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Map the network,Nmap.

Nmap ("Network Mapper") is the default tool in every pentest playbook — an open-source utility for host discovery, port scanning, service detection, OS fingerprinting, and scriptable probes via the Nmap Scripting Engine. This cheatsheet covers scan types, NSE, firewall evasion, and copy-paste recipes for real engagements.

InstallPort scanningNSE scriptingCommon recipes

Authorization first

Scanning networks you don't own or have written permission to scan is illegal in most jurisdictions. Stick to your own infrastructure, CTF ranges, scanme.nmap.org, and engagements with a signed scope.

• Discovery — find live hosts before port scanning.
• NSE — Lua scripts for version, vuln, and brute checks.
• Evasion — fragmentation, decoys, and source-port spoofing.

65535TCP ports

SYNDefault scan

14NSE categories

T0–T5Timing levels

FeaturesFAQRelated

On this page

What this guide covers

Port scanning

SYN, connect, UDP, ACK, and stealth scans — the scan shape that makes Nmap fast or slow.

NSE scripting

The Nmap Scripting Engine — default, safe, vuln, and intrusive Lua scripts with high-value recipes.

Firewall evasion

Source-port spoofing, fragmentation, decoys, and MAC spoofing for sanctioned engagements only.

Common recipes

Subnet inventory, web-server triage, CTF first contact, and continuous monitoring workflows.

Start here

Information

Nmap (“Network Mapper”) is an open-source utility for host discovery, port scanning, service detection, OS fingerprinting and scriptable probes via the Nmap Scripting Engine (NSE). It’s the default tool in every pentest playbook and the first thing oncall reaches for when “is the firewall actually doing what we think.”

Authorization first

Scanning networks you don’t own or have written permission to scan is illegal in most jurisdictions. Stick to your own infrastructure, CTF ranges (HackTheBox / TryHackMe), scanme.nmap.org (provided by the Nmap project for testing), and engagements with a signed scope.

Setup

Install

Debian / Ubuntu

sudo apt update && sudo apt install -y nmap

Fedora / RHEL

sudo dnf install -y nmap

Arch

sudo pacman -S nmap

macOS

brew install nmap

Windows

# Official installer ships Nmap + Zenmap GUI + Ncat + Nping
winget install -e --id Insecure.Nmap

Confirm the version + NSE script database location:

nmap --version
nmap --script-help all | head

Targets

Target Specification

Across every command below, replace TARGET with one or more of:

• A single host: 10.0.0.5 or scanme.nmap.org
• A CIDR block: 10.0.0.0/24
• A range: 10.0.0.1-50 or 10.0.0,1,2.1-10
• A file of targets: -iL targets.txt
• Exclusions: --exclude 10.0.0.1 or --excludefile skip.txt

# Random 100 hosts on the public internet (research only).
sudo nmap -sn -iR 100

Who's up

Host Discovery

Find live hosts without doing a port scan first.

Flag	What it does
-sn	”ping scan” — host discovery, skip port scan
-Pn	skip discovery, treat every host as up
-PE	ICMP echo (requires root)
-PS22,80,443	TCP SYN ping on listed ports
-PA22,80,443	TCP ACK ping
-PU53,161	UDP ping on listed ports
-PR	ARP ping (LAN-only, fastest + most reliable on the local subnet)
-n	skip reverse-DNS for every hit (a lot faster on /16+)

# Ping sweep of a /24, no port scan, no DNS.
sudo nmap -sn -n 10.0.0.0/24

# ARP sweep on a local subnet — bypasses host firewall ICMP filters.
sudo nmap -PR -sn 192.168.1.0/24

# Assume the host is up (some hosts block all ICMP / TCP probes).
sudo nmap -Pn TARGET

Scan types

Port Scanning

The shape of the scan is what makes Nmap fast or slow.

Flag	Scan type	When to use
-sS	TCP SYN (half-open)	default, fast, root only
-sT	TCP connect()	unprivileged or when SYN drops are noisy
-sU	UDP	DNS / SNMP / NTP / VoIP gear
-sA	TCP ACK	map firewall rule sets (stateful vs stateless)
-sN -sF -sX	TCP Null / FIN / Xmas	bypass naïve stateless firewalls
-sY	SCTP INIT	telecom / SS7 gear

Port specifications:

# Top 1000 TCP ports (default).
sudo nmap TARGET

# Top 100 — fastest useful pass.
sudo nmap --top-ports 100 TARGET

# Every TCP port.
sudo nmap -p- TARGET

# Specific ports / ranges / names.
sudo nmap -p 22,80,443,1000-2000,http* TARGET

# Both TCP and UDP, every port.
sudo nmap -sU -sS -p0-65535 TARGET

# Only show open ports in the output (great for diff'ing scans).
sudo nmap --open TARGET

Reading output

Understanding Scan Results

Nmap reports one of six states per port. Knowing the difference between closed and filtered is the single most useful skill when reading output.

