Introduction to Nmap

Nmap (Network Mapper) is a powerful and versatile open-source tool used for network exploration, port scanning, service and version detection, security auditing, and network performance assessment. It caters to a broad spectrum of users, ranging from ethical hackers to network administrators. In this comprehensive guide, we’ll explore the fundamentals of Nmap and provide you with a handy cheat sheet to get started.

Understanding Nmap

Nmap operates by sending packets to the target hosts and analyzing their responses. It can discover hosts available on the network, find open ports, identify services running on those ports, and even determine the operating system of the target.

Target Specification

Switch	Example	Description
	nmap 192.168.1.1	Scan a single IP
	nmap 192.168.1.1 192.168.2.1	Scan specific IPs
	nmap 192.168.1.1-254	Scan a range
	nmap scanme.nmap.org	Scan a domain
	nmap 192.168.1.0/24	Scan using CIDR notation
-iL	nmap -iL targets.txt	Scan targets from a file
-iR	nmap -iR 100	Scan 100 random hosts
–exclude	nmap –exclude 192.168.1.1	Exclude listed hosts

Scan Techniques

Switch	Example	Description
-sS	nmap 192.168.1.1 -sS	TCP SYN port scan (Default)
-sT	nmap 192.168.1.1 -sT	TCP connect port scan (Default without root privilege)
-sU	nmap 192.168.1.1 -sU	UDP port scan
-sA	nmap 192.168.1.1 -sA	TCP ACK port scan
-sW	nmap 192.168.1.1 -sW	TCP Window port scan
-sM	nmap 192.168.1.1 -sM	TCP Maimon port scan

Host Discovery

Switch	Example	Description
-sL	nmap 192.168.1.1-3 -sL	No Scan. List targets only
-sn	nmap 192.168.1.1/24 -sn	Disable port scanning. Host discovery only.
-Pn	nmap 192.168.1.1-5 -Pn	Disable host discovery. Port scan only.
-PS	nmap 192.168.1.1-5 -PS22-25,80	TCP SYN discovery on port x. Port 80 by default.
-PA	nmap 192.168.1.1-5 -PA22-25,80	TCP ACK discovery on port x. Port 80 by default.
-PU	nmap 192.168.1.1-5 -PU53	UDP discovery on port x. Port 40125 by default.
-PR	nmap 192.168.1.1-1/24 -PR	ARP discovery on the local network
-n	nmap 192.168.1.1 -n	Never do DNS resolution

Nmap Port Specification

Switch	Example	Description
-p	nmap 192.168.1.1 -p 21	Port scan for a specific port (e.g., port 21)
-p	nmap 192.168.1.1 -p 21-100	Port range scan (e.g., ports 21 to 100)
-p	nmap 192.168.1.1 -p U:53,T:21-25,80	Scan multiple TCP and UDP ports
-p-	nmap 192.168.1.1 -p-	Scan all ports (1 to 65535)
-p	nmap 192.168.1.1 -p http,https	Port scan using service names (e.g., http, https)
-F	nmap 192.168.1.1 -F	Fast port scan (scan the 100 most common ports)
–top-ports	nmap 192.168.1.1 –top-ports 2000	Port scan the top x most common ports
-p-65535	nmap 192.168.1.1 -p-65535	Scan all ports starting from port 1
-p0-	nmap 192.168.1.1 -p0-	Scan all ports up to port 65535

Service and Version Detection

Switch	Example	Description
-sV	nmap 192.168.1.1 -sV	Attempts to determine the version of the service running on each port
-sV –version-intensity	nmap 192.168.1.1 -sV –version-intensity 8	Specifies the version detection intensity level from 0 to 9, with higher values increasing the possibility of correctness
-sV –version-light	nmap 192.168.1.1 -sV –version-light	Enables light mode for version detection, providing faster results with a lower possibility of correctness
-sV –version-all	nmap 192.168.1.1 -sV –version-all	Enables intensity level 9 for version detection, offering a higher possibility of correctness at the cost of slower scanning
-A	nmap 192.168.1.1 -A	Enables comprehensive scanning, including OS detection, version detection, script scanning, and traceroute

