Free Guide to Network Scanning with Legion

Understanding Network Scanning Fundamentals

Network scanning refers to the process of searching through computer networks to identify devices, open ports, services, and other network characteristics. Legion is a free, open-source tool designed to help security professionals and network administrators perform these scanning tasks. The tool works by sending requests across a network and analyzing the responses to map out what devices exist, what services they're running, and what vulnerabilities might be present.

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At its core, network scanning serves several important purposes in IT security. Organizations use scanning to understand their own network infrastructure, verify that security controls are working properly, and identify unauthorized devices or services. When performed on systems you own or have permission to test, scanning is a legitimate and necessary part of cybersecurity work. However, it's crucial to understand that scanning networks without authorization is illegal in most jurisdictions, including under laws like the Computer Fraud and Abuse Act in the United States.

Legion builds on the foundation of Nmap, one of the most widely used network scanning tools in the industry. While Nmap operates from the command line, Legion provides a graphical interface that makes it easier for users to visualize scanning results, manage multiple scans, and organize findings. The tool was developed specifically to streamline workflows for penetration testers and security professionals who need to conduct authorized security assessments.

The basic workflow in network scanning involves several stages. First, you define the target scope—which networks or IP addresses you want to scan. Second, you configure scan parameters that determine how aggressive or thorough the scan will be. Third, the tool sends probe packets and collects responses. Finally, it processes and displays the results in an organized format. Understanding each stage helps you use scanning tools more effectively and interpret results accurately.

Practical Takeaway: Before using any scanning tool, always verify that you have written permission to scan the target network. Network scanning without authorization violates computer fraud laws. Document your authorization in writing and keep those records for compliance purposes.

Setting Up Legion on Your System

Legion runs on Linux systems and requires certain dependencies to function properly. The installation process involves obtaining the software from its repository, installing required Python libraries, and configuring the system to run the application. Since Legion is open-source software available on platforms like GitHub, you can review the source code before installation, which provides transparency about what the tool does.

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The system requirements for Legion are relatively modest. You need a Linux operating system, Python 3 (typically version 3.6 or higher), and several supporting libraries including Qt5 for the graphical interface and various networking libraries. Many security professionals run Legion on dedicated machines or virtual machines to isolate testing activities from production systems. This isolation prevents accidental interference with live network operations and maintains cleaner records of testing activities.

Installation typically begins with downloading the Legion repository and installing dependencies through your system's package manager. For Debian-based systems, this might involve commands that retrieve Python packages and development libraries. For Red Hat-based systems, the process uses different package management tools but follows similar principles. The exact commands depend on your specific Linux distribution, so consulting the official Legion documentation ensures you follow the correct procedures for your system.

After installation, Legion stores configuration files in user directories and creates a database for storing scan results. This organizational structure allows you to maintain a historical record of scans you've performed, which is valuable for tracking changes in your network over time and documenting your security assessment activities. The database can grow quite large if you perform frequent scans with detailed result collection, so managing storage space may become relevant for long-term use.

Virtual machines provide an excellent environment for running Legion. You can create a dedicated VM with all necessary tools pre-installed, which simplifies the process of conducting authorized security assessments. This approach also keeps your testing environment separate from systems you use for other purposes, reducing the chance of confusion about which systems are involved in assessments versus regular operations.

Practical Takeaway: Document your installation steps and system configuration. Keep records of which version of Legion you used for each assessment, as this information helps explain your methodology if results are ever reviewed or questioned.

Configuring Scans and Understanding Scan Types

Legion provides several different scanning modes, each suited to different purposes and situations. Understanding these options helps you choose the right approach for your specific assessment needs. A basic ping scan simply checks which IP addresses are responding to network requests, providing a fast way to identify active devices without probing for detailed information. This type of scan is useful for understanding network occupancy and device count without the time investment of deeper scanning.

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Port scanning moves beyond simple presence detection to identify which network services are running on devices. Services typically communicate through numbered ports—common examples include port 80 for web traffic, port 443 for encrypted web traffic, and port 22 for administrative access. A port scan sends requests to these ports and notes which ones respond, revealing what services a system is running. This information helps security professionals understand the attack surface and identify which systems need configuration review.

Version detection goes further by attempting to identify not just that a service is running, but which specific software and version is providing that service. A web server port might indicate that Apache web server version 2.4.41 is running, for example. Version information is crucial because security vulnerabilities are typically tied to specific versions, allowing assessors to cross-reference scan results against known vulnerability databases. This connection between version information and vulnerability data makes version detection one of the most valuable scanning approaches.

Operating system detection attempts to identify what operating system a target device is running by analyzing how it responds to network probes. Different operating systems produce slightly different response patterns, and Legion uses these patterns to make educated guesses about what OS is present. This information helps prioritize systems for assessment, as different operating systems have different vulnerability profiles and management approaches. However, OS detection is less reliable than port or version detection, and results should be treated as indicators rather than certainties.

Timing and intensity settings significantly affect how scans behave. Conservative scans take longer but may avoid triggering security alerts and minimize network load. Aggressive scans complete faster but consume more bandwidth and are more likely to be detected by network monitoring systems. Your choice depends on the network environment and your authorization scope. An internal network assessment of systems you fully own might use aggressive settings, while a remote assessment might use more conservative approaches to minimize disruption.

Practical Takeaway: Start with conservative scan settings when working with new networks. You can always run faster scans once you understand how the network responds and have confirmed your assessment parameters are appropriate.

Interpreting Scan Results and Data Organization

Legion displays scan results in a structured format that helps you understand complex information about multiple hosts and services. The tool organizes findings by host, showing for each device the ports that are open, the services detected, and other relevant information. This organization makes it easier to spot patterns—for example, if many systems are running outdated versions of a particular service, you can address them systematically rather than one at a time.

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Understanding port status classifications is essential for correct interpretation. Open ports indicate services actively responding to connection attempts. Closed ports indicate that nothing is listening on that port, though the system is reachable. Filtered ports are those where responses are blocked by firewalls or other network devices, preventing definitive determination of whether services are present. This distinction matters because open ports represent actual exposure, while filtered ports represent blocked ports that are being protected.

Legion maintains a database of scan results that allows you to review historical information and compare scans over time. If you scan the same network periodically, you can track which services appeared or disappeared, which versions were upgraded, and how your overall security posture changed. This historical perspective is valuable for demonstrating the effects of security improvements and identifying unexpected changes that warrant investigation.

The tool provides various export options for sharing results with other team members or including results in reports. Exported data typically includes all detected information organized in formats suitable for documentation. When conducting authorized security assessments, proper documentation of findings is essential for both accountability and for helping network administrators understand what issues need attention. Export formats may include text, CSV, or other structured data formats depending on your version of Legion and your specific needs.

Filtering and search capabilities within Legion help you focus on relevant information. If you're investigating a particular vulnerability, you can filter results to show only systems running the affected software version. If you're interested in a particular type of service, you can display only hosts running that service. These filtering capabilities transform large volumes of raw data into actionable information that points toward specific next steps.

Practical Takeaway: Create a consistent naming scheme for your scans and document what parameters you used for each one. This practice ensures that when you return to results