Essential Nmap Scanning

Overview

Nmap provides various scanning techniques to map a network, discover hosts, and identify open ports. In this module, you will learn:

• Basic Nmap commands and syntax
• Common scan types (SYN, TCP Connect, UDP)
• How to interpret scan results

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Basic Nmap Commands and Syntax

Nmap follows a simple command structure:

nmap [options] [target]

• [options] – Flags that modify the scan (e.g., sS, sT, sU).
• [target] – The IP address, hostname, or subnet to scan.

Example:

Scan a single IP address:

nmap 192.168.1.1

Scan a range of IPs:

nmap 192.168.1.1-100

Scan an entire subnet:

nmap 192.168.1.0/24

Scan a specific domain:

nmap scanme.nmap.org

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Common Scan Types

SYN Scan (-sS) [Stealth Scan]

A SYN scan is the most common and stealthy scan. It sends a TCP SYN packet to check if a port is open but doesn’t establish a full connection.

Example Command:

nmap -sS 192.168.1.1

Sample Output:

Starting Nmap 7.94 at 2025-02-28 12:00
Nmap scan report for 192.168.1.1
PORT     STATE    SERVICE
22/tcp   open     ssh
80/tcp   open     http
443/tcp  open     https

Why use it?

• Faster and stealthier than a full TCP connection.
• Does not log as a full connection on most firewalls.

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TCP Connect Scan (-sT) [Non-Stealthy]

A TCP Connect scan establishes a full three-way handshake with each scanned port.

Example Command:

nmap -sT 192.168.1.1

Sample Output:

Starting Nmap 7.94 at 2025-02-28 12:05
Nmap scan report for 192.168.1.1
PORT     STATE    SERVICE
21/tcp   open     ftp
22/tcp   open     ssh
80/tcp   open     http

When to use it?

• When SYN scans are blocked (e.g., on Windows machines).
• When you don’t need stealth.

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UDP Scan (-sU) [For Non-TCP Services]

A UDP scan checks for open UDP ports, which are used for DNS, SNMP, DHCP, etc.

Example Command:

nmap -sU 192.168.1.1

Sample Output:

Starting Nmap 7.94 at 2025-02-28 12:10
Nmap scan report for 192.168.1.1
PORT     STATE    SERVICE
53/udp   open     domain
67/udp   open     dhcp
161/udp  open     snmp

Key Points:

• UDP scans take longer than TCP scans.
• Some firewalls and routers block UDP responses.

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Interpreting Nmap Scan Results

Nmap scan results include port states:

State	Meaning
Open	The port is actively listening for connections.
Closed	The port is reachable but has no service running.
Filtered	A firewall is blocking access.

Example Command:

nmap -p 22,80,443 scanme.nmap.org

Sample Output:

Starting Nmap 7.94 at 2025-02-28 12:15
Nmap scan report for scanme.nmap.org
PORT     STATE    SERVICE
22/tcp   closed   ssh
80/tcp   open     http
443/tcp  filtered https

Interpretation:

• SSH (22) is closed, meaning no service is running on this port.
• HTTP (80) is open, meaning a web server is running.
• HTTPS (443) is filtered, meaning a firewall is blocking it.

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Summary of Scan Types

Scan Type	Command	Purpose	Stealth Level
SYN Scan	-sS	Fast & stealthy TCP scan	High
TCP Connect Scan	-sT	Full connection (non-stealthy)	Low
UDP Scan	-sU	Scans UDP services	Medium

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