lab Architecture

We will use the default libvirt NAT network:

Host (Ubuntu)
 └── virbr0 (192.168.121.1/24)
        ├── alpine1
        └── alpine2
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  • VMs can talk to each other
  • VMs can access internet via NAT
  • VMs are not reachable from outside

🥇 Step 1 — Download Alpine ISO

Download Alpine (virt edition recommended for KVM):

wget https://dl-cdn.alpinelinux.org/alpine/v3.19/releases/x86_64/alpine-virt-3.19.1-x86_64.iso
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Move it to libvirt image directory:

sudo mv alpine-virt-3.19.1-x86_64.iso /var/lib/libvirt/images/
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Verify:

ls -lh /var/lib/libvirt/images/
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🥈 Step 2 — Create alpine1 VM

Create a 4GB disk and attach ISO:

sudo virt-install \
  --name alpine1 \
  --memory 256 \
  --vcpus 1 \
  --disk path=/var/lib/libvirt/images/alpine1.qcow2,size=4,format=qcow2 \
  --cdrom /var/lib/libvirt/images/alpine-virt-3.19.1-x86_64.iso \
  --os-variant generic \
  --network network=default \
  --graphics none \
  --console pty,target_type=serial
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Explanation:

Option Purpose
--memory 256 Lightweight VM
--disk size=4 Create 4GB disk
--network default Attach to NAT bridge
--graphics none CLI only
--console Serial console access

🥉 Step 3 — Install Alpine Inside VM

Inside VM:

setup-alpine
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Important answers:

  • Hostname → alpine1
  • Network → eth0
  • IP → dhcp
  • Proxy → none
  • SSH server → openssh
  • Disk → sda
  • Install mode → sys
  • Confirm erase → y

After installation:

reboot
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🧹 Step 4 — Verify ISO Removed

On host:

sudo virsh domblklist alpine1
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You should see:

Target   Source
---------------------------------
hda      /var/lib/libvirt/images/alpine1.qcow2
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No ISO attached = good.

🌐 Step 5 — Verify Networking

Connect to console:

sudo virsh console alpine1
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Inside VM:

ip a
ip route
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You should see something like:

inet 192.168.121.100/24
default via 192.168.121.1
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Test internet:

ping 8.8.8.8
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If it works → NAT is working 🎉

🔍 Step 6 — Inspect Networking From Host

Check bridge:

ip a | grep virbr0
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You should see:

virbr0: 192.168.121.1/24
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Check bridge members:

brctl show
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You should see:

virbr0    vnetX
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Check NAT rules:

sudo iptables -t nat -L -n
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Look for:

MASQUERADE  192.168.121.0/24
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This rule allows VM internet access.

🥇 Step 7 — Clone alpine1 to Create alpine2

Do NOT manually copy disk.

Use virt-clone:

sudo virt-clone \
  --original alpine1 \
  --name alpine2 \
  --file /var/lib/libvirt/images/alpine2.qcow2
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This:

  • Copies disk safely
  • Generates new MAC address
  • Prevents network conflict

🥈 Step 8 — Start alpine2 

sudo virsh start alpine2
sudo virsh console alpine2
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Inside alpine2:

ip a
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It should get a different IP:

alpine1 → 192.168.121.100
alpine2 → 192.168.121.101
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🧪 Step 9 — Test VM-to-VM Communication

From alpine1:

ping <alpine2-ip>
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Example:

ping 192.168.121.101
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This traffic:

  • Does NOT go through NAT
  • Stays inside virbr0 bridge
  • Pure Layer 2 switching

🔎 Observe Host Again 

brctl show
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Now you will see:

virbr0
   ├── vnet10 (alpine1)
   └── vnet11 (alpine2)
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Each VM gets its own tap device.

📦 What You Built

You now have:

✔ 2 lightweight VMs (256MB each)
✔ DHCP working
✔ NAT outbound internet
✔ VM-to-VM communication
✔ Real bridge networking
✔ Understanding of packet flow

🧠 Packet Flow Explained

VM → Internet 

VM → vnetX → virbr0 → host → iptables MASQUERADE → Internet
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VM → VM 

VM1 → vnetX → virbr0 → vnetY → VM2
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No NAT involved.

📊 Resource Usage

Each Alpine VM:

  • 256MB RAM
  • 4GB disk
  • ~60MB ISO
  • Very fast boot

You can easily run 6–8 VMs on a normal 8GB machine.

🚀 Next Phase (Preview)

In the next phase, we will:

  • Create isolated network (no NAT)
  • Create internal bridge
  • Make one VM act as router
  • Add static routes
  • Simulate multi-subnet enterprise network

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