State	Meaning
open	An application is actively accepting connections on the port
closed	Port is reachable (host replied) but no application is listening
filtered	A firewall/filter dropped the probe — Nmap can’t tell open vs closed
unfiltered	Port is reachable but Nmap can’t tell open vs closed (ACK scan)
`open	filtered`
`closed	filtered`

PORT     STATE    SERVICE   VERSION
22/tcp   open     ssh       OpenSSH 8.9p1 Ubuntu 3ubuntu0.6
80/tcp   open     http      nginx 1.18.0
443/tcp  filtered https
3306/tcp closed   mysql

Tip

filtered almost always means a firewall. Re-probe with -Pn (skip host discovery), a trusted source port (-g 53), or a different scan type (-sA to map the firewall rules) before concluding the port is dead.

Fingerprinting

Service & Version Detection

# -sV probes open ports and labels them with banner + version.
sudo nmap -sV TARGET

# Stack version detection with default NSE scripts and OS guess.
sudo nmap -A TARGET            # = -sV -sC -O --traceroute

# Tighter probe (faster, less accurate) vs aggressive (slower, more accurate).
sudo nmap -sV --version-intensity 0 TARGET   # light banner only
sudo nmap -sV --version-intensity 9 TARGET   # every probe
sudo nmap -sV --version-all TARGET           # same as intensity 9

Fingerprinting

OS Detection

# Active OS fingerprint (root only, needs at least one open + one closed port).
sudo nmap -O TARGET

# Be conservative — only print a guess when confidence is high.
sudo nmap -O --osscan-limit TARGET

# Guess aggressively (useful for filtered hosts).
sudo nmap -O --osscan-guess TARGET

Scripting engine

NSE — Nmap Scripting Engine

NSE scripts live in /usr/share/nmap/scripts/ (Linux) or /opt/homebrew/share/nmap/scripts/ (macOS, Apple Silicon). Categories: auth, broadcast, brute, default, discovery, dos, exploit, external, fuzzer, intrusive, malware, safe, version, vuln.

# Run all "default" scripts — same as the -sC in -A.
sudo nmap -sC TARGET

# Run every "safe" script.
sudo nmap --script "safe" TARGET

# All vuln-detection scripts.
sudo nmap --script "vuln" TARGET

# A specific script with arguments.
sudo nmap --script http-title --script-args http.useragent="kbve-scanner" TARGET

# Glob multiple categories, exclude one.
sudo nmap --script "default,vuln and not intrusive" TARGET

# Update the local script database (after every nmap upgrade).
sudo nmap --script-updatedb

# Show what a script does before running it.
nmap --script-help http-shellshock

High-value recipes:

# SSL/TLS posture: cipher list, cert expiry, known weak suites.
sudo nmap -sV --script ssl-enum-ciphers,ssl-cert,sslv2,ssl-poodle -p 443 TARGET

# SMB share enumeration on a Windows host / file server.
sudo nmap -p 139,445 --script "smb-enum-shares,smb-os-discovery,smb-vuln-*" TARGET

# HTTP fingerprint pack — title, headers, common dirs, robots, CSP.
sudo nmap -sV --script "http-title,http-headers,http-enum,http-robots.txt,http-security-headers" -p 80,443,8080,8443 TARGET

# DNS recon — zone transfer, NSEC walk, brute subdomains.
sudo nmap -sn --script "dns-zone-transfer,dns-nsec-enum,dns-brute" --script-args dns-brute.domain=example.com TARGET

# Vulnerability sweep (noisy, run only on owned systems).
sudo nmap -sV --script "vuln" TARGET

Save & diff

Output & Reporting

# Normal stdout + grepable + XML in one go.
sudo nmap -oA scan-baseline TARGET
# produces scan-baseline.nmap, scan-baseline.gnmap, scan-baseline.xml

# Save XML for ndiff or downstream tooling.
sudo nmap -oX scan.xml TARGET

# Live progress on a long scan.
sudo nmap -v --stats-every 30s TARGET

# Diff two scans (great for "what changed since last week").
ndiff scan-baseline.xml scan-new.xml

Convert the XML to HTML for sharing:

xsltproc scan.xml -o scan.html

Speed control

Timing & Performance

-T0 (paranoid) through -T5 (insane). Default is -T3.

