Port Scanning

Port Scanning

At a Glance

Port scanning helps you gather additional information about the target by interacting with it. It will give you a better understanding of how different systems interact with each other and host-specific details such as operating systems and available services.

Port Scanners

Port scanners can be classified as synchronous and asynchronous.

Synchronous or connection-oriented scanners, like Nmap, wait for the host response to determine if the port is alive. On the other hand, asynchronous or connectionless scanners, like scanrad, zmap, or masscan, do not wait for a host response as they create separate threads for each port.

Synchronous scanners tend to be slower, especially on large network ranges, but accurate for port discovery. While asynchronous scanners are faster but less accurate since they cannot detect dropped packets. 1


  1. Network Tracing - Infer the network topology.
  2. Network Sweep - Obtain a list of potential targets.
  3. Thorough Hosts Scan - Enumerate OSs and services on the targets.
  4. Vulnerability scans - Enumerate vulnerabilities on the targets’ services.

Network Sweep

masscan 2

sudo masscan -p 7,9,13,21-23,25-26,37,53,79-81,88,106,110-111,113,119,135,139,143-144,179,199,389,427,443-445,465,513-515,543-544,548,554,587,631,646,873,990,993,995,1025-1029,1110,1433,1720,1723,1755,1900,2000-2001,2049,2121,2717,3000,3128,3306,3389,3986,4899,5000,5009,5051,5060,5101,5190,5357,5432,5631,5666,5800,5900,6000-6001,6646,7070,8000,8008-8009,8080-8081,8443,8888,9100,9999-10000,32768,49152-49157 --rate 50000 --wait 0 --open-only -oG masscan.gnmap
  • -p <ports>: port(s) to be scanned.
  • --rate <rate>: rate for transmitting packets (packet per seconds).
  • --wait <seconds>: seconds after transmit is done to wait for receiving packets before exiting. Default 10 seconds.
  • --open-only: report only open ports.
  • -oG <filename>: output scan in grepable format to the given filename.

Host Scan

TCP Scan

This is a good all-purpose initial scan. Scans the most common 1000 ports with service information (-sV), default scripts (-sC), and OS detection.

Note: Verbose mode (-v) not only provides the estimated time for each host, but, it also prints the results as it goes letting you continue with the reconnaissance while scanning.

sudo nmap -v -sV -sC -O -T4 -n -Pn -oA nmap_scan

Similar scan but scans all ports, from 1 through 65535.

sudo nmap -v -sV -sC -O -T4 -n -Pn -p- -oA nmap_fullscan

UDP Scan

During a UDP scan (-Su) -sV will send protocol-specific probes, also known as nmap-service-probes, to every open|filtered port. In case of response the state change to open. 3

sudo nmap -sU -sV --version-intensity 0 -n

Enumeration Scan

In addition to the default scripts that run with -sC or --script=default, NSE (Nmap Scripting Engine), provides multiple types and categories to select from.

It is a common practice to combine the list of ports resulting from a network sweep or a fast nmap scan like:

nmap -T4 -Pn -n

with a more extensive scan against those ports.

sudo nmap -v -sV -O --script="safe and vuln" -T4 -n -Pn -p135,445 -oA nmap_scan

The --script parameter is extremely flexible and full features, refer to the documentation for more about NSE.

Note: Never run scripts from third parties unless you trust the authors or have carefully audited the scripts yourself.

  • -v: verbose mode.
  • -sU: UDP scan.
  • -sV: determine service/version info.
  • -sC: performs a script scan.
  • -O: enable OS detection.
  • -p <port ranges>: scan specific ports.
  • --open: only show open ports.
  • -Pn: no ping.
  • -n: no DNS resolution.
  • -T<0-5>: timing template.
  • oX <filename>: output scan in XML format to the given filename.
  • --version-intensity 0: try only the probes most likely to be effective.


List all Nmap scripts categories

grep -r categories /usr/share/nmap/scripts/*.nse | grep -oP '".*?"' | sort -u

List Nmap scripts for a specific category

List scripts under the default category.

grep -r categories /usr/share/nmap/scripts/*.nse | grep default | cut -d: -f1

Further Reading

  1. Capt. Meelo. “Finding the Balance Between Speed & Accuracy During an Internet-Wide Port Scanning - Hack.Learn.Share.” Hack.Learn.Share, 29 July 2019, https://captmeelo.com/pentest/2019/07/29/port-scanning.html. ↩︎

  2. Graham, Robert David. “GitHub - Robertdavidgraham/Masscan: TCP Port Scanner.” GitHub, https://github.com/robertdavidgraham/masscan. ↩︎

  3. “UDP Scan (-SU) | Nmap Network Scanning.” Nmap: The Network Mapper - Free Security Scanner, https://nmap.org/book/scan-methods-udp-scan.html. ↩︎