OS Detection

Switch	Example	Description
-O	nmap 192.168.1.1 -O	Remote OS detection using TCP/IP stack fingerprinting
-O –osscan-limit	nmap 192.168.1.1 -O –osscan-limit	If at least one open and one closed TCP port are not found, it will not try OS detection against the host, Detecting Operating Systems with Nmap: Uncover OS
-O –osscan-guess	nmap 192.168.1.1 -O –osscan-guess	Makes Nmap guess more aggressively during OS detection
-O –max-os-tries	nmap 192.168.1.1 -O –max-os-tries 1	Sets the maximum number of OS detection tries (x) against a target
-A	nmap 192.168.1.1 -A	Enables comprehensive scanning, including OS detection, version detection, script scanning, and traceroute

Timing and Performance

Switch	Example	Description
-T0	nmap 192.168.1.1 -T0	Paranoid (0) Intrusion Detection System evasion
-T1	nmap 192.168.1.1 -T1	Sneaky (1) Intrusion Detection System evasion
-T2	nmap 192.168.1.1 -T2	Polite (2) slows down the scan to use less bandwidth and target machine resources
-T3	nmap 192.168.1.1 -T3	Normal (3), which is the default speed for scans
-T4	nmap 192.168.1.1 -T4	Aggressive (4) speeds up scans; assumes you are on a reasonably fast and reliable network
-T5	nmap 192.168.1.1 -T5	Insane (5) speeds up scans; assumes you are on an extraordinarily fast network

Switch	Example input	Description
–host-timeout <time>	1s; 4m; 2h	Give up on a target after a specified time
–min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>	1s; 4m; 2h	Specifies probe round trip time
–min-hostgroup/max-hostgroup <size>	50; 1024	Parallel host scan group sizes
–min-parallelism/max-parallelism <numprobes>	10; 1	Probe parallelization
–scan-delay/–max-scan-delay <time>	20ms; 2s; 4m; 5h	Adjusts delay between probes
–max-retries <tries>	3	Specifies the maximum number of port scan probe retransmissions
–min-rate <number>	100	Sends packets no slower than the specified rate per second
–max-rate <number>	100	Sends packets no faster than the specified rate per second

NSE Scripts

Switch	Example	Description
-sC	nmap 192.168.1.1 -sC	Scan with default NSE scripts. Considered useful for discovery and safe
–script default	nmap 192.168.1.1 –script default	Scan with default NSE scripts. Considered useful for discovery and safe
–script	nmap 192.168.1.1 –script=banner	Scan with a single script. Example: banner
–script	nmap 192.168.1.1 –script=http*	Scan with a wildcard. Example: http
–script	nmap 192.168.1.1 –script=http,banner	Scan with two scripts. Example: http and banner
–script	nmap 192.168.1.1 –script “not intrusive”	Scan with default scripts but remove intrusive ones
–script-args	nmap –script snmp-sysdescr –script-args snmpcommunity=admin 192.168.1.1	NSE script with arguments

Useful NSE Script Examples

Command	Description
nmap -Pn --script=http-sitemap-generator scanme.nmap.org	Run the HTTP site map generator script to create a sitemap for a website.
nmap -n -Pn -p 80 --open -sV -vvv --script banner,http-title -iR 1000	Conduct a fast search for random web servers on port 80, retrieving banners and titles for discovered web services.
nmap -Pn --script=dns-brute domain.com	Use DNS brute-force to guess DNS hostnames, attempting to discover subdomains of the specified domain.
nmap -n -Pn -vv -O -sV --script smb-enum*,smb-ls,smb-mbenum,smb-os-discovery,smb-s*,smb-vuln*,smbv2* -vv 192.168.1.1	Run a set of safe SMB (Server Message Block) scripts to gather information about Windows systems on the network, including shares and vulnerabilities.
nmap --script whois* domain.com	Perform a WHOIS query for the specified domain, providing information about domain registration.
nmap -p80 --script http-unsafe-output-escaping scanme.nmap.org	Detect cross-site scripting (XSS) vulnerabilities on web servers running on port 80.
nmap -p80 --script http-sql-injection scanme.nmap.org	Check for SQL injection vulnerabilities in web applications hosted on port 80.

