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Virtual-Lab-Router-VM-Server-VM (KVM)

📌 Overview

This project demonstrates a small virtual infrastructure built with:

  • Router VM (Debian 12)
  • Server VM (Debian 12)
  • Isolated virtual network (labnet)
  • NAT + IP forwarding
  • nftables firewall rules

The goal of this lab was to simulate a minimal production-like network:

Network

⚠️ Warning: The interface names on your system may vary, please check the interface names before configuring the network.

🔁 Traffic Flow

Server → Internet

Server VM
   │
   │ default gateway
   ▼
Router VM (enp7s0)
   │
   │ NAT masquerade
   ▼
Router VM (enp1s0)
   │
   ▼
Internet

Internet → Server (response)

Internet
   │
   ▼
Router VM (enp1s0)
   │
   │ NAT translation
   ▼
Router VM (enp7s0)
   │
   ▼
Server VM

🧾 Network Addressing

Device Interface Network IP
Router VM enp1s0 external DHCP
Router VM enp7s0 labnet 192.168.100.1
Server VM enp1s0 labnet 192.168.100.10

Subnet:
192.168.100.0/24
Gateway:
192.168.100.1

🔧 Virtualization Stack

What is KVM?

KVM (Kernel-based Virtual Machine) is a Linux kernel module that turns Linux into a hypervisor. It allows running virtual machines with near-native performance using hardware virtualization (Intel VT-x / AMD-V). KVM is built into the Linux kernel.

What is libvirt?

libvirt is a virtualization management API. It provides a unified way to manage:

  • Virtual machines
  • Virtual networks
  • Storage pools
  • Bridges

Instead of managing QEMU directly, we manage everything via libvirt.

What does virt-manager do?

virt-manager is a graphical interface for managing libvirt.

It allows:

  • Creating VMs
  • Configuring networks
  • Editing hardware
  • Viewing console

📦 Installed Packages

qemu-kvm
libvirt-daemon-system
libvirt-clients
virt-manager
bridge-utils
nftables

🌐 Network Configuration Files Used

/etc/network/interfaces
/etc/nftables.conf
/etc/sysctl.d/99-ipforward.conf
/etc/resolv.conf

🧠 Important Commands Used

ip a
ip route
ping
traceroute
systemctl status nftables
sysctl -a
sysctl net.ipv4.ip_forward

🖥 Router VM Configuration

Enable IP Forwarding Debian 12 uses systemd-sysctl, which reads configuration from:

/etc/sysctl.d/*.conf

I created:
/etc/sysctl.d/99-ipforward.conf

With:
net.ipv4.ip_forward=1

Apply:
sysctl --system

NAT Configuration (nftables)
Example NAT configuration:

table ip nat {
    chain postrouting {
        type nat hook postrouting priority 100;
        oifname "eth0" masquerade
    }
}

🧪 Problems Faced & Solutions

1️⃣ Server VM in Wrong Subnet

Problem:
Server network interface was not configured in the same subnet as Router VM.

Root Cause:
Incorrect network configuration inside VM.

Solution:
Reconfigured interface to:

192.168.100.10/24
Gateway: 192.168.100.1

2️⃣ IP Forwarding Reset After Reboot

Problem:
After reboot:
net.ipv4.ip_forward = 0

Cause:
Setting was changed manually but not persisted.

Solution:
Created persistent config file in:
/etc/sysctl.d/99-ipforward.conf

Because Debian 12 uses systemd-sysctl.

3️⃣ NAT / Masquerade Not Working

Problem:
Server had no internet access.

Cause:
Incorrect nftables configuration.

Solution:
Studied nftables manual and correctly implemented:

  • NAT postrouting
  • Forward chain rules
  • Masquerade on external interface

4️⃣ Ping to Google Not Working

Problem:
Server VM could not ping 8.8.8.8.

Deep Root Cause:
IP address conflict.

Router VM:
192.168.100.1

Host machine (via virbr1):
192.168.100.1

libvirt automatically created a bridge (virbr1) and assigned the same IP address as my Router VM.

This created a routing conflict.

Solution:

I edited the XML definition of virtual network labnet:
virsh net-edit labnet
And removed host IP assignment from the bridge.

After that:

  • No more IP conflict
  • Correct routing
  • Internet worked

5️⃣ Default Gateway Showing "onlink"

Problem:
default via 192.168.100.1 dev eth0 onlink

Cause:
Improper route configuration.

Solution:
Corrected subnet and gateway configuration in /etc/network/interfaces.

📂 Repository Structure

.
├── README.md
├── router/
│   ├── nftables.conf
│   ├── 99-ipforward.conf
│   └── interfaces
├── server/
│   └── interfaces
└── scripts/
    ├── setup_router.sh
    └── setup_server.sh

🎯 What This Lab Demonstrates

  • Linux networking
  • Routing fundamentals
  • NAT configuration
  • nftables
  • Troubleshooting methodology
  • Understanding of libvirt networking
  • Persistent kernel configuration
  • Route analysis