How to Scan & Find All Open Ports with Nmap

Introduction

Nmap is the world’s leading port security network scanner. The Nmap hosted security tool can help you determine how well your firewall and security configuration is working.

This guide will show you how to use Nmap to scan all open ports on Linux systems.

Prerequisites

• Linux operating system
• Access to a user account with sudo or root privileges
• Access to a command line/terminal window
• The apt package manager, included by default (Debian / Ubuntu)
• The yum package manager, included by default (Red Hat, CentOS)

What Are Ports?

On modern operating systems, ports are numbered addresses for network traffic. Different kinds of services use different ports by default.

For example, regular web traffic uses port 80, while the POP3 email uses port 110. One of the ways that a firewall works is by allowing or restricting traffic over a particular port.

Because the port configuration can cause a security risk, it’s critical to know which ports are open and which are blocked.

How to Scan Nmap Ports

To scan Nmap ports on a  remote system, enter the following in the terminal:

sudo nmap 192.168.0.1

Replace the IP address with the IP address of the system you’re testing. This is the basic format for Nmap, and it will return information about the ports on that system.

In addition to scanning by IP address, you can also use the following commands to specify a target:

To scan a host:

nmap www.hostname.com

To scan a range of IP addresses (.1 – .10):

nmap 192.168.0.1-10

To run Nmap on a subnet:

nmap 192.168.0.1/13

To scan targets from a text file:

nmap –iL textlist.txt

Scan a Single Port, All Ports, or Series

Nmap commands can be used to scan a single port or a series of ports:

Scan port 80 on the target system:

nmap –p 80 192.168.0.1

Scan ports 1 through 200 on the target system:

nmap –p 1-200 192.168.0.1

Scan (Fast) the most common ports:

nmap –F 192.168.0.1

To scan all ports (1 – 65535):

nmap –p– 192.168.0.1 

Other Types of Nmap Port Scans

Different types of scans can be performed:

To scan using TCP connect (it takes longer, but is more likely to connect):

nmap –sT 192.168.0.1

To perform the default SYN scan (it tests by performing only half of the TCP handshake):

nmap –sS 192.168.0.1

To instruct Nmap to scan UDP ports instead of TCP ports (the –p switch specifies ports 80, 130, and 255 in this example):

nmap –sU –p 80,130,255 192.168.0.1

Run a fast scan on the target system, but bypass host discovery. (Host discovery uses ping, but many server firewalls do not respond to ping requests. This option forces the test without waiting for a reply that may not be coming):

nmap –Pn –F 192.168.0.1 

The nmap utility can be used to detect the operating system of a particular target:

nmap –A 192.168.0.1

It can also be used to probe for the services that might be using different ports:

nmap –sV 192.168.0.1

Common Ports

Here is a brief list of standard ports and their designations:

• 21 – FTP
• 22 – SSH
• 25 – SMTP (sending email)
• 53 – DNS (domain name service)
• 80 – HTTP (web server)
• 110 – POP3 (email inbox)
• 123 – NTP (Network Time Protocol)
• 143 – IMAP (email inbox)
• 443 – HTTPS (secure web server)
• 465 – SMTPS (send secure email)
• 631 – CUPS (print server)
• 993 – IMAPS (secure email inbox)
• 995 – POP3 (secure email inbox)

A Linux firewall can be configured to block all traffic on a particular port.

For example, a firewall can be set to block Port 80, but users won’t be able to load any website in that case. You can use firewall rules to allow some ports, but block others. Use a firewall in conjunction with other network security tools and software to scan traffic on a particular port, and to watch for suspicious traffic.

Nmap Scanning Best Practices

You should only use Nmap port scanning on servers that you own, or that you have permission to scan. Often, port-scanning is seen as an aggressive method, or a prelude to a cyber attack. It is also considered a bad practice to tie up a server’s resources by using Nmap to run repeated scans on the same target.

It is possible that during your scan, you may find unusual activity. For example, you may see a service running on an unusual port number. This means there is something strange going on, and should be investigated.

The OS and Service scanning options are helpful for scanning a particular port or service to get more information. If a service is running on a non-default port, it might be by design – or it might suggest there is a security breach.

Ports often have a default usage. Most ports under 1000 are dedicated and assigned to a specific service.

Conclusion

This guide provided an overview of Nmap scanning and how you can use it for testing ports in Linux. You should now understand how ports work, and why it is important to know how they are used.

Nmap adds a versatile tool to any system administrator’s arsenal for debugging and locating security flaws.