Introduction: The Hidden Vulnerability in Modern VPN Networks

In the contemporary digital landscape, the widespread adoption of Virtual Private Networks (VPNs) has revolutionized remote work and secure data transmission. Organizations routinely employ VPNs to extend their private internal networks across public internet infrastructure, allowing remote employees to access sensitive corporate resources securely. However, this architectural convenience introduces a critical vulnerability: if an unauthorized actor compromises a single VPN credential, they gain unfettered lateral access to the entire internal subnet.

Traditional network monitoring tools are often complex, resource-intensive, and difficult to manage on lightweight virtual infrastructure. This is where NetAlertX emerges as a game-changing solution. NetAlertX (formerly known as Pi.Alert) is an open-source, lightweight, and highly customizable network security scanner designed to detect new and unauthorized devices on your network. This comprehensive guide walks you through the step-by-step process of deploying NetAlertX on a Linux Virtual Private Server (VPS) to monitor your VPN network, providing early detection and immediate notification of potential security breaches.

Why NetAlertX is Essential for VPN Security Monitoring

When a remote user connects to a VPN server (such as OpenVPN, WireGuard, or IPSec), their device is assigned a local IP address within a specific subnet. From a network topology perspective, that remote device is now physically present inside your internal corporate infrastructure. If a malicious actor successfully exploits weak authentication or stolen keys, they can silently scan your internal servers, databases, and endpoints.

NetAlertX addresses this threat vector by acting as an automated digital sentry. Key capabilities that make it indispensable for enterprise and small-to-medium business (SMB) infrastructures include:

Active and Passive Scanning: Combines multiple methodologies, including ARP scanning, ICMP pings, DNS resolution, and custom API integrations to discover active hosts.
Real-Time Alerting Architecture: Integrates seamlessly with enterprise communication platforms such as Telegram, Discord, Gotify, Home Assistant, and email (SMTP) to notify system administrators the precise moment an unrecognized MAC or IP address appears.
Historical Device Logging: Maintains a comprehensive database of all connected devices, their connection intervals, and changes in hostnames or vendor attributes.
Minimal Resource Footprint: Designed to run efficiently on Linux environments, making it ideal for deployment on budget-friendly Linux VPS instances without degrading server performance.

Pre-requisites and Environmental Setup

Before initiating the deployment process, ensure your infrastructure meets the following architectural and technical prerequisites:

A Linux VPS Instance: A reliable virtual server running an enterprise-grade distribution such as Ubuntu Server 22.04 LTS or 24.04 LTS, Debian 12, or Rocky Linux 9. A minimum configuration of 1 vCPU and 1 GB of RAM is sufficient.
Established VPN Infrastructure: An active VPN server (e.g., WireGuard or OpenVPN) deployed either on the same VPS or within a routable network segment that the VPS can access.
Root or Sudo Privileges: Administrative access to execute system configuration changes and install software packages.
Docker and Docker Compose: The recommended deployment medium to ensure container isolation, portability, and streamlined dependency management.

Security Best Practice Note: Ensure your VPS firewall (such as UFW or firewalld) is strictly configured. Only expose the necessary VPN ports and the specific port assigned to the NetAlertX web user interface (UI) to trusted IP addresses or via an internal reverse proxy.

Step-by-Step Deployment of NetAlertX via Docker Compose

Step 1: System Optimization and Package Update

First, connect to your Linux VPS via SSH and update the core system repositories to ensure all dependencies are running the latest stable security patches. Execute the following commands:

sudo apt update && sudo apt upgrade -y

If Docker and Docker Compose are not yet installed on your server, install them using the official Docker convenience script:

curl -fsSL [https://get.docker.com](https://get.docker.com) -o get-docker.sh
sudo sh get-docker.sh

Step 2: Designing the Directory Structure

To maintain an organized production environment, establish a dedicated directory for NetAlertX persistent data and configuration files. This guarantees that your historical database and customized parameters remain intact during application container updates.