Firewall/IDS Evasion and Spoofing

Switch	Example	Description
-f	nmap 192.168.1.1 -f	Requested scan (including ping scans) use tiny fragmented IP packets, which can be harder for packet filters to detect.
--mtu	nmap 192.168.1.1 --mtu 32	Set your own maximum transmission unit (MTU) offset size for packet fragmentation.
-D	nmap -D 192.168.1.101,192.168.1.102,192.168.1.103,192.168.1.23 192.168.1.1	Send scans from spoofed IP addresses. This can help hide your identity during scans.
-D	nmap -D decoy-ip1,decoy-ip2,your-own-ip,decoy-ip3,decoy-ip4 remote-host-ip	An explanation of the -D switch, allowing you to send scans from multiple decoy IP addresses.
-S	nmap -S www.microsoft.com www.facebook.com	Scan a target (e.g., Facebook) from a different source (e.g., Microsoft). Use with caution, as this involves IP address spoofing. Additional options like -e eth0 -Pn may be required.
-g	nmap -g 53 192.168.1.1	Use a given source port number (e.g., source port 53).
--proxies	nmap --proxies http://192.168.1.1:8080,http://192.168.1.2:8080 192.168.1.1	Relay connections through HTTP/SOCKS4 proxies. Useful for hiding your true source IP address.
--data-length	nmap --data-length 200 192.168.1.1	Append random data to sent packets, which can help obfuscate traffic and evade certain filters.

Output and Display Options

Switch	Example	Description
-oN	nmap 192.168.1.1 -oN normal.file	Save normal output to the file normal.file.
-oX	nmap 192.168.1.1 -oX xml.file	Save XML output to the file xml.file.
-oG	nmap 192.168.1.1 -oG grep.file	Save grepable output to the file grep.file.
-oA	nmap 192.168.1.1 -oA results	Output in all major formats (normal, xml, and grepable) at once, with the base name results.
-oG -	nmap 192.168.1.1 -oG -	Send grepable output to the screen. -oN - and -oX - are also usable for this purpose.
--append-output	nmap 192.168.1.1 -oN file.file --append-output	Append the results of a scan to a previous scan file.
-v	nmap 192.168.1.1 -v	Increase the verbosity level (use -vv or more for greater detail).
-d	nmap 192.168.1.1 -d	Increase the debugging level (use -dd or more for greater detail).
--reason	nmap 192.168.1.1 --reason	Display the reason a port is in a particular state, providing the same output as -vv.
--open	nmap 192.168.1.1 --open	Only show open (or possibly open) ports in the output.
--packet-trace	nmap 192.168.1.1 -T4 --packet-trace	Show all packets sent and received during the scan for debugging purposes.
--iflist	nmap --iflist	Display information about host interfaces and routes.
--resume	nmap --resume results.file	Resume a scan using the information from a previous scan stored in results.file.

Helpful Nmap Output Examples

Command	Description
nmap -p80 -sV -oG - --open 192.168.1.1/24 | grep open	Scan for web servers and use grep to show which IPs are running web servers.
nmap -iR 10 -n -oX out.xml | grep "Nmap" | cut -d " " -f5 > live-hosts.txt	Generate a list of the IP addresses of live hosts.
nmap -iR 10 -n -oX out2.xml | grep "Nmap" | cut -d " " -f5 >> live-hosts.txt	Append IP addresses to the list of live hosts.
ndiff scan1.xml scan2.xml	Compare output from Nmap scans using the ndiff tool.
xsltproc nmap.xml -o nmap.html	Convert Nmap XML files to HTML format.
grep " open " results.nmap | sed -r 's/ +/ /g' | sort | uniq -c | sort -rn | less	Generate a reverse-sorted list showing how often ports turn up as open in the Nmap results.

Miscellaneous Options

Switch	Example	Description
-6	nmap -6 2607:f0d0:1002:51::4	Enable IPv6 scanning.
-h	nmap -h	Display the Nmap help screen.

Other Useful Nmap Commands

Command	Description
nmap -iR 10 -PS22-25,80,113,1050,35000 -v -sn	Discovery only on specified ports, no port scan.
nmap 192.168.1.1-1/24 -PR -sn -vv	ARP discovery only on the local network, no port scan.
nmap -iR 10 -sn -traceroute	Perform a traceroute to random targets, no port scan.
nmap 192.168.1.1-50 -sL --dns-server 192.168.1.1	Query the internal DNS server for hosts and list targets only.

Conclusion

This Nmap cheat sheet provides you with essential commands to start using Nmap effectively. However, Nmap offers a plethora of advanced and complex scanning options for various scenarios. To delve deeper into Nmap’s capabilities, consider exploring the official documentation.

Remember to use Nmap responsibly and ethically, respecting legal boundaries and permissions when scanning networks or systems.

Nmap is a valuable tool in the world of network security and administration. By mastering its usage, you can enhance your ability to assess network vulnerabilities, identify security risks, and maintain robust network infrastructure.

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