Profile	Use
-T0	IDS evasion, hours-per-port
-T1	sneaky, IDS evasion
-T2	polite, low bandwidth
-T3	normal (default)
-T4	aggressive — assume a fast, reliable network
-T5	insane — local LAN or willing-to-lose-accuracy

Tunables when -T* isn’t precise enough:

sudo nmap \
  --min-rate 1000 --max-rate 5000 \
  --min-parallelism 64 --max-parallelism 256 \
  --max-retries 2 \
  --host-timeout 30m \
  --max-rtt-timeout 200ms \
  -T4 TARGET

Stealth

Firewall & IDS Evasion

Caution

These flags exist because IDS engines like Snort / Suricata watch for exactly this signature. Don’t use them outside a sanctioned engagement.

# Source-port spoof — many firewalls trust 53 / 80 / 443 outbound.
sudo nmap --source-port 53 TARGET
sudo nmap -g 53 TARGET

# Fragmented packets — split TCP header across multiple IP fragments.
sudo nmap -f TARGET
sudo nmap --mtu 16 TARGET

# Decoys — N decoy IPs appear to scan alongside your real one.
sudo nmap -D 10.0.0.1,10.0.0.2,ME,10.0.0.4 TARGET

# Proxy chain (limited support — works for TCP connect scans).
sudo nmap --proxies socks4://127.0.0.1:9050 -sT TARGET

# Randomize host order + spoof MAC vendor on the LAN.
sudo nmap --randomize-hosts --spoof-mac Apple TARGET

# Slow IDS-evasion scan (be patient).
sudo nmap -T1 -sS -f --data-length 24 TARGET

Workflows

Common Recipes

Internal subnet inventory

# 1. Find live hosts.
sudo nmap -sn -PR -oA inv-live 10.0.0.0/24

# 2. Fast TCP top-1000 against live hosts only.
grep "Status: Up" inv-live.gnmap | awk '{print $2}' > live-hosts.txt
sudo nmap -sS --top-ports 1000 -iL live-hosts.txt -oA inv-tcp

# 3. UDP focus on the usual suspects.
sudo nmap -sU -p 53,67,68,69,123,137,138,161,162,500,514,520,1900,5353 -iL live-hosts.txt -oA inv-udp

Web-server triage

sudo nmap -p 80,443,8080,8443,8000,8888,3000,5000,9000 \
  -sV -sC \
  --script "http-title,http-headers,http-methods,http-enum,http-security-headers,ssl-cert,ssl-enum-ciphers" \
  -oA web-triage TARGET

CTF / HackTheBox first contact

# Stage 1: every TCP port, fast, just to see what's open.
sudo nmap -p- --min-rate 5000 -T4 -oN stage1.txt TARGET

# Stage 2: focus on the open ports from stage 1.
PORTS=$(grep ^[0-9] stage1.txt | cut -d/ -f1 | paste -sd,)
sudo nmap -p "$PORTS" -sC -sV -O -oA stage2 TARGET

Egress-firewall test from inside the network

# Which outbound ports actually reach the internet from this host?
sudo nmap -Pn -sS -p- --min-rate 1000 scanme.nmap.org

Continuous monitoring (cron + ndiff)

# Save a daily baseline, alert on drift.
sudo nmap -sS --top-ports 1000 -oX /var/log/nmap/$(date +%F).xml 10.0.0.0/24
ndiff /var/log/nmap/yesterday.xml /var/log/nmap/$(date +%F).xml | tee /var/log/nmap/diff-$(date +%F).txt

Cheatsheet

Quick Reference

nmap -h                              # built-in help
sudo nmap -sn 10.0.0.0/24            # who's up
sudo nmap -p- TARGET                 # every TCP port
sudo nmap -sU TARGET                 # UDP
sudo nmap -sV -sC -O TARGET          # service + default NSE + OS
sudo nmap -A TARGET                  # aggressive (= -sV -sC -O --traceroute)
sudo nmap --script vuln TARGET       # known-CVE checks
sudo nmap -oA scan TARGET            # save all formats
sudo nmap -T4 --min-rate 1000 TARGET # fast pass

Fixes

Troubleshooting

Common failures and the fix.

