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Nmap
tool
Sebastian Oberdorfer

Learn Nmap in Y Minutes

So, you’re connected to a network and want to know what else is connected to it. Maybe you’re trying to find that mystery device eating up bandwidth or check if there are services running you didn’t know about, or you just want to verify what ports are exposed on your machine?

Meet your swiss-army network knife named Nmap!

Introduction

Nmap 101
Nmap is an open-source network scanning tool built by Gordon Lyon. Designed to help you find devices, open ports and services across your network. It’s a swiss-army knife for network admins, security folks, dev's and anyone curious about what’s living on their network.

When to Use It

  • Finding Devices: What’s connected, and what’s running?
  • Network Troubleshooting: Resolve DNS or connection issues.
  • Vulnerability Detection: Spotting potentially risky services.
  • Network Security: Evaluate exposed ports.

When Not to Use It

  • Public Networks: Scanning Starbucks WiFi might land you in hot tea.
  • Corporate Networks: Scanning your corporate network without permission, is potentially not allowed.
  • Global Web: In some cases scanning across the web can be illegal.

Certain scans are intrusive and can trigger security alarms, so stick to **only ** scanning networks or systems where you have permission. Unauthorized scanning can be considered illegal under cybersecurity laws in many regions, and companies might view it as a hacking attempt.

Use Nmap extensively and wisely.

Installation

Installation is straightforward, thoroughly explained on nmap.org - install

The Basics

These are low-key scans that safe to use since they don’t do deep probing.

  • Ping Scan:
    A low-impact scan just to check if devices are online. Typically fine on trusted networks.

    • Scan a single device 
       nmap -sn 192.168.1.1
    • Scan a range of devices 
      nmap -sn 192.168.1.1-100
    • Scan a CIDR range of devices 
      nmap -sn 192.168.1.0/24   # Range 192.168.1.0 to 192.168.1.255
nmap -sn 192.168.0.0/16   # Range 192.168.0.0 to 192.168.255.255
nmap -sn 192.0.0.0/8      # Range 192.0.0.0 to 192.255.255.255

  • Fast Scan:
    Quickly checks the 100 most common ports. Great for a quick peek without probing all 65,535 ports.

    nmap -F 192.168.1.1

  • Operating System Detection:
    OS detection requires some extra probing, which might be detectable by Intrusion Detection Systems (IDS).

    nmap -O 192.168.1.1

  • Output to File
    Specific scanning and saving the output to a file, enables you to scan more thorough without overloading your network.

    • Plain text 
      nmap -oN output.txt 192.168.1.1
    • XML, handy for using elsewhere 
      nmap -oX output.xml 192.168.1.1

Moving Up: More Insightful Scans

These scans dig a bit deeper, so they may trigger alarms on security systems. Use these only on networks where you have explicit permission to scan.

  • Service Version Detection:
    Tries to identify versions of services on open ports. Useful but more invasive.

    nmap -sV 192.168.1.1

  • Aggressive Scan:
    The aggressive scan mode (-A) combines multiple checks, like OS detection, version detection and traceroute. This is likely to be flagged on any network and can be considered illegal on networks you don’t own.

    nmap -A 192.168.1.1

  • Scanning Specific Ports:
    Narrowing scans to specific ports is generally fine.

    • Scan a specific port 
      nmap -p 80 192.168.1.1
    • Scan a range of ports 
      nmap -p 1-100 192.168.1.1

Advanced Scans: When You’re the Power User

So, you’re getting into the advanced stuff—maybe testing your own firewall or finding rogue services. The following scans are loud and intrusive that definitely trigger security defenses.

  • Scripted Scans (NSE)
    Nmap’s script engine is like a toolbox of plugins. Need to check for a specific vulnerability? There’s likely an NSE script for it.

    nmap --script=http-vuln-cve2021-12345 192.168.1.1

  • Aggressive and fastest Scans: -T5 turns up to knob to 11. -A scans all ports. Use it sparse and only if you really need full visibility.

    nmap -T5 -A 192.168.1.1

  • TCP and UDP Combined Scans:
    Combining TCP and UDP scans (-sS for SYN scans and -sU for UDP) gives complete coverage but increases the scan’s footprint, making it detectable.

    nmap -sS -sU 192.168.1.1

  • Spoofing and Decoy Scans:
    Using decoys (-D) or spoofed IP addresses to hide your real IP can be seen as deceptive. These scans are easily flagged by IDS and could lead to legal repercussions if you’re not authorized.

    # 10 random IP decoys
nmap -D RND:10 192.168.1.1

Practical Tips and Tricks

Timing Templates
Nmap has timing options from -T0 (paranoid) to -T5 (insane). Stick with -T2 or -T3 for a good balance between speed and not making too much noise. More on nmap - timing-templates

Check Out Nmap’s Scripts
NSE scripts make Nmap super versatile. From DNS enumeration to vulnerability checks, there’s probably a script for whatever you need. More on nmap - Nmap Scripting Engine

Use aggressive scans and decoys only on networks you own or with formal authorization, such as during a penetration test with client permission. If you’re running scans at work, talk to the network admins first.

Know When to Stop
Once you’ve got the info you need, wrap it up. It’s easy to get scan-happy.

Happy scanning!