mkdir -p ~/netalertx/config ~/netalertx/db
cd ~/netalertx

Step 3: Creating the Docker Compose Configuration File

Using your preferred command-line text editor (such as Nano or Vim), create a docker-compose.yml file within your newly established directory:

nano docker-compose.yml

Populate the file with the following structurally optimized container definition. Ensure you adjust the environment variables, specifically the TZ (Time Zone) and PORT parameters, to match your geographical and infrastructural requirements:

version: '3.8'

services:
  netalertx:
    image: jokobsk/netalertx:latest
    container_name: netalertx
    restart: unless-stopped
    network_mode: host
    volumes:
      - ./config:/app/config
      - ./db:/app/db
    environment:
      - TZ=Europe/London
      - PORT=20211
      - PUID=1000
      - PGID=1000
    cap_add:
      - NET_ADMIN
      - NET_RAW

Crucial Architecture Requirement: The parameter network_mode: host is fundamentally critical. By bypassing the standard Docker bridge network isolation, NetAlertX gains direct access to the host VPS network interfaces, allowing it to accurately intercept and scan the underlying VPN interfaces (e.g., wg0 or tun0). Furthermore, cap_add privileges for NET_ADMIN and NET_RAW are required to execute low-level network packet generation.

Step 4: Executing the Container Deployment

Launch the NetAlertX service in detached background mode by executing the following Docker command:

docker compose up -d

Verify that the container is executing correctly and analyze its initial startup sequence logs to ensure no initialization faults have occurred:

docker compose logs -f netalertx

Configuring NetAlertX to Target the VPN Subnet

Once the container is operational, open a web browser and navigate to the NetAlertX web interface using your VPS IP address and the configured port: http://your-vps-ip:20211.

By default, NetAlertX will attempt to scan the standard physical network interfaces of the VPS host. To pivot its focus toward your virtual private network infrastructure, navigate to the Settings panel within the web UI and configure the core network monitoring variables:

1. Define the Scanning Subnets

Locate the network scanning range parameters. You must explicitly input the Classless Inter-Domain Routing (CIDR) block allocated to your VPN clients. For example, if your WireGuard VPN server assigns client IP addresses within the 10.8.0.0/24 or 192.168.10.0/24 blocks, insert these definitions into the network scan array. This instructs the automation engine to actively trace changes within those specific virtual spaces.

2. Identify Interface Monitors

In the interface mapping configuration, verify that the application points toward the correct virtual network interface card (NIC). Common defaults include tun0 for OpenVPN configurations or wg0 for WireGuard setups. You can verify your exact host interface names by running ip a or ifconfig in your VPS terminal.

Establishing Real-Time Incident Alerting Mechanisms

An intrusion detection system is only as effective as its notification latency. If an adversary connects to your VPN at midnight, waiting until the next morning to check a dashboard is an unacceptable security posture. NetAlertX features native integrations with modern communication systems to bridge this gap.

To configure instant alerts via Telegram, complete the following workflow:

Create a new notification bot by messaging the @BotFather account on Telegram and secure your unique HTTP API Token.
Create a private Telegram channel or group, add your newly created bot to it, and extract the unique Chat ID of that channel.
Access the NetAlertX UI, navigate to Settings > Notifications > Telegram.
Enable the service, input your API Token and Chat ID parameters, and execute a test broadcast.

Once activated, the moment an unmapped, unknown MAC or IP address accesses the VPN tunnel, your security team will receive an instantaneous push notification containing the hostname, IP assignment, and MAC manufacturer profile of the unauthorized asset.

Conclusion: Proactive Network Defense

Deploying NetAlertX on a Linux VPS provides a robust, lightweight, and cost-effective layer of defense for your VPN architecture. By transforming a passive remote access tunnel into an actively monitored network segment, you effectively eliminate the blind spots that sophisticated network intruders rely upon. Combining containerized infrastructure via Docker, targeted subnet mapping, and instant Telegram alerting ensures that your internal corporate or private assets remain insulated from lateral exploitation. Implement this setup today to establish a definitive, proactive security perimeter around your remote operations.

Enhancing Network Security: Deploying NetAlertX on a Linux VPS to Detect VPN Intrusions