Symptom	Cause	Fix
You requested a scan type which requires root privileges	-sS, -sU, -O, -PE need raw sockets	Run with sudo, or fall back to -sT (unprivileged connect scan)
Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn	Host blocks ICMP/TCP discovery probes	Add -Pn to skip discovery and treat the host as up
All ports show filtered	A firewall is dropping probes	Try -Pn, a trusted source port (-g 53), or an ACK scan (-sA) to map rules
UDP scan crawls / takes forever	UDP is rate-limited and retransmit-heavy	Limit ports (-p 53,161,500), raise --min-rate, add --host-timeout
dnet: Failed to open device ethX / no results on LAN	Wrong interface or no L2 access	Pin the interface with -e eth0, or use -Pn to avoid ARP
Failed to resolve "TARGET"	DNS lookup failed	Check the hostname, or scan the IP directly and add -n to skip DNS
Scan is accurate but extremely slow	Default timing too polite for the link	Raise to -T4 and set --min-rate; drop -A/-O if you don’t need them
NSE: 'X' did not match a category, filename, or directory	Script DB is stale or the name is wrong	Run sudo nmap --script-updatedb; verify with nmap --script-help X

Tip

When a scan misbehaves, add --reason (shows why Nmap marked each port a given state) and -d (debug, repeatable -dd) to see the packets Nmap actually sent and received.

Comparison

Nmap vs Other Scanners

Nmap is the most accurate all-rounder, but it’s not the fastest at internet scale. Pick the right tool, then hand off to Nmap for the deep pass.

Tool	Strength	Trade-off
Nmap	Accuracy, NSE scripts, version + OS detection	Slower on huge ranges
masscan	Asynchronous, scans the whole IPv4 internet in minutes	No version detection, accuracy drops at high rates
RustScan	Fast port discovery, then auto-pipes into Nmap	Just a front-end; still needs Nmap for depth
ZMap	Single-port internet-wide research scanning	Built for one port at a time, not host audits
naabu	Fast SYN/CONNECT discovery, scriptable pipelines	Discovery only, no service fingerprinting

A common pattern: discover open ports fast, then deep-scan only those.

# masscan finds open ports fast, Nmap fingerprints them accurately.
sudo masscan -p1-65535 TARGET --rate 10000 -oG masscan.gnmap
PORTS=$(grep -oP 'Ports: \K[0-9]+' masscan.gnmap | sort -un | paste -sd,)
sudo nmap -p "$PORTS" -sC -sV -O TARGET

Bundled tools

Related Companions

• Ncat ships with Nmap — TCP/UDP swiss army knife, replaces nc.

  ncat -lvnp 4444                  # listen on TCP/4444
  ncat --ssl example.com 443       # TLS client

• Nping — packet crafter, for ad-hoc ICMP/TCP/UDP/ARP probes.

  sudo nping --tcp -p 443 --flags syn -c 5 TARGET

• Ndiff — compare two XML scans (already shown under Output).
• Zenmap — official GUI; useful for first-time users + saved scan profiles.

Questions

Frequently asked

Is it legal to use Nmap?

Nmap itself is legal, but scanning networks you do not own or lack written permission to scan is illegal in most jurisdictions. Stick to your own infrastructure, CTF ranges like HackTheBox or TryHackMe, scanme.nmap.org, and engagements with a signed scope.

What is the difference between an -sS and -sT scan in Nmap?

-sS is a TCP SYN (half-open) scan that never completes the handshake, making it fast and stealthier, but it requires root. -sT is a full TCP connect() scan that completes the handshake; it works unprivileged but is slower and noisier.

How do I scan all 65535 ports with Nmap?

Use the -p- flag, for example "sudo nmap -p- TARGET", to scan every TCP port. Combine with --min-rate and -T4 for speed, e.g. "sudo nmap -p- --min-rate 5000 -T4 TARGET".

What is the Nmap Scripting Engine (NSE)?

NSE lets Nmap run Lua scripts for advanced discovery, version detection, vulnerability checks and brute forcing. Scripts are grouped into categories such as default, safe, vuln and intrusive, and run via --script, for example "sudo nmap --script vuln TARGET".

What does filtered mean in an Nmap scan, and how is it different from closed?

"closed" means the port is reachable and the host replied, but no application is listening. "filtered" means a firewall or filter dropped the probe, so Nmap cannot tell whether the port is open or closed. Filtered almost always indicates a firewall; re-probe with -Pn, a trusted source port, or an ACK scan.

How do I detect service versions and the operating system with Nmap?

Use -sV for service and version detection and -O for OS fingerprinting. The -A flag bundles both with default scripts and traceroute, equivalent to "-sV -sC -O --traceroute".

Keep exploring

Related applications

WireGuardSecure the network you just mapped with an encrypted mesh.NginxThe web server you'll fingerprint on ports 80 and 443.Flipper ZeroA hardware companion for the wider security toolkit.